ANALYSIS OF CEMENT CONCRETE OVERLAYS FOR PAVEMENT SLABS

Abstract

Detailed finite element analysis with experimental verification by model tests, have been carried out in the present study for cement concrete overlays with cracked or uncracked base slabs, representing subgrade through several types of foundation models,for understand ing the behaviour of such pavement systems, and in turn developing an analytical approach for suggesting design parameters. Idealised in plane strain, the analysis initially reported here is to gain insight into the mechanics of load-deformation and stress distribution of cracked base with a fully bonded overlay and an overlay with bond broken in the neighbourhood of the crack. Based on plane strain analysis, a formulation is presented for slabs by numerically integrated Hermitian plate bending finite elements. Sub-element concopt is developed to model economically the crack, non-uniformity in subgrade support and consistent nodal loads. Capability to assess the stresses and displacements due to hydro-thermal changes is also incorporated in the computer program. Several tests were applied to check the accuracy of the developed technique and the computer program. Behaviour of a single slab with a finite crack and a full length -XX111- crack with shear transfer is studied and the results are verified. A formulation to modify this program for analysis of a bonded overlay is then presented to evaluate stresses and deflections under action of wheel loads, temperature differential and differential shrinkage. Analyses of some cases, with and without crack in the base is reported here to establish the feasibility of bonded overlays, estimate bond and flexural stresses, and study the mechanics of stress distribution. •Reinforced Key Technique', a method to inhibit the menace of crack reflectance is suggested and its action is brought out based on this study, the funda mental principles of fracture mechanics, and the reported field results. Similar modification is also formulated and incor porated to study the behaviour of sound or cracked slabs having an unbonded Overlay.Comparative behaviour of bonded and unbonded overlays is studied. The computer program was further modified to cons ider the subgrade not only as Winkler model but also an elastic continuum. Results of non-linear analysig to consider the subgrade as deformation dependent are presented, The comparison between Winkler and elastic solid models is made. An effective method to incorporate the generalized foundation model, such as due to Pasternak, is -XXIVsuggested and included in the program. A simple proced ure to estimate the second parameter of the two para meter foundation model is given. Statistical evaluation of proposed method is reported, based on the model tests carried out. Theory of influence surfaces is included by s a simple addition in the existing computer program to evaluate the critical load positions exactly and eco nomically. It is shown that the same influence sur face results can also be used to obtain the influence surfaces for temperature and shrinkage effects for different temperature distributions at any location in a pavement slab. It is also shown that by adopting a modified procedure, the standard solutions e.g. Westergaard's equation or Pickett's influence charts can still be used for stress computations in rigid pavement slabs which have more than one layer of different properties in bonded or unbonded conditions. Similarly, a procedure is also, suggested to correctly evaluate the stresses in base and overlay due to temperature differential by using Bradbury's solution in a modified form. Also, a similar procedure has been suggested for evaluating the shrinkage stresses. For estimating wheel load stresses in cracked base, a procedure based on beams on elastic foundation is suggested. -XXVIt is hoped that the present study would help in evolving rational design of rigid overlays for cement concrete pavements incorporating the effects of wheel loads, temperature and shrinkage with parti cular emphasis on the conditions of the base-overlay systems and their interfaces.