Siew-Ann Tan

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.

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.

Determination of thermal conductivity and diffusivity by transient heating of a thin slab

Abstract Transient heat conduction through a thin slab which is initially at a constant temperature, and subsequently heated by free convection in a thermal bath, is a well established theory. From the theory, it is deduced that a thickness to length ratio of 0.2 is adequate to achieve one-dimentional heat flow in the middle of a square slab. Using the theory, the non-dimensional temperature-time history for the midpoint of the slab is obtained. From these non-dimensional plots, a statistical scheme of finding the best fit between experimental data and the theory is described. By obtaining the best fit, the most probable values of the thermal conductivity and diffusivity are obtained directly.

Nondestructive density measurements of cylindrical specimens by gamma-ray attenuation

Nondestructive density measurement of cylindrical specimens in the laboratory has several important applications. Many specimens in civil engineering laboratory testing are usually in the form of cylinders; undisturbed soil samples and field cores of concrete and asphaltic materials are primary examples. This paper describes a nondestructive laboratory technique for density measurement that can achieve a 95% confidence limit on the order of ± 0.01 g/cm3 for a useful range of density from 0.8 to 2.7 g/cm3, relevant to the above materials. The technique uses a 5 milliCurie (mCi) Cesium 137 source with a scintillation detector and counter assembly to record the radiation transmitted through the specimen. This is converted to a density value by calibration to an empirical radiation attenuation law. The principles of the method are described with consideration to the variables of the measuring system and the specimen geometry. It is shown that for solid cylindrical specimens of the materials tested, the radiation attenuation law is satisfied to a very high degree. From the law, a diameter ratio to count ratio relationship is established. Thus detailed calibration is only necessary for specimens of one diameter to determine the density of specimens of other diameters.

Specimen size effects on the diametrical mechanical testing of cylindrical asphalt mixes

Specimens, 4-in. (101.6 mm), 5-in. (127.0 mm), and 6-in. (152.4 mm) in diameter, of asphalt mixtures were prepared in the laboratory to test for the specimen size effect on the resilient modulus test, the indirect tension test, and the Marshall test. Four asphalt mixtures with different top stone sizes ranging from 15.8 to 31.5 mm were used. They are B1 mix (base course), Airport, W3, and W6 mixes (wearing course). Specimens, 5-in. (127.0 mm) in size, were prepared using the same compaction equipment as 6-in. (152.4-mm) specimens, designed as by Kandhal, P. S. (“Large Stone Asphalt Mixes: Design and Construction,” Proceedings of the Association of Asphalt Paving Technologists, 1990). The height of the 5-in. (127.0-mm) diameter specimen was calculated based on the diameter over the height ratios for the 4- and 6-in. (101.6- and 152.4-mm) specimens. The compaction level required was based on the energy per unit volume ratio of 4- and 6-in. (101.6- and 152.4 mm) diameter specimens. Similar measured densities of the 5-in. (127.0-mm) specimens compared with the densities of the 4- and 6-in. (101.6- and 152.4-mm) specimens indicate that the same level of compaction has been achieved. The stability ratios (stability of 6-in./5-in. specimen, stability of 6-in./4-in. specimen and stability of 5-in./4-in.) were also analyzed. They were found to be close to the theoretical values reported by Kandhal, P. S. (“Large Stone Asphalt Mixes: Design and Construction,” Proceedings of the Association of Asphalt Paving Technologists, 1990). Detailed comparison of flow values for the W3 and B1 mixes were made. The B1 mix gave an unacceptable flow/diameter ratio caused by the effect of large stones. The value was too low in the case of the 4-in. (101.6-mm) specimen for the B1 mix. However, acceptable values were found for all sizes tested using the W3 mix, which had a top stone size less than 1-in. Hence, large stones (stones with diameter greater than 1-in.) in 4-in. specimens tend to produce lower deformation under loading, which also accounts for the higher modulus measured in the resilient modulus test. The influence of the ratios of the diameter of specimens to top stone size were determined for the resilient modulus and the indirect tension test. A general trend of reduction in resilient modulus and indirect tensile strength was observed. The trend suggested that 5- and 6-in. diameter specimens would give more realistic values of tensile strength and stiffness for mixes using large stones (larger than 1-in.). When larger diameter specimens are used, the results obtained from the resilient modulus and the indirect tension test are more representative of the behavior of the mixtures.

Relating triaxial test properties of asphalt mixtures to mix parameters determined by Marshall stability test

The conventional method of asphalt mix design based on the Marshall stability test cannot be used directly for pavement thickness design and performance analysis. On the other hand, the triaxial test that measures fundamental engineering properties of friction-cohesive materials is a well understood and established test in civil engineering, and the authors have reported in an earlier paper that triaxial test properties of asphalt mixtures can be used to study analytically the response of the mixtures under loads. In an effort to establish a link between the two tests, this paper illustrates that the three triaxial test-determined properties of an asphalt mixture, namely cohesion c, angle of friction φ and elastic modulus Ep, are statistically correlated with Marshall stability test properties of the asphalt mixture. The statistical analyses furnish useful information on the trends of variation of various material parameters as a function of mix properties. Potentially useful statistical regression predictive models are derived for predicting material parameters of the Marshall stability tests using the triaxial test properties, and vice versa. Such models make it possible to analyze pavement performance under loads for those pavements constructed based on Marshall mix design.

Analysis of Marshall test behavior with triaxial test determined material properties

The Marshall test is one of the most common methods used for mix design and quality control of asphalt concrete mixtures. However, this method is empirical in nature and does not provide fundamental engineering properties. Fundamental engineering properties provide a basis for rational analysis and design of asphalt concrete pavements. The triaxial test method described in this paper allows engineering properties such as internal angle of friction, [phi], cohesion, c, and elastic modulus, E, to be determined. The method of specimen preparation and the triaxial test setup are briefly described. A numerical simulation of the Marshall test is performed using a plane-stress finite element analysis with triaxial test determined properties as input parameters. A constitutive plasticity model based on the Drucker-Prager yield condition is used to describe the elasto-plastic behavior of the specimen. Analysis shows that the model very well describes the deformation progression before failure and can predict experimental Marshall stability value very closely. There is some underprediction of the Marshall flow, probably due to the idealization of an elastic-perfectly-plastic stress-strain relationship for asphalt concrete.

Compaction of Asphalt Mixtures for Laboratory Testing: Evaluation Based on Density Profile

Cylindrical specimens are commonly used in laboratory testing of asphaltic paving mixtures. This paper describes a study that examined the influence of different compaction methods on the resulted density distributions of cylindrical specimens using a laboratory twin-probe nuclear density gage. Two common sizes of 102 mm (4 in.) cylindrical specimens, namely 64-mm (2.5-in.) high Marshall-size specimens, and 200-mm (7.87-in.) high triaxial test specimens, were considered in the test program. Four compaction methods were studied: drop-hammer compaction, kneading compactions, single-plunger compression, and double-plunger compression. In the case of 200-mm-tall specimens, the effect of compaction in layers was also examined. Test results show that, except for the kneading compaction method, the other three methods could all produce 64-mm-tall Marshall-size specimens of relatively uniform density distributions with density variation within 0.05 g/cm3. For 200-mm-tall specimens, only the double-plunger compression method was able to produce specimens with density distribution of similar uniformity.

Experimental Evaluation of a Laboratory Twin-Probe Nuclear Gage for Specimen Density Measurement

This paper describes a series of tests conducted to establish the operating requirements of a laboratory twin-probe nuclear gage for density measurement of cylindrical specimens of homogeneous materials. The nuclear device is an adaptation of a direct transmission field gage for laboratory density measurement. It is shown in this evaluation program that three forms of analysis are required for the setting up of such an apparatus: calibration analysis, precision evaluation, and accuracy assessment. Three specimen diameters (75, 100, and 150 mm), three materials with densities from 0.8 to 2.7 g/cm3, and nuclear gage source-to-detector spacings ranging from 250 to 450 mm were included in the evaluation program. It was found that density calibration based on radiation attenuation theory is valid, and that a diameter-count rate relationship can be established experimentally so that only a single set of density calibration using one specimen diameter is needed for routine testing. In the accuracy assessment, it was found that there exists a narrow range of source-to-detector spacings for unbiased density estimation. A spacing of 280 mm is recommended for the device studied. At this spacing, the density measurement precision at 95% confidence level, using four 1-min counts per test, is found to vary from 0.010 g/cm3 for 150-mm-diameter specimens to 0.017 g/cm3 for 75-mm-diameter specimens.

Laboratory Wheel Tracking Apparatus for Bituminous Pavement Studies

Rutting deformation is one of the most common forms of pavement distress found on bituminous pavements, especially in the hot, tropical climate of Singapore. These deformations are usually found in the wheel track on roads carrying heavy, slow moving, channeled traffic, such as in the city and industrial areas at traffic light junctions. To and our investigation into the nature of pavement rutting, a laboratory wheel-tracking apparatus was used to simulate channeled wheel traffic loading under controlled conditions. It was upgraded with automated temperature and rut profile measurement capability. This article describes the features of the apparatus, which include temperature control of test specimens from ambient (27°C) to 70°C via a water bath with an adjustable weir, speed control of 0 to 80 wheel passes per minute, and variable wheel loading from 18 to 54 kg giving approximate equivalent tire pressures of 175 to 530 kPa. The apparatus can be used to simultaneously test three standard specimens of dimensions 405 mm by 135 mm by 90 mm-thickness. Single tests of two other sizes, which are two or three times the width of the standard specimen, are permissible. The rut depths are monitored by means of three linear variable displacement transducers (LVDTs), which measure the vertical displacements of each of the three wheel axles independently as rutting progresses. Due to errors induced by machine vibrations, accurate rut profile measurements can be made only under static conditions at selected intervals of wheel passes. With the use of a 200-ton-capacity static press, uniform dense asphaltic concrete beam specimens can be made, giving reasonably repeatable rutting test data on the apparatus, thus making it a valuable tool for evaluating the ratting potential of various mixes.