1. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
385
BENEFIT ANALYSIS OF SUBGRADE AND SURFACE
IMPROVEMENTS IN FLEXIBLE PAVEMENTS
Dr. K.V.Krishna Reddy
Professor & Principal, Chilkur Balaji Institute of Technology, Hyderabad-75, AP, India
ABSTRACT
In the present study, an attempt is made to highlight the benefit of improvements
made in the subgrade and surface layers individually and in combination when compared to
the conventional flexible pavement. FPAVE program has been used to evaluate the design
thickness of various combination of improvements made at the subgrade and surface layers.
The results obtained represent that the strain levels reflected on the top surface and subgrade
are low in case of pavement with improvements made at both the subgrade and surface layers
with same pavement thickness and the design thickness required decrease resulting in a cost
benefit of 50%
Key Words: FPAVE, Subgrade stabilization, Surface course improvements, Cost benefit
analysis, Flexible pavements
1. INTRODUCTION
Flexible pavements have the advantage of easy and simple construction, adaptability
for stage construction and are best suitable for sustainable development. However, they have
the disadvantages of having shorter life, and require constant maintenance. Among the
various types of distress in flexible pavements, block / edge / longitudinal / transverse
cracking, pothole formation, water bleeding, pumping and swelling are load, moisture and
drainage dependent. Fatigue / alligator / edge cracking, shoving, pothole formation,
corrugations are load induced apart from the climatic effect. Rutting, bleeding, polished
aggregates, lane to shoulder drop off, corrugation and depression depend essentially on the
materials, load and moisture.
Most of the pavement failures initiate in the subgrade and some are attributed to the
deficiencies in the surface course. The other layers of the conventional flexible pavements
being aggregate and granular material in the base and subbase course, both of which have
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2. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
386
good material properties and sufficient strength to transfer the loads coming from the top
layers and need no replacement or additives except for strict quality control during
construction, thus leaving the subgrade and surface course for modification to obtain the
desired performance.
This study involves modification of the surface course material with waste tire rubber
and clayey subgrade with pond ash and lime. Evaluate pavement thickness required using
FPAVE program for various combinations of the improvements to understand the benefit of
improvements.
2. RESEARCH METHODOLOGY
2.1 Laboratory analysis
Laboratory experimentation is done to determine the optimum additives content for
stabilizing the clayey subgrade with pond ash and lime and optimum crumb rubber content to
modify the surface course.
2.2 Pavement Analysis and Design
Analysis and design of pavements for full-scale loading has been considered to
determine the pavement thickness. FPAVE, a software program for analysis and design of
flexible pavements is used to evaluate the same. FPAVE program analyses for the stresses
and strain values at various depths of the pavement section based on the input elastic modulus
and Poisson ratio values. For the purpose of design, the program has the input option for the
conventional (80/100-b8, 60/70-b6 and 30/40-b3) penetration grade bituminous mixes. In the
present study the input elastic modulus values have been calculated based on the guidelines
given in IRC37 –2001 and b8 is considered for evaluation. The thickness of the pavements
has been evaluated for 10 million standard axel repetitions. The average pavement
temperature has been considered to be 400
C.
3. DATA ANALYSIS
3.1 Subgrade stabilization
Initially it was determined to check for the improvement in the properties of the clay
soil by addition of pond ash alone. Pond ash was added at the rate of 15, 20, 25 30 and 35%
by weight of soil. IS heavy compaction test has been conducted on three samples of black
cotton soil and that of each modified mix to determine the optimum moisture content (OMC)
and maximum dry density (MDD). CBR test was conducted on three samples of each mix at
OMC, after curing for 7 days by covering with wet sand followed by 96 hours of soaking
under a surcharge weight of 7.5 kg. The CBR test was carried out up to penetration value of
7.5mm, and corrections for initial concavity are made based on the same. CBR of the cured
and soaked samples increased with addition of pond ash content up to 25% to a CBR of 10
and then decreased on further addition. This may be due to the replacement of clay fines by
silty natured pond ash particles. It also indicated that the calcium oxide present in the pond
ash was not sufficient to initiate any reaction in the pond ash soil mix.
The Atterberg limits and swelling characteristics were determined for the 25% pond
ash soil mix and the results depicted that the volume stability needs to be attended. It was
proposed to use lime to cater for the volume stability and additional strength needs. Hydrated
Lime at 3, 4 and 5% was added to the 25% pond ash soil mix. Lime was added in terms of

3. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
387
percentage by weight of clay. The optimum lime content for volume stability was determined
to be 5%. The optimal mix from strength and stability considerations was determined to
constitute 25% of the pond ash and 5% hydrated lime by weight of clay.
Table 1 indicates the Atterberg limits, free swell index, swell potential, CBR and 7-day
unconfined compressive strength (UCC) of the optimal mix. The optimal mix resulted in a
CBR of 16.34 and a 7day unconfined compressive strength of 310kN/m2
.
Table 1 Mix, Atterberg limits, swelling and strength characteristics of pond ash / lime
stabilized Clay soil
S.
No
Property
Type of Mix
Clay
Clay +
25%PA
Clay +
25%PA +
5%lime
1
Atterberg limits
Liquid Limit (%)
Plastic Limit (%)
Plasticity Index
Shrinkage Limit (%)
79.30
31.46
47.84
12.20
60.83
29.28
31.55
19.23
56.50
44.30
12.20
39.80
2 Free swell index (%) 110 52.50 30
3 Swell potential (%) 27.07 11.13 1.20
4 Soaked CBR (%) 2.65 10 16.60
5 UCC kN/m2
61 - 310 (7D)
3.2. Bitumen modification
The conventional bitumen is modified using locally available waste tire rubber
powder (>0.6mm &<1.18mm). 80/100-penetration grade bitumen is considered for
experimentation. Wet process of mixing waste tire rubber was adopted, where in the waste
tire is added to the conventional bituminous binder before incorporating the same into the
final mix. Waste tire rubber is added after heating the bitumen to a temperature of 1630
C. It
was added in various percentages varying from 8 to 14. Laboratory tests, namely,
penetration, softening point, ductility and loss on heating test are conducted to determine the
properties of the waste tire rubber modified bitumen before and after loss on heating test.
Aggregates with grade II specifications as per MORTH code have been collected
from local quarry. Marshall mix design methodology has been considered for mix design of
the bituminous concrete. Marshall stability value has been considered as the value for judging
the optimum bitumen / additive content for the mix and the other properties like air voids and
flow value are checked for to be within limits. It was found that an optimum bitumen and
waste tire rubber content of 4.3 and 11.8% respectively for conventional and modified
bituminous concrete yielded the best results. The properties of the conventional and modified
bituminous concrete are tabulated in Table 2

4. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
388
Table 2 Properties of conventional and waste tire modified bituminous concrete mix
S.No Mix / Property Conventional
Waste tire
modified mix
1 Optimum Bitumen / Waste tire
content
4.3%
11.8%
2 MSV (Kg) 1300 2405
3 Air voids (%) 3.875 3.425
4 Flow value (mm) 2.375 3.750
5 Bulk density (g/cc) 2.520 2.470
3.2 Pavement Analysis and Design
Different trails have been considered, varying the thickness of the bituminous layer
(h1) and the other component layers (h2) through trials 1 to 4 for each combination of
improvements. The combinations being conventional subgrade with conventional surface,
stabilized subgrade with conventional surface, conventional subgrade with modified surface
and stabilized subgrade with modified surface, the layer thickness of the pavements is arrived
at for the various combinations. The tensile strain under the bituminous layer and the
compressive strain on the top of the subgrade are arrived for the optimum design layer
thickness. The input parameters along with the corresponding output are tabulated in Table 3
to 6
Table 3 Summary of input and output parameters for conventional pavement structure
Input
Output
(mm)
Design thickness (mm)
Strain values based
on output
Trailno
CBR(%)
E3(Mpa)
E1(Mpa)
H1(mm)
H2(mm)
E2(Mpa)
h1
h2
BC
DBM
LBM
Gr.Base
Gr.Subbase
epZ epT
1 2.65 26.5 797 50 650 357 285 650 40 110 160 250 400 5.52E-04 4.27E-04
2 2.65 26.5 797 200 750 391 280 750 40 100 160 250 450 4.78E-04 4.21E-04
3 2.65 26.5 797 230 800 409 275 800 40 100 155 250 550 4.48E-04 4.22E-04
4 2.65 26.5 797 175 900 441 265 900 40 100 145 250 650 3.95E-04 4.28E-04
h1: Thickness of bituminous layer h2: Thickness of granular base and subbase
E1: E of Bituminous material E2: E of granular base and subbase
E3: Elastic modulus(E) of Subgrade BC: Bituminous concrete
DBM: Dense Bituminous Macadam LBM: Lean Bituminous Macadam
Gr. Granular epZ: Vertical strain on the top of the subgrade
epT: Tangential strain at the bottom of the bituminous layer

5. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
389
Table 4 Summary of input and output parameters for pavement with Pond ash and
Lime modified subgrade with Conventional surface
Input
Output
(mm)
Design thickness
(mm)
Strain values based
on output design
thickness
Sno
CBR(%)
E3(Mpa)
E1(Mpa)
H1(mm)
H2(mm)
E2(Mpa)
h1
h2
BC
DBM
Gr.Base
Gr.Subbase
epZ epT
1 16 104 797 50 300 271 195 300 50 165 250 150 5.94E-03 3.81E-04
2 16 104 797 100 400 308 110 400 50 80 250 150 5.96E-04 4.24E-04
3 16 104 797 60 450 325 65 450 40 40 250 200 5.94E-04 3.25E-04
4 16 104 797 50 450 325 65 450 40 40 250 200 5.94E-04 3.25E-04
Table5 Summary of input and output parameters for pavement with conventional
subgrade and modified surface
Input
Output
(mm)
Design thickness
(mm)
Strain values based
on output design
thickness
Sno
CBR(%)
E3(Mpa)
E1(Mpa)
H1(mm)
H2(mm)
E2(Mpa)
h1
h2
BC
DBM
Gr.Base
Gr.Subbase
epZ epT
1 2.65 26.5 1200 50 850 110 247 850 40 100 400 550 4.44E-04 3.72E-04
2 2.56 26.5 1200 80 750 104 254 750 40 100 350 550 5.03E-04 3.71E-04
3 2.65 26.5 1200 40 900 113 30 900 40 100 250 550 7.52E-04 4.43E-04
4 2.65 26.5 1200 60 800 107 250 800 40 100 360 550 4.73E-04 3.72E-04

6. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
390
Table6 Summary of input and output parameters for pavement with stabilized
subgrade and modified surface
Input
Output
(mm)
Design thickness
(mm)
Strain values based
on output design
thickness
Sno
CBR(%)
E3(Mpa)
E1(Mpa)
H1(mm)
H2(mm)
E2(Mpa)
h1
h2
BC
DBM
Gr.Base
Gr.Subbase
epZ epT
1 16 104 1200 40 360 294 41 360 40 0 250 200 8.77E-04 1.98E-04
2 16 104 1200 50 350 290 50 350 60 0 250 200 8.73E-04 2.89E-04
3 16 104 1200 60 340 286 61 340 60 0 250 200 8.69E-04 3.52E-04
4 16 104 1200 65 340 286 60 340 40 20 250 200 8.50E-04 3.72E-04
4. RESULTS
4.1 Strain analysis
The vertical strain on the top of the subgrade is evaluated for a constant thickness of
the pavement irrespective of the improvements made at subgrade and surface course. The
pavement thickness provided for the conventional test track section is considered for all
combinations to evaluate at the benefit in life based on the vertical strain at the top of the
subgrade and shear strain at the bottom of the bituminous layer. Table 7 depicts the strain
values at the top of the subgrade and the number of standard load repetitions the pavement
can take. It is found that the strain is least in case of test track section improved at both
subgrade and surface level compared to the other sections resulting in high life benefit.
Table 7 Summary of Strains and load repetitions for a standard thickness despite
improvements made
Secti
on
Type of pavement
Strain on top of
subgrade (epZ)
Tangential
Strain at the
bottom of the
bituminous
surface (epT)
No. of Load
repetitions
based on
fatigue
criteria
No. of Load
repetitions
based on
rutting
criteria
1 Conventional 5.48E-04 5.80E-04 3E6 25E6
2 Subgrade stabilised 1.40E-04 1.19E-04 144E6 12359E6
3 Surface modified 5.39E-04 6.10E-04 1.7E6 27E6
4
Subgrade and surface
improved
0.80E-04 1.29E-04
745E6 156260E6

7. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
391
4.2 Cost Analysis
The cost of the pavement for two lanes has been worked out based on the rates
obtained from Roads and Buildings Department. The layer thickness of the pavements for
various combinations considered along with the cost is tabulated in Table 8. The results
indicate that, for the same design life of the pavements, there is substantial reduction in the
pavement thickness requirement and the cost of pavement construction due to modifications.
Table 8 Pavement layer thickness and Cost of two lane Pavement for Km length
S.
No
Pavement type
BC /
CRM
DB
M
LBM
Granular
Base
Gr.
Subbase
Cost
(Lakhs)
1 Conventional 40 100 155 250 550 117.0
2 Surface modified 40 100 135 250 500 162.0
3 Subgrade Stabilised 65 40 0 250 200 65.0
4
Subgrade and Surface
Improved
40 20 0 250 200 58.0
5. ACKNOWLEDGEMENT
At the outset the author would thank the Head, CED and TE division, and other
professors at NIT Warangal for their valuable guidance and encouragement during
experimentation.
6. CONCLUSION
1) Clayey soils in particular can be effectively stabilized adding 25% Pond ash + 5%
lime by weight of soil to have a CBR of 15.
2) The strain levels of the subgrade stabilized soils indicate that the number of load
repetition the pavement can take with the same thickness as that of conventional
pavements is 48 times.
3) Analysis and design using FPAVE indicated that stabilization of subgrade alone
results in a pavement structure that can be formed with a cost of 55% of that required
for construction of conventional pavement
4) Use of bitumen modified with 12% of tire rubber in formation of bituminous concrete
alone without stabilizing subgrade has not resulted in much improvement both in
terms of cost and number of load repetitions when compared to conventional
pavement structure.
5) Life benefit in pavements is pronounced when the poor subgrades are improved along
with the surface course. Life benefit by modifying both subgrade and surface courses
is 200 times in terms of the number of load repetitions for the same pavement
thickness as that of conventional pavement.

8. International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
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6) Improving both subgrade and surface course results in a pavement structure that can
be formed with a mere 50% amount of that required to form conventional pavement
section.
7) The key to effective performance of flexible pavements is to understand the causes of
failures and the action needed for correction. Sound judgment should be used to
determine which part of the existing pavement structure is weak and if the subgrade is
affected, total reconstruction of the pavement or full depth reclamation should be
considered for best performance rather than improving the surface alone.
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International Journal of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2,
2012, pp. 347 - 352, ISSN Print: 0976 – 6308, ISSN Online: 0976 – 6316.