Slab-on-Ground Defects in Residential Builds (AU)

What it is

A slab-on-ground defect is any failure of a residential concrete slab that is bearing directly on prepared ground rather than being suspended on piers, beams or stumps. In Australia these slabs are almost always designed under AS 2870, poured against the underlying soil profile, and then loaded by the house frame and roof above. Defects show up as cracking, sloping floors, finish failures or, in the case of post-tensioned slabs, sudden cable rupture. Every Australian residential builder will deal with at least one of these on a build.

The cost of slab defects is usually in the rectification, not the original work. A slab that has heaved 25 mm at one corner is a slab that needs underpinning, pier and pin or full demolition. Statutory home warranty schemes (HBCF in NSW, DBI in Victoria, QBCC in Queensland) regularly settle large claims that started as a missed AS 2870 step at pour day.

The five defect families

Heave

Heave is upward movement of the slab caused by clay soils swelling as they take on moisture. The classic Australian heave pattern is a slab that lifts in the centre or along one edge after a wet winter, opening cracks in interior cornices and walls, sticking doors at the top, and showing diagonal cracks in masonry walls outside. Heave is the most common AS 2870 related failure on reactive H1 and H2 sites where:

• The slab was designed to a lower site class than the ground deserves
• Site moisture conditions were abnormal at pour day and not allowed for
• Plumbing or stormwater leaked under the slab after handover, soaking the reactive clay

Settlement

Settlement is downward movement. It is most often a fill issue. The slab sits on fill that consolidates over time, or on soft natural ground that compresses under the building load. Differential settlement (one corner drops more than the rest) produces the most damage. Visual signs are similar to heave but tend to read as cracks opening at the bottom of walls rather than the top, and door heads dropping rather than rising.

Plastic shrinkage cracking

Plastic shrinkage cracking happens in the first few hours after the pour, while the concrete is still wet. Fast surface evaporation pulls water from the slab face faster than bleed water can rise. Cracks form as parallel lines or random map cracking, typically shallow. Hot, dry, windy weather makes it worse. Plastic shrinkage cracking is mostly cosmetic but can be a sign that curing was inadequate, which raises a flag about long term durability.

Drying shrinkage cracking

Drying shrinkage cracks form weeks to months after the pour as the concrete loses water and contracts. They appear as straight lines, often near re-entrant corners (such as the corner of a room or where the slab steps down to a garage), and where the slab has been over-watered at the surface or finished while bleed water was still present. AS 3600 sets minimum reinforcement to control crack widths; many domestic slabs are reinforced just barely enough, so any extra restraint will crack the slab.

Finish defects

Finish defects sit on the slab surface. They include:

• Crazing (fine map cracking on the surface only)
• Dusting (surface that powders under foot traffic)
• Scaling and delamination (top wear layer lifting away from the slab body)
• Bird baths (dips where water pools, common in garage slabs)

Most finish defects trace back to one of three causes: over-troweling while bleed water is present, premature use of the slab before curing is complete, or chemical curing compound applied unevenly or skipped entirely.

Post-tension cable failures

Post-tensioned (PT) slabs are increasingly common in residential subdivisions on reactive sites. The slab is poured around plastic-sheathed steel tendons which are then stressed after the slab has gained strength. Defect modes specific to PT slabs include:

• Tendon corrosion at anchorages where the grout pack failed
• Cable rupture during stressing from a buried defect in the tendon
• Anchorage blowout if the concrete behind the anchor has voids
• Penetration strikes when a homeowner drills or cores into the slab after handover

PT failures are usually catastrophic. A snapped tendon can travel several metres and rupture floor finishes, walls or wet area linings.

How defects get caught

Slab defects are mostly caught either at slab inspection (before pour) or at final inspection (cracks visible). The middle phase between pour and lockup is where most defects develop unseen. Builders who keep an engineer present at slab inspection, a curing checklist on the day and a six-month post-handover check tend to catch defects before they escalate.

If a homeowner reports cracking inside the statutory warranty period (typically six years in most AU jurisdictions), the builder is on the hook for rectification. Insurance under HBCF, DBI or QBCC covers the homeowner if the builder is insolvent, but premiums and claim records follow the builder.