Nevertheless, the harmful effects of water on building materials can hardly be over-emphasized. If they did not exist, the construction of durable buildings would be greatly simplified and the task of the designer made much easier. This Digest is intended to draw attention to these destructive mechanisms and to give a brief account of some of the phenomena involved. With a change in moisture content many building materials show considerable change in dimension, the magnitude of which may be greater than that caused by normal temperature variations. Table I lists the percentage change in length of various materials on immersing a dry specimen in water, together with the thermal expansion for a 100 F deg temperature rise for comparison.
Lightweight aggregates give higher expansions.
Volume changes in brick masonry materials were the subject of study by Palmer of the National Bureau of Standards in 1931. Similar differential dimensional changes, with change in moisture content, can take place in two materials bonded together. A warping effect may be produced similar to that produced on a bimetallic strip by changes in temperature. Precast concrete panels, for example, which have a facing material of a composition different from the backing, may be subject to such warping. A differential moisture content through the thickness of a homogeneous material will also have a warping effect, since the side of higher moisture content will expand more than that of the lower.