TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment Department of Transport

TRRL LABORATORY REPORT 1043

EARLY PERFORMANCE OF SOME EXPERIMENTAL BITUMINOUS OVERLAYS IN KENYA

by

H R Smith and.C R Jones

The work described in this Report forms part of the programme carried out for the Overseas Development Administration,

but any views expressed are not necessarily those of the Administration.

Overseas Unit Transport and Road Research Laboratory

Crowthorne, Berkshire 1982

ISSN 0305--1293

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. The work described in this report forms part of a programme carried out for the Overseas Development Administration, but the views are not necessarily those of the Administration. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Objectives of the experiments

3. The design of the experiments

4. Tests and measurements made before overlaying

4.1 Pavement investigations

4.2 Surface condition assessments

4.2.1 Rut depths

4.2.2 Cracking and patching

4.3 Deflection measurements

4.4 Determination of overlay thicknesses

5. Construction of the overlays

5.1 Tack coats

5.2 Manufacturing and rolling temperatures

5.3 Laying and compaction

5.4 Sampling of the overlay materials

6. Post-construction measurements

6.1 Compaction of the overlays

6.2 Overlay thickness

6.3 Traffic loadings

6.4 Surface condition

6.4.1 Rut depths

6.4.2 Cracking

6.4.3 Surface roughness

6.5 Ageing of the bitumen in the overlays

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6.6 Deflection measurements

6.6.1 Strengthening effect of the overlays

6.7 Radius of curvature of deflected surfacing

6.7.1 Deflection-curvature relationships

6.7.2 Effect of temperature on deflection-curvature relationships

7. Performance of the overlays

7.1 General observations

7.1.1 S i te l

7.1.2 Sites 2 and 3

7.1.3 Site 4

7.1.4 Site5

7.1.5 Sites 6 and 7

7.1.6 Site8

7.2 Relationships between measured parameters and cracking

8. Discussion of results

9. Recommendations for overlay design in Kenya

10. Future work

11. Conclusions

12. Acknowledgements

13. References

14. Appendix: Adaption of British overlay design method for use in Kenya

14.1 Application of the method in Kenya

14.1.1 Roads with crushed stone roadbases

14.1.2 Roads with soil-cement roadbases

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© CROWN COPYRIGHT 1982 Extracts from the text may be reproduced, except for

cDmrnerJcial purposes, proyJded_the~our~e is_ackno~vledged

EARLY PERFORMANCE OF SOME EXPERIMENTAL BITUMINOUS OVERLAYS IN KENYA

ABSTRACT

This report describes the performance of bituminous overlays on roads in tropical and sub-tropical environments in. Kenya.

Considerable hardening of the binder occurred throughout the depth of the overlays during the first five to seven years, but negligible rutting developed. The main form of deterioration was cracking which was initiated at the surface of the overlays.

The reduction in pavement deflections produced by the overlays is shown to be only slightly less than those obtained in Britain despite the higher ambient temperatures in Kenya.

The performance of the overlays is shown to be primarily dependent on the properties of the overlay material itself. No correlation could be established between the development of cracking on each test section and the corresponding values of surface deflection and curvature, traffic loading or overlay thickness. However the overlays have remained serviceable for up to seven years and have carried up to 4.3 x 106 equivalent standard (80 kN) axles.

1. INTRODUCTION

Many of the paved roads in developing countries have, for sound economic reasons, been built on the

'stage construction' principle which requires that a road is strengthened periodically to match the growth

in traffic. Consequently in many countries substantial programmes are now required to reconstruct or

strengthen roads that have reached the end of their original design lives.

It is often more economical to strengthen a road when it has reacheda critical condition rather than

to allow it to fail, necessitating reconstruction of the whole or the upper layers of the pavement. Premixed

bituminous overlays provide a relatively rapid and convenient method of strengthening roads. However,

high quality bituminous materials are expensive and hence it is important that the method of design of

the overlay thickness should be as effective as possible and the 'life' o f the overlay should be predictable

with a high level of confidence.

1,2,3,4,5 Authorities in several countries use deflection techniques for evaluating the structural condition

of pavements and for designing the thicknesses of bituminous overlays. Most of the available overlay design

recommendations have been developed in countries with temperate climates and there is an urgent need to

develop similar recommendations for use in countries with tropical climates.

This report gives details of the construction and early performance of a number o f experimental

overlays placed on a variety of road pavements in the Republic of Kenya between 1973 and 1977 as part of

a cooperative study between the Kenya Ministry of Works and the Overseas Unit o f the Transport and

Road Research Laboratory.

2. OBJECTIVES OF THE EXPERIMENTS

The objectives of the experiments a r e : -

(a) to study the performance of different types and thicknesses of bituminous overlays in tropical and

sub-tropical environments;

(b) to determine the strengthening effect o f the overlays by comparing deflection measurements made

before and after overlaying;

(c) to develop bituminous overlay thickness design charts for use in tropical environments.

3. THE DESIGN OF THE EXPERIMENTS

The experiments comprise different types and thicknesses of premixed bituminous materials laid on different

types of pavement construction in a range of climatic zones.

The Kenyan Ministry of Works arranged for the construction of seven groups of experimental overlays

and an eighth small group was constructed on the Nairobi-Thika road in cooperation with the Nairobi City

Council. Details of the locations of the sites are given in Table 1. Summaries of the available rainfall data

recorded at meteorological stations near to the sites are given in Table 2.

A restriction placed upon the selection of the sites arose from the need to minimise costs by locating

the overlays near to road construction projects where premixed bituminous material was being manufactured.

The sites were chosen so that variations in road alignment and drainage conditions were kept to the minimum.

At each site a number of 100 metre long sections, separated by transitions, were overlaid with premixed

bi tuminous materials 6,7 to the specifications shown in Tables 3 and 4.

Details o f the types of bituminous mixes laid at each site togethe r with section numbers and thicknesses

o f material are indicated in Table 5.

The thicknesses o f the overlays at each site were selected on the basis o f available traffic loading data

and the results of deflection measurements made before overlaying. The mean deflection for each section

was calculated and an estimate of the overlay thickness required for a typical design life of eight years was

obtained f rom the design curves developed in the United Kingdom 8. (These design curves were revised in

19784 and now indicate longer lives for the overlays.)

At each site a 'design' thickness was chosen and each overlay material was laid on asection to this

thickness. In addition an asphaltic concrete mix was also laid on three other sections to thicknesses of

25 m m less than the design thickness and 25 and 50 mm more than the design thickness. A minimum

thickness o f 35 Or 40 m m was stipulated, depending upon the maximum size of stone in the aggregate.

Continuous cast in situ kerbs were constructed along two sections of sites 2 and 3 to determine

whether they provide any benefit by restraining the edges of the unbound roadbases: The kerbs extended

f rom below the roadbases to the surface o f the existing road. Drainage paths were provided from the bo t tom

2

of the roadbase through the kerbs and shoulders. At each site one of these sections was overlaid with the

design overlay thickness of 100 mm andthe other with 75 m m of the current (1973) nominal 20 m m

asphaltic concrete. The overlays extended over the kerbs.

At each site, except for site 1 where there was an insufficient length of uniform road, a 'control '

section of the existing road was included in the overlay monitoring programme.

4. TESTS AND MEASUREMENTS MADE BEFORE O V E R L A Y I N G

At each site the required number of 100m long.sections were marked out with twenty deflection test

points and forty-four levelling points as indicated in Figure 1.

4.1 Pavement investigations

At each site inspection holes were dug in all the transition zones to measure the thicknesses, in situ

CBR strengths and moisture contents of the pavement layers. Summaries of pavement layer thicknesses

and general descriptions of materials are given in Table 6.

Moisture contents and CBR strengths are summarised in Tables 7 and 8. Additional measurements

made after overlaying have been included in the tables.

The results of a number of dynamic cone penetrometer tests carried out on the subgrades and expressed

as equivalent in situ CBR values are summarised in Table 9. These tests were carried out to indicate if there

were any significant changes in subgrade strength with depth. The cone penetrometer had a cone angle of

90 °, and the driving impact was provided by a mass of 10 kg falling through 0.5m.

An illustration of the relationship obtained between in situ CBR and rate of penetration is given in

Figure 2. The equation of the regression line is:

L N (CBR) = 5.45 - 1.01 L N (Rate of penetration)

the regression coefficient r 2 is 0.85.

A large increase in strength with depth was recorded on site 7, whilst large decreases in strength

occurred on sites 4 and 8.

Samples of the subgrade materials were taken from the inspection holes and the results o f compaction,

CBR and Casagrande classification 9 carried out in the laboratory are summarised in Table 10. Samples of

subgrade materials from each site were generally bulked together before testing, but where dissimilar

materials occurred on a site, they were tested separately."

4.2 Surface condition assessments

4.2.1 R u t d e p t h s . Measurements of the depths of ruts in the wheelpaths were made at each deflection

test point, as indicated in Figure 1, by placing a 2m straightedge transversely to the centre-line o f the road

and over the test point. The results of these measurements are given later in this report.

3

4 . 2 . 2 Cracking and patching. Summaries of the surface conditions and of patching work carried out

on the sites before overlaying are given in Tables 11 and 12. The asphaltic concrete surfacing in the verge

side wheelpaths of site 3 had suffered severe fatigue cracking and some roadbase and sub-base failure had

also occurred. Extensive areas of cracked surfacing were cut out with a pavement saw and reinstated with

asphaltic concrete. Where roadbase failure and, in the case of section 1, severe displacement at the road

edge had occurred the roadbase and sub-base were replaced with cement-stabilised soil. Traffic was allowed

to run on the soil-cement patches within a few hours of construction. At a later date the top 50 mm of

these patches were cut out and replaced with bituminous material.

4.3 Deflection measurements

Transient deflection tests using the standard TRRL test method 10'11, were made on the twenty test

points o f each section as indicated in Figure 1. Additional tests were made to measure movement of the

feet of the deflection beam during deflection tests. This movement causes a change in position of the

original reference datum and an error in the dial gauge reading 10. Movement of the beam feet during

testing before and after overlaying were of similar magnitude and were generally small. Only on a few

test points on site 2 were movements large enough to cause the indicated deflection to be increased by as

much as 6 x 10 -2 mm. Corrections for feet movement have not been made to deflection readings referred

to in this report.

Provided that the feet of the deflection beam did not sink into 'rich' surface dressings, deflections

measured on the surface dressed sites were not dependent upon pavement temperature and only small

corrections were found to be necessary on a few test points on the other sites.

Deflection measurement made on areas of site 3 before and after patching showed little change in

magnitude. Small decreases were recorded on the full-depth patches and small increases on the asphaltic

concrete surfacing patches.

4.4 Determination of overlay thicknesses

Optical levelling was carried out on each section before overlaying as indicated in Figure 1 to determine

the thickness of overlay at each test point.

5. CONSTRUCTION OF THE OVERLAYS

All the overlays, with the exception of those on sites 5 and 8, were constructed under normal contract

conditions, using plant and methods employed in the main contract for surfacing work, the only difference

being that there was more control of rolling temperatures on the experimental sites. The overlays on sites

5 and 8 were constructed by direct labour.

5.1 Tack coats

Bitumen emulsion was applied at a rate o f approximately 1 litre/m 2 by hand lance to the existing

road surface. Where multiple layer overlays were completed during a second day's work, a very thin tack

coat was used. A discontinuous application of emulsion was made, which was spread by means of a

pneumatic-tyred roller to give a complete covering. It has been found that where this was done a more

reliable bond between the layers was obtained.

4

5.2 Manufacturing and rolling temperatures

Temperatures specified for manufacturing, delivery and rolling of the overlay materials are given in

Table 13. The temperature of each lorry load of material delivered to site was measured and recorded.

Approximately 75 per cent of the loads delivered to all the sites were within the specified temperature

range and only 5 per cent were more than i0°C outside of this range. A large number of loads of material

delivered to site 5 were below the specified temperature, but because of high ambient temperatures and

the relatively slow rate of delivery, compaction was completed at temperatures above the minimum specified

for rolling.

5.3 Laying and compaction

Overlays of 75-125 mm thickness were laid in two layers, except on sites 2, 3 and 4, where 75 m m

thick layers were laid in one lift. Overlays of 150 mm thickness were laid in three 50 m m layers.

Usually one section was completed each day except for the 150 m m thick overlays where the third

layer was added on the second day. In a few instances where a layer was only laid on one lane, the centre-

line edge was cut back before completion of the second lane of the layer.

Typical compaction plant consisted of a tandem steel-wheeled roller of 8 Mg mass and a pneumatic-

tyred roller of 12 Mg mass. The same plant was used on sites 2, 3, 4, 6 and 7. The steel-wheeled roller

was used to do the initial 'break-down' rolling and finishing. Occasionally this method of compact ion was

found to cause shearing marks or cracks in the mat, some of which subsequently opened up during the

early lives of the overlays concerned. The steel-wheeled rollers also left a smooth surface texture.

On sites 3 and 4 the steel-wheeled roller was restricted as far as possible to the compact ion of the cerLtre-

line joint. An unballasted pneumatic-tyred roller, with tyre pressures of 310 kN/m 2 (45 psi) was used to do

the 'breakdown' rolling and a pneumatic-tyred roller of 14 Mg mass with tyre pressures of 620 kN/m 2 (90 psi)

was used to complete the main compaction. A coarse textured surface was formed which has remained

after trafficking.

5.4 Sampling of the overlay materials

Samples of overlay material were taken shortly after mixing and tested in th¢ site laboratory in

accordance with contract requirements. Additional samples for determination of aggregate gradings,

bitumen content and bitumen hardness were also taken.

Samples of the overlay materials were taken from the end of the paver feed screw, approximately 10m

after the beginning and 10m before the end of each section in each lane for all layers. A comparison was

made between different methods of sampling, which included taking samples from the paver hopper , the

end of the paver feed screw and from the laid mat. No significant differences were found between the

analyses of aggregate gradings and bitumen contents for the samples taken by the three methods.

Samples were tested for aggregate grading and bitumen content by the Road Note 10 extraction

bottle method 12. The most common extraction test used on site was the hot extractor method 13. It

was found that in this method tidier could pass the filter paper particularly where hydrated lime had been

used resulting in inaccurate determinations of fdier and bitumen contents. The cause o f this problem was

not investigated. In the Road Note 10 method a centrifuge is used to ensure that no filler is retained in the

extracted bitumen. 5

Bitumen contents for the asphaltic concretes and bitumen macadam basecourses are summarised in

Table 14, the aggregate gradings for the principal mixes laid on sites 1 - 6 are shown in Table 15 and the

analyses for the hot rolled asphalts are summarised in Table 16.

The results show that the gradings of the asphaltic concrete mixes on site 1 were deficient in sand-sized

material. This was a result o f the unavailability of suitable crushed rock and of the very long haul distance

and expense involved in bringing sand to the site.

The 'current ' 20 m m mix laid on sections 1, 2, 7, 8 and 10 on site 2 did not conform to the design mix.

This resulted from the use o f a high frees content sand, which had not been passed through the mixing plant

drier, for determining the laboratory design mix. Despite established procedures the sand grading had been

determined by dry sieving instead of by wet sieving. By chance, the same proportion of times was lost from

the asphalt plant drier during the manufacturing process as was underestimated by dry sieving of the sand,

so that analyses of the laid material gave the same aggregate gradings and bitumen contents as the design

mix specification. The aggregate in the laid material has a lower surface area than the laboratory design

mix and therefore the bi tumen content is effectively too high. This has provided an opportunity to study

the performance of a bitumen-rich mix which is 'critical' in that it is susceptible to deformation under traffic

but should have very good resistance to cracking.

On site 6, unavailability of suitable materials resulted in the dense bitumen macadams being very

similar in grading to the '20 ram' asphaltic concretes, but they contained approximately 0.5 per cent less

bitumen.

Samples of bi tumen were taken from the heater tanks during the construction periods so that changes

in the hardness of the bi tumens during the mixing process could be determined. This subject is referred to

again in Section 6.5 of this report.

6. POST-CONSTRUCTION MEASUREMENTS

In order to relocate the test spots on the overlays directly above those on the old surface, marker nails

were placed at the edge of the pavement before overlaying and the distances to the test points were recorded.

6.1 Compaction of the overlays

As part of the contract requirements cores were cut from the timished overlays to determine the degree

of compact ion achieved.

It was often impossible to cut integral cores from the overlays within a few days of construction

because distortion of the soft material and damage due to shearing at interfaces occurred. Only in a very few

cases was the density of cores less than 97 per cent of the Marshall design density, which was reported to

give air voids in the mixes of between 3 and 5 per cent. However the mean air voids contents of cores cut

f rom sections 1 - 4 and f rom 9--12 on site 5 were 7.4 and 5.7 per cent respectively.

Aggregates used in the overlays were absorptive with values of up to 4 per cent for the time aggregate

used at sites 2 and 3. Calculations of air voids based upon apparent relative density and the relative density

on an oven dried basis gave a difference in air voids content o f 3 per cent. It is recommended that where

aggregates have absorption values of more than one per cent a mean value o f the two relative densities is used 14.

6

It is apparent that accurate determinations of air voids in mixes, particularly with absorptive aggregates,

is a difficult part of the design process. Whilst contractual agreement on the method of calculation can be

reached, determination of the absolute air voids content to allow comparisons between different mixes

laid under different contracts is much more difficult.

6.2 Overlay thickness

The thicknesses of the overlays were determined by optical levelling which was checked by direct

measurement of cores cut at some levelling points in each section.

6.3 Traffic Ioadings

in order to obtain an estimate of the damaging effect of traffic on the sites, axle-load surveys were

carried out by the Kenya Ministry of Transport and Communications using a portable weighbridge developed

by the Overseas Unit of the Transport and Road Research Laboratory 15.

Factors derived from the AASHO Road Test 16 have been used to express all axle loads in terms of an

equivalent number of standard 80 kN axle loads. The equivalency factors used were calculated f rom the

following formula:

Equivalency factor = (axle load in kgf/8160 kgf) 4"5

A summary of the estimated cumulative number of equivalent standard axles carried by the overlays

is given later in this report.

6.4 Surface condition

6.4.1 Ru t d e p t h s . The measurement of rut depths beneath a 2m long straightedge have been made on all

test points. A summary of rutting measured before and after overlaying on site 2 is given in Table 17, whilst

the results for the other sites are given in Table 18. The results for site 2 are reported separately because

of the high bitumen content of the 'current 20 mm' mix. Rutting in this mix has however been minor and

negligible on all other overlays.

6 .4.2 Crack ing . Cracking was assessed by placing a one-metre square frame parallel to the road edge so

that the maximum amount of cracking was enclosed; the test point was within the square and the frame

did not cross the centre-line of the road. Centre-line cracks were included but were recorded separately

because this type of cracking is usually caused by poor construction practice rather than by the effects of

traffic.

In this study photographic records from which measurements can be taken are being compiled. A

35 mm camera and surveying tripod are set up so that the camera is vertically above the centre o f the frame

and transparencies of 'chalked-in' cracks are taken. The length of cracking can be measured f rom scaled

projections of the transparencies.

Coting surveys have been conducted to monitor changes in crack depths. The predominant mode

of failure has been cracking which has propagated down from the surface o f the overlays. Initiation and

development of cracks to the full depths of the overlays has been very variable, bo th within sections and

7

between the different sites. Measurements indicated that only those cracks which were 0.25 nun or more

in width were likely to be o f significant depth during the early lives of the overlays. Cracking measurements

referred to in this report are therefore for cracks which are 0.25 mm or more in width. Care had to be

taken with these measurements because it was found that the width of cracks could change between early

morning and afternoon when the maximum pavement temperature was reached. On site 5 crack widths

decreased during this period by up to 60 per cent, the mean change being approximately 25 per cent.

An indication of the rates of deterioration of the sections due to cracking is given in Table 19 and

Figure 3. Table 19 also gives details of the overlays and shows the number of test points which have

cracked and the mean cracking for the sections in 1978 and 1981. The results for 1978 are included

because until the oil pipeline was opened in 1978 the annual traffic loading on sites 2, 3, 4 and 5 in the

Mombasa direction was less than half of that in the Nairobi direction.

In Figure 3 the mean length of cracking in m / m 2 measured on the test points is plotted against time.

Sections which had mean cracking of less than 0.2 m/m 2 in 1981 are listed separately in the figure.

6 . 4 . 3 S u r f a c e r o u g h n e s s . The sections were not long enough to permit the use of a towed fifth wheel

bump integrator 17 on individual sections. An indication of the mean overall pavement roughness measured

in the four wheelpaths o f each site before and after overlaying for sites 1 to 7 is given in Table 20.

6.5 Ageing of the bitumen in the overlays

Samples of some o f the overlays have been taken periodically to measure changes in the hardness of

the bi tumens in the mixes. Samples f rom a given section have been taken from positions adjacent to each

other over a period of t ime and the bi tumens recovered using standard test methods 13,18.

Results o f penetration (at 25°C) and ring and ball tests are summarised in Figures 4, 5 and 6. The

results show that with the exception of the bi tumen rich mixes on site 2 which have not hardened

appreciably the bitumen in the overlays has hardened considerably. The rate of hardening is similar in

mixes which did and did not comply with aggregate grading specifications. The data shown in Figure 5

indicate that the bitumens were f rom a common source.

Comparisons are shown in Figure 6 of the penetrations of bitumens recovered from the top and

b o t t o m layers of two course overlays taken at various ages from all o f the sites. It can be seen that the

rate of hardening of the bi tumen in the bo t t om layers is only slightly less than in the top layer.

Bitumens recovered from the top 20 to 35 m m of samples taken in 1981 had very similar penetration

values to the full depth top layer samples taken in 1980.

The two most important results to come from this part o f the study are that firstly the bitumen in the

standard mixes hardened very quickly in contrast to the bitumen in the rich mixes and secondly the rich

mixes have remained uncracked despite carrying the heaviest traffic loading. Samples of bitumen recovered

in 1981 had penetrations significantly higher than 50 at 25°C. Additional work is needed to determine the

variability in rate of hardening within these mixes.

8

6.6 Deflection measurements

Transient deflection measurements were repeated on the overlays at the same test point locations as

were used before overlaying. Several surveys were made on the overlays during the first years of service,

the only exception being site 4 which could only be tested at six monthly intervals for 2 years and annually

thereafter.

Temperature-deflection tests were carried out so that, where necessary, deflections could be corrected

to standard deflections associated with a temperature of 35°C measured at a depth o f 40 m m below the

road surface. Details of the test method and typical results for temperature-deflection tests have been

given elsewhere 11.

6.6.1 Strengthening effect of the overlays. An indication of the strengthening effect o f an overlay

can be obtained by comparing the deflections measured on a test point before and after overlaying.

Ideally, deflection, surface condition and traffic loading surveys would be made on a road f rom the

time of construction. This historical data would help to determine the effects o f rainfall on pavement

strength and provide an indication of the rate, as well as the level, o f deterioration. The rate of deterioration

is important because a pavement which has deteriorated slowly may be basically stronger than one which

has deteriorated more rapidly, even though the levels of deterioration at a given t ime may be similar.

In the present study deflection surveys could only be made during a relatively short period of t ime

before the overlays were constructed. The exception was site 4 where surveys were made during a three year

period before overlaying.

Typical examples of the effect of rainfall upon pavement strength, as indicated by deflection

measurements have been given elsewhere 10,19. It is important to correct deflections for climatic effects

when comparing deflection surveys made at different times or when designing overlay thicknesses.

The results of deflection surveys carried out during periods which had similar ra infaUpat tems, have

been used to determine the reductions in deflection due to overlaying.

It was not possible to use this procedure for site 4 and corrections for the effects of rainfall have been

based upon the evidence obtained from tests on the 'control" section and from a similar 1 km long test

site located immediately adjacent to the overlay site 19. These corrections have been applied uniformly

to each wheelpath, but the actual effect of rainfall on pavement strength will vary, depending upon soil

properties, drainage and pavement condition.

The mean reductions in deflections obtained for complete sections of twenty test points are indicated

in Figure 7 where the mean deflection before overlaying is shown next to each symbol. The superimposed

lines were calculate~d from a multiple linear regression analysis, which gave a correlation coefficient (r 2) of

0.93, the equation for the relationship being

D A = 3.6 + 0.818 D B - 0.0027 DBT

where D A = deflection after overlaying

D B = deflection before overlaying

T = overlay thickness 9

Attempts to fit curved relationships resulted in only very minor deviations from the straight lines

shown in Figure 7.

The results for site 1 have not been included in the figure because the poorly graded mixes on this

site gave approximately 12 per cent less reduction in deflection than the mixes on the other sites.

Whilst there is considerable scatter in the results, the dense bitumen macadam base courses 7 laid on

sites 2, 3 and 4 gave approximately 6 per cent less reduction in deflection than the other mixes. These

results have also been omitted from Figure 7.

The curves have been extrapolated in Figure 8 and are recommended for densely graded bituminous

overlays where it is desired to reduce pavement deflections to a given value.

6.7 Radius of curvature of deflected surfacing

It is generally accepted that the measurement of longitudinal radius of curvature together with

deflection provides a better indication of the structural condition of a pavement than the measurement of

deflection alone 20'21. On a surface-dressed roadbase supported directly on the subgrade the radius of

curvature is sensitive to changes in the modulus of the roadbase, whilst deflection is sensitive to changes in

moduli of both the roadbase and subgrade. The product of radius of curvature and deflection (Rd) is

proportional to the modular ratio of the roadbase and subgrade.

Whilst there are considerable differences between the simple model on which these conclusions are

based and the actual trial sections on which there were considerable variations in thickness, quality and

condition of the pavement layers, combined measurements of radius of curvature and deflections would

be expected to give a better representation of pavement strength than would deflection alone.

Radius o f curvature should be dependent upon the modulus of the overlay and would be expected to

give a good indication o f the tensile strains induced in the overlays 22. This in turn should provide some

correlation with the propagation of cracks in a given overlay.

Reliable measurements of radii of curvature were obtained on the sections early in 1979 and again in

1980. Deflection beams were fitted with linear variable differential transformers (LVDTs) in place of the

dial gauges and their output was fed into a chart recorder driven by a pulse generator so that an influence

line was produced from which a measure of radius of curvature could be obtained. The radius of curvature

obtained in this manner is not an absolute one, it is based upon the difference between the maximum

deflection and the deflection obtained when the loading wheels are 125 mm away from the position of

maximum deflection during the transient deflection test where:

L 2 Radius of curvature = _ _ (m)

2d

L = 125 mm

d = differential deflection (x 10 -2 ram)

6.7.1 Deflection-curvature relationships. Regression lines for values of radius of curvature against

deflection for the verge side wheelpaths of the 'control" sections which were not overlaid are shown in

Figure 9. Values o f the regression coefficient r 2 were in the range 0.67 to 0.90, except for site 6, where a

small range of values was recorded. 10

It can be seen from Figure 9 that a wide range of pavement strengths, as indicated by the range of

values of deflection and radius of curvature, can occur within 100 metre long sections. There are also

considerable differences in the deflection-curvature relationships of some of the sites.

An indication of the changes in the deflection-curvature relationships brought about by overlaying

are given in Figures 10 to 12. These figures show for each site a comparison between measurements made

in 1980 on the control section and on a section which, before being overlaid, had deflection characteristics

similar to those of the control section at that time.

Regression analyses of deflections against radius of curvature for all overlays gave correlation

coefficients of r 2 of greater than 0.5 in 85 per cent of cases. The remaining correlations all applied to

cases where a small range of values was measured.

The .results show that for a given overlay, deflection and radius of curvature correlate well enough

to expect that either parameter might correlate With pavement performance and that the lack of curvature

measurements before 1979 should not seriously affect any deflection-performance assessments.

6.7.2 Effect of temperature on deflection-curvature relationships. The effect o f surfacing

temperature on deflections and curvature are indicated in Figure 13 and mean values of deflection,

curvature and their product Rd, for a wheelpath of a section, are shown plotted against the temperature

measured at a depth of 40 mm. These results were obtained from two surveys carried out in 1980 On

consecutive days at different surfacing temperatures.

Deflections do not decrease with increasing temperature but the value of Rd does, indicating that

the radius of curvature is more sensitive to temperature change than is deflection. It can be seen in

Figure 13 that large changes in Rd values can occur, implying large changes in the modular ratio between

the upper and lower (including the subgrade) pavement layers.

From the limited results available it appears that there might be a relationship between the rate of

change of Rd with temperature and the magnitude of Rd. However the gradients o f the Rd-temperature

relationships for overlays thicker than 50 mm are similar up to Rd values of 130 irrespective of

thickness. The exception is the example shown for site 2 which shows that the Rd value for this

bitumen-rich ~ is very sensitive to temperature change.

Radius of curvature appears to give a better indication than deflection of the differences between the

properties of the bitumen-rich mix and the more conventional mixes. However the occurrence of plastic

flow during transient deflection tests 11 gave a similar indication of differences.

7. PERFORMANCE OF THE O V E R L A Y S

The pavements were generally in good structural condition before overlaying in that only a moderate

amount of rutting had occurred. Part of site 2 exhibited the largest mean rut depths but these ruts had

developed over a long period of time, indicating a basically sound pavement. Some failures had occurred

on site 3 as referred to in Section 4.2.2 of this report.

11

It has been suggested that a roughness value of 3,750 mm/km, measured with a towed bump

integrator, is the level of roughness at which overlaying or reconstruction should be carried out on trunk

roads in Kenya 23. Sites 2, 4 and 5 had levels of roughness within 600 mm[km of this value. There are

likely to be many sections of the paved road network in Kenya where the main requirement will be to

reduce the roughness of the road surface.

It has been shown elsewhere 19 that rougtmess can be more indicative of surfacing failure and the

quality of patching work than of serious structural deterioration. It is more appropriate to overlay a road

which only requires the restoration o f riding quality than it i s t o reconstruct it.

7.1 General observations

7.1.1 Site 1. Traffic restrictions imposed because of pavement failure elsewhere along the road on

which the overlays were situated resulted in little or no traffic being carried by the overlays for a period

o f approximately two years from 1977 to 1979.

The particle size distribution of the asphaltic concretes on this site did not conform to the specifications

(see Section 5.4) and for this reason the performances of the overlays are not directly comparable with

those of the asphaltic concrete overlays on the other sites.

The overlay of dense bi tumen-macadam basecourse is extensively cracked. This 75 mm thick

overlay was laid in two layers on separate days, without a tack coat and a very poor bond was achieved

between the two layers. This lack of bond allowed excessive flexure of the top layer of the overlay to

occur under traffic, causing it to crack. There is a good bond between the original road surface and the

bo t tom layer o f the overlay which, in the few locations investigated, is in good condition. This overlay

would have performed much more satisfactorily if it had been placed in one layer or at least had a thin

tack coat applied between layers.

7 . 1 . 2 S i tes 2 a n d 3. The two hot-rolled asphalt overlays, which were expected to have long lives, have

cracked quite extensively. These mixes derive their strength from the sand-Idler-bitumen matrix and

because a softer bi tumen than is recommended for UK conditions was used it was thought that a higher

filler to binder ratio should be used to resist deformation under traffic at high pavement temperatures.

A f'fller to binder ratio of approximately 1.7:1 in the mixes appears to have reduced their resistance to

cracking.

On all the sites where dense bi tumen macadam basecourse material has been laid some segregation has

occurred. This has resulted in cracking in these overlays because of the low strength of accumulations of

coarse aggregates with normal bi tumen contents.

7 . 1 . 3 S i t e 4 . The filler to binder ratio for the hot rolled asphalts was approximately 1.15:1. Only one

test point in the two sections with this more flexible mix has cracking at the surface after 3 years service.

There is also no significant deformation on these sections.

7 . 1 . 4 S i t e 5. Cracks formed very soon after construction in asphaltic concretes of up to 150 mm

thickness. The appearance o f the cracks suggested reflection cracking from the soil-cement roadbase but

the cracks developed f rom the surface of the overlays and inspection of the surface of the~roadbase under

12

the cracked overlay failed to identify any cracks which might cause this behaviour; in fact sound blocks

of roadbase were cut from beneath cracked surfacing. A factor which may have contributed to the

development of these cracks was the generally higher voids content o f the overlay materials, as stated

in Section 6.1 of this report. The higher voids content may well have resulted in a reduction in the tensile

strength of the material 24.

During cycles of thermally induced movement the asphalt expanded against material which had

been blown into the cracks from the shoulders of the road, resulting in attrition at the edges of the cracks.

Where the cracks have propagated down to the old surface dressing, movement of the edges of the

asphalt has caused the surface dressing to crack. Two areas in section 2 now require patching as a result of

water penetrating through cracks formed in this way. Without the overlay the original surface dressing

would not have cracked at this time. To prevent the cracks widening and water penetrating to the

roadbase an application of a seal soon after construction of the overlay would have been necessary.

7 .1 .5 Si tes 6 and 7. The traffic loadings on these sites were reduced to a very low value with the cessation

of traffic to and from Zambia and Tanzania approximately one and three years respectively after

construction of the overlays. Traffic on site 6 is almost solely generated by the transportation of sugar

cane to the factory at Ramisi. There is negligible traffic on site 7.

7 .1 .6 Si te 8. It was not possible to manufacture continuously graded mixes to specification with the

materials available at this site. The hot rolled asphalt was by comparison, very easy to manufacture and

control. The dense bitumen macadams laid on sites 2, 3 and 4 were very similar but a much greater

variation in aggregate gradings and a greater variation in bitumen content occurred on site 8. In general

the mix has a higher percentage of filler and of aggregate finer than 300/~m, and approximately 0.5 per

cent more bitumen. In the slow lane the mixes have a 'rich' appearance and the DBM mixes have not

suffered the type of cracking caused by segregation which has occurred on the other sites. In the lightly

trafficked fast lane the mixes have a 'drier' appearance and some cracking has occurred, particularly in the

hot rolled asphalt.

7.2 Relationships between measured parameters and cracking

Twenty-eight of the overlay sections were uncracked or had mean cracking of less than 0.2 m / m 2

in early 1981. For the remaining forty overlay sections no correlation had been found between deflection,

radius of curvature or traffic loading and the development of cracking.

Examples are given in Figures 14 and 15 of comparisons between traffic (esa) carried before test

points reached a nominal 'critical' condition (defined as 0.2 m/m 2 of cracking wider than 0.25 ram) and

the predicted number of equivalent standard axles carried to a critical condition obtained from design

charts used in the United Kingdom 4. These design charts relate overlay thickness and deflection before

overlay to the traffic carrying capability of the overlay for different types of pavement.

It can be seen that no pattern is discernible in Figures 14 and 15. This is not surprising because of the

way in which the overlays in Kenya have deteriorated, where cracks have propagated from the surface of

the overlays at varying rates even at individual test points. This is quite different from the normal form of

failure in Britian, which is deformation. This behaviour has made it impossible to define a meaningful

'critical condition' for the Kenyan overlays.

13

There has been no consistent relationship between traffic loading and the development of cracking.

The data in Table 19 shows that on sites 1, 2 and 5 more cracking has generally developed in the more

heavily trafficked lane whilst for sites 3, 4 and 8 the reverse tends to be true.

On site 1 there has been a tendency for linear cracking to develop more rapidly as overlay thickness

decreases. It has taken longer for cracks to penetrate through the full depth of the thicker overlays. On

site 5 where cracks have formed the maximum rate of penetration for eight of the overlays has been of the

order of 22 m m per year, but this particular rate of penetration is unique to this site.

8. DISCUSSION OF RESULTS

The outstanding features of the behaviour of the overlays have been the initiation of cracking from the

surface, the hardening of the binder through the depth of the overlays and the importance of the properties

of the overlay materials.

Cracking has occurred very early in the lives of overlays up to 160 mm in thickness. Similar

behaviour has been observed elsewhere 25 but whilst in this latter case thick overlays with lower deflections

cracked first the development of cracking in the overlays in the Kenya study has been more random. The

occurrence of cracks which have developed from or near to the top of bituminous surfacings has recently

been reported in the United Kingdom 26.

No satisfactory explanation for this behaviour can be given at present. The effect which temperature

may be having upon the overlays is indicated by the way in which crack widths change considerably during

diurnal cycles. It is possible that the application of particular wheel loads and configurations at certain

pavement temperatures coupled with hardening o f the binder at the surface can be more significant in

causing this cracking than the cumulative equivalent standard axles carried by the pavement.

The importance of the properties of the overlay materials has been indicated in several examples.

Hot rolled asphalts laid on sites 2 and 3 with 'high' filler to binder ratios cracked at a very early age.

Similar materials but with lower filler to binder ratios laid on sites 4, 6 and the slow lane of site 8 have

been much more resistent to cracking. Dense bitumen macadam basecourses laid on sites 1, 2, 3 and 4

have not been very durable, whilst the denser, richer mixes on site 8 have performed well.

A most important indication is given by the bitumen-rich mix on site 2. This mix has been very

resistant to cracking and there is a marked tendency for any fine cracks to be full of bitumen. Significantly

this is the only mix which has not suffered hardening of the bitumen binder.

It appears that low air voids are required to reduce bitumen hardening, but as in the case on site 2

the b i tumen film thickness around the aggregate will increase as air voids are filled with bitumen. The

thicker bi tumen film should make the mix more tolerant of induced tensile strains and reduce the

likelihood of cracking.

A mix of the type used on site 2 offers good resistance to cracking, however this mix is at the

upper limit for bi tumen content beyond which severe rutting could be expected. The tolerances required

to reproduce this mix would be too restrictive for normal production methods.

14

9. RECOMMENDATIONS FOR O V E R L A Y DESIGN IN K E N Y A

The mechanism by which the cracking is initiated in the overlays is not fully understood and this prevents

the recommendation of a simple,reliable design method.

Very few conventional overlays can be expected to carry more traffic than that predicted f rom design

curves used in Britain 8 before developing 0.2 m/m 2 of cracking wider than 0.25 mm. Significant areas

of such an overlay can be expected to crack at an early stage and require resealing.

There has been little difference in the performances of the various types of asphaltic concretes laid

in this study provided they were within specification. The hot rolled asphalts which had frier to binder

ratios close to unity have performed well and are recommended for further consideration in areas, such as

the coastal strip, where sand of the correct grading is available and good quality aggregate is scarce.

Dense bitumen macadam basecourse mixes have not been as durable as the denser materials, but

have nonetheless given over six years of service on heavily trafficked roads and are still serviceable. These

mixes, if surface dressed as part of the construction process, may offer a cost-effective method of overlaying.

The dense bitumen macadam wearing course mixes can be expected to give better performance.

Examination of Tables 6 to 9 and of data published elsewhere 19,27,28 will show the wide variations

which occur in pavement layer thicknesses and strengths. The indications are that to design overlays on

the basis of variations in in situ pavement properties will not be precise in most developing countries

because the amount of work and expense which would be involved is prohibitive. Sufficient testing must

however be made to provide the engineer with a good indication of pavement strength and yariability.

The maximum use of surface condition assessments (particularly of historical data as discussed in Section 6.6.1

of this report) backed up with deflection-curvature measurements and pavement investigation pits still

appears to be the only practical means of assessing the present condition of a pavement.

It will be seen that in Figure 8 a minimum overlay thickness of 25 m m has been indicated. Overlay

thickness should be regarded as being in addition to any regulation of the existing road surface.

It is recommended that for the immediate future deflection measurements and curves published in

Britain 4 (or others which give similar thickness design) are used to determine overlay thicknesses and that

provision is made to reseal where cracks occur.

Where asphaltic concrete is designed by the Marshall test method the emphasis should not be placed

on attaining high stabilities, which imply lower bitumen contents. Where the requirement for thick overlays

is indicated the use of an unbound crushed rock regulating course should be considered, particularly on

roads with soil-cement roadbases. This could be cost-effective and may also prevent reflection cracking

from the roadbase.

10. FUTURE W O R K

During future overlay programmes monitoring of changes in surface condition should be undertaken

together with coring on cracked areas to monitor deterioration. Further work to determine a more effective

combination of overlay materials appears to offer the best improvement in overlay design at the present time.

15

The design of a crack resistant overlay material should be attempted. Such a material could be a

'bi tumen-rich' gap or semi-gap graded asphalt into which precoated chippings could be rolled. A 25 mm

thick layer of this material laid on a more stable, cheaper mix would be to some extent self-limiting in

terms o f possible rutting and may prevent the initiation of cracking.

A 'rich' asphaltic concrete may also be a feasible alternative. The specification of a bitumen content

above the Marshall op t imum for this thin wearing course can be checked in relation to the sensitivity of

the Marshall properties to bitumen content. The behaviour of the mix during compaction trials will

provide a valuable assessment of the material. Compaction of this type of mix with steel-wheeled rollers is

likely to produce a very smooth finish. More use of pneumatic-tyred rollers should be seriously considered

in an a t tempt to acquire a coarse surface texture for anti-skid properties.

Lower Idler contents than those found in the asphaltic concrete overlays laid in the current trials

may result in more durable mixes.

11. CONCLUSIONS

. The overlays have been effective in providing good riding quality and have now remained serviceable

for up to seven years under heavy traffic despite cracking which appeared early in the lives of some

of the overlays.

. Cracking has been the predominant form of deterioration of the overlays. Rutting has been

negligible even on or near areas which were heavily repaired before overlaying.

3. Cracking of asphalt overlays up to 160 mm in thickness has occurred after very short service lives.

4 . A feature of the cracking has been its development from the surface down through the overlays at

varying rates even in a nominally uniform overlay.

. Temperature changes can cause crack widths to vary considerably during a day. This behaviour can

lead to considerable attrition of the edges of the cracks.

. Crack propagation down through the thinner overlays (less than 50 mm) has been quite rapid,

occurring within 2 years. On site 5 cracking in the overlay has eventually broken through the

original surface dressing which would have remained intact had the overlay been omitted.

7. No correlations have been found between deflection measurements (and by inference radius of

curvature), overlay thickness, traffic loading and cracking in the overlays.

. Considerable hardening of the bitumen in the overlays occurred. Samples of nominally 80 -100

penetration grade bitumens taken from the heater tanks and recovered after mixing had

penetrations of 70--83 and 43--60 respectively.

Bitumen recovered from the overlays five years after construction had penetrations in the

range 16 -25 . Hardening occurred throughout the depth of two layer overlays of 100 mm thickness.

The bitumen-rich mix laid on site 2 had not hardened significantly after 6 years in service.

1 6

. Care must be exercised to prevent shear cracks forming in the overlay during compaction with steel-

wheeled rollers. Pneumatic-tyred rollers avoided this type of cracking and provided a very good

surface texture.

Compaction trials at the beginning of a surfacing contract are essential to ensure that mix properties

are correct and that the compaction method is satisfactory.

10. The overlays were effective in reducing deflections despite high pavement temperatures. The dense

asphaltic concretes and hot roiled asphalts were the most effective mixes in reducing deflections.

11. Conventional overlays whose thicknesses have been derived from deflection-overlay thickness design

charts similar to those used in the United Kingdom should give satisfactory performance provided

that resealing is carried out if cracks occur at an early stage. It is important that resealing is carried

out before cracks have penetrated to the full depth of the overlay and before crack widening due to

attrition has occurred.

12. Further work is required to design an overlay system and materials which resist cracking at the surface.

12. ACKNOWLEDGEMENTS

The authors are indebted to the Government of the Republic of Kenya for allowing and organising the

construction of the overlay experiments.

Particular thanks are due to Mr S J Mbugua, Permanent Secretary, Ministry of Transport and

Communications and his staff especially those in the Roads and Materials Branches, for the help and

cooperation given during this study.

The work described in this report forms part of the research programme of the Overseas Unit

(Unit Head: Mr J N Bulman) of the Transport and Road Research Laboratory.

13. REFERENCES

. HIGHWAY RESEARCH BOARD. Design of overlays and pavement rehabilitation. Highway

Research Record 300. Washington, I969.

. HIGHWAY RESEARCH BOARD. Evaluation of pavements by deflection studies for maintenance

purposes. Highway Research Record 129. Washington, 1966.

. CANADIAN GOOD ROADS ASSOCIATION. A guide to the structural design of flexible and rigid

pavements in Canada. Canadian Good Roads Association Pavement Design and Evaluation Committee.

Ottawa, 1965 (Canadian Good Roads Association).

. KENNEDY, C K and N W LISTER. Prediction of pavement performance and the design of overlays.

Department of the Environment Department of Transport, TRRL Report LR 833. Crowthorne,

1978 (Transport and Road Research Laboratory).

17

.

.

.

.

.

10.

11.

12.

13.

14.

15.

16.

17.

ORGANISATION FOR ECONOMIC COOPERATION AND DEVELOPMENT. Road Strengthening.

A report prepared by an OECD road research group. Paris, 1976 (Organisation for Economic

Cooperation and Development).

BRITISH STANDARDS INSTITUTION. Rolled asphalt (hot process) for roads and other paved

areas. British Standard No. 594 : 1973. London, 1973 (British Standards Institution).

BRITISH STANDARDS INSTITUTION. Coated macadam for roads and other paved areas.

British Standard No. 4987 : 1973. London, 1973 (British Standards Institution).

LISTER, N W. Deflection criteria for flexible pavements and the design of overlays. Proc. 3rd

International Conference on the Structural Design of Asphalt Pavements, Vol. 1, 1972 (University

of Michigan, Ann Arbor), pp 1206--26.

BRITISH STANDARDS INSTITUTION. Methods of test for soils for civil engineering purposes.

British Standard No. 1377 : 1975. London, 1975 (British Standards Institution).

SMITH, H R and C R JONES. Measurement of pavement deflections in tropical and sub-tropical

climates. Department of the Environment Department of Transport, TRRL Report LR 935.

Crowthorne, 1980 (Transport and Road Research Laboratory).

JONES, C R and H R SMITH. Deflection-temperature relationships for bituminous road surfacings

in Kenya. Department of the Environment Department of Transport, TRRL Report LR 936.

Crowthorne, 1980 (Transport and Road Research Laboratory).

ROAD RESEARCH LABORATORY. Rapid methods of analysis for bituminous road materials.

Ministry of Transport, RoadNote 10. Second Edition. London, 1967 (H M Stationery Office).

BRITISH STANDARDS INSTITUTION. Specification for sampling and examination of bituminous

mixtures for roads and other paved areas. Part 2. Testing. British Standard No. 598 : Part 2 : 1974.

London, 1974 (British Standards Institution).

BRITISH STANDARDS INSTITUTION. Methods for sampling and testing of mineral aggregates,

sands and Idlers. Part 2. Physical properties. British Standard No. 812 : Part 2 : 1975.

London, 1975 (British Standards Institution).

POTOCKI, F P. A portable wheel-weighing unit and data recorder. Department of the Environment, RRL Report LR 391. Crowthorne, 1971 (Road Research Laboratory).

HIGHWAY RESEARCH BOARD. The AASHO Road Test Report 5. Pavement Research. Highway Research Board Special Report 61E. Washington DC, 1962 (National Research Council).

JORDAN, P G and J C YOUNG. Developments in the calibration and use of the Bump-integrator

for ride assessment. Department o f the Environment Department of Transport, TRRL Report SR 604. Crowthorne, 1980 (Transport and Road Research Laboratory).

1 8

18.

19.

20.

21.

22.

23.

4.

25.

26.

27.

28.

INSTITUTE OF PETROLEUM. IP standards for petroleum and its products. Part 1 : Methods of

analysis and testing. London, 1978 (Institute of Petroleum), 37th Edition.

SMITH, H R, T E JONES and C R JONES. Performance of sections of the Nairobi to Mombasa

road in Kenya. Department of the Environment Department of Transport, TRRL Report LR 886.

Crowthorne, 1980 (Transport and Road Research Laboratory).

LEGER, Ph and P AUTRET. The use of deflection measurements for the structural design and

supervision of pavements. Proc. Third Int. Conf. on the Structural Design of Asphalt Pavements.

London, 1972. Ann Arbor, 1972 (University of Michigan).

NATIONAL INSTITUTE FOR ROAD RESEARCH. The measurement of deflection and curvature

of road surfaces. CSIR Manual K16, Pretoria, South Africa, 1970.

HUANG, Y H. Deflection and curvature as criteria for flexible pavement design and evaluation.

Paper presented at 50th Annual Meeting. Highway Research Board. Washington DC, 1971.

HODGES, J W, J ROLT and T E JONES. The Kenya Road Transport Cost Study: research on road

deterioration. Department of the Environment, TRRL Report LR 673. Crowthorne, 1975 (Transport

and Road Research Laboratory).

MONISMITH, C L. Design considerations for asphalt pavements. Proc. of the 1st Conf. on Asphalt

Pavements for South Africa. July 28 - August 1st 1969. CAPSA, Durban.

GRANT, M C, C P MARAIS and D GUYS. An investigation of an asphalt overlay on a lightly

trafficked road showing premature cracking. Proc. Third Conf. on Asphalt Pavements for Southern

Africa University of Natal, Durban, South Africa 1979.

THROWER, E N. A parametric study of a fatigue prediction model for bituminous road

pavements. Department of the Environment Department of Transport, TRRL Report LR 892.

Crowthorne, 1979 (Transport and Road Research Laboratory).

JONES, C R and H R SMITH. Tensile and compressive strength Of some stabilised road bases in

Kenya. Department of the Environment Department of Transport, TRRL Report SR 623.

Crowthorne, 1980 (Transport and Road Research Laboratory).

BULMAN, J N and H R SMITH. Pavement performance and deflection studies on Malaysian roads.

Department of the Environment Department of Transport, TRRL Report LR 795. Crowthorne,

1977 (Transport and Road Research Laboratory).

19

TABLE 1

Locations of overlay sites

Site No. Road No. Location Date overlaid

1 A104 90 km North of Nakuru at Timboroa September 1973

December 1974 to 2 A109 2 km North of C97-A109 Junction

March 1975

3 A109 2 km South of C97-A109 Junction January - February 1975

4 A109 132 km South of Nairobi near Emali January 1977

5 A109 356 krn South o f Nairobi at Maungu September 1973 to January 1974

6 A 14 62 km South of Mombasa at Ramisi February 1974

7 A14 75 km South of Mombasa October 1973

8 A 2 Thika-bound carriageway at Ruaraka August 1975

TABLE 2

Summary of rainfall data

Annual rainfall at the experimental sites t

Site No. mm

i 1973 I 1974 1975 1976 1977 1978 1979 1980

1 1240 1450 1940 650 1930 1110 1990" 1370

790 2 and 3 480 740 520 350 820 770 490*

4 - 580 340 470 820 1100 1080 410

5 307 360 120 40 340 530 240 260

6 1500 890 1000 850 1470 1710 3100 845*

7 930 920 840 1010 1350 1660 1720 873*

8 600 880 660 660 1470 1080 960 900

t Supplied by Kenya Meteorological Department * Excluding rainfall in December 1980

2 0

TABLE 3

Specification for modified hot rolled asphalt 6

in.

BS sieve size

l n l l l

1 25.4

¾ 19.1

½ 12.7

3/16 4.8

No. 7 2.4

No. 25 0.6

No. 72 0.2

No. 200 0.075

Coarse Aggregate

Grading of aggregates per cent passing sieve size

Fine Aggregate

100

85-100

0-60

100

95-100

75-100

15-60

0-5

Bitumen content

Mortar only Stability (kN) Flow (mm)

Coarse aggregate content (per cent)

Voids in mix (per cent)

Voids filled with bitumen (per cent)

From Marshall tests on mortar only

3.6 minimum 2.5--4

45-50 (see Table 16)

4-10

60 minimum

Filler to bitumen ratio 1.3:1

21

TABLE 4

Specifications for asphaltic concretes and dense bitumen macadam

Nominal maximum size stone and per cent passing sieve

Kenya Ministry of Transport Dense bitumen- and Communications macadam basecourse 7

Asphaltic concrete

BS 1621: BS 4987: BS sieve size Current (1973) Old (pre 1973) 1961" 1973

specification specification Table 5 Table 25

in. m m 20 mm 13 mm 20 mm 13 mm 19 mm 20 mm

28 1 25.4

20 ¾ 19.1

14 ½ 12.7

10 3/8 9.5

¼ 6.4 6.3

3/16 4.8 3.35

1/8 3.2 No. 7 2.4 No. 14 1.2 No. 36 0.43

0.30 No. 200 0.075

100

8 5 - 1 0 0

75--90

65--80

4 5 - 6 5

3 5 - 5 5 30---45 2 0 - 3 3

100

8 5 - 1 0 0

7 5 - 9 5

50--70

40--60 33 - 5 0 21--35

100

85--100

75--90

65--80

45--65

3 0 - 5 0

10--22

100

85 -100

75-95

5 0 - 7 0

3 5 - 5 5

12-23

100

9 5 - 1 0 0

7 0 - 9 0

55 -75 40--60

25--40

15 -30

100

95 -100

65 -85

52-72

39-55

32-46

7 -21 2 - 8 4 - 1 0 4--10 4--8 4 - 8 3---6

Bitumen content Designed by Marshall method 4 . 4 - 4.6 t - (per cent) (50 blow test) 5.4 5.4

Voids in mix 3 - 5

(per cent)

Voids in mineral aggregate 19 maximum (per cent)

Voids Idled with bitumen 7 5 - 8 2 (per cent)

Stability (kN) 3.6 minimum

Flow (mm) 2--4

* Laid on site 1 only.

t 0.5 per cent higher than in BS 4987 : 1973

22

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I I I I

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TABLE 6

Details of existing roads

Site No.

2

3

4

Surfacing Roadbase Sub-base Fill material

Type

Multiple surface dressing

Multiple surface dressing

Asphaltic concrete

Multiple surface dressing

Multiple surface dressing

Asphaltic concrete

Asphaltic concrete

Multiple surface dressing

Thickness Material

LilTle

20 stabilised { gravel

Crushed 26

rock

27 Quartzitic gravel

Cement 27 stabilised

gravel

Cement 37 stabilised

gravel

Cement 28 stabilised

gravel

Cement 27 stabilised

gravel

Cement 17 stabilised

gravel

Thickness (ram)

Mean] Range

121 113-128

185 160-220

132 80--155

150 130-16G

132 110-163

124 109-137

116 100-127

138 120-159

Material

LilTle

stabilised gravel

Gravel

Cement stabilised gravel

Gravel

Comp- ! acted subgrade

Gravel

Gravel

Gravel

Thickness (mm)

Mean I Range

121 93--193

212 153--280

137 107-180

109 80--170

131 83--180

125 111-160

134 103-177

i99 135-225

Material

Gravel 156

rhickness(mm)

Mean Range

50-223

"~ The pavement was constructed on top of an existing road, details of which are given below.

Site No.

Surfacing

Thickness Type

1TI1TI

Surface [ _ dressing

i

Roadbase

Material

Cement stabilised gravel

Thickness (mm)

Mean Range

132 90- i87

Sub-base

Material

Gravel

Thickness (mm)

Mean Range

135 87-200

Fill material

Thickness(mm) Material

Mean Range

Clay 156 47-500 subgrade

2 4

. .1 o

I ~o

°~ o ~ o-,

I t-:

~ ~ ÷ ~ I t ~ + . - I

I I I I I I

+ 0 0

I

I I I I

.9.

¢-~ J O~ , -~ t~. t¢~ , - , O . ,..-,

o ~

÷ + & + 0 0 & ÷ 0

83

0 + -I- .4- + ~' 0 0 0 0 ~ o t o o o ,-., , ~ I I I

- I I I I

e ~

o-, o o , , o-, o-, o o o

Z

I

E

u, _~ o ,

I

O~

°~ o ~

I'

0 .4-

¢I1 ~ +

Z

26

TABLE 8

In situ properties of subgrades

Site No. Number o f

tests

CBR (per cent)

Mean Range

Moisture content (per cent)

Mean Range

1 42 29 21 --47 31.1 26 .7 -35 .3

2 48 10 5 - 1 9 18.3 11.0-26.1

3 37 7 2 - 1 2 30.2 23 .1-35.4

11 --31 19.8 18.5-22.6

17 -85 7.8 4 .2-11.8

15--94 6.4 3 .5-10.8

6 - 6 4 5.1 4 .2-5 .8

8 - 5 4 25.8 19.5-28.4

4 29 20

5 69 47

6 47 42

7 24 39

8 14 22

TABLE 9

In situ subgrade strengths measured with a penetrometer

Site No. Number of

tests

CBR at subgrade surface (per cent)

Mean Standard deviation

Range

CBR at 0.5m below subgrade surface (per cent)

Mean Standard deviation

Range

1 17 20 5.1 15-35 15 8.2 6 - 3 2

2 27 8 3.7 4 - 1 6 9 3.9 3 - 1 5

3 21 6 2.2 4 - 1 1 7 2.8 3 - 1 7

4 20 19 11 -35 9 4.2 5 -25 5.4

39.9 5 11 59 22 -155 - - -

6 -12 34 17.3 10--66 27 17.4 10--66

7 8 27 5.1 2 1 - 3 5 48 7.5 39--64

8 5 22 7.2 13-31 8 2.5 6 - 1 2

2 6

n n

b

O

b

r ~

• i v ~ ¢~'1 , ~ ~ ~ - ~ ~t" C~ ~ o o ~t '

b ~

N

¢.)

¢..)

~ )

z z

o

¢)

27

T A B L E 1 1

Condi t ion of road surfaces before overlaying

Site No. Condi t ion o f road surface before overlaying

1 6 pa tches o f 26m 2 to ta l area. Negligible cracking.

2 No patching, negligible cracking, some large deformat ions .

Considerable areas o f asphalt ic concre te surfacing, roadbase and sub-base layers 3

rep laced (see Table 12). Widespread fatigue cracking in verge side wheelpaths.

Surface dressing severely cracked before par t ia l resealing in 1974. Section 7, ch 85 4 to the end o f Sec t ion 10 was no t resealed, the old surfacing was extensively cracked.

Loose mate r ia l was replaced w i th asphalt ic concrete before overlaying.

5 Uncracked b i tumen- r i ch surface dressing.

6 Minor cracking. Sect ions 2 and 3 had areas o f 6 and 5m 2 respectively o f fine cracking.

7 Fine cracks. Mean cracking app rox ima te ly 0.3 m / m 2 in the wheelpaths.

8 Extensive fine cracking in the slow lane. One or two small areas o f crazed surfac ingin each sec t ion rep laced wi th b i tuminous mater ia l before overlaying.

TABLE 12

Pre-overlay patching on site 3

Area re ins ta ted (per cent)

Sec t ion No. Roadbase Surfacing

Towards M o m b a s a Towards Nairobi Towards Mombasa Towards Nairobi

1 - 4 . 6 - 2.0

2 - 3.7 0.1 2.7

3 -- 1.2 -- --

4 -- -- -- 2.6

5 -- -- -- 0.2

6 -- 1.5 1.8 0.9

7 -- 7.0 3.7 1.4

8 -- -- 8.3 2.0

9 -- -- 3.2 2.0

10 - - 1 . 0 6.5 1.7

1 1 " - - - - 6 . 5 2 . 9

* Not over la id

2 8

TABLE 13

Temperatures specified for manufacture and rolling of bituminous materials

Material

Asphaltic concrete 80-100 penetration bitumen

Dense bitumen macadam 80-100 penetration bitumen

Modified hot rolled asphalt 60-70 penetration bitumen

Mixing

Aggregate Bmder

Mill. Max.

140 160

120 140

150 175

Temperature °C

Delivery

Min. Max.

120 140

115 135

125 150

Mill. Max.

140 150

130 160

150 165

Rolling

Min.

85

80

90

29

TABLE 14

Summary of analysis results

Site No.

2

4

6

Overlay material

Current (1973) 20 m m AC Current (1973) 13 m m AC Old (pre 1973) 20 mm AC DBM BS 1621 1961

Current (1973) 20 m m AC Current (1973) 13 m m AC Old (pre 1973) 20 m m AC DBM BS 4987 1973

Current (1973) 20 m m AC Current (1973) 13 mm AC Old (pre 1973) 20 m m AC DBM BS 4987 1973

Current (1973) 20 m m AC Current (1973) 13 m m AC Old (pre 1973) 20 m m AC DBM BS 4987 1973

Current (1973) 13 m m AC Old (pre 1973) 13 m m AC Bitumen -- Gravel

Current (1973) 20 m m AC Current (1973) 13 m m AC Old (pre 1973) 20 m m AC DBM BS 4987 1973

Specified bitumen content

(per cent)

6 . 0 ± 0 . 4 6 . 8 ± 0 . 4 5 . 6 ± 0 . 4 4 . 9 ± 0 . 5

5 . 9 ± 0 . 4 6 . 2 ± 0 . 4 6 . 2 ± 0 . 4 5 . 0 ± 0 . 4

5 . 9 ± 0 . 4 6 . 2 ± 0 . 4 6 . 2 ± 0 . 4 5 . 0 ± 0 . 4

5 . 5 ± 0 . 4 5 . 3 ± 0 . 4 5 . 0 ± 0 . 4 5 . 0 ± 0 . 4

6 . 9 ± 0 . 4 6 . 7 ± 0 . 4

6 . 0 ± 0 . 4 5 . 9 ± 0 . 4 5 . 8 ± 0 . 4 5 . 0 ± 0 . 4

7 Current (1973) 20 m m AC 6.0 + 0.4

8 DBM BS 4987 i973 4.9 + 0.5

Number of samples

Mean

14 6.0 5 6,7 4 5.5 4 4.7

23 5.6 4 6.1 4 6.1 4 4.9

24 5.9 4 6.0 4 5.9 4 5.0

25 5.4 5 5.2 6 4.9 6 4.8

35 7.3 5 6.2

18 6.7

16 6.0 2 2 4 5.5

Actual bitumen content (per cent)

Range

5.5--6.3 6 .4-7 .2 5.3-5.7 4.4-5.1

5.0--6.3 6.0--6.2 5,8--6.4 4 .9-5 .0

5.7-6.7 5.8-6.3 5.7--6.0 4 .6-5 .4

5.0-5.7 5.2-5.3 4.6-5.1 4 .7-5 .0

6 .0-8 .0 5.6-7.5 5 .4-8 .4

5.5--6.6 5.9, 5.9 5.9, 6.2 4 .8-5 .9

No samples taken at this site

6 5.4 [ 5 .1-5.7

30

TABLE 15

Summary of grading analyses for current (1973) asphaltic concretes

Current Mix type Current nominal 20 mm nominal

1 3 m m

Site No. ] 1 2 3 4 6 5

No. samples [ 13 23 24 26 15 35

BS sieve size (in) (mm)

1 25.4

¾ 19.1

½ 12.7

3/8 9.5

3/16 4.8

No. 7 2.4

No. I4 1.2

No. 36 0.43

No. 200 0.075

Percentage of aggregate passing sieve size

100

100

90 -96

80 -90

65 - 8 4

47 -59

32--40

16-22

4 - 7

100

95 -100

83 - 9 4

76 -86

58-71

4 6 - 5 8

39 -48

25 -33

6 - 1 3

100

9 3 - i 0 0

8 5 - 9 3

7 8 - 8 6

64 -71

52-61

3 9 - 4 6

2 2 - 3 0

7 -1 1

100

9 9 - 1 0 0

8 5 - 9 4

73 - 8 5

5 6 - 6 8

4 7 - 5 6

3 9 - 4 8

2 4 - 3 0

6 - 1 0

100

90--100

75--85

68--81

51--69

42--53

34--44

22--27

6--9

100

9 6 - 1 0 0

8 2 - 9 6

4 9 - 7 8

3 9 - 6 0

3 2 - 4 9

2 4 - 3 5

7 - 1 1

31

TABLE 16

Summary of analysis results for hot rolled asphalt

Site No. 2 3

Number of samples 4 5

Mix constituents

Coarse aggregate (per cent)

Filler (per cent)

Bitumen content (per cent)

Filler: Bitumen ratio

Actual Target

Mean Range

48 44.6 42.7--46.0

7.7 9.9 8.5--10.0

5.9 6.1 6 .0 -6 .2

1.3

Correctedt

Mean Range

48

9 . 6 8 .2 -10 .2

5.8 5 .7 -5 .9

1.7

Target

48

7.7

5.9

1.3

Actual Correctedt

Mean Range Mean Range

45.3 42.7---47.5 48 -

10.0 9 .1-10.3 9.7 8.8-10.0

6.1 6 .0-6 .3 5.8 5 .7-6 .0

- 1 . 7 -

Site No.

Number of samples

Mix constituents

Coarse aggregate (per cent)

Filler (per cent)

Bitumen content (per cent)

Filler:Bitumen ratio

4 (Section 2)

Actual Correctedt Target

Mean Range Mean Range

45 40.2 39.1--43.2 45 -

8.2 7.3 6 .9 -8 .3 6.8 6 .5 -7 .8

6.3 6.3 6 .1 -6 .4 5.8 5 .7 -5 .9

1 . 3 - - 1 . 2 - -

Target

45

8.2

6.3

1.3

4 (Section 3)

Actual

Mean Range Mean

43.3 41 .6-44 .0 45

6.2 5 .9-6 .6 6.1

5.9 5.8-6.1 5.7

- 1.1

Correctedt

Range

5.8-6.3

5 .6-6 .0

Site No. 6 8

5 4 Number of samples

Mix constituents

Coarse aggregate (per cent)

Filler (per cent)

Bitumen content (per cent)

Filler:Bitumen ratio

Target

50

7.7

5.9

1.3

Actual

Mean R a n ~

~ . 4 39.2--53.8

5.4 3 .5 -8 .9

5.5 4 .9 -5 .8

Correctedt Target

45

8.1

6.2

1.3

Actual

Mean Range

50

5.1 2 .6 -7 .9

5.1 4 . 6 - 6 . 0

1.0

Mean Range Mean

44.0 39.6-50.0 45

6.8 6 .2-7 .6 6.7

6.5 6.0-7.1 6.5

- 1 . 0

Correctedt

Range

6.3 --7.2

6.2~6.7

? In accordance wi thBS 5946.

3 2

TABLE 17

Rut depth measurements on site 2

Rut depth under a 2m long straightedge (ram)

Before overlay September 1974

Section Towards Towards Mombasa Nairobi

Verge Off Off Verge Verge Off Off side side side side side side side

No.

1" Mean 1 1 . 2 5.8 3.4 12.4 Range 8 - 1 6 2 - 8 0 -11 9--15

After overlay After overlay November 1975 January 1981

Towards Towards Towards Towards Mombasa Nairobi Mombasa Nairobi

Verg~ Verge Off Off Verge side side side side side

0.4 0.8 0 1 . 4 3.0 0--2 0--2 -- 0--3 0--4

Mean 9.4 5 .2 3.2 10.2 0 0 0 0.8 3.6 2*

Range 5 - 1 3 0 - 8 2 - 5 2 - 2 2 -- - - 0--2 2 - 5

Mean 9.0 7.2 6.2 14.8 0 0.4 0.4 0 0 3

Range 6 - 1 1 6 - 8 5 - 8 6--24 - 0 - 2 0--2 -- -

Mean 7.4 6.8 3.6 13.8 0 0 . 4 0 2.4 0.8 4

Range 5 -11 6 - 1 0 2 - 5 13-15 - 0 - - 2 - 0--4 0 - 2

Mean 2.4 5.6 1 .6 3 . 2 0 0 . 4 0.4 0.6 0.8 5

Range 0 - 5 2 - 8 0--4 0--8 - 0 - 2 0 . 2 0 - 3 0 - 2

7*

Mean 4.8 4.0 1.8 6.4 0 0.4 0.4 0 2.2 Range 2 - 7 2 - 6 0 - 5 2 - 9 - 0 - 2 0 - 2 - 0 - 3

Mean 10.2 4.2 3.4 7.0 2.4 0.8 0.4 1 . 4 7.0 Range 3 - 1 5 2 - 7 0 - 6 5 - 1 0 2--4 0 - 2 0 - 2 0 - 4 5 - 9

Mean 7.4 2.2 3.0 5.0 2.6 1.8 0 0.8 7.6

3.0 0.8 4.4 2--4 0--2 3--7

4.2 0.6 3.6 4--5 0--3 0--7

0.8 0.8 0.4 0--2 0 - 2 0--2

0.8 0 4.8 0 - 2 - 4 - 6

0.4 0.4 2.6 0 - 2 0 - 2 0--4

0 0 0.8 - - 0 - 2

4.8 2.2 5.0 3--7 2 - 3 3 - 7

5.8 1.4 4 .8 8*

Range 6 - 9 0 - 5 0 - 7 4 - 6 2---3 0--3 -- 0 - 2 6 - 8 4 - 7 0 - 3 3 - 8

Mean 8.8 2.4 4 . 4 11.6 0 0 0.8 0 Range 5 - 1 3 0 - 5 2 - 7 8 - 1 7 - - 0 - 2 -

0 0.4 0.8 0.6 - 0--2 0--2 0 - 3

10" Mean 10.0 2.4 1.6 10.0 0.4 0 0 0

Range 7 - 1 4 0 - 6 0 - 4 7 - 1 3 0 - 2 - - -- 6.0 5.4 1.2 1.0 5 - 7 4--7 0--2 0 - 3

i1"* Mean 7.4 5.8 5.2 5.4 Range 0 - 1 2 2 - 8 0 - 9 2 -11

8.6 5.6 6.4 4.6 1 1 . 8

0 - 1 4 3--9 3--10 0 - - 1 2 4 - 1 9

6.4 8.6 7.8 3--10 6 - - 1 4 2--16

* Bitumen-rich mixes ** Not overlaid

3 3

TABLE 18

Rut depth measurements before overlay

Site No. Rut depth under a 2m straightedge (mm)

Towards Eldoret Towards Nakuru

Mean Range

Mean 3

Range

4 Mean Range

Mean Range

Mean 6

Range

Mean 7

Range

Mean 8

Range

Verge side

7.5 3 -19

Off-side

4.4 2 - 1 t

Towards Mombasa

Verge side [ Off-side

2.9 0 -12

4.3 0 -19

6.7 2--21

0.2 0 -3

2.5 0 -7

5.2 0-13

Off-side Verge side

5.2 9.1 0 -12 3-18

Towards Nairobi

Off-side Verge side

0 0

2.7 0-10

6.0 0-23

6.8 0-23

2.7 0-12

7.1 0-23

Towards Mombasa Towards Lunga-Lunga

Verge side Off-side Off-side Verge side

1.8 0 - 4

3.0 0--10

1.3 0 -5

0.4 0 - 4

Off-side

Slow lane

2.0 0 -9

0.2 0 - 4

4.5 0-15

0.7 0 -4

Fast lane

Off-side Verge side

1.2 0.6 0 -5 0 -3

Verge side

1.8 0-14

1.3 0 - 8

3 4

TABLE 19

Site No.

Type of roadbase

Lime stabilised

gravel

Crashed rock

Crashed gravel on cement stabilised sub-base on old road

Section No.

1

2

3

4

5

6

7

1

2

3

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

Type of overlay

Current 20 m m AC

Old 20 mm AC

Current 13 m m AC

DBM

Current 20 m m AC

Current 13 m m AC

Current 13 mm AC

Old 20 m m AC

HRA

DBM

Current 20 m m AC

Current 20 mm AC

Current 13 mm AC

Old 20 mm AC

HRA

DBM

Current 20 m m AC

Current 20 mm AC

Direction

Eldoret Nakum

Eldoret Nakuru

Eldoret Nakuru

Eldoret Nakum

Eldoret Nakuru

Eldoret Nakum

Eldoret Nakum

Mombasa Nairobi

Mombasa Nairobi i

Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

f Mombasa Nairobi

Mombasa Nairobi ,

J Mombasa Nairobi

I Mombasa Nairobi :

Mombasa Nairobi

i Mombasa Nairobi

i Mombasa Nairobi

Mean thickness overlay

m m

51 52

76 93

107 106

86 88

62 66

100 92

53 54

70 70

111 110

115 112

101 103

104 96

101 106

79 69

112 109

166 160

122 129

76 75

115 121

106 106

102 100

108 107

104 100

105 111

163 172

128 133

71 76

Estimated Cracking in 1978 Est imated

traffic Number traffic loading cracked Mean loading

cracking by 1981 by 1978 test m / m 2 ESA x 106

ESA x 106 points t i i

0.3 1 0.05 0.68 0.2 6 1.0 0.37

i i i

0.3 2 0.2 0.68 0.2 0 0 0.37

i i i

0.3 1 0 0.68 0.2 0 0 i 0.37

i i i

0.3 3 0 0.68 0.2 2 0.05 0.37

i i i

0.3 3 0.35 0.68 0.2 6 ' 0.15 0.37

i [ i

0.3 10 3.05 0.68 0.2 9 3.75 0.37

i i i

0.3 8 0.7 0.68 0.2 6 0.25 0.37

i i i

0.8 0 0 2.0 2.4 0 0 4.2

i i i

0.8 0 0 2.0 2.4 1 0 4.2

i | i

0.8 0 0 2.0 2.4 0 0 4.2

i i i

0.8 0 0 2 .0 2.4 1 < 0 . 0 5 4.2

i i i

0.8 ~' 3 - 0.05 2.0 2.4 ] 7 0.35 4.2

i i i

0.8 ] 3 0.1 2 ~0 2.4 2 0.05 4.2

i i i

0.8 0 0 2.0 2.4 0 0 4.2

i i i

0.8 0 0 2.0 2.4 i 0 0 4.2

i i i

0.8 0 0 2.0 2.4 0 0 4.2

i i i

0.8 0 0 2.0 2.4 ~ 0 ~ 0 i 4.2

0.8 0 0 2.0 2.2 0 0 3.8

i i i

0.8 0 0 2.0 2.2 1 0.1 3.8

i i i

0.8 0 0 2.0 2.2 O 0 3.8

f f i

0.8 0 0 2.0 2.2 0 0 I 3.8

i I 1 . 0 2.0 0.8 10

2.2 1 0.05 3.8 i i i

0.8 2 0.15 2.0 2.2 1 < 0 . 0 5 3.8

i i 1

0.8 3 0.05 2.0 2.2 0 0 3.8

1 i i 0.8 2 0.15 2.0 2.2 0 0 3.8

i i f

0.8 2 0.15 2.0 2.2 0 0 3.8

i i i

0.8 2 0.1 2.0 2.2 ~ 0 0 3.8

Cracking in 1981

Number Mean

cracked cracking

test m / m 2 points

i 9 1.3 8 1.1

i 7 0.3 1 < 0 . 0 5

i

5 0.5 0 0

i 6 0.3 2 0.1

t 6 1.4 7 0.8

10 [ > 3 (5 pts failed) 10 > 4 (8 pts failed)

8 > 3 (4 pts failed) 8 1.4

0 0 0 0

i

0 0 2 I 0.1

O 0 0 0

i i

0 0 3 0.2

i i

4 0.05 7 0.5

i i

6 0.5 4 0.2

i i

0 0 0 0

i i

0 0 0 0

i i

0 0 1 0 . 1

i i

0 0 0 0

i i

2 0.4 5 ; 0.4

i i

0 0 3 0.2

i i

1 < 0 . 0 5 3 0.I

i i

0 0 2 0.1

i i

10 0.9 i 1 < 0 . 0 5

i 3 0.2 7 0.7

i 3 0.2 1 < 0 . 0 5

t 5 0.4 3 0.2

i 7 0.5 2 < 0 . 0 5

i

5 0.4 4 0.3

35

TABLE 19 (continued)

Site No.

Type of roadbase

Cement stabilised gravel

Cement stabilised gravel

Section No.

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

11

12

13

Type of overlay

HRA

Old 20 mm AC

DBM

Current 13 mm AC

Current 20 mm AC

Current 13 mm AC

Current 13 mm AC on bitumen-gravel

Old 13 mm AC on bitumen-gravel

Current 13 nun AC

Old 13 mm AC

Direction

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa

Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mombasa Nairobi

Mean thickness overlay

mm

101 100

104 103

103 97

102

99

96 101

146 157

121 126

102 110

79 80

33 25

45 42

71 71

95 87

155 144

114 118

108 110

119 114

88 86

75 70

41 40

34 32

35 33

Estimated Cracking in 1978

traffic Number Mean loading cracked cracking by 1978 test I m/m2

ESAx 106 points

0.3 0 0 0.8 0 0

0.3 0 0 0.8 0 0

0.3 0 0 0.8 0 0

0.3 1 0.05

0.8 0 0

0.3 0 0 0.8 0 0

0.3 "0 0 0.8 0 0

0.3 2 <0.05 0.8 2 0.1

0.3 0 0 0.8 0 0

0.3 0 0 0.8 0 0

1.1 4 0.45 2.7 8 1.1

1.1 10 2.25 2.7 6 0.7

1.1 3 0.65 2.7 7 1.15

1.1 9 1.4 2.7 10 1.2

1.1 8 0.95 2.7 10 1.2

1.1 1 <0.05 2.7 6 0.6

1.1 6 0.7 2.7 3 0.1

1.1 0 0 2.7 2 <0.05

1.1 2 0.15 2.7 8 0.55

1.1 5 0.45 2.7 10 1.05

1.1 7 1.1 2.7 9 1.35

1.1 8 1.0 2.7 7 0.5

1.1 0 0 2.7 5 0.4

Estimated Cracking in 1981

traffic Number Mean loading cracked

cracking by 1981 test m/m2

ESA x 106 points

1.4" 1 <0.05 2.3. 0 0

1.4 0 0 2.3 0 0

1.4 5 0.4 2.3 2 0.1

1.4 4 0.7 (1 pt > 5 m/m 2)

2.3 3 0.1

1.4 0 0 2.3 0 0

1.4 2 0.2 2.3 0 0

1.4 5 0.4 2.3 2 0.2

1.4 3 0.1 2.3 ' 0 0

1.4 0 0 2.3 1 0.1

2.3 7 0.7 4.3 10 1.5

2.3 10 2.6 4.3 9 1.1

2.3 4 0.7 4.3 9 1.5

2.3 10 1.8 4.3 10 1.7

2.3 8 1.2 4.3 10 1.3

2.3 3 0.06 4.3 6 0.9

2.3 7 1.1 4.3 5 0.3

2.3 0 0 4.3 2 0.1

2.3 4 0.3 4.3 9 1.2

2.3 5 0.6 4.3 10 1.9

2.3 9 1.3 4.3 9 1.4

2.3 8 1.1 4.3 10 0.7

2.3 4 0.2 4.3 8 1.0

36

TABLE 19 (continued)

Site No.

6

Type of roadbase

Cement stabilised gravel

Cement stabilised gravel

Cement stabilised gravel I

Section No.

1

2

3

4

5

6

7

8

9

10 (40 test points)

Range for 6 similar sections

1

2

3

Type of overlay

DBM

HRA

DBM

Old 20 mm AC

Current 13 nun AC

Current 20 mm AC

Current 20 mm AC on base-course

Current 20 mm AC

DBM

HRA

Direction

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa Lunga- Lunga

Mombasa

Lunga- Lunga

Fast lane Slow lane

Fast lane Slow lane

Fast lane Slow lane

Mean thickness oveday

45

48

47

50

47

52

45

53

45

52

35

41

46

48

65

69

86

96

40

46

40

40

56 61

71 55

59 62

Estimated Cracking in 1978

traftie Number Mean loading cracked

cracking by 1978 test m/m2

!ESA x 106 points i i

0.18 0 0

0.12 i 1 0 i

0.18 0 : 0

0.12 0 0

0.18 0 0

0.12 0 0

1 0.18 0 0

0.12 2 0.05

0.18 0 0

0.12 0 0 i i

0.18 2 0.05

0.12 3 0.05

0.18 6 0.1

0.12 0 0

0.18 1 . 0

0.12 9 0.95

0.18 0 ! 0

0.12 9 0.55

0.18 6 0.15

0.12 6 i - 0.05 i i

0.25-- 0.19 5--9

0.65

0 .7 - 0.13 I 9--10 1 . 5 5

i i

0.33 1 0.05 1.30 1 0.05

i i

0.33 0 0 1 . 3 0 0 0

i i

0.33 8 0.5 1 3 0 0 0

Estimated traffic loading

by 1981 ESAx 106

0.19

0.15

0.19

0.15

0.19

0.15

0.19

0.15

0.19

0.15

0.19

0.15

0.19 7

0.15 6

Cracking

Number cracked

test points

0.19 5

0.15 9

0.19 3

0.15 10

0:19 9

0.05 5 i i

0.19 ! 4--8

0.13 9--10 i i

0.7 2.9

i i

0;7 2 2.9 0

i

0.7 8 2.9 0

in 1981

Mean cracking

m/m 2

0.1

0.1

4 0.1

0 0

1 <0 .05

4 OA

<0.05

0

0.3

0.1

0.2

0.2

0.2

0:9

0.1

0.8

0.2

0.1

0.2-- 0.6

0.9T 1.6

4 0.1 2 0.1

<0.05 0

0.6 0

37

TABLE 20

Mean pavement roughness before and after overlaying

SRe No.

1

2

3

4

5

6" 7

Roughness (ram/kin)

Before overlaying

2700

3400

1600

3300

3200

2300

After overlaying

1400

1600

1400

1900

1800

1700

1800

3 8

0 - - ~ - ( 3

X X X X

0 0

X X X

0 0

X X

0

x X

I

o.

A v

0

A v

C

0

E

0 ° _

e -

I--

k _ - N -

~o~ ° _

° _

, ¢ " ~ ' J e -

x

l oox

0 ° ~

0

0

.-I

I.L

A +-* ¢--

03 ¢J

¢:L

er"

a 3

¢-.

100

80

60

50

40

30

20

10

i

D

m

B

P

D

. 5.4 _ 1 .0 , , N , r a t e of enetrat,on,

• z~C~',~c~c~D~m~

-• % ' ~ •

=0 R,.~O

O

• a ~ ' \ I

c] 8 AUL

I I I I I I I I I I I I I i 1

10 20 30 4 0 50 6 0 8 0 1 0 0

Rate of penetration (ram/blow)

Fig. 2 Relationship between in-si tu subgrade C.B.R. and rate of cone penetration

A

E E E O . B

t ~

r -

e . )

t - t ~

2.o

1.5

1.0

0.5

0

Site 1

'74 "75

Note: 7(54) indicates section No. and overlay thickness (mm)

'76

3.4 3.9

~-J 6(96) / (D.B.M. /

1(52)

5(64)

All asphaltic concretes

'77 '78 '79 '80 '81

Survey date

1.0

0.5

0.5

0

Site 2 B

Sections with < 0.2 m/m2 mean cracking in 1981

1 (7O) A.C. 2 (110) A.C. 3 (114) A.C. 4 (102) A.C. 7 (74) A.C. 8 (111) A.C. 9 (163) A.C.

10 (126) A.C.

I I I I I I I '75 '76 '77 '78 '79 '80 '81

6(104) (D .B.M.)

5(100) (H.R.A.)

Site 3 Sections with ~0.2 m/m2 mean cracking in 1981

2 (118) A.C. 3 (106) A.C. 4 (101) A.C. 7 (108) A.C.

- 7 ~ D ~ 5(108)(H.R.A.) / - ~ 6(102)(D.B.M.)

/ All asphaltic ~ I(0~6)4, / concretes /I-,,

~_ 4 ~ 8(168) i / ~ 9(131)

'75 "76 '77 '78 "79 '80 '81 Survey date

Fig. 3 Rate of d e v e l o p m e n t of surface crack ing

E E

v

t -

O

o 3 ¢ -

2.0

1.5

1.0

0.5

Note: 5 (101) indicates section No. and overlay thickness (ram)

Site 4

Sections with < 0.2 m/m2 mean cracking in 1981

2 (101) H.R.A. 3 (104) H.R.A. 6 (99) A.C. 7 (152) A.C. 9 (106) A.C.

10 (80) A.C.

O

'76.

~ 5(101) D.B.M .i 8(124) 4(100)

I '77 '78 '79 "80 '81

Survey date

2.0

Site 5 Section 8(117) < 0.2m/m2 mean cracking in 1981

4(91 )

1.5

/ f / ~ 10(73) jI/,, :,1: 1.0 y / ~ ~ ~ - ~ 1(29)

9(87) 7(109)

13(34)

0.5 6(116)

0 '74 "75 '76 "77 '78 '79 '80 '81

Survey date

Fig. 3(cont) Rate o f development o f surface cracking

E E E O , B

Q .

O} t -

2.0

1.5

1.0

0.5.

0

Site 6

Sections with < 0.2 m/m2 mean cracking in 1981

1 (47) D.B.M. ~ 2 (49) H.R.A. 3 (50) D.B.M. ~ 4 (49) A.C. 5 (48) A.C.

10 (43) A.C.

Actually similar to A.C.s but with lower bitumen content

All asphaltic concretes

-- 8(67)

9(91)

" O

"~ 6(38) 7(47)

I '75 '76 '77 '78 '79 '80 '81

Survey date

1.C

0.5

0.5

Site 7

O

'73 '74 '75

~ 6

2

All 40mm thick asphaltic concrete

I I I I I I I

'76 '77 '78 '79 "80 '81

Site 8 Sections with < 0.2 m/m2 mean cracking in 1981

1 (59) D.B.M. 2 (63) D.B.M.

, , ~ 3(61 ) ~ H.R.A.

O /

i I ~ 1 I I I I I I

"73 '74 '75 '76 '77 '78 '79 '80 '81

Survey date

Fig. 3 (con t ) Rate o f deve lopment o f surface c rack ing

O.

E o

t-

O.

E

Q~

x o~

E t-

t~ °_

c

E 0_ t~

x °_

E

-8

u_

0

¢0

E t-

I P P l I I I

( , , , tu) ~)oCji~ :i.e ua,,,n:l.!q aq:l. j.o uop.eJ~.auad

0

0

A

"0

Q.

E

c

0

<

¢I}

E °i

,.C

°I

E

0i

"10

cSA >.

C

~E ,_o 04-

C 0

Gb

t,~v

I.I.

O

0

z~

80/100 60/70 80/100 60/70 80/100 60/70

~enetration grade bitumen sampled from heater tank )enetration grade bitumen sampled from heater tank ~enetration grade bitumen recovered after mix ing ~enetration grade bitumen recovered after mix ing )enetration grade bitumen recovered from overlays up to 5 years old )enetration grade bitumen recovered from overlays up to 5 years old

A

E E (J o L~ ¢N

r

.o

o ~ t -

Q .

90

80

70

60

50 -

4 0 -

30 -

20 -

10

O 0

0 0

0

• []

m o ~

Az~

A • AM~

n

0 I I I I I I 0 20 30 40 50 60 70

Softening point ( °C )

8 0

Fig. 5 Re la t ionsh ip between pene t ra t ion and r ing and ball values f o r samples o f b i t u m e n s

D

e e

O)

+ Q. o c- CD Q.

o.

II

E 0

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- N

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N o O

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E O

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C

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0 e',

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U ,

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+./~/++ ~" ® ~ ® m / • •

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," _ " m - / = °m ~ _ m ~ = / ~" + / $ , . - _ - / + ~ " ~ N "+

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I I I I I I I I t O 'rap C~I CD CO ( D ' ~ C~l

(Will) AIZlJaAO JO SSaU>I::>!LI/

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t- O

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rv-'

I " , ,

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/ / / . . " ,

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I I I I I I I

((.uuJ) A e l J e A o ~o s s e u ~ o ! L i l

I .E .-~

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0 0

0

0 ¢,D

A

E E

0 0

X v

¢-

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o ~ ' ~ e--

0 . _

. _

0

0

r-

E e= o

° ~

c

U ° ~ o= o e=

° ~

¢0 m

~D

O

e=

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=O =O

° ~ e= U

C O

U

"C3 C

C O

U

" O

C

E ° ~

"0 C

E E 0 U

e "

1 0 0 0 900

800

700

600

500

400

300

R x D ( x l 0 3 m m 2)

.o\

Verge-side wheelpaths on ly

7 . . . i nd i ca tes Site No.

A

E

P

> -I

n-

200

100 --

9 0 -

8 0 -

70 -

60 -

50 -

40 -

30 -

20 -

10 10 20

I I I I I I I I 30 40 50 60 70 80 90100

Transient deflection (x lO-2mm)

200

Fig. 9 Re la t i onsh ips be tween rad ius o f c u r v a t u r e and d e f l e c t i o n o n the c o n t r o l sec t i ons

1000

900

800

700

600

500

400

300

200

E P

co ,> = 100 -

9 0 -

• -~ 8 0 -

r r 70 -

60 -

50 -

40 --

30 --

20 --

10 10

R x D ( x 103mm 2)

%

- \

I

20

I Site 2 • = " ~ ~ Site 3

Overlays

I I I I I I I I

30 40 50 60 70 80 9 0 1 0 0

Transient def lect ion (x 10-2mm)

200

F ig . 1 0 D e f l e c t i o n - c u r v a t u r e r e l a t i o n s h i p s . f o r t h e " C o n t r o l " s e c t i o n and a 9 0 - 1 0 9 m m t h i c k o v e r l a y

1000 ~ . O O R x D (x 103mm 2) 900

800

700

600

500

Site 5 i Site 4

400

300

200 Overlays

A

E

- i

? 100

N 90

N 80 "O

-~ 70

60

50

40

30

20

10 I 10 20

I I ! I I I I I 30 40 50 60 70 80 90100 200

Transient deflection (x 10-2mm)

Fig. 11 Deflection-curvature relationships for "Control" sections and 90-109mm thick overlays

1 0 0 0 900

800

700

600

500

400

300

200 --

E P 2

-" 100 - ¢J

9 0 - -

~5 80 -- O3

oc 70 --

60 --

50

40 - -

30 - -

20

R x D ( x l O 3 m m 2)

10 I

10 20

Site 7 Site 8

,Overlays

I I I I I I I I 30 40 50 60 70 80 90 100

Transient deflection (x 10"2ram)

200

Fig, 12 D e f l e c t i o n - c u r v a t u r e r e l a t i o n s h i p s f o r "Con t ro l " sec t ions and 40 and 5 0 m m t h i c k o v e r l a y s

A

E E

%

v t -

O om

N--

"O

"O t -

.=

"O o a .

200

180

160

140

120

100

80

60

40 - -

2 0 -~

0 2O

Q ~ 3 (150) denotes Site No. thickness

0)

4(150)

5(86) 5(121)

4(100) 4(75) 5(33)

I I 30 40

Surfacing temperature at depth 40ram (°C)

50

Fig. 13 Relat ionships between the p roduc t o f radius o f curvature and def lect ion and overlay temperature

O0

"5 e E = o_ "~ o --

z~ ~J

N o o c .>__ I-- I - - "~

-~ oO

I

O"

._I

CO

I I I I I

(901 x "V'S'=I ) 6u!~oeJo ~.o~uJ/uJ~" 0 o:l aj! I len~ov

0

0 0

0

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0

0

0

0

0

0

(N

0

O0

A

X

<

LU

c-

O

"0

0

~*="

o o~

0.

i 0

0

i i=.

o

w-

o A

W=

o

E

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d v

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n~ t= 0~

o

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6,1

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I ~ ~ =- I __ "O

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, "

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14. APPENDIX

ADAPTION OF BRITISH OVERLAY DESIGN METHOD FOR USE IN KENYA

In LR 8334 a method of designing overlay thicknesses for roads in the United Kingdom is described and

charts are provided which relate the traffic loading to be carried, pavement deflection before overlay and

overlay thickness. Two levels of probability of achieving the design life are also given.

The method incorporates an estimate of the remaining useful life of a road before the onset of

'critical conditions' which are deemed. This uses 'criteria' curves which have been developed for roads in

t h e United Kingdom. These are basically deflection-performance (surface condition) histories which are

used to predict the future behaviour o f roads of similar construction. The design charts relate to the use of

hot rolled asphalt as the overlay material but conversion factors for bitumen macadams are given.

14.1 Application of the method in Kenya

For Kenyan conditions dense bitumen macadam wearing course material and asphaltic concretes

can be assumed to be equivalent to hot rolled asphalt for thickness design purposes.

It is unlikely that criteria curves of the type referred to in LR 833 will be available. Areas of roads

which are to be overlaid will almost always have reached a 'critical' condition at the time of overlaying.

It is very important that badly damaged areas are dug out and reconstructed and that drainage

deficiencies are rectified before overlaying.

14.1.1 Roads with crushed stone roadbases. Two levels of probability of achieving a given overlay life are provided in thedesign charts two of which are reproduced as Figures 16 and 17. The results of the

present study show that these levels o f probability are not meaningful for Kenyan conditions because of

the possibility o f cracking during the early life of the overlay. The charts do however provide two

acceptable levels of strengthening. It is suggested that the chart for the 0.5 probability level is used in

areas receiving less than 1000 mm of rainfall per annum and that the chart for the 0.9 probability level is

used in wetter areas until more experience of overlay performance is acquired from future overlay contracts.

14.1.2 Roads with soil-cement roadbases. These pavements present special problems in that the

nature and strength of the roadbase at the time of overlaying largely determines the performance of the

overlay. A stabilised roadbase which has cracked into individually strong slabs will probably cause reflection

cracking in the overlay with subsequent failure due to the ingress of water. A stabilised roadbase which has

become highly fragmented will behave more like a granular roadbase. There are many stages of deterioration

between these two examples where it will be difficult to determine whether reflection cracking will occur.

Evidence reported elsewhere 19 has shown the importance of maintaining an unbroken seal throughout the

life o f this type o f road structure and o f the influence of patching work upon serviceability. Surface

dressings are likely to provide the cheapest seals and should be used for as long as possible before overlaying.

A surface dressed bitumen macadam or, for thick overlays or regulating courses, a composite overlay o f crushed rock with an asphalt surfacing will be more effective in resisting reflection cracking than would

the same thickness of a dense asphalt.

58

In the current study the overlays on site 4 have now performed well for four years and whilst the

evidence is far from conclusive the indications are that deflections greater than 50 x 10 -2 mm and a

radius of curvature (as measured in the study) o f less than 60m measured on a surface dressed pavement

indicate a well fragmented roadbase.

It is recommended that the overlay thickness design charts proposed for granular roadbases are also

used for roads with cement-stabilised roadbases but that the use of overlay systems indicated above are

considered as alternatives to dense asphalt overlays.

5 9

(758) Dd8041301 1,400 4/82 H P L t d S o ' t o n G1915 PRINTED IN ENGLAND

ABSTRACT

Early performance of some experimental bituminous overlays in Kenya: H R SMITH and C R JONES: Department of the Environment Depar tment o f Transport , T R R L Laboratory Report 1043: Crowthorne, 1982 (Transport and Road Research Laboratory) . This report describes the performance o f b i tuminous overlays on roads in tropical and sub- tropical environments in Kenya.

Considerable hardening of the binder occurred th roughou t the dep th o f the overlays during the first five to seven years, but negligible rut t ing developed. The main form o f deterioration was cracking which was initiated at the surface o f the overlays.

The reduction in pavement deflections produced by the overlays is shown to be only slightly less than those obtained in Britain despite the higher ambient t empera tures in Kenya.

The performance of the overlays is shown to be primarily dependen t on the propert ies of the overlay material itself. No correlation could be established be tween the deve lopment o f cracking on each test section and the corresponding values o f surface def lect ion and curvature, traffic loading or overlay thickness. However the overlays have remained serviceable for up to seven years and have carried up to 4.3 x 106 equivalent s tandard (80 kN) axles.

ISSN 0305 -1293

ABSTRACT

Early performance of some experimental bituminous overlays in Kenya: H R SMITH and C R JONES: Department of the Environment Depar tment o f Transport , T R R L Laboratory Report 1043: Crowthorne, 1982 (Transport and Road Research Laboratory) . This report describes the performance of b i tuminous overlays on roads in tropical and sub- tropical environments in Kenya.

Considerable hardening of the binder occurred t h roughou t the dep th o f the overlays during the first five to seven years, but negligible rut t ing developed. The main form o f deterioration was cracking which was initiated at the surface o f the overlays.

The reduction in pavement deflections produced by the overlays is shown to be only slightly less than those obtained in Britain despite the higher ambient t empera tures in Kenya.

The performance of the overlays is shown to be primarily dependen t on the propert ies of the overlay material itself. No correlation could be established be tween the deve lopment o f cracking on each test section and the corresponding values o f surface def lect ion and curvature, traffic loading or overlay thickness. However the overlays have remained serviceable for up to seven years and have carried up to 4.3 x 10 O equivalent s tandard (80 kN) axles.

ISSN 0 3 0 5 - 1 2 9 3