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35 The density that can be obtained under normal rolling con- ditions is clearly related to the t/NMAS. For improved com- pactibility, it is recommended that the t/NMAS be at least 3 for fine-graded mixes and at least 4 for coarse-graded mixes. The data for SMA indicate that the ratio should also be at least 4. Ratios less than these suggested numbers could be used, but more compactive effort would generally be required to obtain the desired density. In most cases, a t/NMAS of 5 does not result in the need for more compactive effort to obtain maxi- mum density. However, care must be exercised when the thickness gets too large to ensure that adequate density is obtained. The results of the evaluation of the effect of mix tempera- ture on the relationship between density and t/NMAS indi- cate that one of the reasons for low density at thinner sections (lower t/NMAS) is the more rapid cooling of the mixture. Hence, for thinner layers it is even more important that rollers stay very close to the paver so that rolling can be accomplished prior to excessive cooling. For the conditions of this study, the mixes placed at the NCAT test track at 25-mm thickness cooled twice as fast as mixes placed at 37.5-mm thickness. For thicker sections (larger t/NMAS), the rate of cooling is typi- cally not a problem. The in-place void content is the most significant factor impacting permeability of HMA mixtures. This is followed by coarse aggregate ratio and VMA. As the values of coarse aggregate ratio increases, permeability increases. Permeability decreases as VMA increases for constant air voids. The variability of permeability between various mixtures is very high. Some mixtures are permeable at the 8 to 10 per- cent void range and others do not seem to be permeable at these higher voids. However, to ensure that permeability is not a problem, the in-place air voids should be between 6 and 7 percent or lower. This appears to be true for a wide range of mixtures regardless of NMAS and grading. When laboratory prepared samples having low levels of water absorption were evaluated, the dimensional method resulted in the highest air void contents followed by the gamma ray method. The vacuum-sealing and water displacement (AASHTO T166) methods resulted in similar air void con- tents when the water absorption level was low. The vacuum seal method is an acceptable method to use for low and high void levels. At low levels of water absorption, the water displace- ment method is an accurate measure for bulk specific grav- ity. The error develops when removing the sample from water to determine the SSD weight. When water flows out of the sample, an error occurs. The allowable absorption level to use the displacement test method is specified as 2 percent in AASHTO T166, but this level of absorption can create accuracy problems, as shown in this report. It is recommended that the absorption limit for the displace- ment test method be reduced to 1 percent. If the vacuum- seal method is adopted on a project, the measured voids may now be somewhat higher than with the water displacement method. The water displacement method was accurate for all water absorption levels encountered for mixes that were fine-graded (ARZ gradations). For mixes having gradations near the max- imum density line (TRZ) or coarser (BRZ and SMA), the level of water absorption at which AASHTO T166 began to lose accuracy was between 0.2 and 0.4 percent. For mix design samples and other laboratory samples that are compacted to relatively low voids, the displacement method will provide reasonably accurate answers. However, for field samples where the void levels will typically be 6 percent or higher, it is important to evaluate absorption to determine if the vacuum-seal method needs to be used. Care must be used when using the vacuum sealing method to measure density. Many times the plastic bag develops a leak during the test, leading to an error in the result. Weighing the sample in air after measuring the submerged weight will indi- cate if a leak has developed. If a leak is identified, the test must be repeated until an acceptable test is achieved. There appears to be a need for a correction factor for the vacuum-sealing and water displacement methods to provide equal measured air void contents even when the air void level is low. The correction factor for the mixtures evaluated in this report was approximately 0.2 percent air voids. A better determination of the correction factor can be made for specific dense graded mixes by compacting samples in the Superpave gyratory compactor to approximately 4 percent air voids (design air void content) and testing using the two test methods. The difference between these two tests will be the correction factor for the mix. The in-place air voids of the 20 field projects were high. Fourteen of the 20 mixes tested had average in-place air voids above 8 percent and seven of the mixes had average air voids over 10 percent (based on test results with the vacuum-seal method). This low density on a high percentage of random projects is disturbing because this lower density will most certainly lead to significant loss in pavement life. More emphasis must be placed on obtaining adequate density. Regardless of the method of density measurement CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
used, some cores have to be taken and tested for calibration. The most reliable way to measure density is to take cores for density testing. If the amount of absorption during den- sity measurement exceeds 1 percent, the T166 method will likely provide a higher measured density than the true den- sity. The vacuum seal method is one approach to measure a density more accurately when the water absorption exceeds 1 percent. 36 Even though there is a lot of scatter within and between projects, most field results support the finding that higher t/NMAS ratios generally provide lower void levels. Coarse- graded mixtures generally have higher permeability values than the fine-graded mixtures for a given air void level. Air voids were clearly shown to be a key determinant of perme- ability. However, many times the air voids were reasonably low (5 to 7 percent) and the permeability was still high.
