It has been observed that mortar joints, where precast concrete trim and limestone building blocks are used in conjunction, tend to crack in the course of one or two years to such an extent as to admit water. It seemed possible that a difference in the thermal coefficients of the two materials might cause unequal changes in length along the joint, thereby setting up shearing stresses in the mortar, sufficient in time to destroy the bond. This investigation was undertaken in order to determine the relative rates of expansion of the local materials in question and to consider the validity or the theory that such differences might affect the mortar. The investigation and the conclusions drawn therefrom form the first part of this thesis. Part two consists of an investigation into the effects of different mixes on the thermal coefficient of such a concrete as might be suitable for the manufacture of precast trim. / M.S.