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• W610933437 abstract "The Repeat Load Tri-axial (RLT) apparatus can approximate the loading experienced by a material element in a pavement. RLT tests for at least 50,000 load cycles gives an indication of a materials resistance to permanent strain (deformation/rutting) at the stress level tested. A series of RLT permanent strain tests were conducted on two Northern Ireland unbound granular materials (UGMs) and four New Zealand materials used in pavement tests. The results at each testing stress of accumulated permanent strain versus load cycles were categorised into 3 types of behaviour ranges (A, B and C). Range A is where the incremental increase of permanent strain for each load cycle is decreasing (i.e. stable behaviour). Collapse of the RLT specimen or unstable behaviour (rate of permanent strain is increasing) is categorised as range C and range B is behaviour between the two extremes A and C. For pavement design purposes the Range A behaviour case is ideal. The ABAQUS finite element package was used to apply the Range A behaviour criteria. The granular material was assigned a yield line that represented the boundary between Range A and B behaviour. Various pavements of different asphalt and granular depths were analysed. Results were contours of permanent strain showing regions in the pavement that had yielded. The total amount of yielding was quantified as a total permanent surface deformation. It was found that asphalt cover thicknesses of around 200mm (the actual thickness depended on the granular material type) showed minimal amounts of yielding and thus Range A or stable behaviour in the granular material would be predicted. Other results of interest from this analysis showed how the regions in the pavement that exhibited the highest amount of permanent strain shifted depending on the asphalt and granular thickness. For thin asphalt cover ( 600mm) the maximum amount of permanent strain occurs in the granular material at a depth of 150mm while the subgrade exhibited nil permanent strain. Furthermore, recycled aggregates and other materials considered suitable for use as unbound sub-base pavement layers can often fail the highway agency material specifications and thus restrict their use. There is potential of the permanent strain test in the RLT (or similar) apparatus to assess the suitability of these alternative materials for use at various depths within the pavement (e.g. sub-base and lower sub-base). Thus, current pavement design methods and material specifications should consider the repeated load deformation performance of the UGM layers. Thus, as an approach to overcome the limitations of current practice, the shakedown concept was used as a design method in the ABAQUS finite element package. This design method utilises results from RLT permanent strain tests to modify the Drucker-Prager yield criteria to reflect a stress boundary between different rutting behaviours defined as shakedown behaviour ranges (e.g. reducing or constant rate of rutting with increasing load cycles). 2 SHAKEDOWN CONCEPT The performance of UGMs in permanent strain RLT tests is highly non-linear with respect to stress. There are a range of permanent strain responses to stress level and load cycles that cannot be described by a single equation. Several researchers (Wekmeister et al 2001, Sharp and Booker 1984) who related the magnitude of the accumulated permanent (plastic) strain to shear stress level concluded that the resulting permanent strains at low levels of additional stress ratio, ∆σ1/σ3 eventually reach an equilibrium state after the process of post-compaction stabilisation (i.e. no further increase in permanent strain with increasing number of loads). At slightly higher levels of additional stress ratio, however, permanent deformation does not stabilise and appears to increase linearly. For even higher levels of additional stress ratio, however, permanent deformation increases rapidly and results in failure of the specimen. These range of behaviours are illustrated in Figure 1 and can be described using the shakedown concept. Cumulative Loads Pe rm an en t s tr ai n Range B Range C Range A Increasing Stress Ratio (or reducing material strength) Figure 1. Shakedown range behaviours for permanent strain versus cumulative loading. Dawson and Wellner (1999) have applied the shakedown concept to describe the observed behaviour of UGMs in the RLT permanent strain test. The results of the RLT permanent strain tests are reported as either shakedown range A, B or C. This allows the determination of stress conditions that cause the various shakedown ranges for use in defining stress boundaries between the various behaviour types. The shakedown ranges are: Range A is the plastic shakedown range and for this to occur the response shows high strain rates per load cycle for a finite number of load applications during the initial compaction period. After the compaction period the permanent strain rate per load cycle decreases until the response becomes entirely resilient and no further permanent strain occurs. This range occurs at low stress levels and Werkmeister et al (2001) suggest that the cover to UGMs in pavements should be designed to ensure stress levels in the UGM will result in a Range A response to loading. Range B is the plastic creep shakedown range and initially behaviour is like Range A during the compaction period. After this time the permanent strain rate (permanent strain per load cycle) is either decreasing or constant. Also for the duration of the RLT test the permanent strain is acceptable and the response does not become entirely resilient. However, it is possible that if the RLT test number of load cycles were increased to perhaps 2 million load cycles the result could either be Range A or Range C (incremental collapse). Range C is the incremental collapse shakedown range where initially a compaction period may be observed and after this time the permanent strain rate increases with increasing load cycles. 3 RLT PERMANENT STRAIN TESTS A series of Repeated Load Tri-axial (RLT) permanent strain tests were conducted on six unbound granular materials (UGMs) and one silty-clay subgrade soil as detailed in Table 1. The aggregates chosen for testing were those used in a Northern Ireland road trial and in full scale accelerated pavement tests at CAPTIF located in Christchurch New Zealand. This allowed validation of any predictions of rutting behaviour to actual field trial results. Table 1. Materials tested in the Repeat Load Tri-axial apparatus. Material Name Description NI Good Premium quality crushed rock graded aggregate with a maximum particle size of 40mm from Bandridge, Northern Ireland, UK. NI Poor Low quality crushed quarry waste rock graded aggregate (red in colour) with a maximum particle size of 40mm from Banbridge, Northern Ireland, UK. CAPTIF 1 Premium quality crushed rock – graded aggregate with a maximum particle size of 40mm from Christchurch, New Zealand. CAPTIF 2 Same as CAPTIF 1 but contaminated with 10% by mass of silty clay fines. CAPTIF 3 Australian class 2 premium crushed rock – graded aggregate with a maximum particle size of 20mm from Montrose, Victoria, Australia. CAPTIF 4 Premium quality crushed rock – graded aggregate with a maximum particle size of 20mm from Christchurch, New Zealand. CAPTIF Subgrade Silty clay soil used as the subgrade for tests at CAPTIF from Christchurch, New Zealand. The aim of the RLT tests was to determine the range of stress conditions that cause the various shakedown range responses either A, B or C. In the RLT permanent strain tests the cell pressure (confinement) was held constant while the vertical load was cycled. It is usual to use a new specimen per permanent strain RLT test at a particular stress level because of the effects of stress history. However, to save time, multi-stage permanent strain tests, where several different stress conditions on the same specimen are conducted were employed. For the same sample, maximum p (mean normal stress) was kept constant while maximum q (principle stress difference) was increased for each subsequent test of 50,000 load cycles. This method of testing allows the full spectra of stresses to be tested while only using 3 samples at 3 values of maximum p of 75, 150 and 250 kPa. Multi-stage testing was considered appropriate when the aim is to determine the type of permanent strain behaviour (i.e. range A, B or C) for each stress level. It is considered that stress history is more likely to affect the magnitude of permanent strain rather than the type of behaviour, however this point will require further research. For each material three multi-stage RLT permanent strain tests were conducted. The results were analysed to determine cumulative deformation as shown for one test in Figure 2. To determine which stress level caused the various shakedown ranges (A, B or C) cumulative permanent strain versus permanent strain rate plots were produced (Figure 3). For calculation of cumulative permanent strain it was assumed that at the start of each new test stage the deformations were nil (or zero). Figure 3 shows the cumulative permanent strain plot versus permanent strain rate for the test results shown in Figure 2. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 5000" @default.
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• W610933437 date "2004-01-01" @default.
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• W610933437 title "Deformation behaviour of granular pavements" @default.
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