Brian D. Prowell

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.

Evaluation of Circular Texture Meter for Measuring Surface Texture of Pavements

The circular texture (CT) meter is a laser-based device for measuring the mean profile depth (MPD) of pavement at a static location. MPD measurements from the CT meter and mean texture depth measurements from the sand patch test were obtained in five random locations in each of 45 sections of the 2000 National Center for Asphalt Technology (NCAT) test track. The NCAT test track provides a wide range of surface types, including coarse and fine dense graded Superpave® mixes, Hveem mixes, stone mastic asphalt, and Novachip. Testing indicated that the CT meter produced results comparable with the ASTM E965 sand patch test. When open-graded mixtures were excluded, this study indicated that the offset was nonsignificant between CT meter and sand patch test results. Previously developed equations to predict macrotexture were found to be inadequate for the wide range of mix types and aggregate types found at the NCAT test track. An equation was developed to relate fineness modulus to macrotexture. This equation was validated with independent data collected by the Virginia Transportation Research Council. Testing conducted as part of a mini round robin indicated that two readings should be averaged to represent a single CT meter measurement. The within-lab coefficient of variation for the CT meter is estimated to be 2.3%. The between-lab coefficient of variation for the CT meter is estimated to be 4.2%. Both estimates are based on the average of two tests being reported as a single measurement.

Comparison of Laboratory Wheel-Tracking Test Results with Wes Track Performance

In 1990, members of FHWA participated with other industry and highway agencies in a European asphalt study tour to examine pavement technology. One of the technologies brought back from this tour was laboratory wheel-tracking devices. However, the ability of these devices to predict or rank field performance has been questioned. WesTrack provides a unique opportunity for comparing accelerated laboratory wheeltracking devices with full-scale pavement performance under controlled conditions. The three devices examined here are the asphalt pavement analyzer (APA), the French pavement rutting tester (FPRT), and the Hamburg wheel-tracking device (HWTD). WesTrack is a 2.9-km oval test track with each tangent consisting of 13- to 70-m sections. Samples for laboratory evaluation using three wheel-tracking devices were taken from the 5-m sampling segments of the rehabilitated sections: 10 sections placed in the summer of 1997. Based on the data and analyses in this study, the three laboratory devices examined satisfactorily correlate with the permanent deformation of the WesTrack sections studied. The correlations are 89.9, 83.4, and 90.4 percent for the APA, FPRT, and HWTD, respectively. The devices, although not perfect in predicting or ranking section performance, did exhibit increased variability with poorer performance. This increased variability with mean permanent deformation or creep rate allows for a rational method to establish mixture design specifications for all three devices. The procedure exhibited here is flexible, allowing an agency to select the level of performance desired. The selection of an appropriate test temperature that reflects the in-service temperature at which the pavement will be expected to perform was found to be necessary as well.

Evaluation of Measurement Techniques for Asphalt Pavement Density and Permeability

A pavement quality indicator device developed through an NCHRP project, the Corelok device, and laboratory and field permeability index devices were evaluated for their potential to improve the Virginia Department of Transportation’s density specification or to be designated for use in a replacement specification. Fifteen samples were taken from six projects representing three nominal maximum aggregate sizes. Densities measured by a nuclear gauge correlated well with core densities measured in accordance with AASHTO T166 and by the Corelok device. Pavement quality indicator readings appeared to be repeatable but did not correlate with the densities measured in accordance with AASHTO T166 or with the Corelok device. A fair correlation was obtained for two projects. Laboratory permeability and field permeability indices were correlated with pavement density. A good correlation was found between the laboratory permeability and field permeability indices. The relationship appears to be linear in the range agencies typically specify.

Corrosion inhibiting repair and rehabilitation treatment process for reinforced concrete structures

A repair and rehabilitation treatment process for reinforced concrete structures involves the removal of concrete from above rebar or other metal reinforcement material in the concrete structure. After removal of concrete, the metal reinforcement materials are saturated with corrosion inhibiting agents. Saturation is best achieved by multiple spray applications of the corrosion inhibitor. The cavity in the concrete structure with the treated rebar or other metal reinforcement materials is then backfilled and/or overlaid with repair concrete. Preferably, the repair concrete includes corrosion inhibitors which will diffuse to the rebar over time or is a low permeability concrete that reduces the rate of diffusion of chloride corrosion causing agents to the rebar. The repair and rehabilitation process significantly increases the concrete structures service life.

Virginia's Experience with 9.5-mm Nominal-Maximum-Aggregate-Size Stone Matrix Asphalt

Construction data from two Virginia stone matrix asphalt (SMA) field projects that used 9.5-mm nominal-maximum-aggregate-size (NMAS; Superpave® definition) gradations are presented. Analyses were conducted to evaluate breakpoint sieve size, rut resistance, design compaction effort, and permeability. Rut resistance and permeability results for 9.5-mm NMAS mixes were compared with those for more conventional Virginia 19.0- and 25.0-mm NMAS SMA mixes produced by the same contractor and with the same aggregate source. A sieve size of 2.36 mm was confirmed as being the appropriate breakpoint for 9.5-mm NMAS SMA. For this gradation, 9.5-mm NMAS SMA with both performance-graded binder (PG) 70-22 and PG 76-22 was as rut resistant as previous Virginia SMA mixtures, as tested with the Asphalt Pavement Analyzer. The data for these sections indicate that 9.5-mm NMAS SMA should be designed with 75 gyrations instead of the currently recommended 100 gyrations. SMA with a 9.5-mm NMAS was also shown to be less permeable at the same air void level as the more commonly used 19.0- and 25.0-mm NMAS SMA.

Evaluation of New Test Procedures for Determining Bulk Specific Gravity of Fine Aggregate by Automated Methods

Two automated methods were evaluated for determining the dry bulk specific gravity (Gsb) of fine aggregates: Thermolyne SSDetect and InstroTek CoreLok. Each proposed method was evaluated against the standard method described in AASHTO T-84. The evaluation was based on a round-robin study with 12 laboratories and six materials, four crushed fine and two uncrushed (natural) fine aggregate sources. The CoreLok and SSDetect methods of determining fine aggregate specific gravity offer significant timesavings over AASHTO T-84. Both the CoreLok and SSDetect methods generally produce Gsb results similar to AASHTO T-84. It is believed that AASHTO T-84 may not produce accurate results for angular materials with high dust contents. More frequent statistical differences exist between the CoreLok and SSDetect apparent specific gravity (Gsa) and water absorption results than those produced by AASHTO T-84. However, Gsa and water absorption are not used in volumetric calculations for hot-mix asphalt. SSDetect offers improved precision over AASHTO T-84. The precision of the CoreLok method is slightly worse than that of AASHTO T-84, but it is expected to improve as technicians become more familiar with the procedure.

DESIGN, CONSTRUCTION, AND EARLY PERFORMANCE OF VIRGINIA'S HOT-MIX ASPHALT STABILIZER AND MODIFIER TEST SECTIONS

Three modifiers and two types of fibers (stabilizers) for hot-mix asphalt commonly available in Virginia were evaluated. Ten test sections were built on I-66, west of Manassas, Virginia. The modified asphalts complied with the specifications for performance grade 76-22 and were placed in dense-graded Marshall and coarse-graded Superpave mixes. The fibers were placed only in the dense-graded Marshall mixes. During design the dense-graded Marshall mix complied with all of the Superpave requirements except for the gradation, which passed through the restricted zone. Samples from the sections were tested for asphalt content, gradation, and Superpave and Marshall volumetrics and were rut studied using both the Georgia loaded-wheel tester and the asphalt pavement analyzer. Field samples generally passed Marshall volumetrics and generally failed Superpave volumetrics. Neither laboratory compaction method correlated with the ultimate field densities. Both tests indicated that the sections were rut resistant. All of the sections have been rut resistant and have performed well in the field after 45 months of service.

PRECISIONS OF FLAT AND ELONGATED PARTICLE TESTS: ASTM D4791 AND VDG-40 VIDEOGRADER

The design procedures for Superpave and stone-matrix asphalt mixtures emphasize the importance of aggregate shape. The current test procedure for characterization of aggregate shape, ASTM D4791, lacks a precision statement and is extremely time-consuming. A round-robin analysis was conducted with 15 laboratories, five materials, and two replicates to determine a precision statement for ASTM D4791. The results from the round-robin analysis, and for two additional materials obtained by the current procedure, were compared with results from the VDG-40 videograder, a promising automated device for measurement of aggregate shape. Although a correlation was indicated, further research is needed to relate the measurements of aggregate shape obtained by both tests to hot-mix asphalt performance.

EFFECT OF LIME ON IGNITION FURNACE CALIBRATION

This study investigated mix components, other than aggregate source, that were perceived to affect the ignition furnace mixture calibration. Four sets of experiments with one aggregate were designed and run to evaluate the effects of the amounts of lime, sulfur, calcium carboxylates, and fines. Five asphalt binders with different chemistries were evaluated. Two binders had high and low sulfur contents, respectively; two other binders contained relatively high concentrations of carboxylates. Various concentrations of hydrated lime (0 to 3 percent by weight) were added to mixtures and were found to have a significant effect on the ignition furnace correction factor for all five binders. The magnitude of the effect was large enough to cause the quality control tests to fail the tolerances established by the Virginia Department of Transportation. The lime appears to react with the sulfur dioxide (SO2) formed from the combustion of organic sulfur to generate calcium sulfate. The amount of sulfur present in the asphalt can significantly affect the ignition furnace correction factor. Initial thoughts that calcium carboxylates are reacting with carbon dioxide (CO2) to produce some form of calcium carbonate were not substantiated. The influence of carboxylic acid groups, however, does come into effect in the presence of lime. This suggests that lime reacts with the combustion gases to produce some form of carbonate. Basalt fines and portland cement did not have a significant effect on the ignition furnace correction factor. Additional testing should be conducted to assess the effects of fines representing other aggregates, particularly carbonates.