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42 Project 9-37, "Using Surface Energy Measurements to Select Some western U.S. states and European countries use basalts Materials for Asphalt Pavements," can be used to apply the containing high-plasticity expansive clay minerals. energy surface theory in the future. 2.7.1 Aggregate Tests Related to Abrasion 2.7 TESTS RELATED TO AGGREGATE Resistance and Breakdown DURABILITY 2.7.1.1 LA Abrasion Aggregate durability generally encompasses two categories of tests: tests that measure aggregate abrasion resistance and Aggregates must be resistant to crushing and abrasive wear breakdown during handling, mixing, laydown, and under traf- to withstand handling during stockpiling, shipping, mixing at fic and tests that address aggregate weathering when aggre- the HMA plant, laydown, and compaction. Once the HMA is gate is exposed to freezing and thawing or wetting and dry- in place, the aggregates need to be sufficiently hard or tough ing. These tests are employed in concert to ascertain that the to transfer load through contact points. This is especially true aggregate used in the production of HMA will be durable. of aggregates used in gap-graded mixtures such as SMA. Specifically, tests related to durability are selected to address Aggregates must also withstand surface abrasion and polish- the following: ing from traffic. The SHRP aggregate expert task group identified the Los Aggregate breakdown during handling, mixing, and Angeles Abrasion Test, AASHTO T96 (ASTM C131), as the placement. Such breakdown can alter the HMA grada- fourth most important aggregate property in both the first- tion, resulting in a mixture that does not meet volumetric and second-round questionnaires used in the Delphi process properties. This breakdown can generally be accounted (1). The LA abrasion test was included as a source property for in the design process. in the Superpave mix design system. The specification val- Abrasion or weathering of the aggregates in the ues for source properties were to be set by the agency to pavement structure. Gross aggregate wear or weath- allow for variations in locally available aggregates. Based on ering can occur in the form of raveling, popouts, or pot- the survey conducted as part of this study, 96% of the 48 U.S. holes. An example of extensive surface loss caused by states and Canadian provinces that responded use the LA popouts in a 2-year-old Superpave-designed pavement abrasion test. is shown in Figure 10. The LA abrasion test was originally developed by the Freeze-thaw durability, which is more closely associ- Municipal Laboratory of the city of Los Angeles in the 1920s. ated with the performance of aggregate base, Portland The LA abrasion test procedure requires that an aggregate cement concrete, and surface treatments. This may be sample be placed inside a rotating steel drum containing a due to the fact that aggregate particles in HMA should specified number of steel balls or charge. As the drum rotates, be coated with asphalt. Popouts of surface aggregates a shelf inside the drum picks up the aggregate and steel may be related to freeze-thaw durability. spheres. The shelf lifts the aggregate and steel balls around Other forms of abrasion on the pavement surface, until they drop approximately 27 in. on the opposite side of such as polishing or loss of microtexture of coarse aggre- the drum, subjecting the aggregate to impact and crushing. gate particles. Polishing is beyond the scope of this The aggregate is subjected to abrasion and grinding as the report and will only be treated briefly. drum continues to rotate until the shelf picks up the contents, and the process is repeated. The drum is rotated for a spec- ified number of revolutions, typically 500. Afterward, the aggregate is removed from the drum and sieved over a No. 12 (1.7-mm) sieve to determine the degradation as a percent loss. Kandhal and Parker (2) conducted a literature review on the early LA abrasion research as part of NCHRP Project 4-19. Their review indicated only a fair correlation with field performance for coarse aggregates; however, they did note that early developmental studies, most notably by Woolf (110) and Melville (111), indicated good correlations with perfor- mance. Testing conducted as part of NCHRP Project 4-19 indicated that LA abrasion loss was not related to historical pavement performance ratings (112). Limited recent research has been conducted on the LA abrasion test by Amirkhanian et al. (113). A survey con- ducted as part of the study indicated that the majority of state Figure 10. Popouts and raveling in 2-year-old pavement. DOTs specified the LA abrasion test, similar to the current

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43 study. The survey also determined that most agencies believed Xie and Watson conducted a study to evaluate the break- that the LA abrasion results were most related to breakdown down of SMA mixtures during laboratory compaction (114). during compaction and that the majority were satisfied with Five aggregate sources with a range of LA abrasion loss from their current specifications. The study investigated the break- 17% to 36% were selected for the study: crushed gravel, gran- down of four granite aggregates with LA abrasion values ite (two sources), limestone, and traprock. Aggregates sizes ranging from 28 to 55. The study evaluated indirect tensile were combined to produce 9.5-mm, 12.5-mm, and 19.0-mm strength, resilient modulus, and aggregate breakdown for sam- NMAS mixtures. Samples were compacted with either a 50- ples compacted using 25, 50, 75, or 100 Marshall blows. The blow Marshall or 100-gyration Superpave Gyratory Com- indirect tensile strength and resilient modulus tests were per- pactor (SGC) effort. Ignition furnace extractions were used formed on both conditioned and unconditioned samples. The to compare the batched, loose-mix, Marshall-compacted, and test results indicated that the indirect tensile strengths for the SGC-compacted gradations. Comparisons were made based three mixtures produced with the granite having an LA abra- on the critical or breakpoint sieve for the SMA mixtures. For sion loss greater than or equal to 30% were significantly lower the 12.5-mm and 19.0-mm NMAS mixtures, the 4.75-mm for both the conditioned and unconditioned samples than the (No. 4) sieve was used; for the 9.5-mm NMAS mixtures the strengths produced with the aggregate having an LA abrasion 2.36-mm (No. 8) sieve was used. As shown in Figure 11, the loss of 28%. Further, the tensile strength and resilient modu- 50-blow Marshall compaction effort resulted in greater break- lus ratios were generally lower for the mixture produced with down than the SGC compaction. The data from this study an aggregate having an LA abrasion loss of 55%. Interest- correlated well with similar data from NCHRP Project 9-8 ingly, for the dense-graded mixes tested, aggregate break- (36). Unfortunately, this study was not correlated to the actual down was only significant on the 0.150 and 0.075 (No. 100 breakdown that occurred in the field. Breakdown for larger and No. 200) sieves. Unfortunately, the level of breakdown sieve size is expected with gap graded mixes such as SMA that occurs in the field was not investigated. because there are more coarse aggregate contact points dur- As previously discussed, work by Aho et al. (39) indicated ing compaction. Increased breakdown for open-graded mix- the interrelationship between F&E, LA abrasion, and expected tures was also noted in NCHRP Project 4-19 (2) when assess- breakdown in the field. The LA abrasion of the sources in this ing dry aggregate breakdown in the SGC. study represented a very narrow range (24% to 26%). This study concluded that breakdown in the gyratory compactor generally exceeded the breakdown that occurred in the field 2.7.1.2 Other Tests Related to Aggregate for dense-graded mixtures used in Illinois. In conjunction Breakdown with their FAA study, Roque et al. (51) recommended includ- ing LA abrasion limits to differentiate between good- and In Europe, a number of alternative tests are used to assess poor-performing fine aggregates with borderline FAA values. aggregate breakdown: the Aggregate Impact Value Test (BS SGC This Study SGC NCHRP 9-8 Mars hall This Study Mars hall NCHRP 9-8 Linear (SGC) Linear (Mars hall) 26.0 Mars hall: y = 0.3192x + 1.6599 22.0 R2 = 0.6324 4.75 mm Sieve Breakdown, % 18.0 14.0 10.0 6.0 SGC: y = 0.3732x - 3.3991 R2 = 0.8609 2.0 -2.0 10 20 30 40 50 60 LA Abrasion value, % Figure 11. Critical sieve breakdown versus LA abrasion by compactor type (114).

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44 812), the German Schlagversuch Impact Test (DIN 52115) and handling. The aggregate impact value test was stated to and the Aggregate Crushing Value (BS 812). The aggregate have the following advantages: it requires a small sample; it impact value test uses a graded sample between 10.0 mm and requires less expensive, portable equipment; and samples may 14.0 mm. The sample is subjected to 15 blows of a 100-mm be tested in a wet condition (115). (4-in.) diameter hammer weighing 13.5 to 14.0 kg that falls Kandhal and Parker (2) found better correlations between 380 5 mm. The aggregate impact value is reported as the LA abrasion loss and both the aggregate impact value and the percentage of the initial sample weight passing the 2.36-mm aggregate crushing value (R = 0.932 and 0.934, respectively); (No. 8) sieve after impact. A lower value indicates a stronger however, no clear trends were noted between the results of aggregate. The German Schlagversuch impact test is similar the LA abrasion loss, aggregate impact value, or aggregate to the British aggregate impact value test except that the crushing value, and subjective field performance ratings. equipment is more expensive and less portable (109). Both the USACE and Superpave gyratory compactors have The aggregate crushing value test applies a compressive been investigated as means of simulating aggregate break- load to an approximately 2-kg sample in a steel container. A down during construction and handling. Ruth and Tia (116) total load of either 400 kN is applied to a 150-mm-diameter investigated the use of USACE Gyratory Testing Machine to piston or 100 kN is applied to a 75-mm-diameter piston in a simulate the breakdown that occurs in drum plants. Samples 10-min period. The aggregate crushing value is expressed as were tested with an initial angle of gyration of 3, ram pres- the percentage of fines passing the 2.36-mm (No. 8) sieve sure of 690 kPa, and air roller pressure of 62 kPa. Samples based on the original sample mass. were tested to either 25, 50, 100, or 200 revolutions. Initial Woodward (109) notes that with the impact and crushing- results indicated that the majority of the breakdown (greater type tests, if the sample consists of a blend of good- and poor- than 50%) occurred in the first 25 revolutions. Three aggre- performing particles, the stronger particles can often carry the gate sizes were tested: No. 67, No. 89, and screenings. Good load masking the weaker particles. Conversely, with what correlations were observed between LA abrasion loss and Woodward describes as fragmentation tests like LA abrasion, the percent passing the No. 10 sieve from the Gyratory Test- weaker particles are equally exposed to fragmentation forces. ing Machine for the coarse aggregates (R2 = 0.893 to 0.985). Woodward (109) produced a correlation matrix among the Relatively good correspondence was found between the aggregate impact value, German Schlagversuch impact, breakdown of samples blended to meet actual HMA job mix aggregate crushing value, and LA abrasion tests as shown in formulas tested at 25 gyrations and the breakdown that Table 10. The data in Table 10 is based on a range of aggre- occurred when the same gradings were processed dry (no gates commonly used in Great Britain and Ireland. There were asphalt cement) through a drum plant. Two key advantages 246 common aggregates tested with the aggregate impact were observed for the Gyratory Testing Machine procedure: value and aggregate crushing value tests, 75 to 91 common the distribution of fines passing the No. 12 sieve is investi- aggregates tested for LA abrasion, and 13 common aggregates gated and samples may be tested wet. tested with the German Schlagversuch Impact test. All of the The use of the SGC to evaluate aggregate toughness was relationships were statistically significant except the relation- evaluated by Kandhal and Parker (2). AASHTO No. 8 stone ship between the LA abrasion test and the German Schlagver- and a limited number of AASHTO No. 57 stone sources were such impact test. There appears to be a reasonable fit between tested in the SGC. The actual number of gyrations used to sim- LA abrasion and the aggregate crushing value tests. Senior and ulate breakdown is not reported (2, 112). Aggregate break- Rogers (115) also compared the results from the LA abrasion down was assessed in two ways: gradation change for a single and aggregate impact value tests for use in Ontario. They sieve, such as the 4.75-mm (No. 4), or the sum of the grada- found a correlation (R = 0.797) based on 98 aggregate sources. tion changes on all sieves before and after compaction. Kand- Senior and Rogers concluded that the aggregate impact value hal and Parker concluded that the SGC can be used to differ- test could be a practical substitute for the LA abrasion test to entiate between tough and weak aggregates (2). They also assess the extent of breakdown expected during processing noted that breakdown is greater for open graded mixtures, TABLE 10 Correlation (R) matrix for aggregate strength tests (109) Test Aggregate German Aggregate LA Impact Schlagversuch Crushing Abrasion Value Impact Value Aggregate Impact Value 1.0 0.607 0.822 0.731 German Schlagversuch 1.0 1.0 0.683 0.403 Impact Aggregate Crushing 1.0 0.861 Value LA Abrasion 1.0

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45 which would tend to have more contact points. Comparisons The micro-deval test has been proposed as an alternative between the degradation in the SGC and field performance to LA abrasion in North America (2). The micro-deval test indicated no obvious breakpoints or correlations (2). was developed in France in the 1960s (115). To perform the micro-deval test on coarse aggregate, a 1500-g sample is first soaked in 2 liters of water for at least 1h. A 5-kg charge of 2.7.1.3 Aggregate Tests Related to Abrasion 9.5-mm (3/8-in.) diameter balls is placed in a jar along with Resistance the sample and the water it was soaked in. The jar is then rotated at 100 rpm for 2 h. The sample is then washed and A number of studies have evaluated alternative tests to mea- oven dried. The micro-deval abrasion loss is the percent of sure aggregate degradation and abrasion resistance. Senior and material passing the 1.18-mm (No. 16) sieve expressed as a Rogers summarize some of the concerns with the LA abra- percentage of the original sample mass. A reference material sion test that led to the investigation of alternatives: "The Los is available for periodic calibration of the loss. Angeles test is not always appropriate because the steel balls Senior and Rogers investigated alternative tests for assess- impart a severe impact loading on the test sample, overshad- ing coarse aggregate toughness and durability in Ontario owing interparticle abrasion, which is the predominant process (115). The alternative tests included the unconfined freeze- in pavement subject to traffic stress" (115). Senior and Rogers thaw test for coarse aggregate, micro-deval abrasion test, note that some coarse grained granites and gneisses tend to aggregate impact value test, polished stone value test and be brittle resulting in high LA abrasion losses but adequate aggregate abrasion value test. Results were compared with LA field performance. By contrast, some softer aggregates such abrasion loss, magnesium sulfate soundness loss, and water as carbonates and shales will absorb the impact of the steel absorption. The micro-deval test produced similar results to balls, resulting in acceptable performance. These types of the sulfate soundness test (R = 0.85 for 106 samples) with "soft" aggregates tend to be susceptible to slaking and to par- greater precision (115). Parker and Kandhal (2) also reported ticle degradation when wet. Woodward (109) emphasizes the a reasonable correlation between magnesium sulfate sound- concerns that road surfaces are frequently wet and that there ness loss and micro-deval loss (R = 0.848, p = 0.0001). The can be a significant reduction in the wet strength of some improved precision is indicated by comparing the single aggregates. Samples cannot be tested wet in the LA abrasion operator standard deviations as a function of test value shown machine because it would be difficult to remove the fines that in Figure 12. Senior and Rogers recommended the micro- would cake along the shelf and drum. deval test, polished stone value, and unconfined freezing and Figure 12. Standard deviation versus magnesium sulfate soundness or micro-deval abrasion (115).

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46 thawing in concert to ascertain the performance of aggre- not report the correlation between aggregate abrasion value gates for HMA (115). Recommendations for modifications and micro-deval. However, based on the figure in Senior and to the micro-deval test were developed for testing fine aggre- Rogers (115), the relationship appears reasonable. Senior gates (117). The goal of these modifications was to replace and Rogers (115) recommend the micro-deval test over the the sulfate soundness test. Modifications included a smaller aggregate abrasion value because the micro-deval test is less sample (500 g), small charge (1250 g), less water (750 ml), expensive and less time consuming. and a shorter test time (15 min). Loss was reported as the per- NCHRP Project 4-19 recommended both the micro-deval cent passing the 0.075-mm (No. 200) sieve. Based on the test and magnesium sulfate soundness as the two tests most results of this study, micro-deval losses of less than 20 and related to HMA performance in terms of popouts, raveling, 25 were established for high-quality and lower-quality HMA and potholing (2). This recommendation was based on single surfaces, respectively (109). variable correlations between both test results and subjective Woodward compared the abrasion results from the micro- field performance rankings. Figures 13 and 14 indicate the deval test and aggregate abrasion value test (109). In the relationships, respectively, between both micro-deval loss aggregate abrasion value test (BS 812), aggregate particles and magnesium sulfate soundness and field performance. An are held in a mold and the exposed aggregate is placed on a 18% maximum loss was recommended for both test methods. flat, rotating steel plate. A standard weight is placed on the The correlation between micro-deval loss and magnesium mold, and silica sand is metered onto the steel plate. The test sulfate soundness was not addressed in NCHRP Project 4-19. is generally performed dry. The aggregate abrasion value is Based on the recommendations from NCHRP Project 4-19, based on the loss determined from the sample mass before Cooley et al. (118) evaluated the micro-deval loss, LA abra- and after abrasion normalized for the density of the aggre- sion loss, and sodium sulfate soundness of 72 aggregate gates. Based on testing of 133 samples, Woodward indicated sources from the southeastern United States. No statistically a significant correlation (R = 0.799) between the aggregate significant results were found between either LA abrasion or abrasion value and micro-deval loss. Senior and Rogers did sodium sulfate soundness and micro-deval loss. Of the 72 Figure 13. Pavement performance and micro-deval abrasion loss (2).

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47 Figure 14. Pavement performance and magnesium soundness loss (2). sources studied, six were characterized as poor performers. micro-deval test represents inter-particle abrasion within the Two of these six aggregates produced micro-deval losses less HMA (109). Sulfate soundness tests are meant to represent than 6%. Also, 33% of the aggregate sources characterized the degradation that may occur because of freezing and thaw- as fair exceeded the 18% micro-deval loss criterion recom- ing; however, several studies representing a large range of mended by NCHRP Project 4-19 (118). The authors recom- aggregate types have indicated that the tests are strongly cor- mend consideration of specifications for micro-deval loss related. Therefore, as proposed by Senior and Rogers (115), based on parent aggregate type. Woodward (109) also rec- it appears advisable that only one such test be used to screen ommended specifications based on rock type. Ontario has aggregates. Rismantojo (23) also indicated correlations implemented a specification for micro-deval loss based on between the micro-deval loss and both LA abrasion loss and aggregate type (119). For high-volume roads, the maximum water absorption. The relationship with LA abrasion broke micro-deval loss for igneous gravel is 5%; for dolomitic down when the NCHRP Project 4-19 and 4-19(2) data sets sandstone, 15%; for traprock, Diabase, and andesite, 10%; and were combined. The strength of some of the aggregates for meta-arkose, meta-gabbro, and gneiss, 15%. tested as part of NCHRP Project 4-19 appear to be greatly Rismantojo (23) tested six aggregate sources for micro- affected by water. deval loss, LA abrasion loss, and magnesium sulfate sound- Eighteen aggregate sources in Oklahoma were evaluated ness as part of NCHRP Project 4-19(2). Figure 15 shows the by Tarefder et al. (120) to develop a baseline specification for relationship between the micro-deval value and magnesium micro-deval loss. The aggregates tested were predomi- sulfate soundness loss for 22 aggregates representing the nantly limestone and sandstone. LA abrasion loss, freeze- combined results from NCHRP Projects 4-19 and 4-19(2). thaw soundness, water absorption, and aggregate durability Figure 15 indicates a good correlation (R2 = 0.76) between index were also performed. A fair correlation (R2 = 0.63) was the two tests. This matches a similar finding by Senior and indicated between micro-deval loss and LA abrasion loss. Rogers (115). In theory, the two tests should indicate differ- The authors noted that different micro-deval abrasion loss ing modes of deterioration. Woodward suggests that the values may be required, depending on the parent aggregate