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Paving: Concrete - Limestone Concrete

Published: 29 July 2011

CONTROL JOINT SPACING AND REINFORCEMENT

“Limestone concrete” or “liquid limestone” as is commonly called is used to achieve a decorative (colour and pattern) paving surface. Whilst it is a pre-mixed material, it is a Special (S) mix that is likely to have a lower strength (probably 15MPa) than Normal (N) concrete (20MPa and higher).

AS 3727 Guide to Residential Pavement is the nearest applicable “guide” to “recommended practice”. Whilst AS 3727 only refers to concrete with an N20 or higher grade, it does reference work practices related to pre-mixed material.

Specification Note

The Specification of Nominal Building Work uses the following descriptions for: Concrete / Paving:

Materials: When a Special (S) cream cement mix is used it will not comply with AS 1397 strength requirements but will be properly batched and delivered.

Laying: Control joints will be located at nominal spacings and a foam rubber expansion joint will be installed at wall junctions in the Garage, Store and the like.

Whilst no cracking is the ideal outcome, AS 3727 sets serviceability limits that allow for cracks up to 1.5mm wide. Both AS 3727 and AS 2870 Residential Slabs and Footings recognise that minor movement and cracking will occur on most sites and, whilst the intention is to reduce the risk of cracking, the nominated “recommended practices” do not guarantee that cracking will not occur.

Once laid concrete looses moisture or “dries” and shrinks quicker than it develops strength. Control joints divide the paved area into smaller panels and create “lines of weakness” at which shrinkage cracking is promoted.

In addition to control joint spacing and/or the cost of reinforcement material, “recommended practice” will also require additional set-out work to position and separate reinforcement panels, isolation joints at and around built in objects (additional to control joints), and a caneite rubber foam (or similar) expansion joint filer between the limestone concrete and abutting structures such as walls and steps.

For the builder, serviceability (cracking) disputes are typically harder to resolve if cracking occurs after concrete paving has been laid outside “recommended practice”.

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