Strategic Highway Research Program A407, "The Superpave Mix Design Manual for New Construction and Overlays" recommends that aggregate gradations pass below the restricted zone as traffic level increases. The aggregate gradation curve of most of the SUPERPAVE mixes used today, especially in the southeast region of the U.S., follows a typical ""S"" shape with fines lying slightly below the restricted zone. Such a gradation yields a relatively coarse mix; however, the use of gradation with fines slightly above the restricted zone is technically acceptable according to SHRP specifications. Since such a mix is similar to the I-1 or I-2 surface mixture used by the NCDOT for surfacing of the secondary roads, it will be more economical as well as more amenable to acceptance by field engineers. The use of natural sand will reduce the build-up of fines in the stone quarries and encourage the use of reclaimed asphalt pavement (RAP) material.
This study investigated the use of natural sand in the fine and coarse gradations for the surface course mixtures. The mixtures were designed using the SUPERPAVE mix design approach and were evaluated for their performance in terms of resistance to rutting, fatigue, and moisture damage. In addition, the accelerated performance of these mixtures was also evaluated using the Asphalt Pavement Analyzer (APA).
The following are the specific objectives of the proposed study:
- Study the aggregate gradation curves and investigate the use of natural sand in the fine and coarse gradations for the Surface Course mixtures.
- Design the mixtures using the SUPERPAVE mix design approach and conduct testing of the selected asphalt mixtures, including the SUPERPAVE performance tests to evaluate their performance in terms of resistance to rutting, fatigue, and moisture damage.
- Compare the accelerated performance test results of the selected asphalt mixtures using the Asphalt Pavement Analyzer (APA).
It can be concluded from the test results that:
- Maximum size of aggregate and type of gradation influence the performance of mixtures containing natural sand.
- The performance of 12.5mm mixtures, in terms of fatigue and rutting, is not adversely affected by incorporation of natural sand in the mixture gradation.
- The rutting performance of 9.5mm mixtures is not affected by the addition of natural sand, while the fatigue life of these mixtures is adversely affected. The 9.5mm mixtures containing natural sand have 50% to 70% of fatigue life of the mixtures made with manufactured sand.
- The rutting test results indicate that the minimum service life of all the mixtures is about 3 million ESALs, while the minimum fatigue life is to the tune of approximately 1 million ESALs. Therefore, it is quite evident that even 9.5mm mixtures containing natural sand can adequately serve as a surface course for the low volume secondary roads.
- The fatigue life of a given pavement structure is a function of the thickness of the asphalt concrete layer; thicker the asphalt concrete layer, higher the fatigue life. Therefore, an increase in thickness of 9.5mm mixtures containing natural sand may improve the fatigue life to a desired level.
- The mixtures containing natural sand exhibit relatively higher moisture susceptibility.
- The GLPA test is conducted at the compaction temperature (approximately 150oC) at which the behavior of the mixtures could be significantly different from the behavior at the maximum service temperature of 60oC. Furthermore, standard protocols call for APA tests to be conducted on specimens compacted to approximately 7% air voids and at maximum service temperature of the performance graded asphalt, i.e. 64oC. On the other hand, the shear tests are conducted on specimens compacted to Ndes and at 54oC. The discrepancy in these test conditions may obscure a meaningful comparison of the performance of mixtures as evaluated by these tests.
This study has provided an objective evaluation of the Superpave mix design for mixtures containing natural sand. The results provide guidelines for designing such mixes and allow NCDOT to develop and utilize relatively economical Superpave surface course mixes for secondary roads.
