Pavement Design Revised

7/21/2019 Pavement Design Revised

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Pavement design forms an important part of detailed engineering study. The economic returns are

directly dependent on the performance of the pavement. The scope of pavement design in this project

can be divided into the following sections.

Design of Flexible Pavement in Concentric/ Eccentric Widening portions,

Design of Rigid Pavement

Design of overlay on the existing pavement

Design of service roads

Flexible Pavement Design

Design methods outlined in IRC: 37-2001 has been used for design of the new pavement.

Flexible pavement design in our country generally is based on the guidelines given in the IRC: 37-

2001. The revised code is a modification to the IRC-37-84 incorporating a mechanistic approach. In

this new code, pavement designs have been extended to cover upto 150 Msa and CBR strengths

upto 10%.

Flexible pavement design has been modified as a layer structure. Stresses and strains at critical

locations have been computed using the linear elastic model developed under the aegis of MORT&H.

To give proper consideration to the aspect of performance, the following three types of pavement

distress resulting from repeated application of traffic loads are considered.

Vertical compressive strain at the top of subgrade

Horizontal tensile strain at the bottom of bituminous layer

Pavement deformation within the bituminous surface

By meeting the mix design requirements, the permanent deformation within the bituminous

layer can be controlled, whereas the thicknesses of granular and bituminous layers are

selected using the analytical design approach so that the strains at the critical points are

within the allowable limits.

Appropriate designs can be furnished based on the following data:

Design traffic in terms of cumulative number of standard axles

CBR value of Subgrade

Traffic Growth and calculation of Msa

The method considers traffic in terms of cumulative number of standard axles (8160 Kg) to be carried

by the pavement during the design life. Design traffic will be estimated based on the following

parameters:

Initial traffic after construction in terms of number of commercial vehicles per day

Traffic growth rate during the design life in percentage

Design life in number of years

Vehicle damage factor (VDF)

Distribution of commercial traffic over the carriageway

Soil Properties

Subgrade: The soil used for subgrade should be well compacted to utilise its full strength toeconomise the overall thickness of the pavement. Most of the specifications prescribe use of selected


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material and stiffer standards of compaction in the subgrade, and the top 500mm portion of the

roadway. As per the MORT&H specifications, the subgrade has to be compacted to 97% of dry

density, using heavy compaction (modified Proctor density).

Subbase: Granular base materials conforming to Clause 401 of MORT&H specifications for Road and

Bridge works are recommended for use. These specifications suggest three gradings each for close

and coarse graded granular sub-base materials and specify that the materials passing 425 microns

sieve should have liquid limit and plasticity index more than 25 and 6 respectively. These

requirements and the specified grain size distribution of the sub-base material should be strictly

enforced to meet stability and drainage requirements of the granular sub-base layer. The sub-base

should have a minimum CBR of 20% for cumulative traffic upto 2 Msa, and 30% for traffic exceeding 2

Msa. The thickness of Subbase should not be less than 150mm for design traffic less than 10 Msa,

and 200mm for design traffic of 10 Msa and above.

Base: Materials to be used in base course must satisfy the grading and physical requirements

prescribed in the IRC/MORT&H specifications. The recommended minimum thickness of granular

base is 225mm for traffic upto 2 Msa and 250mm for traffic exceeding 2 Msa.

Bituminous Surfacing: Bituminous surfacing shall consist of either a wearing course or a binder

course with a wearing course depending upon the traffic to be carried. The most commonly used

wearing courses are surface dressing, open graded premix carpet, mix seal surfacing, semi-dense

bituminous concrete and bituminous concrete. Choice of the appropriate type of bituminous wearing

course will depend on several factors like design traffic over the service life, the type of base/binder

course, rainfall and other related factors. The grade of bitumen will be selected keeping in view the

traffic, rainfall and other environmental and climatic conditions

Design

Flexible pavement design has been carried out using IRC: 37-2001. The revised code is a

modification to the IRC: 37-1984 incorporating a mechanistic approach. In this new code, pavementdesigns have been extended to cover upto 150Msa and CBR strength upto 10%. The design values

are compared with the values obtained by the AASHTO method for their adequacy.

a. Design Life

A 20 year design life has been considered for the flexible pavement.

b. Vehicle Damage Factors (VDF)

The vehicle damage factor is a multiplier to convert the number of commercial vehicles of different

axle loads and axle configuration. To the number of standard axle load repetitions. It is defined as

equivalent number of standard axles per commercial vehicle. Since, the axle load data was not

available, a value of 4.5, as given in IRC: 37-2001 has been adopted.

c. Growth Rate

Traffic growth rate should be estimated from either by studying the past trends of the traffic growth or

by establishing the econometric models. However average growth rate of 7.5% has been assumed for

carrying out the design.

d. Traffic Considerations

Classified volume count was conducted from 18 th Feb to 24th Feb of 2006 at km 413+100. The details

of the volume count are given below.

Date Car/Jeep/Van Bus Mini Bus LCV2 Axle

Truck

Multi-Axle

Vehicles18.02.06 11532 1451 546 1174 1604 180


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19.02.06 13200 1517 329 888 1210 100

20.02.06 8759 1045 185 639 1702 172

21.02.06 8234 1026 149 738 1645 245

22.02.06 8183 1014 228 800 1617 187

23.02.06 7710 1024 162 786 1683 172

24.02.06 6206 894 101 632 819 116Total 63824 7971 1700 5657 10280 1172

ADT 9118 1139 243 808 1469 167

After projecting the traffic with the above discussed parameters, a design lane Msa of 100 has been

considered for the pavement design. The projected traffic is given below.

YearProjected Traffic

MsaCumm

Msa

2006 3826 2.36 2.36

2007 3984 2.45 4.81

2008 4148 2.56 7.37

2009 4321 2.66 10.03

2010 4519 2.78 12.81

2011 4726 2.91 15.72

2012 4943 3.04 18.77

2013 5196 3.20 21.97

2014 5462 3.36 25.33

2015 5742 3.54 28.87

2016 5972 3.68 32.54

2017 6211 3.83 36.372018 6459 3.98 40.35

2019 6717 4.14 44.49

2020 6986 4.30 48.79

2021 7236 4.46 53.25

2022 7496 4.62 57.86

2023 7765 4.78 62.65

2024 8043 4.95 67.60

2025 8332 5.13 72.73

2026 8632 5.32 78.05

e. Subgrade Strength

The widened/new pavement will have to be laid on embankment constructed by the material from the

borrow areas. Soil with subgrade CBR of 8% is considered in the design. The test results of the

borrow samples is given below.

CHAINAGE SIDEDISTANCEFROM NH-1

FSI(%)

GRAVEL(%)

SAND(%)

SILT /CLAY (%)

LL(%)

PI(%)

MDD(gm/cc)

OMC(%)

CBR(%)

433+100 RHS 1.10 Km 11.00 0 56.10 43.90 18 2.0 2.12 7.50 33.50

430+000 LHS 0.40 Km 13.64 0 39.80 60.20 19 2.0 2.13 8.50 20.60

432+000 LHS 0.60 Km 15.00 0 51.70 48.30 19 2.0 2.03 9.00 8.90


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431+000 RHS 4.20 Km 11.11 0 64.00 36.00 - NP 2.13 8.50 18.20

431+000 LHS 5.50 Km 0.00 0 61.80 38.20 - NP 2.14 8.00 42.50

431+000 LHS 5.50 KM 0.00 0 62.16 37.84 - NP 2.12 6.50 60.00

f. Flexible Pavement Design Approach

New pavement is designed for subgrade strength of 10%. Pavement will be laid on subgrade, which

has a 4 days soaked CBR more than 10%. IRC pavement designs are given for a maximum sub-

grade CBR of 10%. In the present case, a thickness of 500mm select sub grade with 10% is assumed

as sub grade, and over which the pavement crust will be built. The flexible pavement for new

carriageway has been designed as per the IRC: 37-2001. The pavement composition has been

designed for a life of 20 years. The pavement composition has been designed for a traffic level of 100

Msa and subgrade CBR of 10%.

The proposed pavement composition for the main carriageway is given below:

BC 50mm

DBM 130mm

WMM 250mm

GSB 200mm

The pavement is designed for 20 years and supposed to withstand the traffic till the end of the design

life. But the pavement needs functional overlays every 5 th year from date of commencement. Hence it

is proposed to provide the thickness for 50msa in the beginning and the balance thickness soon after

the traffic reaches 50msa, which shall be verified during 1st and 2nd overlay.

The proposed pavement composition for the main carriageway for 50 msa is given below:

BC 40mm

DBM 120mm

WMM 250mm

GSB 200mm

However, the concessionaire has to follow the O&M operations given in Schedule L. As per the

requirement he has to carry out the Roughness and Benkelman Beam deflection studies from time to

time to ensure a good riding surface.

g. Design of Service Roads

As per the technical schedules, service roads should be designed for 25msa, which seems to be in

higher side. While reviewing the DPR it was found that the following thickness is provided for service

roads, hence the same is followed.

The following pavement is recommended for Service Roads:

SDBC 25 mm

BM 50 mm

WMM 250 mm

GSB 200 mm

DESIGN OF RIGID PAVEMENT

IRC: 58-2002

This design method is based on limiting the flexural stress of the cement concrete layer. The critical

stress is a combination of stress induced by traffic and temperature. The fatigue effects in the


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concrete, due to repetitive loading of the pavement, are incorporated in the design method by an

adjustment factor, dependent on the appropriate traffic class.

Traffic Parameters

According to IRC: 58-2002, the roads have been classified under various categories depending on the

number of commercial vehicles (>3 Ton) plying on them in a day. The 98% weight of commercialvehicles, as recorded in the axle load survey, will be considered and the design calculations will be

carried out.

Environmental Parameters

h. Temperature Effects

Temperature differential between the top and bottom of the concrete pavement is dependent upon the

slab thickness, wind velocity, and also depends upon the geographical location. Recommendation

regarding the temperature differentials and the geographical locations is given in the IRC: 58-2002.

Spacing of the expansion and contraction joints is also dependent on the changes in temperature.

i. Sub-grade and Sub-base

The strength of the subgrade is expressed in terms of K, which is defined as pressure per unit

deflection of the foundation determined by the plate-bearing test. The subgrade soil strength and

strength of foundation will be effected by the presence of moisture. A correlation between the K value

and the CBR strength has been given in the IRC: 58. From this table one can estimate the

approximate K value, depending on the CBR of soil.

Design Procedure

After choosing the input values for the various parameters and a selected design thickness, the

temperature stress in the critical edge region will be computed. The residual strength of concrete

strength for supporting traffic loads will be calculated. Finally, the edge load stress will be calculated

and compared with the available strength. In case the thickness is not adequate the calculation has tobe repeated with an adjusted slab thickness.

Design

Dry lean concrete (DLC) subbase is suggested for use. For the expected traffic situation a 150mm

thick sub-base layer is deemed appropriate. The DLC should have a characteristic 7 day compressive

strength not less than 10 MPa which corresponds to a flexural strength of approximately, 2 MPa.

Below DLC, GSB layer of 150mm thick will be provided which will not only act as drainage layer but

also serves as a stable working platform on which the construction equipment can be operated.

The combined subgrade and subbase support is taken in terms of Westergaards subgrade modulusK, the value of which has to be obtained from plate bearing tests. The Austroads design procedure

utilizes the corresponding CBR strength derived from the CBR of subgrade and the thickness of the

subbase to estimate the K-value

After estimating the K-value as 48.667 N / mm2 / mm x 10-3, the Axle load spectrum as obtained from

the Axle load survey was taken and the percentage category of commercial vehicles in each load

category was calculated. The projected traffic for the design period of 30 years has been used to

estimate number of repetitions of individual category of load.

The following composition is adopted for rigid pavement in toll plaza.

GSB 200mm

Dry lean concrete (DLC) 150mm


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Pavement quality concrete (PQC) 280mm

Length of Dowel bar 500mm

Diameter of Dowel bar 32mm

Spacing of Dowel bar 300mm

DESIGN OF OVERLAY

Benkelman Beam Deflection Survey

General

Pavement evaluation involves a thorough study of various factors such as sub-grade support,

pavement composition and its thickness, traffic loading and environmental conditions. The primary

objective of pavement condition evolution is to assess as to whether and to what extent the pavement

fulfils the intended requirements so that the maintenance and strengthening jobs could be planned in

time. A well compacted pavement section or one which has been well conditional by traffic deforms

elastically under each wheel load application such that when the load moves away, there is an elastic

recovery or rebound deflection of the deformed pavement surface. This is the basic principle ofdeflection method pavement evaluation or overlay design.

The maximum deflection under a design wheel load depends on several factors such as sub-grade

soil properties, moisture in the sub-grade, pavement thickness and its composition, temperature of the

pavement, loading particulars, etc. Therefore the amount of pavement deflection under a design

wheel load or its rebound deflection on removal of this load is a measure of the structural stability of

the pavement system under the prevailing condition of the test. Larger rebound deflection indicates

weaker pavement structure, which may require earlier strengthening or higher overlay thickness. It is

desirable to carry out the deflection studies soon after the monsoon when the pavement system may

be at the weakest condition due to maximum sub-grade moisture content.

Structural Evaluation of Pavement

Field investigations and tests carried out in various countries have shown that the performance of a

flexible pavement is closely related to be elastic deflection under loads or its rebound deflection.

Measurement of transient deflection of pavement under design wheel loads serves as an index of the

pavement to carry traffic loads under the prevailing conditions. Out of the various equipment used for

the purpose, Benkelman Beam is most commonly used, as the measurements are simple and

reliable. Assessment of flexible pavement overlay thickness requirement by Benkelman Beam method

is explained below. It is one of the non-destructive testing techniques for assessing the load carrying

capacity of the pavement.

Benkelman Beam

Benkelman Beam is a device, which can be conveniently used to measure the rebound deflection of a

pavement due to a dual wheel load assembly or the design wheel load. The equipment consists of a

slender beam of length 3.66m, which is pivoted to a datum frame at a distance 2.44m from the probe

end. The datum frame rests on a pair of firm levelling legs and a rear leg with adjustable height. The

probe end of the beam is inserted between the dual rear wheels of the truck and placed on the

pavement surface at the centre of the loaded area of the dual wheel load assembly. A dial gauge is

fixed on the datum frame with its spindle in contact with the other end of the beam in such a way that

the distance between the probe end and the fulcrum of the beam is twice the distance between the

fulcrum and the dial gauge spindle. Thus the rebound deflection reading measured at the dial gauge

is to be multiplied by two to get the actual movement of the probe end due to the rebound deflection of

the pavement surface when the dual wheel load is moved forward. A loaded truck with rear axle load


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of 8170 kg is used for the deflection study. The design wheel load is a dual wheel load assembly of

gross weight 4085kg with an inflation pressure of 5.6 kg/cm2.

Procedure

The stretch of road length to be evaluated is first surveyed to assess the general condition of the

pavement with respect to the ruts, cracks and undulations. The loading points on the pavement fordeflection measurements are located along the wheel paths on a line 0.9metre from the pavement

edge in the case of pavements of total width more than 3.5 m; the distance from the edge is reduced

to 0.6metre on narrower pavements. Deflection measurements were taken at every 500m staggered

in each lane. After marking the deflection observation points, the study is carried out in the following

steps:

The truck is driven slowly parallel to the edge and stopped such that the left side rear dual

wheel is centrally placed over the first point for deflection measurement.

The probe end or the Benkelman Beam is inserted between the gap of the dual wheel and is

placed exactly over the deflection observation point.

When the dial gauge reading is stationary or when the rate of change of pavement deflection

is less than 0.025 mm per minute, the initial dial gauge reading D0 is noted. Both the readings

of the large and small needles of the dial gauge may be noted; the large needle may also be

set to zero if necessary at this stage. (The least count of the large needled is 0.01 mm and

that of small needle is 2.0 mm).

The truck is moved forward slowly through a distance of 2.7 m from the point and stopped.

The intermediate dial gauge reading D1 is noted when the rate of recovery of the pavement is

less than 0.025 mm per minute.

The truck is then driven forward through a further distance of 9.0 m and the final dial gauge

reading D2 is recorded as before.

The three-deflection dial reading D0 D1 and D2 form a set of readings at one deflection point

under consideration. Similarly the truck is moved forward to the next deflection point, the

probe of the Benkelman Beam inserted and the procedure of noting the set of three-deflection

observations is repeated. The deflection observations are continued at all the desired points.

The temperature of the pavement surface is recorded at intervals of one hour during the

study. Tyre pressure is checked and adjusted if necessary, at intervals of about three hours

during the deflection study. The moisture content in the sub-grade soil is also to be

determined at prescribed locations.

The rebound deflection value D at any point is given by one of the following two conditions:

(a) If D0-D1 < 2.5 divisions of the dial gauge or 0.025 mm, D = 2 (Do D2) divisions of

0.01 mm units = 0.02 (Do- D2) mm

(b) If D0- D1 > 2.5 division, this indicates that correction is needed for the vertical

movement of the front legs. Therefore, D = 2 (D0- D2) +2 K (D1 D2) divisions.

The value of K is to be determined for every make of the Benkelman Beam and is given by the

relation:

f

edK

23 =


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Where

d = distance between the bearing of the beam and the rear adjusting leg

e = distance between the dial gauge and rear adjusting leg

f = distance between the front and rear legs

The value of K of Benkelman Beam generally available in India is found to be 2.91. Therefore, the

deflection value D in case of leg correction is given by:

D = 0.02 (D0- D2) + 0.0582 (D1 D2) mm

Correction for Pavement Temperature and Sub-Grade Moisture Variations

When the pavement consists of relatively thick bituminous layers like the bituminous macadam or

asphaltic concrete in the base/binder/surface course, variations in temperature of pavement surface

course cause variation in pavement deflection under the standard load. The IRC has suggested a

standard pavement temperature of 35 C and a correction factor of 0.0065 mm per C to be applied

for the variation from this standard pavement temperature. The correction will be negative when the

pavement temperature is above 35 C and positive when it is lower. However, it is suggested that

deflection studies should be carried out when the pavement temperature is above 30 C, if this

correction factor is to be applied.

The seasonal variations cause variation in sub-grade moisture. It is always not possible to conduct

deflection studies during monsoon season when sub-grade moisture content is the highest. IRC has

suggested tentative correction factors of 2.0 for clayey soils and 1.2 to 1.3 for sandy sub-grade soils

may be adopted if the deflection observations are made during dry seasons. The deflection under the

worst sub-grade moisture may therefore be estimated by multiplying the summer deflection value by

the appropriate correction factor.

Analysis of Data

The rebound deflection values are determined in millimeters after applying the leg corrections. The

mean value of the deflections and the standard deviations are calculated. The Characteristic

deflection is arrived by using the following equation.

tDDe +=

Here the value of t is to be chosen depending upon the percentage of the deflection values to be

covered in the design. When t = 1.0, De = D + covers about 84 percent of the cases; when t = 2.0,

De = D + 2 covers about 97.7 percent of the cases of deflection values on the pavement section,

assuming normal distribution of rebound deflection values. The necessary corrections for pavement

temperature and sub-grade moisture may be applied to the characteristics deflection values, D e

before designing the overlay thickness.

Design

Benkelman Beam Deflection Survey & Analysis of Data

Benkelman Beam deflection measurements were made in the month of July 05. The testing approach

and measurement was essentially according to IRC: 81-1997, the test points being located 0.9 metre

inside the edge of the road. Testing was carried out both for main line and control sections. In main

line, the deflections reading were taken at 500m interval, and in control sections, measurements were

taken at 10m interval, staggered for a length of 100m per kilometer. Pavement and air temperatures

were recorded for the purpose of temperature correction. Subgrade moisture data was also obtained


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from field moisture measurements in the test pits for applying seasonal corrections to deflection

measurements.

Benkelman beam deflections were carried out at sections where the condition of the pavement is

good to fair. Where the condition of the pavement is distressed, testing was not carried out as the

deflections obtained in the distressed sections will not give consistent deflection measurements. In

stretches where the pavement is severely cracked, such sections were considered for overlay by

scarifying the top distressed layer.

(a) Temperature Correction

Pavement temperatures at the time of BBD measurements were on an average above the standard

temperature (IRC) of 350C. Appropriate temperature corrections were made based on the

recommendations in IRC: 81-1997.

j. Correction for Seasonal Variation

The pavement for most of the length is laid on embankment. From the test pits, soil samples were

collected and the field moisture content was estimated. Based on the field moisture content, type of

soil, and the average intensity of the rainfall, appropriate moisture correction factors were applied assuggested in IRC: 81-1997.

k. Homogeneous Sections

Based on the cumulative differences approach, entire project corridor has been divided into 10

homogeneous sections. For these sections, characteristic deflections have been calculated taking the

mean plus 2 standard deviations. The homogeneous sections and the overlay thickness for each

section are given in the Table.

Table: Homogeneous Sections and Overlay Thickness

Sl. NoChainage Length

(Km)

Overlay Thickness (mm)

From To BC DBM Total

1 407.00 415.00 8.00 50 75 125

2 415.00 417.00 2.00 50 85 135

3 417.00 418.00 1.00 50 90 140

4 418.00 425.00 7.00 50 100 150

5 425.00 431.00 6.00 50 75 125

6 431.00 433.00 2.00 50 105 155

7 433.00 436.00 3.00 50 75 125

8 436.00 438.00 2.00 50 70 120

9 438.00 445.00 7.00 50 75 125

10 445.00 449.00 4.00 50 50 10011 449.00 452.00 3.00 50 75 125

12 452.00 456.00 4.00 50 55 105

BBD analysis is given in Appendix-1

Pavement Design Revised

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