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Concrete blocks regardless of manufacturer will dry out and shrink slightly once the building is complete. This is unavoidable and perfectly natural for most construction products based on natural materials. This drying out can sometimes cause very slight differential movement and stresses between building elements and material types, which may result in hairline cracks in blocks or mortar joints.
These cracks are rarely anything more serious than a visual nuisance and are very easy to avoid completely if this movement is accommodated for in the design stage and during the build. The guidelines in this section are industry-standard and have been proven to work for many decades.
Internal walls of low-rise dwellings and houses do not normally require movement joints and the use of bed-joint reinforcement is usually sufficient and movement joints are never usually required below dpc. Movement joints are particularly suitable at the interfaces between dissimilar materials; at intersecting walls for example. For acoustic party walls, vertical movement joints should never be used.
Wherever possible, blockwork should be designed as a series of ‘panels’ separated by movement joints. Generally, length should not exceed 3 x height of the panel, except where bed joint reinforcement is incorporated.
Care must be taken when introducing wide window openings as the blockwork immediately above and below may exceed the 3 to 1 length/height ratio. Where possible storey height openings should be designed, forming the masonry into discrete panels without interfering with the overall design aspect.
General Guidelines for Vertical Movement Control:
Bed joint reinforcement is particularly suitable for houses and low-rise dwellings wherever a vertical movement joint is impractical and for acoustic separating party walls where vertical movement joints are prohibited.
Horizontal movement control can be in the form of steel ladder-type mesh or flexible movement control mesh (particularly designed for use with Airtec thin joint mortar construction).
Horizontal movement control is particularly effective at relieving stresses around areas of point loading such as joists and lintels as well as above and below openings such as doors and windows.
General Guidelines for Horizontal Movement Control
Wall ties should be flexible enough to accommodate the relative movement between both leaves of a cavity wall by stiff enough to transmit axial loads. Stainless steel wall ties should be specified conforming to BS EN 845-2 or BS DD 140-2.
General Guidelines for Wall Ties
Wall Tie Spacing and Positioning:
Thomas Armstrong (Concrete Blocks) Ltd
Part of the Thomas Armstrong Group
T: 01900 68211
F: 01900 602672