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Backcalculation Analysis of Pavement-Layer Moduli Using Genetic Algorithms

Most existing iterative backcalculation programs for pavement layer moduli arrive at their solutions by minimizing an objective function related to the differences between computed and measured surface deflections. Unfortunately, the solution surface of the backcalculation problem of pavement-layer moduli is known to contain many local minima. A potentially good backcalculation procedure would be one that has a strong global search ability to overcome the problem of local minima. The genetic algorithm (GA) is a technique that satisfies this requirement. The development of a backcalculation program known as NUS-GABACK using the genetic-algorithm approach is presented, along with the formulation and operations of the program. A detailed performance evaluation of the GA-based method is made against four other programs by solving five backcalculation problems with different structural composition. It was found that NUS-GABACK performed comparably well against the other programs and demonstrated consistency in the accuracies of backcalculated moduli.

Pavement Maintenance Prioritization Using Analytic Hierarchy Process

The prioritization of maintenance activities is commonly applied in pavement maintenance planning. A widely adopted practice is to express maintenance priority in the form of a priority index, computed by means of an empirical mathematical expression. Though convenient to use, empirical mathematical indices often do not have a clear physical meaning and cannot accurately and effectively convey the priority assessment or intention of highway agencies and engineers. In an attempt to overcome this limitation, this study explores the use of an analytic hierarchy process (AHP) for the prioritization of pavement maintenance activities. The main aim is to identify an approach that can reflect the engineering judgment of highway agencies and engineers more closely. Three forms of AHP are examined, namely, the distributive-mode relative AHP, the ideal-mode relative AHP, and the absolute AHP. The applications of the three methods are illustrated by using an example problem, and the results are compared with the priority assessments obtained by a direct assessment method in which the raters make the evaluation by directly comparing all maintenance activities together. The study concludes that the absolute AHP is suitable for the pavement maintenance prioritization process, on the basis of its ability to provide priority assessments for pavement maintenance activities in good agreement with the priority assessments obtained by the direct assessment method and its operational advantage in evaluating a large number of maintenance activities.

INFLUENCE OF PAVEMENT MATERIALS ON THE THERMAL ENVIRONMENT OF OUTDOOR SPACES

Abstract About 80% of the population of Singapore live in public housing estates built by the Housing and Development Board of Singapore. A typical neighbourhood in a public housing estate consists of several multi-storey buildings (from 8 to 25 storeys) laid out in a quadrangle enclosing an open space for outdoor play-grounds, car parks and private access roads into the neighbourhood. As car parks and access roads form the bulk of open spaces in these estates, it is likely that the outdoor thermal environment is much influenced by the materials used in their pavement construction. This is of special concern to the housing authority and community organizations since most community activities occur in the open spaces at ground level on weekends, and after working hours (5 pm) on weekdays. The present study does not attempt to quantify the effects of paving materials on air temperature, or on some comfort index for people inhabiting the outdoor space. However, this paper describes the experiments and analysis used to evaluate the comparative performance of several different pavement types with respect to their surface temperature behaviour and heat output to the environment as a result of daily exposure to solar irradiation in the Singapore climatic condition. It is found that pavement materials such as granite slab, terracotta bricks and coloured concrete interlocking blocks give lower surface temperatures and lower heat output to the environment than the conventional asphaltic concrete pavements used presently. A definite improvement to the thermal environment can be expected when any of these alternative pavement materials are used for the car parks and access roads in the housing estates of Singapore.

Modeling Hydroplaning and Effects of Pavement Microtexture

Hydroplaning on wet pavement occurs when a vehicle reaches a critical speed and causes a loss of contact between its tires and the pavement surface. This paper presents the development of a three-dimensional finite volume model that simulates the hydroplaning phenomenon. The theoretical considerations of the flow simulation model are described. The simulation results are in good agreement with the experimental results in the literature and with those obtained by the well-known hydroplaning equation of the National Aeronautics and Space Administration (NASA). The tire pressure-hydroplaning speed relationship predicted by the model is found to match well the one obtained with the NASA hydroplaning equation. Analyses of the results of the present study indicate that pavement microtexture in the 0.2- to 0.5-mm range can delay hydroplaning (i.e., raise the speed at which hydroplaning occurs). The paper also shows that the NASA hydroplaning equation provides a conservative estimate of the hydroplaning speed. Th...

Permeability Measurement of Base Materials Using Falling-Head Test Apparatus

The development of a new falling-head procedure for laboratory permeability measurement of pavement base materials is described. The main advantage of the falling-head test over the conventional constant-head test is its ability to determine permeability properties of the test material at different levels of hydraulic gradients in a single test. The test setup is simpler and the test duration is much shorter. Each test takes only a few minutes to complete. A large number of tests have been conducted with the proposed procedure on different materials, including compacted crushed stones, glass spheres, and cement stabilized base materials. The falling-head test results were verified by performing constant-head permeability tests on the same specimens at different hydraulic gradients. Analysis on the falling-head test data revealed that the relationship between specific discharge and hydraulic gradient over the range studied could not be represented by a single linear relationship as stated by Darcy’s law. The proposed approach can be a useful tool for determining the drainage properties of base materials within the range of hydraulic gradients encountered in actual pavements.

Determination of thermal properties of pavement materials and unbound aggregates by transient heat conduction

A laboratory procedure for determining the thermal conductivity (k) and diffusivity (a) of pavement materials and unbounded aggregate beds by means of a transient heat conduction experiment is described. It is first established that the plane-wall theory of heat conduction can be applied to a finite-slab problem provided that the thickness-to-width ratio is kept within 0.2. The procedure is to obtain the k and α values that would match the theoretical temperature-time history response with the measured response. An analytical curve-fitting technique is used to match the inflection points of the measured to the theoretical temperature- √t curves. The heat experiment is conducted in a controlled convection oven with parallel air flow at constant velocity over a horizontal test specimen bed. This allows for the testing of unbounded aggregate beds made into a slab by placement of aggregates in an insulated polystyrene box that fits into the base of the oven. The test method is first validated by comparing steady-state heat conduction results with the transient test predictions of k for a solid acrylic slab, two bituminous slabs, and four concrete slabs, with good agreement in the values of k determined by both methods. For the unbounded aggregates, it is observed that there is trend of decreasing values of k and α, with increase in particle size. Also, wet aggregates exhibit higher thermal conductive properties than dry aggregate beds. The test method will be useful for obtaining thermal properties of pavement materials to allow for thermal analysis in pavement layers subjected to solar heating.

Analysis of process energy use of asphalt-mixing plants

We present the results of a study on process energy use of two asphalt-mixing plants. The production process involves the mixing of granite aggregates and bitumen to give asphalt mixtures. The mixtures are widely used as a road-paving material. A large part of the process energy use is for drying and heating the aggregates. Statistical analyses using historical energy-consumption data show a high correlation between specific energy requirements and rainfall level. Field sampling and laboratory investigations indicate that aggregate moisture content and rainfall level are closely related and that aggregate moisture content is a critical determinant of process-energy requirements. Measures for maintaining a low aggregate moisture level, such as isolating aggregate stockpiles from rain, offer great potential for energy savings. We present estimates of these savings, establish process-energy flows, and compare the performances of the two plants.

A study on the fuel-consumption characteristics of public buses

Using the regression technique, we have studied the fuel-consumption characteristics of a fleet of public buses under actual operating conditions. The fuel consumption was measured on a daily basis and the effects of the service routes, vehicle models, engine overhaul, vehicle speed and loads, number of stops, and other factors were established statistically. The analysis shows that service routes and vehicle models had a significant influence on fuel efficiency, and that the average travel speed and loaded weight together could be used to explain up to two-thirds of the variations in fuel efficiency. We also show indirectly that a significant portion of the efficiency variations is attributable to driver behavior and the daily scheduled times of the buses. Based on our results, we estimate the differences in the vehicle fuel efficiency between peak and non-peak operations.

Prediction of Wet-Pavement Skid Resistance and Hydroplaning Potential

The current means of predicting the skid resistance of a wet pavement and the speed at which hydroplaning would occur are based on empirical models or relationships derived from experimental studies. These models and relationships are applicable only for the conditions specified, and extrapolations beyond the applicability range of parameters (e.g., vehicle speed, tire load, tire inflation pressure, water film thickness, and type of tire and pavement surface) are not advisable. Such restrictions could be overcome by developing an analytical model based on theoretical considerations. An analytical model also would provide a more in-depth understanding of the relative influence of different parameters. A three-dimensional finite element model is presented to predict wet-pavement skid resistance and hydroplaning speeds (i.e., wheel speed at which hydroplaning occurs) under different magnitudes of passenger-car wheel load, tire inflation pressure, water film thickness, and vehicle speed. The analysis shows that hydroplaning speed increases (i.e., hydroplaning risk decreases) with wheel load and tire inflation pressure but decreases with the depth of water film thickness. The skid resistance measured in terms of skid number decreases as the sliding-wheel speed or the water film thickness increases but increases with the magnitude of the wheel load and is affected marginally by the tire inflation pressure. Within the normal passenger-car operation range of each of the parameters, the hydroplaning speed is affected most by tire inflation pressure followed by water film thickness and is least influenced by the wheel load; the skid resistance is most influenced by sliding-wheel speed followed by water film thickness and wheel load and is least affected by the tire inflation pressure.

A new apparatus for measuring the drainage properties of porous asphalt mixes

A new apparatus is designed to measure the permeability of porous asphalt road mixes under laboratory conditions. This apparatus works on the falling head principle, from which the falling head with time through a porous specimen is measured with a pracise pressure transducer at very close time intervals (10 readings/s). Using the falling head data, she specific discharge through the specimen can be computed and the penneability of the material obtained from the velocity versus hydraulic gradient relationship. Tests on unbound specimens made with glass spheres showed excellent agreement with constant head test results. The influence of specimen diameter and thickness in relation to particle size is examined, and recommendation for appropriate specimen size proposed. Tests with three types of common porous asphalt mixes were conducted and it is demonstrated that specification of void content alone is not adequate for describing the drainage property of a porous mix.