B. W. ADDERSON and J. M. HART Building Research Establishment

Wall ties play a key part in ensuring the structural integrity of a wide range of masonry and masonry-clad buildings. Omission or ineffectual bonding of ties or the use of insufficiently durable ot inappropriate types of tie is a frequent fault in a wide range of building types and has resulted in the need to carry out very expensive remedial work and, in rare cases, collapses. The recent trend towards wider cavities to accomodate insulation has increased the risk of insufficiently long ties being specified or used. Although electrical detectors can give an indication of the presence/absence of ties what is wanted is a non-destructive test which will give an indication of condition and effectiveness of ties.
Infra-red thermography is capable of identifying “hot spots” in walls and thus indicating the position where the tie is acting as a thermal bridge across the cavity. Obviously there will be no hot spot if the tie does not bridge because of corrosion, omission or because it is only buried in one leaf. A reduced spot is given if wire ties have been substituted for strip ties. This offers a potential non-destructive check on workmanship and a means of surveying existing structures for provision and condition. The current programme is aimed at establishing the necessary techniques and the base data such as heat transfer rates, detection limits, threshold thermal gradients/temperature differential, etc., for normal cavity masonry walls.