Concrete defects in Australian residential construction

What it is

Concrete defects in residential work cover slabs, footings, driveways, paths and suspended elements. The two governing standards are AS 3600 (Concrete structures) and AS 1379 (Specification and supply of concrete). NCC Volume Two adopts both for Class 1 and Class 10 buildings.

Common defect categories

Inadequate cover to reinforcement is the most common structural failure. AS 3600 specifies minimum cover by exposure class. For a slab on ground in inland NSW the minimum is 30 mm. For an exposed coastal slab in WA it can reach 65 mm. When bar chairs are missing or crushed by the crew walking across the mesh during the pour, cover drops to zero. Tribunals send drilled core samples to a NATA accredited lab and order full slab replacement when cover is below 20 mm.

Hot weather concreting is a routine failure across QLD and northern NSW. AS 1379 requires concrete temperature at delivery to stay below 32 degrees and the placing crew to start curing within 15 minutes of finishing. Slabs poured in 38 degree summer afternoons without continuous water spray develop plastic shrinkage cracks within hours. These cracks are non-structural but trigger the two-year warranty.

Cold weather concreting fails in southern VIC and TAS. AS 1379 sets minimum delivery temperature at 5 degrees. Concrete poured below this stops hydrating, stays soft, and never reaches design strength.

Honeycombing appears in footing edges and pier tops where the crew did not vibrate properly. The voids are visible after stripping forms. Surface patching does not fix the underlying structural weakness.

Shrinkage cracking is the cosmetic counterpart. All concrete shrinks as it cures. Control joints at maximum 6 metre spacing handle this. Crews who omit joints get random cracking instead.

Who is liable

Cover failures and strength failures sit inside the six-year major defect warranty in every state. Concrete supplier carries co-liability under the AS 1379 supply chain when delivery dockets show the wrong mix design or temperature. Builder carries the placing and curing obligation. Engineer is co-liable if the slab design did not specify the right exposure class.

NCAT and QCAT decisions on concrete defects consistently order independent core testing as evidence. Where strength is below design the tribunal awards full element replacement plus associated demolition costs.

TradeLens risk flags

Files should be flagged when any of these are missing. Delivery docket showing slump, temperature and mix code matching the engineering specification. Bar chair count and spacing record from pre-pour inspection. Curing method statement (membrane curing compound or continuous moisture). Temperature log on pour day. Control joint plan for paths and driveways. These five drive most concrete tribunal claims.

Typical rectification cost

Patching honeycombing is 1500 to 4000 dollars per element. Cutting in missing control joints is 200 to 400 dollars per linear metre after the fact. Full slab replacement on a single dwelling runs 60000 to 150000 dollars including demolition, disposal and reinstatement. Concrete claims are among the most expensive residential defect categories in AU tribunal data.