Slab on Ground

A slab-on-ground is a concrete floor cast directly on prepared ground, usually over a vapour barrier and compacted fill. In houses it is often also the footing, stiffened with edge and internal beams. In warehouses and industrial buildings it is a heavy-duty working surface carrying racking, forklifts and stored goods, with joints to control cracking and surface tolerances set for the equipment that runs on it.

Warehouse floors are increasingly specified for flatness because very narrow aisle forklifts and tall racking need level surfaces; a floor that was fine for its original use can be inadequate for a new tenant's equipment.

What shortens life:

• Hard-wheeled forklifts crossing open joints, which break the edges.
• Loads beyond design: heavier racking, point loads, storage.
• Soil movement beneath the slab: reactive clay, fill settlement, leaking pipes.
• Moisture rising through slabs without vapour barriers.
• Chemicals: acids, sugars and oils attacking concrete surfaces.

The slab itself outlasts almost everything on it; the joints, surface and the ground beneath decide how long it stays serviceable for its use.

Shrinkage cracking

Concrete shrinks as it dries. Fine random cracks and cracks at re-entrant corners are normal; they matter only if they widen, displace or let water and termites through.

Curling

Slabs dry faster at the top than the bottom, so panel edges lift (curl). Joints then rock under traffic, break down and crack the slab beneath the curled edge.

Joint deterioration

Forklift wheels hammer open joints, breaking the arrises (edges); sealants fail; load transfer dowels are missing or ineffective so panels move independently. Broken joints damage forklifts and goods and become trip hazards.

Settlement and heave

Poorly compacted fill settles; reactive clay shrinks and swells; leaking under-slab pipes soften the ground. Slabs crack, dish or dome, and pull away from walls.

Moisture and flooring failure

Moisture vapour rising through slabs without vapour barriers, or with damaged ones, debonds adhesives, swells timber and blisters vinyl and epoxy coatings. It is a common cause of flooring failure when old slabs get new finishes.

Post-tensioned tendon damage

Tendons in post-tensioned slabs carry very high forces. Drilling or cutting into them during alterations can release them violently and weaken the slab; anchorages corrode where water reaches them.

Surface deterioration

Dusting, scaling and delamination of weak or poorly finished surfaces; chemical attack in food and industrial plants; abrasion in heavy-traffic aisles.

Termite entry

In termite regions, cracks, slab edges and service penetrations are hidden entry routes for subterranean termites into wall framing.

On site

• Crack pattern: fine random, at re-entrant corners, along a line, or widening and displaced.
• Joints: broken arrises, rocking panels, failed sealant, trip edges.
• Floor levels: dishing, doming, steps; racking out of plumb.
• Damp patches, efflorescence and blistered or debonded floor finishes.
• Gaps between slab and walls; skirtings lifting.
• Termite mud tubes at slab edges and penetrations.

In the records

• Slab design and loading, including racking and forklift loads.
• Flatness specifications and surveys.
• Post-tensioning drawings.
• Floor finish failure history.
• Termite inspection records.

• Forklift operators avoiding certain joints or aisles, and goods damaged in transit across the floor, which are early operational signs of joint failure.

Cracks that widen, displace or step across their length are moving; cracks that stay hairline and even are not. Gauge studs across a few representative cracks for several months settle the question.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A structural or pavement engineer, with a flooring specialist for moisture problems.

• Crack and joint mapping across the floor.
• Level and flatness survey against the tolerance the use requires.
• Joint condition: arris damage, load transfer, rocking under load.
• Moisture testing by in-situ relative humidity probes before new flooring.
• Cores for thickness, strength and reinforcement.
• Scanning by ground-penetrating radar for tendons and services before cutting.
• Under-slab services: CCTV and pressure testing where leaks are suspected.
• Load assessment against racking and equipment loads.

The report should give the condition, flatness and capacity against the use, causes of distress, and repair options.

• Joint movement monitoring with gauges where panels appear to be moving, to separate curling from settlement.
• Termite inspection at slab edges, penetrations and expansion joints in termite regions.

• Joint repair: rebuild broken arrises with polymer or epoxy mortars, install steel armoured joints in heavy-traffic aisles, and fill with semi-rigid fillers.
• Curl correction: grout under lifted panels and grind high edges.
• Crack repair by routing and sealing or epoxy injection.
• Slab lifting and stabilisation by grout or polyurethane injection.
• Grinding and toppings to restore flatness.
• Moisture mitigation with surface membranes before new flooring.
• Local replacement of broken panels.
• Termite management at slab edges and penetrations.

• Load management: relocate heavy racking or point loads away from weak areas, add baseplates to spread loads, or restrict loads until strengthening.
• Coatings and hardeners renewed in chemical and abrasion zones to protect the surface.
• Under-slab pipe repair by relining or rerouting above the slab, avoiding breaking out the floor.

Surveys are lump sums. Joint repairs are priced by linear metre; armoured joints by length; panel replacement by area; grinding and toppings by area; moisture membranes by area. In operating warehouses, out-of-hours work and racking relocation add significantly.

Joint and surface renewal are repairs and maintenance or lifecycle; tenant-driven flatness upgrades sit with the tenancy; safety fixes are immediate.

Repairs timed around tenant shutdowns or between leases are usually far cheaper than work in a fully operating warehouse.

• Australia. AS 2870 governs residential slabs and AS 3600 concrete structures; Concrete Institute and CCAA guides cover industrial floors; AS 3660.1 covers termite management at slab edges and penetrations.
• United Kingdom. The Concrete Society's TR34 sets industrial floor design and flatness; BS 8204 covers screeds; BS EN 1992 concrete design.
• United States. ACI 360 covers slabs on ground and ACI 302.1 floor construction; PTI publishes design of post-tensioned slabs on expansive soils; ASTM F2170 sets in-situ moisture testing.

In all three jurisdictions, warehouse leases commonly specify the floor loading and flatness a tenant may rely on, and fit-out works that cut or core the slab need engineering approval. Racking installers and operators also have safety obligations for racking on uneven or damaged floors under workplace safety law, which can force floor repairs at short notice.

• Slab design, loading and flatness specification.
• Post-tensioning drawings.
• Flatness and level surveys.
• Floor finish failure history.
• Termite inspection records.
• Racking and forklift specifications.
• Previous repair records.

• Lease clauses on floor loading and flatness.
• Racking inspection reports noting floor issues.
• Records of coring, cutting or slab penetrations.