TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 144

THE SKIDDING RESISTANCE OF CONCRETE: PERFORMANCE

OF LIMESTONE AGGREGATE EXPERIMENT AFTER 10 YEARS

by R E FRANKLIN

The views expressed in this Report are not necessarily those of the Department of Transport

Pavement Materials and Construction Division, Highways Group, Transport and Road Research Laboratory, CROWTHORNE, Berkshire 1988

I S S N 0266-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Site

3. Design of experiment

3.1 Selection of aggregates

3.1.1 Coarse aggregates

3.1.2 Fine aggregates

3.2 Concrete mixes

3.3 Laboratory tests

3.3.1

3.3.2

3.3.3

Accelerated Wear Test

Aggregate properties

Calcium carbonate in the fine aggregate

4. Performance of road sections

4.1 Skid resistance tests

4.2 Texture measurements

4.3 Analysis of results

4.3.1 Influence of coarse aggregate properties

4.3.2 Influence of fine aggregate properties

4.3.3 The Accelerated Wear Test

4.3.4 Effect of traffic

4.3.5 Multiple correlations

5. Discussion of results

6. Conclusions

7. Acknowledgements

8. References

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© CROWN COPYRIGHT 1988 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged.

THE SKIDDING RESISTANCE OF CONCRETE: PERFORMANCE OF LIMESTONE AGGREGATE EXPERIMENT AFTER 10 YEARS

ABSTRACT

A full-scale experiment was constructed on the Windover to Funtley section of M27 and opened to traffic in 1976. The experiment comprised twelve sections, each 150 m long, containing a variety of concretes chosen to study the effect of using a range of qualities of limestone coarse aggregate and also to determine the effect of using different proportions of limestone or shell in the fine aggregate.

Measurements of Sideway Force Coefficient have been made annually and after 10 years these have been analysed to determine the influence of various aggregate and concrete properties tested in the laboratory.

The factor having the most effect on skid resistance was the acid-soluble content of the fine aggregate. Results of the Polished Mortar Value test correlated with both the acid-soluble content and the Sideway Force Coefficient on two of the three lanes but were relatively insensitive to changes in material quality. The Polished Stone Value of the coarse aggregate was a significant factor but, as previous studies also have shown, its effect on skid resistance was limited. The results of the Accelerated Wear Test were inconsistent with the fine aggregate properties and in general did not relate to skid resistance as well as individual aggregate properties.

1 INTRODUCTION

Earlier work reviewing the low-speed skidding performance of concrete roads carried out by Weller (1970) concluded that, on heavily trafficked roads, caution was necessary in the use of sands containing a proportion of limestone and recommendations were made for further studies. Information was gained from laboratory studies by Weller and Maynard (1970b) on the effect of mix design and materials including the abrasion resistance of fine aggregate and the polishing resistance of coarse aggregate. These studies utilised the then recently developed Accelerated Wear Test by Weller and Maynard (1970a) and together with the site review data were the basis for the introduction of skid resistance criteria into the Department of Transport's Specification for Road and Bridge Works (1976). Confirmation of the relative effects of fine and coarse

aggregate on the skid resistance of concrete was gained under full-scale trafficking using a technique of core-inserts reported by Franklin and Calder (1974). The importance of resistance to both abrasion and polish in the fine aggregate shown by the core- inserts led to the development by Franklin (1978) of a test procedure to measure these properties by the adaption of the PSV test apparatus.

In order to investigate further the appropriateness of the restrictions on aggregate properties in the Specification a full-scale experiment was constructed on Motorway M27 in March 1975. The experiment was designed to study the effect of using a range of qualities of limestone coarse aggregate and also to determine the effect of using different proportions of limestone or sea-shell in the fine aggregate. This Report examines the performance of the experimental sections after 10 years of trafficking and compares the results with those from previous studies.

2 SITE

The experiment was constructed as part of the Windover to Funtley section of M27. Twelve experimental sections, each 150 m long, were sited on the westbound carriageway where it passes through the Parkgate Interchange. The design of the scheme included the provision of an extra-long 2-lane slip road for traffic leaving the westbound carriageway at Parkgate. Originally intended to provide access to a planned Service Area alongside the interchange, the slip road also offered a convenient diversionary route for traffic so that inspections and measurements could be carried out on the main carriageway.

When this length of M27 was first opened in 1976 the motorway route was incomplete and t¢affic amounted to only 1145 commercial vehicles per day. Since the completion of the route from Portsmouth to Cadnam, traffic has increased substantially and in 1986 was estimated to be approximately 3150 commercial vehicles per day on the westbound carriageway with 85 per cent of these in the nearside lane.

The experimental carriageway which comprised 3 lanes and a hardshoulder was constructed in two passes (or rips). The first rip, designated as rip A, provided the hardshoulder and nearside lane and the

Neg. no. CB 692/75

Plate 1 Experimental sections under construction

second rip (rip B) provided the centre and offside lanes. Paving was carried out using a CMI slip-form paving train consisting of a spreader, a compacter- finisher and a joint-forming machine (Plate 1).

The slab was of reinforced concrete, 280 mm thick, laid on a granular sub-base.

In addit ion a section was included in wh ich the coarse aggregate comprised a 20 to 10 mm fract ion of a Iow-PSV l imestone combined wi th a 10 to 5 mm fraction of a h igh-PSV igneous rock in order to establish the relative effect on the resistance to skidding of these two sizes of coarse aggregate.

Each of the above six selections of coarse aggregate was used in conjunct ion wi th a commonly used sand from the Thames Val ley containing a minimal amount of l imestone contaminat ion.

3 DESIGN OF E X P E R I M E N T

3.1 SELECTION OF AGGREGATES The experimental concrete mixes were considered in two basic groups, one containing limestone coarse aggregates and the other containing limestone in the fine aggregate.

3.1.1 Coarse aggregates Limestones from three sources were selected to provide a range of polish resistance, as measured by the PSV test, representative of the limestones that could be used for concrete road construction. As a control, a f l int gravel from the Thames Valley was selected that represented a commonly used material with no special attributes towards resistance to skidding, and in contrast, a crushed igneous rock of high PSV from Leicestershire.

3.1.2 Fine aggregates The present specif icat ion limits the amount of l imestone permitted in the f ine aggregate by requiring that no more than 25 per cent by mass of acid soluble material may be present in either the fract ion retained on or the fract ion passing the 600/~m sieve. This l imit was examined in the exper iment by incorporat ing sections wi th blended f ine aggregate that were intended to contain nominal ly 10 and 30 per cent of an ool i t ic l imestone f rom Gloucestershire and 20 and 40 per cent of shell using a marine aggregate obtained f rom the North Sea.

Each of these four blends was made using the same Thames Valley f ine aggregate as was used wi th the dif ferent coarse aggregates.

The coarse aggregate used w i th these blended sands was also from the Thames Valley source.

TABLE 1

Types and sources of aggregates

Section number

10 11 12

Coarse aggregate Fine aggregate

Proportion Type Source (per cent) Type Source

Limestone Limestone Limestone Limestone Granite Granite Granite Flint

Flint

Flint

Flint Flint Marine

Churchwood Stowfield Holcombe Holcombe

100 100 100 100

Flint Flint Flint Flint

Chertsey Chertsey Chertsey Chertsey

Croft Croft Croft Chertsey

Chertsey

Chertsey

Chertsey Chertsey Northfleet

100 100

10 90 30 70 50 50

100 100 100

Flint Granite Limestone Flint Limestone Flint Marine Flint Marine Flint Marine

Chertsey Croft Cerney Chertsey Cerney Chertsey Northfleet Chertsey Northfleet Chertsey Northfleet

Two additional sections were also included. One combined the igneous coarse aggregate with fine aggregate from the same source in order to determine whether such fines are, in fact, as unsuitable as had been concluded from laboratory studies by Weller and Maynard (1970b). The other section combined coarse aggregate from the same source as the marine dredged fine aggregate to give a mix containing aggregate entirely of marine dredged material. A schedule of the various fine and coarse aggregates is given in Table 1.

3.2 C O N C R E T E M I X E S Once the various sources of aggregate had been selected, laboratory trial mixes were made at TRRL aimed at identifying suitable mix proportions for the paving. A target workability level represented by a Compacting Factor of 0.85 was chosen together with an average cube strength of 40 MN/m 2 at 28 days. The fine aggregate content was chosen as 35 per cent and the ratio of 20 mm to 10 rnm aggregate was chosen as 2:1. Details of the resulting mix proportions were offered to the contractor for information but the final choice of proportions was left to the contractor in view of his contractual responsibilities. The final mix proportions as used on site are given in Table 2.

3.3 L A B O R A T O R Y TESTS During the period that the experimental sections were being paved, samples of each of the selected aggregates were taken and used for the following laboratory tests.

3.3.1 Accelerated Wear Test The Department of Transport specification requires that, when coarse aggregate produced from limestone rock is used in the top 50 mm of a slab, the mix shall be subjected to the Accelerated Wear Test. Appropriate aggregate samples were prepared and delivered to the Department of Transport Materials Testing Laboratory at Ruislip where specimens were prepared to the mix proportions given in Table 2 and tested in the usual way for concretes used in Department contracts. Samples were not provided for sections 3B, 4B or 5B because it was found that the Holcombe 20 mm stockpile had been contaminated with Croft granite and the Croft 10 mm stockpile had been contaminated with Holcombe limestone.

A second series of Accelerated Wear Tests were carried out on specimens made on site from samples of concrete taken from the carriageway. These samples were obtained as the concrete was delivered to the spreading machine. No site-made specimens were made for sections 1A to 8A nor for section 11A. However a full set of specimens was made for the second rip ie, sections 1B to 12B. Results of both series of tests are given in Table 3.

3.3.2 Aggregate properties Studies of core-inserts on M4 and A12 by Franklin and Calder (1974) had shown that both the resistance to polish and the resistance to abrasion of the fine aggregate were important properties in determining the skid-resistance of concrete. The abrasion resistance of the coarse aggregate was found to

Section number

1A 1B

2A 2B

3A 3B

4A 4B

5A 5B

6A 6B

7A 7B

8A 8B

9A 9B

10A 10B

11A 11B

12A 12B

Agg. /cement ratio

5.53 5.57

5.56 5.53

5.56

5.45

5.45

4.53 4.52

5.21 5.23

5.20 5.27

5.84 5.85

6.04 6.08

5.63 5.63

5.84 5.84

TABLE 2

Mix proportions of experimental sections

Aggregate proportions (per cent) Water/

cement ratio 20 mm 10 mm Fine 1 Fine 2

0.44 0.43

0.41 0.44

0.41

0.40

0.39

0.44 0.42

0.37 0.37

0.37 0.37

0.41 0.41

0.41 0.43

0.41 0.41

0.38 0.38

42 43

43 43

43

43

43

49 49

43 43

43 43

43 43

43 42

43 43

43 43

21 20

20 20

20

20

20

17 17

21 20

21 20

21 21

21 21

21 21

21 21

37 37

37 37

37

37

37

34 34

3.5 3.6

10.7 10.5

17.9 17.8

36 37

36 36

36 36

32.5 33.4

25.3 26.5

18.1 18.2

Note: Proportions have been expressed for aggregate in a saturated-surface dry condition.

make a minor contribution in these studies and in another analysis of similar data the polish resistance of the coarse aggregate was found to make a small contribution. In order to permit further analysis, samples from each of the different 10 mm aggregate stockpiles were used to prepare test specimens for the Polished Stone Value Test. These were made and tested in accordance with BS 812: Part 3(1975).

Samples from each of the 20 mm aggregate stockpiles were similarly taken and used to make specimens for the Aggregate Abrasion Test, also in accordance with BS 812: Part 3(1975).

As a result of the studies by Franklin and Calder (1974), a Polished Mortar Test was developed by Franklin (1978) which correlated well wi th skid resistance when tested over a range of widely di f ferent aggregates. This new test procedure was carried out therefore on the samples of the various fine aggregates used in the M27 experiment. The results are given in Table 3; it can be seen that the values obtained were all very similar.

3.3.3 Calcium carbonate in the fine aggregate

In order to confirm that the amount of calcium carbonate present in the different mixes was as intended by the design of the experiment, samples from the fine aggregate stockpiles were subjected to a test for acid solubility. The results were used in conjunction with proportions of the various mixes to give an estimate of the percentage of limestone present in the fine aggregate fraction. These estimates are given in Table 3.

During construction of the second rip (the centre and offside lanes) the opportunity was taken of obtaining a second measure of the limestone in the fine aggregate using material remaining after completion of the concrete fresh analysis.

These analysis tests had been carried out by the materials testing consultant on samples taken as the concrete was delivered to the spreading machine. The fine aggregate obtained from these analyses was tested also for acid solubility; the results thus

4

Section number

1A 1B

2A 2B

3A 3B

4A 4B

5A 5B

6A 6B

7A 7B

8A 8B

9A 9B

10A 10B

11A 11B

12A 12B

Results of laborator'

Accelerated Wear Tests

Site-made specimens

60

55 55

Laboratory- made

specimens

53

57

56

55

53

53

57 55 54

53 54 49

63 55 60 58

58 52 59 54

56 60

60 53

T A B L E 3

tests on aggregates and concrete

AAV PMV

CaCO3 Content

62 62

60 59

61

60

58

38 38

PSV

53 53

36 36

32 32

10.5 8.2

7.8 8.5

11.5 9.3

10.7 9.1

4.9 5.3

4.8 4.4 0.8 0.5

0.8 0.5

0.8 0.5

0.8 0.5

0.8 0.5

0.6 0.7

55 55

55 55

55 55

55 55

55 55

55 56

54 54

55 55

53 53

52 52

55 55

52 52

Concrete samples

56 56

56 56

32 32

32 32

32 32

32 32

32 32

32 32

6

6

6

6

6

0 0

9

11

21

29

6

29

Stockpile samples

5 4

5 4

5 4

5 4

5 4

0 0

12 13

27 27

18 18

30 33

5 4

38 33

obtained were considered to give a more reliable estimate of limestone fines in the road than those from the first method based on stockpile samples. The results of these tests are given also in Table 3 and show good agreement between the two sets of values for sections 7B, 9B, 10B, 11B and 12B but a marked difference for section 8B. The figure of 11.4 per cent for section 8B was an average of 5 values ranging from 8.9 per cent to 13.8 per cent and was considered the more representative value of limestone fines in the concrete.

4 PERFORMANCE OF ROAD SECTIONS

4.1 SKID RESISTANCE TESTS Since their construction, the experimental sections have been monitored each year to determine their low-speed skidding resistance using SCRIM to measure values of Sideway Force Coefficient in each of the three traffic lanes.

Test runs were made three t imes each year between April and September and the average of these three test results was designated the Mean Summer Value. A different level of SFC was quickly established for each of the three traffic lanes which then reduced as rates of traff ic increased. As an indication of the level of SFC that each section reached after 10 years of traff icking, the average of the values for 1985 and 1986 is given in Table 4.

4.2 TEXTURE M E A S U R E M E N T S On one of the three occasions each year when SCRIM was used to measure SFC values, the brushed texture of the pavement surface was measured in the wheel track of four of the twelve sections using the sand patch test as specified by the Department of Transport (1976). The results, given in Table 5, show that during the f irst four years of trafficking the texture, particularly in the nearside lane, decreased markedly but in the successive two years little change occurred and measurements were therefore discontinued.

TABLE 4

Mean SFC for 1985/86

Section Nearside Offside number lane Centre lane lane

1 2 3 4 5 6 7 8 9

10 11 12

0.50 0.49 0.47 0.48 0.53 0.51 0.47 0.46 0.46 0.44 0.46 0.46

0.51 0.52 0.51 0.53 0.53 0.58 0.52 0.50 0.44 0.41 0.49 0.44

0.58 0.58 0.57 0.58 0.58 0.63 0.58 0.55 0.52 0.50 0.54 0.52

4.3 A N A L Y S I S OF R E S U L T S The two basic variables used in the design of the experiment were the amount of calcium carbonate in the fine aggregate and the polish resistance of the coarse aggregate. The first analysis was made therefore by evaluating the performance of the appropriate sections with respect to these factors.

For each year's results of the skid-resistance measurements, relationships between these two variables and the Mean Summer Value of SFC were tested for significance and the change in correlation coefficient was noted. In addition, a broader analysis was carried out using the mean SFC values for 1985/86 from Table 4 which were compared against the values obtained from the various aggregate and concrete tests as detailed in Table 3.

4.3.1 Influence of coarse aggregate properties

The sensit ivi ty of coarse aggregate to polishing under the action of traff ic is measured principally by the

• Nears ide lane

O Cen t re lane

• O f f s i de lane

E

t~

0.3

0.2

0.1

PSV of coarse aggregate

J I i I I

76 78 80 82 84 86

0.5

0.4

0.3

0.2

tT

0.1

Acid soluble content of fine aggregate

O

- /

o/° D

f " I S

/

O i

O

0 I I I I I 76 78 80 82 84 86

Year

Fig. 1 Change of regression coefficients wi th time

PSV test. Therefore, a subset of results for Sections 1 to 5 inclusive plus Section 11 was taken and tested for correlation With each year's Mean Summer Values of SFC. Significant correlations were found for several of the yearly results from the nearside

TABLE 5

Surface texture depths (ram)

Lane Nearside Offside

Section number 2 11 2 6 10 11

1975" 1976 1977 1978 1979 1981

0.74 0.62 0.50 0.46 0.46 0.41

6 10

1.14 0.76 1.00 0.67 0.75 0.54 0.71 0.42 0.58 0.46 0.57 0.40

0.77 0.75 0.58 0.41

0.45

0.78 0.78 0.74 0.61

0.60

1.02 1.30 0.97 0.87 0.70

0.86 0.92 0.76 0.52 0.59 0.55

0.72 0.86 0.66 0.49

0.54

"1975 values were obtained before the Motorway was opened to traffic

Dependant variable

(y)

SFC nears•de SFC centre SFC offside

SFC nears•de SFC centre SFC offside

T A B L E 6

Regression equations for coarse aggregate properties (of form , = m x + c )

Independant variable

(x)

PSV PSV PSV

AAV AAV AAV

0.88 0.62 0.68

0.46 0.52 0.57

Significance level

(per cent)

0.03 3.1 1.5

12.9 7.8 5.3

Regression coefft.

(m) x 100

0.217 0.288 0.241

0.598 0.494

Intercept (c) x 100

39.5 38.7 46.8

47.3 54.0

r

PSV AAV

denotes Correlation Coefficient denotes Polished Stone Value denotes Aggregate Abrasion Value

lane but the centre and offside lanes gave no significant relationships for any of the individual years. A significant relationship was first obtained for the nears•de lane after 3 years trafficking and with further periods of trafficking the effect of the PSV, as indicated by the correlation coefficient, became greater, reaching a value in the order of 0.2 after 9 years. A plot of this change in correlation coefficient is shown in Figure 1. The effect was such that a difference in PSV of 25 units, which was approximately the range between the fl int gravel and the crushed granite, would lead to a difference in SFC of 0.05 after a period of 10 years trafficking.

The full data set for the 1985/86 results from Tables 3 and 4 were similarly tested for correlation between SFC and PSV and a very significant relationship was found for the nears•de lane values. Similar relationships were also found for the centre and offside lanes but at much less significant levels; the details are given in Table 6 and the relationships are shown in Figure 2.

As mentioned above, earlier studies had shown that at times the Aggregate Abrasion Value could also influence skidding resistance. However, analysis of the results in Tables 3 and 4 showed that in this instance there was no effect on the nears•de lane skid resistance and only weak trends for the other two lanes. Details are given in Table 6.

Section 4 had been included in the experiment to determine the relative effect of the 10 mm and 20 mm fraction on skid resistance. The results indicated that in the nears•de lane where more polishing would have occured, it was the nature of the 10 mm aggregate that was determining the SFC of the concrete. Differences of SFC between sections 3 and 5 in the other two lanes were too small to enable any effect to be clearly identified.

0.60

0.50

u. c/)

0.40

(a) Nears•de lane

r = 0.88

0.30 I I I 2O 30 40 50 60

0.60

co

0.50 o0 03

u_

~n 0.40 g

(b) Centre lane • r = 0.62

•(2)

0.30 I I I 20 30 40 50 60

~D eO

O~

0.70

0.60

0.50

(c) Offside lane

r = 0.68

0.40 I I I 20 30 40 50 60

P S V o f c o a r s e aggregate

Fig. 2 Relat ions between PSV of coarse aggregate and S F C

7

4.3 .2 I n f l u e n c e of f ine a g g r e g a t e p r o p e r t i e s

As was ment ioned in 3.3.3, the better estimates of calcium carbonate were considered to have been given by the acid soluble contents of the sand samples recovered from the fresh concrete analyses. Using these results for the centre and offside lanes and the results from the stockpile samples for the inner lanes, correlations were sought wi th the Summer Mean Values of SFC as before. The analyses showed that the most frequent occurrence of s igni f icant relationships was in the centre lane. The change in regression coeff ic ient where the relationships were better than 5 per cent is shown in Figure 1. It can be seen that the effect of the carbonate content took 4 to 5 years to become ful ly established and was such that a content of 25 per cent reduced the SFC by 0.11 compared to an aggregate w i th no acid soluble material.

The mean values for 1985/86 were similarly tested over the ful l data set and similar results were obtained. These are given in Table 7 which shows that stronger correlations were obtained wi th values based on the recovered sand samples from the fresh analysis tests support ing the v iew that such samples gave a better representation of the road concrete than stockpi le samples. The relationships are shown in Figure 3.

The Polished Mortar Value (PMV) test was designed to measure the integrated effect of all fine aggregate properties that could inf luence skidding resistance. However, the PMV results gave very similar degrees of correlat ion w i th SFC, as indicated by the correlation coeff icients r in Table 7, to those found for the acid soluble content. This Would seem to indicate that, for the range of f ine aggregates used in this exper iment, the acid soluble material was the only factor of any consequence in determining skid resistance. This can be explained by the fact that, apart f rom the section contain ing Croft granite fines, all the sections used sands that who l l y or part ly consisted of f l int. In such a si tuat ion it was not surprising to f ind that there was a strong correlation between the PMV results and both sets of acid soluble contents.

4 . 3 . 3 T h e A c c e l e r a t e d W e a r T e s t Each of the two sets of Accelerated Wear Test results was compared to the 1985/86 SFC results. The init ial analysis wh ich used all available data from the laboratory-made specimens produced no correlations, but a plot of the values clearly showed that the results for Sect ion 6 was an isolated value. When this value was omit ted f rom the data set a s igni f icant relationship was obtained for the nearside lane values but the centre and offside lane values cont inued to show no correlat ion.

0.60 (a) Nearsidelane,

• r = 0.54

o0 0.50 oo ob

I.L

0.40

0.30 I I I 0 10 20 30 40

co o3 LO CO (33

CJ

0"60 i~ll (b) Centre lane ] L concrete samples 1

L 0.40 -- • ~ /

030 I I I r = 0.93 /

0 10 20 30 40

(~ (c) Offside lane 0.60 I ~ ¢ = ~ ~ r e t e samples

050 I-- r = 0.88

LL

0.40

0.30 I I I 0 10 20 30 40

Acid-soluble content (per cent)

Fig. 3 Relations between acid-soluble content and SFC

Analysis of the results from the site-made specimens gave a correlation, albeit of low significance, wi th a negative regression coefficient. Clearly this is not logically acceptable as it indicates that a better result f rom the Accelerated Wear Test is associated wi th a worse skid resistance on the road. This set of values was therefore rejected. The results of the correlation analyses on the Accelerated Wear Test results are given in Table 8.

Dependant variable

(y)

SFC nearside SFC centre SFC offside

SFC nearside SFC centre SFC offside

SFC nearside SFC centre SFC offside

T A B L E 7

Regression equations for fine aggregate properties (of form , = r e x + c )

Independant variable

(x)

ASSA ASSB ASSB

m

ASCB ASCB

PMV PMV PMV

0.54 0.81 0.79

0.93 0.88

0.54 0.91 0.85

Significance level

(per cent)

6.8 0.2 0.2

0.006 0.03

6.8 0.01 0.08

Regression coefft.

(m) x 100

-0 .109 - 0.309 - 0.232

m

- 0.457 - 0.329

1.04 3.30 2.34

Intercept (c) x 100

49.3 53.6 58.9

55.0 59.8

- 8 . 4 - 129.7

- 7 1 . 7

N.B. r denotes Correlation Coefficient ASSA denotes Acid soluble content of stockpile sample in rip A. ASSB denotes Acid soluble content of stockpile sample in rip B. ASCB denotes Acid soluble content of concrete sample in rip B.

T A B L E 8

Regression equations for Accelerated Wear Test results (of form y = m x + c)

Dependant variable

(y)

Including section 6 SFC nearside SFC centre SFC offside

SFC centre SFC offside

Omitting section 6 SFC nearside SFC centre SFC offside

Independant variable

(x)

AWLA AWLB AWLB

AWSB AWSB

AWLA AWLB AWLB

0.06 0.4"5 0.45

0.65 0.68

0.63 0.20 0.29

Significance level

(per cent)

85.7 22.3 22.0

2.2 1.4

3.7 63.7 50.2

Regression coefft.

(m) x 100

m

- 1 . 1 7 - 0.936

0.457

Intercept (c) x 100

D

m

115.0 108.4

21.3

N.B. r denotes Correlation Coefficient AWLA denotes Accelerated Wear Test on laboratory-made specimens for rip A. AWLB denotes Accelerated Wear Test on laboratory-made specimens for rip B. AWSB denotes Accelerated Wear Test on site-made specimens for rip B.

4 . 3 . 4 E f f e c t o f t r a f f i c

Inspection of Table 4 shows that, as would be expected, the more heavily trafficked nearside lane had polished to a greater extent than the centre and offside lanes. The average SFCs of the three lanes were 0.48, 0.50 and 0.56 for the nearside, centre and

offside lanes respectively. Comparisons between individual results in the three lanes indicates that across some sections a comparable concrete was not produced in both rips. Individual differences between the centre and offside lanes ranged from 0.05 to 0.09, whereas between the nearside and the centre

lanes the differences ranged from - 0 . 0 4 to 0.07. This means that the concrete laid in one or other of the two rips was not as intended in some sections and could explain why some correlations were not signif icant for the inner lane. Unfortunately samples of fine aggregate recovered from fresh analysis tests had not been available during the construction of the first rip to enable confirmation of the acid soluble contents. These would have provided checks on the concrete composit ion and would have been part icularly appropriate as it was the mixes containing marine sand that gave the anomalies between the two rips.

4.3.5 Multiple correlations In v iew of the significant correlations found for the various fine and coarse aggregate properties several combinat ions of properties were tested for mult iple correlations. Although the correlation coefficients were increased slightly above those for the simple correlations one factor proved to be dominant in each case with the other factors becoming insignificant.

as significantly influencing skid resistance. The size of the effect was limited and this was considered to be a reflection of the limited exposure of the coarse aggregate particles in the surface. On M27 the coarse aggregate was, contrary to expectations, as well exposed in the centre lane as in the nearside lane and although the correlation was much stronger for the nearside lane all three lanes were equally affected, as indicated by the regression coefficients. The abrasion resistance of the aggregate, although showing a trend towards relating to SFC, was not a significant factor.

It was to be expected that the Accelerated Wear Test would correlate with skid resistance but of the eight relationships that were tested only one yielded a significant or plausible correlation and this was dependent on the omission of an anomalous result for one of the concretes. Of the 11 remaining concretes the one giving the lowest AWT result was that for Section 8B which contained a nominal 30 per cent of oolitic limestone in the fine aggregate. However, the SFC results were considerably lower for Sections 10B and 12B which had higher contents of acid-soluble material but in the form of sea shells in marine aggregate. This seems to indicate that for some reason the Accelerated Wear Test was not as sensitive to shell as it was to oolitic limestone although the SFC was apparently equally sensitive to both forms of acid-soluble material.

5 DISCUSSION OF RESULTS

The foregoing analyses indicate again the importance of f ine aggregate properties in determining skid resistance of concrete roads and confirm the need for appropriate quality criteria to be included in Specif ications. The fact that stronger relationships were derived from samples obtained from the road than those from stockpiles shows also the sensit ivi ty of skid resistance to variations in aggregate quality.

The present Department of Transport Specification limits could result in differences in SFC of up to 0.11 although in general a smaller range would be expected. This is because in practice acid soluble material present in the form of a contaminant in the fine aggregate is unlikely to be uniformly distributed over all fractions of the grading.

Results of the PMV tests were well correlated to skid resistance but their range was extremely narrow for the materials used in the experiment. This is probably due to the fact that the selected sands, wi th one exception, were based on the same type of f l int aggregate. A small variation within such a closely grouped set of values can cause large differences in the equation of the best-f i t line through these values and this was found to be the case in this study.

Earlier studies by Weller and Maynard (1970b) and by Franklin (1978) identified the coarse aggregate PSV

6 CONCLUSIONS

After 10 years of trafficking on the various concretes incorporated in the experiment the following conclusions can be made:

1. The most influential and significant materials factor affecting the skid resistance was the amount of acid-soluble material in the fine aggregate. The limits in the current Department of Transport Specification could give a range of SFC of up to'0.11 although in practice the non-uniform distribution of acid-soluble material normally found throughout the fine aggregate grading would result in a smaller effect.

2. The Polished Mortar Value test related well to the amount of acid-soluble material and to skid resistance on two of the three lanes but was relatively insensitive to changes in material quality.

3. The PSV of the coarse aggregate in the concrete had a significant effect on the pavement's skid resistance and was most strongly correlated in the most heavily trafficked lane. However, the SFC was unlikely to change by more than 0.05 over the range of coarse aggregates normally used in concrete pavements.

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4. The results of the Accelerated Wear Test were inconsistent with the fine aggregate parameters. Correlation was established for the nearside lane but was not related to the pavement skid resistance as well as individual aggregate properties.

7 ACKNOWLEDGEMENTS

The work described in this report was carried out in the Pavement Materials and Construction Division (Head: Mr G F Salt) of the Highways Group of the Transport and Road Research Laboratory.

8 REFERENCES

BRITISH STANDARDS INSTITUTION (1975). Methods of sampling and testing of mineral aggregates, sands and filler. British Standard BS 812: part 3: British Standards Institution, London.

DEPARTMENT OF TRANSPORT (1976). Specification for road and bridge works. HMSO, London.

FRANKLIN, R E (1978). The skidding resistance of concrete: development of a polishing test for fine aggregate. Department of the Environment Department of Transport TRRL Report SR dA.d: Transport and Road Research Laboratory, Crowthorne.

FRANKLIN, R E and CALDER, A J J (1974). The skidding resistance of concrete: the effect of materials under site conditions. Department of the Environment TRRL Report LR 640: Transport and Road Research Laboratory, Crowthorne.

WELLER, D E (1970). A review of low-speed skidding resistance of a number of concrete roads containing various aggregates. Ministry of Transport RRL Report LR 335: Transport and Road Research Laboratory, Crowthorne.

WELLER, D E and MAYNARD, D P (1970a). The use of an accelerated wear machine to examine the skidding resistance of concrete surfaces. Ministry of Transport RRL Report LR 333: Transport and Road Research Laboratory, Crowthorne.

WELLER, D E and MAYNARD, D P (1970b). The influence of materials and mix design on the skid resistance value and texture depth of concrete. Ministry of Transport RRL Report LR 334: Transport and Road Research Laboratory, Crowthorne.

Printed in the United Kingdom for Her Majesty's Stationery Office (1946/88) Dd8222739 7/88 CI0 G426 10170

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