PERFORMANCE EVALUATION OF CEMENT
STABILISED ROAD BASE USING FALLING WEIGHT
DEFLECTOMETER
Gary Chai, Pengurusan Lebuhraya Berhad, Kuala Lumpur
Asmaniza Asnan, Projek Lebuhraya Utara-Selatan Berhad, Kuala Lumpur
ABSTRACT
Falling Weight Deflectometer (FWD) is widely used as a non-destructive device for the evaluation
of flexible pavements. In Malaysia, the FWD has been employed for the study of structural
characteristics and performance evaluation of pavement subgrade during construction. The FWD
usage for compliance testing of in-situ recycling of road base material using cement has also been
explored. This paper presents a case study where the FWD was used to determine the in situ
stiffness of the cement stabilised road base at a trial section along the North-South Expressway in
the Peninsular Malaysia. The FWD deflection measurements were used to check whether the
design assumptions such as the compressive strength and the recommended material stiffness of the
cement stabilised base layer (CTB) have been achieved during stabilisation. The improvement in
the stiffness of the stabilised base layer was monitored during the trial. An engineering relationship
between the compressive strength and the FWD deflection parameter d1 has also been derived. The
FWD was found to be useful for the structural assessment and performance evaluation of the
cement stabilised base layer prior to the placement of asphalt layers. Results from the FWD testing
were also used to check if the assumptions used in the pavement design were met, thus ensuring
that the finished pavement would achieve the required design service life.
INTRODUCTION
The FWD has been widely accepted as a non-destructive device for the evaluation of pavements in
Malaysia for the determination the structural condition of the pavements. The FWD has also been
used during pavement construction for assessing the performance of the pavement foundations
along an expressway in the Peninsular Malaysia (Chai & Faisal, 2000). The research study
demonstrated that when falling weight deflectometer tests are performed on the pavement
foundation several information can be gathered from the FWD deflection basin. The FWD
deflection measurements can be used directly to check whether the design assumptions such as the
level of compaction and the recommended material stiffness have been achieved during subgrade
construction. This approach may also serve to provide a supplementary performance specification
to subgrade construction in highway projects.
The practice of cement bound aggregate is recognized in Malaysia for the construction of road
bases for both concrete and flexible composite pavements. The terminology used to describe the
cement bound aggregate may differ internationally. In the United Kingdom, the term “cementbound material” or CBM is for mixtures from gravel-sand, a washed or processed granular
material, crushed rock, all-in-aggregate, blastfurnance or any combination of these. In Chapter 6 of
the Austroads Guide to the Structural Design of Road Pavements (Austroads, 1992), the material is
described as cemented material. In Malaysia, the common terminology used is cement stabilised
base or CTB.
This paper describes the performance testing works carried out using FWD on a flexible composite
pavement test section on the Southbound Carriageway of the North-South Expressway near Port
Dickson Toll Plaza in Malaysia. The pavement rehabilitation involved in-situ stabilisation of the
existing road base material using cement. The pavement works involved milling of the existing
bituminous layers to a depth of about 175mm, in-situ cement stabilisation of the road base to a
thickness of 200mm and finally overlay with 230mm asphaltic concrete. Performance testing
including FWD testing and laboratory tests were carried out on the cement stabilised base (CTB).
Core sampling was also part of the testing work.
The trial section was 100m in length over a 3.65m (one lane) width. The purpose of the trial was to
assess the suitablility of the construction plant and the construction procedure for the stabilisation.
Monitoring works carried out during the test will allow any initial problems to be identified and
rectified before a full scale stabilisation works. For example, the calibration of the water content
and the depth of the stabilisation can be adjusted accordingly during the trial test and thus enabling
the recycling process to be modified. In addition, the in situ and laboratory testing using recycled
material from the trial area was used to verify the mix design and to demonstrate compliance with
the specification. The use of the FWD during the trial enabled the stiffness modulus of the existing
base layer to be verified before recycling and the trial was intended to demonstrate that the design
parameters such as the in-situ effective stiffness of the CTB layer had been achieved on site. Thus
the expected design life, based on the actual in-situ properties of the pavement, could be
determined.
CEMENT STABILISATION WORKS
The first operation involved in the in-situ recycling was milling the existing 175mm bituminous
materials using a Wirtgen W1000. The in-situ aggregate moisture content was determined by
drying a sample of aggregate in a pan at the verge of the road located next to the paved shoulder.
The balance water content (the difference of the optimum and in-situ aggregate moisture contents)
was then calculated. Cement was then manually spread on the surface of the existing road base
material at a quantity of 2 cement bags (90 kg per bag) for every 1.75 lane-metre. The rate of
spreading of cement was based on the mix design requirements. A summary of the design
parameters adopted in the CTB design is as follows:
• Design water content within 4.5+0.5% of the dry mass of aggregate and cement.
• Design cement content was 3.5% (by mass of the dry aggregate).
• A minimum effective stiffness modulus of 1000MPa to be achieved after 28 days of
curing.
• The average 7-days compressive strength determined from a group of 5 cubes of the CTB
roadbase shall be between 4 and 8MPa.
• The average in-situ wet density shall not be less than 94% of the average wet density of the
corresponding group of 5 cubes.
The in-situ recycling was then performed to a depth of 200mm using a Caterpillar CAT 350
stabilizer machine. The aggregate and cement were mixed in the mixing chamber of the stabilizer
machine, where the balance water was added. The CTB was then leveled using a Mitsubishi motor
grader. The surface level of the CTB was measured with respect to the existing ACWC surface
level of the fast lane and shoulder by pulling a string-line between road pins, which had previously
been set-out and measuring down ward. The measurements allowed the thickness of asphalt to be
determined. Compaction of the recycled base layer was then carried out using a Dynapac vibratory
roller immediately after the CTB was recycled and leveled. After a curing period of 7-days,
bituminous materials to a nominal thickness of 190 mm (130mm DBM + 60mm ACBC) were laid
over the CTB base. ACWC layer of 40mm thick was then laid on the ACBC as a wearing course.
Field Testing Programme
A testing programme was prepared for the purpose of monitoring the quality of workmanship and
performance of the stabilised pavement at various stages during and after the CTB construction.
The testing comprised density measurements using sand replacement method, FWD, coring and
laboratory tests on the CTB cores and cube specimens prepared during construction.
Density measurement were made shortly after the CTB had been compacted using using sand
replacement test. The density results were used to ensure the CTB layer had achieved the minimum
94% relative density during the stabilisation work.
FWD testing was performed at 5m spacing along two FWD runs, which were positioned
approximately 0.7m from both edges of the trial area. The FWD loading used at the various stages
of the FWD testing were computed as those expected from a standard axle loading on a completed
pavement. The contact pressures adopted were 200, 350 and 700kPa for testing performed on the
existing granular road base, CTB and completed asphalt surfaces respectively. For stage 1, FWD
testing on the granular road base surface just before recycling was carried out to determine the
present condition of the granular road base and foundation layer. In Stage 2 and 3, FWD testing on
the CTB after 3-days of curing and after 7-days of curing was also carried out to monitor the
increase in stiffness of the CTB layer. FWD testing was also carried out on the asphalt layer
surface at 28-days after the CTB construction. The testing on the completed pavement layers would
enable the effective stiffness of each of the various pavement layers to be determined and the
assumed design parameters to be verified.
A total of 12 cores were taken from the trial section. 4 cores were taken from the CTB pavement
after a curing period of 3-days, 5 cores at 7-days and 3 cores through the combined thickness of
bituminous and CTB layers at 28-days, at various locations of the test site. The CTB cores were
transported to the laboratory where the recycled thickness was checked. In addition to the thickness
measurement, the cores were tested for in-situ compressive strength.
A total of 12 cube specimens were also prepared from the recycled CTB on the day of construction.
The samples of the recycled CTB were taken before compaction from four locations along the
length of the trial area. Cubes were prepared and tested in the laboratory for compressive strength.
RESULTS AND DISCUSSION
The in-situ cube compressive strength was determined from the prepared cube specimens taken at
3, 7 and 28 days after CTB construction and tested on the subsequent day. The results are
summarized in Table 1, which indicate that the average compressive cube strength of the CTB at 7-
days was 6.00MPa, which was within the specified limits of 4.0 to 8.0MPa.
The results of in-situ compressive strength measured from the prepared core samples are also
summarized in Table 1. The results show that an average of the 7-day compressive strength
obtained from the core samples is 6.0MPa which is equivalent to the in-situ compressive strength
obtained from the cube specimens.
The FWD data obtained from the field test were normalized to a pressure 200, 350 and 700 kPa for
the testing performed on the existing granular road base, CTB and completed asphalt surfaces,
respectively. The seven normalized deflection readings were measured by geophones at distances
(0, 300mm, 600mm, 900mm, 1200mm, 1500mm and 2100mm) from the center of the loading
plate.
FWD test carried out on the existing road base show a center deflection reading of 900 micron at
85 percentile value. For tests performed on the CTB, the deflection were observed to decrease
between 3 and 7 days due to curing of the CTB base. The FWD center deflection value at 85
percentile for the 3-days and 7 days are 650 micron and 400 micron respectively. The profiles of
the center deflection parameter before and after the stabilisation have been plotted against chainage
and are shown in Figure 1.
The FWD data were back-analysed using ELSYM5 (1986) computer program to determined the
effective stiffness after each stage of testing. A three-layer pavement structure was used to model
the CTB base, granular subbase and subgrade layers for the 3 and 7-days strength evaluation. For
the 28-days, a 4-layer model was used for the asphalt, CTB base, granular subbase and subgrade
layers. The effective stiffness modulus at 85 percentile values for the various pavement layers at
different stages of construction are presented in Table 2. The stiffness of the CTB layer increased
from 700 MPa at 3-days to 1350MPa at 28-days after curing. It was also noted that the stiffness of
the CTB is greater than the adopted design stiffness of 1000MPa and thus the CTB stiffness value
had been achieved on site.
0
100
200
300
400
500
600
700
800
900
1000
0 20 40 60 80 100
Chainage (m)
FWD Deflection (micron)
| d1(After Cement Stabilisation) D1(Before Cement Stabilisation) | |
| after | 8 days |
Figure 1 : FWD Center Deflection Profiles on the Asphalt Layer
before and after Cement Stabilization
Table 1: In-Situ Compressive Strength of the CTB Layer
| Age at Test (days) | In-Situ Compressive Strength (MPa) from Core Samples | In-Situ Compressive Strength (MPa) from Cube Specimens |
| 1 | – | 3 |
| 3 | – | 5.5 |
| 4 | 4.5 | – |
| 7 | – | 6.0 |
| 8 | 6.0 | – |
| 29 | 7.5 | – |
Table 2: Effective Stiffness Modulus of the CTB Layer
| Stages of Testing | Effective Stiffness Modulus (MPa) at 85 Percentile Values | |
| Pavement Layers | CTB | Road base |
| Granular Road base Before Recycling | 280 | |
| CTB after 3 days | 700 | – |
| CTB after 7 days | 1150 | – |
| Asphalt Surface after 28 days | 1350 | – |
From the compressive strength and FWD deflection data gathered at the test site, a relationship
between the compressive strength-deflection D1 can be derived. Statistical regression analyses
have been performed to establish the mathematical relationship. The relationship is:
Su = 7.4543Ln(D1) + 51.002 …………………………(1)
| where | Su D1 | = = | Compressive Strength of CTB (MPa), and FWD deflection Sensor No.1 (micron) |
Another useful engineering relationship between the stiffness modulus and compressive strength of
cement stabilised base (CTB) can also be derived in this study. The relationship is:
E = 381 * Su 0.6047 ……………………………………(2)
| where | E Su | = = | Backcalculated Stiffness Modulus (MPa), and Compressive Strength of CTB (MPa) |
The mathematical relationships are depicted in Figure 2 and 3.
Cement Stabilised Granular
| Road base | y = -7.4543Ln(x) + 51.002 R2 = 0.9997 |
8 6 4 2 0
10
10 100 1000
FWD Deflection (micron)
Compressive Strength
(MPa)
Figure 2: Compressive Strength versus FWD Deflection D1
Stiffness Modulus versus In-Situ
Compressive Strength
| y = 381.38×0.6047 R2 = 0.9245 |
0
500
1000
1500
0 2 4 6 8 10
Compressive Strength (MPa)
Stiffness Modulus (MPa)
Figure 3: Stiffness Modulus versus Compressive Strength
CONCLUSIONS
A pavement section, 100m in length over the Slow Lane, on the Southbound Carriageway of the
North-South Expressway near Port Dickson Toll Plaza has been rehabilitated by strengthening the
existing granular road base using cement stabilisation. The performance of the completed pavement
was investigated through in-situ FWD, coring and laboratory testing. An essential detail of the insitu assessment of the cement stabilised base layer using Falling Weight Deflectometer has been
presented. For tests performed on the CTB, the deflection were observed to decrease between 3 and
7 days due to curing of the CTB base. The use of cement stabilized base leads to a significant
improvement in the structural capacity of the pavement. This has been demonstrated by a marked
increase in the stiffness modulus of the CTB layer.
An engineering relationship between the in situ compressive strength and the deflection D1 of the
CTB layer has been derived. The study also shows that there is a unique relationship between the
stiffness modulus and the in-situ compressive strength of the CTB. These two engineering
relationships can be useful for the monitoring the performance of the CTB layer when stabilisation
is in progress.
The significant finding from the trial test is that the use of the FWD would enable design
parameters such as the in situ effective stiffness modulus of the CTB layer be verified during the
construction stage. The FWD test can also be used to demonstrate that the required compressive
strength and stiffness modulus of the CTB had been achieved on site. Thus the expected design
life, based on the actual in-situ properties of the pavement, could be determined in greater
confidence.
ACKNOWLEDGMENT
The authors would like to express their sincere gratitude to PLUS for granting the permission to
publish this technical paper. They are also grateful to Pengurusan Lebuhraya Berhad (PLB), UE
Construction/Scott Wilson Pavement Engineering, Soil Centralab for various technical contribution
to the testing works.
REFERENCES
Austroads (1992), Pavement Design, Austroads Guide to the Structural Design of Road Pavements,
Austroads, Sydney. pp 6.1 – 6.17.
British Standards Institution (1990), “Methods of test for Cement-Stabilised and lime-stabilised
materials,” Stabilised Materials for Civil Engineering Purposes, Section 2.1.6 and 4.2, BS
1924:Part 2:1990.
Chai, G. W. K., and Faisal. H. A., (2002), “In Situ Assessment of Pavement Foundation using
Falling Weight Deflectometer”, Special Proceeding of Invited Papers of the 2nd World Engineering
Congress, Kuching, Sarawak, Malaysia, 22-25 July 2002.
Chai, G. W. K., and Faisal. H. A., (2000), “Determination of Stiffness Modulus and Density of
Pavement Foundation using Falling Weight Deflectometer during Pavement Construction”, 10th
REAAA Conference, Tokyo, Japan, 4-9 September, 2000.
Croney, D., and P. Croney., (1991), The Design and Performance of Pavements, Transport and
Road Research Laboratory, Her Majestic’s Stationery Office, 2nd Edition, London, 1991.
ELSYM5, (1986), Computer Program for Determining Stresses and Strains in a Multiple-layer
Asphalt Pavement Sysyem, by Gale Ahlborn, Institute of Transportation and Traffic Engineering,
University of California at Berkeley, California, U.S.A.
PLUS, (2001), Specification for Highway Works, North-South Interurban Toll Expressway and
New Klang Valley Expressway, Projek. Lebuhraya Utara-Selatan Berhad, Kuala Lumpur,
Malaysia.
UEM/SWPE (2001), Report on Field Trial of In-Situ Recycled Cement Bound Roadbase, NorthSouth Expressway, Kuala Lumpur, Malaysia, November 2001
AUTHOR’S BIOGRAPHY
Dr Gary Chai received his education from Malaysia, the United Kingdom and the United States of
America. Dr Chai holds a Bachelor and Master of Science degree in Civil Engineering and a Ph.D
degree in Pavement Engineering. His areas of specialisation and research interest include pavement
evaluation and performance study using Falling Weight Deflectometer, pavement design using
mechanistic approach and soil mechanics of pavement. His area of interest also include network
planning and management of pavement using HDM-4 program.
Dr Gary Chai is presently a Senior Pavement Manager at the Pengurusan Lebuhraya Berhad (PLB)
in Kuala Lumpur, Malaysia. Prior to that, he has served the Kansas Department of Transportation
in the USA as a Pavement Design Engineer.
He is now advising PLUS Berhad on the various rehabilitation designs in pavement strengthening
works along the North-South Expressway in Malaysia.