Study of load transfer efficiency of airfield rigid pavement joints based on stresses and deflections

The concept of joint load transfer efficiency is very important and fundamental to Federal Aviation Administration\u27s (FAA) airfield rigid pavement thickness design procedures. The FAA procedure assumes 25% of stress applied to the edge is transferred to the adjoining slab. Moreover, since it is not convenient or practical to measure stress-based load transfer efficiency [LTE (S)], field measurement of load transfer efficiency includes computation of the ratio of unloaded slab deflection to loaded slab deflection. Falling weight deflectometer (FWD) generally serves the purpose of measurement of deflection-based load transfer efficiency [LTE (6)]. The true load transfer is defined by stress (or strain) ratio [LTE (S)]. The current FAA specification prescribes the evaluation of LTE (S) from deflection based load transfer efficiency [LTE (6)] which suggests that LTE (S) of 25 % is same as LTE (6) of 70-90 %. However, the equivalency of LTE (6) and LTE (S) depends on the effect of single plate loads of FWD versus multiple gear loads of aircraft and short duration impulse loads of FWD verses a comparatively longer duration moving aircraft wheel loading. In addition, unknown differences may exist due to differential slab bending phenomena under different aircraft gear configurations and various gear positions as an aircraft traverses a joint. There is a need to determine the sensitivity of appropriate variables such as pavement structure, static or moving modern aircraft gear loads in different positions along the joint etc. on the LTE (S) of the rigid airfield pavement joints. The FAA currently uses a single slab model for thickness design using FAARFIELD. The design philosophy is now being extended to multi-slabs. Hence there is also a need to study the above effect considering multi-slabs in finite element modeling. The three objectives of the study were: 1) To determine how 25% stress-based load transfer efficiency compares considering above mentioned variables; 2) To study the effect of various load types such as static versus moving loads, various aircraft gear configurations and position of gears with respect to the joint on the LTE of joint; 3) Justify the commonly used correlation between LTE (S) and LTE (S) considering the above mentioned effects. The full scale test data collected at National Airport Pavement Test Facility (NAPTF) was used in this study. Available strain gage records obtained during the slow rolling tests were analyzed to obtain LTE (S) under moving aircraft gear loading. Deflection data from FWD was analyzed to obtain LTE (6) of the test item joints. The pavement stresses and deflections under static aircraft loading were also determined using 2D and 3D-finite element analysis programs. The pavement configuration and the aircraft gear similar to those at NAPTF was simulated in a 2Dfinite element program JSLAB and LTE (S) under static load was determined and compared with that obtained under the moving loads. Finally, a 3D-finite element program FEAFAA developed by the FAA, was used to study the effect of different modern day aircrafts with different gear configurations in various positions along the joint on joint load transfer. Thus, the effect of static versus dynamic loading, footprint shapes, gear configurations and gear positions on joint load transfer was studied using the full scale data as well as finite element analysis programs. Overall the results demonstrated that stress based LTE under a moving aircraft gear was significantly higher than that under a static aircraft gear loading. Under static loading, when the main axis of aircraft gear was perpendicular to the joint, LTE (S) under a single wheel was lower by 27% as compared to the same under a 6-wheel, 4-wheel and 2-wheel gear configuration. It was also observed that number of loaded areas along a joint also governed the LTE of joint however; the difference in LTE was statistically insignificant. Overall, the 25% LTE (S) criterion was met in all the cases while it was highly conservative in case of moving aircraft gear loading