Falling-weight deflectometer sensor location in the backcalculation of concrete pavement moduli

This paper discusses the principles of two closed-form algorithms, ILLI-BAC K and NUS-BACK, for backcalculating layer moduli of concrete pavements and their implications for selecting sensor locations in falling-weight deflect ometer tests. It is found that sensor location has an effect on the results backcalculated using these two closed-form algorithms. Use of sensors at l arge radial distances may produce results with high variations since a smal l variation in the deflections recorded by these sensors will result in a s ignificant change of layer modulus. Use of close sensor spacing will produc e a ratio of deflections recorded by two consecutive sensors close to 1, le ading to significant variations in backcalculation results in those cases w here errors are involved in deflection measurements. The Strategic Highway Research Program (SHRP) FWD sensor configuration with sensors located at 0, 203, 305, 457, 610, 914, and 1524 mm is examined. Care should be taken in backcalculation analysis of concrete pavements using the SHRP sensor config uration in conjunction with the closed-form algorithms. Backcalculation res ults from deflections measured by SHRP and on Indiana US52 quantitatively c onfirm these findings.