Farhana Rahman

Soil Stiffness Evaluation for Compaction Control of Cohesionless Embankments

Mechanistic pavement design procedures based on elastic layer theory require characterization of pavement layer materials including subgrade soil. This paper discusses the subgrade stiffness measurements obtained from a new compaction roller for compaction control on highway embankment projects in Kansas. Three test sections were compacted using a single, smooth steel drum intelligent compaction (IC) roller that compacts and simultaneously, measures stiffness values of the compacted soil. Traditional compaction control measurements such as, density, in-situ moisture content, stiffness measurements using a soil stiffness gage, surface deflection tests using the light falling weight deflectometer (LFWD) and falling weight deflectometer (FWD), and penetration tests using a dynamic cone penetrometer (DCP), were also done. The results show that the IC roller was able to identify the locations of lower soil stiffness in the spatial direction. Thus, an IC roller can be used in proof rolling. IC roller stiffness showed sensitivity to the field moisture content indicating that moisture control during compaction is critical. No universal correlation was observed among the IC roller stiffness, soil gage stiffness, back-calculated subgrade moduli from the LFWD and FWD deflection data, and the California bearing ratio obtained from DCP tests. The discrepancy seems to arise from the fact that different pieces of equipment were capturing response from different volumes of soil on the same test section.

Aggregate Retention in Chip Seal

Chip seal is a widely used preventive maintenance treatment for flexible pavements. However, a major problem with chip seal is the damage caused by loose aggregates from newly placed seals, partly because of a lack of compatibility between aggregate and asphalt emulsion. In this study, limestone, crushed gravel, synthetic lightweight aggregates, and recycled asphalt pavement materials were studied in the laboratory with two polymer-modified asphalt emulsions to find the aggregate–emulsion combination that would result in maximum chip retention. Replicate specimens were tested in the ASTM D7000 sweep test for each aggregate–emulsion combination. Lightweight aggregates and gravel were also tested in a newly developed test setup for simulating the sweep test. Test results show that lightweight aggregates perform better than gravel. However, lightweight aggregate sources play an important role in chip retention. Emulsion is the most significant factor that affects chip loss. Statistical analysis identified the influential design factors affecting chip retention. Analysis of variance shows that aggregate and emulsion types and aggregate–emulsion interaction are significant factors affecting chip retention. Aggregate precoating and its interaction with emulsion type are insignificant factors as far as chip retention is concerned.

Evaluation of Superpave Mixtures with High RAP Content

The Kansas Department of Transportation (KDOT) is currently evaluating Superpave mixture performance with higher percentage of reclaimed asphalt pavement (RAP) materials. Hamburg Wheel Tracking Device (HWTD) tests were done on Superpave mixes with high RAP content sampled from a number projects across the state. Each mixture was subjected to 20,000 repetitions or 20-mm rut depth, whichever came first. The HWTD test results were also correlated with the volumetric parameters and other potentially significant factors. The analysis results showed that the rutting performance was significantly affected by the asphalt source and binder grade regardless of RAP content. The surface mixes with higher percentage of RAP material were not significantly susceptible to rutting compared to the mixes with moderate percentage of RAP material. Volumetric mix design parameters of these mixtures, except voids in mineral aggregate (VMA), were not significant in determining rutting performance. The rut depth decreased with increasing percent VMA for the mixes with moderate percentage (25%) of RAP materials.

Kansas Experience with Stiffness-Based Quality Control/Quality Assurance Specifications for Compaction of Highway Embankments

Compaction of highway embankment is a key factor influencing premature pavement distresseses. The current specifications of the Kansas Department of Transportation (KDOT) control embankment compaction are based on in-situ density and moisture content. In the recent past, KDOT participated in a number of projects that studied feasibility of performance-based specifications, specifically in terms of in-situ soil stiffness measured by non-destructive tests and/or produced by the intelligent compaction (IC) rollers, for quality control/quality assurance of highway embankments. Kansas experience shows that in-situ tests with Lightweight Deflectometer result in stiffness that is highly variable like other available tests. Correlation among different test methods is fair. The intelligent compaction results in very uniform compaction in the spatial direction. However, no existing test results are correlated with the stiffness parameters yielded by such rollers. The IC rollers also can detect soft spots in compacted subgrade. Thus such device can be used in proof rolling of compacted highway embankments.

Experience with Thin Superpave Mixture Overlay of Small Aggregate Top Size in Kansas

Superpave® asphalt mixture with small (4.75-mm) nominal maximum aggregate size (NMAS) is a promising, low-cost pavement preservation treatment for rural roads. This mixture has the potential to provide a smooth riding surface, be applied in thin lifts, correct surface defects, decrease construction time, and provide an economical surface mixture for low- to medium-volume facilities. Because of budget constraints in highway construction and preservation programs and the economic advantages of such a mixture, many highway agencies are considering implementing it for rural roads. Two rural highway pavements in Kansas were overlaid with a 15- to 19-mm-thick 4.75-mm NMAS Superpave mixture in 2007. Overall, the experience in Kansas with the 4.75-mm NMAS mixture was varied. Both projects had construction issues. Experience on one project showed that extended aggregate drying and drum cleaning would be necessary when producing these mixtures. Performance history on both projects after overlay construction showed that roughness of the road surface was significantly improved and was consistent for the 3 years after the overlay. Transverse cracks seem to be the major concern, although the rutting performance of this small-sized aggregate mixture is satisfactory. Crushed gravel aggregates seemed to perform better in the mixture than crushed limestone aggregates. A binder with a higher grade may be needed to address transverse cracking issues.

Optimized Design for 4.75-mm NMAS Superpave Mix Thin Overlay

Transportation infrastructure facilities such as highways consume large quantities of materials in initial construction and periodic maintenance and rehabilitation. A Superpave asphalt mixture with 4.75-mm nominal maximum aggregate size (NMAS) is a promising, low-cost pavement preservation treatment that can be applied in thin lifts for low- to medium-volume facilities. This study evaluates the performance of 4.75-mm NMAS Superpave asphalt mixture and proposes an optimized mixture design for a long-lasting ultra thin hot-mix asphalt (HMA) overlay. Twelve different 4.75-mm NMAS mix designs were developed in the laboratory using two binder grades and three different percentages of natural (river) sand in combination with finer quarry materials. Test results from two laboratory performance tests show that rutting and moisture damage of such mixes during Hamburg Wheel Tracking Device and Lottman tests are aggregate-type and source specific. Anti-stripping agent affects moisture sensitivity test results irrespective of natural sand content and binder grade. Finally, optimum combinations of aggregates and binder were developed for 4.75-mm NMAS mixture based on the statistical analysis of performance data.