TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport RESEARCH REPORT 58 SUBSTITUTION OF BITUMINOUS ROADBASE FOR GRANULAR SUB-BASE By M E NUNN and D LEECH (A paper presented to the 3. Eurobitume Symposium, The Hague, 1985) The views expressed in this report are not necessarily those of the Department of Transport Pavement Design and Maintenance Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1986 ISSN 0266-5247
Transcript
TRANSPORT A N D ROAD RESEARCH LABORATORY Department of Transport
RESEARCH REPORT 58
SUBSTITUTION OF BITUMINOUS ROADBASE FOR
GRANULAR SUB-BASE
By M E NUNN and D LEECH
(A paper presented to the 3. Eurobitume Symposium, The Hague, 1985)
The views expressed in this report are not necessarily those of the Department of Transport
Pavement Design and Maintenance Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1986
ISSN 0266-5247
Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I 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. Laying bituminous materials on a weak sub-grade
3. A test to assess the performance of full- depth asphalt
except for commercial purposes, provided the source is acknowledged.
SUBSTITUTION OF BITUMINOUS ROADBASE FOR GRANULAR SUB-BASE
M E Nunn and D Leech
ABSTRACT
Trials were carried out to investigate the strength of subgrade needed to allow rolled asphalt and dense coated macadam roadbases to be laid satisfactorily without a sub-base. Having established that a roadbase could be laid on a relatively weak foundation test pavements were constructed to enable the performance of conventional and full-depth pavements to be compared through the measurement of stress, strain and pavement deflection under load.
Stress, strain and deflection calculated from linear elastic theory were not in good agreement with the measured values but theory indicated the same sensitivity of stress and strain to changes of asphalt thickness and dynamic modulus. It was therefore concluded that theory could be used to produce tentative designs by comparison with conventional designs. However, extensive road trials would be necessary to confirm the suitability of these theoretical designs and examine the practical problems of trafficking weak subgrades and compacting the lower pavement layers under difficult site conditions in wet weather.
1 INTRODUCTION
In conventional road construction a sub-base of well graded granular material is used to protect the earthworks from the effects of weather and provide a working platform over which material for the roadbase is hauled. The roadbase is the main structural layer and in bituminous roads consists of either bitumen macadam or rolled asphalt. Surfacing layers are placed on top of the roadbase to provide a safe and durable running surface. In situations where good quality aggregates are expensive or construction depth is limited, as for example in reconstruction, there may be a technical and economic case for replacing some or all of the sub-base wltl~ a structurally equivalent but less thick bituminous roadbase. This form of construction is often referred to as full depth asphalt, but may be more correctly described as asphalt substitution since it may be neither necessary nor desirable to replace all of the granular sub-base.
On trunk roads the specifications of the Department of Transport are mandatory, and on local roads they
provide guidance. These specifications are based on the performance of experimental sections of road and the resultant catalogue of designs does not facilitate the introduction of non-standard designs. In particular, it is diff icult to make best use of new materials and design concepts that have not been proven in practice; also it is diff icult to produce designs for the much higher levels of traffic anticipated in the future. Nevertheless a non-standard design was proposed by Lister, Powell and Goddard (1982) and has been incorporated into the specifications for the reconstruction of very heavily trafficked roads designed to carry more than 80 million standard axles, where construction depth is often severely limited and a premium structure is required. The need to construct stronger pavements to withstand increased traffic loading, coupled with limitations on the depth of construction, identifies a niche ideally suited to designs incorporating a larger proportion of bituminous roadbase. There are practical advantages in reducing exposure of the subgrade and considerable economies could be realised as a result of reduced tonnages of excavation and haulage and reduced traffic delays arising from shorter contract durations.
The present research was designed to investigate the strength of subgrade needed to permit satisfactory laying and compact ion of asphalt wi thout a sub-base. Test pavements were constructed to compare the structural quality of a conventional pavement wi th a sub-base and a full depth asphalt pavement. The measured stresses, strains and deflections beneath a rolling wheel load were compared with the corresponding calculated values, to determine whether theory could be used for the design of pavements in which bituminious roadbase is substituted for granular sub-base. In practice trials of proposed construct ion methods would be required to evaluate any difficulties of construction on an unprotected, moisture susceptible subgrade.
2 LAYING BITUMINOUS MATERIALS ON A WEAK SUBGRADE
To investigate the quality of bituminous pavement that could be laid over subgrades of different strength, rolled asphalt and dense coated macadam
roadbases were laid on foundations of three strengths: a clay with a CBR of 2 to 3 per cent, another clay wi th a CBR of 12 to 15 per cent and a rigid concrete slab.
The clay foundations were prepared, to a total thickness of one metre, in layers according to the requirements of the Department of Transport (1976) Specification for Road and Bridge Works. eenetrometer measurements were made to determine the strength of the clays, both during preparation and at the t ime of the trial. Results of soil tests on samples of the two clays removed after the trial are given in Table 1.
TABLE 1
Properties of clay subgrades
In-situ Foundation CBR
Strong clay 12-15 Weak clay 2 - 3
Moisture Liquid Plastic Plasticity content l imit l imit Index
% % % %
24 64 27 37 29 59 28 31
The two types of roadbase material, both containing 50 pen bitumen, were mixed at 150-160°C and were laid on each of the three foundations to a compacted thickness of 150 mm. On each foundation, areas of both roadbase materials were given 6 and 12 passes of a 9 tonne deadweight tandem roller. A conventional wheeled paver was used on the rigid foundation and on the strong clay wi thout dif f iculty but a tracked paver and t imber roadway for the asphalt lorries were found necessary to prevent excessive rutting of the weak clay foundation. Plate 1 shows the ruts formed by the paver with its hopper empty and by a loaded 10 tonne asphalt lorry. When
loaded the paver weight was more uniformly distributed over the track and little rutting occurred in the actual trial.
Cores were removed from the laid material to determine layer thickness, material composition and density. The composition of the dense roadbase macadam complied with BS 4987 (1973a) with the exception that it contained a 50 pen bitumen and the hot rolled asphalt roadbase complied with BS 594 (1973b).
Table 2 shows that after 6 passes of the roller the average levels of compaction achieved on the three foundations with each material were not very different. Additional rolling resulted in improved compaction except with the dense bitumen macadam on the weak foundation where the density of the material did not increase and the cracks shown in Plate 2 developed.
Six beams cut from the trial pavements had values of dynamic stiffness modulus in the range 2.8-4.5 GPa (20°C and 5 Hz) indicating a high level of stiffness even in the materials laid on the weakest foundation.
One week after the material had been laid deflection beam measurements were made on the pavements on the clay foundations. The average result for each trial is given in Table 3. The cracks that developed in the dense bitumen macadam where it was given 12 roller passes had an appreciable weakening effect on the pavement, showing up as an increase in deflection from 1.65 mm to 2.04 mm. Some cracking is to be expected with thick lifts but it is not normally a serious problem, the cracks penetrating only a few millimetres into the material. However, the severe cracks in the trial macadam penetrated more than 20 mm and it must be concluded there would be less risk in using hot rolled asphalt in preference to dense bitumen macadam for the first pavement layer on weak foundations. Hot rolled asphalt performed better and was also found to compact more uniformly in depth.
TABLE 2
Summary of compaction results
Foundation
Rigid Strong clay Weak clay
Average Rolling
Temp °C
120 125 115
DBM HRA
Percentage refusal density
6 passes
93.6 94.3 93.6
12 passes
95.9 95.7 93.2
Average Rolling
Temp °C
150 130 135
Percentage refusal density
6 passes
97.1 97.1 96.6
12 passes
98.2 99.2 97.5
Neg. no. R287 /82 /4
Neg. no. R287 /82 /3
Neg. no. R287/82/1
Plate 1 Ruts made in weak clay foundat ion by paver and 10 tonne asphalt lorry
Summary of deflection beam results corrected to 20°C and 155 mm thickness
Foundation
Strong clay Weak clay
DBM Deflection mm
6 pass 12 pass area area
0.44 0.41 1.65 2.04
HRA Deflection mm
6 pass 12 pass area area
0.42 0.42 1.67 1.64
3 A TEST TO ASSESS THE PERFORMANCE OF FULL- DEPTH ASPHALT
3.1 C O N S T R U C T I N G THE TEST P A V E M E N T S
Having established that roadbase could be laid and compacted on a relatively weak subgrade, the two main roadbase types, rolled asphalt and dense bitumen macadam, were laid side by side with and without the use of a sub-base. This was done to allow the performance of the conventional and full- depth pavements to be directly compared for both types of roadbase through measurement of stress, strain and pavement deflection under load. Also, by measuring the physical properties of the materials a comparison was possible of the measured performance of the pavements with that predicted by linear elastic theory, which it was hoped would ultimately provide an acceptable basis for design.
The layout of trial pavements is shown in Figure 1. They were laid on a prepared subgrade of heavy London Clay having a strength of approximately 12 per cent CBR determined with the in-situ CBR test. Over this subgrade two conventional pavements each 15 m long and 4 m wide was constructed, with 40 mm of hot rolled asphalt wearing course laid over two nominally 90 mm thick layers of bituminous roadbase, and 150 mm of Type 1 sub-base. In one pavement the roadbase was dense bitumen macadam (pavement A), in the other it was hot rolled asphalt (pavement B). The sub-base was crushed limestone laid in a single layer and compacted with a vibratory roller to the requirements of the Department of Transport (1976) Specification for Road and Bridge Works.
Alongside the conventional pavements corresponding full-depth bituminous pavements (pavements C and D) were constructed directly on the subgrade. The roadbases of these pavements were laid in three nominally 80 mm thick layers, the total thickness having been estimated from linear elastic theory to
provide protection to the subgrade the same as that for the conventional pavement. Each layer of the roadbase was laid with a paver and compacted with 10 passes of a 9 tonne deadweight tandem roller. The same roller was used to compact the wearing course, which was laid at a temperature in the range 130-150°C and had coated chippings applied. The compositions of the bituminous materials complied with the appropriate British Standards specified in BS 4987 and BS 594 with the exception of the dense roadbase macadam layers overlying the bottom most layer which contained a 200 pen binder instead of the required 100 pen binder.
3.2 T E S T I N G THE P A V E M E N T S In each pavement section 8 foil gauges were installed to measure horizontal strain at the bottom of the bituminous material and 5 gauges to measure vertical stress 100 mm down in the subgrade as shown in Figure 1. Unfortunately compaction of the first layer of roadbase caused the failure of all the strain gauges in pavement B and 3 in each of the remaining sections: also one stress gauge in pavement B failed. Temperature was measured at all layer interfaces.
E O o3
l
Gauge line
DBM road base Foil strain
[ ~ gauges
Pavement A Pavement C / ~ Soil stress
I I - o
Pavements with subbase
Pavement B
o ! Pavements
wi thout subbase O
Pavement D
HRA road base t
Fig. 1 Layout of trial pavements and instrumentation
A rolling wheel load of 31 kN on a single tyre was used to induce transient stresses and strains in the pavements; the same wheel load, but applied to two tyres, was used for the deflection beam testing. The lorry was driven over the pavement at 15 kph with one rear wheel passing immediately above the line of stress and strain gauges. The output from the gauges was taken via amplif iers to an ultra-violet recorder and peak values of stress and strain were measured from the records. Measurements of pavement deflection were also carried out above each of the buried stress gauges. Most of the measurements
carried out during the construct ion of the pavements were for pavement temperatures close to 20°C. To examine the validity of linear elastic modell ing fur ther tests were carried out at temperatures of 6 ° and 30 ° on the completed pavements. The low temperatures were obtained naturally on a cold day in winter and infra-red heaters and polystyrene boards were used to obtain high temperatures.
TABLE 4
Measured values of elastic modulus for the bi tminous materials (GPa) (Test f requency 2 Hz)
Pavement
A
C
D
Pavement Temperature
oc
6 o
20 ° 30 °
6 ° 20 ° 30 °
6 ° 20 ° 30 °
6 ° 20 ° 30 °
Wearing Course
7.8 2.1 0.61
7.8 2.1 0.61
7.8 2.1 0.61
7.2 2.1 0.61
Roadbase layers
1 2 3
4.3 6.7 -- 0.80 2.4 -- 0.18 0.72 --
8.3 7.1 -- 2.4 2.4 -- 0.70 0.67 --
4.8 5.5 9.2 0.70 0.93 3.1 0.20 0.22 i0.84
8.2 ,6.8 4.4 2.4 1.8 1.3 0.72 0.561 0.35
1° l
Q.
E3
Frequency
8Hz
4Hz
2Hz
1Hz
O.2Hz
I I I I 1
5 10 15 20 25 30 Temperature (°C)
Fig. 2 Effects of frequency and temperature on the dynamic modulus of a rolled asphalt test specimen
TABLE 5
Mean structural properties for each trial
Pavement A
Thickness of wearing course (mm)
53
Roadbase layer 1 2
Thickness (ram) 94 96
Rolling temperature (°C) 120 120
Percentage refusal density
Void content
Pavement
Thickness of wearing course (mm)
:~oadbase layer
Thickness (mm)
Rolling temperature (°C)
Percentage refusal density (%)
Void content (%)
94.4 92.6
8.1 9.9
C
47
1 2 3
97 68 100
100 120 120
95.3 92.8 94.3
6.1 7.7 7.1
42
1 2
85 79
120 115
97.6 98.0
1.5 3.6
1
81
105
D
45
2 3
80 93
115 100
97.5 97.1 98.2
3.7 4.5 1.9
Having completed measurements of stress, strain and deflection, beams cut from the roadbase layers and tested in three-point bending gave the wide range of values of elastic modulus shown in Table 4. A typical set of test results is shown in Figure 2.
Average layer thicknesses of the four pavements were estimated from cores cut at the sites of stress, strain and deflection measurements after the tests had been completed. Thicknesses, average rolling temperatures, percentage of refusal density test results and void contents for the roadbase layers are shown in Table 5.
3.3 A N A L Y S I N G THE RESULTS The measurements of stress, strain and deflection made on the four pavement sections could not be directly compared because of variations in modulus of the bituminous layers and in thickness of pavements; this comparison is necessary to calculate the value of asphalt substitution. However, by comparing the measurements wi th those calculated from linear elastic theory it was possible to test the applicability
of the theory to this form of construction and hence to compare the pavements indirectly.
A multi-layer linear elastic model was used to calculate stress, strain and deflection axially beneath a uniformly loaded circular area representative of the wheel load that produced the stresses and strains in the pavements: a 31 kN load applied uniformly over a circle 0.29 m in diameter. Further input to the model comprised thickness and dynamic modulus for each of the pavement layers. The values of dynamic modulus of the bituminous materials used to calculate stress, strain and deflection were determined from the tests on beams at frequencies of loading of 2 Hz for the stress and strain measurements and 0.2 Hz for the deflection measurements, corresponding to vehicle speeds of 15 kph and 2 kph respectively; these frequencies are in accord with recommendations recently published by Powell, Potter, Mayhew and Nunn (1984) and are supported by examination of the stress and strain records. Values of Poisson's Ratio recommended in the same publication were also adopted, 0.35 for the bituminous layers, and 0.45 for sub-base and subgrade. A dynamic modulus of 150 MPa was used for the sub-base as recommended by Powell et al (1984). The modulus for the subgrade of 90 MPa was
. . . . Calculated { ----o--- Measured
~, 40
2O ~_
5
0. 40
,~ 2 0
,o
P a v e m e n t A / /
/ / /
/ / j
10 20 30
P a v e m e n t B / / '
/ /
I I
10 20 30 Temperature (°C)
4 0
2 0
10
5
0
P a v e m e n t C / / / ,/ / /
/ / /
/ / /
10 2 0 3 0
40
20
10
5
0
Pav
I i 10 20 30
Temperature (°C)
Fig. 3 Comparison of measured and calculated stresses in the subgrade beneath the completed pavements at different temperatures
I O X v .E o0
E ~J
g
o T
400
2 0 0
1 O0
50.
25
0
. . . . Calculated Measured
P a v e m e n t A ] 400
/ / / J ~ / / 200
P a v e m e n t C
/
/ / /
/ / /
.< i
1=0 20
A 400 ,%
x 2OO
.E
-~ I00 E
c 5 0 o N
-i- 2 5 0
25
30 0 0 i 1 20 30
Pavement D
/ / / ' ~
/ / / / / / ' ~ t 10 20 30
Temperature (°C)
Fig. 4 Comparison of measured and calculated st~rains at bottom of the bituminous layer of the completed pavements at different temperatures
estimated from the mean measured CBR near the surface. The Falling Weight Deflectometer, which responds to the strength of a greater depth of subgrade, indicated a higher modulus of 110 MPa. Both tests showed that the sub-grades beneath the four test pavements were of equal strength.
Figures 3 -5 show measured and calculated relationships between stress, strain and deflection and pavement temperature for the completed pavements while Figures 6 -8 show similar results at about 20°C at different stages in the construction of the pavements. The measured strains are those from gauges placed transversely. The longitudinally placed gauges showed strain pulses approximately half the amplitude of that measured by the gauges placed transversely whereas linear elastic theory predicts that these two strain components should be equal. Even allowing for other possible sources of disagreement between measured and calculated results such as uncertainty regarding the subgrade modulus and scatter in the measurements, agreement between measured and calculated stresses, strains and deflections is poor. However both the measured and calculated stresses and strains show the same sensitivity to changes in temperature and thickness of asphalt. This correlation is equally good for the conventional and full-depth pavements.
A 80
0
x
E E c 40 o
" 0
~ 2 0
o
0
x
E E
- - 4 0 g
~ 2 0
g, 0
I iO 210 30
Pavement B
j J * '
f /
/ /
I I
10 20 30 Temperature (Oc)
. . . . Calculated I Measured
80
40
20
o
8o~
40
Pavement C
20.
// /
-/ /
i i
10 20 30
Pavement D
/ i
/ j
i j f
I I
10 20 30 Temperature (°C)
Fig. 5 Comparison of measured and calculated surface deflections of the completed pavements at different pavement temperatures
The correlation between measurement and theory suggests that theory can be employed wi th some conf idence to adjust the measured stresses and strains for the effect of temperature and hence modulus, or for thickness. Hence the measured stress or strain can be adjusted to the value expected if the road contained a material of a standard modulus by mult ip ly ing the measured value by the ratio between the stress or strain calculated using the standard modulus and using the measured modulus. This procedure was employed to adjust the measured results for pavements A and C to those corresponding to a single modulus for the roadbase layers equal to the average value; measurements of pavements B and D were treated similarly. The adjusted results shown in Figure 6 and 7 al low
estimates to be made of the thickness of dense bitumen macadam or hot rolled asphalt roadbase required to substitute for the sub-base of the conventional pavements on the basis of equal pavement strength as indicated by stress and strain. The estimates are expressed in Table 6 as the ratio of the thickness of bound material equivalent to a given thickness of granular sub-base. Table 6 suggests that equivalence ratio is not dependent on the thickness of bituminous material.
The agreement between measured and calculated values of deflection shown in Figure 8 was less satisfactory, particularly for pavements A and C. This may be explained in part by the very low values of dynamic modulus measured in the laboratory for most of the dense bitumen macadam. Figure 5 shows that
(3- V
80
N 4O
Q. E 0 o 2 0
4,,-'
% a .
,=
. . . . Calculated Adjusted measurements
= 40
E oo -- 20
> 10
Pavement A
100 150 200 250
Pavement B
80
80
401
20
10
Pavement C
I I I I
100 150 200 250 300
8o e'e no 40 '%~ ~'~'~,
20
10 I I I I I I I
100 150 200 250 100 150 200 250 gO0
Thickness of bituminous layer (mm)
Fig. 6 Effect of thickness of bituminous layer on measured and calculated stresses in the subgrade
TABLE 6
Ratio of the thickness of b i tuminous material to the structural ly equivalent thickness of granular sub-base
Vert ical stress on sub-grade (Kpa) 50 40 30 25 20 15 12 Mean
Pavements A and C (DBM) -- 0.26 0.26 0.24 0.26 0.26 0.26 0.26
Pavements B and D (HRA) 0.31 0.29 0.29 0.32 0.48 -- - - 0.34
Strain at bot tom of bound layers ( x 10 -6) 300 250 200 150 120
Pavements A and C (DBM) 0.37 0.29 0.26 0.30 0.37 0.32
o 8 0 0 x
E
400
E 200
E O N "c 100 o 3"
J .... Calculated ---o---- Adjusted measurements
800 Pavement A
- 400
200
'100 I I I
100 150 200 250
-~ 400
~ b
E v 200 O .N L O "1-
100
Pavement C
i i i i
100 150 200 250 300
8OO Pavement D
100 150 200 250 300 Thickness of bituminous layer (ram)
Fig. 7 Effect of thickness of bituminous material on measured and calculated strains at bottom of bituminous layer
100
~" 5O O
x E E v c 25 O
lO0 " o
3
~ 5 0
25
Pavement A
I I I
100 150 200 250
.~ %~~Pavemant B
l I I
100 150 200 250
. . . . . Calculated o Adjusted measurements
I 10C ~ q ~ Pavement C
X
5 0 ~" "~ " ~ ~ ~ ,~
25 = = i I 100 150 200 250 300
10oi , ~ , Pavement D
2~ t I I I 1 O0 150 200 250 300
Thickness of bituminous layer (mm)
Fig. 8 Effect of thickness of bituminous layer on measured and calculated surface deflections
the abil i ty of linear elastic theory to predict deflection is worst at 30°C, at wh ich temperature the measured dynamic modulus of much of the bi tuminous material is less than that of sub-base; it is evident that the dynamic modulus that should be used to model deflection must be greater than that measured in the complex modulus test. Furthermore, under these condit ions of low test f requency and low stiffness of the bound pavement layers, high levels of stress in the sub-base could increase its dynamic modulus substantially as indicated by recently published work by Mayhew (1983). Such increases in modulus would bring measured and calculated deflection results into better agreement but have little effect on the stress and strain results.
4 APPLICATION OF RESULTS
The correlation between the experimental measurements and the calculated values for the test pavements implies that despite obvious l imitations linear theory might play some part in generating alternative designs. For a convent ional pavement, it could be used to calculate the horizontal strain at the bottom of the b i tuminous material and the vertical strain on the subgrade under a standard wheel load. The same calculations could then be carried out to produce an equivalent design wi th reduced thickness of sub-base, or no sub-base at all. Linear elastic theory can thus be used as the first stage in modify ing the design of convent ional pavements. It cannot on its own support the introduct ion of a new design, which wou ld require comprehensive trials to establish practicabil i ty and evaluate variabil ity. Exploitation of partial or ful l subst i tut ion of granular sub-base wi th b i tuminous roadbase depends on the advantages being substantial and not likely to be lost amongst other construct ion variables.
To examine the appl icat ion of linear elastic theory standard designs for a b i tuminous roadbase and a subgrade CBR of 5 per cent were selected from the work of Powell et al (1984) to give lives of 1, 10 and 80 msa. Values of maximum vertical strain at the top of the subgrade and horizontal strain at the bottom of the asphalt arising from a load of 40 kN applied over a circle of diameter 0.15 m, were calculated for each design using the layer modul i recommended by Powell et al (1984). The thickness of the bi tuminous layer when roadbase material is subst i tuted for the granular sub-base to give levels of strain no higher than these values was then calculated. Table 7 contains standard designs and the equivalent designs for asphalt subst i tut ion together wi th an estimate of the increased requirement for bi tumen and savings in the use of aggregate that might result f rom asphalt substi tut ion. Between 50 and 75 tonne of aggregate are saved for each addit ional tonne of bi tumen used.
9
The mean equivalence ratios observed in the trials and given in Table 6 are at the upper end of the calculated range in Table 7. However the values from the trials were obtained by comparing the stress and strain developed in the pavement under a loaded wheel travelling at a relatively slow speed of 15 kin/hr. At speeds more typical of commercial traffic the effective modulus of the bound layer would be higher and the equivalence ratio would be reduced. In addition some layers of the pavements containing dense roadbase macadam included 200 penetration binder instead of the required 100 penetration and as a consequence these layers had an unrealistically low modulus which would result in an increase in the measured equivalence ratio. The reduction in the calculated equivalence ratio given in Table 7 resulting from removing the last 75 mm of sub-base may be due to linear elastic theory being inadequate to describe the behaviour of the pavement in some circumstances. Suitable designs can therefore only be established by extensive road trials.
layer or some other soil improvement measure is normally used. If it were intended to extend asphalt substitution into this area of subgrade improvement the dangers involved in exposing a moisture susceptible subgrade and seeking to compact the asphalt over it would require special consideration.
Values of stress, strain and deflection calculated from linear elastic theory were not in good agreement with measurements made on conventional and full depth asphalt pavements. However, quite large changes of asphalt thickness and dynamic modulus have been shown to have the same relative effect on the measured stress and strain as on the calculated values, indicating that linear elastic theory could be used to generate tentative designs so long as these are not too different from a conventional design.
Suitable designs for full depth asphalt or limited asphalt substitution will only be established by extensive road trials.
5 CONCLUSIONS In the trials it was found that thick lifts of bituminous roadbase, particularly rolled asphalt, were properly compacted even on a subgrade as weak as 3 per cent CBR; a foundation too weak to support a normal paver or the supply vehicles. In practice there should be no difficulty compacting thick lifts because on subgrades weaker than 5 per cent CBR a capping
6 ACKNOWLEDGEMENTS The work described in this report forms part of a joint programme of research between Transport and Road Research Laboratory, British Aggregate Construction Materials Industries and the Refined Bitumen Association. It was carried out in the Pavement Design and Maintenance Division (Division Head: Mr J Porter) of the Highways and Structures Department of TRRL.
TABLE 7
Equivalence analysis
Design Traffic (msa)
10
80
Design recommended by Powell et al (1984)
Bound layer thickness
(mm)
190
280
390
Sub-base thickness
(mm)
225
225
225
Equivalent designs
Bound layer thickness
(mm)
Sub-base thickness
(ram) Equivalency
ratio
Increase in bitumen
requirement (tonnes/m 2)
Saving in aggregate
(tonnes/m)
208 150 0.24 0.0025 0.13
228 75 0.25 0.0052 0.25
230 0 0.18 0.0055 0.42
300 150
75
0.27
320 0.27
0.19
0.0027
323
0.0055
0.0059
150 0.29 0.0030
75 0.27 0.0055
0 0.20 0.0062
412
430
435
0.12
0.25
0.41
0.21
0.25
0.41
10
7 REFERENCES
British Standards Institution (1973a). Coated macadam for roads and other paved areas. BS 4987: 1973 (British Standards Institution).
British Standards Institution (1973b). Rolled asphalt (hot process) for roads and other paved areas. BS 594:1973 (British Standards Institution).
Department of Transport (1976) Specification of Road and Bridge Works. London (HMSO).
Mayhew, H C. (1983) Resilient properties of unbound roadbase under repeated triaxial loading. Department of the Environment Department of Transport, TRRL Report LR 1088. Crowthorne, (Transport and Road Research Laboratory).
Powell, W D, J F Potter, H C Mayhew and M E Nunn. (1984) The design of bituminous roads. Department of the Environment Department of Transport, TRRL Report LR 1132. Crowthorne, (Transport and Road Research Laboratory ).
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