A numerical model for the computation of concrete pavement moduli: a non-destructive testing and assessment method J.N. Karadelis * School of the Built Envir onmen t, Unive rsit y of Coventry, Priory Street, Coventry CV1 5FB, UKReceived 15 March 1999; received in revised form 28 June 1999; accepted 30 June 1999 Abstract The falling weight deflectometer (FWD) non-destructive testing technique has been used to monitor and assess the behaviour and performance of rigid pavement systems. In addition to the full-scale site investigation, tests were also carried out with the aid of a specifically developed laboratory scaled model of the FWD. A rigorous finite element model was developed to analyse a multi-layered pavement system with various material and geometric properties and to relate the surface deflections as measured to the computed values. Evidence of non-linearity and deviation from classical linear elastic theory led to a more complex mathematical solution to fit the experimental data more accurately. The laboratory and field test results were compared with the computed values. This paper includes extensive discussion of these results and the conclusions drawn from them. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Falling weight deflectometer; Young’s modulus; Pavement quality concrete 1. Introduction The falling weight deflectometer [1–3] was first intro- duced in Fra nce about 30 yea rs ago to test the flexibl e road networks. It has since gained increasing acceptance as one of the most effective methods for evaluating flexible roads. Recently, it has also been used to quantify the condi- tion of joints in concrete pavements and detect deterioration in cement treated layers below the surface [4]. Essentially, a weight impa ct s th e pa ve ment and the de flections ar e measured by a series of seven geophones: one at the centre of the impact plate and six at other positions equally spaced along a radius. A heavy version of the FWD can produce a maximum instantaneous dynamic force, as measured by a load cell of up to 250 kN, to simulate one wheel of a fully loaded Boeing 747. The impact time is between 20 and 25 ms. 1.1. Objectives The primary research objectives were as follows: first, to mea sur e simulta neo usly under a giv en load the surface deflections and the stresses and strains in the layers of a pavement; and second, to correlate these results with theo- retical values obtained from a finite element analysis, for the same loading pattern, using experimentally determined elas- tic modulus values for each layer. If the correlation between theoretical and expe rimental stress, strain and deflec tion values could be achieved, then the assumed moduli should be representative of particular pavement materials. An infi nit ely rigi d slab wou ld the ore tica lly dis tri bute concentrated loads to the full extent of its boundaries and would not deform. However, in its simplest form a rigid pavement refers to a concrete slab resting on one or more soil layers and it is called ‘rigid’ because the modulus ofelasticity of the slab is several hundred times greater than that of the underlying soil. The overall ob ject ives of the inves ti ga tion we re as follows: • Examine the suitability of the FWD-system for asse ssing rigid pavements in general and a specific multi-layered rigid pavemen t airpo rt site in parti cular [1,3,5]. • Examine the possibilities of regarding pavement layer stiffness as an index of the structural condition of the pavement, by reviewing the existing methodology [6–9]. • Examine the importance of relevant parameters such as critical stresses and strains within the layers of the pave- ment, the moisture content, the duration of loading, the geometry of the pavement structure, the drainage condi- tions and the stress history [10]. • Rel ate the sti f fne ss mentio ned abo ve, to the sur fac e NDT&E International 33 (2000) 77–84 0963-8695/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0963-8695(99)00034-1 www.elsevier.com/locate/ndteint * Tel.: 44-1203-63 -1313; fax: 44-1203-83-8485.
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A numerical model for the computation of concrete pavement moduli:a non-destructive testing and assessment method
J.N. Karadelis*
School of the Built Environment, University of Coventry, Priory Street, Coventry CV1 5FB, UK
Received 15 March 1999; received in revised form 28 June 1999; accepted 30 June 1999
Abstract
The falling weight deflectometer (FWD) non-destructive testing technique has been used to monitor and assess the behaviour and
performance of rigid pavement systems. In addition to the full-scale site investigation, tests were also carried out with the aid of a specificallydeveloped laboratory scaled model of the FWD.
A rigorous finite element model was developed to analyse a multi-layered pavement system with various material and geometric properties
and to relate the surface deflections as measured to the computed values. Evidence of non-linearity and deviation from classical linear elastic
theory led to a more complex mathematical solution to fit the experimental data more accurately. The laboratory and field test results were
compared with the computed values. This paper includes extensive discussion of these results and the conclusions drawn from them. 2000
inordinately long and severely limited the number of FE
analyses which could be made. The values shown for E 1were obtained at an intermediate stage and those for E 2were the values when further adjustment was discontinued.
Similarly, the sensitivity analysis indicated that although
the values for layers 2 or 3–5 changed fairly dramatically
from E 1 to E 2, the theoretical surface displacements changed
relatively little at all locations. This is shown in Fig. 4. It is
clear therefore that the static E -value of 30 GN/m2 taken
throughout for the PQC (layer 1) was too low and that a
higher, dynamic E-value should have been assumed and
then adjusted as necessary for the best fit. In addition,
while the conical load distribution theory provides a useful
guide in the case of rigid pavements, the influence of the
J.N. Karadelis / NDT&E International 33 (2000) 77– 84 81
Fig. 3. Comparison between theoretical and laboratory results for impact load of 35 kN.