Brickwork and Paving by Michael Hammett

Author:Michael Hammett [Hammett, Michael]
Language: eng
Format: epub
Publisher: Crowood
Published: 2013-10-02T21:00:00+00:00

Movement Control

Although masonry seems to be robust and static, it does move very slightly in response to changes in moisture content and temperature. Clay bricks are fired at very high temperature during manufacture and for a number of years afterwards they expand very slightly as moisture from the atmosphere is taken into the molecules of the material. In addition, the brickwork also expands and contracts as its temperature rises and falls in response to environmental changes. The net effect of these movements is a slight expansion over years.

In brickwork of modest dimensions, stress caused by this expansion is accommodated without problems, but brickwork longer than about 12m (40ft) should be interrupted by movement joints. These are vertical joints that are continuous for the full height of the wall. They separate the wall into discontinuous lengths to prevent cumulative stress of expansion that could cause cracking, movement and possible instability. In detached houses, the dimensions of uninterrupted clay brickwork are rarely so large that they require movement joints.

The standard recommendations are that the joints should be located at not more than 6m (20ft) from a corner or end and the distance between them should not exceed 12m (40ft). If there are changes of level in the length of the wall, movement joints can conveniently be located at those positions. Short returns in brickwork can be rigid and if they interrupt long straight runs they can concentrate stress and cause cracking. Treat short returns as corners, even if they are only a half brick offset.

Some brick manufacturers recommend more frequent movement joints in new free-standing walls. This is because free-standing walls have less structural restraint than the walls of buildings because they are not subjected to vertical loading and they have little self weight, particularly if they are low in height. Also they are generally comparatively thin and built with cement-based mortar. Furthermore, some clay bricks have greater potential for moisture expansion than others and although the differences are not great, more frequent movement joints are considered advisable when using them. A manufacturer’s advice could be obtained for specific proposals, but as a general guide the following provisions are recommended. When using soft-mud, stock bricks and fletton bricks, the standard guidance can be followed, but when using low water absorption, engineering types of brick, reduce the distance of joints from ends and corners to 4m (13ft) and spacing between joints to 8m (26ft).

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