Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt (2005) / Chapter Skim
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Pages 14-59
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From page 14... ...
Four consensus aggregate properties were specified: coarse aggregate angularity, flat and elongated particles, uncompacted voids content in fine aggregate, and sand equivalent. Specification levels for the consensus property tests depend on design traffic level and depth in the pavement structure (for tests related to permanent deformation)
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From page 15... ...
Rut depths were measured and cores were recovered within and between the wheel paths. Cores were tested for density, voids filled, asphalt content, coarse aggregate fractured face count, and uncompacted void content in fine aggregate.
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From page 16... ...
The percent crushed face count (PCP Composite) as measured by CRD-C 171 for the composite coarse and fine aggregate as well as the uncompacted voids in coarse aggregate are also good predictors.
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From page 17... ...
For comparing gradations, the authors recommended testing the individual size fraction and then calculating the result for a target gradation. Flat and elongated particles tend to increase the measured uncompacted voids content and aggregate particle index.
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From page 18... ...
and rutting performance. Trends indicate that higher particle index values or uncompacted voids contents produce more rut-resistant pavements.
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This indicates that the tests are highly influenced by particle shape. The uncompacted voids test is also sensitive to the texture of coarse aggregate particles.
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From page 20... ...
The same manufactured sand, natural sand, and mineral filler were used for both designs. The dolomite mixture used 55% coarse aggregate with the fine aggregate being an 80% to 20% blend of manufactured and natural sand.
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From page 21... ...
The rut depths for the granite mixtures produced in the vertical shaft impact crusher were significantly less than the rut depth for the asreceived mixture; however, the granite mixes produced using the aggregate from the vertical impact crusher have failing VMA values and therefore lower asphalt contents. Constant strain fatigue tests were performed according to AASHTO T321 at two strain levels.
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From page 22... ...
. The difference between two individual test results with a 95% confidence interval is determined by multiplying the standard deviation by 2√–2; therefore, results of two properly Tests Flat or Elongated Particles 2:1 and Uncompacted Voids Method A Micro-Deval and Magnesium Sulfate Soundness Flat or Elongated Particles 5:1 and LA Abrasion Flat and Elongated Particles 5:1 and LA Abrasion Micro-Deval and Bulk Specific Gravity LA Abrasion and Bulk Specific Gravity Micro-Deval and Water Absorption Magnesium Sulfate Soundness and Water Absorption 0.786, 0.0641 0.863, 0.027 0.832, 0.040 0.844, 0.035 –0.877, 0.022 –0.812, 0.049 0.961, 0.002 0.894, 0.016 1Not significant at the 5% level, significant at the 10% level.
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From page 23... ...
, the results indicate the tremendous variability of the test method. 2.3.4 Summary of Research Related to F&E A limited number of studies have been conducted to relate the percentage of F&E to performance since the implementation of the Superpave method.
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From page 24... ...
2.4.2 Uncompacted Voids Content in Fine Aggregate The Superpave method specifies AASHTO T304 (ASTM C1252) , "Uncompacted Void Content in Fine Aggregate, Method A," to ensure that the blend of fine aggregates in an HMA mixture has sufficient internal friction to provide rut24 resistance in an HMA mixture (47)
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From page 25... ...
. The blended uncompacted voids contents were on average 2.4% lower when the material retained on the 2.36-mm sieve was included (49)
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From page 26... ...
A rutting model with an R2 = 0.77 was developed between rate of rutting and aggregate properties with data from pavements with in-place air voids in excess of 2.5%. The aggregate properties considered included coarse aggregate crushed faces, uncompacted voids in fine aggregate, gradation parameters, and both nominal and maximum aggregate size divided by lift thickness (17)
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From page 27... ...
Five methods were used to characterize the sands: NAA uncompacted voids Method A, DST, ASTM D3398, Michigan Test Method 118-90, and a flow rate method. Michigan test method 118-90 is similar to the NAA uncompacted voids test in that the volume of voids in a loosely compacted sample is used to determine the air voids–to–solids ratio and, in turn, an angularity index.
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From page 28... ...
. The uncompacted voids contents of the fine aggregate portion of the 11 blends as measured by ASTM C1252 Method A ranged from 38.4% to 47.1%.
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From page 29... ...
SST testing for frequency sweep at constant 29 height and repeated shear at constant height were performed according to AASHTO TP7-94. Correlation analysis between the three fine aggregate tests and rutting performance based on both repeated shear at constant height and the PurWheel rut depths indicated that the uncompacted voids content was most correlated with rutting performance (64)
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From page 30... ...
sections, the limestone sand mixtures performed best in both 30 the PurWheel and the APT facility. However, it should be noted that the air void contents of the natural sand and granite fine aggregate sections were close to the 2.5% level identified by Cross and Brown (10, 17)
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From page 31... ...
The study was conducted for the International Center for Aggregate Research. The study evaluated 23 fine aggregates using seven different procedures: uncompacted voids content (AASHTO T304)
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From page 32... ...
This may be due to the packing characteristics of the fine aggregates with higher uncompacted voids contents. The authors conclude that "although FAA had some influence on the shear strength, aggregate toughness and gradation appeared to overwhelm its effects, confirming that FAA alone was not a good predictor of fine aggregate shear strength" (51)
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From page 33... ...
The trend between uncompacted voids and APA rut depths indicated decreased rutting with increasing uncompacted voids. Two fine aggregates with uncompacted voids less than 45 and high toughness (LA abrasion < 35%)
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From page 34... ...
There is clear evidence that good-performing mixes can be designed with uncompacted voids contents between 43% and 45%, but evaluation of these mixes using a rutting performance test is recommended. • Higher uncompacted void contents generally resulted in higher VMA and lower densities at Ninitial.
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From page 35... ...
The relatively poor precision statements for the uncompacted voids in fine aggregate (AASHTO T304)
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The Aggregate Imaging System (AIMS) contains both a fine aggregate and a coarse aggregate module (83)
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From page 37... ...
Fine aggregate measurements and coarse aggregate measurements of the longest and intermediate axis and information for coarse aggregate angularity are accomplished using backlighting of the aggregate tray. All other measurements are accomplished with top-lighting.
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From page 38... ...
Based on their review of potential technologies, digital image analysis, and laser scanning were recommended for further research. A continuation of the study evaluated five automated gradation devices: LCPC VDG-40 Videograder, W.S.
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From page 39... ...
Ten fine aggregates representing a wide range of mineralogical compositions and plasticity characteristics were used. Their plasticity characteristics were evaluated by three test methods: sand equivalent test, plasticity index, and methylene blue value.
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From page 40... ...
Results from ongoing project NCHRP Project 4-19(2) , "Validation of Performance-Related Tests of Aggregates for Use in Hot-Mix Asphalt Pavements," showed a very significant relationship between the rutting performance of the wet pavements and the MBV (23)
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From page 41... ...
It appears that the MB test may be the best method to quantify the amount of harmful clays in fine aggregate. The NAT was developed during SHRP to evaluate the interaction between the asphalt binder and aggregate in the presence of water; however, validation work conducted as part of SHRP indicated a poor predictive ability for the test, and it has not been widely used since.
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From page 42... ...
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)
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From page 43... ...
when assessing dry aggregate breakdown in the SGC. 2.7.1.2 Other Tests Related to Aggregate Breakdown In Europe, a number of alternative tests are used to assess aggregate breakdown: the Aggregate Impact Value Test (BS SGC: y = 0.3732x - 3.3991 R2 = 0.8609 Marshall: y = 0.3192x + 1.6599 R2 = 0.6324 -2.0 2.0 6.0 10.0 14.0 18.0 22.0 26.0 10 20 30 40 50 60 LA Abrasion value, % 4.
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From page 44... ...
There were 246 common aggregates tested with the aggregate impact value and aggregate crushing value tests, 75 to 91 common aggregates tested for LA abrasion, and 13 common aggregates tested with the German Schlagversuch Impact test. All of the relationships were statistically significant except the relationship between the LA abrasion test and the German Schlagversuch impact test.
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From page 45... ...
. The alternative tests included the unconfined freezethaw test for coarse aggregate, micro-deval abrasion test, aggregate impact value test, polished stone value test and aggregate abrasion value test.
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From page 46... ...
. In the aggregate abrasion value test (BS 812)
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From page 47... ...
A fair correlation (R2 = 0.63) was indicated between micro-deval loss and LA abrasion loss.
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From page 48... ...
2.7.2 Aggregate Tests Related to Weathering and Freeze-Thaw Durability There is some question as to whether aggregates used in HMA need to be resistant to freezing and thawing. Soundness tests such as magnesium or sodium sulfate soundness have commonly been used to assess degradation from freezing and thawing as well as from wetting and drying.
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From page 49... ...
Initial macrotexture is a function of gradation, although tests for abrasion resistance -- such as the aggregate abrasion value and micro-deval abrasion loss -- are related to the change in macrotexture with time (119)
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From page 50... ...
In some cases, natural sands, which are predominantly −600μm sieve material, are added. Natural sands have been identified as a cause of decreased resistance to permanent deformation and of tender mix problems during construction (121)
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From page 51... ...
FAf Ratio: fine aggregate fine ratio, which describes gradation of fine portion of fine aggregate. Changes to any of these parameters will affect the air voids, VMA, constructability, and performance of the resulting asphalt mixture.
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From page 52... ...
summarized the influences mineral filler can have on the performance of HMA mixtures as follows: • Depending on the particle size, fines can act as a filler or an extender of asphalt cement binder. In the later case, an over-rich HMA mix can lead to flushing and rutting.
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From page 53... ...
Two mineral fillers, quartz and calcite, were added to four asphalt cements, and the rheological properties and failure properties of the resulting mastics were determined using the test methods developed by SHRP. DSR, flexural creep, and direct tension were found to be applicable to voidless filler–asphalt cement mastics.
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From page 54... ...
2. A poor correlation existed between the percent rut depth measured by the LCPC pavement rut tester and the free binder content.
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From page 55... ...
2.9.2 Summary of Research Related to Fines and Fillers It is widely believed that depending on the particle size, fines can act as a filler or as an extender of asphalt cement binder. Some fines have a considerable effect on the asphalt cement, making it act as a much stiffer grade of asphalt cement compared with the neat asphalt cement.
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From page 56... ...
within the fine aggregate fraction. 2.10.1 Types of Crushers There are four major types of crushers used to produce aggregate for HMA: jaw, gyratory, cone, and impact.
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From page 57... ...
and are often used for crushing fines. An example of an autogenous vertical shaft impact crusher is shown in Figure 21.
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From page 58... ...
One potential drawback of using impact-type crushers to produce coarse aggregate is the resulting effect on the fine aggregate. Iowa DOT is conducting research to compare the aggregate properties and properties of the resulting aggregate produced with a cone or hammermill (a form of horizontal shaft impact crusher)
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From page 59... ...
Cubical coarse aggregates are believed to be desirable for the production of HMA. However, the production of cubical coarse aggregates may result in • Aggregate base that packs more tightly, reducing drainage capacity; • More cubical fine aggregate, resulting in lower uncompacted voids; • Reduced LA abrasion loss; • More cubical aggregates may pack closer in HMA, resulting in lower VMA; • More cubical fine aggregate, which packs closer, resulting in higher density block; and • More cubical fine aggregate that reduces water demand for Portland cement concrete, resulting in higher strength.
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