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Deflection testing is a common nondestructive test procedure used by pavement engineers to evaluate the structural properties of the pavement structure and the loadbearing capacity of the underlying soil system. In dynamic deflection testing, the most common analytical models are based on linear elastic layer theory. While pavement systems are neither linear nor elastic, these models have been shown to adequately simulate in-situ response of pavement structures.
A critical element in linear elastic layer analysis is accurate determination of individual layer thickness. Minor changes in layer thickness can produce significant changes in analytical results of materials properties.
Traditionally, determination of layer thickness has relied on either construction records or on destructive (coring) calibration. Either method has now been shown to be capable of significant error. Construction records have been shown to vary from actual conditions by an order of magnitude or more, and actual pavement thicknesses have been shown to vary by an order of magnitude within only 100 m, a rate of change not detectable by coring procedures.
Since variations of only about 25 mm (1 in.), in extreme conditions, can skew deflection testing results by 100%, these traditional methods are frequently no longer adequate, and extreme care must be taken to employ methods which assure that accurate layer thicknesses are obtained in order to optimize pavement rehabilitation design.
One nondestructive technique, ground-penetrating radar (GPR), has now been demonstrated to provide such layer thicknesses within tolerances of ±6 mm (1/4 in.), a level of accuracy which lends an increased level of confidence to deflection testing results. In addition, GPR can be used to locate reinforcing steel, dowel bars, and voids, and to evaluate the deterioration of joints and load transfer systems within rigid pavements.
Case histories for a number of airport projects will be used to show the value of coupling GPR to a deflection testing program. Projects included are Des Moines International Airport, El Paso International Airport, and Chicago-O'Hare International Airport.
ground-penetrating radar, deflection testing, airfield capacity, pavement evaluation
President, Eckrose/Green Associates, Inc., Madison, WI