Graham C Hurley

Field Performance of Warm-Mix Asphalt at National Center for Asphalt Technology Test Track

Warm-mix asphalt (WMA) mixes produced by an emulsion process were evaluated under accelerated loading in three total sections of the National Center for Asphalt Technology Test Track and used as the surface mix for two of the sections. Evotherm was incorporated into the same mixes used previously on the track. In-place densities of the WMA surface layers were equal to or better than the hot-mix asphalt (HMA) surface layers, even when compaction temperatures were reduced by 8°C to 42°C (15°F to 75°F). Laboratory rutting-susceptibility tests conducted in the asphalt pavement analyzer indicated similar performance for the WMA and HMA surface mixes with the PG 67-22 base asphalt. However, laboratory tests indicated an increased potential for moisture damage with the WMA mixes. The two WMA sections and the HMA section showed excellent rutting performance in the field after the application of 515,333 equivalent single-axle loads in a 43-day period. One of the WMA sections was also evaluated for quick turnover to traffic and showed good performance.

EFFECT OF SUPERPAVE DEFINED RESTRICTED ZONE ON HOT MIX ASPHALT PERFORMANCE

The effect of the Superpaver defined restricted zone on hot-mix asphalt rutting performance was evaluated. One gradation that violated the restricted zone (TRZ) and two gradations that did not violate the restricted zone (BRZ and ARZ) were evaluated. Evaluated mixes represented a range of maximum aggregate sizes (MASs), design traffic levels, and aggregate types. Three laboratory tests—asphalt pavement analyzer, rotary-loaded wheel tester, and Marshall test—were used to evaluate the rutting performance. From the analysis, it was found that mixes with gradations violating the restricted zone performed similarly to or better than the mixes with gradations passing outside the restricted zone with respect to laboratory rutting tests. This conclusion was drawn from the results of experiments with 12.5-, 19.0-, and 25.0-mm MAS gradations at Ndesign (design number of gyrations) values of 100, 75, and 50 gyrations. This conclusion is confirmed and supported by the recently completed NCHRP Project 9-14: The Restricted Zone in the Superpave Aggregate Gradation Specification. The results also showed that rutting performance of mixes having gradations below the restricted zone, which was commonly recognized to be rut resistant, appeared to be more sensitive to aggregate properties than mixes having gradations above or through the restricted zone.

EVALUATION OF INFRARED IGNITION FURNACE FOR DETERMINATION OF ASPHALT CONTENT

The Troxler Model 4730 infrared ignition furnace was compared with a standard Thermolyne ignition furnace. Comparisons conducted with a single unit of each furnace type were based on the correction factor for aggregate loss during ignition, accuracy, and the variability of the measured asphalt content and aggregate degradation during ignition. Forty-eight samples representing two nominal maximum aggregate sizes (9.5 and 19.0 mm), four aggregate types (granite, crushed gravel, limestone, and dolomite), and two asphalt contents (optimum and optimum plus 0.5% asphalt content) were tested in each furnace. The results indicated that the correction factors for aggregate loss during ignition were significantly different for each type of furnace, thus requiring a separate calibration for each type of furnace. In practical terms, the differences for all but the 9.5-mm nominal maximum aggregate size (NMAS) limestone and both dolomite mixtures were less than 0.1%. The samples with the optimum plus 0.5% asphalt content were tested by using the calibration factors developed for a particular mix–furnace combination. The results were analyzed in terms of accuracy (bias) and variability (standard deviation). Neither the measured biases nor the standard deviations for the two types of furnaces were significantly different. The results obtained with four sieve sizes (NMAS and 4.75, 2.36, and 0.075 mm) were evaluated for aggregate breakdown. A comparison of the aggregate gradations recovered from both furnaces indicated no significant difference in the degree of aggregate degradation. A round-robin investigation should be conducted to confirm that the precision of the infrared furnace is similar to the precision of the standard furnace.

Refinement of the Hot-Mix Asphalt Ignition Method for High-Loss Aggregates

Four methodologies for determining the asphalt content of mixtures containing high-loss aggregates in the ignition furnace were evaluated: the standard method using the Thermolyne furnace (control), the Troxler NTO infrared furnace, the Ontario method, and a Tempyrox glasscleaning oven. Six aggregate sources with high ignition furnace aggregate corrections were obtained from around the country: four dolomites, a basalt, and a serpentine/chlorite. Calibration factors were determined for each method at optimum asphalt content. Additional samples were then tested at optimum plus 0.5% asphalt content, and the measured asphalt content was calculated by using the correction factor determined for that method and aggregate source. The Tempyrox Pyro-Clean furnace, commonly used for cleaning laboratory glassware, produced the lowest aggregate correction factors. The standard method and the Ontario method, both using the Thermolyne ignition furnace, produced the smallest bias or error in measured asphalt content. Th...

Evaluating Nonnuclear Measurement Devices to Determine In-Place Pavement Density

The air void content of both in-place and laboratory-compacted hot-mix asphalt may be the factor that most affects the performance of a properly designed mixture. A mediocre mix, well constructed with good in-place air voids, often will perform better than a good mix that has been poorly constructed. In-place density may be monitored by using three methods: cores, nuclear density gauge measurements, and nonnuclear density gauge measurements. A study was done to evaluate and compare two non-nuclear density devices, the pavement quality indicator (PQI) Model 301 and the PaveTracker, to in-place core densities. Testing and analyses of 20 projects showed that uncorrected gauge measurements (from each gauge) provided reasonable correlation (significant at a 5% level of significance) with core density measurements approximately 75% of the time. Average coefficients of determination for the relationships between uncorrected gauge measurements and core densities were approximately .5. Simulation analyses indicated that the error in measuring in-place air voids using calibrated PQI Model 301 gauge measurements based on the constant offset method can be 1% to 2.7% air voids for the average of 10 measurements. The numerical experiments also indicated that calibration of PQI Model 301 by simple offset or by slope and offset did not improve the correlation between the gauge and core densities.