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225 1.57.4 Construction Practices Tan et al did not discuss construction practices. 1.57.5 Maintenance Practices Tan et al did not discuss maintenance practices. 1.57.6 Rehabilitation Practices Tan et al did not discuss rehabilitation practices. 1.57.7 Performance Tan et al did not discuss performance of permeable asphalt mixtures. 1.57.8 Structural Design From the finite element analysis that Tan et al performed, a series of drainage design charts were constructed. These charts allow the user to determine the ratio of porous surface asphalt thickness to width (mm/m) given the cross slope (%) of the road and the maximum rainfall intensity (m/s). Tan et al recommends the following procedure for using these charts: 1. Determine the vertical permeability of the porous asphalt that will be used as the surface course (kv) 2. Select or determine the maximum allowable rainfall intensity (m/s) for the design storm 3. Adjust the rainfall intensity by multiplying by a factor equal to 20/ kv. This corrects for the fact that the design drainage charts were constructed assuming the vertical permeability was 20 mm/s. 4. Find the appropriate design chart based on longitudinal slope. Then select the appropriate cross slope curve on this chart and read the ratio of porous surface asphalt thickness to width (mm/m). Tan et al present drainage design charts for longitudinal slopes of 0, 2, 4, 6, 8, and 10 percent. Each of these charts has curves corresponding to 0, 1, 2, 3, and 4 percent cross slope. 1.57.9 Limitations Tan et al did not discuss any limitations of permeable asphalt mixtures. 1.58 Watson, D. E., J. Zhang, R. B. Powell. âAnalysis of Temperature Data for the NCAT Test Track.â Transportation Research Record No: 189. Transportation Research Board. National Research Council. Washington, D.C. 2004. 1.58.1 General The study reported in this paper was carried out to determine the maximum and minimum temperatures in pavements with different types of asphalt mix surfaces, compare these temperatures with those predicted by Strategic Highway Research Program (SHRP) and
226 Long Term Pavement Performance (LTPP) models, and evaluate the effect of mix type, aggregate type, asphalt binder type and layer thickness on the maximum and minimum temperatures. Watson et al indicates that a number of combinations of materials, mix and layer thicknesses were used in the National Center for Asphalt Technology (NCAT) Test Track near Auburn, Alabama. The different types of mix include Open-Graded Friction Course (OGFC) (3 sections), Stone Matrix Asphalt (SMA) (6 sections), Superpave mix (36 sections) and a mix designed with the Hveem method. High and low temperatures in each and every section were determined from temperature probe data, recorded over a period of two years. The authors describe the process of predicting temperatures with the use of different models, as well as the effect of temperature on rutting. In this review, only that portion which is relevant to permeable friction courses, is discussed. Watson et al conclude that the open surface texture of OGFC and SMA mixes result in underlying layers being about 2ºC (3.6ºF) cooler than when conventional dense-graded surface mixes are used. 1.58.2 Benefits of Permeable Asphalt Mixtures No information is provided on benefits of permeable asphalt mixtures. 1.58.3 Materials and Design No information is provided on materials and mix design. 1.58.4 Construction Practices Watson et al mentions that the OGFC mix in the three sections was placed 18 mm (0.7 in) thick. 1.58.5 Maintenance Practices No information is provided on maintenance practices 1.58.6 Rehabilitation Practices No information is provided on rehabilitation practices 1.58.7 Performance Watson et al presents the temperature data at different depths for the different pavements, as well as results of comparison of these data with data predicted from SHRP and LTPP models. Temperature was recorded with probes at the top and mid-depth of the surface course, and at the top and bottom of the binder course in each section. The authors mention that the Datalogger receives temperature data every minute and then records the minimum, maximum, and average pavement temperature every hour.
227 The temperature data was used to evaluate measured versus predicted temperatures using SHRP and LTPP temperature models, evaluate the effect of mix type on pavement temperature, and compare the effect of surface layer thickness on pavement temperatures. Watson et al mentions that most of the sections had a surface layer 50 mm (2 in) thick while some had surface layers of 18 and 38 mm (0.75 and 1.5 in) thick. Since the temperature gauge was located at the interface of layers, the depth of each temperature gauge below the surface was varied based on the thickness of each layer. Pavement temperature at depths up to 250 mm (10 in) was measured at the NCAT test track. The general conclusions from comparison of predicted versus measured temperatures are shown in Tables 109 and 110. Table 109: Conclusions from Comparison of SHRP Model Predicted Versus Measured Temperatures Parameter SHRP Model Comments/Explanation High temperature 1. SHRP high pavement model at 50 percent reliability estimates pavement temperatures fairly close, but in some cases slightly underestimates the measured temperature. At depths of 250 mm (10 in) the model varies from the measured temperature as much as 9.6ºC (17.3ºF). With 98% reliability, this model gave a close prediction of pavement temperatures in the upper layers in 2002, but overestimated the pavement temperatures in 2001. However, the SHRP model still significantly underestimated the pavement temperature at 250 mm (10 in) depth. Assumption of maximum pavement temperature at a wind speed of 4.5 m/sec may not be proper; research suggests that consideration of wind speed of 1 m/sec to be more appropriate. Low Temperature SHRP low temperature model does not closely agree with measured values at either 50 percent or 98 percent reliability. At 50 percent reliability the SHRP model overestimated cold temperatures as much as 10.3ºC (18.5ºF) at a depth of 38 mm. SHRP low temperature model does not consider the effect of latitude; surface temperature of the pavement was assumed to be equivalent to the low air temperature. This does not consider that heat will be radiated to the pavement surface from underlying layers during the early morning hours when air temperatures are generally lowest.
