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The current practice in pavement design is to use a circular uniformly distributed load as the input to ascertain the maximum stresses in the pavement. This is not the reality; tyre-pavement contact stress distributions are very complex. The distress on the surface of the pavement in the form of rutting and surface initiated cracking is very much dependent on these complex stresses. This study investigates the effects that non-uniform contact pressure distributions have in comparison to the traditional circular loading on the initiation and rate of accumulation of this distress. The problem has been modelled using the CAPA-3D finite element software. The traditional circular load was modelled for two different asphalt materials with different moduli. The stresses in a number of key locations were recorded and measured. Then the non-uniform loading was modelled using the same procedure. What was of particular interest was the difference in the peak surface stresses and positions between the two different methods of loading. The non-uniform loading created significantly larger stresses on the surface in comparison to the circular loading. The non-uniform loading also created significant shearing forces close to the surface. This leads to a greater propensity for the surface to develop rutting and cracking to develop at the surface. The differences started to become less evident with depth and/or distance from the loading area for the principal stresses. It is recommended that for the design of surface layers non-uniform contact pressures should be used.

The saturation ageing tensile stiffness (SATS) procedure is the first laboratory asphalt mixture durability protocol to combine moisture damage and oxidative ageing during the conditioning of asphalt mixtures prior to mechanical testing. However, the application of the SATS procedure has tended to be predominantly targeted at base materials with relatively ‘hard’ binders and overall high stiffness modulus requirements and is considered potentially too severe for more standard asphalt mixtures. This research study focused on understanding the effect of changes to the SATS conditioning parameters on the relative moisture damage assessment performance of asphalt mixtures, particularly ‘softer’ binder, high binder content and, finally, low air void content mixtures. Using four different aggregate types, together with ‘hard’ 10/20 pen and ‘softer’ 40/60 pen bitumen, the parameters of pressure, temperature and duration were altered, either one at a time or in combination, to arrive at a suitable combination for...

Discrete element modeling has been used to simulate constant strain rate compressive tests for an idealized asphalt mixture comprising approximately single-sized sand particles. A range of constant strain-rate compressive tests to failure have been undertaken in the laboratory and the axial stress-strain response has been carefully measured. The peak stress (compressive strength) of the material was found to be as a power-law function of the equivalent (temperature compensated) strain rate in the viscoelastic region of behavior. The internal geometry of the idealized asphalt mixture has been reproduced in PFC-3D and internal damage (cracking) in the material was modeled by allowing bond breakage between adjacent particles. Elastic contact properties have been used to investigate the effect of random variations in internal sample geometry, the distribution of bond strengths between adjacent particles and the coefficient of friction between particles where the bond has broken. A simple viscoelastic Burger's model was used to introduce time dependent shear and normal (tensile) contact stiffnesses and an elastic contact has been assumed for the compressive normal contact stiffness. To reduce the computation time, both viscosities in the Burger's model were scaled which has been shown to have the same effect as scaling the loading velocity (strain rate) by the same factor. A strain rate dependent bond breakage criterion has been developed and model results were found to compare well with the experimental data.

A recently developed large-scale triaxial test apparatus for railway ballast testing comprises a double-cell arrangement for measuring volume change by differential pressure. Monotonic and cyclic tests were performed on limestone ballast samples. Axial and volumetric strains and breakage were determined from both types of test. Resilient modulus and Poisson’s ratio were obtained only from the cyclic tests. The permanent axial strain and breakage results from the cyclic tests are compared with the simulated traffic loading in the railway test facility (RTF) which comprises three sleepers embedded in ballast over a subgrade. The traffic loading in the RTF was applied by hydraulic actuators with built-in displacement transducers. A column of painted ballast was placed under a rail seat of the middle sleeper to ease sample collection for sieve analysis at the end of the test. The stress condition in the RTF is predicted by a simple calculation based on findings of previous literature. It was found that the results from the cyclic triaxial test with conditions similar to the predicted conditions in the RTF were comparable to those results from the RTF tests.

This paper describes the laboratory measurement of shear interface properties between asphalt layers using the Leutner test. Results are presented and compared for both laboratory prepared specimens and field cores. The standard Leutner test was modified by the introduction of a 5 mm gap into the shear plane to reduce edge damage caused by misalignment of the specimen and specimens that incorporate a thin surfacing material were extended using a 30 mm thick grooved metal cylinder to eliminate dependence of the shear strength on surfacing thickness. The laboratory produced surfacing/binder course combinations incorporating the 20 mm Dense Bitumen Macadam (20 DBM) binder course showed the highest average shear strengths when nothing was applied at the interface and the lowest average shear strengths when the tack coat was applied at the interface. The average shear strength from field cores was found to increase as the class of the road increases for both surfacing/binder course interfaces and binder course/base interfaces.

Discrete Element Modelling (DEM) has been used to simulate constant strain rate tests on an idealised asphalt mixture containing approximately single-sized sand particles under monotonic compressive loading. Results have been compared to a range of constant strain-rate compressive tests to failure that have been undertaken in the laboratory. The internal geometry of the idealised asphalt mixture has been reproduced in PFC-3D and internal damage (cracking) in the material was modelled by allowing bond breakage between adjacent particles. Elastic contact properties have been used to investigate the effect of random variations in internal sample geometry, the distribution of bond strengths between adjacent particles and the coefficient of friction between particles where the bond has broken on the stress-strain behaviour. A simple viscoelastic Burger's model was introduced to give time dependent shear and normal (tensile) contact stiffnesses and an elastic contact has been assumed for the compressiv...

This paper presents the results from discrete element modelling (DEM) of an idealised (sand) asphalt mixture under constant strain rate (velocity) compressive loading using the Burgers viscoelastic model. Using dimensional analysis it is shown that the effect of scaling applied velocity is the same as that of scaling both viscosities in the Burgers model by the same factor. This has been shown to be the case for a bonded aggregate, and it also has been shown to be applicable even when bond breakage occurs within the sample. This scaling method will be of great benefit in modelling tests on asphalt under different constant strain rates, simply by scaling both viscosities to minimise computational time.

This paper describes a comparison between an indirect tensile fatigue method and the currently preferred European standard two-point bending, trapezoidal fatigue test. A study was undertaken using both tests to determine the fatigue properties of a standard UK asphalt mixture at two temperatures of 10 and 30°C. The fatigue life, in addition to using the traditional 90% reduction in initial stiffness, was also computed using a fatigue failure point based on the transition between the quasi-stationary phase (associated with a uniform stiffness decrease and the development of micro-cracks) and the failure phase (associated with localised crack propagation). This fatigue failure point, determined by taking the peak of the product of loading cycles and stiffness versus loading cycles, was also converted into an alternative form used to determine the fatigue failure point for the indirect tensile fatigue test. Separate fatigue relationships were produced for the four combinations of test method and temperature based on the phenomenological initial tensile strain versus fatigue life relationship. Although the indirect tensile fatigue test produced shorter fatigue lives compared to the two-point bending fatigue test, it was also possible to combine all four fatigue relationships to produce one unique fatigue function for the 20 mm dense bitumen macadam asphalt mixture.

Moisture damage in an asphalt mixture can be defined as the loss of strength, stiffness and durability due to the presence of moisture leading to adhesive failure at the binder–aggregate interface and/or cohesive failure within the binder or binder–filler mastic. Various test methods exist to identify the susceptibility of asphalt mixtures to moisture damage, such as the AASHTO T283 procedure. This paper describes a new combined ageing/moisture damage laboratory test known as the Saturation Ageing Tensile Stiffness (SATS) test that has been successfully used to quantify the moisture damage of a range of UK asphalt mixtures. The test consists of initial saturation prior to placing compacted asphalt mixture cylindrical specimens in a moist, high temperature and pressure environment for an extended period of time. The stiffness modulus measured after the test divided by the stiffness modulus measured before the test (retained stiffness modulus), and the specimen saturation after the test (retained saturation), are used as an indication of the sensitivity of the compacted mixture to moisture damage. In this paper, the sensitivity of the SATS test to different aggregates, fillers, binders and volumetric proportions as well as mixture types has been assessed. The results show that the SATS test is able to discriminate between different asphalt mixture combinations in terms of their moisture damage resistance. Compared to AASHTO T283, the SATS test was found to be a more aggressive conditioning protocol, although both tests ranked mixtures in a similar order with respect to moisture damage.

This paper investigates the influence of asphalt mixture type (aggregate gradation) and compaction energy level on the degradation of aggregate during laboratory compaction. The most severe form of laboratory compaction in terms of potential aggregate degradation, i.e. Marshall (impact) compaction has been used together with a standard UK aggregate and two asphalt mixture gradations consisting of a continuous and gap-graded mixture design. The results of aggregate degradation have been determined by comparing the aggregate gradation in terms of retained aggregate per individual sieve size before and after compaction. The aggregate gradation after compaction has been determined by recovering the aggregate from compacted asphalt mixture specimens and then re-grading using the series of sieve sizes used to batch the original material. The results show that asphalt mixture type and therefore aggregate gradation has a far greater influence on aggregate degradation under compaction than alterations to the level of compaction energy within the normal compaction levels experienced in the laboratory. The gap-graded asphalt mixture experienced a greater amount of aggregate degradation after compaction compared to the continuously graded mixture. The effect of increased compaction energy level did not alter the pattern of aggregate degradation for the continuously graded asphalt mixture and only increased the degree of degradation by a small amount.

This note outlines the experimental investigation of the factors affecting bond development between pavement layers. The tests involved the use of an apparatus known as the Nottingham shear box. To establish a realistic stress distribution at the interface, which would facilitate the analysis of the test results, finite element models of bonded and weakly bonded specimens were developed. It was concluded that care must be taken when interpreting test results from the shear box experiments. Several materials and four interface conditions were investigated. Bond stiffness and strength were assessed under repeated dynamic and monotonic static test conditions. The analysis of the experimental results showed that the interface properties depend more seriously on the type of materials in contact, rather than on the amount of the applied tack coat and the interface condition. It was suggested that the interface bond should be described by introducing a vertical shear reaction modulus, as well as using the horizontal shear reaction modulus.

Currently in the UK, the mechanical properties of bituminous (asphalt) mixtures are determined by means of a suite of tests (stiffness modulus, resistance to permanent deformation and resistance to fatigue cracking) performed by means of a pneumatic apparatus known as the Nottingham Asphalt Tester (NAT). This paper presents a methodology for an alternative experimental characterisation and modelling procedure for bituminous (asphalt) mixtures using a dynamic plasticity based constitutive model. The paper briefly describes the material response model and details the monotonic, uniaxial compression and tension testing at various temperatures and strain rates that are necessary to determine the basic model parameters. Two standard UK wearing course mixtures have been included in the testing programme comprising a continuously graded, Dense Bitumen Macadam (DBM), and a gap graded, Hot Rolled Asphalt (HRA). The uniaxial test data has been used to determine mathematical functions for the compressive and tensile...

It is important to be able to distinguish easily between ballasts of poor quality and good quality in terms of strength. For a range of uniformly graded granular materials compacted to the same relative density, there will be two important factors influencing the strength of the aggregate: particle strength and voids ratio (which will be a function of particle shape). Large oedometer tests have been performed in addition to box tests and single particle crushing tests and a new parameter, called the relative strength index, is defined to incorporate the effects of both particle strength and voids ratio. It is found that the relative strength index correlates well with the amount of degradation of ballast in both the large oedometer tests and the box tests, and therefore shows promise as a suitable method for comparing ballast types without having to apply thousands of cycles of load on each ballast.

Different laboratory compaction methods can produce volumetrically identical asphalt mixture specimens but with widely varying mechanical properties. Provided that the mixture design is constant, variations in mechanical properties are probably due to differences in the structure of the aggregate-bitumen matrix. The objective of this study is to investigate the structure of the internal aggregate-bitumen matrix created by different laboratory compaction methods and compare it with mechanical performance. Three types of laboratory compaction are considered: gyratory, vibratory, and slab. Image analysis techniques have been used to provide quantitative information on the orientation and distribution of aggregates on horizontal planes within asphalt mixture specimens, and the results indicate that circumferential alignment of aggregate particles occurs in gyratory and vibratory compacted specimens. This behavior is more pronounced for larger aggregate particles and in those with an aspect ratio (maximum length/maximum width) greater than two. Slab-compacted specimens display a more random particle orientation. The distribution and segregation of aggregates has been considered relative to the center of each horizontal asphalt-specimen cross section. While overall levels of aggregate particle density are similar across all compaction methods considered, greater segregation occurs in vibratory-and gyratory-compacted specimens. Repeated load axial testing was done on specimens compacted by each method. The results indicate higher resistance to deformation in the vibratory- and gyratory-compacted samples than in volumetrically identical slab-compacted samples.

The tensile strengths of six types of ballast for various size fractions were investigated and quantified using Weibull statistics. For a given ballast and a given size fraction, the strengths follow the Weibull distribution reasonably well. However, it was found the size effect on strength implied by Weibull statistics for bulk failure of particles, is not consistent with measured values. Various possible reasons for this are explored, namely that the grinding process at the quarry may offer a proof test for ballast, that some ballasts may not be homogeneous or isotropic, and finally that surface fracture rather than bulk fracture may dominate.

Almost all mechanistic flexible pavement design procedures implicitly assume that fatigue cracks originate at the bottom of the bound layers and propagate upwards towards the surface of the pavement. However, observations from cracked roads in many countries have shown that the cracking originated from the surface of the pavement rather than at the base, particularly for thick flexible constructions. Many researchers have been trying to confirm that surface-induced cracking can be explained by the presence of tensile and/or shear stresses (strains) at (or near to) the pavement surface. A standard analysis of a pavement structure loaded by a uniform vertical contact stress shows that small horizontal stresses and strains can be generated due to negative curvature effects although it should be noted that the maximum (principal) strain at the pavement surface (at approximately 45o to the surface) can be of a similar magnitude to the horizontal tensile strain at the base of the asphalt layers. In add...

This paper describes the interaction between crumb rubber and various penetration grade bitumens in terms of the absorption of the light fractions of the bitumen by the rubber and the chemical composition and rheological properties of the residual binder. Eight bitumens from two crude oil sources and four penetration grades ranging from 200 pen to 35 pen have been mixed with 2 mm to 8 mm sized crumb rubber at three rubber to binder ratios of 1:8, 1:6 and 1:4 by mass. The increased mass of the crumb rubber was used to determine the loss of volatiles and light fractions that have been absorbed from the different bitumens under constant temperature and equiviscous temperature conditions. The residual binders were then subjected to dynamic mechanical analysis using a dynamic shear rheometer to determine their rheological properties following the interaction with crumb rubber. The results show that the rate of adsorption as well as the total amount of absorption is directly related to the penetration ...

In the 1970s, the U.K. Transport Research Laboratory published a series of reports dealing with wearing course slippage due to debonding. At present, however, no accepted nondestructive test ~NDT! technique exists for a reliable detection of poor bond. This paper outlines the research undertaken in developing an NDT method for quantifying bond condition beneath thin surfacings ~10-50 mm!. Laboratory experiments were conducted in the Pavement Test Facility at Nottingham University. A generic type of surfacing material was used and several interface conditions were compared: ~1! bonded; ~2! debonded; and ~3! partially bonded. Different surfacing thicknesses were considered. An instrumented impulse hammer was used to provide the transient excitation, and vertical dynamic responses close to the point of impact were monitored using accelerometers. The results were used to identify suitable parameters for identification of bond condition. Cores taken at test points showed good agreement between the Leutner test results and the predicted condition of the interface. Finite-element ~FE! analysis has also been performed to provide understanding of the response of thin surfacings with different degrees of bond. Results from the FE analysis confirm the trends established from the testing.

This paper describes the development of a combined ageing/moisture sensitivity laboratory test to evaluate the performance of coated macadam binder course and base mixtures with relatively low binder contents and relatively high air void contents. The test is known as the saturation ageing tensile stiffness (SATS) test, and consists of initial saturation prior to placing compacted asphalt core samples in a high-temperature and high-pressure environment in the presence of moisture for an extended period of time. The stiffness modulus measured after the test divided by the stiffness modulus measured before the test (retained stiffness modulus), and the specimen saturation after the test (retained saturation), are used as an indication of the sensitivity of the compacted mixture to combined ageing/moisture effects. The test was used to reproduce in the laboratory the loss of stiffness modulus observed on a trial site where high modulus base (HMB) had been used. Two different aggregate types (basic and acidic) and four different binders were used in the HMB mixture that was investigated. For the mixtures containing the acidic aggregate, the general trend is for the retained stiffness modulus to reduce from approximately 0·6 at a retained saturation level of 10% to 0·2 at a retained saturation level of between 80% and 100%. At a retained saturation level of 40% the retained stiffness modulus is approximately 0·4, which compares favourably with the 60% reduction in stiffness modulus observed from the HMB trial site (for nominally identical mixtures). For the mixtures containing the basic aggregate, the retained stiffness modulus remains at an approximately constant value of 0·7 over a wide range of retained saturation levels (between 15% and 80%), indicating a lower sensitivity to moisture. For the low binder content, high air void content mixtures investigated, the effect of changing the binder (source and grade) was small compared with the effect of the aggregate type.

It is important to have consistent ballast testing methods that provide results reflecting the performance of different ballast materials in the railway trackbed. Extensive laboratory tests have been conducted to investigate the correlation between simple ballast index tests and box tests simulating ballast field loading conditions in a simplified and controlled manner. Ballast is tamped using a vibratory poker, which causes particles to rearrange as the level of the sleeper is raised. The index tests used include single particle crushing tests, oedometer tests, and attrition tests in a revolving drum (WAV, MDA and LAA tests), in addition to water absorption and particle shape characteristics and petrographic analysis. The attrition tests performed in a revolving drum (WAV, LAA and MDA) have been found to give the best correlations with the performance of ballast in the box tests. It is considered that this may be due to the rearrangement of particles in the box test caused by the simulated tamping.

The development and finite element (FE) implementation of a stress-dependent elastoviscoplastic constitutive model with damage for asphalt is described. The model includes elastic, delayed elastic, and viscoplastic components. The strains (and strain rates) for each component are additive, whereas they share the same stress (i.e., a series model). This formulation was used so that a stress-based nonlinearity and sensitivity to confinement could be introduced into the viscoplastic component without affecting the behavior of the elastic and delayed elastic components. A simple continuum damage mechanics formulation is introduced into the viscoplastic component to account for the effects of cumulative damage on the viscoplastic response of the material. The model is implemented in an incremental formulation into the CAPA-3D FE program developed at Delft University of Technology in the Netherlands. A local strain compatibility condition is utilized such that the incremental stresses are determined explicitly from the incremental strains at each integration point. The model is demonstrated by investigating the response of a semirigid industrial pavement structure subjected to container loading. Results show that the permanent vertical strains in the non-stress-dependent case are significantly lower than the permanent vertical strains in the stress-dependent case. Results also show that in the stress-dependent case, there is a more localized area of high permanent vertical compressive strain directly under the load at approximately halfdepth in the asphalt compared with the non-stress-dependent case, in which the distribution is more even.

Pavement structures comprise several layers of different materials. The overall strength and stiffness of the pavement depends not only on the strength and stiffness of each individual layer, but also on the bond between them. Should the bond at an interface be inadequate, the strains throughout the pavement may increase (under trafficking) and its life may consequently be reduced. This paper investigates the use of the Leutner test to measure bond condition between surfacing and binder course materials and binder course and base materials. A range of bond conditions have been investigated. Results show that, for a particular combination of materials, there is an approximate linear relationship between interface shear strength and corresponding shear displacement that is independent of the bond condition. Results also indicate that, for HRA/20DBM, SMA/20DBM and 20DBM/28DBM combinations, the type and quantity of tack coat commonly applied in practice are sufficient to approach maximum attainable bond strength. For the 20DBM/28DBM combination a much reduced interface shear strength occurred if tack coat was not used, but extra tack coat appeared to compensate to some extent for the presence of dirt. In the 20DBM/CBM combination, it proved impossible to achieve a good bond, reflecting common experience on site.

This paper describes the use of a dynamic plasticity based constitutive model to represent the response of gap-graded and continuously graded asphalt mixtures commonly used in the UK. The constitutive model is under development at Delft University of Technology, where it will be implemented in the finite element code, INSAP. The paper briefly describes the material response model and details the monotonic, uniaxial compression and tension testing at various temperatures and strain rates that is necessary to determine the basic model parameters. The test data has been used to determine mathematical functions for the compressive and tensile strengths of the asphalt mixtures as well as functions describing their hardening and softening behaviour. The model was then successfully used to simulate the compressive response of the asphalt mixtures. For the covering abstract see ITRD E117423.

This paper describes work undertaken to investigate the mechanical performance of road ironwork installations in highways, concentrating on the chamber construction. The principal aim was to provide the background research which would allow improved designs to be developed to reduce the incidence of failures through improvements to the structural continuity between the installation and the surrounding pavement. In doing this, recycled polymeric construction materials (Jig Brix) were studied with a view to including them in future designs and specifications. The paper concentrates on the finite-element (FE) analysis of traditional (masonry) and flexible road ironwork structures incorporating Jig Brix. The global and local buckling capacity of the Jig Brix elements was investigated and results compared well with laboratory measurements. FE models have also been developed for full-scale traditional (masonry) and flexible installations in a surrounding flexible (asphalt) pavement structure. Predictions of response to wheel loading were compared with full-scale laboratory measurements. Good agreement was achieved with the traditional (masonry) construction but poorer agreement for the flexible construction. Predictions from the FE model indicated that the use of flexible elements significantly reduces the tensile horizontal strain on the surface of the surrounding asphaltic material which is likely to reduce the incidence of surface cracking.

This paper describes the linearity limits of various unmodified and modified bituminous binders as well as asphalt mixtures incorporating these binders. The linearity limits of the binders and mixtures have been determined using a dynamic shear rheometer (DSR) and a purpose-built dynamic, uniaxial tension-compression, servo-hydraulic testing apparatus respectively. Both continuously graded as well as gap-graded asphalt mixtures have been tested together with a fines/filler mortar (mastic). The results show that there are strain dependent linearity criteria for both binders and asphalt mixtures at high binder stiffness values as well as a high temperature strain dependent linearity criterion for elastomeric modified binders at temperatures greater than 60°C. The linearity strain criterion for the asphalt mixtures was found to be in the order of 100 microstrain with the criterion for the binders being at least 100 times greater at just over 10,000 microstrain (1% strain) and the elastomeric polymer...

This paper describes a method for determining viscoelastic material properties from the Indirect Tensile Stiffness Modulus (ITSM) test undertaken using the Nottingham Asphalt Tester (NAT). A number of viscoelastic material idealizations have been fitted to ITSM results and the effects of the order of the viscoelastic model (i.e. the number of elements), and the details of the load pulse (rise time, number of load pulses utilized in the fit) have been investigated. Results indicate that the use of a model comprising a Maxwell element in series with two Voigt elements can yield unrealistic and more variable fitted viscoelastic parameters from the simpler model comprising a Maxwell element in series with one Voigt element. The fitted viscoelastic parameters for the model comprising a Maxwell element in series with one Voigt element have been found to show a slight dependence on the rise time in the ITSM test. Results also indicate that the fitted elastic bulk and shear moduli tend to be slightly lower and the shear viscosity and time constants tend to be slightly higher in the case where 5 consecutive load and deflection pulses have been used in the fitting procedure rather than single load and deflection pulses. For the covering abstract see ITRD E117423.

As bituminous materials are viscoelastic in nature, their performance must be characterised using test methods and analytical techniques that account for time (or rate) of loading and temperature. In addition, it is usually advisable to confine the characterisation of a bitumen to its linear viscoelastic response (small strains) to simplify the mathematical modelling of the material, as non-linear response, particularly for viscoelastic materials, is extremely difficult to characterise in the laboratory and model in practical engineering problems. This paper describes an investigation of the linearity limits of a range of unmodified and modified bituminous binders and mixtures using a dynamic shear rheometer and a purpose-built dynamic, direct tension-compression, servo-hydraulic testing apparatus. The results show that there are strain dependent linearity criteria for both binders and asphalt mixtures at high stiffness values (low temperatures for binders and low to intermediate temperatures for mixtures) as well as a high temperature strain dependent linearity criterion for elastomeric modified binders. The linearity strain criterion for the mixtures was found to be in the order of 100 microstrain with the criterion for the binders being at least 100 times greater at just over 10,000 microstrain and the polymer network strain criterion at 1,000,000 microstrain. For the covering abstract see ITRD E117423.

This volume contains contributions from international experts, reflecting the rapid advances in the design of new improved bitumen and hydraulic bound composites, the trends in the use of waste and recycled materials and up-to-date methods of testing and evaluation.

The paper describes the use of a purpose built hydraulic tension-compression apparatus to investigate the linearity limits and linear viscoelastic (LVE) rheological characteristics of a range of unmodified and modified (polymer and process) asphalt mixtures. The asphalt mixtures consist of dense bitumen macadam (DBM), hot rolled asphalt (HRA) and HRA mortar mixtures with conventional 50 pen bitumen, radial and linear SBS PMBs, EVA PMB and multigrade bitumen. Cylindrical specimens of the mixtures were subjected to sinusoidal loading conditions at temperatures between 10 and 40 degrees C, firstly, under stress sweep conditions and, secondly, under frequency sweep conditions. The results indicate that the linearity limits of behaviour for the asphalt mixtures were strain dependant and found to be between 20 and 100 microstrain for the DBM and HRA mixtures and between 50 and 150 microstrain for the HRA mortars. In general, the LVE rheological characteristics of the asphalt mixtures were similar with differences in complex modulus and phase angle only being found at intermediate temperatures and frequencies and attributed to the unique rheological characteristics of the binders. For the covering abstract see ITRD E117840.

This paper describes results from a laboratory investigation into the effects of the interaction between a styrene-butadiene-styrene (SBS) modified bitumen and recycled crumb rubber on the mechanical performance of impact absorbing asphalt (IAA), an asphaltic material that has applications as a sports and safety surface. Curing tests at high temperatures have been undertaken using different rubber-bitumen ratios to assess the amount of bitumen absorbed by the rubber. The changes in bitumen constitution through this interaction have been monitored and related to changes in the rheological characteristics of the residual bitumen. The results show that significant absorption takes place and that the residual bitumen undergoes alterations in terms of its stiffness and viscoelastic balance, together with a reduction in cohesion. A significant finding has been the loss of the polymeric nature of the residual SBS polymer modified bitumen. This has been due to the precipitation of the SBS polymer as the compatibi...

This paper investigates the way in which traffic data are used in flexible pavement design and analysis procedures. A deterministic long-term pavement performance model (LTPPM) is used to calculate flexible pavement damage caused by trafficking from three realistic fleets of commercial vehicles. The fleets have been modelled using seven-axle group models representing steer axles, drive axles (single or tandem) and trailer axles (tandem or tridem) with either steel suspensions or air suspensions. Results are compared with the calculated flexible pavement damage caused by a fleet of 80 kN standard axles and predictions from the currently used method of traffic characterization for UK pavement design procedures. Results show that pavements that fail by rutting last longer when trafficked by the realistic vehicle fleets compared with the fleet of 80 kN standard axles whereas the pavements that are predicted to fail by fatigue last longer when trafficked by the fleet of 80 kN standard axles. Results also show that current UK pavement design procedures are more sensitive to differences in the vehicle fleet compared with LTPPM predictions.

This paper describes research carried out to investigate pavement material-stiffness modulus variability. Detailed pavement surface deflection data were obtained from two sections of newly constructed flexible pavement in the United Kingdom, by performing tests with a falling weight deflectometer. The deflection data were back-analyzed using the computer program PADAL to obtain estimates of the in situ pavement layer stiffness moduli. The results were statistically analyzed to identify suitable test sampling regimes for end product specifications using the falling weight deflectometer. Results show that the mean values of key deflection parameters are not very sensitive to test spacing for a nominally uniform (as specified) pavement construction. However, more variability in the standard deviations of these parameters, particularly when data were sampled at more than 50 m centers (

This paper presents a model for statistically correcting asphalt layer stiffness moduli determined from the Falling Weight Deflectometer (FWD) test to account for asphalt layer thickness variations. Both random variations in asphalt layer thickness and a difference between the mean asphalt layer thickness and the constant value of asphalt layer thickness used in the back-analysis procedure (ie mean thickness error) are included and the model has been validated using published data. The model shows that the mean value of back-calculated asphaltic material stiffness modulus is primarily influenced by incorrect assumptions regarding the mean asphalt layer thickness, whereas the standard deviation of the back-calculated asphaltic material stiffness modulus is also sensitive to variations in the thickness of the asphalt layer around the mean level.

The increased speeds of modern trains are normally accompanied with increased transient movements of the rail and ground, which are especially high when train speeds approach some critical wave velocities in the track-ground system. These transient movements may cause large rail deflections, as well as structural vibrations and associated noise in nearby buildings. There are two main critical wave velocities in the track-ground system: the velocity of the Rayleigh surface wave in the ground and the minimum phase velocity of bending waves propagating in the track supported by ballast, the latter velocity being referred to as the track critical velocity. Both these velocities can be exceeded by modern high-speed trains, especially in the case of very soft soil where both critical velocities become very low. The discussion in this paper focuses on the effects of transient rail deflections on associated ground vibrations in the cases of train speeds approaching and exceeding Rayleigh wave and track critical velocities. The obtained theoretical results are illustrated by numerical calculations for TGV and Eurostar high-speed trains travelling along typical tracks built on soft soil.

In this paper, current knowledge of artificial playing surfaces is reviewed. Research status in the fields of sports medicine, engineering and biomechanics is described. A multidisciplinary approach to the study of artificial sports surface properties is recommended. The development of modelling techniques to characterise fundamental material properties is described as the most appropriate method for the unique specification of material properties such as stiffness and damping characteristics. It is suggested that the systematic manipulation of fundamental surface material properties in biomechanics research will allow the identification of subject responses to clearly defined surface variation. It is suggested that subjects should be grouped according to characteristic behaviour on specific sports surfaces. It is speculated that future biomechanics research will identify subject criterion related to differing group responses. The literature evidence of interactions between sports shoes and sports surfaces leads to the suggestion that sports shoe and sports surface companies should work together in the development of ideal shoe - surface combinations for particular groups of subjects.

The effects of spatial repeatability of dynamic tyre forces on the long-term performance of three typical British pavement constructions are investigated. Long-term pavement performance is calculated using a ‘whole-life pavement performance Model’ (WLPPM). The WLPPM is capable of predicting deterministic pavement damage due to realistic traffic and environmental loading, throughout the life of the pavement. Particular attention is given to modelling dynamic tyre forces and patterns of loading applied to the pavement by a typical fleet of heavy vehicles. A method is described for simulating vehicle fleets with varying degrees of spatial repeatability using a small number of dynamic tyre force histories. Results indicate that thinner pavements are most sensitive to the level of spatial repeatability exhibited by the vehicle fleet. Pavement damage predictions made without assuming an appropriate level of spatial repeatability can be in error by 20–150 per cent, the higher values being for thinner pavements that fail by fatigue damage.

The dynamic tyre forces generated by heavy goods vehicles are measured with a portable load measuring mat and used to investigate ‘spatial repeatability’. The mat is 56 m long, 13 mm thick and has 141 capacitative strip sensors, spaced at 0.4 m intervals. Two sets of experiments are described. In the phase 1 experiments the measured dynamic tyre forces generated by fourteen articulated vehicle combinations (three tractors and five trailers) are used to confirm previous theoretical predictions of spatial repeatability. In the phase 2 experiments the dynamic tyre forces generated by over 1500 heavy goods vehicles travelling on a British trunk road are used to estimate the degree of repeatability exhibited by a typical highway fleet. Spatial repeatability is assessed using the ‘spatial repeatability index’ (SRI), and a method is developed to correct the SRI for sensor noise. Approximately half the vehicles tested were found to contribute to a spatially repeatable pattern of road loading.

This paper examines theoretically the possible mechanisms of fatigue cracking in asphalt pavements using linear elastic fracture mechanics (LEFM). The effects of a non-uniform transverse contact traction between the tyre and pavement on longitudinal surface fatigue cracking are examined theoretically using a finite element (FE) pavement model. Fatigue cracking due to thermal loading is also examined using a simplified linear viscoelastic pavement model. Results indicate that the non-uniform transverse contact traction produces short surface cracks (10 mm) but does not lead to significant pavement fatigue. However, for these short surface cracks, the combined vehicle and thermal load cycles may produce conditions that are favourable for fast fracture. Fatigue cracking (both longitudinal and transverse) at the base of the asphalt layer is found to be mainly dependent on the pavement geometry and the magnitude of the normal load.

Permanent deformation (rutting) in bituminous pavements is treated as a linear viscoelastic flow phenomenon and is calculated using the linear elastic layered pavement model VESYS, reported by Kenis in 1978, supplied with equivalent viscous material properties. After an initial compaction period, the model predicts that the permanent deformation per wheel pass is proportional to the static axle load and inversely proportional to the vehicle speed. The predicted permanent deformation field is realistic and independent of the material elasticity. To determine the viscosity of asphalt mixtures operating under a wide range of environmental conditions, a simple new theoretical method is introduced which is based on routine test data and requires only one parameter to be fitted to experimental data. Published wheel-tracking tests were used to determine this parameter. Rut depth predictions are compared with experimental data from a recent full-scale test in France. Agreement is generally good, indicating the applicability of this linear rutting model.

A wheel tracking test was modelled to gain better understanding of the deflection and stress–strain distribution in an overlaid cracked pavements with and without membrane interlayer (SAMI). For this purpose, commercial finite element software Abaqus 6.7-1 was used. Two different models were considered, one incorporating stress absorbing membrane interlayers (SAMIs) and the other without SAMI. In the study, full bond condition was assumed for the boundaries between the layers, and a linear elastic model was used for the analysis. The results show that introduction of SAMI caused greater deflection of the pavement. It is found that although with SAMIs, low stiffness is required, a very low stiffness may yield undesirable results. The results show that the introduction of SAMIs results in high strain concentration around the crack region, whilst the strain in the overlay is smaller than the values predicted in the models without SAMIs.

Current practice in pavement design is to use a uniformly distributed load over a circular contact patch. However, this is not the reality; tyre-pavement contact stress distributions are very complex. There are vertical, longitudinal and transverse stress components that affect the response of the pavement. The formation of surface initiated cracks is known to be caused by a number of factors, but primarily by the traffic loading, these cracks propagate from the surface into the pavement. The study investigated the Stress Intensity Factors (SIF) of mode one and mode two cracking at the tip of a short crack and as such their influence on the continued propagation further into the pavement. The CAPA-3D finite element software was used to model the two scenarios of uniformly distributed loading and uniformly distributed loading with symmetrical inward shear. This analysis was performed for two different subbase moduli. The analysis was 2D and used plane strain conditions. The loading was moved to a number of different positions relative to the crack for both load cases. The study showed that the SIF for both KI (stress intensity factor for mode one cracking) and KII (stress intensity factor for mode two cracking) was dependant on the load type and also on its location relative to the crack tip. It also showed the increase in the ratio of asphalt modulus to subbase modulus increases the SIF values at all locations. It was shown that the shearing force only had a noticeable impact on the SIF values when the distance from the crack tip was small or zero.

The mechanical characterisation of the asphalt concrete in terms of both the fatigue resistance and the stiffness modulus is necessary to use any design method of the flexible road pavements. Even more the determination of these mechanical properties is necessary if the asphalt concrete investigated is an innovative material as a dry asphalt rubber concrete DARC (i.e. a bituminous mixture with rubber via dry process). Such material is less known and investigated than the asphalt rubber concrete via wet process, even if its application implies peculiar economical and environmental advantages such as no specialized equipment or significant plant modifications and large quantity of recycled waste tires compared to the wet process. This paper focused on the mechanical behaviour of some DARCs with different rubber content that varies within 0, 3.0% by weight of the aggregates; these materials are designed to the base layer of the flexible pavements. In particular an experimental survey was carried out in order to determine the stiffness modulus of the DARCs by means of the four point bending test on prismatic specimens (UNI EN 12697-26:2004).