T.G. HUGHES AND R.J. HARVEY University of Wales, Cardiff

In a normal environment, masonry constructed from fired clay units will expand as moisture is taken into the dry units from their surroundings; this expansion can lead to considerable compressive forces within the masonry, which, if restrained in, for example, a tunnel lining may be great enough to cause crushing failure of the material. Conversely, if the masonry is constructed from precast concrete units, the composite will tend to shrink with time and restraint of the masonry will lead to the build up of tensile forces instead, with the possibility of cracking damage. In both cases, creep, in the guise of stress relaxation, can act usefully to reduce the generated forces to levels below those critical for material failure to occur. This paper reports the results of tests carried out to measure the build up of tensile forces in concrete block masonry fully restrained in the bed joint parallel direction, and compares this with both free shrinkage measurements and measurements of the direct tensile strength of masonry. In addition to providing more data to assist in the prediction of tensile cracking failure of the material, this type of test can also be used to provide information on the viscous material constants of the masonry, in order to model, for example, post-tensioned masonry action, by rheological analogy.