Creep of clay masonry exhibiting cryptoflorescence: Fid-Bau Portal

Creep of clay masonry exhibiting cryptoflorescence

Forth, John P. / Brooks, J. J.

Abstract To determine creep of clay masonry in the laboratory, total time-dependent strain readings are recorded from a wall subjected to a constant load and from a corresponding control wall not subjected to load. Creep is then calculated by subtracting the moisture movement strain of the control wall plus the elastic strain from the loaded wall, from the total strain recorded on the loaded wall. It is assumed that the moisture movement behaviour is the same in both the loaded and the control walls. The paper questions that assumption for certain types of clay brickwork. Small control walls built from low strength bricks having high water absorption can undergo an enlarged moisture expansion. The enlarged expansion is greater than the irreversible moisture expansion of the unbonded clay brick and is thought to be the result of crystallization of salts at the brick-mortar interface, a process known as cryptoflorescence. In a loaded wall, cryptoflorescence is suppressed or restrained so that the effect of an enlarged expansion of the control wall is to overestimate creep and considerably so in some cases. The paper demonstrates the problem by examining the effect of enlarged expansions when ascertaining creep of masonry built from different types of clay bricks. Experimental and design recommendations are made for more reliable methods of determining creep.