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X-ray computed tomography (CT) and laboratory permeability techniques were applied to study the internal properties of porous asphalt mixture. Specimens containing aggregate of different gradations and of different mineral compositions were prepared in the laboratory. The air voids of all the specimens were determined in the laboratory and the specimens were then scanned in X-ray CT. The same samples were then tested for laboratory permeability. The active air voids and contact points of aggregate with nearby aggregate particles were determined from X-ray CT images. The laboratory permeability test results were found in comparison with X-ray CT values of voids’ internal connectivity. A comparatively high permeability value was observed for porous asphalt with maximum aggregate size irrespective of its mineral composition. The size of air voids and volume of active (connected) air voids were also found high for mixtures made up of large aggregate size. The difference in mineral composition of aggregate was...

The application of finite element modelling for assumed mesh elements is limited for asphalt mixtures because of the inclusion of air voids within the structure. A multiphase finite element (FE) model was applied to consider the effect of air voids on the properties of asphalt mixtures. The virtual microstructure of an asphalt mixture was incorporated into the FE model from the X-ray CT images of asphalt specimens. Considering the asphalt mixture as viscoelastic, the mechanical properties determined from laboratory tests were applied to the FE model. Analysis was performed on a representative volume element using the concept of continuum damage mechanics. It was found that X-ray CT images can be converted into a finite element mesh of the real geometry of the asphalt sample. It was also found from the analysis that the mesh generated from the actual geometry account of the stress concentrations due to air void distribution compared with the assumed simplified geometry. The FE model was also analysed using CAPA 3D to give a validation of the results.

This study investigated damage in asphalt due to moisture by using the modified saturation ageing tensile stiffness (SATS) test along with image analysis. Specimens made from both granite and limestone aggregate with a 40/60 penetration grade bitumen were tested in the modified SATS test. The samples were scanned within an X-ray CT facility before the SATS testing, after the initial saturation and at the completion of the SATS test. Bitumen was extracted from tested specimens and scanned separately in a high resolution Nanotom (high resolution X-ray CT). The extracted bitumen was tested in a DSR for shear rheological properties. The specimens made from granite (acidic) and limestone (basic) aggregate were found to have similar permeability and this was found to be high for specimens having high design air void content. Specimens made from basic aggregate had higher stiffness than the specimens made from acidic aggregate. The master curves of the aged binder are different from those of the virgin binder (bitumen). Also the microstructure of binder observed was different for aged and virgin binder.

In this paper mixing of reclaimed material with virgin asphalt material has been studied non-destructively. Specimens prepared with virgin asphalt mix and with asphalt mix containing reclaimed material were scanned in X-ray Computed tomography (CT). The obtained Images were analysed for particles and pores (air voids) distribution. All the specimens were also tested in compression test and X-ray scanned while testing. It was found that distribution of reclaimed is not uniform through the samples. There is no considerable effect of increasing the mixing time on particles’ distribution. Similar trend of increase in air voids (cracks) with the increase in load was found for both virgin mix and for asphalt mix containing reclaimed material. Only at failure the increase in voids was found higher for specimens containing reclaimed material.

This paper describes the development of a laboratory-based automatic torque bond test that is capable of quasi-static and repeated load interface testing. Results from quasi-static testing undertaken using either a controlled torque rate or a controlled rotation rate showed that the shear strengths and shear reaction moduli increased as the temperature decreased. Samples with interfaces treated with the cationic emulsion were found to display the highest shear strengths and those treated with no emulsion were found to display the lowest shear strengths. A reasonably good correlation was found between results from testing performed at 600 N m/min and testing performed at 180°/min. The nominal shear strength measured from a test performed at 180°/min was found to be approximately 1.9 times higher than the nominal shear strength measured from a test performed at 600 N m/min. The nominal shear reaction modulus from a test performed at 180°/min was found to be approximately 1.6 times higher than the nominal shear reaction modulus from the test performed at 600 N m/min. The effect of material compliance above and below the interface on the rotation of the interface and shear reaction modulus of the interface is likely to be relatively small unless the shear stiffness of these materials is very low and/or the thickness is large. Results from repeated load (fatigue) testing showed a higher fatigue life and greater sensitivity to shear stress level at the lower temperature.

X-ray Computer Tomography (CT) has been used to quantify the damage induced during constant strain rate and fatigue tests on asphalt. Using image analysis techniques grading of the aggregate was carried out from the X-ray images of compacted asphalt specimens. It was found that particles larger then 5mm can be seen in comparison to the mix design curve of the asphalt mixture. It was observed from X-ray scanning of the specimens tested in mechanical testing that the increase in the volume of void content is not uniform throughout the specimen. It was found from the constant strain rate tests that the damage (air voids) were found to increase with increasing strain as damage accumulates in the specimen. It was also found that there is a greater increase in damage (void content) for the specimens tested at the higher temperatures and lower strain rates. Similar behaviour was observed from the fatigue tests with the greatest increase in damage at the higher test temperature.

The field performance of dry process crumb rubber–modified (CRM) asphalt mixtures has been reported to be inconsistent with stripping and premature cracking on the surfacing. One of the concerns is that, because achieving field compaction of CRM material is difficult due to the inherent resilient nature of the rubber particle, nonuniform field compaction may lead to a deficient bond between rubber and bitumen. To assess the influence of compaction, a series of CRM and control mixtures was produced and compacted at two levels: 4% (low, optimum laboratory compaction) and 8% (high, field experience) air void content. The long-term durability, in regard to moisture susceptibility of the mixtures, was assessed by conducting repeated moisture conditioning cycles. Mechanical properties (stiffness, fatigue, and resistance to permanent deformation) were determined in the Nottingham Asphalt Tester. Results indicated that compared with conventional mixtures, the CRM mixtures, regardless of compaction effort, are more susceptible to moisture with the degree of susceptibility primarily depending on the amount of rubber in the mixture, rather than the difference in compaction. This behavior is different from that of conventional mixtures in which, as expected, poorly compacted mixtures were found to be more susceptible to moisture than were well-compacted mixtures.

This paper describes the linear viscoelastic (LVE) dynamic oscillatory properties of synthetic binders, as well as synthetic binder—bitumen blends, that have been synthesised from acrylic monomers by atom transfer radical polymerization (ATRP). The rheological properties of these binders have been determined using a dynamic shear rheometer (DSR) and frequency sweep tests. The results indicate that acrylic polymers, such as poly(ethyl acrylate) (PEA) and poly(methyl acrylate) (PMA), have rheological properties similar to that of 100/150 and 10/20 penetration grade bitumens, respectively. However, polyacrylates such as poly(butyl acrylate) (PBA) tend to be highly viscous at ambient temperatures and are therefore unsuitable as an asphalt mixture binder. The synthetic polymer binder—bitumen blends tended to be more compatible and stable when produced with a softer bitumen (70/100 pen or 100/150 pen) combined with a hard synthetic binder (PMA).

The spherical indentation response of a dense bitumen macadam asphalt mixture with two different volume fractions of bitumen binder is investigated both experimentally and via an analytical model. The model for the indentation of bitumen developed by Ossa et al. in 2005, was used to study the spherical indentation behavior of the mixtures with good agreement when compared to experimental results. An extensive experimental study of the monotonic and recovery spherical indentation behavior is reported for a range of temperatures. In line with the predictions of the model, the monotonic indentation response of the mixtures exhibits a power-law dependence on the indentation force. The model is also successful in capturing the indentation recovery behavior of the mixtures. A comparison of the material parameters obtained from uniaxial compression and indentation tests showed that indentation tests can be used in an easy and reliable way to obtain the fundamental asphalt parameters. Further, parameters found from indentation tests implicitly account for the confining conditions generated by the aggregate particles below the indenter.

The use of discrete-element modelling (DEM) to simulate the behaviour of a highly idealized bituminous mixture under uniaxial and triaxial compressive creep tests is investigated in this paper. The idealized mixture comprises single-sized spherical particles (sand) mixed with bitumen and was chosen so that the packing characteristics are known and the behaviour of the mixture is dominated by the bitumen. The bitumen is represented as shear and normal (tensile and compressive) contact stiffnesses. Numerical sample preparation procedures for specimens containing spherical particles or clumps have been developed to ensure that the final specimen is isotropic and has the correct volumetric proportions. An elastic contact was used for the compressive normal contact stiffness and a viscoelastic contact was used for shear and tensile normal contact stiffness. Simulation results show that the idealized mixture is found to dilate when the ratio of compressive to tensile contact stiffness increases as a function of...

This paper investigates the use of discrete element modelling (DEM) to simulate the behaviour of a highly idealised bituminous mixture under uniaxial and triaxial compressive creep tests. The idealised mixture comprises single-sized spherical (sand-sized) particles mixed with bitumen and was chosen so that the packing characteristics are known (dense random packing) and the behaviour of the mixture will be dominated by the bitumen and complex aggregate interlock effects will be minimised. In this type of approach the effect of the bitumen is represented as shear and normal contact stiffnesses. A numerical sample preparation procedure has been developed to ensure that the final specimen is isotropic and has the correct volumetrics. Elastic contact properties have been used to investigate the effect of the shear and normal contact stiffnesses on bulk material properties. The bulk modulus was found to be linearly dependent on the normal contact stiffness and independent of the shear contact stiffness. Poisson’s ratio was found to be dependent on only the ratio of the shear contact stiffness to the normal contact stiffness. An elastic contact has been assumed for the compressive normal contact stiffness and a viscoelastic contact for shear and tensile normal contact stiffness to represent the contact behaviour in idealised mixture. The idealised mixture is found to dilate when the ratio of compressive to tensile contact stiffness increases as a function of loading time. Uniaxial and triaxial viscoelastic simulations have been performed to investigate the effect of stress ratio on the rate of dilation with shear strain for the sand asphalt. The numerical results have been validated with experimental data.

An extensive experimental study of the dilation behaviour of asphalt mixtures was performed. The effect of mixture composition (volume fraction, gradation, shape and size of aggregate) and test conditions (loading/unloading, temperature and confinement pressure) of idealized asphalt mixtures of various volume fractions of aggregate was performed. The volume fraction and angularity of aggregate was found to play an important role on the dilation behaviour whereas the experimental conditions were found to have no effect on the dilation behaviour of asphalt mixtures. A generic DBM mixture was fabricated with two different volume fractions of bitumen. It was found that an increase in bitumen content decreases the dilation gradient. An analytical model proposed by Goddard and Didwania (1998) was successfully used to predict the dilation gradient for mixtures with sub-spherical aggregate, but failed to predict the dilation behaviour of specimens with angular aggregate.

This paper investigates the use of distinct element modelling to simulate the behavior of a highly idealized bituminous mixture in an uniaxial compressive creep test. The effect of bitumen is represented as shear and normal contact stiffnesses. Elastic contact properties have been used to investigate the effect of sample size and the effect of the values of the shear and normal contact stiffnesses on bulk material properties. It was found that a sample containing at least 4,500 particles is required for Young's modulus and Poisson's ratio to be within 2% of the values calculated using a much larger number of particles. The bulk modulus was found to be linearly dependent on the normal contact stiffness and independent of the shear contact stiffness. Poisson's ratio was found to be dependent on only the ratio of the shear contact stiffness to the normal contact stiffness. A simple elasto-visco-plastic Burger's model was introduced to give time dependent shear and normal contact stiffnesses.

This paper describes laboratory work undertaken to measure the resistance to crack propagation of bituminous materials comprising a range of binder grades from different sources. The compact tension (CT) test has been adapted for use with bituminous materials, and an image analysis technique, based on digital photographs of the specimen taken during testing, has been used to quantify crack length. Linear elastic fracture mechanics (LEFM) principles have been used to calculate the mode I stress intensity factor (SIF), and the Paris law has been used to quantify crack growth. Results show that the general trend is for the Paris law exponent n to decrease and for the multiplier A to increase with increasing temperature for each of the materials, indicating that, the harder the bitumen, the greater is the sensitivity to overloading. A good correlation was found between the values of A and n, and also between the values of A in the Paris law and the stiffness modulus of the asphaltic mixture. It was found that mixtures containing the nominal 15 penetration grade binders were least sensitive to variations in stiffness of the mixture, and mixtures containing the nominal 35 and 50 penetration grade binders were most sensitive to variations in stiffness of the mixture. The number of standard wheel loads required to cause a crack to propagate through the asphalt layer of a typical minor and major flexible pavement structure have been estimated using data from the CT testing. Results show that there is a good correlation between the stiffness modulus of the asphalt mixture and the predicted number of load applications to failure. The predicted number of load applications to failure increases as the stiffness modulus of the asphalt increases.

Interest in the rheological and fracture characteristics of low penetration grade bitumens has increased in recent times due to greater use of progressively stiffer base materials and the growing concern over the durability of these extremely hard, high stiffness modulus materials. This paper investigates the high and low temperature rheological characteristics, tensile fracture modes and cohesive properties of a series of semi-blown, polymer modified and straight-run (direct distillation) low (15) penetration grade bitumens using a Brookfield (rotational) viscometer, Dynamic Shear Rheometer (DSR), Bending Beam Rheometer (BBR), Direct Tension Tester (DTT) and Vialit Pendulum Cohesion apparatus. The binders have been tested in their virgin (unaged) condition and after short-term, Rolling Thin Film Oven Test (RTFOT) and long-term, High Pressure Ageing Test (HiPAT) ageing. The results show considerable differences in the high and low temperature rheological and fracture properties of the 15 penetrat...

This paper describes an evaluation of the absorption of different sources and grades of bitumen into particles of crumb rubber using a basket drainage method. The effect of the rubber–bitumen interaction has been investigated in terms of the absorption of the light fractions of the bitumen into 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 to 35 pen have been mixed with 2–8 mm sized granulated crumb rubber at three rubber to binder ratios of 1:4, 1:6 and 1:8 by mass. The increased mass of the crumb rubber was used to determine the loss of volatiles and light fractions absorbed from the different bitumens. The residual binders were then subjected to asphaltene content tests, high temperature viscosity and dynamic mechanical analysis using a dynamic shear rheometer to determine the chemical composition and rheological properties of the binders following their interaction with c...

This paper describes the use of the Dynamic Shear Rheometer (DSR) to characterise the steady-state shear deformation properties of two grades of bitumen over a wide range of stresses and strain rates. The steady-state shear strain rate has been used to characterise the quasi-static deformation behaviour of the bitumen. Results indicate that at low levels of shear stress the behaviour of the bitumen is linear whereas at higher levels of shear stress the behaviour is non-linear. A simple model has been developed to compare results from the DSR test to results that would be obtained from a test with uniform shear stress and shear strain rate conditions. The model assumes linear behaviour at low stress levels ( 100 kPa). Between these two stress levels there is a transition region where the behaviour changes from linear to power law creep. A power law creep exponent of n =2.4 was used for both bitumens. Results from the model show reasonably good a...

Compaction forms an integral part in the formation of the aggregate orientation and structure of an asphalt mixture and therefore has a profound influence on its final volumetric and mechanical performance. This article describes the influence of various forms of laboratory (gyratory, vibratory and slab-roller) and field compaction on the internal structure of asphalt specimens and subsequently on their mechanical properties, particularly stiffness and permanent deformation. A 2D image capturing and image analysis system has been used together with alternative specimen sizes and orientations to quantify the internal aggregate structure (orientation and segregation) for a range of typically used continuously graded asphalt mixtures. The results show that in terms of aggregate orientation, slab-compacted specimens tend to mimic field compaction better than gyratory and vibratory compaction. The mechanical properties of slab-compacted specimens also tend to be closer to that of field cores. However, the results also show that through careful selection of specimen size, specimen orientation and compaction variables, even mould-based compaction methods can be utilised with particular asphalt mixtures to represent field-compacted asphalt mixtures.

This paper investigates the quasi-static permanent deformation (rutting) behaviour of two idealised bituminous mixtures (sand asphalts). The permanent deformation behaviour is assessed using simplified quasi-static uni-axial and tri-axial laboratory tests performed over a range of temperatures, stress levels and strain rates. The results are compared to results from a standard repeated load laboratory test and a more simulative laboratory scale wheel tracking test performed at different temperatures and load levels. Results from the quasi-static uni-axial testing show that the deformation behaviour of the idealised mixtures follows the same pattern as the deformation behaviour of the bitumen used in the idealised mixtures (previously measured). At high stress levels the response is non-linear with a power law creep exponent of approximately 2.4. At lower stress levels the effective power law exponent decreases and the behaviour becomes more linear. The effect of the aggregate is to reduce the str...

This paper describes tests undertaken to characterize the deformation behaviour of idealized bituminous mixtures and the development of a model to describe the permanent deformation exhibited by these mixtures in a wheel-tracking test. Steady state uniaxial and triaxial tests are used to characterize the deformation behaviour of the idealized mixtures over a range of stress levels, strain rates and temperatures. Results show that at low stress levels the deformation mechanism is linear viscous flow whereas at high stress levels the deformation mechanism is non-linear viscous flow with a power-law creep exponent of approximately 2.4. Wheel tracking tests have been performed over a range of applied stress levels and temperatures. Results indicate that the accumulation of permanent deformation in these idealized mixtures is non-linear. The measured uniaxial and triaxial properties have been used in a viscous model to predict the permanent deformation measured from wheel tracking tests. A method is presented for incorporating the observed non-linear behaviour of these materials in a linearized viscous approach. The results indicate good agreement between predictions and measurements, indicating the applicability of this type of model.

Imaging techniques have been used to characterise laboratory fatigue testing on asphalt mixtures. Specimens were tested in tension-compression cyclic testing at different temperatures. Testing was stopped at predefined stages, and specimens were scanned in X-ray computed tomography. A three-dimensional image of the original specimens was developed for analysis. It was found from the image analysis that the damage (air voids) were found to be increasing with an increase in strain as damage accumulated in the specimens. An energy-based damage mechanics model was used to calculate the increase in damage. Dissipated pseudo-strain energy and adhesion between binder and aggregate were used in the model. Pseudo-strain energy was calculated from laboratory testing, and the adhesion parameter was determined from surface energies of aggregate and bitumen (binder). Values obtained from a numerical model were found to have a similar trend as that of the data from X-ray computed tomography and image analysis.

This study examines the performance of stress-absorbing membrane interlayer (SAMI) under traffic loading. Accelerated pavement test facility at the University of Nottingham was used for the study. It involves construction and trafficking of a pavement with sections having SAMI and without SAMI. The study indicates that the SAMI is able to delay reflective cracking. It shows that the performance of the SAMI depends on the thickness of the SAMI and the interface bonding between the overlay and the SAMI. Also, it reveals that when SAMI is used between an existing cracked pavement and an overlay, the interface shear stiffness of the overlay and SAMI plays a role in isolating the overlay from the relative deflection of the cracked underlying layer due to traffic loading.

This study has investigated the crack resistance potential of selected SAMIs subjected to wheel loading. Tests were carried out in a wheel tracking facility and temperature-controlled environment to simulate field stress and strain conditions. The results indicate that the potential of SAMIs to retard crack growth depends on factors such as the SAMI stiffness and thickness, the overlay thickness, the load level and temperature. It was found that the relative benefit of SAMIs was higher when lower SAMI and overlay thicknesses were used. The study also showed the SAMIs to be effective at 20 °C and 30 °C but relatively ineffective at 10 °C.

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.

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 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...

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 ...

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.

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 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.

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 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.

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).