Asphalt surfacing to bridge decks

Prepared for SSR Directorate (Highways Infrastructure)

Pavement Engineering Team, Highways Agency

J C Nicholls, R W Jordan and K E Hassan

TRL Report TRL655

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First Published 2006ISSN 0968-4107ISBN 1-84608-654-XCopyright TRL Limited 2006.

This report has been produced by TRL Limited, under/as partof a contract placed by the Highways Agency. Any viewsexpressed in it are not necessarily those of the Agency.

TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

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CONTENTS

Page

Executive Summary 1

1 Introduction 3

2 Questionnaire 3

2.1 Format 3

2.2 Responses 3

3 Laboratory test programme 3

3.1 Asphalt mixture properties 3

3.1.1 Objectives and test programme 3

3.1.2 Air voids content 4

3.1.3 Permeability 5

3.1.4 Indirect Tensile Stiffness Modulus 6

3.1.5 Wheel-tracking 8

3.1.6 Review of asphalt results 8

3.2 Asphalt flexibility 10

3.2.1 Objectives and test programme 10

3.2.2 Indirect tensile fatigue test 10

3.2.3 4-point bending fatigue test 10

3.2.4 Semi-circular bending test 10

3.2.5 Asphalt bending test 12

3.2.6 Discussion 12

3.3 Waterproofing system properties 15

3.3.1 Objectives and test programme 15

3.3.2 Torque bond tests 15

3.3.3 Interface permeability 16

4 Considerations for specification of surfacing forbridge decks 17

4.1 General approach 17

4.2 Drainage 17

4.2.1 Drainage requirements 17

4.2.2 Removal of water 17

4.2.3 Sub-surface drainage 18

4.3 Waterproofing system 18

4.3.1 Bond to asphalt 18

4.3.2 Membrane stiffness 18

4.3.3 Effect of laying and compaction temperatures onthe waterproofing system 19

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Page

4.4 Asphalt properties 20

4.4.1 Deformation resistance 20

4.4.2 Texture depth 21

4.4.3 Skid resistance 21

4.4.4 Flexibility and fatigue 21

4.4.5 Permeability and air voids content 21

4.4.6 Protection for waterproofing systems 22

4.5 Joints and interface between layers 22

4.5.1 General 22

4.5.2 Joints 22

4.5.3 Bond between layers 22

4.5.4 Bond to waterproofing system 23

4.5.5 Permeability of interface between asphalt andwaterproofing system 23

5 Conclusions 24

6 Acknowledgements 24

7 References 25

Appendix A: HA draft notes for bridge-deck overlays 27

Appendix B: Questionnaire 28

Appendix C: Permeability tests 32

Appendix D: Semi-circular bending test 33

Abstract 40

Related publications 40

1

Executive Summary

equal ranking. Material optimisation is usually avoidingany excessively adverse property rather than getting thebest performance in all.

The flexibility test programme involved applying fourdifferent tests to three control mixtures and six stonemastic asphalt mixtures. The tests undertaken were theindirect tensile fatigue test, the 4-point bending fatigue testat low temperature and low frequency, the semi-circularbending test from the Netherlands and an asphalt bendingtest from China. The results obtained from the testprogramme are reviewed together with theoreticalconsiderations and the semi-circular bending test isidentified as the most promising. This test was found to bepractical, equivalent to a controlled stress fatigue test andranks materials similarly to binder content, a knowncomponent of flexibility if other things are constant. It issuggested that initially values of 3.0 N/mm2 and 19 N/mm3/2

could be set as the minimum values required for tensilestrength and fracture toughness, respectively. These limitsare considered practical because they were exceeded bythe majority of the mixtures tested.

The results from the torque bond test on specimensincluding waterproofing systems without secondarycompaction varied by an order of magnitude, with the twowaterproofing systems used ranking the surfacingmaterials differently. In all cases, the failures were at theinterface between the waterproofing membrane and thetack/bond coat, so the failure stresses were more dependenton the properties of the waterproofing systems than theasphalt mixtures. For one waterproofing system, thehighest failure stresses were measured with a tack coat andsand carpet whilst, for the other waterproofing system, thefailure stresses differed for two different tack coats. Thehigh values for mastic asphalt were probably due to thehigh temperature at which the material was laid andcompacted and at which the tack/bond coat was activated.

Based on these findings and other considerations,various additions and changes to the Design Manual forRoads and Bridges, Specification for Highway Works andNotes for Guidance on the Specification for HighwayWorks have been proposed. The main changes include:

! Sub-surface drainage is emphasised.

! Bond requirements are strengthened.

! Deformation requirements are specified for all mixtureswithin 100 mm of the surface.

! Maximum air voids content limits on all asphaltmixtures.

Aspects that were not fully covered are permeabilitytesting of the asphalt and at the interfaces. Potential testshave been identified that could be developed forstandardisation if these aspects are considered critical. Theasphalt permeability can be covered by the surrogate of airvoids content, but this approach is more difficult atinterfaces where more than one material is involved.

The current requirements in the Specification for HighwayWorks require waterproofing systems on concrete bridgedecks to be overlaid with a 20 mm thick sand asphaltprotection layer and binder and surface courses so that thetotal thickness of the three layers is 120 mm. Theserequirements include that the material that directlyoverlays the waterproofing system should be sand asphaltas well as considerations about the bond of the surfacing tothe waterproofing system and the sub-surface drainage. Inservice, it has been found that the performance ofsurfacing on concrete bridges is generally satisfactory ifthe total thickness of the asphalt layers is at least 120 mm.However, the total thickness on some bridges has to bereduced for practical and/or economic reasons and, in suchcases, a number of premature failures have occurred whenthe asphalt has broken up and potholes have developed.Therefore, HA commissioned TRL to develop aspecification for the asphalt surfacing on bridge decks thatis suitable even when the surfacing has a total thickness ofless than 120 mm. The objective of this work was toinvestigate parameters that would allow the developmentof a specification for surfacings on concrete bridges thatenhances the probability of achieving reasonable durabilitywhen they are less than the standard thickness.

The research has included a literature search, aquestionnaire and laboratory test programmes. Theliterature search provided little useful information. Theresults from the questionnaire, sent to individuals andcompanies involved in the manufacture and laying of thecurrent materials used on bridge decks, weredisappointing, with only eight responses received.

The laboratory test programme of asphalt mixtures thathave been, or could be, used for surfacing bridge decks hasidentified some differences in their properties that have beenused as the basis for the specification. A supplementary testprogramme was also undertaken to look at tests formeasuring the flexibility of asphalt materials, an importantproperty for both bridge deck surfacings (particular thosewith a thin deck) and other situations with relative thinpavements over soft substrates. Also, tests were undertakenon composite samples of concrete, waterproofing andasphalt to assess the bond achieved.

From the tests on a series of twelve asphalt mixtures,mastic asphalt was found to have the most suitable airvoids content, permeability and stiffness modulusproperties and was fifth at wheel-tracking, making it thebest material overall for these tests. However, masticasphalt is a relatively expensive mixture, a factor thatcannot be excluded. A dense 0/10 SMA was the next bestdespite having the 8th highest air voids content whilst anopen 0/10 SMA was the worst, showing that the precisemixture design can be critical. The remaining mixturesshowed relatively similar overall ratings, but with the sandcarpet only ranked 8th overall. Nevertheless, whenconsidering the appropriate materials, the choice is often atrade off between properties and they will not usually have

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3

1 Introduction

The current clause in the Specification for Highway Works(MCHW 1) requires waterproofing systems on concretebridge decks to be overlaid with a 20 mm thick sand asphaltprotection layer and binder and surface courses so that thetotal thickness of the three layers is 120 mm. A previousresearch project by TRL for the Highways Agency (HA)found that the performance of surfacing on concrete bridgesis generally satisfactory if the total thickness of the asphaltlayers is at least 120 mm. However, the total thickness onsome bridges has to be reduced for practical and/oreconomic reasons and, in such cases, a number of prematurefailures have occurred when the asphalt has broken up andpotholes have developed. The failures have been attributedto a number of factors, including:

! The accumulation of sub-surface water in the asphalt.

! Poor bond of the asphalt layers to the waterproofingsystem.

! Low compressive modulus of the waterproofing system.

! Low stiffness modulus of the asphalt layers.

Therefore, HA commissioned TRL to develop aspecification for the asphalt surfacing on bridge decks thatis suitable even when the surfacing has a total thickness ofless than 120 mm. The specification is intended to developfurther the existing HA advice for bridge deck surfacing,which is reproduced as Appendix A.

The research has included a literature search,questionnaire and two laboratory test programmes. Theresults from the literature search and the questionnaire tothose that manufactured and laid the current materials usedon bridge decks were disappointing, with only eightresponses received from the questionnaire. However, theinitial laboratory test programme of asphalt mixtures thathave been, or could be, used for surfacing bridge decks hasidentified some differences in their properties that havebeen used as the basis for the specification. The secondlaboratory test programme identified a test method forassessing the flexibility of asphalt materials.

2 Questionnaire

2.1 Format

A questionnaire was issued to individuals and companiesinvolved in the manufacture and laying of the currentmaterials used on bridge decks in order to ascertain if theywere aware of any deficiencies in the present systems and,probably more importantly, whether they considered thatsome new materials, or combination of materials, couldproduce better results. It was anticipated that the replieswould focus on practical aspects. There was a deliberatepolicy to keep the questionnaire relatively short but toallow recipients to extend their thoughts if they wished.The questionnaire, which is reproduced as Section B.1 ofAppendix B, was sent to a total of 32 people from 23different organisations.

2.2 Responses

The reaction to the questionnaire was very limited withonly eight responses received, of which only fourcontained useful comments. The contents of theconstructive replies are given in Section B.2 of Appendix B.However, any conclusions from the survey can only beregarded as indicative.

The replies were not consistent other than that the aimshould be to produce surfacings with a service life of about15 years. This aim is consistent with the top end of thedurability found for thin surfacing systems in conventionalsituations (Nicholls and Carswell, 2004) given that thesurfacing will need to be maintained at, if anything, ahigher standard than typically on a highway. Nevertheless,it is an indication of what engineers, let alone the drivingpublic, expect.

One reply indicated reservations about currently usedmaterials but the respondent did not think anything elsecurrently available would produce a practical alternativethat was economic. A second respondent offered epoxyasphalt, but that tends to be an expensive product thatpresents considerable risks if there is any distance betweenthe batching plant and the site. It is, therefore, onlyappropriate for important and/or conveniently locatedbridges, although it was for such site that the responder putit forward.

Another respondent did identify red sand carpet as aparticular material that should be replaced. Thisreplacement was already being considered because thelimited deformation resistance it provides will becomemore of a liability as the thickness of material over itreduces. Nevertheless, some material with relatively smallaggregate particles will be needed to protect some types ofthe waterproofing systems from being punctured.

3 Laboratory test programme

3.1 Asphalt mixture properties

3.1.1 Objectives and test programmeThe object of the initial laboratory test programme onasphalt mixtures was to identify the values achieved byvarious materials in tests used to measure the propertiesassumed to be relevant to bridge deck surfacings. Theseproperties are air voids content, permeability, stiffness anddeformation resistance. Air voids content and permeabilitywere investigated because drainage of water is particularlyimportant on bridges whereas the stiffness and deformationresistance are required as standard properties needed for allsurfacings. Flexibility was not included within thisprogramme because there are no accepted tests to definethe property, but is covered separately (Section 3.2).

The results were required in order to be able to specifyachievable levels of performance for future surfacing. Theknown limitations on the site performance of some of thematerials for specific parameters can be used to ensure thatthe requirements do not allow it to be used where it wouldnot perform satisfactorily.

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The materials tested had to represent those that havebeen, or could be, used on bridge decks. The mixtures thatwere considered covered the range of potential mixturetypes that might be used on bridge decks whilst not beingtoo extensive. The mixtures included a variety of hot rolledasphalts (HRA), dense bitumen macadams (DBM), stonemastic asphalts (SMA) and mastic asphalts. From these,the following twelve mixtures were selected for studyingin the initial test programme:

! Mastic asphalt BS 1447 (BSI, 1988).

! 0 % 0/2 HRA (Sand carpet) Column 6/1 in BS 594(BSI, 2002).

! 30 % 0/10 HRA – surface Column 6/3 in BS 594course (BSI, 2002).

! 35 % 0/14 HRA – surface Column 3/3 in BS 594course (BSI, 2002).

! 55 % 0/14 HRA – base Column 2/2 in BS 594(roadbase), binder course (BSI, 2002).and regulating course

! 55 % 0/10 HRA – base Column 2/1 in BS 594(roadbase), binder course (BSI, 2002).and regulating course

! 0/20 DBM – binder course Tables 15 and 16 inBS 4987 (BSI, 2001).

! 0/6 SMA Table 2, column 3 ofprEN 13108-5 (CEN, 2000a).

! 0/10 SMA (open) Table 2, column 5 ofprEN 13108-5 (CEN, 2000a).

! 0/10 SMA (dense) Table 2, column 5 ofprEN 13108-5 (CEN, 2000a).

! 0/14 SMA (open) Table 2, column 6 ofprEN 13108-5 (CEN, 2000a).

! 0/14 SMA(dense) Table 2, column 6 ofprEN 13108-5 (CEN, 2000a).

For convenience, the sieve sizes current at the start of thework were used to define the gradings for the first sevenmixtures rather than those implemented in 2004. Similarly,the SMA mixtures were derived from gradings given in thedraft for development of the harmonised European Standard(CEN, 2000a) after conversion to the ‘old’ sieve sizes. Thetarget gradings, based on the mid point of the envelopes formost of the mixtures but on the envelope boundaries for thetarget gradings of 0/10 and 0/20 SMA mixtures to give thepermitted extremes, are given in Table 3.1.

Four 300 mm by 300 mm by 50 mm thick slabs weremixed to BS EN 12697-35 (CEN, 2004a) and compactedby roller-compactor to BS EN 12697-33 (CEN, 2003a) foreach of the mixtures. Five 100 mm diameter cores werethen cut from two of the slabs of each material inaccordance with BS EN 12697-27 (CEN, 2000b).

One core from each cored slab was tested, consecutively,for bulk density to BS EN 12697-7 (CEN, 2002a), indirecttensile stiffness modulus at 20 °C and then at 40 °C toBS DD 213 (BSI, 1993) and maximum density to BS EN12697-5 (CEN, 2002b). The air voids contents were thencalculated in accordance with BS EN 12697-8 (CEN, 2003b)using the bulk density and maximum densities.

A separate core from the cored slab was tested for bulkdensity and the air voids content calculated in accordancewith BS EN 12697-8 (CEN, 2003b) using the maximumdensity for the previous core from the same slab. Thepermeability of the cores was then measured using the TRLpermeability cell, as described in Section C.1 of Appendix C.

The remaining two slabs of each mixture were tested forwheel-tracking in accordance with BS EN 12697-22(CEN, 2003c) using Procedure A with the small sizedevice, one slab at 45 ºC and the other at 60 ºC.

3.1.2 Air voids contentThe results of the density and air voids contentmeasurements are given in Table 3.2, where ‘ITSM’ and‘Permeability’ refer to the other test carried out on thesamples on which the measurements were made.

The bulk densities measured varied within a range of2.18 to 2.46 Mg/m³ and the maximum densities within arange of 2.27 to 2.51 Mg/m³. These values are of limitedvalue in themselves because they will be more dependenton the density of the aggregate than the properties of theasphalt. Such variation is hidden for this investigationbecause aggregate from the same source was used for allmixtures. However, the combination of bulk andmaximum density to produce air voids content is

Table 3.1 Composition of mixtures

30% 35% 55% 55%Mastic Sand 0/10 0/14 0/14 0/10

Type asphalt carpet HRA HRA HRA HRA

28 mm – – – – – –20 mm – – – 100 100 –14 mm – – 100 93 95 10010 mm 100 – 92 71 82 956.3 mm 95 100 75 – – –3.35 mm 80 – – – – –2.36 mm 60 97 66 61 45 450.600 mm – 90 58 53 35 350.300 mm – – – – – –0.212 mm – 47 32 28 17 170.075 mm 30 15 10 9 5.5 5.5

Binder content 8.0 10.3 7.8 6.4 6.5 6.5Binder grade 15 pen 50 pen 50 pen 50 pen 100 pen 100 pen

0/10 0/10 0/14 0/140/20 0/6 SMA SMA SMA SMA

Type DBM SMA (open)* (dense) (open) (dense)

28 mm 100 – – – – –20 mm 97 – – – 100 10014 mm 75 – 100 100 90 10010 mm 62 100 90 100 60 756.3 mm 47 95 35 50 40 503.35 mm 39 – 24 34 24 342.36 mm – 37 21 26 19 290.600 mm – – – – – –0.300 mm 14 – – – – –0.212 mm – – – – – –0.075 mm 5.5 10.5 9 11 9 11

Binder content 4.7 7.7 6.7 6.9 7.4 7.6Binder grade 100 pen 50 pen 50 pen 50 pen 50 pen 50 pen

* Also 0/10 PMB SMA (open) mixture except for binder grade for theflexibility programme (Section 3.2).

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Table 3.2 Air voids content results

Sample Bulk density Max density Air voids content

Material ID ITSM Permeability ITSM ITSM Permeability

Mastic asphalt A 2.38 2.37 2.38 0.2 % 0.4 %B 2.38 2.38 2.37 -0.4 % -0.4 %Mean 2.38 2.38 0.0 %

0/2 HRA A 2.19 2.22 2.27 3.3 % 2.1 %(Sand carpet) B 2.20 2.18 2.27 3.2 % 3.9 %

Mean 2.20 2.27 3.1 %

30 % 0/10 HRA A 2.34 2.36 2.39 2.2 % 1.4 %B 2.29 2.32 2.38 3.9 % 2.5 %Mean 2.33 2.39 2.5 %

35 % 0/14 HRA A 2.28 2.31 2.42 5.9 % 4.7 %B 2.26 2.31 2.37 4.9 % 2.7 %Mean 2.29 2.40 4.6 %

55 % 0/14 HRA A 2.37 2.38 2.45 3.0 % 2.7 %B 2.41 2.36 2.45 2.0 % 3.7 %Mean 2.38 2.45 2.8 %

55 % 0/10 HRA A 2.39 2.38 2.44 2.0 % 2.4 %B 2.38 2.40 2.45 2.6 % 1.9 %Mean 2.39 2.45 2.2 %

0/20 DBM A 2.46 2.44 2.51 2.0 % 3.0 %B 2.44 2.43 2.50 2.6 % 2.8 %Mean 2.44 2.51 2.6 %

0/6 SMA A 2.38 – 2.42 3.8 % –B 2.33 – 2.42 2.6 % –Mean 2.35 2.42 3.2 %

0/10 SMA A 2.22 2.21 2.41 8.2 % 8.3 %(open) B 2.21 2.21 2.42 8.6 % 8.7 %

Mean 2.21 2.42 8.5 %

0/10 SMA A 2.35 2.37 2.44 3.5 % 2.8 %(dense) B 2.35 2.35 2.43 3.5 % 3.6 %

Mean 2.35 2.43 3.3 %

0/14 SMA A 2.27 2.24 2.42 6.0 % 7.2 %(open) B 2.25 2.25 2.42 7.1 % 7.1 %

Mean 2.25 2.42 6.8 %

0/14 SMA A 2.36 2.37 2.45 3.5 % 3.0 %(dense) B 2.30 2.37 2.43 5.3 % 2.6 %

Mean 2.35 2.44 3.6 %

significant. For the mixtures tested, the air voids contentranged from zero for the mastic asphalt to 8.5 % for the0/10 SMA (open) mixtures. The width of the range and theranking of mixtures within it are consistent with what wasexpected, with the denser, more binder rich mixtureshaving less air voids content than the more open mixtures.

The relationships between the binder content of the variousmixtures and their densities are shown in Figure 3.1 andthat between the binder content and air voids content inFigure 3.2. Included on the figures are also linear trend lines,the equations for which are given as Equations (3.1) to (3.3).

ρbulk

= 2.56 – 0.033 bc

(R2= 0.34) ....(3.1)

ρmax

= 2.69 – 0.039 bc

(R2= 0.84) ....(3.2)

νair

= 5.03 – 0.198 bc

(R2= 0.014) ....(3.3)

where: ρbulk

= bulk density (Mg/m³)ρ

max= maximum density (Mg/m³)

νair

= air voids content (%)b

c= binder content (%)

There is little correlation between binder content and airvoids content because the binder contents are selectedbased on the mixture type and aggregate skeleton.However, the correlation between binder content andmaximum density is surprisingly robust.

3.1.3 PermeabilityThe permeability testing was carried out on asphalt cores(100 mm diameter and height between 70 mm and 100 mm).Air was applied at the desired pressure (P) and the flow

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rate was calculated using a bubble flowmeter with diameter,D

f, length, L

f, and the average flow time, t

average. The flow

rates were used for the calculation of the air permeabilities,as described in Section C.1 of Appendix C, and the resultsare given in Table 3.3.

The permeability ranking of the different asphalt mixturesis shown in Table 3.4. The lowest permeability values wereobtained from the mastic asphalt mixture, which exhibitedthe lowest air voids content of around 0 %. The permeabilityof the HRA mixtures varied over a range of two orders ofmagnitude for air voids content between 2.3 % and 5.4 %.The permeability and air voids content of the sand carpetand the 50 % stone HRA mixtures were lower than those ofthe 30 % and 35 % stone HRA mixtures. In fact, for thesame aggregate size (0/14), increasing the aggregate contentfrom 35 % to 55 % reduced the air voids content from 5.4 %to 2.4 % and the permeability by approximately two ordersof magnitude. The DBM and dense SMA mixtures gavepermeability values within the lower band of HRA. Incontrast, the high air voids content of the 0/6 and open SMAmixtures with >5 % air voids contents resulted in highpermeability values. These values were out of the limit ofthe flowmeter used with the TRL permeability cell.

The results in Table 3.4 shows that the permeabilityranking, from low to high, of the different asphalt mixturesis generally as follows: mastic asphalt, HRA, DBM, dense

SMA and open SMA. The most obvious outlier is 35 % 0/14HRA, which has a higher air voids content and, to a greaterextent, permeability than the other HRA mixtures.Nevertheless, this ranking is quite similar to that reportedearlier (Daines, 1995). It is important to highlight that thepermeability results are obtained from laboratory specimenswith high quality control, which may not represent theactual values of the same mixtures constructed on site.However, the ranking is expected to be the same.

Figure 3.3, with the data points plotted on a previouslyfound relationship (Zoorob, 1999), shows the influence ofair voids content on the permeability of asphalt. Whilst thereare not adequate data to establish a reliable relationship,particularly in terms of mixtures with high air voidscontents, a similar trend can be fitted to that found forconcrete. Regardless of the mixture type, low permeabilityvalues are obtained when the air voids content is 4 % orbelow and the permeability increases rapidly for air voidscontents above 5 %. In the absence of a specification limitfor permeability, it appears reasonable to use the air voidscontent as an indicator for permeability.

3.1.4 Indirect Tensile Stiffness ModulusThe results of the Indirect Tensile Stiffness Modulus(ITSM) test are given in Table 3.5.

2.15

2.20

2.25

2.30

2.35

2.40

2.45

2.50

2.55

4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Binder content (%)

Den

sity

(M

g/m

³)

Max density

Bulk density

0.0

2.0

4.0

6.0

8.0

10.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Binder content (%)

Air

void

s co

nten

t (%

)

Figure 3.1 Relationship between binder content and density

Figure 3.2 Relationship between binder content and air voids content

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Table 3.3 Permeability results

Sample Flow meter Permeability (10-17 m2)

Df Lf P taverage

Material ID (mm) (mm) (bar) (s) Sample Mean

Mastic asphalt A 1.7 50 0.8 25.0 0.55 0.73B 1.7 50 0.8 15.0 0.91

0 % 0/2 HRA A 1.7 100 0.8 13.3 1.96 2.27B 1.7 100 0.8 10.1 2.58

30 % 0/10 HRA A 1.7 100 0.6 3.3 12.52 8.09B 1.7 100 0.6 10.1 3.66

35 % 0/14 HRA A 10 100 0.4 7.6 276 219B 10 100 0.4 14.0 162

55 % 0/14 HRA A 1.7 50 0.4 20.0 1.56 1.85B 1.7 50 0.4 14.9 2.14

55 % 0/10 HRA A 1.7 100 0.6 21.2 1.84 2.50B 1.7 100 0.6 12.4 3.16

0/20 DBM A 10 – 0.8 – Permeable 3.41B 1.7 100 0.8 8.0 3.41

0/6 SMA A 10 – 0.8 – Permeable PermeableB 10 – 0.8 – Permeable

0/10 SMA (open) A 10 – 0.8 – Permeable PermeableB 10 – 0.8 – Permeable

0/10 SMA (dense) A 1.7 100 0.8 20.5 1.34 2.93B 1.7 100 0.8 6.0 4.52

0/14 SMA (open) A 10 – 0.8 – Permeable PermeableB 10 – 0.8 – Permeable

0/14 SMA (dense) A 1.7 100 0.8 4.8 5.64 4.47B 1.7 100 0.8 9.0 3.30

The ITSM of the mixtures ranged from 1.0 GPa to 5.8 GPaat 20 °C and 0.09 GPa to 0.55 GPa at 40 °C. However, theresults for mastic asphalt are effectively outliers, with therange reducing to 1.0 GPa to 2.2 GPa at 20 °C and 0.09 GPato 0.27 GPa at 40 °C without them. The high value formastic asphalt is probably due to the high binder contentgiving relative high tensile strength. Ignoring mastic asphalt,the lack of any correlation between ITSM and air voidscontent is shown in Figure 3.4. The plotted trend line at20 °C, including mastic asphalt, has a correlation coefficientof R² = 0.23, but the value reduces to 0.0006 when themastic asphalt is omitted.

Table 3.4 Permeability ranking of the asphalt mixtures

Air voids Air permeabilityMixture content (%) (10-17 m2)

Mastic asphalt < 1 0.73HRA 2.3 – 5.4 1.85 – 219DBM 2.3 3.4Dense SMA 3.5 – 4.4 2.9 – 4.5Open SMA 5.2 – 8.4 Permeable

Table 3.5 ITSM results

ITSM @ 20 °C (GPa) ITSM @ 40 °C (GPa)

Material A B Mean A B Mean

Mastic asphalt 5.13 6.38 5.75 0.55 0.54 0.550/2 HRA 1.43 1.54 1.48 0.11 0.15 0.1330 % 0/10 HRA 2.12 2.29 2.20 0.17 0.24 0.2135 % 0/14 HRA 3.00 2.22 2.61 0.32 0.22 0.2755 % 0/14 HRA 0.99 1.07 1.03 0.15 0.15 0.1555 % 0/10 HRA 1.08 1.25 1.16 0.15 0.18 0.16

0/20 DBM 1.84 1.47 1.65 0.15 0.19 0.17

0/6 SMA 1.89 2.08 1.99 0.16 0.16 0.16

0/10 SMA (open) 1.38 1.43 1.40 0.10 0.09 0.090/10 SMA (dense) 2.10 2.05 2.08 0.17 0.16 0.160/14 SMA (open) 1.57 1.41 1.49 0.12 0.10 0.110/14 SMA (dense) 1.51 1.64 1.58 0.10 0.19 0.15

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8 9

Air voids (%)

Air

perm

eabi

lity

(10-1

7 m

2 )

Figure 3.3 Influence of air voids content on thepermeability of asphalt

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The effect of temperature on ITSM is also shown inFigure 3.4. The ratio of ITSM at 20 °C to ITSM at 40 °Cranged from 6.9 to 15.4 for the mixtures tested with theranking order being marginally different for the twotemperatures, as shown in Table 3.6.

worst at deformation resistance with the other HRAmixtures in a band between the sand carpet and the other,almost non-deforming mixtures. Therefore, sand carpetsare not suitable for use in the top 100 mm of surfacingwhilst other HRA mixtures should be designed againstdeformation to SHW clause 943 (which was not the casefor the trialled mixtures) for use in that region.

An inconsistency among the mixtures is the 0/14 SMA(dense) at 45 ºC, in which the rate of deformation startedhigh before reducing considerably so that the rut depth isin the range for the HRA mixtures tested but the wheeltracking rate is only marginally higher than the other SMAmixtures. Therefore, this result is assumed to be an outliercaused by excessive bedding in at the beginning of the test.

3.1.6 Review of asphalt resultsThe rankings from the various tests can be combined toproduce a rough overall ranking as in Table 3.8. However,when considering the appropriate materials, the choice isoften a trade off between these properties and they will notusually have equal ranking. Material optimisation is usuallyavoiding any excessively adverse property rather thangetting the best performance in all. Nevertheless, mastic

Table 3.6 Ranking of ITSM results

ITSM (GPa) ITSM Ranking Ratio

@ @ @ @ 20 °CMixture 20 °C 40 °C 20 °C 40 °C Mean / 40 °C

Mastic asphalt 5.8 0.55 1 1 1 10.535 % 0/14 HRA 2.6 0.27 2 2 2 9.630 % 0/10 HRA 2.2 0.21 3 3 3 10.70/10 SMA (dense) 2.1 0.16 4 5= 4 12.7

0/20 DBM 1.7 0.17 6 4 5 9.80/6 SMA 2.0 0.16 5 5= 6 12.30/14 SMA (dense) 1.6 0.15 7 8= 7 10.855 % 0/10 HRA 1.2 0.16 11 5= 8 7.2

0/2 HRA 1.5 0.13 8= 10 9 11.50/14 SMA (open) 1.5 0.11 8= 11 10 13.855 % 0/14 HRA 1.0 0.15 12 8= 11 6.90/10 SMA (open) 1.4 0.09 10 12 12 15.4

The ranking orders are similar for the two temperatures,but not identical. The top three materials are the same inboth with the material that has the greatest change inranking being 55 % 0/10 HRA, which is 11th at 20 °C but5th= at 40 °C. The differences demonstrate that the choiceof test temperature will be important if the asphalt stiffnessneeds to be a parameter when specifying the stiffness ofthe waterproofing system.

3.1.5 Wheel-trackingThe results of the wheel tracking test are given in Table 3.7with the plots of the tests undertaken at 45 ºC and 60 ºCbeen shown in Figure 3.5 and Figure 3.6, respectively.

The plots for the tests at 60 ºC show dramatically thatthe HRA mixtures, with the possible exception of the 35 %0/14 HRA, were less deformation resistant than the DBMand SMA mixtures. At the lower temperature of 45 ºC, theplots show that 0/2 HRA (sand carpet) is significantly the

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 2.0 4.0 6.0 8.0 10.0

Air voids content (%)

Indi

rect

tens

ile s

tiffn

ess

Mod

ulus

(kP

a)

ITSM @ 20oC

ITSM @ 40oC

Table 3.7 Wheel tracking test results

Testing @ 45 ºC Testing @ 60 ºC

Wheel Wheeltracking Rut tracking Rut

rate depth rate depth OverallMaterial (µm/cycle) (mm) (µm/cycle) (mm) ranking

0/10 SMA (dense) 0.7 3.2 2.7 5.6 10/14 SMA (open) 1.0 3.1 3.6 6.6 20/6 SMA 1.2 3.4 3.3 8.0 30/10 SMA (open) 1.2 3.0 4.1 7.1 4

Mastic asphalt 0.8 2.3 6.7 11 50/20 DBM 0.9 2.7 6.7 7.7 60/14 SMA (dense) 2.1 11 4.6 9.4 735 % 0/14 HRA 7.6 9.6 14 17 8

55 % 0/10 HRA 6.0 10 55 84 955 % 0/14 HRA 4.1 7.1 44 71 1030 % 0/10 HRA 7.9 12 70 98 110 % 0/2 HRA 18 25 76 103 12

Figure 3.4 Stiffness modulus variation with air voids content and temperature

9

0

2

4

6

8

10

12

14

16

18

20

0 100 200 300 400 500 600 700 800 900 1000

Number of load cycles

Rut

dep

th @

45o C

(m

m)

0% 0/2 HRA 30% 0/10 HRA 35% 0/14 HRA 55% 0/14 HRA 55% 0/10 HRA 0/20 DBM

0/6 SMA 0/10 SMA open 0/10 SMA dense 0/14 SMA open 0/14 SMA dense Mastic Asphalt

0 100 200 300 400 500 600 700 800 900 1000

Number of load cycles

0% 0/2 HRA 30% 0/10 HRA 35% 0/14 HRA 55% 0/14 HRA 55% 0/10 HRA 0/20 DBM

0/6 SMA 0/10 SMA open 0/10 SMA dense 0/14 SMA open 0/14 SMA dense Mastic Asphalt

0

2

4

6

8

10

12

14

16

18

20

Rut

dep

th @

60o C

(m

m)

Figure 3.5 Plot of wheel-tracking results @ 45 oC

Figure 3.6 Plot of wheel-tracking results @ 60 oC

10

asphalt is demonstrated to be the best material for all theproperties other than deformation resistance, when it wasstill in the top half. However, mastic asphalt is a relativeexpensive mixture, a factor that cannot be excluded.

The maximum density (and, to a lesser extent, bulkdensity) correlated well with the binder content, but notwith the air voids content because the binder content wasselected based on the mixture type and aggregate skeleton.Therefore, assuming air voids content is considered to beindicative of permeability, the binder content cannot beused as a surrogate for permeability without reference tothe mixture type.

3.2 Asphalt flexibility

3.2.1 Objectives and test programmeFlexible asphalt has obvious advantages for use insituations where there is potential movement. Suchsituations include roads on ‘soft’ foundations as well asbridge decks. However, there are currently no testrequirements for flexibility in either the British Standardsfor asphalt or in the British Board of Agrément (BBA)Highway Authorities Product Approval Scheme (HAPAS)for thin surfacing systems. The inclusion of such a test asan option in the BBA/HAPAS scheme has been requestedby representatives from the County Surveyors Society.Because of the synergy with the bridge deck issue, theHighways Agency extended the scope of this work toinclude a programme of tests to identify a suitable test andtypical values for the property.

The test programme devised consisted of undertakingfour different tests that were proposed by members ofBBA-HAPAS Specialist Group 3 on a series of asphaltmixtures. The known ranking for the properties required ofsome of the mixtures tested was hoped to provide aninsight into which of the tests had potential to assess theflexibility of asphalt mixtures.

From the mixtures used in the asphalt material trials(Section 3.1.1), the mastic asphalt, 30 % 0/10 HRA and 55 %0/14 HRA mixtures were selected as the controlmixtures for the flexibility programme with the SMAmixtures, including a variant of the 0/10 PMB SMA(open) mixture that used styrene-butadiene-styreneblock co-polymer (SBS) modified binder instead offibres, as the trial materials.

The test methods undertaken on the nine mixtures were:

a Indirect tensile fatigue test.

b 4-point bending fatigue test.

c Semi-circular bending test.

d Asphalt bending test.

The indirect tensile fatigue tests were carried out inaccordance with Annex E of BS EN 12697-24 (CEN,2004b) at 20 °C except that the deformations were measuredvertically rather than horizontally because Cooper ResearchTechnology Limited, manufacturer of the fatigue equipmentused, do not offer a horizontal deflection measurementsystem. The specimens had to go through the test two orthree times before they fractured because the test stops at adeflection of 8 mm. The results are the cumulative numberof cycles and deflections combined.

The 4-point bending fatigue tests were carried out inaccordance with Annex D of BS EN 12697-24 (CEN,2004b) with a loading device capable of imparting a fixedadjustable amplitude up to ± 5 mm with a frequency of6 cycles/h at a temperature of 5 ºC and a sample size of300 mm × 40 mm × 40 mm.

The semi-circular bending tests were carried out inaccordance with Appendix D, repeated on separatesamples with and without a notch, at a temperature of15 °C and a deformation rate of 5.1 mm/min. The draftmethod is based on a method developed in the Netherlandsand has recently been put forward for standardisation as aEuropean test method to assess the crack propagationproperty of asphalt.

The asphalt bending tests were carried out in accordancewith Appendix E at temperatures of 0 °C and 15 °C and adeformation rate of 50 mm/min. The draft method is based onPRC Requirement T0715-93 (Peoples Republic of China,1993) except that, for practicality, the deformation wasmeasured at the top of the specimen rather than the bottom.During the tests, it appeared that compression of thespecimens was minimal (also evidenced by the low test loadsachieved before failure) which would imply very similardeformations at the bottom and the top of the specimens.

3.2.2 Indirect tensile fatigue testThe mean fatigue lives of the various asphalt mixturestested for different applied tensile stresses are summarisedin Table 3.9. An approximate ranking of the mixtures hasbeen included.

These values are plotted on Figure 3.7, from where therough ranking order can be seen.

3.2.3 4-point bending fatigue testThe mean fatigue lives in terms of time of the variousasphalt mixtures tested are summarised in Table 3.10. Anapproximate ranking of the mixtures has been included.

3.2.4 Semi-circular bending testThe values calculated for the tensile strength (from the un-notched samples) and the fracture toughness (from thenotched samples) of the asphalt mixtures tested are givenin Table 3.11. An approximate ranking of the mixtures has

Table 3.8 Overall rankings for asphalt properties

Air voids Perme Stiffness Wheel Sum ofTest content -ability modulus -tracking rankings

Mastic asphalt 1 1 1 5 80/10 SMA (dense) 8 5 4 1 180/20 DBM 4 6 6 6 2255 % 0/10 HRA 2 4 8 10 24

30 % 0/10 HRA 3 8 3 11 250/6 SMA 7 11= 5 3 2655 % 0/14 HRA 5 2 11 9 2735 % 0/14 HRA 10 9 2 8 29

0/2 HRA 6 3 9 12 300/14 SMA (open) 11 11= 7 2 310/14 SMA (dense) 9 7 10 7 330/10 SMA (open) 12 11= 12 4 39

11

Table 3.9 Summary of mean fatigue lives in ITFT

Applied tensile stress (kPa)

Material 100 150 250 350 500 570 750 Ranking

Mastic asphalt – – – – 155,000 106,000 5,280 10/14 SMA(dense) – 176,000 11,900 6,810 – – – 2=0/10 SMA (dense) – 162,000 15,600 7,250 – – – 2=30 % 0/10 HRA – 117,000 16,900 6,470 – – – 2=0/6 SMA – 94,300 12,500 5,130 – – – 2=

0/14 SMA (open) – 64,200 9,440 2,360 – – – 60/10 SMA (open) 43,200 20,200 4,670 – – – – 7=55 % 0/14 HRA 95,000 19,400 3,100 – – – – 7=0/10 PMB SMA (open) 27,000 5,620 1,670 – – – – 9

Table 3.10 Summary of fatigue lives in 4-point bending

Fatigue life (h) for sample:

Material 1 2 3 Mean Ranking

0/10 PMB SMA (open) 4.87 8.39 6.65 6.63 155 % 0/14 HRA 4.85 6.41 3.25 4.83 2=0/6 SMA 10.0 1.81 2.00 4.61 2=0/10 SMA (open) 5.87 2.82 1.94 3.55 430 % 0/10 HRA 3.53 2.92 1.68 2.71 5

0/10 SMA (dense) 2.92 1.48 1.81 2.07 60/14 SMA (dense) 1.99 1.03 1.55 1.52 7=0/14 SMA (open) 1.05 1.86 0.76 1.22 7=Mastic asphalt 0.77 1.38 1.05 1.07 9

Table 3.11 Summary of semi-circular bending test results

Tensile strength Fracture toughness

CombinedMaterial (N/mm²) Ranking (N/mm3/2) Ranking ranking

Mastic asphalt 8.29 1 50.24 1 130 % 0/10 HRA 3.86 2 24.68 2= 20/14 SMA (dense) 3.24 3= 23.03 2= 30/14 SMA (open) 3.35 3= 19.95 4= 40/6 SMA 3.03 5= 19.63 4= 5=

0/10 SMA (dense) 3.07 5= 19.88 4= 5=0/10 SMA (open) 2.43 7 17.35 7 755 % 0/14 HRA 1.88 8= 14.45 8 8=0/10 PMB SMA (open) 1.89 8= 11.87 9 8=

3.0

3.5

4.0

4.5

5.0

5.5

0 100 200 300 400 500 600 700

Applied tensile stress (kPa)

Log

(Fra

ctur

e lif

e)

30 % 0/10 HRA 55 % 0/14 HRA Mastic asphalt

0/6 SMA 0/10 SMA (open) 0/10 PMB SMA (open)

0/10 SMA (dense) 0/14 SMA (open) 0/14 SMA (dense)

Figure 3.7 Plot of mean fatigue lives

12

been included for both properties, which were similar,together with an overall ranking by rounding the averageof the two rankings.

3.2.5 Asphalt bending testThe values calculated for the bending strength, maximumbending strain and bending stiffness modulus of theasphalt mixtures tested are given in Table 3.12 for a testtemperature of 0 ºC and in Table 3.13 for a testtemperature of 15 ºC. An approximate ranking of themixtures has been included for each property plus anoverall ranking combining them for each temperature.

The bending strength and bending stiffness modulusrankings are in approximately the reverse order of therankings for maximum bending strain. An overall rankingcan be derived by averaging the rankings for the bendingstrength and bending stiffness modulus and the inverse(i.e. 10 minus the actual ranking) of the ranking for themaximum bending strain, as shown in Table 3.14.

3.2.6 Discussion3.2.6.1 PracticalityThe complexity of the potential flexibility tests differ. TheITFT took considerably longer than the other tests because thefatigue life has to be between 103 and 106 cycles with eachcycle lasting 0.5 s (0.1 s loading time and 0.4 s rest period),giving a total duration of between 8 min and 6 dayscontinuous working for a single determination with threedeterminations required per test. Of the other three tests, thefour-point bending fatigue test was the most complex to setup whilst the semi-circular bending and asphalt bending testswere both the simplest to set up and quickest to perform.

Table 3.12 Summary of asphalt bending test results@ 0 ºC

Bending Maxm Bendingstrength bending strain stiff. modulus

Value Ran- Ran- Value Ran-Material (MPa) king Value king (GPa) king

0/14 SMA (dense) 11.0 1= 0.00238 6= 4.69 10/10 PMB SMA (open) 8.3 5= 0.00229 6= 3.75 2=30 % 0/10 HRA 10.9 1= 0.00295 4 3.70 2=0/10 SMA (dense) 8.4 5= 0.00223 6= 3.85 2=

0/10 SMA (open) 6.6 9 0.00231 6= 2.85 5=55 % 0/14 HRA 10.7 1= 0.00380 2 2.91 5=0/14 SMA (open) 7.8 7= 0.00255 5 3.05 5=

Mastic asphalt 8.0 7= 0.00329 3 2.43 8

0/6 SMA 9.3 4 0.00501 1 1.88 9

Table 3.13 Summary of asphalt bending test results@ 15 ºC

Bending Maxm Bendingstrength bending strain stiff. modulus

Value Ran- Ran- Value Ran-Material (MPa) king Value king (GPa) king

0/14 SMA (dense) 14.86 1 0.0101 9 1503 10/10 SMA (dense) 6.19 3= 0.0190 7= 348 2=0/14 SMA (open) 6.71 2 0.0205 4= 331 2=0/10 PMB SMA (open) 6.26 3= 0.0203 4= 314 4Mastic asphalt 4.81 6= 0.0174 7= 276 5

30 % 0/10 HRA 6.27 3= 0.0289 3 218 6=0/10 SMA (open) 4.80 6= 0.0217 4= 228 6=55 % 0/14 HRA 3.83 8 0.0324 1= 121 80/6 SMA 2.41 9 0.0354 1= 72 9

A plot of the ranking of the six different measures (threeproperties at two temperatures) is shown in Figure 3.8.The order of the materials has been revised in accordancewith their average ranking from the bending strength andbending stiffness modulus at both temperatures to assistthe comparison.

0/6

SM

A

0/10

SM

A (

open

)

Mas

tic a

spha

lt

55 %

0/1

4 H

RA

0/14

SM

A (

open

)

0/10

PM

B S

MA

(o)

30 %

0/1

0 H

RA

0/10

SM

A (

dens

e)

0/14

SM

A (

dens

e)

Bending stiffness modulus @ 0oC

Bending stiffness modulus @ 15oC

Bending strength @ 0oC

Bending strength @ 15oCMax bending strain @ 0oC

Max bending strain @ 15oC

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Ran

kin

g

Material

Test

Figure 3.8 Relative rankings of the properties measured in the asphalt bending test

13

3.2.6.2 Comparison of rankingsThe overall rankings derived from each of the four testsare compared in Table 3.15 with the ranking plotted inFigure 3.9.

The mastic asphalt and 0/10 PMB SMA tend to be theextremes on the rankings, certainly for the first three testsalthough, compared to the ITFT and semi-circular bendingtests, the order is reversed for the 4-point bending and, to alesser extent, the asphalt bending test. The reason is thatthe tests undertaken measure different properties atdifferent temperatures.

The properties and temperatures are:

! ITFT: Fatigue life under controlled stress (force)conditions at 20 ºC.

! 4-point bending: Fatigue life under controlled strain(displacement) conditions at 5 ºC.

! Semi-circular bending: Tensile strength and/or fracturetoughness at 15 ºC.

! Asphalt bending: The bending strength, maximumbending strain and/or bending stiffness modulus at both0 ºC and 15 ºC.

Controlled stress and controlled strain tests are known toresult in reversed rankings. Therefore, it can be assumedthat the tensile strength and the fracture toughness in thesemi-circular bending test are aligned to constant stressrather than constant strain conditions.

Figure 3.10 shows the ranking orders when the materialsare listed in order of their semi-circular bending test resultswhilst Figure 3.11 shows them when ordered for the 4-pointbending test. Both these figures show that there a consistencyin the order for three of the tests (even if the 4-point bendingtest order has to be reversed), but that the overall rankingderived from the asphalt bending test shows no consistencywith the others.

Plotting the individual rankings from the asphaltbending test on graphs with the same axes as Figure 3.10and Figure 3.11 did not produce any obvious equivalence.Therefore, the properties derived from the asphalt bendingtest are not considered to be well correlated with thosefrom the other three tests.

Mastic asphalt could be expected to be the most flexiblebecause of its high binder content, although that binder is

Table 3.14 Development of an overall ranking for theasphalt bending test

Ranking

Max Bendingbending Bending stiff.strain strength mod.

@ @ @ @ @ @Material 0ºC 15ºC 0ºC 15ºC 0ºC 15ºC Overall

0/14 SMA (dense) 6= 9 1= 1 1 1 10/10 SMA (dense) 6= 7= 5= 3= 2= 2= 20/10 PMB SMA (open) 6= 4= 5= 3= 2= 4 30/14 SMA (open) 5 4= 7= 2 5= 2= 4=30 % 0/10 HRA 4 3 1= 3= 2= 6= 4=

Mastic asphalt 3 7= 7= 6= 8 5 6=0/10 SMA (open) 6= 4= 9 6= 5= 6= 6=55 % 0/14 HRA 2 1= 1= 8 5= 8 80/6 SMA 1 1= 4 9 9 9 9

Table 3.15 Ranking order by different tests

4-point Semi- AsphaltMaterial ITFT bending circular bending

0/14 SMA(dense) 2= 7= 3 10/10 SMA (dense) 2= 6 5= 20/10 PMB SMA (open) 9 1 8= 3

30 % 0/10 HRA 2= 5 2 4=0/14 SMA (open) 6 7= 4 4=

Mastic asphalt 1 9 1 6=0/10 SMA (open) 7= 4 7 6=55 % 0/14 HRA 7= 2= 8= 80/6 SMA 2= 2= 5= 9

30%

10

HR

A

55%

14

HR

A

Mas

tic a

spha

lt

0/6

SM

A

0/10

SM

A (

o)

0/10

PM

B S

MA

0/10

SM

A (

d)

0/14

SM

A (

o)

0/14

SM

A (

d)

ITFT

4-point bending

Semi-circular

Asphalt bending

0

1

2

3

4

5

6

7

8

9

Ran

kin

g

Material

Test

Figure 3.9 Plot of observed rankings for each test

14

harder than that for other mixtures which could lead to lessflexibility, particularly at colder temperatures. If the bindercontent is the overwhelming criteria for flexibility, then theranking would be as follows:

1 Mastic asphalt (8.0 % binder)

2 30 % 0/10 HRA (7.8 % binder)

3 0/6 SMA (7.7 % binder)

4 0/14 SMA (dense) (7.6 % binder)

5 0/14 SMA (open) (7.4 % binder)

6 0/10 SMA (dense) (6.9 % binder)

7= 0/10 SMA (open) (6.7 % binder)

7= 0/10 PMB SMA (open) (6.7 % binder)

9 55 % 0/14 HRA (6.5 % binder)

This ranking is very close to that obtained with the semi-circular bending test. The differences are that 0/6 SMAwas moved up two places and 55 % 0/14 HRA swappedplaces with 0/10 PMB SMA (open). These differencescould be due to an improvement in flexibility with smalleraggregate size.

What is unexpected is that the styrene-butadiene-styrene(SBS) modified-binder did not enhance the flexibility of the

mixture compared to the fibres, as measured by the semi-circular bending test. A possible for this lack of enhancementis that the SBS binder was used as a replacement rather thandesigning a new mixture with SBS binder.

The binder content ranking was not consistent with thatobtained from the 4-point bending test.

3.2.6.3 Theoretical considerations

There is a need for flexibility in bridge-deck surfacing andsurfacings over weak substrates if they are to overcomeslow but repeated movements, particularly at lowtemperatures. Therefore, the concept of the 4-point bendingtest at 5 ºC appears be appropriate. However, the need forcontrolled stress or control strain conditions in that testdepend on the overall structure – constant strain conditionsare indicative of thick, stiff pavements (where other layerswill take more of the load when the surfacing weakens withfatigue) while constant strain conditions are indicative ofthinner pavements (where the deflections will increase asthe material weakens and flexes to resist the applied forcewithout much help from other layers). Therefore, any testfor flexibility should be based on controlled stress ratherthan controlled strain, implying the ITFT rather than the

0

1

2

3

4

5

6

7

8

9

10

Masticasphalt

30 %0/10 HRA

0/14 SMA(dense)

0/14 SMA(open)

0/6 SMA 0/10 SMA(dense)

0/10 SMA(open)

55 %0/14 HRA

0/10 PMBSMA(open)

Ran

king

ITFT 4-point Semi-circular Asphalt bending

0

1

2

3

4

5

6

7

8

9

10

0/10 PMBSMA

55 %0/14 HRA

0/6 SMA 0/10 SMA(open)

30 %0/10 HRA

0/10 SMA(dense)

0/14 SMA(open)

0/14 SMA(dense)

Masticasphalt

Ran

king

ITFT 4-point Semi-circular Asphalt bending

Figure 3.10 Relative rankings in order of semi-circular bending ranking

Figure 3.11 Relative rankings in order of 4-point bending ranking

15

4-point bending fatigue test or, by default, the semi-circulartest. However, the ITFT takes considerable time and,therefore, is not particularly practical.

Therefore, the recommendation from theoreticalconsiderations aligned to the results of the test programme isto use the semi-circular bending test. The median value ofresults obtained were a tensile strength of 3.07 N/mm2 and afracture toughness of 19.88 N/mm3/2, so suggested initiallimits that are known to be attainable are a tensile strengthof 3.0 N/mm2 and a fracture toughness of 19 N/mm3/2.

3.3 Waterproofing system properties

3.3.1 Objectives and test programmeThe objective of the laboratory study of waterproofingsystems was to assess the variation of, and typical valuesfor, the bond strength between them and possibleoverlaying asphalt materials. It was hoped that theknowledge would be useful in setting realistic specificationlimitations which become more important as the thicknessof the layers is reduced. In addition, the opportunity wastaken to investigate the interlayer permeability becausewater will always find the weakest link, and that is usuallythe joints. A pavement structure will not be impermeable,even when constructed of impermeable materials, if thehorizontal and/or vertical joints between those materialsare permeable.

Eight 300 mm by 300 mm by 55 mm thick blocks weremanufactured using C40 concrete and waterproofingsystems applied. Two different membranes were used,each with two different tack coats. The waterproofingsystems were then overlaid with 40 mm depth of asphalt,compacted using a roller compactor to give an overalldepth of around 100 mm (block plus waterproofing plussurfacing). The mixtures used were mastic asphalt, 0/2 HRA(sand carpet), 30 % 0/10 HRA and 0/10 SMA (dense) fromSection 3.1.1 such that there was one slab with each of thecombinations of waterproofing manufacturer and asphalt

type. Both waterproofing manufacturers offer at least twodifferent tack/bond coats for different types of asphalt, sothey selected tack/bond coats that were deemed mostsuitable for these mixtures. Two cores were taken fromeach composite specimen and tested for torque bond inaccordance with the HAPAS thin surfacing guidelines(BBA, 2000). The holes were then refilled and thecomposite specimens re-compacted for 250 passes with theroller compacter set at 1 bar in order to try to simulatetrafficking before two further cores were taken and testedfor torque bond.

Each composite specimen was sliced vertically down thecentre to provide 300 mm by 100 mm cut faces with thewaterproofing system along the centre. These faces werethen tested for permeability at the interface between asphaltand the waterproofing system using apparatus developed formeasuring the initial surface absorption of concrete (BSI,1996), as shown in Section C.2 of Appendix C.

3.3.2 Torque bond testsThe results of the initial torque-bond test are given inTable 3.16 together with the results after simulatedtrafficking in Table 3.17.

In order to estimate the ultimate shear stress of a failuresurface from a torque bond test, assumptions must be madeconcerning the stress-strain curve of the material at thefailure surface. If the material yields so the shear stress atfailure is uniform across the failure surface, the maximumshear stress can be calculated from the theoreticalrelationship given in Equation (3.4).

T

d3

12 000τπ

×=×

…. (3.4)

where: = shear stress in N/mm²T = maximum torque in N.md = diameter in mm

Table 3.16 Torque-bond results before simulated trafficking

Waterproofing systemDiameter Height Max. torque Shear stress Failure

Membrane Tack coat Surfacing type ID (mm) (mm) (N.m) (N/mm²) location

B S Mastic asphalt 1 98.49 46.13 72 0.29 – 0.38 12 98.20 46.15 50 0.20 – 0.27 1

B R 0 % 0/2 HRA 1 98.27 35.15 118 0.47 – 0.63 12 98.33 35.83 145 0.58 – 0.77 1

B S 30 % 0/10 HRA 1 97.87 40.21 70 0.29 – 0.38 12 98.47 40.13 45 0.18 – 0.24 1

B S 0/10 SMA (dense) 1 98.54 41.31 42 0.17 – 0.22 12 98.26 40.53 20 0.08 – 0.11 1

A P Mastic asphalt 1 97.91 43.17 310 1.26 – 1.68 12 97.87 44.76 320 1.30 – 1.74 1

A P 0 % 0/2 HRA 1 98.60 45.93 180 0.72 – 0.96 12 98.62 46.62 205 0.82 – 1.09 1

A Q 30 % 0/10 HRA 1 97.95 45.86 20 0.08 – 0.11 12 98.28 46.51 48 0.19 – 0.26 1

A Q 0/10 SMA (dense) 1 98.02 43.83 130 0.53 – 0.70 12 98.02 44.00 125 0.51 – 0.68 1

Location of failure 1: Debonded at interface between tack/bond coat and waterproofing membrane.

16

If the stress is proportional to the strain at the failuresurface, the shear stress at failure will increase from zero atthe centre to a maximum at the outer diameter. Themaximum stress will be given by Equation (3.5).

T

d3

16 000τπ

×=×

…. (3.5)

In practice, the stress-strain curve of the material at thefailure surface will fall somewhere between the fullyelastic and fully plastic behaviour – strain hardening isunlikely. Therefore, the true shear stress will be within therange calculated by Equations (3.4) and (3.5) that areshown in Table 3.16 and Table 3.17.

Of the initial results, the torque bond varied considerablyby an order of magnitude with the two waterproofingsystems giving different rankings for the different materials.For all materials other than 30 % 0/10 HRA, the results werehigher for Membrane A systems than for Membrane Bsystems. Overall, the results can be ranked as follows:

1 Membrane A and tack coat P with (c.300 N.m)mastic asphalt.

2 Membrane A and tack coat P with (c.200 N.m)sand carpet.

3= Membrane B and tack coat R with (c.130 N.m)sand carpet, andMembrane A and tack coat Q with0/10 SMA.

5= Membrane B and tack coat S with (c.60 N.m)30 % 0/10 HRA, and

Membrane B and tack coat S withmastic asphalt.

7= Membrane B and tack coat S with (c.30 N.m)0/10 SMA, andMembrane A and tack coat Q with30 % 0/10 HRA.

According to BD 47/99 (DMRB 2.3.4), the minimumshear bond strength of an asphalt layer to a bridge deckwaterproofing system should be 0.2 MPa at 23 ºC. Table 3.16shows that, for each membrane, the failure stresses beforesimulated trafficking exceeded the minimum for threemixtures but were below it for the fourth mixture. In allcases, the failures were at the interface between thewaterproofing membrane and the tack/bond coat, so thefailure stresses were more dependent on the properties ofthe waterproofing systems than the asphalt mixtures. Forthe Membrane B systems, the highest failure stresses weremeasured with the tack coat R tack coat and sand carpet.For the Membrane A systems, the failure stresses werehigher for tack coat P than for tack coat Q. The high valuesfor mastic asphalt were probably due to the hightemperature at which the material was laid and compactedand at which the tack/bond coat was activated.

The additional passes of the roller compactor atelevated temperature to simulate trafficking did not provesuccessful, with debonding occurring, particularly for theMembrane B systems, rather than the gain in adhesion.Therefore, the results after simulated trafficking havebeen ignored from any analysis. However, it should benoted that most of the samples that debonded had a lowshear stress before trafficking.

3.3.3 Interface permeabilityThe permeability at the interface between the asphaltsurfacing and the waterproofing was carried out on thefollowing three composite systems with the results beinggiven in Table 3.18:

Table 3.17 Torque-bond results after simulated trafficking

Waterproofing systemDiameter Max. torque Shear stress Failure

Membrane Tack coat Surfacing type ID (mm) (N.m) (N/mm²) location

B S Mastic asphalt 1 – – – 22 – – – 2

B R 0 % 0/2 HRA 1 97.83 40 0.16 – 0.22 12 – – – 2

B S 30 % 0/10 HRA 1 – – – 22 – – – 2

B S 0/10 SMA (dense) 1 – – – 22 – – – 2

A P Mastic asphalt 1 97.65 340 1.39 – 1.86 12 97.73 340 1.39 – 1.86 1

A P 0 % 0/2 HRA 1 97.92 260 1.06 – 1.41 12 97.73 200 0.82 – 1.09 1

A Q 30 % 0/10 HRA 1 – – – 22 – – – 2

A Q 0/10 SMA (dense) 1 97.99 137 0.56 – 0.74 12 97.73 117 0.48 – 0.64 1

Location of failure 1: Debonded at interface between tack/bond coat and waterproofing membrane.

Location of failure 2: Material debonded during trafficking.

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! Mastic Asphalt – Membrane A and tack coat P

! 0 % 0/2 HRA – Membrane A and tack coat P

! 0/10 SMA (dense) – Membrane A and tack coat Q

likely to have the greatest effect on the durability of thesurfacing on bridges concern the following properties:

! The permeability of the asphalt and the control andremoval of water.

! The bond of the asphalt to the waterproofing system.

! The resistance of the asphalt to fatigue cracking.

! The compatibility with the waterproofing system.

4.2 Drainage

4.2.1 Drainage requirementsWhereas sub-surface water can permeate downwardsthrough several bound and unbound layers as it flowstowards the sub-surface drainage systems on pavements,sub-surface water on bridges can only flow downwards asfar as the waterproofing system. It must then flowhorizontally towards any sub-surface drainage systems thatare located at the low points and may be well over tenmetres away from where the water entered the surfacing.Therefore, drainage and the movement of water throughasphalt have a greater impact on the durability of asphalton bridge decks than on pavements.

Clause 4.1 of BD 47/99 (DMRB 2.3.4) requires surfacewater to be removed from bridge decks by the provision offalls and suitable surface drainage systems. Clause 4.2 alsorequires sub-surface drainage where natural drainage is notpossible. Some expansion joints provide sub-surfacedrainage by incorporating 20 mm square slotted drainagechannels that run across the carriageway. However, thesedrains can become clogged over a period of time so theyno longer drain away the water quickly enough. Through-deck drains at the low points are considered to be a moreeffective means of sub-surface drainage. Whilst these maybe installed on new bridges, they are not always installedon old bridges during maintenance works unless specificproblems have been encountered.

Therefore, Specifications for the asphalt surfacing onbridge decks should be ensure:

! Minimum accumulation of water by identifying suitablerequirements for the asphalt used in each layer.

! Efficient surface and sub-surface drainage systems tofacilitate drainage, aided by longitudinal gradients andcrossfalls of the bridge deck and the overlaying asphalt,as are currently specified.

4.2.2 Removal of waterThe accumulation of water within an asphalt layer,whether on a bridge or a pavement, has a detrimentaleffect in areas trafficked by heavy good vehicles. This isbecause wheel loading induces high hydrostatic pressuresin saturated asphalt that are sufficient to weaken the layerand break it up so that potholes are formed. Also, thepressures have an adverse effect on the bond betweenlayers and, in particular, the bond of asphalt to bridge deckwaterproofing systems and, hence, on the structuralintegrity of the asphalt layers on bridge decks. There havebeen a number of failures of the surfacing on bridge deckswhere water has accumulated in the asphalt, particularly on

Table 3.18 Flow rate at interface between asphalt andwaterproofing

Average flow rate (ml/m2/s)

Time Mastic 0% 0/10 SMA(min) asphalt 0/2 HRA (dense)

1 0.043 7.05 1362 0.044 7.78 1253 0.043 7.34 1355 0.042 6.99 13110 0.046 7.14 11815 0.052 6.90 11430 0.047 6.83 11060 0.045 7.88 111

Mean 0.045 7.18 123

The test specimens were taken from the surfacedconcrete blocks used for the torque bond tests. Thediameter of the test areas was 85 mm and they wereapproximately 25 mm deep. The measured flow rate wasdependent on the permeability of the concrete, thepermeability of the interface between the asphalt and thewaterproofing system, and the permeability of the asphaltwithin the test section. Because the permeability of theconcrete was low, most of the flow was through theinterface and the asphalt itself.

A very low flow rate was measured on the mastic asphaltspecimen because both the interface and the asphalt were oflow permeability. The flow rate was, on average, about 160times higher on the sand asphalt specimen than on themastic asphalt specimen. Because of the nature of sandasphalt, it is unlikely that the permeability of the interfacewas significantly different to that of the asphalt itself.However, it is possible that the permeability of the base ofthe layer of sand asphalt was less than that at mid-layer or atthe top of the layer, the latter having an influence on theresults shown in Table 3.3.

The flow rate was, on average, about 2700 times higheron the 0/10 (dense) SMA specimen than on the masticasphalt specimen. Because the waterproofing membranewas overlaid with a thick layer of tack coat Q, the interfacepermeability should not have been significantly higherthan that of the SMA. Therefore, most of the flow shouldhave been through the SMA itself.

4 Considerations for specification ofsurfacing for bridge decks

4.1 General approach

The Specification for the surfacing on bridge decks mustinclude the requirements normally applied to asphaltsurfacings, e.g. skidding resistance, resistance todeformation and surface regularity, as well as furthercriteria specific to asphalt on bridges. The specific criteria

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the high side of certain types of expansion joint that form abarrier to the flow of water. Many of these failures haveoccurred after periods of heavy rain.

The main criterion influencing the penetration andmovement of water is the permeability both of the asphaltitself (see also Section 4.4.5) and of the interface betweenlayers (see also Section 4.5.5). Permeability is a measureof the connectivity of pores/voids within the binder matrix,aggregate and the interface between them. The voidcontent at the interface between the asphalt layers and thewaterproofing system is dependent on the characteristics ofthe asphalt layer and the waterproofing system, so theyhave been considered together.

4.2.3 Sub-surface drainageIf the surface course material is permeable (see Section 4.4.5),significant amounts of water could reach the lower asphaltlayer(s) so that the potential for water to enter these layer(s)through air voids, defects and improperly sealed joints andreach the waterproofing system will be greater. Therefore, tominimise the risk of premature failure, an edge ofcarriageway drainage system should be provided to drain thefull depth of a permeable surface course. Furthermore, therisk should be reduced further by specifying a surface coursesystem which incorporates a thick bond coat that whenapplied to the lower asphalt layer helps to seal it.

4.3 Waterproofing system

4.3.1 Bond to asphaltAccording to Clause B4.2 (l) of BD 47/99 (DMRB 2.3.4),the minimum tensile bond of an asphalt layer to a bridgedeck waterproofing system should be 0.1 MPa at 23 ºC.According to Clause B4.2 (k), the minimum shear bondshould be 0.2 MPa at -10 ºC and 23 ºC, and 0.1 MPa at40 ºC. However, because BD 47/99 requires the totalthickness of the asphalt layers to be 120 mm, it is impliedthat these bond strength requirements are applicable forsurfacing of this standard thickness.

The bond of asphalt to a waterproofing system isdependent on the characteristics of both the waterproofingsystem and the asphalt, e.g. the adhesive and cohesiveproperties of the membrane, tack coat and asphalt, not justthe ‘thickness’ of the tack coat. It is also dependent on thetemperature at which the asphalt is laid and compacted andwhether any tack coat is activated.

The bond strength requirements in BD 47/99(DMRB 2.3.4) refer to the initial bond, but Clause 2005.5 ofthe Specification for Highway Works (MCHW 1) states that:

‘The additional protection layer1 or surfacing laid onthe waterproofing system shall be fully bonded to thesystem for the life of the system.’

The bond is prone to failure in service when the surfacingbecame saturated and high hydrostatic pressures aregenerated by wheels of heavy goods vehicles. Surfacing of

standard thickness with a sand asphalt layer performs welleven if, apparently, it may be poorly bonded to thewaterproofing system. However, surfacing of less thanstandard thickness has not been durable when it has notremained firmly bonded. In these circumstances, water hasaccumulated on the waterproofing system and weakened/failed the bond of the surfacing to the waterproofing systemand/or the surfacing itself. The durability of the surfacingappears to be more reliant on the bond when the thickness,and hence dead weight, of the surfacing is low.

Where the surfacing is less than 120 mm thick, the bondstrength requirements are much higher in other countriescompared to those in the UK. For example, the currentJapan Highway Public Corporation specification requireswaterproofing systems to be overlaid with asphalt of totalthickness 75 mm. The 35 mm thick layer directly overlayingthe waterproofing system is SMA with a 0/5 mm aggregatesize. The 40 mm thick surface course is a drainage (porousasphalt) layer with a 0/13 mm aggregate size. Performancetests on waterproofing systems include tests to measure theshear and tensile bond strength of the weakest interface ofspecimens comprising a concrete block, the waterproofingsystem and an asphalt layer. The shear bond strength mustnot be less than 0.8 MPa at -10 °C and 0.15 MPa at 20 °C.The tensile bond strength must not be less than 1.2 MPa at-10 °C and 0.6 MPa at 20 °C.

The current German specification requires waterproofingsystems on major roads to be overlaid with surfacing of totalthickness from 70 mm to 80 mm. A 35 mm thick protectivelayer of Gussasphalt with a 0/8 mm aggregate size directlyoverlays the waterproofing system. The surface course is35 mm to 45 mm thick. The tensile bond strength of theweakest interface of specimens comprising a concrete blockand the waterproofing system must not be less than 0.7 MPaat 8 °C and 0.4 MPa at 23 °C. The shear bond strength ofthe weakest interface of specimens comprising a concreteblock, the waterproofing system and an asphalt layer mustnot be less than 0.15 MPa at 23 °C when the shear force isapplied at an angle of 15º to the plane of the specimen.

Therefore, it is proposed that the minimum tensile andshear bond requirements are increased from the valuesgiven in BD 47/99 (DMRB 2.3.4) when the surfacing isless than 120 mm thick. Furthermore, if the surfacing is120 mm or more thick, higher bond strength requirementsshould be specified if the waterproofing system is notoverlaid with an additional protective layer of sand asphalt,because of the increased risk of water accumulating on themembrane and weakening the bond. The higher bondstrength should not be achievable unless the contact areabetween the asphalt and waterproofing system is high, i.e.there are few voids at the interface. To ensure that thebond is not susceptible to the presence of water or varieswith time, the bond strength should be measured bothbefore and after exposure to water.

4.3.2 Membrane stiffnessAlthough well-designed pavements should not sufferfatigue cracking, there has been concern that the surfacingoverlaying waterproofing systems on bridge decks can besusceptible to fatigue when the asphalt layers are thin.

1 The additional protection layer is specified in the DMRB and SHWas a 20 mm layer of sand asphalt.

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The susceptibility of asphalt layers to fatigue isdependent on the strains induced in the asphalt and itsfatigue properties. For a given wheel load, the strainsinduced are dependent on the thickness and stiffnessproperties of the waterproofing system, the stiffnessproperties of each asphalt layer, the combined thickness ofthe layers, and the bond of the asphalt layers to each otherand to the waterproofing system. In addition, cracking ofthe upper asphalt layers can be induced because of thebreak-up of the lower layers by high hydrostatic pressureswhen they become saturated.

The strains induced by wheel loading are enhanced bythe local and global deformation of the substrate to whichthe waterproofing system and asphalt are applied. Thedeformation of the substrate of steel bridge decks (the deckplate) is significant. However, concrete bridge decks havea local stiffness that is considerably higher than that ofsteel decks, so strains induced by the local deformation ofthe substrate are relatively low. Similarly, strains inducedby the global deformation of decks are considered to belower that those due to the other factors listed above.

The stiffness moduli and Poisson’s ratio of themembranes of waterproofing systems have been measuredat different temperatures. The membranes were found tobehave elastically or visco-elastically, with one type stifferat lower temperatures and the other type stiffer at highertemperatures at the strains and strain rates encountered onbridge decks. A series of finite element analyses is beingundertaken to determine the significance of the propertiesof the membrane on the strains induced in the asphaltlayers overlaying waterproofing systems on bridge decks.The findings, yet to be published, indicate thesusceptibility to fatigue of surfacing overlaying differenttypes of waterproofing system and the significance of thestiffness properties of the waterproofing membrane.

4.3.3 Effect of laying and compaction temperatures onthe waterproofing system

The DMRB and SHW specify requirements that areintended to:

! prevent waterproofing systems from being damagedwhen they are overlaid with hot asphalt; and

! ensure that asphalt remains bonded to the waterproofingsystems over their service life.

Clauses B4.2 (i) and (j) of BD 47/99 (DMRB 2.3.4)require waterproofing systems to pass tests that simulatethe conditions when they are overlaid with hot asphaltmaterials. The test specified in Clause B4.2 (j) assesses theeffects of high temperatures encountered during surfacingon the crack bridging ability of the waterproofingmembrane by overlaying it with hot material to achieve atemperature of 145 °C on its surface. Clause B4.2 (i)assesses the resistance to aggregate indentation during thecompaction of the asphalt. Currently, all membranes mustbe permanently indented by no more than half theirthickness after a force of 500 N has been applied by anaggregate indentor heated to a temperature of 80 °C. Thistest is designed to simulate the compaction of sand asphalt.

If systems are to be overlaid with mixtures containinglarger sized aggregates, the membrane must pass the testswith the aggregate indentator heated to 125 °C. Not all ofthe waterproofing systems currently registered for use onHighways Agency bridges have passed this test at 125 °Cand can be overlaid with coarse mixtures.

Allied to the aggregate indentation requirements are thosein Clause 901.9 of the SHW (MCHW 1), which states that:

‘With the exception of sand asphalt carpet, bituminousmaterials with a temperature greater than 125 °Cshall not be deposited on a bridge deck waterproofingsystem unless adequate precautions are taken to avoidheat damage in accordance with a good industrypractice. A maximum temperature of 145 °C ispermitted for sand asphalt carpet.’

The BBA Roads and Bridges Agrément Certificates forthe different waterproofing systems currently registeredinclude the following statements:

‘Temperature of the APL or HRA surfacing whenapplied should exceed the minimum reactivationtemperature of 100 °C required for the tack coat Rtack coat.’

‘The rolling temperature of the surfacing must notfall below the minimum reactivation temperature of85 °C required for Tack coat P, and 90 °C for Tackcoat SA 1030.’

‘Temperature of the surfacing when applied shouldexceed the minimum reactivation temperature of 80 °Crequired for Britdex MDP Tack Coat.’

‘Temperature of the APL when applied should be asspecified in BS 594-1: 1992 and BS 594-2: 1992.’

Clause 2005.5 of the SHW (MCHW 1) states that:

‘The additional protective layer of surfacing laid onthe waterproofing system shall be full bonded to thesystem for the life of the system. The bond shall beachieved by either: (i) the binder within the directlyapplied additional protective layer of surfacing; or(ii) a separate tack coat … details of which are givenon the BBA Roads and Bridges Agrément Certificate.Where the tack coat is of the type activated by the heatof the succeeding bituminous layer the temperature ofthis layer shall be sufficient to ensure adhesion.’

When a layer of asphalt is laid onto a substrate such as awaterproofing system, the base of the layer cools rapidlyas heat is transferred from the layer into the waterproofingsystem and concrete substrate. After a short period of time,the temperature of the waterproofing system will haverisen so that it is similar to that of the base of the layer.However, the waterproofing system will then cool as heatis lost by conduction to the concrete substrate and also byconvection and radiation through the top of the layer.

The asphalt layer must be laid at a sufficiently hightemperature so that the rolling/compaction temperature is

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high enough to activate the tack coat and form a denselayer. Table 4.1 and Table 4.2 show, respectively, thetimes after laying for 20 mm and 40 mm thick layers toreach a given temperature at mid-layer that were estimatedusing the method by Nicholls and Daines (1993). Thetemperature at the base of a layer (i.e. at the tack coat)could be about 10 °C below the mid-layer temperature.Therefore, the current Specification gives little time tocomplete compaction at a sufficiently high temperaturewhen the activation temperature of the tack/bond coat is100 °C, especially when the asphalt layer is only 20 mmthick and is laid at 145 °C. There is more time to completethe compaction of a 40 mm layer, even if it is laid at only125 °C. Much more time would be available if mixturescontaining coarse aggregates were laid at temperatureshigher than 125 °C, which probably already happens on anumber of bridges. The implication from the aggregateindentation test is that rolling should not occur when thetemperature at the waterproofing system is greater than125 °C. Therefore, the Specification could be changed toprevent ‘rolling’ when the mid-layer temperature is 125 °C(rather than specify a laying temperature of 125 °C).However, because the temperature at the waterproofingsystem is lower than that at mid-layer, rolling could bepermitted when the temperature at the waterproofingsystem is 125 °C or lower, or rolling could be permitted

when the mid-layer temperature is, say, 135 °C or less.Clearly, the contractor should provide details of thetemperature at the tack/bond coat above which compactionshould be completed or, preferably, the wording in theBBA Roads and Bridges Agrément Certificates should bechanged accordingly.

The above may appear to be unnecessarily complicated,but it is important because the bond of asphalt towaterproofing systems tends to increase with the rollingtemperature.

An aggregate indentation test could be carried out at atemperature up to a maximum of 145 °C, the temperatureat which the crack bridging ability of the waterproofingmembrane is assessed, if it is necessary to compactasphalt when the waterproofing membrane is at suchtemperatures. If the surfacing is likely to produce highertemperatures at the waterproofing system, it would benecessary to carry out a crack bridging test during whichsuch temperatures were induced during the thermal shockpreconditioning phase.

4.4 Asphalt properties

4.4.1 Deformation resistanceDeformation resistance of the component materials of apavement is more important for those materials nearest thesurface where the loads caused by traffic are highest. Theloads are distributed by the pavement layers so that theyreduce with the thickness of overlying pavement. Theapproach used to be to consider only the surfacing layers,with a more stringent requirement on the surface coursethan on the binder course. However, with the introductionof proprietary thin surfacings and hence thinner surfacecourses, it becomes more logical to apply the stringentrequirement for deformation resistance to compositesamples for the top 50 mm and the less stringentrequirement to the next 50 mm. It is proposed to take thisapproach for bridge deck surfacings.

Requirements for deformation resistance should becomemore important as the overall thickness of the surfacinglayers is reduced. Using the conventional constructionapproach, if in thin layers, the red sand carpet would comemore within the critical depth. Sand carpet is notparticularly deformation resistant, being the worst of thematerials tested (Section 3.1.5). Therefore, it is proposedto apply the current surface course requirements (asdefined in tables NG 9/28 and NG 9/29 of the Notes forGuidance on the Specification for Highway Works(MCHW 2)) for deformation resistance to (50 ± 10) mmthick samples of each material type that occurs in the top50 mm. In addition, it is proposed to apply therequirements at the next level down to (50 ± 10) mm thicksamples of materials that occur in the next 50 mm of thepavement. Where materials are nominally laid at athickness less than 40 mm, it is proposed to permit thetesting of composite samples made up of more than onedifferent layers to be used in the pavement provided nopart layer has a thickness less than twice its maximumaggregate size.

Table 4.1 Effect of environmental conditions on timeavailable to compact a 20 mm thick layer

Time after laying toWind reach given temperature

Air speed at Laying at mid-layer (min)temperature 2 m height temperature(°C) (km/h) (°C) 120 °C 110 °C 100 °C

5 0 145 2.5 3.3 4.35 10 145 2.0 2.7 3.55 40 145 1.7 2.2 2.9

20 0 145 2.9 3.9 5.120 10 145 2.3 3.1 4.120 40 145 1.9 2.6 3.3

Table 4.2 Effect of environmental conditions on timeavailable to compact a 40 mm thick layer

WindAir speed Laying Time after laying to reach giventemper- at 2 m temper- temperature at mid-layer (min)ature height ature(°C) (km/h) (°C) 120 °C 110 °C 100 °C

5 0 125 3.9 6.9 10.25 10 125 3.2 5.7 8.35 40 125 2.6 4.7 6.920 0 125 4.6 8.1 12.020 10 125 3.7 6.5 9.620 40 125 3.0 5.3 7.95 0 145 8.6 11.6 15.15 10 145 7.0 9.5 12.45 40 145 5.8 7.8 10.220 0 145 10.1 13.6 17.820 10 145 8.0 10.9 14.220 40 145 6.6 8.9 11.6

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4.4.2 Texture depthTexture depth is a standard requirement for the surfacecourse of high-speed trunk roads, and the requirements ofclause 921 of the Specification for Highway Works(MCHW 1) should be applied when appropriate for thespecification of surfacings to bridge decks.

4.4.3 Skid resistanceAlthough it is only referenced explicitly in clause 918(slurry surfacing), 919 (surface dressing), 922 (surfacedressing), 938 (porous asphalt) and 942 (thin surfacecourse systems) of the Specification for Highway Works(MCHW 1), the skid resistance of the surface course isdefined by the polished stone value of the coarse aggregateas laid out in Appendix 7/1 of the job specification. Adviceon values to specify in Appendix 7/1 is given in AdviceNote HD 36/99 (DMRB 7.5.1). This approach should beused for the specification of surfacings for bridge decks.

4.4.4 Flexibility and fatigueFlexibility has been identified as an important factor overbridge decks (Section 4.3.2). However, there are doubtsabout the current fatigue tests for asphalt because they donot provide a consistent ranking. In particular, the choicebetween constant stress and constant strain can reverse theorder. For bridge decks, it is assumed that constant strainwill apply because the support is so much stiffer than theasphalt. The obvious fatigue tests are the indirect tensile,four-point bending and two-point bending, the first havinghad some use in the UK whilst the latter two are themethods that will be available for CE marking the propertywhen the European standards are implemented. However,none of these tests are currently used for routine testing ofasphalt in the UK.

An alternative to fatigue is flexibility. The research intoa possible test for flexibility identified the semi-circularbending test from several put forward. This test was foundto be practical, equivalent to a controlled stress fatigue testand ranks materials similarly to binder content, a knowncomponent of flexibility if other things are constant. Thetest is currently being considered for Europeanstandardisation as a measure of crack propagation. For theBBA HAPAS scheme for thin surfacings, it was suggestedthat initially the values of 3.0 N/mm² and 19 N/mm3/2

could be set as the minimum values required for tensilestrength and fracture toughness, respectively. These limitswere considered practical because the majority of theresults obtained from trial mixtures exceeded them.

Whilst it would be preferable to have fatigue and/orflexibility requirements that get more severe as the asphaltthickness gets less, there is currently no agreement on thetest to be used or the limits to be achieved. Therefore, it isproposed not to set any fatigue requirements at this time,but to consider latter inclusion if subsequent researchidentifies more precisely what is appropriate.

4.4.5 Permeability and air voids contentPrevious research at TRL has shown that the layer ofasphalt directly overlaying the waterproofing system must

be impermeable if surfacing on bridge decks is to remaindurable. If the lower asphalt layers on bridge decks arepermeable, any water that passes through the surfacecourse through the air voids in the layer and any defects(cracks and/or improper sealing around joints) willpercolate down towards the waterproofing system whileflowing across the deck towards the low points. If thewater encounters a barrier on its way, such as an expansionjoint or a less permeable area, it will accumulate in theasphalt until it is drained by a sub-surface drainage system.Though-deck drains will be required at all low points onthe deck, including large hollows and depressions that arenot free draining. Furthermore, the lower asphalt layer willneed to remain sufficiently permeable throughout itsservice life to drain the sub-surface water quickly in theheaviest periods of rain. However, premature failures haveoccurred on a number of bridges when the lower asphaltlayer has been permeable, even when there has been somesub-surface drainage (Section 4.2.3), because highhydrostatic pressures have been generated by the wheels ofheavy goods vehicles in the surfacing when saturated.Therefore, the lower asphalt layer should be impermeableand sub-surface drainage should be provided to drain thesmall amounts of water that will inevitably enter the layerand permeate down to the waterproofing system.

In spite of the great importance of permeability inrelation to the durability performance bridges, there iscurrently no permeability specification to control thequality of asphalt surfacing. This deficiency may beattributed to the controversy regarding the repeatabilityand reproducibility of permeability tests using differenttechniques and the relatively long time required to produceresults. A new European Standard, EN 12697-19 (CEN,2004c), describes a method for measuring the permeabilityof porous asphalt to water, but this method would be time-consuming for partially permeable and unsuitable forimpermeable asphalt mixtures.

Permeability testing has received more attention from theconcrete industry and has been widely used within thedurability specifications of major concrete constructions,such as the Jubilee Line Extension Project in the UK.Hassan and Cabrera (1997) demonstrated that thepermeability of concrete is dependent on many factors thatare all related to the volume of open pores, as thepermeability increases with higher volume of open porosity.A concrete with a volumetric proportion of open pores ofless than 4 % exhibited low permeability values, whereasabove 5 % there is a very rapid increase in permeability.This change indicates that an interconnected system of poresis established when the volume of open pores is greater thanabout 3 % or 4 %. A similar trend of permeability results isobtained from the asphalt mixtures, as shown in Figure 3.3.Regardless of the mixture type, low permeability values areobtained when the air voids content is 4 % or below and thepermeability increases rapidly for air voids contents above5 %. This hypothesis is supported from results of otherinvestigations (Zoorob et al., 1999).

The permeability test, under a pressure gradient, is aneasy test and takes a short time to be conducted. The testresults are found reliable in ranking the permeability of

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various asphalt mixtures and, therefore, could be adoptedin the durability specifications of asphalt surfacing tobridge decks. However, there is still no agreement aboutthe test method, the associated limits of permeability to bespecified or the precision of the results. In the absence of agenerally accepted standard permeability test, it appearsreasonable to use the air voids content as an indicator forpermeability.

Any specified maximum values for air voids contents, asa surrogate for permeability, should depend on thepresence and effectiveness of sub-surface drainage. If thereis adequate drainage that can be properly maintained, adesign air voids contents of up to 6 % could be acceptablefor asphalt layers other than that directly over thewaterproofing system whereas, for the bottom layer andwhere there is no adequate sub-surface drainage, an airvoids contents of not more than 4 % should be required.Both requirements can be met by all of the materials tested(Section 3.1.2) other than the open SMAs and the 35 %0/10 HRA (despite 35 % 0/14 HRA having been usedsuccessfully on bridges in the past), the latter being theonly one to fall between the two proposed limits. If thoselimits are used for the mean, a tolerance of 2 % should beincluded to give maxima on individual readings of 8 % and6 %, respectively. It would be ideal to use the individualmaxima on site, but it is appreciated that it is best not totake cores when impermeability is sought.

4.4.6 Protection for waterproofing systemsThe current Specification requires waterproofing systemsto be overlaid with a 20 mm thick additional protectionlayer of sand asphalt (0/2 HRA). Because the layer has lowresistance to deformation, it performs satisfactorily only ifit is overlaid with sufficient thickness of deformationresistant surfacing. Therefore, it should be omitted whenthe overlaying surfacing is less than 100 mm thick.

However, the benefits of laying sand asphalt, even whenit is overlaid with 100 mm or more of surfacing, have beenquestioned by many in the industry. The original intentionwas to lay the sand asphalt immediately afterwaterproofing in order to protect the waterproofing systembefore and during surfacing operations. However, anumber of waterproofing systems have passed theaggregate indentation test at 125 ºC (see Section 4.3.3) anddo not require the protection of sand asphalt provided duecare is taken before and during surfacing operations.

Appendix B to BD 47/99 (DMRB 7.2.3) requires that allpermitted waterproofing systems are tested for aggregateindentation at 40 °C and 80 °C with a compliancerequirement that the indentation after a recovery periodshall not exceed 50 % of the initial system. Waterproofingsystems may be overlaid with asphalt containing coarseaggregates of maximum aggregate size greater than thatfound in sand asphalt if they have passed an aggregateindentation test at 125 °C. Not all of the registered systemshave passed the test at 125 °C, which could disadvantagethem in the marketplace, particularly when a specificationrequires that the asphalt directly overlaying thewaterproofing system should contain large sized coarseaggregate, reducing the number of waterproofing systems

that can comply. The material itself and the interfacebetween the material and waterproofing system wouldneed to be impermeable. Also, the resistance of thewaterproofing system to aggregate indentation whenoverlaid with the material would need to be demonstrated.

The use of red oxide in the sand carpet could bereproduced in the alternative mixtures if required.However, there have been hearsay reports that often thecolour has been lost by the time the material is exposedand, with a thinner overall surfacing, the protective layerwill be reached sooner so that greater care will be needed.It has been found that the red sand carpet cannot be usedreliably as an indicator layer to enable a bridge to beresurfaced without it being re-waterproofed, which was itsmain purpose. Therefore, the requirement to use red oxideshould become optional.

4.5 Joints and interface between layers

4.5.1 GeneralThe use of impermeable asphalt layers will be inadequatewithout ensuring proper sealing at any details, interfaces orjoints between the rips, and any variations in permeabilityassociated with defects such as cracks. The relevant detailsare kerbs, parapets and expansion joints. The relevantinterfaces are between the asphalt layers and between thelowest asphalt layer and the waterproofing system.

4.5.2 JointsThe joints between adjacent rips of asphalt should besealed effectively in order to prevent an easy path forwater to flow vertically through an asphalt layer, as set outin both BS 598-2 and BS 4987-2. However, joints that arefully sealed will prevent the flow of water horizontallyacross a bridge so provision must be made so there is nodanger that water can accumulate in a layer, which couldlead to high hydrostatic pressures being generated by thewheels of heavy goods vehicles. However, given theintention to ensure that the materials used are relativelyimpermeable (Section 4.4.5), it is proposed to require allcold vertical joints to be painted with bitumen before beinglaid against.

4.5.3 Bond between layersThe bond between layers has become more important withthe use of thin surface course layers because the absence ofthe weight supplied by a thick layer needs to be replaced bypositive adhesion with the layer below in order to minimisethe potential for de-bonding. However, there is no generallyaccepted method for measuring the property in-situ on aroutine basis. There are two aspects to bond, the adhesionachieved by the binder present and the aggregate interlockcreated by texture of the underlying material being filled byparticles from the overlay. Different test methods that havebeen proposed to measure bond are affected differently bythe two aspects, with pull-off tests (as used for bridge water-proofing and high-friction surfacing system) ignoringaggregate interlock whilst shear tests (as used for thinsurfacing systems) incorporate that aspect. Provided they arenot saturated, the adhesion generally improves with

23

trafficking because the two layers are pressed together witheach wheel-pass, so the timing of any measurements of theproperty is also important.

Bond, based on a torque shear test, is a requiredproperty for thin surfacing systems in order to obtain aHighway Authorities Product Approval Scheme (HAPAS)certificate from the British Board of Agrément (BBA).However, there is no pass/fail rating under the currentguidelines (BBA, 2000), only the need to report the result.The test, which is carried out after between 28 and 56 daysof trafficking, is intended as a type test because of thedisruption caused by closing the road to obtain the samplesso soon after being re-opened. However, the resultsrecorded to date typically vary between 400 and 1500 kPa,so that a specification requirement for 400 kPa wouldallow all currently certified thin surfacing systems to beused whereas a limit of, say, 700 kPa would exclude someof those that have performed least well in the test. If a limitis set, any non-proprietary surfacing would also need to betested before it was permitted for use on bridge deckoverlays. The limits proposed will be dependent on thedepth of the interface with 700 kPa if within 20 mm of thesurface, reducing to 400 kPa at 50 mm and no requirementbelow that. The tests should be performed with therelevant tack or bond coat to be used on site with theappropriate materials as substrate and overlay.

Given the variable approach, it is proposed here to raisethe general requirement, of either a tack or bond coatbetween each layer, to a requirement for bond coats at allinterfaces. This approach does not affect the current bondrequirements for waterproofing systems or thin surfacingsystems in order to obtain their BBA-HAPAS certificates.

4.5.4 Bond to waterproofing systemAccording to clause 2003.2 of the Specification forHighway Works (MCHW 1), a waterproofing systemneeds to have a BBA Roads and Bridges certificate inorder to be permitted for use on trunk road bridges. Part ofthe laboratory test procedure to gain certification is tocheck the bond between the waterproofing system and theunderlying concrete substrate by means of a tensileadhesion (pull-off) test and to check the bond betweenwaterproofing system and the overlying asphalt by meansof shear-adhesion and tensile bond tests. These tests aredescribed in Appendix B to BD 47/99 (DMRB 7.2.3).There are limits for the tests, with minima of:

! 0.3 N/mm² at -10 °C, 0.3 N/mm² at 23 °C and 0.2 N/mm²at 40 °C for the tensile adhesion test on thewaterproofing system to concrete substrate interface;

! 0.2 N/mm² at -10 °C, 0.2 N/mm² at 23 °C and 0.1 N/mm²at 40 °C for the shear adhesion test on the surfacing towaterproofing system interface; and

! 0.1 N/mm² at 23 °C for the tensile bond test on thesurfacing to waterproofing system interface.

These limits apply to overlaying asphalt of minimumthickness 120 mm where the asphalt directly overlayingthe waterproofing system is 0 % 0/2 mm HRA (sandasphalt) or 50 % 0/10 HRA. However, higher limits areconsidered necessary when mixtures containing coarse

aggregates directly overlays the waterproofing system.Also, higher limits are appropriate for thicknesses less than120 mm, and the proposal is to increase them in steps of30 mm down to 60 mm. Any total depth of surfacing lessthan 60 mm is regarded as a special case that will requireexpert advice. The resultant limits are given in Table 4.3.

Table 4.3 Bonding limits for waterproofing systemswhen overlaid by coarse mixtures

Surfacing thickness ≥120 mm <120 mm; ≥90 mm < 90 mm; ≥60 mm

Tensile adhesion test@ -10 °C 0.30 N/mm² 0.50 N/mm² 0.70 N/mm²@ 23 °C 0.30 N/mm² 0.50 N/mm² 0.70 N/mm²@ 40 °C 0.20 N/mm² 0.30 N/mm² 0.30 N/mm²

Shear adhesion test@ -10 °C 0.30 N/mm² 0.30 N/mm² 0.40 N/mm²@ 23 °C 0.30 N/mm² 0.30 N/mm² 0.40 N/mm²@ 40 °C 0.10 N/mm² 0.15 N/mm² 0.15 N/mm²

Tensile bond test@ 23 °C 0.40 N/mm² 0.45 N/mm² 0.50 N/mm²

4.5.5 Permeability of interface between asphalt andwaterproofing system

Most waterproofing systems comprise a primer to optimisethe bond of the system to the concrete substrate, amembrane to prevent water and chlorides from reachingthe concrete, and a tack/bond coat to optimise the bond ofthe overlaying asphalt to the system.

When asphalt with coarse aggregates is compacted onto a‘hard’ surface, the bulk of the material may have a low voidcontent but there may be large voids at the base of the layer.The voids tend to be larger with larger aggregate sizes andwith higher proportions of coarse aggregate. Above a certainproportion of coarse aggregate, the voids may beinterconnecting. When there are voids at the interfacebetween the lowest asphalt layer and the waterproofingsystem, water can accumulate and there is a risk of prematurefailure. Therefore, the void content at the interface should below and voids should not be interconnecting.

The permeability of the interface between thewaterproofing system and the asphalt is dependent on theproperties of both the asphalt and the tack/bond coat.Whereas a tack/bond coat for a waterproofing system maycomprise more than one layer, the upper layer that is incontact with the asphalt and which aggregates canpenetrate can be described as ‘thin’ (generally <0.2 mm) or‘thick’ (generally >1.0 mm). When the tack/bond coat is‘thin’, an asphalt layer with a small aggregate size and lowproportion of coarse aggregate will yield no largeinterconnecting voids. However, any mixture that containslarge aggregates will result in some voids where water mayaccumulate and a reduction in contact area between themixture and the waterproofing system. As discussedbelow, a reduction in the contact area may adversely affectthe tensile bond of the asphalt to the waterproofing system.

A ‘thick’ tack/bond coat can (partially) fill the voids atthe base of an asphalt layer with coarse aggregates and,

24

thereby, limit the accumulation of water andinterconnecting voids and, potentially, improve the tensileadhesion. Therefore, a waterproofing system incorporatinga thick tack/bond coat should be specified for mixtureswith coarse aggregates. The tack/bond coat must not be toothick otherwise ‘bleeding’ of the excess binder through theoverlying asphalt layer may occur during its laying andcompaction. Also, the asphalt layer will be moresusceptible to fatigue if it ‘floats’ on a thick layer of ‘soft’tack/bond coat. Ideally, the coarse aggregates shouldalmost fully penetrate the tack/bond coat as the tack/bondcoat material fills the voids at the base of the layer.

Therefore, the specification needs to ensure that thelower asphalt layer overlaying the waterproofing system isimpermeable. The mixture design and type of tack/bondcoat should ensure that the air voids contents at theinterface between the waterproofing system and the baseof the layer is low without interconnecting voids (seeSection 3.3.3). To minimise the amount of water that canenter the lower asphalt layers, the full depth of permeablesurface courses should be drained at the edges. If this isnot possible, an impermeable surface course should be laidthat directs water to a suitable surface drainage system.These requirements are covered in Section 4.4.5. However,frequent changes in surface texture and road noiseassociated with a change in surface course materials frombridge to pavement should be avoided.

The total thickness of surfacings on bridges may rangefrom 40 mm to over 120 mm. Such surfacings may requireone, two or three asphalt layers, each with a differentmaterial. On most bridges, there are small variations in thelevel of the deck that can be accommodated withoutvarying the number of asphalt layers. However, it has beennecessary on a few bridges to vary the number of layers,requiring some degree of compromise in the regions wherethe number changes. Under these circumstances, the lowerlayer directly overlaying the waterproofing system shouldbe of reasonably uniform thickness and of the sameimpermeable material. Changes in thickness should beaccommodated in the upper layers, with tapered layerstrimmed so material too thin to have been compactedsufficiently is removed.

5 Conclusions

From the tests on a series of twelve asphalt mixtures,mastic asphalt was found to have the most suitable airvoids content, permeability and stiffness modulusproperties and was fifth at wheel-tracking, making it thebest material overall for these tests. However, masticasphalt is a relatively expensive mixture, a factor thatcannot be excluded. The dense 0/10 SMA was the nextbest despite having the 8th highest air voids content whilstthe open 0/10 SMA was the worst, showing that theprecise mixture design can be critical. The remainingmixtures showed relatively similar overall ratings, but withthe sand carpet only being ranked 8th out of 12 overall.Nevertheless, when considering the appropriate materials,the choice is often a trade off between properties and they

will not usually have equal ranking. Material optimisationis usually avoiding any excessively adverse property ratherthan getting the best performance in all.

The initial results from the torque bond test varied by anorder of magnitude, with the two waterproofing systemsranking the surfacing materials differently. In all cases, thefailures were at the interface between the waterproofingmembrane and the tack/bond coat, so the failure stresseswere more dependent on the properties of thewaterproofing systems than the asphalt mixtures. ForMembrane B systems, the highest failure stresses weremeasured with tack coat R and sand carpet. For MembraneA systems, the failure stresses were higher for tack coat Pthan for tack coat Q. The high values for mastic asphaltwere probably due to the high temperature at which thematerial was laid and compacted and at which the tack/bond coat was activated.

Based on these findings and other considerations,various additions and changes to the Design Manual forRoads and Bridges, Specification for Highway Works andNotes for Guidance on the Specification for HighwayWorks have been proposed. The main changes include:

! sub-surface drainage is emphasised;

! bond requirements are strengthened;

! deformation requirements are specified for all mixtureswithin 100 mm of the surface; and

! maximum air voids content limits on all asphalt mixtures.

Aspects that were not fully covered are permeabilitytesting of the asphalt and at the interfaces. Potential testshave been identified that could be developed forstandardisation if these aspects are considered critical. Theasphalt permeability can be covered by the surrogate of airvoids content, but this is more difficult at interfaces wheremore than one material is involved.

For flexibility, the test selected from those put forwardis the semi-circular bending test. This test was found to bepractical, equivalent to a controlled stress fatigue test andranks materials similarly to binder content, a knowncomponent of flexibility if other things are constant.

It is suggested that initially the values of 3.0 N/mm² and19 N/mm3/2 should be set as the minimum values requiredfor tensile strength and fracture toughness, respectively.These limits are considered practical because the majorityof the results obtained exceeded them.

6 Acknowledgements

The work described in this report was carried out in theInfrastructure and Environment Division of TRL Limited.The authors are grateful to Kevin Green, Mario Patchettand Jon Harper, who carried out all the laboratory testingthat underpin this work, and to Val Atkinson, who carriedout the quality review and auditing of this report.Particular thanks are given to the two manufacturers whoapplied their waterproofing systems to concrete samplesand to the four positive responders to the questionnaire.

25

7 References

British Board of Agrément (2000). Guidelines documentfor the assessment and certification of thin surfacingsystems for highways. BBA-HAPAS SG3/98/169, WorkingDraft 3. Watfird: British Board of Agrément.

British Standards Institution (1988). Specification formastic asphalt (limestone fine aggregate) for roads,footways and paving in building. BS 1447: 1988. London:British Standards Institution.

British Standards Institution (1993). Methods fordetermination of the indirect tensile stiffness modulus ofbituminous mixtures. British Standard Draft forDevelopment DD 213: 1993. London: British StandardsInstitution.

British Standards Institution (1996). Testing concrete –Recommendations for the determination of the initialsurface absorption of concrete. BS 1881-208: 1996.London: British Standards Institution.

British Standards Institution (2001). Coated macadam(asphalt concrete) for roads and other paved areas –Part 1: Specification for constituent materials and asphaltmixtures. BS 4987-1: 2001. London: British StandardsInstitution.

British Standards Institution (2002). Hot rolled asphaltfor roads and other paved areas – Part 1: Specification forconstituent materials and asphalt mixtures. BS 594-1:2002. London: British Standards Institution.

Comité Européen de Normalisation (2000a). Bituminousmixtures – Material specification. Part 5, Stone masticasphalt. Draft BS EN 13108-5, DPC No. 00/100954DC.London: British Standards Institution.

Comité Européen de Normalisation (2000b). Bituminousmixtures – Test methods for hot mix asphalt – Part 27:Sampling. BS EN 12697-27: 2000. London: BritishStandards Institution.

Comité Européen de Normalisation (2002a). Bituminousmixtures – Test methods for hot mix asphalt – Part 7:Determination of bulk density of bituminous specimens bygamma rays. BS EN 12697-7: 2002. London: BritishStandards Institution.

Comité Européen de Normalisation (2002b). Bituminousmixtures – Test methods for hot mix asphalt – Part 5:Determination of the maximum density. BS EN 12697-5:2002. London: British Standards Institution.

Comité Européen de Normalisation (2003a). Bituminousmixtures – Test methods for hot mix asphalt – Part 33:Specimen prepared by roller compactor. BS EN 12697-33:2003. London: British Standards Institution.

Comité Européen de Normalisation (2003b). Bituminousmixtures – Test methods for hot mix asphalt – Part 8:Determination of void characteristics of bituminousspecimens. BS EN 12697-8: 2003. London: BritishStandards Institution.

Comité Européen de Normalisation (2003c). Bituminousmixtures – Test methods for hot mix asphalt – Part 22:Wheel tracking. BS EN 12697-22: 2003. London: BritishStandards Institution.

Comité Européen de Normalisation (2004a). Bituminousmixtures – Test methods for hot mix asphalt – Part 35:Laboratory mixing. BS EN 12697-35: 2004. London:British Standards Institution.

Comité Européen de Normalisation (2004b). Bituminousmixtures – Test methods for hot mix asphalt – Part 24:Resistance to fatigue. BS EN 12697-24: 2004 BritishStandards Institution, London.

Comité Européen de Normalisation (2004c). Bituminousmixtures – Test methods. Part 19, Permeability of porousasphalt specimen. BS EN 12697-19: 2004. London: BritishStandards Institution.

Daines M E (1994). Tests for voids and compaction inrolled asphalt surfacings. Project Report PR78.Wokingham: TRL.

Grube H and C D Lawrence (1984). Permeability ofconcrete to oxygen. Proceedings of the RILEM Seminar on‘Durability of concrete structures under normal outdoorexposure’. Hanover, pp. 68-79.

Hassan K E and J G Cabrera (1997). Controlling thequality of concrete by measuring its permeability.Proceedings 13th International Conference on ‘BuildingMaterials’, Volume 3. Weimar-Germany, pp. 3/005-3/0017.

The Highways Agency, Scottish DevelopmentDepartment, The National Assembly for Wales and TheDepartment for Regional Development NorthernIreland. Manual of Contract Documents for HighwayWorks. London: The Stationery Office:

Volume 1: Specification for Highway Works (MCHW 1).

Volume 2: Notes for Guidance on the Specification forHighway Works (MCHW 2).

Highways Agency, Scottish Executive DevelopmentDepartment, National Assembly for Wales andDepartment for Regional Development, NorthernIreland. Design Manual for Roads and Bridges. London:The Stationery Office:

BD 47/99: Waterproofing and surfacing of concretebridge decks (DMRB 2.3.4)

HD 36/99: Surfacing materials for new and maintenanceconstruction (DMRB 7.5.1)

26

Nicholls J C and Carswell I (2004). Durability of thinasphalt surfacing systems: Part 2: Findings after 3 years.TRL Report TRL606. Wokingham: TRL.

Nicholls J C and Daines M E (1993). Acceptable weatherconditions for laying bituminous materials. Project ReportPR13. Wokingham: TRL.

Peoples Republic of China (1993). Complete ultimatestrain at a low temperature –10 ºC: > 6 x 10-3; AsphaltBending Test. PRC Requirement T0715-93.

Zoorob S E, Cabrera J G and Suparma L B (1999). Agas permeability method for controlling quality of densebituminous composites. Proceedings of the 3rd EuropeanSymposium on Performance and durability of bituminousmaterials and hydraulic stabilised composites. pp. 549-572.Leeds.

27

Appendix A: HA draft notes for bridge-deck overlays

waterproofing. However they are resilient and althoughthey are designed to be applied in very thin layers,concrete bridge decks to which they are applied are oftenquite rough. This can result in thicker areas of membrane,increasing the resilience. Premature failures have occurredwith both hot rolled asphalt and stone mastic asphalt due atleast in part to fatigue of a sub-standard thickness overlay.Such failures are always associated with debonding of theoverlay and it has not been possible to determine theprimary cause of these failures – poor bond, watersaturation and traffic generated pressure, or fatigue. It islikely that each plays some part.

A.5 Actions to be considered before applying for adeparture from standards to use a sub-standardbituminous overlay less than 120 mm in thickness

i Re-assessment of the structure to maximise thethickness of the bituminous overlay.

ii Specification of the highest modulus approvedproprietary waterproofing system obtainable. (note: theneed for this will diminish with increasing thickness ofoverlay and where the thickness of the membrane canbe kept to a minimum.)

iii Omission of the sand asphalt layer above thewaterproofing system.

iv Specification of a waterproofing system which includesthe provision of a bond coat for the asphalt overlay.Where the waterproofing system offers alternativebond promoting treatments, the specification of thetreatment claimed to provide the maximum bond shallbe specified. Any proprietary bond coat between thewaterproofing system and the overlay shall be a ‘tack-free’ material, such that it does not adhere to tyres ofvehicles delivering asphalt to the paver.

v Provision of sub-surface, edge and joint drainage asappropriate, to reduce or eliminate water pressureunder traffic.

vi Specification of a paver-laid hot rolled asphalt layer orlayers, containing an elastomeric polymer and anappropriate aggregate size, to form the overlay bindercourse, all in accordance with SHW Clause 943, (butomitting any coring over the bridge-deck!).

vii Specification of a thin surface course system whichincorporates a heavy elastomeric-polymer modifiedbond coat.

The risk of premature failure of sub-standard thicknessbituminous overlays on bridge decks is considerable. Forfurther advice or clarification, please contact JamesGallagher or John Williams of SSR RDS HighwaysInfrastructure Group.

James Gallagher: Tel: 0161 930 5527 (GTN 4315 5733)

John Williams: Tel: 01234 796116 (GTN 3013 6116)or 01438 718487 (GTN 6492 0008)

A.1 Sub-surface drainage

All blacktop leaks to some extent, through joints etc – andthe use of macadam bases with void contents that may be(at worst) up to 7 or 8 % and more porous quiet surfacingsmay well exacerbate this. Advice is given in DMRB Vol 2Section 3 Part 4 BD 47/99 Chapter 4 ‘Drainage’. Inparagraph 4.2 on sub-surface drainage, to quote:

‘Bituminous surfacing is porous and can retain surfacewater. Where the geometry of the deck or deck movementjoints prevents this water from draining naturally throughsurface drainage, sub-surface drains shall be provided.Advice on sub-surface water drainage is given in BA 47(DMRB 2.3.5)’

The term surfacing in this instance is referring to thebituminous overlay above the waterproofing not just thesurfacing course.

Further information is given in TRL Application Guide 33‘Water Management for Durable Bridges’ in the drainagesection. Other useful references are BD 33/94 and BA 26/94both concerning expansion joints and covering their drainage.

Bridge-deck overlays (the surface course and base layers)should always be regarded as porous and drainage should beprovided below the overlay at low points over the bridgedeck waterproofing. Edge drainage should be provided atjoints – where compaction is likely to be least efficient – andsub-surface drainage installed where interstitial pondingmay occur. There have been one or two instances of frettingin thin surfacings at such locations, caused by poorcompaction. It is important to note that this may not be onthe bridge deck side of the joint, depending on the gradient.

A.2 Bond to the waterproof membrane

Another issue is the bond of the overlay to thewaterproofing membrane. This tends to be relatively lowwith the modern waterproofing systems now in use andmay be disrupted by pressures generated under traffic ifthe overlay becomes saturated. There have been instanceswhere this appears to have occurred; in particular wherethe blacktop overlay, usually in the past surfaced withHRA, is less than the standard 120 mm in thickness.

A.3 The thin surface course

It is government policy to use quiet surfacings on all trunkroads in England, including motorways. The capacity ofthese proprietary systems to waterproof the base layersbelow often appears to depend more on the bond or tackcoat applied than the apparent ‘porosity’ of the surfacecourse. Anecdotal evidence suggests that thin surfacingsystems with an open texture laid on a heavy polymermodified bond coat can be more effective at sealing andwaterproofing the base layers than thicker, less opensystems (SMAs) laid on a thin tack coat.

A.4 Sub-standard overlay thickness

A bridge-deck overlay is more vulnerable the thinner it is.Modern waterproofing systems are very effective at

28

Appendix B: Questionnaire

29

30

31

32

Appendix C: Permeability tests

C.1 TRL permeability test

The TRL permeability cell (Figure C.1) is similar to thatdeveloped by the Cement and Concrete Association(C&CA, since renamed the British Cement Association,BCA) (Grube and Lawrence, 1984) and Leeds University(Hassan and Cabrera, 1997) for concrete under differentialpressure techniques. Measurement of air permeability wasconducted by sealing the curved surface of 100 mmdiameter cores and applying a desired pressure on one sideof the specimen. The pressure gradient across the specimenresults in a flow, which is measured at the other side usinga flowmeter. The test duration is quite short, less than30 min, depending on the air voids content and thecontinuity of voids of the tested asphalt.

The intrinsic oxygen permeability can be determinedfrom the measurements of flow rate according to themodified D’Arcy’s equation, as given in Equation (C.1).

( )L P

KA P P

2

2 21 2

2ν η=

− ….. (C.1)

where: K = intrinsic air permeability (m²)

ν = flow rate (m³/sec)

η = viscosity of air (1.82 × 10-5 N.s/m² at 20 °C)

L = length of the specimen (m)

A = cross-sectional area of the specimen (m²)

P1

= inlet absolute applied (gauge) pressure (bar)

P2

= outlet pressure at which the flow rate ismeasured (bar), usually 1 bar

C.2 Interface permeability test

The water permeability at the interface between the asphaltsurfacing and the waterproofing system was measuredusing the apparatus described in BS 1881-208: 1996 formeasuring the initial surface absorption of concrete. Theprinciple of the test is to determine the time taken for aquantity of water to flow through a calibrated glass tubeonto a given area of the test specimens.

The composite slabs (concrete, waterproofing andasphalt) were sliced vertically down the centre to providecut faces specimens with the waterproofing system alongthe centre. The test cap was sealed into the slicedspecimens using silicon rubber and a clamp mechanism, asshown in Figure C.2. Water was then introduced throughthe reservoir, funnel, to fill the test cap. The amount ofwater leaking through the sliced specimens was recordedperiodically to determine the flow rate per unit area of thetest specimen.

Figure C.2 The initial surface absorption test for measuring permeability at the interface

Figure C.1 The TRL permeability cell

33

Appendix D: Semi-circular bending test

D.3.7

Symbols

D Diameter in millimetres (mm).

W Height in millimetres (mm).

t Thickness in millimetres (mm).

a Notch depth in millimetres (mm).

F Force in newtons (N).

Fmax

Maximum force in newtons (N).

∆W Vertical displacement in millimetres (mm).

Ki c

Fracture toughness in newtons per metre to thepower of 1.5 (N/m1.5).

ε Strain.

σ Stress in newtons per metre squared (N/mm²).

σhor

Horizontal stress in newtons per metre squared(N/mm²).

σmax

Maximum stress at failure in newtons per metresquared (N/mm²).

f(a/W) Geometric factor.

D.4 Principle

A half-cylinder test piece is loaded in three-point bending insuch a way that the middle of the base of the test piece issubjected to a tensile stress. During the test, the deformationincreases at a constant rate of 0.085 mm/s. The correspondingload increases to a maximum value, F

max, that is directly

related to the fracture toughness of the test sample.

D.5 Apparatus

D.5.1 Universal test machine

The machine should have a range of at least 50 kN andhave a drive with which a constant deformation rate of(5 ± 0.5) mm/min is maintained during the test.

NOTE: Contact with moving parts should be avoidedwhen the machine is in operation. In case of emergency,the machine should be capable of being stopped bypressing an emergency button.

D.5.2 Load cell or other force-measuring gauge

Capable of measuring loads up to 50 kN with an accuracyof ±0.2 kN.

D.5.3 Roller bearings

With a support length (centre to centre) of 120 mm and adiameter of 35 mm, as shown in Figure D.1.

D.5.4 Metal loading strip

With a width of (10 ± 0.2) mm, as shown in Figure D.1.

D.1 Scope

This annex describes the semi-circular bending (SCB) testmethod to determine the tensile strength or fracturetoughness of an asphalt mixture. The results of the test canbe used to calculate:

! The maximum load that the material containing a notch(crack) can resist before failure.

! When the presence of a notch is critical.

D.2 Normative references

The following referenced documents are indispensable forthe application of this annex. For dated references, onlythe edition cited applies. For undated references, the latestedition of the referenced document (including anyamendments) applies.

BS EN 12697-27, Bituminous mixtures – Test methods –Part 27: Sampling.

BS EN 12697-33, Bituminous mixtures – Test methods –Part 33: Specimen preparation by slab compactor.

BS EN 12697-35, Bituminous mixtures – Test methods –Part 35: Laboratory mixing.

D.3 Terms and definitions

For the purposes of this annex, the following terms anddefinitions apply.

D.3.1SCB test piecesample obtained by sawing an asphalt cylinder through adiameter

D.3.2strainrelative deformation of the SCB test piece

D.3.3stressthe force per unit area

D.3.4horizontal stressthe tensile stress prevailing at the base of the SCB test piece

D.3.5tensile strengththe maximum stress occurring during the test on the baseof the SCB test piece

D.3.6fracture toughnessresistance to failure by breaking

34

D.5.5 Monitoring equipment

Capable of continuously logging the loading and thevertical deformation of the SCB test piece.

D.5.6 Sliding callipersWith a reading accuracy of ±0.1 mm

D.5.7 Climatic chamberWith a temperature range of 0 °C to 50 °C and an accuracyof ±2 °C.

D.6 Sample preparation

D.6.1 ManufactureD.6.1.1Prepare asphalt cylinders in accordance with either D.6.1.2or D.6.1.3.

NOTE: If the cylinders are cored, it is important thatthey are drilled as near perpendicular to the surfaceof the asphalt as practicable.

D.6.1.2Prepare asphalt slabs by either:

! Manufacturing sufficient asphalt in the laboratory inaccordance with BS EN 12697-35 and compacting theasphalt into slabs 50 mm thick by roller compactor inaccordance with BS EN 12697-33.

! Manufacturing sufficient asphalt from the asphalt plantand compacting the asphalt into slabs 50 mm thick byroller compactor in accordance with BS EN 12697-33.

Cut 150 mm diameter cores from the slabs in accordancewith BS EN 12697-27.

D.6.1.3Cut 150 mm diameter cores from site-compacted asphalt inaccordance with BS EN 12697-27. Ensure that the top andbottom surfaces of the SCB specimen are flat and parallelby slicing if required.

D.6.1.4

Cut each core into two equal semi-circular SCB specimensthrough the middle (Figure D.2).

D.6.1.5If required for determining the fracture toughness, cut anotch nominally to a width of (5 ± 0.3) mm and a depth of10 mm (Figure D.2).

NOTE: Depending on the parameters to bedetermined, either a notched cylinder (for fracturetoughness) or an un-notched cylinder (for pullingforce) may be used.

D.6.2 Dimensional checkD.6.2.1Ensure that the SCB specimen is dry.

D.6.2.2Measure the diameter of the SCB specimen with thecallipers at two places along the longest side of the piece.Record the average diameter, D, to an accuracy of 0.1 mm.

120mm

Rollerbearings

Loading strip

Figure D.1 Schematic of semi-circular bend test

Figure D.2 SCB specimen dimensions (with the nominal dimensions for a standard test)

h1h2

a

W

t

D

½D

Notch

D = 150mm t = 50mmW = 75mm a = 10mm

35

D.6.2.3Measure the height of the SCB specimen with the calliperson each side, h

1 and h

2. Discard any SCB specimen for

which (h1 – h

2) > 0.5 mm. Record the average height, W,

to an accuracy of 0.1 mm.

D.6.2.4Measure the thickness of the SCB specimen with thecallipers at the two ends of the base and once at the top.Record the average thickness, t, to an accuracy of 0.1 mm.

D.6.2.5

If cut, measure the depth of the notch, a, to an accuracy of0.1 mm.

D.6.3 StorageSCB specimens that are not to be tested directly shall bestored on a flat surface in a cool area at (7 ± 3) °C.

D.7 Procedure

D.7.1Place the SCB specimen in the climatic chamber at the testtemperature of 15 ºC for at least 4 h.

D.7.2Position the load cell and loading strip on the upper beamand the roller bearings on the lower beam of the universaltest machine.

D.7.3Remove the SCB specimen from the climatic chamber andinstall in the universal test machine between the loadingstrip and the roller bearings (Figure D.1) in as short a timeas practicable. The overhangs of the SCB specimen fromthe roller bearings shall be equal to ±2 mm.

D.7.4As soon as the specimen is in place, raise the lower beamof the universal test machine until the SCB specimen justtouches the top load strip. Set the vertical deformation atzero and then apply a load to the specimen sufficient toproduce a deformation rate of 0.085 mm/s = 5.1 mm/min.Record the force and the vertical displacement at intervalsof not more than 5 s until the specimen fails.

NOTE: The time between the removal of the SCBsample from the climatic chamber and the samplefailing should be as short as possible (preferably lessthan 60 s).

D.7.5Plot both the force, F, and the vertical displacementagainst time from the start of the test.

D.8 Calculations

D.8.1Determine the maximum load, F

max, and the vertical

deformation, ∆W, from the plot.

D.8.2Calculate the strain, ε in accordance with Equation D.1.

W

W100%ε ∆= × …. (D.1)

where: W = Height in millimetres (mm)

∆W = Vertical displacement in millimetres (mm)

D.8.3Calculate the horizontal stress, σ

hor, in accordance with

Equation D.2.

24.263N/mmhor

F

D tσ ×=

× …. (D.2)

where: D = Diameter in millimetres (mm)

t = Thickness in millimetres (mm)

F = Force in newtons (N)

D.8.4Calculate the maximum stress at failure, σ

max, in accordance

with Equation D.3.

2maxmax

4.263N/mm

F

D tσ ×

=× …. (D.3)

where: D = Diameter in millimetres (mm)

t = Thickness in millimetres (mm)

Fmax

= Maximum force in newtons (N)

D.8.5For un-notched SCB specimens, the tensile strength isequal to the maximum stress.

D.8.6For notched SCB specimens, calculate the fracture toughness,K

ic, of the material in accordance with Equation D.4.

3/2max N/mmic

aK a f

Wσ π ⎛ ⎞= × × ⎜ ⎟

⎝ ⎠…. (D.4)

where: W = Height in millimetres (mm)a = Notch depth in millimetres (mm)σ

max= Stress at failure in newtons per

millimetre squared (N/mm²)f(a/W) = Geometric factor in accordance with

Equation D.5

a a af

W W W

2

0.623 29.29 171.2⎛ ⎞ ⎛ ⎞ ⎛ ⎞= − + × − ×⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

…. (D.5)

a a a

W W W

3 4 5

457.1 561.2 265.54⎛ ⎞ ⎛ ⎞ ⎛ ⎞+ × − × + ×⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

36

D.9 Test report

The test report shall include the following information:

a Identification number, type of mixture andcomposition.

b Date and time of testing and name of operator.

c Reference to this document.

d Sample code.

e Date sample was taken.

f Test temperature.

g Average sample diameter, D, to an accuracy of 0.1 mm.

h Average sample height, W, to an accuracy of 0.1 mm.

i Average sample thickness, t, to an accuracy of 0.1 mm.

j Notch depth, a, to an accuracy of 0.1 mm.

k Maximum force, Fmax

, to an accuracy of 1 N.

l Maximum stress at failure, σmax

, to an accuracy of0.1 N/mm².

m Fracture toughness, Kic, to an accuracy of 0.1 N/mm².

n Any observation which may have an influence on theevaluation.

D.10 Precision data

No precision data is currently available for this method.

37

Appendix E: Asphalt bending test

E.1 Scope

E.1.1This annex describes a method to determine themechanical properties of hot mixed asphalt whenbending failure occurs at a specified temperature andloading rate. Unless stated otherwise, the recommendedtest temperature is (15.0 ± 0.5) ºC and loading rate is(50 ± 1) mm/min. For the evaluation of the lowtemperature tensile properties of the asphalt, therecommended test temperature is (–10.0 ± 0.5) ºC andloading rate is (1.0 ± 0.1) mm/min.

E.1.2This annex applies to prismatic beams cut from slabscompacted by roller-compactor. The dimensions of thespecimen are (250 ± 2.0) mm in length, (30 ± 2.0) mm inwidth and (35 ± 2.0) mm in height. The span shall be(200 ± 2.0) mm.

E.2 Normative references

The following referenced documents are indispensable forthe application of this annex. For dated references, onlythe edition cited applies. For undated references, the latestedition of the referenced document (including anyamendments) applies.

BS EN 12697-5, Bituminous mixtures – Test methods –Part 5: Maximum density.

BS EN 12697-6, Bituminous mixtures – Test methods –Part 6: Bulk density, measurement.

BS EN 12697-8, Bituminous mixtures – Test methods –Part 8: Air voids content.

BS EN 12697-27, Bituminous mixtures – Test methods –Part 27: Sampling.

BS EN 12697-33, Bituminous mixtures – Test methods –Part 33: Specimen preparation by slab compactor.

BS EN 12697-35, Bituminous mixtures – Test methods –Part 35: Laboratory mixing.

E.3 Terms and definitions

For the purposes of this annex, the following terms anddefinitions apply.

E.3.1bending stiffness modulus in the elastic rangeratio of the bending strength to the bending strain in theelastic range in megapascals (MPa)

E.3.2bending stiffness modulus at failureratio of the bending strength to the bending strain at failurein megapascals (MPa)

E.4 Principle

The test is conducted on a standard beam by exertingconcentrated load on the midspan point of the specimenuntil failure occurs. The bending strength of the test piecein megapascals (MPa) and the bending strain of the asphaltat failure can be worked out are calculated from themaximum load at failure and the midspan deflection,respectively. The ratio of the bending strength to thebending strain is the bending stiffness modulus at failure inmegapascals (MPa).

E.5 Equipment

E.5.1 Universal test machineThe load shall be measured by a transducer and themeasuring range shall be at not less than 125 % and notgreater than five times the maximum load. The machineshall have a beam pedestal and the centre-to-centre spacingof the lower pedestal shall be 200 mm. The upper pressurehead shall be located in the middle and the upper pressurehead and pedestal shall be fixed steel cylindrical rods witha radius of (10 ± 1) mm. The upper pressure head shall bemovable and capable of coming into close contact with thetest specimen. The strain rate shall be adjustable.

Note 1: A loading range of 1 kN or 5 kN isdesirable with divisions at 10 N.

Note 2: It is desirable for the testing machine to beservo system.

E.5.2 Ambient temperature cabinetCapable of controlling the temperature to an accuracy of± 0.5 ºC (optional).

E.5.3 Midspan displacement measuring device

Note: An LVDT, electronic dial gauge or similardisplacement meter can be used

E.5.4 Data acquisition system or X-Y recordinginstrument

Capable of automatically acquiring the electric signalsfrom the transducer and displacement meter and eitherstoring them in the data acquisition system or plotting thecurves of the load against the midspan torsion on therecording instrument.

E.5.5 Water thermostat or refrigerator and dry ovenHaving a temperature range meeting the test requirementswith an accuracy of ± 0.5 ºC. When the test temperature isbelow 0 ºC, a 1:1 methyl alcohol water solution orantifreeze solution may be adopted as the refrigerant mediafor the water thermostat. The liquid in the water thermostatshall be able to circulate.

38

E.5.6 Circular sawCapable of cutting asphalt.

E.5.7 Callipers

E.5.8 Stopwatch

E.5.9 ThermometerWith the division value being 0.5 ºC.

E.5.10 BalanceWith the sensitivity being no greater than 0.1 g.

E.5.11 Miscellaneous plate glass

E.6 Sample preparation

E.6.1

Prepare asphalt slabs by either:

! Manufacturing sufficient asphalt in the laboratory inaccordance with BS EN 12697-35 and compacting theasphalt into slabs by roller compactor in accordancewith BS EN 12697-33.

! Manufacturing sufficient asphalt from the asphalt plantand compacting the asphalt into slabs by rollercompactor in accordance with BS EN 12697-33.

! Cutting slabs from site-compacted asphalt in accordancewith BS EN 12697-27.

Cut the slabs into prismatic beams of length (250 ± 2) mm,width (30 ± 2) mm and height (35 ± 2) mm.

Note: Up to 8 testing piece may be cut down from a300 mm × 300 mm × 50 mm plate.

E.6.2Measure the dimensions of the test beams at sections of themidspan and the two bearing points by means of callipers.A test beam shall be rejected when the two height or widthmeasurements differ by more than 2 mm. The width, b,and height, h, of the midspan section shall be the mean ofthe relevant measured values with an accuracy of 0.1 mm.

E.6.3Measure the maximum density, bulk density and air voidscontent of the test beams in accordance withBS EN 12697-5 Procedure A, BS EN 12697-6Procedure C and BS EN 12697-8, respectively.

E.6.4Place the test beams either in the water-bath at thespecified temperature for at least 45 minutes or in theconstant temperature air bath for at least 3 hours until thetest beam reaches the required testing temperature± 0.5 ºC. During the heating, the test beams shall be on thesupported plate glass and the spacing between the testbeams shall be not less than 10 mm.

E.7 Test procedure

E.7.1When the loading rate is less than 50 mm/min, raise thetemperature in the ambient temperature cabinet of theuniversal test machine, if available, to the value requiredby the test.

E.7.2Position the beam pedestal of the universal test machinewith (200 ± 0.5) mm between the bearing points. Make boththe balance between the upper and lower pressure heads andthe distance on each side equal; then fix the position.

E.7.3Take the test beams out of the water-bath or air bath andimmediately put them symmetrically on the pedestal.

E.7.4Place the displacement-measuring device at the centre of thebottom edge of the beam span and fix the pedestal on thebody of the universal test machine. Put the measuring headof the displacement meter at the centre of the top edge of themidspan of the test beam or (in the case of two displacementmeters) put the measuring heads on both sides. Select themeasuring range such that the effective range isapproximately 1.2 times the expected maximum deflection.

E.7.5Connect the transducer and displacement meter to the dataacquisition system or the X-Y recording instrument withthe displacement on the X-axis and the load on the Y-axis.Set the measuring range and zero the readings.

Note 1: The midspan torsion can be measured bymeans of LVDT, electric dial gauge or similardisplacement meter.

Note 2: When the displacement of the pressure head ofthe precision electro-hydraulic servo testing machineis taken as the torsion of the beam, the torsion can becalculated from the loading rate arid the time recordedby the X-Y recording instrument.

Note 3: In order to record the midspan torsion curvecorrectly, the paper moving speed (or scanning speed)of the Axis X of the recording instrument should bechosen to be between 500 mm/min and 5000 mm/min,depending on the temperature, when the loading rate of50 mm/min is chosen.

E.7.6Start the universal testing machine, exerting a concentratedload on the centre of the span at the specified rate until thetesting piece fails. During the loading, the strain rate shallremain constant. At the same time, the recordinginstrument shall record the load-midspan torsion curve.

Note: Examples of the load-midspan torsion curve areshown in Figure E.1.

39

E.7.7When the testing machine is not equipped with an ambienttemperature cabinet, the time period between taking the thetest beams out of the water-bath or air bath and thecompletion of the test shall not be greater than 4 s.

E.8 Calculations

E.8.1As shown in Figure E.1, extend the linear region of theload-deflection curve to cross the abscissa, taking theintersection as the origin of the curve. Measure themaximum load, F

m, and the midspan deflection, d

0, at the

peak point from the graph.

E.8.2Calculate the bending strength, R

B, the beam bottom

maximum bending strain, EB, and the bending stiffness

modulus, SB, at the time of the failure in accordance with

equations (E.1), (E.2) and (E.3), respectively.

BB

L FR

bh2

3

2= …. (E.1)

B

h dE

L2

6= …. (E.2)

BB

B

RS

E= …. (E.3)

where: RB

= bending strength of the asphalt inmegapascals (MPa).

EB

= maximum bending strain of the asphalt.

SB

= bending stiffness modulus at failure ofthe asphalt in megapascals (MPa).

b = width of the midspan section testingpiece in millimetres (mm).

h = height of the midspan section testingpiece in millimetres (mm).

L = span length of the testing piece inmillimetres (mm).

FB

= the maximum load when the testingpiece fails in newtons (N).

d = the midspan torsion when the testingpiece fails in millimetres (mm).

Note: In the calculation, the self weight of the beam isignored and, therefore, this method does not applywhen the test temperature is greater than 30 ºC.

E.8.3Calculate the stress, strain and stiffness modulus at anytime during the loading process using the method in E.8.2.

Note: It is only necessary to take readings of load anddeformation at the moment of failure and substitutethem for the maximum load and failure deformation inthe equations.

E.8.4When the linear region of the load-deformation curve is inthe range of 0.1 to 0.4 times the maximum test load,calculate the bending stiffness modulus in the elastic rangeusing E.8.1 to E.8.3.

E.8.5If the difference between a single measurement and the meanvalue is greater than ‘k’ times of the standard deviation, thismeasured value should be rejected and the mean value of theremaining measured values is to be taken as the test result.The value k shall be 1.15, 1.46, 1.67 and 1.82 when thenumber of tests, n, is 3, 4, 5 and 6, respectively.

E.9 Test report

The test report shall include the following information:

a Identification number, type of mixture and composition.

b Date and time of testing and name of operator.

c Reference to this document.

d The method of manufacture of each test beam.

e The dimensions of each test beam.

f Maximum density, bulk density and air voids content ofeach beam.

g The test temperature and loading rate.

h The bending strength, maximum bending strain andbending stiffness modulus at failure of each test beam.

i The bending stiffness modulus in the elastic range, ifcalculated, of each test beam.

j Any observation which may have an influence on theevaluation.

E.10 Precision

No precision data is currently available for this method.

Brittle example

Medium example

Ductile example

Load

F o

r st

ress

S

Torsion d or strain E

Figure E.1 Bending test load – midspan torsion curve

40

Abstract

The current clause in the Specification for Highway Works requires waterproofing systems on concrete bridgedecks to be overlaid with a 20 mm thick sand asphalt protection layer and binder and surface courses so that thetotal thickness of the three layers is 120 mm. However, the total thickness on some bridges has to be reduced and, insuch cases, a number of premature failures have occurred. The objective was to develop a specification forsurfacings on concrete bridges that will enhance the probability of achieving reasonable durability when they areless than the standard thickness. The research has included a literature search, a questionnaire and a laboratory testprogramme. The laboratory test programme identified some differences in their properties that have been used toidentify the basis for the specification. A secondary test programme was undertaken specifically to look at tests formeasuring the flexibility of asphalt materials. Based on the findings and other considerations, various additions andchanges to the Design Manual for Roads and Bridges, Specification for Highway Works and Notes for Guidance onthe Specification for Highway Works have been proposed.

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TRL494 The behaviour of asphalt in adverse hot weather conditions by J C Nicholls and I G Carswell.2001 (price £35, code E)

TRL250 Design of long-life flexible pavements for heavy traffic by M E Nunn, A Brown, D Weston andJ C Nicholls. 1997 (price £65, code L)

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PR13 Acceptable weather conditions for laying bituminous materials by J C Nicholls and M E Daines.1993 (price £35, code E)

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Prices current at August 2006

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