It’s 10 o’Clock: Do You Know Where Your Sensors Are?

More than 400 falling weight deflectometer (FWD) devices are presently in use throughout the world, and deflection reading accuracy is very important. Deflections are measured in microns, or hundredths of a mil, and even very small errors in the deflection readings can have a profound effect on the results of backcalculation. One question that has somehow escaped scrutiny is the one alluded to in the title to this paper—exactly where along the deflection basin are the FWD deflection sensors positioned? This is an extremely important issue for proper definition of the deflection basin as a function of distance from the center of the loading plate. A review of the FWD load-deflection data in the Long Term Pavement Performance (LTPP) study found that in at least 7 percent of some 4 million lines of FWD deflection data in the National Information Management System (NIMS) database, the sensors were not positioned as reported. This problem is not limited to LTPP and NIMS, and it occurs all too frequently on FWDs everywhere. How sensor positioning errors influence backcalculated moduli, even if all other facets of the FWD data are 100 percent correct, is described. Examples of the errors found in NIMS are also presented—real-life illustrations of what can go wrong and how much influence these errors can have on pavement analysis. A method of scanning for sensor positioning errors without carrying out backcalculation is presented. By use of the suggested transform, sensor positioning errors are clearly evident when suspect data are compared with correct data along the same, or other, pavement sections.