Masonry facades

An increase in moisture content of a material is usually accompanied by a volumetric change. Two types of movement need to be considered:

• irreversible or permanent changes (usually occurring during the early life of a component); and
• reversible changes.

Irreversible changes usually affect clay brickwork because during the burning process moisture is removed from the brick. Once the brick cools the moisture content gradually increases until it reaches equilibrium with its surroundings. Most movement of this kind occurs within the first 3 months of the life of the brick.

Reversible changes are cyclical and take place as a result of daily or seasonal changes in moisture content.

Unlike the temperature based changes of most materials, movements due to moisture change are not necessarily the same in each axis. Furthermore, moisture changes are not necessarily uniform throughout the thickness of a material - the outer surface may be saturated while the inner surface remains dry, thereby imparting a moisture and stress gradient through the material.

According to BRE Digest 228 Part 2 (1979) an estimation of the percentage increase (or decrease) is given by:

(Factor x dimension)/100 = R (change in size)

Factors are listed in the Digest and for brickwork, irreversible expansion is shown as +0.02 to +0.07, while reversible moisture movements are shown as 0.02. So, for a brick panel of length 6,000mm, the possible change in length occurring between extreme wet and extreme dry conditions could be:

(0.07 x 6000)/100 = 4.2mm (irreversible expansion)

(0.02 x 6000)/100 = 1.2mm (reversible movement)

The example above assumes the highest figure given for brickwork in Digest 228. If the lower factor (0.02) is used a smaller irreversible expansion will be estimated.

Calcium silicate brickwork, unlike clay, usually undergoes an initial irreversible shrinkage on laying (clay brickwork tends to expand). So long as the propensity for movement is understood and catered for in the design, there is no reason why the brickwork should not perform adequately. Calcium silicate bricks often exhibit reversible movements of between 0.02-0.2% and irreversible shrinkage of 0.02-0.06%.

Calcium silicate bricks should not be used in solid work with clay facings or backings. This is due to the propensity of the bricks to shrink in contrast with the expansion of clay brickwork. If solid walling is being contemplated, backings of concrete bricks or blocks should be used, as these have similar movement characteristics to calcium silicate bricks.

In cavity work, if differing materials are used, and providing the cavity is in excess of 50mm, there should be sufficient flexibility in the ties to permit the differing movements. However, care must be taken to ensure discontinuity around cavity closers to prevent cracking.

The estimation of dimensional changes in brickwork is rarely as straightforward as making the calculations described above. Expansion due to rise in temperature may be partially offset by a reduction in moisture content and subsequent shrinkage. Furthermore, movement in brickwork is often restrained, and considerable frictional restraint can be provided by mortar bed joints. Since masonry is weak in tension, cracking can occur instead of contraction of the whole panel.

However, the calculations do produce a guide as to what is likely to be the dominant movement effect, and this information can help to explain movement or crack patterns in brickwork. If movement is arising from distinctly separate causes, the total movement can be assumed to be the sum of the moisture and thermal movements.