Properties of Asphalt used in road pavement structures in Sri Lanka is analysed under different traffic and climatic conditions.

1. Dense Graded Asphalt for Sri Lanka:
Considerations Related to Climate
and Different Traffic Conditions
Presented by
W. W. Bandara and R.P.M.M. Premathilaka

2. Outline
 Introduction
 Methodology
 Conclusion
 Recommendation
 References

3. Introduction
 High standard crushed rock aggregates,
and 60/70 penetration grade bitumen
which have a high stiffness at 25°C
(testing temperature) are used to produce
dense graded asphalts (DGAs) in Sri
Lanka.
 Hence it is considered in pavement
structure design that asphalts have a high
stiffness (Modulus) approximately 2500
MPa.
 Alligator (or map) cracking in wheel path
and formation of waves at intersections
are observed on asphalt surface of
moderate-to-heavily trafficked roads after
several years from construction
Those load related defects occur due to
insufficient resistance to fatigue and
permanent deformation effected by the asphalt
modulus.

4. Modulus of Asphalt Our study is conducted to review;
 a selection of the bitumen class for Sri
Lankan climate (pavement temperature)
and different traffic conditions, and
 mixing and compaction temperatures of
Asphalt.
Finally, design modulus of Asphalt is estimated
for pavement structure designs.
“Austroads Guide to Pavement Technology”
(Austroads) has been principally used for the
study.
Stiffness
Bitumen
Mixing and
Compaction
Quality
and
Grading
Aggregate
s
Bitumen
Class
Pavement
Temperature
Loading
Time (Heavy
Vehicle
Speed)

5. Methodology
 Defined Pavement Temperatures in Sri Lanka according “Austroads”
 Selected the bitumen class from “Austroads” for Pavement Temperature and
different traffic conditions in Sri Lanka.
 Estimated stiffness and mixing & compaction temperatures of the selected
bitumen and bitumen used in Sri Lanka
 Compared those two bitumen class.
 Estimated the stiffness modulus of Asphalt used in Sri Lanka for deferent
pavement temperature and traffic conditions.
 Calculated stiffness moduli of a good condition asphalt from field surface
deflections measured by a falling weight deflector meter (FWD).
 Compared estimated stiffness modulus and calculated stiffness modulus.

6. Estimation of Weighted Mean
Annual Pavement Temperature
(WMAPT)
Pavement temperature is measured in terms of
Weighted Mean Annual Pavement Temperature
(WMAPT).
The WMAPT is estimated from the monthly average
air temperatures by averaging the maximum and
minimum air temperatures (Tair) in an area.
1. Temperature weighting factor (WF) is calculated
for each month by substituting Tair in Equation
1,
2. The weighted mean annual air temperature
(WMAAT) are estimated by substituting WFs in
Equation 2 and averaging.
3. The WMAPT is estimated by substituting the
WMAAT in Equation 3.
Historical average monthly temperature data of all
districts in Sri Lanka is obtained from the URL
https://www.msn.com/en-gb/weather/today/.
Colombo Ave High Ave Low Rec High Year Rec Low Year Ave WF WMAAT WMAPT
January 29 24 34 2016 20 2011 26.5 2.505
February 30 24 34 2014 20 1997 27 2.676
March 32 25 36 1997 20 1992 28.5 3.258
April 32 26 36 2001 23 2012 29 3.478
May 31 27 36 1987 23 2013 29 3.478
June 30 26 33 2002 20 1991 28 3.052
July 29 26 32 2002 21 1990 27.5 2.858
August 29 26 36 2010 21 1992 27.5 2.858
September 30 26 32 1988 23 1992 28 3.052
October 30 26 33 2014 21 1992 28 3.052
November 29 25 34 2008 20 2002 27 2.676
December 28 24 33 2015 19 2011 26 2.345
Average 30 25 2.941 27.6 40.2
According to Metrological Department of Sri Lanka, Average
High and Low Temperatures in Colombo are 30.0°C-32.5°C
and 22.5°C-25°C.
Estimated WMAPT in Colombo is 40 °C

7. Weighted Mean Annual Pavement
Temperature (WMAPT) in Sri
Lanka WMAPT values calculated from the URL for Melbourne City in
Australia is compared with the actual figure given in Austroads
for Melbourne City.
WMAPTs in Sri Lanka is between 35°C and 41°C.
Melbourne Ave High Ave Low Ave WF WMAAT WMAPT
January 27 15 21 1.199
February 27 15 21 1.199
March 24 14 19 0.912
April 21 11 16 0.603
May 17 9 13 0.396
June 14 7 10.5 0.278
July 14 7 10.5 0.278
August 15 7 11 0.298
September 17 8 12.5 0.369
October 20 10 15 0.524
November 22 12 17 0.693
December 25 13 19 0.912
0.638 16.4 24.6
Estimated WMAPT for Melbourne City is 24.6°C
WMAPT given in Austroads for Melbourne City is 24°C
Historical Tempetature Data Validation

8. Bitumen Product Standards
• Sri Lankan Standard is based on
Penetration/Softening Point
• Australian Standard (Austroads) is based
on Viscosity
Need to compare properties of Penetration
Grade Bitumen used in Sri Lanka and
Australian Viscosity Grade Bitumen
selected for Sri Lanka.
Bitumen Product Standards
Used Across the world

9. Australia
 Bitumen is classified by its consistency expressed as viscosity
at 60°C and 135°C and comprise three major grades: Class
170, Class 320 and Class 600.
 Class 170 bitumen is the softest grade and provides the
greatest flexibility and durability for use in asphalt in cool
climates, and for light traffic.
 Class 320 is a slightly harder grade of bitumen that is the most
commonly used binder for hot mix asphalt in a wide range of
applications.
 Compared to Class 170 bitumen, Class 320 provides greater
resistance to rutting and deformation at high service
temperatures or heavy traffic.
 Class 600 is a harder grade of bitumen than Class 320. The
use of Class 600 is generally limited to heavy-duty asphalt
base layers where greater asphalt mix stiffness is required.
 Class 600 is not generally recommended for use as a wearing
course layer as increased stiffness can result in reduced
resistance to cracking and fatigue in thin layers.
Sri Lanka
 DGAs are manufactured from 20mm nominal
maximum size aggregates of high standard
crushed rocks and 60/70 penetration grade
bitumen.
 The bitumen and aggregate are mixed at
temperatures within absolute limits of 145 °C
and 170 °C.
 The final rolling are completed before the
temperature of mix falls below 90 °C.
 Air void in compacted aggregate are from 3% to
7%. However, it is generally less than 5% in the
mix design.

10. According to Austroads
The selection of bitumen for hot mix asphalt
applications is based on traffic levels and expected
maximum temperatures.
Mampearachchi in 2012 revealed that 7-day
average maximum pavement temperature in
different geographical areas of Sri Lanka is between
52°C and 58°C.
There are many practical difficulties to use two
grade of bitumens in deferent layers of pavement or
different regions in Sri Lanka.
Since maximum pavement temperature is between
52°C and 58°C, a suitable viscosity grade bitumen
class for Sri Lanka climate and traffic up to 20
million ESAs (heavy traffic) is C320.
For very heavy traffic, polymer modified bitumen
(PMBs) shall be used.
Select the Bitumen Class for Sri Lanka

11. 60/70 Penetration Grade Bitumen
used in Sri Lanka
C320 Australian Viscosity Grade Bitumen
selected for Sri Lanka

12. Estimation of Mixing &
Compaction Temperatures
The bitumen test data chart (BTDC) (developed by
Heukelom in 1969), can be used for comparing the
temperature, penetration and viscosity
characteristics of different type of bitumen.
The optimum bitumen viscosity for compaction is
between 2Pa.s and 20Pa.s.
When the aggregate and the bitumen are mixed
together, the viscosity should be approximately
0.2Pa.s for satisfactory coating and not be below
0.5Pa.s.
BTDC is a useful tool for estimating the correct
operating temperatures to achieve the appropriate
viscosity of any grade of bitumen.
CompactionLimit
MixingLimit

13. Estimation of Stiffness Modulus
of a Bitumen
The monograph is used to estimate the stiffness
modulus of a bitumen from:
 Time of loading (heavy vehicle speed),
 Pavement temperature,
 Temperature (°C) at which the penetration of the
bitumen is 800 (0.1mm) (T800Pen) and
 Penetration Index (PI).
PI =
20 − 500A
1 + 50A
where A is the change in log (penetration) per °C
change in temperature.
Time of loading = 1/ Heavy vehicle traffic speed
WMAPTs of different geographic locations in Sri Lanka are
calculated to be between 35°C and 41°C.

14. 60/70 Penetration Grade Bitumen C320 Viscosity Grade Bitumen
A stiffness range of both class of bitumen at 40°C is almost equal (0.4 to 0.7 MPa).
Hence it can be concluded that modulus of asphalt made with both classes of bitumen is equal.

15. 60/70 Penetration Grade Bitumen
There are no large variations in upper and lower temperatures
of mixing and compaction.
Hence temperature limits can be specified for C320 bitumen.
C320 Viscosity Grade Bitumen
Variations of mixing and compaction temperatures are
huge, and some limits are out of specification limits.
Then there is a risk to produce poor DGA mixes due to the
poor coating of bitumen with aggregates, compaction and
consistency in the mix and it leads the insufficient
resistance to fatigue and permanent deformation.

16. Conclusions
 To mitigate the mixing and compaction issues, viscosity grade bitumen class
smiler to 60/70 bitumen class should be used in Sri Lanka for quality service of
road pavements.
 A stiffness range of both class of bitumen at 40°C is almost equal. Hence it can be
concluded that modulus of asphalt made with both classes of bitumen is equal.
 Then modulus of Sri Lankan DGA, made with 60/70 bitumens and 20mm nominal
maximum size aggregates, are considered as the modulus of DGA given for C320
bitumen and 20mm nominal maximum size aggregates.

17. Estimation of Modulus of DGA
Modulus of Sri Lankan DGA, made with 60/70
bitumens and 20mm nominal maximum size
aggregates, are considered as the modulus of DGA
given for C320 bitumen and 20mm nominal
maximum size aggregates.
Modulus is corrected for the in-service air voids,
pavement temperature (WMAPT) and loading rate.
Modulus of Australian DGAs, made with 20mm nominal maximum size
aggregate, determined on laboratory manufactured samples using the
indirect tensile test procedure and standard test conditions and 5% air
voids.
Standard test conditions are 40ms rise time and 25 °C test temperature.
Modulus of DGAs can be taken from 1000MPa to
2200MPa for WMAPTs of Sri Lanka and 30km/h
to 70km/h heavy vehicle speed.
Laboratory Moduli of DGAs is taken as 5500Mpa

18. Validation Modulus of DGA
 The Falling Weight Deflectometer (FWD) is an
excellent device for evaluating moduli of
pavement layers and subgrade.
 FWD data at 50 m intervals and roughness and
rutting data at 100m intervals of the section
from 58+000 to 62+400 of Katugastota -
Kurunegala - Puttalam (A10) road (Variyapola to
Padeniya) are obtained from RDA.
 10th percentile values of the PSI calculated
from IRI values are 3.13 and 3.18 for LHS and
RHS respectively, and 90th percentile value of
rut depth is 5.07 mm.
 Then calculated IRI and rutting values are well
matched to a good condition road (i.e. 4.0 IRI
and 10 mm maximum rutting for a newly
constructed road). Hence, it is confirmed that
the road pavement is in good condition.
According to asphalt coring result that is carried out at 250 m,
thicknesses of asphalt at coring location are more than 200 mm.

19. Modulus values of upper 200 mm asphalt
layer at test locations are back calculated
by BAKFAA program using FWD data.
10th percentile value of Modulus is 880
MPa.
Measured temperatures of the pavement
surface along this road section are varied
from 31°C and 37°C.
Distance Load D0 D1 D2 D3 D4 D5 D6 E Mr
m kN µm µm µm µm µm µm µm MPa MPa
58000 41.1 167 97 64 36 18 6 2 1,245 644
58050 42.8 221 144 115 81 64 43 30 1,881 328
58102 41.1 446 349 281 210 152 84 51 1,219 126
58150 42.4 434 322 266 195 149 87 53 1,266 137
58200 41.0 390 305 248 186 135 80 52 1,449 141
58250 42.7 242 145 106 68 48 29 18 1,153 381
58300 41.0 435 337 272 207 154 94 62 1,327 126
58350 42.7 306 186 122 69 46 26 15 781 335
58400 41.2 401 312 261 205 156 99 66 1,699 127
58450 42.6 241 142 104 66 47 31 21 1,137 387
58500 41.3 409 308 242 175 124 71 44 1,152 150
58550 42.5 270 160 116 70 47 25 17 955 358
58601 41.0 516 391 307 220 153 87 54 895 119
58650 42.0 282 185 139 90 65 42 31 1,186 280
58700 41.0 407 326 270 206 152 93 61 1,571 126
58750 42.1 360 241 177 106 67 31 18 827 234
58800 41.4 382 301 243 186 138 82 51 1,558 142
Mr-Resilient
Modulus
E- ModulusD0 D1 D4

20. Recommendations
 The thickness of the asphalt for decided ESAs depends on the modulus of the asphalt. Since Sri Lankan
DGAs varies from 1000MPa to 2400MPa and precise modulus shall be selected according to pavement
temperature (WMAPT) and heavy vehicle traffic speed. Then only road pavements in Sri Lanka provide their
service without a premature failure.
 Although 60/70 bitumen can be used to produce GDAs up to 20 million ESA traffic loads, excessive asphalt
thickness has to be used for high traffic loads with low heavy vehicle speed or high WMAPT.
 High-grade bitumen cannot be recommended for use as a wearing course layer as increased stiffness can
result in reduced resistance to cracking and fatigue in thin layers. Then a polymer modified bitumens or
composite pavement structure can be used to reduce asphalt thickness. For this reason, typical pavement
structures used in Sri Lanka for high traffic have to be reviewed.
 Low asphalt modulus at low heavy vehicle speed and high pavement temperature in Sri Lanka together with
a poor coating of bitumen, compaction and consistency in the mix, causes insufficient resistance to fatigue
and permanent deformation in loaded road and leads to alligator cracking and formation of waves and
corrugation.
 In this condition, defects can occur on loaded areas even after several years from construction. Hence, it is
very essential to control viscosities of bitumen at mixing and compaction temperatures.
 Since viscosity grade bitumen controls viscosities at 60°C and 135°C (near mixing), viscosity grade
bitumen shall be introduced to Sri Lanka for the better performance of roads.

21. References
Robert N. Hunter, Andy Self, & John Read, The Shell Bitumen Handbook, 6th ed., ICE Publishing, One Great
George Street, Westminster, London, 2015,pp. 87-145, 503-542, 750-752.
Austroads, Guide to Pavement Technology Part 2: Pavement Structural Design, Sydney, 2012, pp. 64-83.
Austroads, Guide to Pavement Technology Part 4F: Bituminous Binders, Sydney, 2008, pp. 10, 23-25.
ICTAD, Standard Specification for Construction and Maintenance of Road and Bridges, 2th ed., Colombo,
2009, pp. 143-151, 459-463.
Mampearachchi, W. K., " Review of asphalt binder grading systems for hot mix asphalt pavements in Sri
Lanka”, Journal of the National Science Foundation of Sri Lanka 40 (4), 2012, pp. 311-320.
https://www.msn.com/en-gb/weather/today/, Visited, January 2017.
AASHTO, Pavement Guide for Design of Pavement Structure, Washington, 1993, ii-10 p.
Transport & Road Research Laboratory, Overseas Road Note 18: A guide to the pavement evaluation and
maintenance of bitumen-surfaced roads in tropical and sub-tropical countries, Berkshire, 1999, 34 p
http://www.airporttech.tc.faa.gov/Downoad/Airport-Pavement-Papers-Publications-
Detail/dt/Detail/ItemID/34/BAKFAA-version-20-update-04012013, Visited, February 2017.

22. Acknowledgement
Prof. W.K. Mampearachchi, Department of Civil Engineering, University of Moratuwa
and other Engineers are highly acknowledged for sharing knowledge.
The Authors express their gratitude to Asian Development Bank (ADB) Division of Road
Development Authority (RDA) and Engineering Design and Project Management
Consultants (ED&PMC) (Pvt) Ltd for providing relevant data for the study.