Abstract
Lean concrete has, on account of its high stiffness, excellent load-spreading properties when used as a roadbase material in flexible pavements, although on the same account, appreciable stresses arise when movements are restrained by the subgrade or sub-base. Consequently, owing to the low tensile strength of the material, temperature falls result in the formation of cracks which, if sufficiently wide, may be transmitted through the superimposed bituminous surfacing. Developments in the design of flexible pavements and in the specification and use of lean concrete roadbases are reviewed, followed by a consideration of the causes of cracking. Expressions for the spacing and width of cracks resulting from restrained thermal contraction of a cementitious roadbase are derived in terms of the temperature fall, material properties and a constant frictional coefficient, in order to identify the critical parameters involved in each case. Thermal movement measurements were carried out on a number of cement stabilised materials in various moisture conditions and an apparatus which would cause cracking of lean concrete by restrained thermal movement was designed and built. A programme of tests was devised principally to determine the susceptibility of the material to cracking as a function of age, the results indicating that cracking in lean concrete roadbases is likely to occur within one day of placing. Additional tests were carried out to investigate the effects of inadequate compaction, the use of mixes of higher water content and cement content and the incorporation of steel fibres. It was found that mixes of increased tensile strength were more resistant to cracking, especially at early ages, but that they ultimately give rise to wider cracks which would be more likely to reflect through the bituminous surfacing. Experimental results and published values of subgrade restraint were combined to give a more accurate estimate of crack spacing and crack width in a lean concrete roadbase at a given age and for a given temperature fall. Temperatures in a newly constructed roadbase were measured and the results used for the prediction of cracking patterns which were found to be- in broad agreement with those detected. The thesis concludes with a review of methods for controlling reflection cracking in flexible roads with cementitious bases.