Your slab is the one part of your house you can't easily fix if it goes wrong. Everything else sits on it. Knowing a little about what concrete is and how it gains strength helps you spot the shortcuts that cause cracking, dusting and weak slabs.
C5 - Pre Pour Slab Inspection Checklist
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Last updated 5 Oct 2026 · Revision 1 · Free updates for life
Checklist Type: Major (Base Stage)
Form Factor: Fillable, print-ready PDF (A4, 17 pages)
When to use: Base stage: after the slab is prepped, before concrete is poured
A 98-point pre-pour slab checklist. Check set-out, levels, reinforcement, services and formwork after the slab is prepped and before concrete is poured, while fixes are still simple.
The ingredients
Concrete is a simple recipe:
- Coarse aggregate (crushed rock or gravel, usually up to 20 mm) and fine aggregate (sand). Together they make up roughly 60 to 80% of the volume and give concrete its bulk and much of its strength.
- Cement, usually general purpose cement or a blend that includes fly ash or slag. It's only around 10 to 15% of the mix but it's the active ingredient that binds everything together.
- Water, which reacts with the cement. The amount relative to the cement (the water to cement ratio) has a big effect on strength.
- Admixtures, chemicals added at the plant to adjust workability or setting time, such as water reducers, plasticisers, and retarders for hot days.
Steel reinforcement (mesh, bars and trench mesh) isn't part of the concrete, but it's what gives a slab its tensile strength. Concrete is strong in compression and weak in tension, so the two work together.
How concrete hardens
Concrete doesn't dry out to harden. It hardens through hydration, a chemical reaction between the cement and water that forms crystals binding the aggregate together.
That's why water matters so much:
- Too much water in the mix makes concrete easier to pour but weaker, more porous and more prone to shrinkage cracking.
- Too little water during curing stops the reaction early, so the surface never reaches its proper strength and can end up dusty and crack-prone.
Strength grades for house slabs
In Australia, concrete is specified by its compressive strength in megapascals (MPa) at 28 days. A house slab is typically:
- N20: 20 MPa, the minimum for house slabs under the NCC Housing Provisions.
- N25: 25 MPa, common for slabs and footings, often specified by engineers.
- N32: 32 MPa, used for more aggressive soils, coastal exposure or particular engineering designs.
Your engineer specifies the grade on the slab drawings, designed under AS 2870 for your site's soil classification. Make sure what's ordered matches.
Concrete reaches roughly 65 to 70% of its strength at 7 days. It's specified by its 28-day strength, but it keeps gaining strength slowly for months after that, as long as there's moisture to keep the reaction going.
What to check on pour day
Homeowners don't mix concrete. It turns up in a truck, already mixed. Your job on pour day is to make sure what arrives is what was ordered, and that it isn't watered down on site.
- Check the delivery docket. Each truck comes with a docket showing the grade (for example N25), the slump, the aggregate size and the time it was batched. Match it to the engineering drawings.
- Check the slump. Slump measures how runny the concrete is. Around 80 to 100 mm is typical for house slabs as ordered.
- No water added on site. Adding water from the hose to make concrete easier to spread is the classic shortcut. It weakens the concrete, and the NCC Housing Provisions say water must not be added to raise the slump above what was specified. If you see it happening, ask questions and take photos.
- Check timing. Concrete should be placed within a reasonable time of batching. A truck that's been sitting around for hours in summer is a problem.
- Proper compaction. Concrete should be vibrated to remove air pockets, especially around beams and edges.
Before any of that, the slab itself needs inspecting: reinforcement, bar chairs, vapour barrier, termite protection, plumbing penetrations and edge beams. Our C5 Pre Pour Slab checklist covers what to check, and C6 Post Pour Slab covers the finished slab. Both are at constructor.net.au/checklists.
Curing: the step most often skipped
Curing means keeping the concrete moist so hydration can continue. It's the cheapest step in the process and the one most often ignored.
The NCC Housing Provisions call for slab surfaces, including edges, to be moist cured for 7 days. On site that usually means one of:
- Keeping the surface wet with sprinklers or regular hosing.
- Covering it with wet hessian or plastic sheeting.
- Spraying on a curing compound straight after finishing.
Curing should start the same day as the pour. In Queensland summer heat and wind, a slab left uncovered can lose surface moisture within hours, and that's when plastic shrinkage cracks and crazing appear.
Things that affect curing:
- Temperature. Hot weather speeds early setting but dries the surface faster. Cold weather slows strength gain.
- Wind and sun. Both pull moisture out of the surface.
- Cement type. Blended cements with fly ash or slag gain strength more slowly early on but often end up more durable, so good curing matters even more.
Useful properties of concrete
- Thermal mass. Concrete stores heat and releases it slowly, which evens out temperature swings inside the house. It is not an insulator, though: heat passes through it readily, so slab edge insulation matters in cooler climates.
- Fire resistance. It doesn't burn.
- Sound. It's dense and heavy, which helps reduce noise transfer.
- Durability. Properly mixed, placed and cured concrete lasts for decades with little maintenance.
The bottom line
Concrete is simple stuff, but the shortcuts are easy to take and hard to see later. Check the docket, keep the hose out of the truck, and make sure curing starts the day of the pour.
Test yourself
Five quick questions on this topic. Click a question to see the answer.
What is the primary role of aggregate in concrete?
To provide strength and bulk.
Why is the water-to-cement ratio carefully controlled in concrete mixing?
It affects strength and workability.
Why is it important to keep concrete damp during the curing process?
To allow for maximum hydration.
What is the main chemical reaction involved in the hardening of concrete?
Hydration.
What property of concrete helps with soundproofing?
Its density and heaviness.
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