Fatigue experiments for the design of plain concrete pavements

In the design of plain (unrelonforced) concrete pavements the influences of traffic and temperature, among others, are taken 1.nto account. These sue j ect, the pavements to load var1.a1:.l.onBse.cause the traffl.c comprises vehicles W1.th widely differing wheel loads, the loading pattern due to it is very erratl.c: the frequency dnd amplitude of the variations as functions of time are not constant. Besides, the pavement is subjected to an additional varying load due more particularly to differences between day and night temperatures. Changes in the temperature of the surface of the concrete cause differences in temperature between the top and the underside of the pavement. In consequence of a temperature rise on its upper face the concrete slab will tend to 'hog', i.e., develop an upward camber, wh1.ch is counteracted by its own weight. Tensile stresses are produced in the bottom part of the slab; these must be taken into account in the design (Fig. 1). dead load and traffic load ~ Deformation of the concrete pavement due to heating at 1.tsupper surface Cooling durlJlg the ru.ght causes a slight temperature gradient in the concrete, but this gradient is often neglected in deslogn calculations. Seasonal temperature variations between summer and winter do not cause a temperature gradient. Tensile stresses will, however, occur in the concrete pavement as a result of overall cooling. To limit such stresses the pavement is provided with (contraction) joints. These tensile stresses are therefore often not explicitly taken into account. Sinoe the frequency of the temperature variation is much lower than that of the traffic, the loading pattern can be represented as a long cyclic or wave motion (due to tempera~re) upon which a 'traffic ripple' is superimposed. The amplitude of this ripple may be either greater or smal.ler than that of the temperature wave. Fig. 2 shows a loading pattern of this kind. The above mentioned load variations can cause material fatigue, leading to cracking al: stresses which are lower than the static strength. For assessing the fatigue behaviour of roads it is necessary to have information on the stresses that occur and furthermore on the relevant material characteristics. For plain concrete pavements the fatigue behaviour of the concrete subjected to tensile stress is the governing feature. The Stevin Laboratory of the Delft university of Technology has carried out comprehensive research on these matters. Besides the effect of regular stress variations, the effect of stress var~ations such as those whloch occur in actual practloce has been studl.ed. stress In slab bottom time ~ Random loading W1.th varyl.ng amplitude values, with two discernible fundamental frequencloes The Palmgren-Miner rule (more briefly referred to as Miner's rule) is often applied in calculating the effecl: of these practically occurring stresses. This paper concisely reviews the way in which concrete fatigue is taken into account in the so-called VNCmethod of pavement desi