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Movement control is an important aspect in achieving a visually acceptable masonry wall. All concrete products shrink and unless provision is made to control this movement unsightly, although not usually structurally important, cracking may occur. Where possible masonry should be designed as a series of panels, separated by movement control joints. Ideally panels should be square, but in general the length should not exceed 3 x the height. The inclusion of full panel height openings and/or the inclusion of bed joint reinforcement can assist in lowering the risk of un-planned cracking.
As a rule of thumb, it is normal to consider movement joints at approximately 6 m centres in straight lengths of autoclaved aerated concrete block walls. Greater distances between movement joints (9 m and sometimes longer) are possible in aggregate concrete blockwork walls depending on the location and block type. Block manufacturers should be consulted for guidance. At corners, consideration should be given to the effect of restraint and may require the first movement joint being positioned closer to the corner than the general rule of thumb suggests.
There is an increased risk of cracking at changes in thickness, loading and height. Where movement joints cannot be sensibly located, the inclusion of bed joint reinforcement should be considered, particularly above and below openings and extending beyond the opening limits.
Acknowledgement: Concrete Society