(Received 25 February 1999; accepted 11 February 2000)
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This paper discusses the principles of two closed-form algorithms, ILLI-BACK and NUS-BACK, for backcalculating layer moduli of concrete pavements and their implications for selecting sensor locations in falling-weight deflectometer tests. It is found that sensor location has an effect on the results backcalculated using these two closed-form algorithms. Use of sensors at large radial distances may produce results with high variations since a small variation in the deflections recorded by these sensors will result in a significant change of layer modulus. Use of close sensor spacing will produce a ratio of deflections recorded by two consecutive sensors close to 1, leading to significant variations in backcalculation results in those cases where 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 configuration in conjunction with the closed-form algorithms. Backcalculation results from deflections measured by SHRP and on Indiana US52 quantitatively confirm these findings.
Research engineer, Indiana Department of Transportation, West Lafayette, IN
Professor, Mississippi State University, MS
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