PERFORMANCE OF WATERBOUND MACADAM PAVEMENTS CONSTRUCTED LABOUR INTENSIVELY: CASE STUDY

Transcription

1 PERFORMANCE OF WATERBOUND MACADAM PAVEMENTS CONSTRUCTED LABOUR INTENSIVELY: CASE STUDY R.J. Moloisane 1 and W.A. van Wyngaard 2 1 Africon Engineering International (Pty) Ltd. PO Box 905, Pretoria, 0001, South Africa. jonesm@africon.co.za 2 Tshwane University of Technology Department of Civil Engineering, Private Bag X680, Pretoria, 0001, South Africa. vanwyngaardwa@tut.ac.za ABSTRACT Waterbound Macadam (WM) layers were used extensively during the first half of the previous century and have been reintroduced in the 1980 s, focusing on re-engineering labour-intensive construction techniques during the 1990 s. In 1994 and 1995 the former Johannesburg Transitional Metropolitan Council (TMC) in consultation with AFRICON and the University of the Witwatersrand constructed two pilot projects in Johannesburg with a labour-based method utilising WM base courses. The design and construction of various WM roads in Soweto followed in The construction processes were monitored throughout and data collected to investigate certain technical issues; among them the in-situ WM properties, the construction tolerances and expected performance. Since that time, a number of WM projects followed towards the development of sound guidelines for the design and construction of these pavements. A case study was conducted in 2001 on eight of these waterbound macadam pavements to assess the in-service short-term performance, and re-visiting the expected structural capacity by applying state of the art techniques in the analysis of these layers. The investigation involved a visual condition assessment and non-destructive mechanical measurements. This paper present the results obtained, highlight findings on the behaviour of these pavements, discusses the expected performance, and comments on the product obtained using labour-intensive construction techniques. It was found that the pavements generally performed well with the rare occurrence of only surface related distresses. It appears that the elastic modulus of most of the WM layers increased, with consequent increase in structural contribution. This confirms that post-construction densification of these layers have taken place due to applied traffic loads and environmental factors. Keywords: Waterbound Macadam, Structural capacity, Structural performance 1. INTRODUCTION The introduction of macadam pavements dates back to the birth of modern road building (Horak, 1983). The majority of earliest pavements in South Africa were built according to the macadam principles (Hefer, 1997). Waterbound and penetration Macadams were however constructed up to the 1960s-1970s (Hefer, 1997; Horak, 1983). Waterbound macadam (WM) pavements were used extensively during the first half of the twentieth century (Calitz et al, 1995), but interest in these pavements declined in the 1960s when mechanisation construction became popular (Visser and Hattingh, 1998). A considerable number of studies (Theyse, 2000; Hattingh and Potgieter, 1999; Hefer, 1997; Calitz et al, 1995; Phillips, 1994; 1992) have been Proceedings of the 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) September 2004 ISBN Number: Sun City, South Africa Proceedings produced by: Document Transformation Technologies cc

2 conducted in South Africa over the last decade, attempting to address design issues of WM such as the in-situ layer stiffness, mechanistic transfer functions, construction tolerances and expected performance. With the implementation of the reconstruction and development programme (RDP) during the 1990s, the use of WM pavement layers gained prominence (Calitz et al, 1995). The main advantage of the use of WM layers in terms of the RDP was the potential for increased employment in road construction (Calitz et al, 1995; Phillips et al, 1993; Phillips, 1992). There has been a renewed interest in South Africa over the last decade in the strength properties of large aggregate materials (Hoffmann and Smit, 1996; Calitz et al, 1995). The major focus of the RDP was the need for road construction of (more heavily trafficked) bus routes in underdeveloped rural and urban areas and township upgrading (Van Wyk & Louw Inc., [S.A.]). These road constructions were carried out through pilot projects (Horak, 1995). Horak et al (1996) reported that the work was carried out as fast-track labour-intensive projects. This contradiction in terms meant that roadbed preparation was to be done with machine-intensive construction techniques to ensure the realisation of the fast-track feature. Focussing on proven high quality end-product labour-intensive construction techniques ensured the high labourintensive component of road base and surfacing. Large-stone base course construction was favoured since WM has proven itself over the years as a high quality, durable base type with high labour-intensive content (Horak, 1995). Macadam pavements consist of high quality base layers constructed with large-sized aggregate (typically railway ballast stone) of approximately 37 to 75 mm, filled with fine aggregate, which is less than 4 to 5 mm (Visser and Hattingh, 1998; Calitz et al, 1995). The fine aggregate can either be vibrated into the large aggregate (dry method) or washed into the large aggregate with water (wet method) (Calitz et al, 1995). With dry method the filler is swept into the voids between the large aggregate and vibrated with rollers whereas with the wet method the filler is washed with water into the voids between the large aggregate. The product developing from the vibration of dry fine aggregate into the voids without the use of water is called drybound macadam, whereas with the use of water is called waterbound macadam. Subbases are mostly stabilised to provide good work platforms, but historically prior to 1960s and 1970s subbases were not stabilised (Horak and Triebel, 1986) Typical gradings and material qualities of WM are specified in TRH14 (1985). There are two designs that are included in the TRH4 (1985) and the Draft TRH4 (1996). These designs, known as WM1 and WM2, differ in respect of the final densities. The WM1 layer should be compacted to 88-90% of apparent density (AD) while the WM2 should be compacted to 86-88% AD. In the Draft TRH4 (1996) catalogue, the required thickness of the WM layers are equated to crusherrun bases in terms of strength and thickness. WM designs are available for all design traffic classes and all road categories (Theyse, 2000; Hefer, 1997; Draft TRH4, 1996) The study by Burrow (1975) on roads in the former Transvaal Province indicated that waterbound and penetration macadams pavements performed better than other pavement types. For a given base course thickness, WM roads tolerated weaker subgrades without failing. The recommendation made by Burrow (1975) was the use of a thinner WM layer rather than a crushed-stone layer for the same pavement strength. WM base courses were reported (McCall et al, 1990) to be excellent drainage layers to be used in cases of high water tables or subsurface water from other sources. Analysing the WM base course under Heavy Vehicle Simulator (HVS) test conditions, Horak (1983) found that the WM layers noticeably densified under loading. The pavements investigated contained 150 mm and 235 mm WM layers on thick stabilised subbases, and Horak (1983) concluded that the WM layers performed well. The elastic modulus of the WM was found by Horak (1983) to be non-linear and stress dependent.

3 Studies using HVS tests on macadam pavements during the 1990s included sites near Cullinan on Road 2388 (Theyse, 1999) and at Louis Trichardt on N1-28 (Du Plessis, 1999). The Cullinan project was an investigation research programme while the Louis Trichardt project was a reconstruction. The reports of these studies showed that macadam pavements somehow tend to deform during the initial exposure to traffic loading, whereafter the rate of deformation decreases significantly (Theyse, 2000; 1999). Both HVS testing projects experimented with the influence of water and different repetitions that influenced the permanent deformation. The poor riding quality on Road 2388 (composite and waterbound macadam sections) was ascribed to insufficient compaction (Theyse, 1999). These HVS tests indicated that most of the permanent deformation takes place in the macadam layers (Du Plessis, 1999; Theyse, 1999). It was noted by Hefer (1997) that the HVS test can be aggressive, as it uses the continuous application of the wheel load throughout the period of testing over a fairly restricted area. WM material provided a slightly lower bearing capacity in both HVS tests (Du Plessis, 1999; Theyse, 1999). Field density of the WM was, however, very low and it did not affect the performance of the material. The performance of the composite and waterbound macadam layers on the Cullinan project was satisfactory (Theyse, 1999). Thinner composite- (75 mm) and waterbound macadam (100 mm) base layers performed better than the thicker composite- (125 mm) and waterbound macadam (150 mm) with reference to rut rate and bearing capacity. Du Plessis (1999) reported that good quality control resulted in a well-compacted WM base layer on the N1-28 project, compacted to what realistically can be achieved from labourintensive construction methods. Based on the preliminary results it was clear that the pavement would be able to carry up to a maximum of 49 million E80s (Du Plessis, 1999), which was significantly bigger than the expected traffic of 20 million. Theyse (2000) reported that the interesting aspects of the resilient behaviour of WM material were that, as the thickness of the layer decreased, the effective stiffness of the layer increased. This was ascribed to the relatively fewer load transfer points in a thin layer containing large aggregate particles. Theyse (2000) analysed WM base layers under HVS testing conditions and found that the base-bearing capacity was determined by the amount of bedding-in deformation that occurred during initial trafficking and the eventual linear rate of increase in deformation of the base layer. The results also indicated that the internal angle of friction of WM material was a function of the relative density and saturation of the material. The aim of this paper is to give an overview of the study on WM bases pavement built during pilot projects of the former Johannesburg Transitional Metropolitan Council in the mid 90 s. The pavements were studied with particular emphasis on the in-service performance thereof. This paper presents some of the results obtained during the study on the in-service performance of these bases. Some highlight findings on the behaviour of these pavements are addressed. Finally some practical recommendations when WM is used, as the pavement s base, are addressed. 2. OBJECTIVE The objective of the study was to investigate pavements with WM bases with the aim of assessing their in-service performance, with the guideline of the following three sub goals; Investigation and comparison of possible trends in current non-destructive surveillance mechanical measurement tests with the available data from initial construction and current performance criteria, Evaluation of the current structural capacity by using the latest research findings on life prediction of WM layers, and Integration of the results of the current non-destructive surveillance mechanical measurement tests and current structural capacity in order to assess the in-service performance of these WM pavements.

4 3. PILOT PROJECTS In 1994 and 1995 the former Johannesburg Transitional Metropolitan Council (TMC) constructed two pilot projects with a labour-based method utilising WM base courses. Horak et al (1996) reported that the work was carried out as fast-track labour-intensive projects. This contradiction in terms meant that roadbed preparation was to be done with machine-intensive construction techniques to ensure the realisation of the fast-track feature. Focussing on proven high quality end-product labour-intensive construction techniques ensured the high labour-intensive component of road base and surfacing. One WM base course was placed on a granular subbase (Xavier Street) and the other on a bituminous subbase (Club Street). Calitz et al (1995) reported that early indications were that the WM layers would perform as expected in the design, despite lower densities, some low Dynamic Cone Penetrometer-California Bearing Ratio (DCP-CBR) values and different longitudinal profiles than specified. The Greater Soweto pilot projects of 1996 were divided into two phases (Horak et al, 1996). Phase 1 included Naledi, Naledi Extension 1 and Tladi Townships, while Phase 2 included Phiri, Mapetla and Chiawelo Townships. The majority of new base courses for Phases 1 and 2 consisted of WM (AFRICON, 1996). Most of the subbases were either stabilised in-situ or material imported from commercial sources (Balmaceda, 1996). The selected WM pavements were; Nape Street in Naledi Township; Maphoto and Sekese Streets in Tladi Township (Phase 1); Phathuthi Street in Mapetla Township and Mulilo Street in Chiawelo Township (Phase 2). These selected in-service WM base course layers were all placed on stabilised subbases. Technologies in Phase 2 were similar to Phase 1 with the inclusion of machine-intensive technologies (Balmaceda, 1996). Implementation methods were a combination of Phase 1 methods (labour-intensive) and conventional methods (machine-intensive). Road K25, which is part of the road-over-rail bridge pilot project of 1997 in Ga-Rankuwa also formed part of this study. Table 6 contains the description of pavement structures of the selected WM pavements of the study. 4. FIELD INVESTIGATIONS 4.1 Traffic Analysis A probable traffic load was estimated (Calitz et al, 1995) for design stage of the seven selected WM roads. The standerd axles (E80s) were less than 0,2 million for the two roads in Johannesburg and 0,05-0,2 million for the five roads in the Greater Soweto. These calculations were based on the pavement design guidelines of Draft UTG3 (1993). The design traffic for Road K25 was a calculated 4 million E80s. The pavement of this road is a class V according to the former Transvaal Provincial Administration Design Catalogue and E3 for TRH4 (1985) Design Catalogue. Two manual 12-hour traffic counts were conducted on 19 April 2001 on Road K25 and on 8 May 2001 on Club Street. These roads seemed to be carrying more traffic than the designed loading. The traffic was not counted on the other six selected WM roads. Assumptions were made to calculate the average daily traffic on the two aforementioned WM roads. An expansion factor of between 0,8 and 1,1 was utilised to obtain the average daily traffic (24-hour traffic) from the 12-hour traffic counts. The results of the traffic counts are summarised in Table 1. Heavy vehicle traffic on Road K25 indicated 120 heavy vehicles/direction/day while the traffic on Club Street indicated 64 heavy vehicles/western lane/day. The traffic growth calculated on Road K25 from 1994 to 2001 amounted to a percentage of 0,86%. Although each of the traffic counts was carried out on a single day, the impression was that the traffic simulated the general behavioural magnitude of traffic encountered on Club Street and Road K25.

5 Table 1. Summary of the results of traffic counts. ROAD K25 CLUB STREET TRAFFIC Northbound Southbound Western lane COMPOSITION 12 hr 24 hr 12 hr 24 hr 12 hr 24 hr Light vehicles Heavy vehicles Total % Heavy vehicles 9,1 8,9 1,4 5. PAVEMENT PERFORMANCE ASSESSMENT The functional and structural attributes of pavement performance adopted for evaluation are listed in Table 2. Table 2. Criteria for pavement performance evaluation. PARAMETER CRITERIA SOUND WARNING SEVERE Riding quality (IRI)* <4,6 4,6 5,8 >5,8 Rut depth (mm)** < >20 Deflection (µm)*** < >1400 *TRH22 (1994) **Draft TRH12 (1997); TRH22 (1994); Draft UTG3 (1993) ***Draft TRH12 (1997; 1986) Legend Sound: Present condition is adequate Warning: Uncertainty exists about the adequacy of the present condition Severe: Present condition is inadequate 5.1 Visual Condition Assessments Visual assessments of all eight selected in-service WM roads of the study were done according to TMH9 (1992) in March 2001 to determine typical distresses. The roads were divided into 50-m segments, and each segment evaluated separately. The WM base on Xavier and Maphoto Streets were surfaced with a slurry seal during the initial construction of the road, while Road K25 and Club Street had asphalt overlays. The latter was given a new single seal on top of the existing asphalt overlay during the past few years. The surfacing of Sekese, Phathuthi and Mulilo Streets was sand-mix premix. The predominant distress encountered on Xavier and Phathuthi Streets was failure of the surfacing. Rare occurrences of patches as well as potholes were also noted. It was postulated that surface failures had developed into potholes, which eventually needed to be patched. The overall binder condition of the roads (with exception of Club and Nape Streets and Road K25 which all had asphalt surfacing) was described as dry, resulting in surface failures. No defects were identified on Road K25 and Nape, Maphoto, Sekese and Mulilo Streets. It was therefore concluded that the patches were not as a result of structurally related failures. 5.2 Riding Quality Measurements The riding quality of the WM pavements of the study was measured in June 2001 by means of high-speed profilometer (HSP). Xavier Street was measured in January 1995, while Nape, Maphoto, and Sekese Streets were measured in July 1996 at 20-m intervals. No measurements

6 were taken on Road K25, Club, Phathuthi, and Mulilo Streets. The measured results were recorded in present serviceability index (PSI). The riding quality of these roads was re-measured in June Historic PSI data were converted to the new roughness scale to ensure that the wealth of available performance data was not lost. The January 1995 results of Xavier Street could not be converted to the international roughness index (IRI) format. The conversion relation for PSI to IRI (mm/m) after Kannemeyer (1997) and Draft TRH4 (1996) as shown in Equation 1 was utilised. 5 IRI = 1,31+ 3,16ln (Eq.1) PSI Kannemeyer (1997) reported that the adoption of the IRI as the new roughness statistic for South Africa resolved many of the problems experienced in the past with the PSI, which was unique to the country. The previous converted and 2001 results are presented graphically in Figure 1. XAVIER STREET CLUB STREET Riding Quality (IRI) 8.00 Severe Warning 4.00 Sound Riding Quality (IRI) Severe Warning 4.00 Sound SEKESE STREET PHATHUTHI STREET Riding Quality (IRI) Severe Warning Sound Riding Quality (IRI) Severe Warning Sound MULILO STREET ROAD K25 Riding Quality (IRI) Severe Warning Sound Riding Quality (IRI) Severe Warning 4.00 Sound NAPE STREET MAPHOTO STREET Riding Quality (IRI) Severe 6.00 Warning 5.00 Sound Chainage (m ) Riding Quality (IRI) Severe Warning Sound Chainage (m) IRI (Jul '96) IRI (Jun '01) IRI (Jul '96) IRI (Jun '01) Figure 1. Riding quality measurements.

7 The riding quality results obtained on Xavier Street signalled a severe condition, which was expected, based on the initial construction information on the longitudinal profile. WM pavements are known for the difficulty to obtain smooth surfaces during construction (Hattingh et al, 1999), and for requiring rectification with an asphalt overlay. If densification had taken place, impaired riding quality was to be expected. This, however, did not imply any structural deficiencies. 5.3 Rut Depth Measurements Rut depth measurements were carried out with the Smart Rutbar of the automatic road analyser (ARAN) during June 2001 in the left and right wheel paths at 10 m intervals. Figure 2 shows a graphical representation of the results. No measurements were taken at the initial construction of the roads. All the roads reflected sound conditions, which showed that poor riding quality previously discussed was not related to structural deformation of the pavements due to shear of compression of layers. Figure 2. Rut depth measurements.

8 5.4 Deflection Measurements Impulse deflection testing was conducted on all eight selected WM roads in May 2001 with the Dynatest 8000 Falling Weight Deflectometer (FWD) machine. The FWD is a device that measures a series of deflections induced in a pavement under a specified impulse load (Horak, 1988). This dynamic loading device that measures pavement deflection bowl to determine elastic properties, have been used popular during the last two decades (Hefer, 1997). The deflection basin parameters (Horak, 1988) commonly used to evaluate measured FWD deflections are listed in Table 3. Table 3. Deflection bowl parameters (Horak, 1988). PARAMETER DESCRIPTION FORMULA Y-Max Maximum deflection δ 0 BLI Base layer index δ 0 - δ 300 MLI Middle layer index δ δ 600 LLI Lower layer index δ δ 900 Douter Deflection at outer sensor δ 1500 δ r Deflection (µm) at offset r from the 40 kn (550 kpa) load. The deflection measurements were taken at 50-m staggered intervals. At every test position the load was dropped three times. The first drop was considered a seating drop. The second drop was at a dynamic load of approximately 60 kn and the third at approximately 80 kn. Because of the stress-sensitive effect of the pavement materials, both drops were analysed to determine the deflections at a dynamic load of 40 kn. Deflections were measured to validate the pavement bearing capacity. These results were compared with the 1994 (Xavier Street), 1995 (Club Street) and 1996 (Nape, Maphoto and Sekese Streets) measurements. The 50th percentile reliability was adopted for the Johannesburg and Greater Soweto roads, while the 90th percentile was adopted for Road K25 in Ga-Rankuwa. The 50th percentile was used for the category D roads, which are local access roads (UTG3, 1993) and the 90th percentile for the category C road, which are collector roads (Draft TRH4, 1996). The results of the statistical analysis of the deflections bowl parameters are given in Table 4. Table 4. Statistical analysis of the deflection bowl parameters. DEFLECTION BOWL PARAMETERS (µm) STREET NAME Y-MAX BLI MLI LLI D outer Avg 50th Avg 50th Avg 50th Avg 50th Avg 50th Xavier Club Nape Maphoto Sekese Phathuthi Mulilo Road K * * 48 65* 25 35* 24 30* *90th percentile reliability

9 Statistics of the deflection data of maximum deflection (Y-Max) and base layer index (BLI) of five of the eight selected WM pavements indicated remarkable decrease. Three of the pavements, namely Phathuthi and Mulilo Streets and Road K25, were not previously measured. Statistics of the Xavier Street data indicated a decrease of 50% (over seven years), while those of Club Street indicated a decrease of 71% (over six years) in the BLI parameter. Generally the May 2001 values were below the recommended criteria. Both pavements indicated a remarkable decrease in the maximum deflection (Y-Max). Other pavement layers, however, showed no significant changes at average deflection. Nape Street data indicated a decrease of 18% (over five years), while those of Maphoto Street indicated a decrease of 34% in the BLI parameter. Both pavements indicated a remarkable decrease in the maximum deflection (Y-Max). Table 5 gives a comparison of previous and 2001 deflection bowl parameters. Only the maximum (Y-Max) and base layer index (BLI) parameters were considered. The roads showed decrease over the years. Table 5. Comparative deflection bowl parameters. DEFLECTIONS DEFLECTIONS IN/DECREASE STREET (Previous)* (2001) (%) NAME Y-Max BLI Y-Max BLI Y-Max BLI Xavier Club Nape Maphoto Sekese Phathuthi Mulilo Road K *Previous pilot projects - Xavier (1994), Club (1995), Nape, Maphoto and Sekese Streets (1996) The Sekese Street data indicated a decrease of 27% (over five years). Sekese Street showed a remarkable decrease in the maximum deflection (Y-Max), while other pavement layers (of all the streets) showed no significant changes at average deflection. 6. STRUCTURAL CAPACITY ANALYSIS The mechanistic-empirical procedure was used to determine the pavement remaining life, the origin of pavement distress and short-term performance. The deflection basins measured under the FWD were used to determine the elastic moduli through the back-calculation process. 6.1 Characterisation of in-situ Elastic Moduli The eight selected WM pavements with their structures and those from the design catalogue developed by Theyse (2000) are given in Table 6. The selected WM pavements of the study as outlined in Table 6 were designed to carry less than 0,2 million E80s (category D roads) and for Road K25 designed to carry 4 million E80s (category C road). The aforementioned WM pavement structures were closer to categories A and C presented in the design catalogue (Theyse, 2000). It was therefore expected that these pavements of lower category would yield higher structural capacity, at least 3 million E80s (1,0 3,0x10 6 ). The latter was confirmed by a detailed structural capacity analysis.

10 Table 6. Comparison of selected WM pavement structures with design catalogue. STREET NAME SURFACING BASE SUBBASE Xavier 10 slurry 100 WM2 150 G5 Club 40 AC 150 WM2 150 RA Nape 20 AC 100 WM 150 C3 Maphoto 5 slurry 100 WM 150 C3 Sekese 5 sand-mix 100 WM 125 C3 Phathuthi 5 sand-mix 100 WM 150 C3 Mulilo 5 sand-mix 100 WM 150 C3 Road K25 20 AC 150 WM 150 C4 Similar WM structures from design catalogue (Theyse, 2000) 0,3 1,0x10 6 Category C* Seal 100 WM1 125 C4 100 WM2 150 G5 1,0 3,0x10 6 Category A** 30 AC 125 WM1 150 C3 RA-Reclaimed asphalt, *80% approximate design reliability, **50% approximate design reliability 6.2 Characterisation of Critical Pavement Responses In the evaluation of elastic moduli of the layers, two linear elastic back-calculation computer programs were used, namely BISAR (Shell, 1987) and ELMOD (Dynatest, 1999). The backcalculated elastic moduli derived from the two programs are listed in Table 7. Table 7. Summary of average back-calculated layer elastic moduli. BISAR ELMOD STREET NAME Back-calculated moduli (MPa) Back-calculated moduli (MPa) Base S/B L/SB SEL S/G Base S/B L/SB SEL S/G Xavier Club Nape Maphoto Sekese Phathuthi Mulilo Road K S/B - Subbase, L/SB - Lower subbase, Sel - Selected, S/G - Subgrade Back-calculated effective elastic moduli for WM layers compared well with the range of typical values suggested by Hefer (1997) for macadam layers back-calculated from FWD deflection basins, except for Road K25 WM base course of the over-weak cemented layer (C4). The WM pavement structures selected for this study were over strong cemented layers (C3), over weak cemented layers (C4), natural gravel (G5) and bituminous (reclaimed asphalt) subbase. Backcalculation values revealed were low for the strong cemented layers in pavements that were subjected to limited traffic loading. Elastic moduli for the previous pilot projects Xavier Street (1994) and Club Street (1995) were determined from deflection that had been carried out directly on the base and subbase layers during construction (Calitz et al, 1995). These values were recalculated to obtain the 50th percentile moduli using deflections that were performed on the base layers. A remarkable difference was detected for the elastic moduli of the subbases. Other roads were calculated on the 90th percentile level. The previous and 2001 back-calculated elastic moduli are given in Table 8.

11 Table 8. Comparative back-calculated effective elastic moduli. STREET NAME E-MODULI (MPa) (PREVIOUS)* E-MODULI (MPa) (2001) Base Subbase Subgrade Base Subbase Subgrade Xavier Club Nape Maphoto Sekese Phathuthi Mulilo Road K *Previous pilot projects: Xavier (1994), Club (1995), Nape, Maphoto and Sekese Streets (1996) 6.3 Structural Performance Analysis The back-calculated elastic moduli were subjected to a mechanistic analysis to determine the critical stresses and strains. A standard 80 kn wheel load at 350 mm spacing between centres and a uniform contact pressure of 550 kpa as described by De Beer et al (1997) were considered. The analysis was done with the linear elastic layer program ELSYM5M (CSIR, 1995). Critical pavement responses (stresses and strains) for the first phase, calculated from ELSYM5M program, are listed in Table 9. STREET NAME BASE WM Table 9. Critical responses for the first phase. CRITICAL PAVEMENT RESPONSES LOWER SUBBASE SELECTED SUBBASE CEMENTED GRAN/ GRAN. RA GRAN. EF CI SOIL σ1 σ3 ht σvc σ1 σ3 ht σ1 σ3 σ1 σ3 vc Xavier ,6 476 Club 251 6,6* Nape Maphoto Sekese Phathuthi Mulilo Road K ,8* 35 8,0 95 RA - Reclaimed asphalt, Gran. - Granular, S/G - Subgrade, EF - Effective fatigue, CI - Crushing initiation, ht horizontal tensile strain, σ1 major stress, σ3 minor stress, Vc vertical compressive strain, σvc vertical compressive stress, *stress in tension Critical pavement responses for the second phase, where the cemented layers material were in equivalent granular state (with assumed reduced elastic moduli), were also calculated from ELSYM5M program and are listed in Table 10. S/G

12 STREET NAME BASE (WM) Table 10. Critical responses for the second phase. CRITICAL PAVEMENT RESPONSES LOWER SUBBASE (Granular) SUBBASE (Cemented in EGS) SELECTED (Gran/Soil) σ1 σ3 σ1 σ3 σ1 σ3 σ1 σ3 vc Nape Maphoto Sekese Phathuthi Mulilo Road K ,3* *Stress in tension, EGS-Equivalent granular state 6.4 Structural Performance Analysis SUB- GRADE Three concepts were considered in the pavement structural capacity. The first was to predict the individual layer structural capacity or life of each of the layers in the pavement structure. Secondly the occurrence of crushing (effective fatigue and crushing initiation) in cemented layers was investigated. Thirdly the occurrence of cemented layers in equivalent granular state was investigated and the ultimate bearing capacity of the pavements determined. STREET NAME Table 11. Structural capacity of the overall layers in first phase. DESIGN TRAFFIC (million E80s) RELATIVE STRUCTURAL CAPACITY OF DIFFERENT PAVEMENT LAYERS (million E80s) SUBBASE L/SB SEL. BASE Cemented S/G* Gran. RA Gran. Gran. WM EF CI Xavier <0,2 > >10, >10 Club <0, > Nape 0,05-0,2 >10 0,60 > >10 Maphoto 0,05-0,2 >10 0,15 > >10 Sekese 0,05-0,2 >10 0,65 > >10 Phathuthi 0,05-0,2 >10 2,13 > >10 Mulilo 0,05-0,2 >10 2,17 > >10 Road K25 4,00 >10 1,26 > >10 >10 >10 *Based on 20 mm rut, L/SB-Lower subbase, SEL-Selected, S/G-Subgrade, Gran.-Granular, RA-Reclaimed asphalt, EF-Effective fatigue, CI-Crushing initiation The empirical transfer functions or distress models in the SAMDM (Theyse et al, 1996) were considered for pavement layers other than WM, while the transfer function for WM material derived by Theyse (2000) were considered in the evaluation of these layers. Table 11 summarises the structural capacity of the layers of the selected eight in-service WM pavements in first phase. The fatigue life of the cemented layer is the life up to a state where the layer breaks up into an equivalent granular layer. Effective fatigue life in the pre-cracked phase of the cemented layers is considered very short (Theyse et al, 1996) in relation to the other phases and is therefore not included in predicting the structural capacity of the layer for the cemented layer.

13 The structural capacity is calculated as the sum of the duration of crushing initiation and effective fatigue life and the predicted equivalent granular layer life for the original cemented layer, whichever is the smaller. Table 12 summarises the expected structural capacity of the second phase of the six selected in-service WM pavements with cemented layers in equivalent granular state. Table 12. Structural capacity of the overall layers with cemented subbase layers in second phase. RELATIVE STRUCTURAL CAPACITY OF DIFFERENT DESIGN PAVEMENT LAYERS (million E80s) STREET TRAFFIC L- NAME (million BASE SUBBASE SELECTED SB *SUB- E80s) GRADE WM EGS Granular Nape 0,05 0,2 >10 >10 >10 Maphoto 0,05 0,2 >10 7 >10 Sekese 0,05 0,2 >10 >10 >10 Phathuthi 0,05 0,2 >10 >10 >10 Mulilo 0,05 0,2 >10 >10 >10 Road K25 4,00 >10 >10 >10 >10 >10 *Based on 20 mm rut, L-SB - Lower subbase The structural capacity or life was calculated for the first phase where the cemented layers (subbases) were in the pre-cracked condition. The second phase structural capacity was also considered where cemented layers were in the post-cracked condition (equivalent granular state) with the assumed reduced effective modulus of elasticity. Structural capacity values obtained reflected the current state of these pavements, which indicated good structural performance. The obtained structural capacity values (range of between 7 and more than 10 million E80s) were higher than the design values (less than 0,2 million E80s). These higher values were however expected as the selected WM pavements of pilot projects were closer to the design catalogue with categories A (1,0-3,0) and C (0,3-1,0) million E80s. This indicated that the selected WM pavements of pilot projects were over designed for category D roads. The performance properties of the eight selected in-service WM pavements of the study are summarised in Table 13. STREET NAME Table 13. Structural indicators and short-term structural capacity. RIDING QUALITY (IRI) RUT DEPTH (mm) DEFLECTION Y-MAX (µm) EXPECTED STRUCTURAL CAPACITY (million E80s) Xavier 8,34* 8,0 683 >10,0 Club 4,59 5, ,0 Nape 3,48 4,9 790 >10,0 Maphoto 2,81 4,7 818** 7,0 Sekese 2,84 5,1 792 >10,0 Phathuthi 4,59 4,1 711 >10,0 Mulilo 2,49 5,1 349 >10,0 Road K25 3,17 6,6 280 >10,0 *Severe condition value, **Warning condition value, ESC Expected Structural Capacity

14 7. CONCLUSIONS The study concentrated on the capacity of the WM layers, its contribution to the pavement system, and a comparison between the initially expected and the short-term remaining life. Consideration was also given to the analyses of other layers in the pavement structures. Based on the investigations and the findings of the study, the following conclusions are made: An analysis of the traffic confirmed that both roads (Club Street and Road K25) carried traffic that warranted a design traffic loading of less than 0,2 million E80s (Club Street) and 4 million E80s (Road K25). The WM pavements of the study generally performed well with the rare occurrence of surface-related distresses. The riding quality results obtained on Xavier Street signalled a severe condition, while the other WM pavements performed well. Based on the initial construction information on the longitudinal profile this condition on Xavier Street was expected. This unevenness was therefore not structural in nature and can generally be rectified by constructing an asphalt overlay. No significant trends were noted between the current results and the data available from initial construction. The overall rutting of the WM pavements was below the sound rut level, which indicated that the roads performed well. Statistics of the measured deflections indicated decreases in the bowl parameters. A decrease in the BLI implied that the occurrence of stiffening of the WM layers can probably be ascribed to post-construction traffic-related densification and environmental factors. Back-calculated elastic moduli showed a significant increase in the stiffness of WM layers. No significant trends were noted between the 2001 results and the data available from initial construction. The evaluated back-calculated effective elastic moduli of the WM pavements were in accordance with the suggested ranges specified by Hefer (1997) and De Beer et al, (1997). The cemented layer values were, however, slightly lower than the specified range. The calculated capacity or remaining life values were above 10 million E80s. The assumed design traffic loading for the Johannesburg and Greater Soweto roads was less than 0,2 million E80s, while the Ga-Rankuwa road design traffic loading was determined as 4 million E80s. Club and Maphoto Streets showed structural capacity values of 10 and 7 million E80s respectively. These values were more than the adequate design values and reflected the current state of the pavements, which indicated good structural performance. These higher values were however expected, as the selected WM pavements structures of pilot projects were closer to the design catalogue which are for category A and C roads. It is therefore concluded that these roads are over designed for category D roads. The pilot projects utilising WM base courses illustrated that high quality road can be constructed using labour-intensive methods and that conventional specifications may not be appropriate in ensuring good performance. These WM pavements that were less than seven years old and indicated no sign of deterioration proved this. 8. ACKNOWLEDGEMENTS Tshwane University of Technology and Africon Engineering International (Pty) Ltd. are thanked for the financial assistance that made it possible to carry out this research study. The latter organisation is also thanked for provision of data and its staff for willingness to share their experience.

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