Number Of Load Applications Research Articles

Detection and Monitoring of Cracks in Asphalt Pavement Under Texas Mobile Load Simulator Testing

The Texas mobile load simulator (TxMLS) is a newly developed accelerated pavement testing device used to evaluate pavement performance under real trafficking loads. This evaluation is performed by applying trafficking loads and monitoring surface distress, such as cracking and rutting, in conjunction with a number of other measurements of the pavement, such as those conducted with the falling weight deflectometer, multidepth deflectometer, strain gauge, pressure cells, and seismic (stress-wave) tests. A procedure for monitoring the progressive degradation of the asphalt surface was developed using the spectral-analysis-of-surface-waves (SASW) technique. This procedure was applied with the TxMLS and proved to be equally effective. SASW tests that were performed intermittently between trafficking phases on trafficked and untrafficked areas show ( a) the effect of temperatures and frequencies on the asphalt moduli, ( b) the importance of temperature and frequency corrections in analyzing the degradation of the asphalt surface layer, and ( c) the long-term trends in surface-wave velocities (and hence, moduli) of the surface layer with increasing number of load applications. It was found that stiffness of the asphalt layer in the longitudinal direction was progressively reduced under trafficking. Concurrently, surface cracking progressively increased. The reduction in longitudinal stiffness occurred at a faster rate than the crack growth. In contrast, the reduction in the stiffness of the asphalt layer in the transverse direction was slower, probably because the main mode of cracking was transverse. The feasibility of using SASW testing as a predictor of degradation and imminent cracking was confirmed with these studies.

Neural Networks and AASHO Road Test

The American Association of State Highway Officials (AASHO) road test, conducted during the period of 1958 through 1960, was a factorial test of pavement durability that considered layer depths, axle load, and number of load applications as the primary variables. These data were processed using traditional statistical techniques. The AASHO formula is the resulting databased model of the road-test data. In the present paper, we reexamine the AASHO road-test data, using the Monte Carlo Hierarchical Adaptive Random Partitioning (MC-HARP) neural-network model developed by Banan and Hjelmstad (1995), and show that an MC-HARP model can represent the data far better than the AASHO formula can. We conclude that the MC-HARP neural network may be an appropriate tool for the development of databased models of pavement performance in the future.

Influences on Permanent Deformation Behavior of Unbound Granular Materials

The results of a research program aimed at characterizing the permanent deformation behaviors of different types of unbound aggregates under repeated loading are summarized. Tests were performed in a triaxial or hollow-cylinder apparatus, depending on the grading of the materials. The scope of the research was to assess the relationship between total permanent axial strain and both the number of load applications and the stress level. The test results were used to verify a model recently developed in France in which attempts are made to predict the long-term behaviors of granular materials. The French model is examined with particular reference to its validity on the effect of stress level on permanent deformation of unbound granular materials. The results of the study indicate that the French model is generally successful in predicting the permanent strain for a given number of load applications. For one of the materials, however, the model did not seem to fit the data well. The French model also suggests that the variation in total permanent strain with stress level is related to the static failure line and could be determined by comparing the maximum shear stress ratio with the slope of the estimated failure line. This is discussed and shown to be questionable because it results in either unreasonable failure parameters or a very low level of correlation with the observations. An attempt was made to modify the French model, but that also proved to be unsuccessful. The concept of relating the permanent deformation behavior to the static failure condition of the material is therefore questioned.

Influences on Permanent Deformation Behavior of Unbound Granular Materials

The results of a research program aimed at characterizing the permanent deformation behaviors of different types of unbound aggregates under repeated loading are summarized. Tests were performed in a triaxial or hollow-cylinder apparatus, depending on the grading of the materials. The scope of the research was to assess the relationship between total permanent axial strain and both the number of load applications and the stress level. The test results were used to verify a model recently developed in France in which attempts are made to predict the long-term behaviors of granular materials. The French model is examined with particular reference to its validity on the effect of stress level on permanent deformation of unbound granular materials. The results of the study indicate that the French model is generally successful in predicting the permanent strain for a given number of load applications. For one of the materials, however, the model did not seem to fit the data well. The French model also suggests that the variation in total permanent strain with stress level is related to the static failure line and could be determined by comparing the maximum shear stress ratio with the slope of the estimated failure line. This is discussed and shown to be questionable because it results in either unreasonable failure parameters or a very low level of correlation with the observations. An attempt was made to modify the French model, but that also proved to be unsuccessful. The concept of relating the permanent deformation behavior to the static failure condition of the material is therefore questioned.

Mechanistic Evaluation of Rigid Airfield Pavements

A structural deterioration relationship is developed using the U.S. Army Corps of Engineers accelerated traffic tests for plain concrete airfield pavements. The relationship predicts the performance of the rigid pavement in terms of the structural condition index from the pavement's Westergaard edge stresses, flexural strength, and number of load applications. The model is compared to other design criteria and unfailed test sections and gives reasonable agreement. The relationship between the structural condition index and the coverages at initial failure of the pavement is determined, and an alternative fatigue curve is suggested for use in the thickness design of rigid airfield pavements with joints capable of load transfer. The relationships developed only account for the tensile fatigue-loading-induced deterioration of the rigid airfield pavement. Actual in-service pavements will show additional deterioration because of other jointed rigid pavement distresses such as pumping, settlement or faulting, d...

Permeability of geotextile-included railroad bed under repeated load

This paper reports some experiments which were used to evaluate the change in permeability of a simulated railroad system with two variables—time and number of load applications. The change in permeability is due to clogging of soil fines in and around the geotextile. Test results have shown that the coefficient of permeability of the simulated railroad system decreases with the increase in time and decreases with the increase in the number of cycle of repeated load. Efforts were also made to unclog the geotextile by flushing water from the top, and the increase in permeability after flushing was evaluated. Reasons for the pumping action are discussed, and the sequence of the processes for clogging for the simulated railroad system is proposed.

A strategy for periodic inspection based on defect growth

Abstract A rational approach is developed for prediction of failure risk or probability of survival of structure components. In the proposed model there are three possible failure modes during any given life cycle of a member of the population. The risk of failure for each mode is derived and the combined total risk is determined as a function of time (or number of load applications). The effects of various inspection schemes on the risk of failure are then investigated. As an example, the probability of failure of a pipe segment containing a defect is evaluated during its service life. An inspection strategy is then outlined whereby the total risk of failure at any time is kept below a predetermined value.

Improvements to Pavement Subgrade Strain Criterion

Relationships between maximum allowable elastic strain on the subgrade and number of load repetitions have been used extensively in pavement design procedures. Their development is reviewed to allow a proper perspective on their correct use and as a background for describing improvements which take account of the differing deformation resistance offered by various asphalt mixtures. These improvements are derived from both field data and the results of an extensive testing program during the last four years at the University of Nottingham using repeated load triaxial tests. A continuously graded dense bitumen macadam base can carry more than 1‐1/2 times the number of load applications before developing the same limiting rut depth as a gap graded hot rolled asphalt of equal elastic stiffness. Factors on the design life against permanent deformation are given for four typical materials for use in conjunction with the subgrade strain criterion.

Reliability of Single-and Multi-member Structure subjected to Fluctuating Load

The growth of the fatigue crack under fluctuating load of which amplitude was assumed to be a random variable following Rayleigh distribution was discussed. The probability distribution function of the fatigue crack length after a number of load applications was given and the momentary risk and reliability functions were obtained using Griffith-Irwin criterion for catastrophic failure. Numerical simulations were carried out to check the result and compare it with Palmgren-Miner rule. It was shown that the fatigue lives estimated from Palmgren-Miner rule were much longer than those of the present model.The result was applied to a multiple-member structure to see its fail-safe capacity. Two methods were proposed to determine upper and lower bounds of reliability function of the structure.