Void Detection Underneath Rigid Pavements: Numerical Simulation Of The Impulse Response Method

Abstract

The Impulse response (IR) technique is a seismic method used in evaluation and condition monitoring of rigid pavements. It enables nondestructive evaluation of in situ pavement material properties and detection of several defects within the pavement structure. Voids or loss of support underneath rigid pavements can be detected due to changes in the flexibility spectrum and damping characteristics of the pavement system. Numerical simulation of the IR technique for void detection underneath concrete slabs in rigid pavements is presented. The simulations were conducted using the finite element method. Effects of several pavement parameters on the pavement response and the ability to detect voids are presented and discussed. Introduction Most of the distresses that occur in pavements are not directly related to their design, but are caused by deficiencies in construction, materials and maintenance. Proper pavement management requires the ability to nondestructively evaluate pavement conditions. The Impulse Response (IR) technique is one of a number of nondestructive techniques used in evaluation of pavement systems. It is based on a measurement of the dynamic response of a pavement to an impact applied on the surface of the system. The IR method has been successfully used in detection of voids or loss of support under rigid pavements by monitoring the changes in damping characteristics of the system [1,2]. This technique is also used in evaluation of the subgrade modulus and to it correlated properties, like e.g. variation in the subgrade moisture content. The modulus of subgrade reaction is evaluated from the mechanical impedance or a flexibility spectrum of the pavement system [3]. A finite element study was conducted with an objective of improving interpretation of the field data collected by the Seismic Pavement Analyzer [2,4]. Rigid pavements with voids of three different sizes are investigated for this purpose. Results from the simulation are described in terms of time histories, and response and flexibility spectra. The first part of the paper discusses fundamentals of the IR used in pavement condition monitoring. The second part of the paper concentrates on issues related to finite element simulation of the IR test, effect of various pavement parameters on the response and flexibility spectra, and the ability to detect voids beneath rigid pavements. 2 Fundamental Principles of the IR Method The IR technique is based on an application of an impact at the pavement surface and evaluation of the response detected at a closely placed receiver. From the applied impact and the response, the shear modulus of the subgrade for rigid pavements and the modulus of the overall system for flexible pavements is evaluated. Presence of voids or loss of support beneath rigid pavements can also be determined. Use of the IR in evaluation of subgrade modulus is illustrated in Fig. 1. A load cell and a close lowfrequency geophone measure both the load and time histories. The flexibility spectrum, representing the ratio of the response and impact load spectra, is then matched by a response spectrum for a single degree of freedom (SDOF) system. Modal parameters of the SDOF system: natural frequency, gain factor and damping ratio are obtained and used in pavement characterization. The natural frequency and gain factor are used to evaluate the shear modulus. If the modulus takes low values, that is typically an indication of a poor support. Low damping indicates the presence of voids beneath a rigid pavement. Figure 2 is an illustration of this phenomenon for a rigid pavement with a void beneath the joint of a concrete slab. A slab with good support or sound contact has a response that is characterized by a high damping ratio, because the impact energy is radiated towards the interior of the medium. On the other hand, voids cause trapping of the wave energy within the slab, resulting in a response characterized by a low damping ratio. Loss of support or presence of voids beneath slab joints is typically indicated by a damping ratio in the range of 10 to 40%, while loss of support beneath the middle of a slab is typically indicated by a ratio of 30 to 60% [2]. Figure 1. Evaluation of subgrade modulus by the IR technique A t A t