Construction of full-scale trials to evaluate the performance of induced cracked CBM roadbases

The use of cement bound material (CBM) roadbases in flexible composite pavements in the United Kingdom is limited by the reflective cracking that occurs due to thermal shrinkage. Inducing cracks during construction is a technique proposed to minimise reflective cracking. This report describes full-scale trials that have taken place and discusses results of the initial monitoring that was carried out during construction. Four trial sites were located with different roadbase materials and upper pavement thicknesses. All sites had cracks induced by using a simple vibrating plate with welded blade. In addition at individual sites the OLIVIA and CRAFT automated cracking processes were brought over from France and a guillotine technique as proposed by the Transport Research Laboratory were used. Initial monitoring during construction has been carried out using the Falling Weight Deflectometer and core samples. Load transfer characteristics of the cracks and elastic modulus of the induced cracked roadbase have been assessed by comparing with the naturally cracked control sections. Core samples have been taken at the cracks to confirm the results obtained for load transfer and assess the size and orientation of the cracks. Analysis of the initial results has proved promising enabling the development of a draft specification. In particular the sensitivity of induced cracking with respect to mix design, depth of induced crack and compactive effort is discussed. (A)