Falling weight deflectometer (FWD) testing is a nondestructive pavement structural evaluation technique routinely performed on highway and airfield pavements to estimate pavement layer properties from measured deflection basins. This paper presents a methodology based on analyzing FWD test data between the trafficked and non-trafficked lanes to determine degradation and rutting potential of asphalt pavement unbound aggregate layers in comparison to the subgrade damage. The validity of the approach is demonstrated by analyzing the heavy weight deflectometer (HWD) data obtained from the Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF) flexible airport pavement test sections built with substantially thick unbound aggregate base/subbase courses. The modified Base Damage Index and Base Curvature Index defined from HWD pavement deflection basins were used to determine relative base to subgrade damage, which clearly showed evidence of the increased base damage induced in the NAPTF airport pavement layers during trafficking partly due to the applied gear load wander. This was in accordance with both the individual pavement layer recovered and unrecovered (inelastic or residual) deformation trends identified from analyzing the multi-depth deflectometer data collected during trafficking and the post traffic forensic analysis results, which indicated that a majority of the permanent deformation occurred in the unbound aggregate layers and not in the subgrade. The methodology presented for the detailed analyses of the FWD (or HWD) test data between trafficked and non-trafficked lanes can be effectively used in asphalt pavements to detect unbound aggregate layer deterioration and its pavement damage potential. For unbound aggregate layers in highway and airport flexible pavements, rutting is the only failure mechanism of relevance as no bound layers are involved for a fatigue failure to occur. With the recent Federal Aviation Administration (FAA) and AASHTO moves towards a mechanistic-empirical pavement design procedure, the validation and field calibration of proper unbound aggregate rutting damage models has gained attention. Using the data from the FAA's National Airport Pavement Test Facility (NAPTF) flexible pavement test sections, Kim and Tutumluer (1) successfully developed unbound aggregate base/subbase permanent deformation models that consider layer stress states and the effects of dynamic loading by a moving wheel in addition to the number of load applications. Their findings indicate that to accurately predict both recoverable and unrecoverable (residual) deformations per pass of the aircraft gear the effects of gear wander locations and wander patterns are critical, and have to be considered in the modeling of rut development. Pavement unbound aggregate layers are said to shakedown under channelized traffic applications which means they consolidate and gain strength with time. This process is seen in the field as well as with repeated load triaxial testing in the laboratory. The recent successful research efforts on the aggregate shakedown concept by Werkmeister et al. (2) identified three zones of shakedown as: A -plastic shakedown, B -plastic creep, and C -incremental collapse. It is likely that for all shakedown ranges, any particle rearrangement that occurs due to load wander, such as in the case of NAPTF pavement test sections, will no doubt relieve some of the residual compressive stresses locked in the granular layer due to shakedown; which in turn will cause additional rutting. Such a mechanism was recently observed by Hayhoe and Garg (3) in the NAPTF pavement test sections. Their post traffic forensic analyses indicated that a majority of the permanent deformation occurred in the unbound aggregate layers and not in the subgrade. As part of the ongoing research efforts at the FAA's Center of Excellence for Airport Technology (CEAT) at the University of Illinois, individual pavement layer dynamic response data from the NAPTF pavement test sections were analyzed due to passing of aircraft gear for various combinations of applied load magnitudes and loading sequences (application order and stress history effects), traffic directions, gear spacings, wander positions, and wander sequences. Nondestructive heavy weight deflectometer (HWD) tests, similar to FWD tests with higher dropped weights, were also used to collect pavement deflections and evaluate pavement profiles due to trafficking. There was significant movement of the unbound aggregate particles in the base and subbase layers during trafficking due to transverse load wander. The permanent deformation during a complete wander cycle was negated due to aircraft wander, indicating recurring particle movements and rearrangements in the unbound pavement layers (4,5). As a
There is an analytical study in finding relationship between the data measured by FWD in trafficked and non-trafficked lanes using statistic methods to determine the degradation and rutting potential of flexible pavement unbound aggregate layers in comparison to the sub grade damages REF .
Falling Weight Deflectometer Testing to Determine Relative Damage in Asphalt Pavement Unbound Aggregate Layers
Transportation Research Record
Engineering