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Joints are necessary to minimise cracking in slabs. In the USA and UK the maximum joint spacing for unreinforced slabs is typically 4.5m but there are several variations to the ´rule of thumb´ for control joint spacing in relation to the slab thickness.
In the USA, joints are commonly spaced at distances 24 to 30 times the slab thickness whereas the Concrete Advisory Service often quote up to 40 times the thickness between joints. This is presumed to relate to Technical Report TR34 Concrete Industrial ground floors although is not stated. The first edition (1988) and associated publications of the time considered unreinforced slabs of 100 to 150mm thickness and maximum distance of 4.5m between joints. Joint spacing in nominally reinforced slabs (or to be more precise sawn-restrained movement joints) could be up to 10m, but with a risk of mid-bay cracking. TR34 sets a maximum distance between control joints in nominally reinforced slabs of 6m and a minimum slab thickness is 150mm, hence a maximum ratio of 1:40.
Why the difference? Possibly because TR34 relates to internal slabs and requires a well prepared and trimmed sub-base surface with a polythene separating layer. This is important to minimise surface restraint to concrete as it dries and shrinks, allowing the joint distance to be greater. Where the sub-base is less controlled and separating layer is not advocated, the base restraint is greater and therefore the need for closer ‘control joints’.
The other rule of thumb to consider is the ratio of length to width (aspect ratio) of the unreinforced and nominally reinforced slabs; the more slender the slab, the greater the risk of mid-bay cracking. Ideally the slab should be as square as possible, i.e. an aspect ratio near 1:1, although up to 1.5 is acceptable.
Acknowledgement: The Concrete Society