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20 the rubber tire roller and two passes of the steel wheel roller The t/NMAS should be set at 4.0 for coarse-graded mixes due in the static mode. to the significant increase in voids when reducing the t/NMAS A total of 23 cores were obtained from the side that utilized from optimum down to 3.0. only the steel drum roller and 26 cores from the side that used the rubber tire roller. To determine the minimum t/NMAS for this mix, the relationship of air voids from the vacuum seal 4.5 EVALUATION OF THE EFFECT OF device and t/NMAS was evaluated for each rolling pattern. TEMPERATURE ON THE RELATIONSHIP The results are illustrated in Figure 24. BETWEEN DENSITY AND t/NMAS The best-fit lines indicate that the air voids decreased as the thickness increased to a point where additional thickness Three locations were selected for temperature measure- resulted in increased air voids. The plots also suggested that ments for each section in the field experiment; one near the side utilizing only the steel drum compactor had higher the beginning of the section, one near the middle, and one density. As shown in Figure 24, the minimum t/NMAS range near the end of the section. To determine the effect of mix for 19.0-mm coarse-graded with modified asphalt mix was temperature on the density, the temperature at 20 minutes 3.4 to 4.8. The effect of t/NMAS on the measured density after placement of the mix at each location was selected was determined from Figure 24. Data in the figure indicate because this provides a reasonable compaction time. Because that the lowest in-place air voids (5.6 percent air voids for the mixes in this study used two different types of asphalt the steel wheel roller only and 7.4 percent air voids for the binder, PG 67-22 and PG 76-22, the temperatures at 20 min- steel and rubber tire rollers) occurred at t/NMAS of 4.2 for utes were normalized by subtracting the high temperature the steel wheel roller and 5.3 for the rubber and steel wheel grade of the asphalt type from the temperatures at 20 min- roller. Table 11 shows the air voids at various t/NMAs as utes. Table 13 presents the t/NMAS, the average tempera- related to this minimum. ture readings at 20 minutes, the asphalt high temperature grade, and the difference between mix temperature and high temperature grade. The differences in temperature were plot- 4.4.8 Summary ted against the t/NMAS together with the core densities for each section, as shown in Figures 25 through 31. In summary, the data for the seven sections appear to be The relationship between density and t/NMAS for all reasonable and to match past experience. A summary of the sections is shown in Figure 32. The best-fit line has an R2 results compared to the t/NMAS for lowest voids is provided of 0.26 and indicates that the density increased as the thick- in Table 12. These results indicate that the t/NMAS should be ness increased to a point where additional thickness resulted somewhere between 3 and 5 for best results. Based on the lim- in a decrease in density. The effect of the layer thickness and ited data, a t/NMAS of 3 is probably reasonable for fine-graded cooling time on mix temperature is provided in Figure 33. mixes, because there is less than 1 percentage point change in The data were obtained from the thermocouples installed in density when the t/NMAS is reduced from optimum to 3.0. the pavement. This plot indicates that, during hot weather, 15.0 14.0 13.0 Steel/Rubber Tire Roller 2 12.0 R = 0.81 11.0 Air Voids, % 10.0 9.0 Steel Roller 2 R = 0.65 8.0 7.0 6.0 5.0 4.0 3.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 t/NMAS Figure 24. Relationships of air voids and t/NMAS for 19.0-mm coarse-graded mix with modified asphalt.
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21 TABLE 11 Relationship of air voids and t/NMAS for 19.0-mm coarse-graded mix with modified asphalt compacted with steel roller and with steel and rubber tire rollers Steel roller Steel and rubber tire rollers t/NMA Percentage t/NMA Percentage points points above above lowest lowest 4.2 (lowest air voids, 5.6 %) 0.0 5.3 (lowest air voids, 7.4 %) 0.0 2 4.9 2 6.1 3 1.3 3 3.4 4 0.0 4 0.8 5 0.8 5 0.0 compaction time for a layer thickness of 1.5 in. is approxi- same amount of compactive effort on an HMA mixture mately twice that for a 1-in. layer. This clearly shows that prior to cooling to some defined temperature will take twice one of the problems in obtaining density is layer thickness as many rollers at a 1-in. thickness as that required for a 1.5-in. regardless of the t/NMAS. If the amount of compaction surface. It is likely to be significantly more difficult to compact time is reduced by 50 percent, it may be very difficult to a 1-in. layer than to compact a 1.5-in. layer simply because of compact the mixture to an adequate density. To place the the cooling rate. TABLE 12 Effect of t/NMAS on compactibility of HMA Increase in Air Increase in Air Increase in Air Increase in Air Description of Voids for Voids for Voids for Voids for Mix t/NMAS=2 t/NMAS=3 t/NMAS=4 t/NMAS=5 Section 1-9.5mm Fine Graded-- 2.5% 1.0% 0.1% 0.1% Steel Roller Section 2-9.5mm Coarse Graded- 2.5% 1.0% 0.5% 0.0% Steel Roller Section 2-9.5mm Coarse Graded- 2.0% 0.5% 0.0% 1.0% Steel and Rubber Roller Section 3-9.5mm SMA(mod AC) 5.5% 2.0% 0.2% 0.2% Steel Roller Section 3-9.5mm SMA(Mod AC) 1.2% 0.2% 0.0% 0.5% Steel & Rubber Roller Section 4- 12.5mm SMA 11.3% 3.3% 0.3% 0.5% (mod AC) Steel Roller Section 4- 12.5mm SMA 6.5% 3.5% 0.5% 0.0% (mod AC) Steel & Rubber Roller Section 5-19mm Fine Graded 3.1% 0.6% 0.0% 1.3% Steel Roller Section 6-19mm Coarse Graded 1.8% 0.6% 0.1% 0.1% Steel and Rubber Roller Section 7-19mm Coarse Graded 4.9% 1.3% 0.0% 0.8% (mod AC) Steel Roller Section 7-19mm Coarse Graded (mod AC) Steel 6.1% 3.4% 0.8% 0.0% & Rubber Roller