The CSIRO says a recent study by a joint task force of its sustainable materials engineering and entomology research groups found that concrete slabs complying with Australian Standard AS2870-1996 ‘residential slabs and footings – construction’ can form part of an effective barrier against termites.
Many termite barrier systems, both chemical and physical, incorporate the building’s concrete slab as part of the protection. However, most slabs have some cracks in them caused by movements induced by stress, drying shrinkage or temperature variations.
Even though the practice of using the slab as part of the barrier has been common for years, there was no detailed knowledge of the crack sizes that any particular termite species could penetrate. CSIRO has embarked on field studies to clarify these crack sizes and to determine whether termites can widen existing cracks.
Slabs of strengths ranging from 25 Mpa to as little as 10 Mpa, and with rough sided cracks through the matrix of the concrete and some simulated smooth sided cracks that occur at cold joints, have been used in the study. The slabs incorporate cracks over which a blcok of mountain ash protected by a cage of Termi-Mesh is placed, so that the only way of gaining access to the wood is through a crack in the slab.
Tt has been shown to date, that a crack width of greater than about 1.4mm is required for termites to pass through a slab. This width seems to be the same for both the smallest termites and for mid-sized termites. To date, none of the termite species has penetrated slabs made with 15b Mpa or stronger concrete by widening the cracks through the concrete. However, one simulated cold joint crack was widened in the weaker 10 Mpa concrete, a strength that is not usually specified for the construction of concrete slabs. Further, the concrete along the sides of this crack had suffered from leakage of cement paste from the formwork when the slabs were cast. This would result in even weaker strength concrete on the sides of the slabs where the termite widening of the crack had occurred. This means that the strength in the attacked areas would have been below that of the 10 Mpa concrete that the slab had been cast.