Danish Building Research Institute, DK-2970 Hørsholm, Denmark


The use of weak mortar has a number of advantages (e.g. elimination of expansion joints, environmental issues). However, according to EC6, the strength of masonry tends to zero when the compressive strength of the mortar approaches zero. In reality the presence of even unhardened mortar kept in place in the joint will ensure a certain level of load-carrying capacity. This is due to the interaction between compression in the weak mortar and tension in the adjacent bricks.

This paper proposes an expression for the compressive strength of masonry made with weak lime mortars (fm<1N/mm2) in which the strength of masonry depends only on the strength of the bricks. A compression failure in masonry made with weak mortars occurs as a tension failure in the bricks, as they seek to prevent the mortar from being pressed out of the joints. The expression is derived by assuming hydrostatic pressure in the mortar joints, which is the most unfavourable stress distribution with respect to tensile stresses in bricks. The expression is compared with the results of compression tests of masonry made with weak mortars. It takes into account bricks with arbitrary dimensions as well as perforated bricks. For a stronger mortar (fm?6N/mm2) compression tests on masonry with perforated bricks show that the EC6 expression is not always safe for Danish masonry. This is probably because the tensile strength of the bricks and hence the compressive strength of masonry is affected whether the mortar is stronger or a very weak lime mortar. An extended version of the EC6 expression for the compressive strength of masonry is proposed, which includes an allowance for the effect in question.