Scott Shuler

Relationships among Gradation Curve, Clogging Resistance, and Pore-Based Indices of Porous Asphalt Mixes

ABSTRACT When properly designed, porous asphalt functions both to make safer the driving in wet condition, and to provide hydrologic restoration while functioning as a filtration interface. This study examined the role of aggregate granulometry both on the pore-based indices of porous asphalt and on the clogging resistance. Three groups of porous asphalt gradations comprised of 12 series of samples and separated based on gradation size indices have been examined. For all samples the vertical (kv) and horizontal (kh) permeability were measured with the commensurate total void contents (e) and total porosity (n). Also clogging resistance was evaluated by measuring hydraulic conductivity before and after 1 year of exposure of the samples to dust intrusion. Marshall tests were also performed in order to control the stability of the mixes. There was a strong relationship between the permeability and the void content. For a given aggregate gradation, each increment of asphalt content increase caused a decrement in permeability and porosity. As for the clogging resistance potential, it seems to be more related to the initial permeability than to the other pore-based parameters. Results indicated that in addition to the coefficient of uniformity (U) and the total void content, a size-based granulometric index (P5·P2·Dmax) provided a relationship with vertical permeability and the total void content that were very consistent for the entire range of porous asphalt mixes. Results of this study allow the role of mix design on the permeability to be examined. These mix design results are a necessary precursor to examination of in-situ permeability after placement, and degradation of such permeability as a function of time and rainfall-runoff hydraulic and particulate loadings.

Surface Performance-Grading System to Grade Chip Seal Emulsion Residues

The performance-graded (PG) asphalt binder specification was developed to characterize asphalt binder properties that are directly related to the performance of hot-mix asphalt concrete in pavements. However, the PG specification cannot be applied directly to binders for use in chip seals. To address this need, the surface performance grading (SPG) specification was developed with the same equipment required for the PG system but with different limiting values for test parameters at high and low pavement surface design temperatures. NCHRP Project 14–17 used the PG and the SPG systems to grade base binders and recovered residues from five laboratory emulsions and from recovered residues for three emulsions for chip seal construction projects. Two residue recovery methods were used and compared: hot oven evaporation with nitrogen blanket and stirred can with nitrogen purge. Researchers compared the grades from the PG and the SPG systems and found that the grades were similar for the two residue recovery methods but slightly different from the base binder. On the basis of these results, a strawman specification for emulsion residues in chip seals is recommended for use with the stirred can recovery method. The current limiting values in the SPG system were developed for the Texas Department of Transportation. Limiting criteria should be developed for other regions and climatic conditions.

When to Broom or Remove Traffic Control Safely on Fresh Emulsified Asphalt Chip Seals

The optimal time to allow brooms to sweep a fresh chip seal is usually determined by experienced field personnel. Factors that affect this decision include aggregate type; aggregate moisture content; emulsion type and grade; application rates of emulsion and aggregate; weather conditions at the site, including humidity, air and pavement temperature, and wind; and number of rollers. Given all these factors, and because changes in one factor can have a significant effect on how an emulsion gains strength and how soon it can resist the forces of brooms and traffic, the decision as to when to allow brooms and traffic on fresh chip seals becomes difficult, at best, to predict. The results of this research suggest that moisture content of the chip seal system is related directly to the strength of an asphalt emulsion residue. Laboratory and field testing confirm that the strength of an asphalt emulsion residue as measured by a modification to the ASTM D7000 sweep test can be used to predict the moisture content level at which a chip seal developed enough strength to resist damage by brooms and vehicular traffic. Three full-scale test pavements were constructed in differing climates, and the results of moisture content testing in the field were compared with modified sweep test results in the laboratory. Results indicated that the three field tests were capable of resisting brooming and traffic damage when the moisture content of the chip seal system ranged between 15% and 25%. This finding correlated well with results of laboratory testing with the modified sweep test on the materials from field tests as well as on experimental laboratory materials.

Correlation of Moisture Loss and Strength Gain in Chip Seals

One of the most subjective decisions made during chip seal construction concerns when to allow brooms and traffic on the newly placed surface. If traffic is allowed too early, damage to the surface and to vehicles may occur. If the chip seal is opened too late, traffic is disrupted and motorists are inconvenienced. The curing of asphalt emulsions in the field is related to many factors, all affecting how fast the asphalt emulsion cures. Three laboratory test methods that measure adhesive strength gain as a function of moisture loss are presented. Two of the methods were sweep tests, one performed according to ASTM D7000 and the other according to a modified method. The third test used frosted marbles to measure adhesive strength gain. Results of all tests were similar and indicated that strength in emulsion residues increased as the total moisture in the system was reduced. This finding is important because the moisture content is independent of the mechanism reducing it. Therefore, prediction of strength gain should be possible by measurement of the moisture loss of a given chip seal system. The time required to obtain that strength gain varies in seals because of differences in emulsion, aggregate, interaction factors, weather, ambient temperature, and other environmental factors. Test results indicated that as moisture loss approached 75% to 90%, strength gain was significantly enhanced.

Effect of Performance Warranties on Cost and Quality of Asphalt Pavements

Colorado began experimenting with performance warranties on asphalt pavements in 1998. The warranty specification on the first projects was developed with the help of the asphalt paving industry and consisted of a 3-year period in which the contractor was responsible for several forms of pavement distress. In exchange for this risk, the contractor was allowed to choose materials and methods for constructing the pavements. A study documented the cost–benefit relationship for these projects during the warranty period and beyond and compared the performance of these projects with comparable nonwarranty pavements. Cost comparisons include the initial hot-mix asphalt, maintenance, pavement evaluation team, weigh-in-motion station, and construction engineering. An analysis was also conducted on the competition, performance, and use of experimental features. The results of this analysis showed no significant difference in competition or performance of the warranty projects when compared with the control projects, and the difference in cost of the warranty projects when compared with the nonwarranty projects was determined to be negligible. The greatest difference in cost identified between the warranty and nonwarranty projects was for weigh-in-motion station installation and maintenance. These stations were installed to verify traffic levels during the warranty period on the first projects. This cost was approximately 3% of the overall project cost. Many lessons were learned on these projects, and recommendations were made to improve the current specification and increase the warranty periods to 5 and 10 years.

Life Cycle Analysis for Sustainable Development: A Case Study of Parking Lot Pavements

The use of porous pavement allows water to drain into an aggregate storage layer beneath the pavement for temporary storage as it percolates into the aquifer below. When using this system in a parking lot application, the need for curb and gutter to channel the stormwater into a nearby retention basin is eliminated, as is the retention basin itself. An initial analysis of the porous system appears to solve some of the environmental issues related to stormwater runoff and aquifer recharge. However, these systems have not been evaluated using both life-cycle cost and life-cycle assessment to fully understand the costs and environmental implications of choosing one system over another. A case study was used to compare the two asphalt systems in a large parking lot application. The results show that the conventional asphalt system is both more cost effective and environmentally benign. The emissions area identified as having the greatest CO2 impact is the manufacturing/mining and extraction of aggregates making it an area that could be targeted for increased efficiencies and emission reductions.

Forensic Analysis of Premature Surface Cracking in Full-Depth Asphalt Pavements on Plastic Clays

Premature cracking in new asphalt pavements before the pavements are opened to public traffic, before the development of the sites is completed, and before surrounding buildings are occupied is the focus of this paper. Previous studies have been conducted at individual sites to determine the cause of this phenomenon and have resulted in a variety of probable, but not conclusive, causes. This paper describes a forensic analysis that used interviews with principals experienced with this form of distress and the results of field and laboratory tests intended to expose commonality by soil type, pavement materials properties, construction sequence, and pavement design among several sites where cracking occurred and where cracking was not present. Six factors that were common at each site with cracking were detected: all cracks began at the surface of the pavement and propagated downward; the moisture contents of the subgrade soils beneath the cracks were higher than optimum, as measured by AASHTO T99; the densities of the subgrade soils beneath the cracks were lower than optimum, as measured by AASHTO T99; subgrade soils were classified as AASHTO A7; all pavements were full-depth asphalt placed directly on the subgrade; and all pavements were constructed in private developments with typical curb and gutter sections. In addition, similar pavements constructed on crushed stone bases in the vicinity of the full-depth pavements with the same construction and materials characteristics did not display this cracking. The results warrant a reduction in compaction moisture levels so that compaction densities can be increased when full-depth asphalt is used to pave over plastic clays.

Achieving 4% Air Voids in Real Pavements with Superpave

The Superpave® mix design method has been used by the Colorado Department of Transportation since 1995. Initially, the number of design gyrations used was as recommended by SHRP research. However, the asphalt contents of these preliminary paving mixtures were lower than what had been historically successful for similar dense graded mixtures under similar conditions. This lower asphalt content began leading to premature moisture damage failures. Therefore, a study was initiated to determine how the pavements constructed according to the original Superpave compaction criteria densified under traffic. An experiment was designed to evaluate the in situ voids in 22 full-scale pavements over a 6-year period. Projects were selected on the basis of variation in traffic, environment, temperature, and elevation. Results of this research indicate that the original number of design gyrations recommended by SHRP for field-mixed–laboratory-compacted materials results in pavements that did not compact sufficiently under traffic, resulting in higher air voids than desired after 3 years service. An adjustment to the original SHRP design gyration levels was done in a subsequent study based on NCHRP research. The result of this revision has been a decrease in voids levels after 3 years of service with a corresponding improvement in performance. Further fine-tuning is needed and will be implemented in the future.

Factors Related to the Appearance of Transverse Bumps in Asphalt Overlays Placed over Crack Sealant

Crack sealants often are used as a preservation tool in asphalt pavements. These sealants are placed in cracks to prevent water intrusion into the pavement foundation. Through the reduction of water intrusion, the strength of foundation layers is maintained, and acceptable pavement performance is extended. However, when a hot-mix asphalt overlay is placed on top of a pavement that contains crack sealants, a bump and additional transverse cracks sometimes occur in the new overlay asphalt. These bumps and sometimes transverse cracks are initiated during breakdown rolling and become progressively more severe on further compaction. This paper presents the results of a 5-year study designed to identify factors that related to the appearance of these bumps and consequent cracks. The results of the study indicated that vibratory breakdown rolling, pavement gradient, sealant geometry, tack coat application rate, and tack coat adhesivity were factors that contributed most to the occurrence of bumps and transverse cracks during asphalt overlay construction over crack sealants. Observations suggested that transverse bumps and consequent cracks occurred in proportion to the size of the bow wave of asphalt concrete present immediately in front of the breakdown roller. The increase in the bow-wave size depended on asphalt mixture properties, breakdown roller size, speed, vibration characteristics, and pavement gradient. Three pavement test sections also indicated that the tack coat application rate had an effect on reduction in the appearance of transverse bumps.

Evaluation of Short-Term Warranty and Prescriptive Specifications for Hot-Mix Pavements After 8 Years

Asphalt concrete pavements constructed according to conventional construction specifications were compared with projects constructed according to performance warranty specifications. Ten projects of each type were compared to assess relative costs and benefits after 8 years. Pairs of projects were selected with similar characteristics regarding preoverlay repair work, functional class, design life, and other features to minimize bias. More than 214 lane miles of performance warranty projects were compared with 277 lane miles of prescriptive specification (control) projects. The average initial construction cost of the warranty projects was