o Performance of sections Mombasa road in Kenya by H. R. Smith, T. E. Jones of the Nairobi to RESE..~( H :.lBoRATORY .TECI-13JI(. i INFORMATION ANB LIBRIRY :RVICES ~ubr) and C. R. Jones TRANSPORT and ROAD RESEARCH LABORATORY Department of the Environment Department of Transport TRRL LABORATORY REPORT 886 PERFORMANCE OF SECTIONS OF THE NAIROBI TO MOMBASA ROAD IN KENYA by H R Smith, T E Jones 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 1980 ISSN 0305–1 293 CONTENTS Abstract 1. 2. 3. 4. 5. 6. 7. Introduction Detafls of the sections investigated Chmate Materials testing Traffic Road performance measurements 6.1 Roughness measurements 6.2 Rut depths 6.3 Cracking and patching 6.4 Deflection measurements Discussion of results 1 7.1 Deterioration of sealed roads with cement stabflised road bases 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 Surface roughness Rut depths Cracking and patching Deflections Performance of sections 1 to 9 7.2 Deterioration ofsections withasphdtic concrete surfaces mdcrushed rock road bases 7.2.1 Surface roughness 7.2.2 Rut depths 7.2.3 Cracking 7.2.4 Deflections 7.2.5 Performance of sections 10, 11 and 12 Page 1 1 1 2 2 2 2 3 3 3 3 4 4 4 5 5 5 6 7 7 7 7 7 8 8: Implications of the results for the relationships used in the Road Transport Investment Model 9. Conclusions 9.1 Sections tithsoil-cement road bases and bituminous seals 9.2 Sections withcrushed rock road bases andasphdtic concrete surfacings 10. Acknowledgements 11. References Page ‘8 9 9 9 10 10 @ CROWN COPYRIGHT 1980 Extracts porn the text may be reproduced, except for commercial purposes, provtied the source is ackno wledged PERFORMANCE OF SECTIONS OF THE NAIROBI TO MOMBASA ROAD IN KENYA MSTRACT Details are given of the performance of sections of the Nairobi-Mombasa road in Kenya. Nine sections of road with cement-stabilised bases surfaced with multiple seals, andthree sections with crushed rock bases andasphaltic concrete surfacings have been studied for seven years. These road sections are located in a dry area, on strong subgrade soils. It was found that the lightly constructed sections with cement-stabilised bases had suffered little structural damage after carrying 6 x 106 equivalent standard (80 kN) axles, considerably more than would be predicted by current methods of pavement design. The more heavily constructed sections with asphaltic concrete surfacings are performing broadly as would be expected. For both types of pavement, deterioration has been mainly confined to the surfacings, and timely resealing has a powerful effect in limiting the amount of patching required and in maintaining an acceptable level of surface roughness. The lives of the better qu~ity seals have been of the order of four to five years. Deflection measurements clearly show how variations in overall pavement strength are affected by changes in annual rainfall, and the wide variation in strength which can occur on a nominally uniform pavement. Deflection measurements did not give early warning of pavement surfacing faflures. 1, INTRODUCTION This report describes a study of the performance of twelve one-kilometre sections of the Nairobi to Mombasa road in Kenya. This road is the principal trunk road in the country linking the capital city with the main port at Mombasa. The twelve sections of the road were the subject of detailed investigations during the period 1971 to 1973, as part of a larger ‘Road Transport Cost Study ‘1‘2 which was undertaken to provide relationships / for use in a computer model capable of calculating totrd transport costs3. Monitoring of the performance of these road sections has continued on a regular basis during the period 1973-1979. 2. DETAILS OF THE SECTIONS INVESTIGATED The locations and type of construction of each of the twelve sections of the road that were studied are summarised in Table 1. The one-kilometre sections were constructed normally and are representative of much longer lengths of the road. Geometric constraints imposed by the investigations for the Kenya ‘Road Transport Cost Study’ 1 governed the locations of the sections. Considerable variations in the crossfall and vertical alignment of the road occur within the length of certain sections, notably Sections 3, 5, 6 and 7. ASa consequence the drainage conditions are not uniform within these sections. Ml of the sections have been resealed periodically with surface dressings. Slurry seals were also placed on Sections 5 to 8 in 1974. 1 3. CLIMATE The sections are located in areas which experience low rainfall. Detads of total annual rainfall recorded near to the sections are summarised in Table 2. 4. MATERIALS TESTING In order to compare the performance of the road sections, a number of inspection holes were dug within each section and measurements were made of layer thicknesses, in-situ CBRSand moisture contents. Selection of the points where inspection holes were dug has been described elsewhere1. The majority of these inspection holes were dug at the points where the largest deflection was measured. At these points it is likely that the overall strength of the pavement or the subgrade was weaker than elsewhere in the section. The maximum size of aggregate used in the crushed stone road bases precluded the use of the in-situ CBR test on this layer. The results of measurements and tests of the pavement layers, together with Casagrande classifications of the Subgrade soilsaresummarised in Tables 3 and 4. The results of grading tests and aggregate strength tests made on samples taken from the crushed stone road bases are summarised in Tables 5 and 6. 5. TRAFFIC The damage caused to a road pavement by vehiclesis dependent upon the magnitude of the individual wheel loads and the number of times these loads pass over the pavement. Many ade load surveys have been carried out on the Nairobi to Mombasa road, initially by the OverseasUnit and in recent years by the Kenya Ministry of Works. In these surveys vehicles were weighed on a portable weighbridge developed by the OverseasUnit of the Transport and Road Research bboratory4. From the results of these surveys estimates of the distribution of the axle loads of commercial vehicles using the road have been made and are reported elsewhere5. Factors derived from the AASHO Road Test6 haw been used to express dl tie loads in terms of an equivalent number of ‘standard’ (80 kN) axle loads7. The equivalence factors used were calculated from the fo~owing formula: ~uivrdence Factor = (axle load in kgf/8160 kg~4.5. The estimated traffic loading histories for the sections are summarised in Table 7. 6. ROAD PERFORMANCE MEASUREMENTS On each section test points were located in the four wheelpaths at eleven chainages spaced at one hundred metre intervals. Periodicdly roughness measurements were made over the length of each wheelpath, and the transient deflection, rut depth and cracking were measured at each test point. The total area of patching in each lane was dso measured. 2 Section 7 was used to study the performance of a pavement that received no normal maintenance. Between September 1971 and August 1973 the repair work was restricted to the filling of potholes with soil. 6.1 Roughness measurements Since August 1972 changes in the roughness of the sections have been monitored with a towed fifthwheel bump integrator. Prior to this date measurements were made with a bump integrator unit mounted over the rear axle of a car. These early results were subsequently converted to equivalent towed bump integrator vduesl. The mean roughness for each lane is shown plotted against time in Figure 1. 6.2 Rut depths The rut depth was measured at each test point under a 2m straight-edge, placed transversely to the centre line of the road. 6.3 Cracking and patching Cracking at a point was measured within a one metre square frame placed over the test point with one edge of the frame parallel to the edge of the road. A tape measure was used to determine the length of cracking which was expressed in metres per square metre (m/m2). Simdarly the amount of patching was expressed in m2/km. The maintenance policy in Kenya is to patch areas in which the cracking is in excess of 5m/m2. A measure of the degree of fadure of the surfacing has been obtained by summing the area of patching and the area of pavement with cracking in excess of 5m/m2 for each lane. The results have been related to the dates at which each successive seal was apphed and are shown in Figure 2. In Figure 3 the mean cracking per square metre measured only in the verge side wheelpaths is plotted. Table 8 lists the mean cracking, cracking plus patching and mean rut depths measured on the sections immediately before each resealing operation. 6.4 Defledion measurements It is generally accepted that the magnitude of the surface deflections of flexible pavements can be correlated with the subsequent performance of the pavements under traffic 89Y. Deflection tests can dso be used for designtig the thickness of bituminous overlays. From field studies it has been possible to relate the magnitude of the deflection before overlaying, the thickness of overlay and the subsequent performance of the overlaid pavement 10. In the present study measurements of deflection were made on each test point 11,12 using the Transport and Road Research bboratory method of measuring transient deflections . Typical deflection histories, for sites 2 and 6, are shown in Figures 4 and 5 where the mean deflection in each wheelpath has been plotted against time, traffic loading and total monttiy rainfall. To develop deflection criterion curves8 for a given pavement, deflections should be measured from the time that the road is opened to traffic. This type of relationship could not be developed for the first nine sections because they were in service for a number of years before the study commenced. 3 It might be expected that the fife of a particular seal could be related to the total traffic loading and the magnitude of deflections measured in a standard manner. To investigate relationships of this kind, deflection histories for individurd test points were plotted, using the classification of road surface conditions Ishown in Table 9 and the symbols shown in Figure 6 to denote the degree of cracking (but not rut depth) in the verge-sidewheelpaths. The data for the verge-sidewheelpaths were used for this because the proximity of the off-side wheelpaths to each other means that in places the damaging effects of both wheelpaths are superimposed. It is not usua~y possible to identify the locations along the road where this regularly happens. Typical deflection histories of individurd test points for Sections 2 and 6 for the period commencing just before the application of new sealsin 1972-73 are shown in Figures 7 and 8. Sections 10, 11 and 12 carried an appreciable amount of traffic during the few months between their formal opening to traffic and the start of the study. Whilst deflection values remained reasonably constant between September 1971 and March 1972, it is by no means certain that these values are representative of conditions before the study started. The ‘early life’ deflections may have been larger and influenced the subsequent performance of the surfacings. Deterioration of the sections has been evident only in the verge-sidewheelpaths. Figure 9 shows deflection histories for individual test points on Section 11, plotted against the cumulative traffic loading using the symbols indicating road surface condition shown in Figure 6. Sections 10, 11 and 12 were resurfaced with slurry sealsin 1976. 7. DISCUSSION OF RESULTS 7.1 Deterioration of sealed roads with cement Wabilised road bases 7.1.1 Surface roughness. Although there is considerable scatter in the roughness values obtained with a vehicle-mounted integrator unit during the period September 1971 to September 1972 (see Figure 1) a marked increase in Toughnessof the sections is evident during this period. This was particularly true for Section 7 on which no normal maintenance was carried out. Between September 1972 and June 1978 roughness values only increased on Sections 2 and 3, where faflures in the surface dressing occurred and potholes formed. It will be seen from Figure 1 that mean values of roughness measured’in each lane on each section followed the same trend even though the rates of traffic loading towards Mombasa and towards Nairobi are very different. Aso the difference in roughness measured in the verge-sidewheelpaths of all sections remained virtually constant from August 1972 to June 1978 with the roughness in the Nairobi direction being between 100 and 380 mm/km greater than in the Mombasa direction. Therefore traffic loadings during the period 1972 to 1978 did not play a major part in changing roughness levels on the sections. From observation of the surface condition of these sections, it has been evident that the most important factor affecting roughness has been the durability of the surface dressings and their ability to prevent the ingress of water which leads to separation of the surfacing and localised degradation of the top of the base. Mere surfacing faflures have occurred (invariably after periods of prolonged rainfall) the quality of finish . achieved in the patching work has been of paramount importance in determining the subsequent surface roughness of the road. With the exception of Section 7 the level of roughness on dl the sites is lessthan 3750 mm/km, regarded as the critical level of roughness at which remedial work is warranted 1. The large increase in roughness of Section 7 can be attributed to the ‘nil-maintenance’ policy adopted during the period 1971-73, and the difficulty in obtaining a smooth finish to the numerous patches which had to be made at the end of the nfl-maintenance period. ‘7.1.2 Rut depths. The results summarised in Table 8 show that rutting on the sections has been rninimd and there has been little change in rut depths during the five year period to 1979. 7.1.3 Cracking and patching. There has been a tendency for the most serious cracking and pothohng to occur along the centre of the road. The two offside wheelpaths overlap to varying degrees, depending upon road alignment, so that the opposing traffic flows both contribute to the cumulative traffic loading along the centre of the road. This would be expected to produce more rapid deterioration in the centre of the road than elsewhere. A cent ributory factor may be pavement weakness due to a construction joint on the centre line of the road, but no circumstantial evidence was found to substantiate this supposition. Small differences in the amounts of cracking occurring in the verge-sidewheelpaths are evident on some sections (see Figure 3). Between 1971 and the date of resealkg more cracking occurred on Sections 1, 5 and 8 in the verge-sidewheelpath in the direction towards Nairobi than in the other direction. However the difference in time for mean cracking in the two wheelpaths to reach 1 m/m2 was only four months, hence the differences are not very significant. On Sections 2, 3,4,6 and 7 there was little or no difference in the amount of cracking in the two verge-sidewheelpaths, whtist on Section 9 no significant cracking occurred. This is surprising since the cumulative equivalent de loading in the Nairobi direction is approximately three times greater than in the Mombasa direction. The cracking may be the result of localised surfacing failure caused either by the number of vehicles using the road or by climatic conditions or a combination of these two factors. The overa~ structural integrity of the pavements has been little affected by the cumulative axle loads. The data given in Table 7 and Figures 2 and 3 show that whilst, in general, each successiveseal has been more durable than the previous one, resealing has not been carried out when the existing seals had deteriorated to a common condition. It can dso be seen that the slurry seals have been especially effective in preserving surface condition and that the nd maintenance policy adopted for Section 7 resulted in a very difficult maintenance problem, in 1979 the condition of the slurry sed on this section was poor compared with those on the other sections. It is noticeable that the slurry seals appear to be ‘rich’ in bitumen in the wheelpaths, which suggeststhat the bitumen content is sufficient to ensure a durable and flexible surfacing but is not so high as to cause instabdity in the relatively thin layer of surfacing. This durability may have been obtained at the cost of reduced stid resistance. 7.1.4 Defletiions. It can be seen from Figures 4 and 5 that the deflections measured in the verge-side wheelpaths respond to changes in the seasonal pattern of rainfall. The process of resealkg can significantly affect deflection values. Very marked reductions in deflection resulted from the application of slurry seals on Sections 5, 6, 7 and 8 in June 1974. It is possible that these seals have provided a more impervious surface than the surface dressings, allowing the top of the base to dry out. In particular, where slurry seals have been used, the long term reductions in deflections measured in the off-side wheelpaths, where most of the patching work was carried out, is very marked. Section 4, which deflection measurements indicate is the strongest pavement, was least affected by changes in rainfall. The results of in-situ tests carried out on the sections,indicate that the road base of 5 Section 4 contributes more to the strength of the pavement than do the road bases on the other sections. Trends in the mean deflections measured in the verge-sidewheelpaths on each section are very similar despite the heavier traffic loading in the direction towards Nairobi. The largest changes in deflection have been caused by variation in rainfall, although dl the sections have been strong enough to carry the imposed traffic loadings without suffering appreciable damage below the surface layer. However if the marked increase in rainfall during 1977 and 1978, which is reflected in the increased deflections measured in the verge-sidewheelpaths, continues through 1979, then serious weakening of the overall pavement structure could occur. It can be seen from Figures 7 and 8, that the magnitudes of deflections measured at the time of resealing did not give an indication of the ‘life’of the new serd. The quality of the seal at a given point and any lack of bond between the various seals, patches and road base are important in determining the performance of the seals. Whilstthe data shows considerable scatter, the amount of cracking and patching that occurred in the 2nd sed on the sections after approximately 1.25 x 106 ESA tends to be inversely related to the mean subgrade CBRof the sections and directly related to the mean deflections measured in 1972/74. 7.1.5 Performance of se~ions 1 to 9. The range of mean layer thicknesses of the sections are compared in Table 10 with recommended layer thicknesses from the pavement design guides Road Notes 2913 and 3114 Road Note 31 is applicable for roads in tropical and sub-tropical climates for traffic loadings up to 2.5 x 106 equivrdent standard axles. Road Note 29 is applicable for roads in Britain, carrying a maximum of 1.5 x 106 equivalent standard axles for soil-cement road bases surfaced with premixed bituminous materials. Graded crushed rock road bases are appropriate for greater traffic loadings. For both design Notes the implication is that at the end of the ‘design life’ of the road, strengthening by overlaying and possibly partial reconstruction wfll be necessary to extend the life of the road. Sections 1 to 9 are of simflar total thickness to those recommended in Road Note 31 but their road bases are thinner. The sections have carried nearly three times the traffic loading suggested in the Road Note. This has been achieved at the cost of some patching work and two or three resealing operations. In addition the sections appear to be capable of sustaining present traffic loads for some time into the future provided that these maintenance practices are continued. Whilstthe in-siti CBR measurements shown in Table 4 indicate that Sections 4,5 and 9 have very strong subgrades, which could explain their good performance, the other sections have strong, but not exceptionally strong, subgrades in comparison with most roads in East Africa. Comparisons with the recommendations of Road Note 29 are not really valid because these pavements would have suffered far greater damage if they had been subject to frost and had weaker subgrades more typical of those found in the United tigdom. Since there is no obvious means of predicting the performance of these sections, the regular monitoring of surface deformation and cracking offers the best method of givingwarning of the need for substantial patching work or resealing. Attention should be given to the quality of patching since this is important in determining the roughness of these pavements. 6 I Deflection surveys of representative lengths of pavement carried out initially on an annual basis or after periods of rain and thence biennially, would help in the design of overlays in the future. Wst present knowledge does not permit such surveys to be used to predict failure, they would indicate variations in strength along the road, and the annual trends in deflections would assistin the design of overlays. In addition periodic measurements of roughness would indicate when an overlay is required to restore an acceptable level of riding quality. 7.2 Deterioration of sections with asphaltic concrete surfaces and crushed rock road bases 7.2.1 Surface roughness. There was no significant change in roughness on these sections before the application of slurry sealsin 1976. It appears that the surface finish of the seals caused a sm~ increase in roughness. 7.2.2 Rut depths. In all cases where critical or failure conditions occurred, cracking of the surfacing was the initial indication of deterioration. Rutting increased at a later date and was dependent upon the effect which water, having entered the structure through the cracks, had upon the pavements’ strength. Mere rutting occurred, further deterioration in the verge-sidewheelpath in the Nairobi direction was rapid under the influence of the heavier traffic travelltig in tfis direction. 7.2.3 Cracking. It has been found that cracking in the bituminous surfacings on other sites and on bituminous overlays in Kenya begins at the top of the layer. This means that there can be a considerable delay between the initial appearance of a crack and its propagation to the fun depth of the asphalt. Climatic effects cause considerable hardening of the bitumen in asphalt layers and a consequent reduction in the ability of the surfacing material to resist fatigue faflure. Cracking of the asphalt on these sites, without any associated deformation indicates that the surfacings have faded due to fatigue. It is noticeable that the onset of cracking in the verge-side wheelpaths in both directions occurred at a sitiar time, despite the considerable difference in traffic loadings. As stated previously, where structural deterioration occurred the heavier traffic traveling towards Nairobi caused additiond rapid deterioration. 7.2.4 Deflections. Deflection measurements on these sections did not give an early indication of pavement fatiure. On Section 10, one test point in each of the verge-sidewheelpaths fafled to the extent that patching was required. In both these areas cracking was followed by an increase in rut depth, and then by an increase in deflection. In Section 11 one test point in each verge-sidewheelpath showed marked increases in deflection of which only the point in the more lightly trafficked lane ‘fafled. Wdst Section 12 has a simflar deflection history to Sections 10 and 11 in terms of magnitudes, this section has suffered far less cracking. 7 7.2.5 Performance of setiions 10, 11 and 12. The total thicknesses suggested in Road Note 29 are comparable with those in Sections 10 to 12, rdthough layer thicknesses were variable. Section 12 had a very variable but generally thicker sub-base layer, and a thicker road base than Sections 10 and 11. The asphalt surfacings on the sections suffered fatigue failure and the seal applied in 1976 was needed to waterproof the pavements. This seal was applied before serious structural damage became apparent on Sections 11 and 12. Section 10 had suffered some serious damage in part of the verge-sidewheelpath of the Nairobi-bound lane by 1976 and the application of the seal was ineffective in preventing further cracking and entry of water into the structure. Further deterioration has occurred and has spread along the wheelpath and now requires partial reconstruction and streilgthening. Application of a seal at an earlier date would probably have prevented all of this deterioration. Direct comparisons between these sections and the recommendations in Road Note 29 are not possible without bearing in mind the differences in thicknesses and types of the surfacing materials and the climatic conditions for which the Note is relevant. The performance of the sections is broadly in agreement with the recommendations in the Road Note. These sections have carried in excess of 6 x 106 equivalent standard axles in the Nairobi direction, but Sections 10 and 11 required resealing after carrying approximately 3.5 x 106 equivalent standard ties, and Section 12 required resealing after carrying more than 5 x 106 equivalent standard axles. It islikely that with additiond resealing these sections, except for part of Section 10, wtil continue to provide good service for some years. 8. IMPLICATIONS OF THE RESULTS FOR THE RELATIONSHIPS USED IN THE ROAD TRANSPORT INVESTMENT MODEL3 The sections which are the subjects of this report (referred to as OBS17 to 25 and P.8 to 10 in reference 1) formed only a part of the original investigation, but the pavement deterioration relationships in the model that were derived from Sections 1 to 9 will have to be modified. In the model the pavement strength of each section is expressed in terms of a modified structural number 1 and pavement deterioration relationships are related to the cumulative equivalent standard axles carried by the sections. Different relationships have been developed for a range of modified structural members. It has been shown that between 1973 and 1979 the sections have been strong enough to carry the imposed traffic loadings without suffering serious structural damage. Little change in rut depths has occurred and roughness only increased when potholes were allowed to develop or the surfaces of patches were not finished to the same level as the adjacent pavement. Significant deterioration has been confined to the surfacings, but each successiveseal has apparently been more durable than the preceding one. This could reflect variations in the qualities of the seals, but their performances may dso have been affected by the drier weather between 1973 and 1977. Before any amendments are made to the existing deterioration relationships additional measurements are required on the sections to determine the effects of the wetter weather which started in 1977 and the durability of the slurry seals. 9. CONCLUSIONS 9.1 Se@ions with soil-cement road bases and bituminous seals i) Traffic loading at the rate of three-quarters of a million equivalent standard axles per year has had negligible effect on thecondition of these lightly-constructed andnormally maintained pavements built on strong subgrades in a ‘dry’ climate. ii) It is likely that climatic effects have a much stronger influence than traffic loading on these pavements (climate clearly influences deflection), but it is too early to tell whether the recent wet weather will result in significantly accelerated pavement deterioration. iii) The deterioration that has occurred, initially in the surfacing, is most prevalent in wet weather and can be remedied by simple patching, waterproofing and periodic resealing. iv) Timely resealing has a powerful effect in limiting the amount of patching required and maintaining an acceptable level of roughness. Subsequent seals appear to have longer ‘lives’than earlier seals. v) The lives of the better quality seals have been of the order of four to five years, but the life of a sed isinfluenced by the quality of previous patching and the bond achieved between the serds. vi) Deflection measurements clearly showed how changes in overall pavement strength can be affected by rainfall and the wide variation in strengths which can occur on a nominally uniform pavement. It is unlikely that deflection measurements can be used to predict pavement performance in these circumstances, where climate and the quality of the surfacings greatly affect pavement deterioration. Deflection histories would however be of vrdue for overlay thickness design. vii) The pavement deterioration relationships used in the Road Transport Investment Model for surfacedressed roads with cement stabilised road bases need to be revised, but further evidence of the combined effects of climate and traffic on the deterioration of these pavements is required. 9.2 Sections with crushed rock road bases and asphaltic mncrete surfacings i) Two sections have carried some 7 x 106 equivalent standard axles and the third has carried 8 x 106 equivalent standard axles in the Nairobi direction by mid-1979. ii) Measurable deterioration has been confined to the verge-sidewheelpaths and has been consistent with fatigue failure of the surfacings. iii) Simflar values of mean linear cracking of the surfacings were recorded on the sections in both verge-side wheelpaths, despite the fact that the traffic loading in the Nairobi direction wasthree times greater than in the Mombasa direction. There is evidence that cracking propagates down through the surfacing more rapidly under the heavier traffic, resulting in accelerated deterioration in the form of rutting when water is able to enter the road base. iv) As with the other sections studied the application of seals at the appropriate time has a great effect in preserving the structural strength of the pavement. v) Measurement of surface deflection did not provide an indicator of subsequent pavement deterioration in the form of cracking. 10. ACKNOWLEDGEMENTS The work described in this report forms part of the research programme of the Overseas Unit (Unit Head: J N Bulman) of TRRL. The authors wish to thank colleagues in the Laboratory, particularly those who were involved in the Kenya Highway Study, and the local staff employed in Kenya for their help in these studies. The cooperation of the Government of the Republic of Kenya is gratefully acknowledged for allowing the conduct of field experiments on a public road. Particular thanks are due to the Ministry of Works (Roads and Materials Branches) for their cooperation and assistance. 1. 2. 3. 4, 5. 6. 7. 8. 10 11. REFERENCES 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 bboratory). HIDE, H,SW ABAYNAYAKA,I SAYER and R WYATT. The Kenya Road Transport Cost Study: research on vehicle operating costs. Department of the Environment, TRRL Report LR 672. Crowthorne, 1975 (Transport and Road ,Research hboratory). ROBINSON, R, H HIDE, J WHODGES, S WABAYNAYAKAand J ROLT. A road transport investment model for developing countries. Department of the Environment, T~L Report LR 674. Crowthorne, 1975 (Transport and Road Research bboratory). POTOCKI, F P. A portable wheel-weighing unit and data recorder. Department of the Environment, RRL Report LR 391. Crowthorne, 1971 (Road Research Laboratory). JONES, T E. Axle-loads on paved roads in Kenya. Department of the Environment Department of ~ansport, TML Report LR 763. Crowthorne, 1977 (Transport and Road Research bboratory). HIGHWAYRESEARCH BOARD. The AASHO Road Test Report 5. Pavement Research. Highway Research Board Special Report 61 E. Washington, DC. 1962 (National Research Council). LIDDLE, WJ. Application of AASHO Road Test results to the design of flexible pavement structures. Proc. Int. Confi on the Structural Desiw of Asphalt Pavements held at the University of Michigan. Ann Arbor, 1963 (University of Michigan), pp 42–5 1. LISTER, N W. Deflection criteria for flexible pavements. Department of the Environment, TRRL Report LR 375. Crowthorne, 1972 (Transport and Road Research hboratory). 9. CANADIAN GOOD ROADS ASSOCIATION. A guide to the structural design of flexible and rigid pavements in Canada. Canadian Good Roads Association Design and Evaluation Committee. Ottawa, 1965 (Canadian Good Roads Association). 10. KENNEDY, C K and N W LISTER. Prediction of pavement performance and the design of overlays. Department of the Environment Department of Transport, T~L Report LR 833. Crowthorne, 1978 (Transport and Road Research bboratory). 11. SMITH, H R and C R JONES . Measurement of pavement deflections in tropical and sub-tropical climates. Department of the Environment Department of Transport, TUL Report LR 935. Crowthome, 1980 (Transport and Road Research Laboratory). 12. KENNEDY, C K. Pavement deflection: operating procedures for use in the United Kingdom. Department of the Environment Department of Transport, T~L Report LR 835. Crowthorne, 1978 (Transport and Road Research Laboratory). 13. ROAD RESEARCH LABORATORY. A guide to the structural design of pavements for new roads. Department of the Environment, Road Note 29. Third edition. London, 1970 (H M Stationery Office). 14. TRANSPORT AND ROAD RESEARCH LABORATORY. A guide to the structural design of bitumen-surfaced roads in tropical and sub-tropical countries. Department of the Environment Department of Transport, Road Note 31. bndon, 1977 (H M Stationery Office), 3rd Edition. 11 TABLE 1 Detafls of the road sections Site hcation Number Kflometres from Mombasa 1 355.0–356.0 2 353.0–354.0 3 349.3 –350.3 4 331.1 –332.1 5 269.0–270.0 6 262.1 –263. 1 7 261.1 –262.1 8 254.4–255.4 9 210.6–21 1.6 Date opened to Type of construction traffic* 1967 I * Surface treatment on cementstabtiised base. Granular subbase 1966 Asphaltic concrete on crushed 1971 stone road base. Stabilised granular sub-base * Before 1970, when freight was transferred from rail to road transport, there was only light traffic on the road. TABLE 2 Summary of annual rainfall recorded near to the sections Sections I 1–9 I 10,11 I 12 Year Range Mean Mean Mean (mm) (mm) (mm) (mm) 1971 1972 1973 1974 1975 1976 1977 1978 490– 780 490– 680 270– 560 460– 580 170– 340 420– 560 420– 820* 740–1 250 600 660 410 500 250 500 690 980 373 ’873 492 521 511 626 790 — 580 15’19 982 804 901 902 1447 — * Incomplete data for 2 sections. 12 TABLE 3 Pavement layer thicknesses Section Mean and range of layer thicknesses measured (mm) No. Surfacing Base Sub-base 1 2 3 4 5 6 7 8 9 29, 21, 26, 29, 32, 34, 34, 27, 26, 20–35 20–25 25–30 20–40 25–50 25–40 25–55 25–30 20–30 147, 128, 126, 133, 131, 130, ‘150, 150, 139, 120–1 60 120–140 105–140 110–150 115–140 110–160 130–160 130–180 100–180 131, 115, 100, 103, 95, 116, 105, 112, 124, 75–250 100–140 80–1 20 80–1 30 65L150 80–170 80– 140 80–150 90–180 10 11 12 32, 20–40 31, 30–35 26, 20–40 130, 80– 190 167, 120–270 155, 135–180 114, 70–140 279, 200–320 248, 70–380 Base + Sub-base 278, 210–400 243, 250–260 226, 185–240 236, 210–260 , 226, 180–290 246, 210–280 255, 210–300 262 225–320 263. 210–360 297, 240–370 269, 230–320 527, 370–580 Note: The surfacings on”Sections 1 to 9 comprise multiple seals. The surfacings on Sections 10 to 12 were asphrdtic concrete. 13 TABLE 4 In-situ materials test data Base Sub-base SubWade Moisture content Casagrande classification CBR Moisture content CBR (Per cent ) Section No. CBR (Per cent) Moisture content ( Per cent) (Per cent ) (Per cent) ~er cent) Mean 14.1 13.3 7.7 7.3 8.8 8.8 10.7 13.1 12.1 10.8 Unge 8–18 9–18 7– 8 4–lo 8–10 7–11 8–14 11–15 11–13 8–12 E— 3.2 3.2 0.5 2.0 0.9 1.5 2.0 1.3 0.9 1.3 — SD 18.5 26.3 11.1 — 24.8 21.5 16.0 19.3 13.3 26.5 — — — !ean 16 17 16 7 8 8 11 12 12 9 T 5 3 — SD— 4 4 2 4 2 1 3 2 1 3 T 2 1 dean - 16 21 13 39 65 52 27 25 26 65 62 80+ 25t Range 8–29 12–32 10–17 21–50 40–95 35–loo 15–50 15–35 15–40 19–100 25–70 W–>loo 5–>100 K— 8 5 3 9 24 24 12 9 10 28— 28t 27t 38t Mean 21 19 20 20 9 8 9 9 10 5 10 9 11 Range 18–23 16–2 1 19–22 15–24 5–1 1 6–10 6–12 6–12 9–13 3–6 7–12 7–1 1 5–15 31 2 1 1 3 2 1 2 2 1 1 ; 1 5 Range 35–75 20–100 50–80 23–> 100 19–80 30–90 12–50 12–60 15–30 20–95 50–> 100 80–> 100 30–> 100 Range 9–22 10–24 14–18 3–15 4–11 7-8 7–14 7–14 10–14 5–12 2–13 3–7 2–4 Mean Range 91+ 70–> 100 96t 65–> 100 >100 – >100 – >100 – 96t 80–> 100 96t 75–> 100 86t 75–> 100 84t 75–> 100 >100 – tiean (19711/2) (19;5/6) 55 48 65 5ot 45 60 30. 30 28 60 GC GC GC GC GC SF Sc Sc SF Sc 2 3 4 5 6 7 8 9 10 11 12 85t 95t 77+ GC GC 1 GF Not measured SD = Standard deviation TABLE 5 Percentage of material passing various sieve sizes for crushed stone road bases Sieve size (mm) 63 50 37.5 28 20 14 . 10 5 Kenya MOW Specification 100 88–98 78–96 66–89 54–79 40-70, 32–62 Road Note 31 Specification 14 100 100 95–100 60–80 40-60 25–40 Section 10 99 84 71 58 46 39 30 - Section 11 100 88 81 72 64 58 48 Section 12 100 90 83 74 63 54 40 , 2;36 2 0.6 0.425 0.212 0.075 0.063 20–44 16–33 15–30 11–24 5–15 15–30 8–22 5–12 I I I I I I — 25 18 15 11 7 — 43 32 27 17 6 — 28 16 9 6 1 TABLE 6’ ~ Results of materials tests on crushed stone bases Section No. I 10 I 11 I 12 I Kenya Ministry of Works Specification Aggregate crushing value 35 35 36 I 35 @s Angeles value 43 41 II 45 ! 50 P.I. of fines Non plastic I <6 . TABLE 7 Estimate of traffic carried by the sections m Dates of sedin[ Section Year of No. opening 1St 2nd to traffi reseal reseal 4 1967 1969 March 1973 5 I 1966 I 1970 I August 1972 6 I 1966 I 1970 I August 1972 7** 1966 1970 June 1974 8 1966 ~970 September 1972 9 1966 1968 October 1974 * 12 1971 June 1976 – 0verlaid7 Fe;;;;y (:::) (::8) ‘verlaid - June 0.28 1.51 3.42 0.44 1978 (0.10) (0.54) (1.21) (0.20) — 0.28 4.92 0.44 (0.10) (1.75) - (0.20) June 0.28 1.47 3.46 1978 (0.10) (0.53) (1.22) May 0.45 1.12 0.96 3.28 1974 (0.20) (0.34) (0.26) (1.25) June 0.45 1.12 1.00 3.24 1974 (0.20) (0.34) (0.30) (1.21) - 0.45 2.12 3.24 — (0.20) (0.64) - (1.21) June 0.45 1.17 0.95 3.24 1974 (0.36) (0.28) (1.21) 0.25 2.42. 3.06 — (0.10) (0.76) - (1.19) 4.60 2.10 — (1.27) “ - (0.89) — 4.60 2.10 (1.27) - - (0.89) 5.33 2.43 — (1.70) - - ,(1.09) Total to an. 1979 — 5.64 (2.05) 5.64 (2.05) 5.64 (2.05) 5.81 (2.05) 5.81 (2.05) 5.81 (2.05) 5.81 (2.05) 5.73 (2.05) 6:70 (2.16) 6.70 (2.16) 7.76 (2.79) * No. of equivalent standard (80 kN) ade loads. T Used for experimental overlays. ** Effect of nfl mainten~nce studied. Section extensively patched in 1973. Traffic towards Mombasa shown in brackets. 16 TABLE 8 Surface condition of the sections immediately before resealing Mean cracking Cracking and patching Mean rut depth Section Date of (m/m2) (Per cent) (mm) No. reseal Towards Towards Towards Towards Towards Towards Nair?bi Mombasa Nairobi Mombasa Nairobi Mombasa 1 1974 6.9 4.6 38 35 4 4 1977* ‘o o 0 0 2 2 2 1973 4.4 3.5 26 18 4 5 1978 1.6 1.6 20 8 3 3 3 1978 3.8 4.5 48 64 2 2 4 1973 0.9 0.1 3 0 4 3 1978 0 0 1 1 2 1 1972 1.9 1.8 1 12 3 4 5 1974 0.1 0.3 19 25 2 3 (1979) o 0.1 0 o“ 4 5 1972 I 2.5 3.4 14 27 4 4 6 1974’ 0.1 0.7 6 10 3 3 (1979) o 0.2 0.4 0 5 3 7 1974 3.5 2.0 74 64 5 8 (1979) 0.7 0.7 8 7 7 7 1972 1.9 1.7 25 32 6 4 8 1974 0.1 0.1 16 10 5 3 (1979) 0.1 0.1 0.3 0 6 3 9 1974 0.2 0.3 2 2 4 3 1979t o 0 0 0 7 4 10 1976 1.0 1.8 6.5 1.5 11 ;ad- 1 ingof21) 11 1976 0.8 0.7 ~ o o.i 1 0 12 } 1976 0.3 0.3 0 0 0 1 * Overlaid. ( ) Condition in 1979 of slurry seals applied in 1974. t Condition in 1979 of surface dressing applied in 1974. , 17 ‘ TABLE 9 Classification of road surface condition Transverse deformation under a 2m long straightedge Classification Index D1 D2 D3 D4 D5 Deformation bss than 10 mm 10mmto 14mm 15mmto19mm 20 mm to 25 mm Greater than 25 mm Degree of cracking (visible cracks) Classification Index C5 Crack len~h/unit area Not greater than 1 m/m2 Greater than 1 m/m2 but not greater than 2 m/m2 Greater than 2 m/m2 but not greater than 5 m/m2 Greater than 5 m/m2 (ravefling and potholing imminent, immediate maintenance required) 18 . . . TABLE 10 Pavement layer Surfacing Road base Sub-base Total thickness (excluding surface dressings) Comparison of actual and recommended layer thicknesses from design guides Range of mean layer thicknesses (mm) Sections 1–9 Surface dressings 126–150 95–131 226–278 Sections 10–1 2 26–32 130–279 114–248 300–553 Recommended layer thicknesses (mm) from design guides for total traffic loadings Of – 2.5 X 106 ESA Road Note 31 for subgrade CBR (per cent) of:– 8–24 24+t Surface dressing 200 200 100 Nfl 300 200 Road Note 29* for subgrade CBR (per cent 8–29 “80 170 150 400 of: – 29+t 80 170 Nil 250 * Sofl-cement road base acceptable for 1.5 x 106 ESA. Graded crushed rock road base acceptable for greater traffic loading. 6 X lb6 ESA Road Note 29* for subgrade CBR (per cent) of– 8–29 i 29+~ 100 100 200 200 150 Nd 450 300 t Apphcable for Sections 4,511 and parts of the other sections. O Towards Nairobi 3900 3700 3500 3300 3100 2900 2700 3700 < 3300 + ~ 3100 2300 SECTION 1 0 0 0 00 ● o -o ● ‘o o 0 000 ● . . ● ● 000 Resealed Overlaid ‘o I ● 1 I I 1 I 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5io To Nairobi I To Mombasa 210 1.0 SECTION 2 0 0 Pot holes 0/S Wheelpaths o 0’0 : 00 0 0 ● Needs patching o & resealing - @@ o ● ~e5ealed ● ● ● ° ● ● 8 Resealed i i 1 I 1 + I T 1 + 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.f To Nairobi 1 I 1.0. To Mombasa 20 Cumulative equivalent standard axles (x 106) Fig. 1 ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADING O Towards Nairobi . Towards Mombasa i 3900 ~700 SECTION 3 z < ● ● 3500 - m o 0 ~ 3300 - ~ O.” 0 Needs patching m 3100 - : ● 8° &resealing m 2900 - e: o > 0 Pot-holes . Resealed : 2700 ● “ -$Q@g , 0/S W-T 2500 “ I ● 1 1 v I 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 To Nairobi 1 I 1.0 To Mombasa ~lo 3600 z 3400 - SECTION 4 e ~ 3200 -. ● $ 3000 - 00 0 ● a 00 : 2800 - & e :e,ealed 00 0 0 m3 @Q : :, z 2600 - ● 0 ● Resealed , 2400 I I I 1 1 1 i 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4!0 5.0 To Nairobi I 1 I t 1.0 To Mombasa ~ o ‘ Cumulative equivalent standard axles (x 106) Fig. 1 (CONT.) ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADING O Towards Nairobi ● Towards Mombasa I 3800 3600 - SECTION 5 z : 3400 - ● g @o 3200 - ● . 8 zw 3000 - *O o Q8 8 ‘5 m o 3 2800 - Resealed Slurry sealed m : 2600 -**8Q1 2400 A 1 1 1 1 1 1 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 ‘ 3.0 4.0 5.0 TO Nairobi ‘ 1, 1:0 ‘o Mombasa 2.0 3900 3700 3500 3300 3100 2900 2700 2500 SECTION 6 0 g Oo 0 ● ● o ● o ● o m: ● . o ● ● ● O ● Resealed 0. - .Q Slurry sealed 1 1, 1 I 1 I 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5io TO Nairobi I 1 1.0 ‘o Mombasa 2.0 Cumulative equivalent standard axles (x 106) Fig. 1 (CONT.) ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADING 4800 4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 I O Towards Nairobi ● Towards Mombasa I SECTION 7 0 ● o ● o 0 ● o ,0 ● ● ✎ o 0 ● ● o ● &\” Patched - 00 Slurv sealed II ● 1 I *I + I 1 1 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5io TO Nairobi 1 1.0 To Mombasa 2 ‘o Cumulative equivalent standard axles (x 106) Fig. 1 (cONT.) ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADtNG I O Towards Nairobi ● Towards Mombasa . 3600 3400 3200 3000 2800 2600 2400 3800 3600 SECTION 8 0 00 0 0. 00 0 ● o. ● o - :0 Oej ● ● ’ ● . SlurrV . Resealed ● ● . sealed ● * 1 1 1 1 1 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5io TO Nairobi 1 i 1.0 ‘o Mombasa 2:0 SECTION 9 ● Q eo~ 0: ● o 0 ● ● ● o 2 z 2600 - ~ Resealed 2400 - 2200 1 1 1 11 1 1 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 TO Nairobi I 1 1.0 To Mombasa 2.0 Cumulative equivalent standard axles (x 106) Fig. 1 (CONT.) ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADING IO Towards Nairobi z* 2200 ~ooo SECTION 10 @ ~ 8 0 % 1800 %a ‘m@880g0 00 2 1600 - ● ● m . Slurry sealed 3 1400 1 1 1 ; I 1 i I I 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 To Nairobi I To Mombasa 1 1.0 2.0 — 2200 2000 1800 1600 1400 2400 2200 2000 1800 1600 SECTION 11 9. 0 9.” -Om8 @ ‘8 g : @@ Slurry sealed 1 1 1 1 1971 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5;0 To Nairobi I ( 1.0 ‘o Mombasa 2.0 SECTION 12 8 0 u’ eQm Q** 0% ● o . Slurry sealed 1400 I I I I ● 1 I 1 1971 1972 .1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 6.0 7.0 To Nairobi I 1.0 210 To Mombasa Cumulative equivalent standard axles (x 106) Fig. 1 (CONT.) ROUGHNESS VERSUS CUMULATIVE TRAFFIC LOADING . . u cm 1600 SECTION 1 1200 - ‘o m O 2nd Seal @ . 3rd Seal o “o 800 - 0 00 400 - 0 Overlaid (1977) o - . 1 A 1 I I L o 1.0 2.0 3.0 4.0 5.0 SECTION 2 1200 - ,0 0 2nd Seal \_ ● 3rd Seal .. 800 - 00 0 400 - 0 00 ● 0 ● ‘o o ~ 1 1 1 ! o 1.0 2.0 3.0 4.0 5.0 u cm 1600, 1 SECTION 3 1200 0 m’ . ~ .- 800 I 00 r v 0 G 00 a 0 400 00 000 0 0 1.0 2.0 3.0 4.0 5.0 Cumulative equivalent standard axles in Nairobi diredion (x 106) Fig. 2 CRACKING AND PATCHING VERSUS CUMULATIVE CARRIED BY EACH SEAL. TRAFFIC LOADING ‘ , 1600 1200 800 400 SECTION 4 .0 0 2nd Seal . 3rd Seal o 0 1.0 2.0 3.0 4.0 5.0 1600 1200 800 SECTION 5 ❑ O 2nd Seal ● 3rd Seal A 4th Seal 400 “ 0-0 ●A*OQ I A I A 1“ I o 1.0 2.0, 3.0 4.0 5.0 1600- SECTION 6 n O 2nd Seal 1200 ● 3rd Seal A 4th Seal 800 400 - 00 0 0 o~ 1 A 1 A 1 L o 1.0 2.0 ‘, 3.0 4.0 5.0 Cumulative equivalent standard axles in Nairobi dire~ion (~ 106) Fig. 2 (CONT.) CRACKING AND PATCHING VERSUS CUMULATIVE TRAFFIC LOADING CARRIED BY EACH SEAL. n 240C 2000 1600 1200 800 400 0 SECTION 7 0 ‘n O 2nd Seal o ● 3rd Seal Oo 0 0 000 0 0 .,. 00 ● ● ● 0 ● ’ o 1.0 2.0 3.0 4.0 5.0 1200 SECTION 8 n O 2nd Seal 800 - 0 0 ● 3rd Seal # ●0 A 4th Seal 400 -- 0 1.0 2.0 3.0 4.0 5.0 1200 SECTION 9 m O 2nd Seal 800 - ● 3rd Seal 400 - 0 - 00 QI$ n o 001 0 1 1 1 0 1.0 2.0 3.0 4.0 Cumulative equivalent standard axles in Nairobi dire~ion (x 106) 5.0 Fig. 2 (CONT.) CRACKING AND PATCHING VERSUS CUMULATIVE TRAFFIC LOADING CARRIED BY EACH SEAL. 5 4 3 2 1 0 3 2 1 0 — Verge side wheelpath towards Nairobi -— -- Verge side wheelpath towards Mombasa SECTION 1 . Resealed Overlaid 1972 1973 1974 1975 1.0 1976 2.0 1977 1978 3.0 4.0 5.0 To Nairobi I 1.0 To Mombasa 2.0 Cumulative equivalent standard axles (x 106) SECTION 2 Resealed Resealed / 1972 1973 1974 1.0. 1975 2.0 1976 1977 3.0 1978 4.0 5.0 To Nairobi I 1.0 ‘o Mombasa 2.0 Cumulative equivalent standard axles [(x 106) Fig. 3 MEAN CRACKING IN VERGE SIDE WHEELPATHS VERSUS CUMULATIVE TRAFFIC LOADING “”“,,.,,,,.,,” — Verge side wheelpath towards Nairobi — — — Verse side wheelDath towards Mombasa 6 5 4 3 2 1 0 3 2 1 0 . /4% /“ y _- / \ ------ Patching Resealed I 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5io TO Nairobi I To Mombasa 210 1.0 Cumulative equivalent standard axles (x 106) 1 SECTION 4 P\ Resealed 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 To Nairobi I 1.0 ‘o Mombasa 2.0 Cumulative equivalent standard axles (x 106) Fig. 3 (cont.) MEAN CRACKING IN VERGE SIDE WHEELPATHS VERSUS CUMULATIVE TRAFFIC LOADING “”“” — Verge side wheelpath towards Nairobi ---- Verge side wheelpath towards Mombasa 3 2 1 0 3 2 1 0 5 4 3 2 1 0 SECTION 5 <,-c ,, , , , _p . Resealed z Resealed (slurry) ,/4 ~@---- A\ ●d Nil SECTION 6 Resealed Resealed (slurry) 1 I Nil I 1 1 I 1 SECTION 7 . . 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 To Nairobi 1 1 1.0 To Mombasa 2.0 Cumulative equivalent standard axles (x 106) Fig. 3 (cont.) MEAN CRACKING IN VERGE SIDE WHEELPATHS VERSUS CUMULATIVE TRAFFIC LOADING “, ,.,,”, “, . ,,, ,,. ,,,,.,,” 3 2 1 0 3 2 1 — Verge side wheelpath towards Nairobi — — - Verge side wheelpath towards Mombasa ‘)/, Resealed Resealed (slurw) /’ .4 p~ ~ -3 I , Nil -y 1 1 - 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 . To Nairobi I 1.0 To Mombasa 2.0 Cumulative equivalent standard axles (x 106) . Resealed I 1972 1973 1974 1975 1976 1977 1978 1.0 2.0 3.0 4.0 5.0 To Nairobi 1:0 To Mombasa 1 2.0 Cumulative equivalent standard axles (x 106) Fig. 3 (cont.) MEAN CRACKING IN VERGE SIDE WHEELPATHS VERSUS CUMULATIVE TRAFFIC LOADING “”“,,.,,,,.,,” ( 1, I ./ I I / / /’ / / /. ‘~\/ (, . 90 80 70 60, 50 40 30 20 10 Surface dressef &1971 1972 1.0 Slurry seal 1 2io v. 3.0 4.0 5.0 To Nairobi 1.0 To Mombasa 2.0 Cumulative equivalent standard axles (x 106) Fig. 5 DEFLECTION HISTORY FOR SECTION 6 “”“”300 200 100 0 C – Cracking D – Deformation P – Patching Fig. 6 SYMBOLS USED FOR RECORDING PAVEMENT CONDITION ON DEFLECTION HISTORY CHARTS (Figs. 7- 10) 150 140 130 120 110 100 90 8C 70 60 50 40 . . . VERGE SIDE WHEELPATH TOWARDS NAIROBI Resealed Resealed o 0 0 0 Trend lines 1972, 1973 I 1974 1 1975 1 1976 I 1977 1 1978 ,1979 I I I 1:0 Fig. 7 DEFLECTION 2;0 3.0 4.0 5.0 Cumulative equivalent standard axles (x 106) HISTORY FOR INDIVIDUAL TEST POINTS ON SECTION 2 14( 13[ 12C Ilc 100 9a 8a 70 60 50 40 30 20 . VERGE SIDE WHEELPATH TOWARDS NAIROBI Resealed .1 Slurry sealed 4 0 1972 , 1973 1 1974 I 1975 1 1976 1 1977 I 1978 ,1979 I I I I I 2.0 3.0 4.0 5.0 6.0 Cumulative equivalent standard axles (x 106) Fig. 8 DEFLECTION HISTORY FOR INDIVIDUAL TEST POINTS ON SECTION 6 \/ / I I \ I Ooomm o 006 0 0 @o ( ~-01 x ww) uo!laal~ap lua!sueJl ● \*rr’ \ mo 0 00 Mo x ,.m 0 m 0 m 0 0 .-m L (825) Dd0536380 1,500 5/80 HPLtd ~o’ton G1915 PRINTEDIN ENGLAND ABSTRACT Performance ofsetiions of the Nairobi to Mombasa road in Kenya: HRSMITH, TEJONES and C R JONES: Department of the Environment Department of Transport, TRRL Laboratory Report 886: Crowthorne, 1980 (Transport and Road Research Laboratory). Details are given of the performance of sections of the Nairobi-Mombasa road in Kenya. Nine sections of road with cement-stabilised bases surfaced with multiple seals, and three sections with crushed rock road bases and asphaltic concrete surfacings have been studied for seven years. These road sections are located in a dry area, on strong subgrade soils. It was found that the lightly constructed sections with cement-stabilised bases had suffered little structural damage after carrying 6 x 106 equivalent standard (80 kN) axles, considerably more than would be predicted by current methods of pavement design. The more heavily constructed sections with asphaltic concrete surfacings are performing broadly as would be expected. For both types of pavement, deterioration has been mainly confined to the surfacings, and timely resealing has a powerful effect in limiting the amount of patching required and in maintaining an acceptable level of surface roughness. The lives of the better quality seals have been of the order of four to five years. Deflection measurements clearly show how variations in overall pavement strength are affected by changes in annual rainfall, and the wide variation in strength which can occur on a nominally uniform pavement. Deflection measurements did not give early warning of pavement surfacing failures, ISSN0305–1293 ABSTRACT Performance of seetions of the Nairobi to Mombasa road in Kenya: H R SMITH, T E JONES and C R JONES: Department of the Environment Department of Transport, TRRL Laboratory Report 886: Crowthorne, 1980 (Transport and Road Research Laboratory). Details are given of the performance of sections of the Nairobi-Mombasa road in Kenya. Nine sections of road with cement-stabilised bases surfaced with multiple seals, and three sections with crushed rock road bases and asphaltic concrete surfacings have been studied for seven years. These road sections are located in a dry area, on strong subgrade soils. It was found that the lightly constructed sections with cement-stabilised bases had suffered little structural damage after carrying 6 x 106 equivalent standard (80 kN) axles, considerably more than would be predicted by current methods of pavement design. The more heavily constructed sections with asphaltic concrete surfacings are performing broadly as would be expected. For both types of pavement, deterioration has been mainly confined to the surfacings, and timely resealing has a powerful effect in limiting the amount of patching required and in maintaining an acceptable level of surface roughness. The lives of the better quality seals have been of the order of four to five years. Deflection measurements clearly show how variations in overall pavement strength are affected by changes in annual rainfall, and the wide variation in strength which can occur on a nominally uniform pavement. Deflection measurements did not give early warning of pavement surfacing failures. ISSN 0305–1 293