Moisture Movement

Moisture movement falls into two categories: irreversible movements as new materials acclimatise to the environment; and reversible cyclical movements due to climatic variations. Many building materials, especially concrete and mortars, exhibit an initial contraction during the drying-out process. Incorrectly seasoned timber will also shrink but new bricks used too quickly after manufacture will expand. After these initial effects, all materials which absorb moisture will expand and contract to varying degrees in response to changes in their moisture content. Depending upon climatic conditions, moisture and thermal movements may oppose or reinforce each other. Typical irreversible and reversible moisture movements are shown in Table 16.2.

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