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8 CHAPTER 2 Findings State-of-the-Art Summary The following observations concerning the effects of aggre- gate gradation and size on HMA mixture properties and per- The summary focuses on literature published after formances were reported in the literature: NCHRP Report 405. Multiple databases were used; 46 papers were reviewed. This summary provides a synopsis of the lit- · HMA mixtures with gradations passing through the erature in terms of the reported effects of aggregate proper- restricted zone exhibit higher bulk density and lower air ties on rutting, fatigue life, and durability of HMA pavements voids than mixtures with gradation plotting below and and organized by effect of gradation and nominal maximum above the zone (4); size; particle shape, angularity, and surface texture; and prop- · Fine-graded HMA mixtures have better fatigue perfor- erties of p0.075 materials. mance than do more coarsely graded mixtures (5); · HMA specifications that allow gradations to pass through the restricted zone produce the best performing pavements Effects of Aggregate Gradation and Size in Georgia (6); Selection of aggregate gradation for use in HMA pavement · In accelerated loading tests, HMA mixtures with grada- is important to pavement performance. Recent guidance for tions above the restricted zone generally exhibit less rutting HMA gradation has been defined in terms of the Superpave than those with gradations below and through the restricted zone. This zone is located along the maximum den- restricted zone (7, 8); sity line between the 0.300-mm and 4.75- or 2.36-mm particle · Triaxial test results indicate that fine-graded HMA mix- sizes (depending on the maximum nominal size of the aggre- tures have greater shear strength than those with coarser gate). Avoiding this zone was intended to limit the amount of gradations (2); and rounded natural sand that can contribute to mixture instabil- · The SST indicated that an aggregate gradation passing ity. Some research suggests that aggregate gradations plotting through the restricted zone had no significant effect on below this zone produce more rut-resistant mixtures. How- HMA mixture performance (4). ever, other studies have shown that gradations plotting above the restricted zone provide equal or even more rut-resistant HMA performance is also influenced by the maximum mixtures (2). aggregate size. HMA mixtures with larger maximum aggre- Comparing HMA pavement performance based only on gate sizes were reported to exhibit better rutting performance different aggregate gradations is not a simple matter. The than those with smaller maximum aggregate sizes (9, 10). interrelationships among aggregate gradation, aggregate Khedaywi and Tons (11) also found that smaller coarse aggre- characteristics, and HMA volumetric properties are complex. gate particles provided more aggregate interlocking and In general, dense-graded HMA mixtures with adequate Voids resulted in increased HMA shear strength. in the Mineral Aggregate (VMA) provide improved resistance Accelerated pavement testing seems to delineate the effect to degradation and improved resistance against fatigue crack- of maximum aggregate size on HMA performance much ing when used in thick pavements (1). In a recent study, more effectively than do the laboratory methods. It was coarse-graded HMA mixtures were found to be more sensi- reported that differences between mixtures with different tive to variations in asphalt binder content and p0.075 mate- nominal maximum aggregate sizes (NMAS) that had shown rial than were fine-graded mixtures (3). significant performance differences when tested in the FHWA

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9 Accelerated Loading Facility (ALF) could not be detected by result in more aggregate breakdown, thus exposing the laboratory methods (9). The following observations regard- aggregate surface and creating a potential durability prob- ing aggregate size effects were reported in the literature: lem. The VMA was also found to increase slightly with an increasing percentage of flat and elongated coarse aggregate · Increasing the maximum aggregate size in a gradation will particles. improve the mixture quality with respect to creep per- The uncompacted voids content of fine aggregate or fine formance, resilient modulus, and tensile strength, but will aggregate angularity (FAA), as affected by particle shape, decrease the Marshall stability and flow (10); angularity, and surface texture, is determined in accordance · Pavement sections tested with the FHWA ALF indicated with ASTM C 1252 by measuring the voids ratio of loosely that mixtures with an NMAS of 37.5 mm perform better placed fine aggregate in a standard cylinder. In general, fine than those with a 19.0 mm NMAS (9); and aggregates with high FAA values result in higher internal fric- · A national pooled-fund study found that mixtures with a tion and stronger bonds with asphalt binder that leads to bet- NMAS of 9.5 mm and 19.0 mm performed similarly when ter stability and rut resistance of HMA mixtures. tested with the Purdue APT and the Purdue laboratory Kandhal and Parker (1) indicated that FAA value is an wheel tracking device (PurWheel) (7). important factor in HMA mixtures performance with aggre- gate gradations above the restricted zone. They concluded that the higher the FAA, the greater the resistance of HMA to permanent deformation. For coarse-graded mixtures, a Particle Shape, Angularity, and Surface study showed that FAA did not impact the HMA perform- Texture ance as measured by the Asphalt Pavement Analyzer (APA), Generally, aggregate shape, angularity, and surface texture Couch Wheel Tracker, and SST (16). However, studies con- characteristics influence HMA performance; however, ducted using the APT, PurWheel, and triaxial tests found that some state agencies and aggregate producers have expressed FAA of coarse-graded mixtures correlated well with HMA concerns with the validity and practicality of the specifica- rutting performance. These results also indicated that very tions and methods used to determine these characteristics-- high FAA values do not necessarily provide better perform- generalizing these specifications to all types of aggregates may ance than do sands with more typical FAA values in the range be inappropriate. For instance, HMA performance problems of 40 to 45 (7, 17). associated with some aggregate types (e.g., igneous aggregate) Stiady (7) reported that design asphalt binder content is are usually not related to particle shape (12). affected by FAA; higher FAA is associated with increased Numerous studies have been conducted on the effects of resistance to compaction, higher VMA, and higher asphalt particle shape, angularity, and surface texture on HMA per- binder content. Higher asphalt binder content results from formance. Results of a study indicated uncompacted voids the fixed 4-percent air voids criteria. Very high FAA values content of the coarse aggregate, often referred to as the coarse also tend to indicate slivered particle shape and/or extremely aggregate angularity (CAA), did not correlate with the actual rough texture. During mixture design compaction, slivered observed performance of either the fine- or coarse-graded particles orient themselves randomly resulting in high VMA mixtures (8, 13). An accelerated pavement testing study and asphalt binder content. When such a mixture is put into revealed that the amount of crushed gravel in HMA mixtures service, traffic breaks or turns the slivered particles flat and affected rutting performance. Tests on two mixtures made the original binder content becomes excessive. Rough aggre- with gravel aggregate showed a mixture with 40-percent gate such as slag resists both laboratory compaction and in- crushed gravel was more sensitive to rutting than a mixture service traffic densification and thus maintains a higher with 70-percent crushed gravel (14). Also, increasing the per- VMA. This high VMA accommodates high design binder centage of crushed particles in HMA mixtures increased the content. Marshall stability (14). Brown and Cross (18) conducted field studies of rutted The particle shapes of coarse aggregates used in HMA HMA pavements in several states. They concluded that the mixtures have been found to affect both performance and initial air voids of HMA mixtures have a strong correlation workability of the mixtures. A study conducted using with rutting. They also concluded that aggregate proper- the Superpave Gyratory Compactor (SGC) showed that ties have little effect on the rutting rate for HMA mixtures increasing the amount of flat and elongated particles with in-place voids below 2.5 percent. Sousa and Weisman increased the required compaction energy (15), indicating (19) reported that, for HMA mixtures with air voids con- that HMA mixtures with high percentages of flat and elon- tent below 2 to 3 percent, the binder acts as a lubricant gated particles are less workable. In addition, increased between the aggregates and reduces point-to-point contact amounts of flat and elongated particles in HMA mixtures pressure.