Andy Collop

Surface effects on ground reaction forces and lower extremity kinematics in running.

INTRODUCTION Although running surface stiffness has been associated with overuse injuries, all evidence to support this suggestion has been circumstantial. In the present study, the biomechanical response of heel-toe runners to changes in running surface has been investigated. METHODS Six heel-toe runners performed shod running trials over three surfaces: a conventional asphalt surface, a new rubber-modified asphalt surface, and an acrylic sports surface. The surfaces were categorised according to impact absorbing ability using standard impact test procedures (BS 7044). RESULTS The rubber-modified asphalt was found to exhibit the greatest amount of mechanical impact absorption, and the conventional asphalt the least. The comparison of peak impact force values across surfaces for the group of subjects demonstrated no significant differences in magnitude of force. However, a significant reduction in loading rate of peak impact force was detected for the rubber-modified surface compared with conventional asphalt (P < 0.1). Although analysis of group data revealed no significant differences in kinematic variables when running on the different surfaces, a varied response to surface manipulation among runners was demonstrated, with marked differences in initial joint angles, peak joint angles, and peak joint angular velocities being observed. DISCUSSION For some subjects, the maintenance of similar peak impact forces for different running surfaces was explained by observed kinematic adjustments. For example, when running on the surface providing the least impact absorption, an increased initial knee flexion was observed for some subjects, suggesting an increased lower extremity compliance. However, for some subjects, sagittal plane kinematic data were not sufficient for the explanation of peak impact force results. It appears that the mechanism of adaptation varies among runners, highlighting the requirement of individual subject analyses.

Absorption of Bitumen into Crumb Rubber Using the Basket Drainage Method

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 crumb rubber. The results show that the rate of adsorption is directly related to the penetration grade (viscosity) of the binders as well as to the chemical composition of the bitumen (crude source) but that the total amount of absorption is controlled by the nature of the crumb rubber. In terms of the rheological properties of the residual bitumen, all the binders showed an increase in viscosity, stiffness (complex modulus) as well as elastic response with these changes being consistent for both crude sources and all four penetration grades.

Development and Finite Element Implementation of Stress-Dependent Elastoviscoplastic Constitutive Model with Damage for Asphalt

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.

VISCOELASTIC LINEARITY LIMITS FOR BITUMINOUS MATERIALS

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.RésuméÉtant donné que les matériaux bitumineux sont naturellement viscoélastiques, la caractérisation de leur tenue en service doit faire appel à des méthodes expérimentales et des techniques analytiques qui tiennent compte du temps (ou de la vitesse) de chargement et de la température. En outre, il est généralement recommandé de se baser sur la réponse viscoélastique linéaire (petites déformations) dans la caractérisation du bitume. Ceci permet de simplifier la modélisation mathématique du comportement matériel, vu que la réponse non linéaire des matériauxviscoélastiques est difficile à caractériser expérimentalement et à modéliser dans le cas des problèmes pratiques de l’ingénieur. Cet article décrit une étude des limites de linéarité concernant une gamme de liants et mélanges bitumineux non modifiés et modifiés, à l’aide d’un rhéomètre de cisaillement dynamique et d’une machine à commande hydraulique, conçue en vue d’effectuer des essais dynamiques de tension-compression directe. Les résultats montrent que des critères de linéarité dépendant de la déformation existent aussi bien pour les liants que pour les mélanges bitumineux, pour des valeurs élevées de la rigidité, à de basses températures pour les liants et à des températures basses ou intermédiaires pour les mélanges. Dans le cas des liants élastomères modifiés, un critère de linéarité dépendant de la déformation à haute température est également mis en évidence. Le critère de linéarité dépendant de la déformation pour les mélanges s’est avéré de l’ordre de 100 micro-déformation. Le critère correspondant aux liants étant au moins 100 fois supérieur et dépasse les 10000 micro-déformation et celui du réseau de polymère se situe à 1000000 micro-déformation.

USE OF THE DISTINCT ELEMENT METHOD TO MODEL THE DEFORMATION BEHAVIOR OF AN IDEALIZED ASPHALT MIXTURE

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 Youngs modulus and Poissons 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. Poissons 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 Burgers model was introduced to give time dependent shear and normal contact stiffnesses.

Aggregate Orientation and Segregation in Laboratory-Compacted Asphalt Samples

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.

On the use of discrete element modelling to simulate the viscoelastic deformation behaviour of an idealized asphalt mixture

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 loading time. Uniaxial and triaxial viscoelastic simulations have been performed to investigate the effect of stress ratio on dilation and the numerical results have been verified with experimental data. The effects of introducing a proportion of frictional contacts and a more complex particle shape (clump) on dilation have been examined.

Compensatory adjustments in lower extremity kinematics in response to a reduced cushioning of the impact interface in heel–toe running

The response of heel-toe runners to changes in cushioning of the impact interface was investigated. Ground reaction force and sagittal plane kinematic data were collected for six heel-toe runners performing barefoot running trials on a conventional asphalt surface and an asphalt surface with additional cushioning. Statistical analysis indicated that similar peak impact force values were maintained when running on the two surfaces (p < 0.1). When running on the less cushioned surface, significant reductions were detected in ankle dorsi-flexion angle immediately prior to ground impact and peak ankle plantar-flexion velocity immediately following impact (p > 0.1). In addition, individual subjects demonstrated reductions in heel velocity and increases in knee flexion immediately prior to ground contact. The observed reduction in ankle dorsiflexion at impact, resulting in a flatter foot at ground contact, supports previous suggestions that this is a strategy to reduce local plantar pressure loads. The additional use of adjustments in heel velocity and initial knee flexion highlights the ability of some subjects to adopt compensatory measures to reduce peak impact loading. However, some subjects appear unable to make these adjustments, resulting in higher impact loading on the less cushioned surface for these subjects. This study provides experimental evidence to support the theoretical potential of heel impact velocity and initial knee flexion to influence impact loading in running.

Spatial repeatability of measured dynamic tyre forces

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.

Discrete Element Modeling of Constant Strain Rate Compression Tests on Idealized Asphalt Mixture

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 Burgers 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 Burgers 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.