Cracking in concrete and masonry from Electronic Blueprint

This article discusses cracking in buildings, with particular emphasis on reducing the cracks in concrete slabs and associated masonry walls. Such cracks are unsightly, and may permit water to enter the building fabric, causing pest infestation, mould, efflorescence and staining. The prolonged drought in Australia caused a significant increase in the incidence of cracked buildings, particularly housing, due to the drying out of foundation soil. Although the drought has now "broken," cyclical periods of wet and dry will exacerbate soil movements over the years to come.

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Crack Limits in Australian Standards - Concrete and Masonry
The relevant Australian Standards for considering cracking in buildings are:

• AS 2870-1997 Residential slabs and footings.
• AS 3661.1-2000 Termite management Part 1 New building work.
• AS 3700-2001 Masonry structures.

The performance requirement for residential dwellings is set out in AS 2870 Clause 1.3. This states that the footing systems used to support the masonry walls shall be such that there is "usually no damage, a low incidence of damage category 1" [Fine cracks which do not need repair, Crack < 1 mm] "and occasional incidence of damage category 2." [Cracks noticeable but easily filled. Doors and windows stick slightly. Crack < 5 mm].

Cracking of Suspended Concrete Slabs
When a concrete slab is suspended, it will bend under the action of its self weight and any imposed gravity loads. This will cause cracks to form at the top of the slab over supports and at the bottom of the slab at the centre of the span. It is at these locations the main tensile reinforcement is placed. There is other reinforcement placed in concrete slabs and beams to control shrinkage cracking, support the main reinforcement and to control diagonal shear cracking near supports.

Cracking of Concrete Slab-on-Ground
When a concrete slab is placed on the ground, it is initially supported uniformly by the soil. Steel fabric reinforcement is placed in the top face to control shrinkage. Reinforced concrete edge beams and/or footings are incorporated to strengthen and stiffen the slab, to resist cracking if the foundation soil should shrink or expand around the edges of the slab.

• AS 2870 Appendix C defines the limiting crack widths fro concrete slabs.
• To prevent termite penetration, AS 3661.1 places a limit of 1 mm on the permissible width of cracks.

Cracking of Masonry
As the foundations shrink or swell and the concrete footings, beams and/or slabs respond, the following can occur.

• Unreinforced masonry superstructures of weak masonry will form multiple small cracks.
• If an unreinforced masonry superstructure has relatively high strength, it will remain intact for small movements, but will eventually crack. Most likely, this will be a single large crack, the most undesirable outcome.
• Single leaf reinforced hollow concrete masonry superstructures built integrally with the concrete footings, incorporating steel starter bars, vertical "wide spaced" reinforcement and a continuous horizontal bond beam, are capable of cantilevering and spanning large distances without cracking.

AS 2870 Appendix C specifies crack limits for walls, including brickwork and blockwork. These limits relate to the overall performance of the building, and are not necessarily relevant to the water resistance of the building. AS 3700Clause 2.5.2 .1 places an upper limit on crack width for masonry of, "1 mm for "masonry which is not subject to aesthetic limitations". Because it is often difficult to assess the crack widths which result from particular load actions (e.g. wind, earthquake and foundation movement), the 1 mm limit is not intended to cover all situations. However it does define a quantifiable limit for use in design, construction and post-construction assessment.