Kenneth H Stokoe Ii

Evaluating Potential for Reflection Cracking with Rolling Dynamic Deflectometer

A common rehabilitation strategy used for repairing aged concrete pavement is to place a hot-mix asphalt (HMA) overlay on the existing concrete pavement. However, reflection cracks are often found to propagate from the underlying cracks and joints through the HMA layer. As such, much reflection cracking is believed to be caused by differential vertical and horizontal movements in the concrete pavement. A common method of determining the differential vertical movements is by measuring the load transfer efficiency (LTE) at the joints by using nondestructive deflection testing devices. A study was conducted with a rolling dynamic deflectometer (RDD) to evaluate the movement of joints in concrete pavements. Evaluation of joint movements by RDD testing permits estimation of the LTE of each joint or transverse crack. On the basis of the assumption that reflection cracks are more likely to form at joints or cracks with low LTE than with high LTE, pavement engineers can use the results to identify areas with low LTE and perform necessary repairs at these locations to reduce the potential for creating reflection cracking. Field data collected before rehabilitation work on US-82 near Gainesville, Texas, are presented as a case study, and the benefits of continuous deflection profiling for use in the district’s rehabilitation strategy are discussed.

Detection and Monitoring of Cracks in Asphalt Pavement Under Texas Mobile Load Simulator Testing

The Texas mobile load simulator (TxMLS) is a newly developed accelerated pavement testing device used to evaluate pavement performance under real trafficking loads. This evaluation is performed by applying trafficking loads and monitoring surface distress, such as cracking and rutting, in conjunction with a number of other measurements of the pavement, such as those conducted with the falling weight deflectometer, multidepth deflectometer, strain gauge, pressure cells, and seismic (stress-wave) tests. A procedure for monitoring the progressive degradation of the asphalt surface was developed using the spectral-analysis-of-surface-waves (SASW) technique. This procedure was applied with the TxMLS and proved to be equally effective. SASW tests that were performed intermittently between trafficking phases on trafficked and untrafficked areas show (a) the effect of temperatures and frequencies on the asphalt moduli, (b) the importance of temperature and frequency corrections in analyzing the degradation of the asphalt surface layer, and (c) the long-term trends in surface-wave velocities (and hence, moduli) of the surface layer with increasing number of load applications. It was found that stiffness of the asphalt layer in the longitudinal direction was progressively reduced under trafficking. Concurrently, surface cracking progressively increased. The reduction in longitudinal stiffness occurred at a faster rate than the crack growth. In contrast, the reduction in the stiffness of the asphalt layer in the transverse direction was slower, probably because the main mode of cracking was transverse. The feasibility of using SASW testing as a predictor of degradation and imminent cracking was confirmed with these studies.

INCREASED SINGLE-LIFT THICKNESSES FOR UNBOUND AGGREGATE BASE COURSES

A study was conducted to evaluate the feasibility of compacting unbound aggregate base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test sections using a variety of material types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone. Three crushed granite test sections were built as part of a gravel production facility near Memphis, Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of-Surface-Waves (SASW) technique, was used to evaluate stiffness profiles within the compacted lifts. Cyclic plate load tests were accomplished by means of the Rolling Dynamic Deflectometer (RDD), modified for this static application. Results showed that compaction targets could be attained for lifts up to 21 inches thick. Density and stiffness results for 13-inch thick lifts in the Georgia tests were equal to, or better than, the results for the base placed in two lifts, a 7-inch lift followed by a 6-inch lift. Higher moisture contents during compaction yielded lower shear wave velocity and Youngs modulus values. Seismic results show that the upper 3 inches of the final test pads had lower stiffness values, presumably from lower effective stresses near the surface and possibly from some disturbance caused by the compaction equipment. This zone of lower stiffness and slightly less compaction is less evident in the density measurements.

PREDICTION OF WORKING LOAD DISPLACEMENTS UNDER PLATE LOADING TESTS FROM SEISMIC STIFFNESS MEASUREMENTS

A study was conducted to evaluate the feasibility of compacting unbound aggregate base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test section using a variety of material types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone. Three crushed granite test sections were built as part of a road widening project in Georgia, and two test pads were constructed of uncrushed and partially crushed gravel with loess fines at a gravel production facility near Memphis, Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of-Surface-Waves (SASW) technique, was used to evaluate stiffness profiles within the compacted lifts. Plate load tests were conducted on the surface of the crushed limestone test pads by means of the Rolling Dynamic Deflectometer specially modified for this fixed site application. Low frequency cyclic loads were applied to determine axial stiffness under transient working loads of varying magnitude. The base courses were tested at two moisture contents. The results were evaluated and compared with small strain seismic tests results. Strain amplitudes in the plate load tests led to a 5% to 25% reduction in measured stiffness as compared to the seismic results.