Concrete Slab Preparation: Why It's the Most Important Part of Any Pour

At Concrete Concepts Group, we often say that 80% of a great concrete job happens before the concrete truck arrives. Preparation is everything. A perfectly poured slab on poorly prepared ground will crack, sink, and fail. A well-prepared base with average finishing will last decades.

Here's what goes into proper slab preparation — and why cutting corners here always costs more in the long run.

Step 1: Site Assessment

Before we dig, we need to understand what we're working with:

• Soil type — Brisbane's reactive clay soils expand and contract with moisture changes. This affects how deep we need to dig and what sub-base is required.
• Drainage — Where does water flow? We need to ensure water drains away from the slab, not under it.
• Services — Underground water, sewer, gas, and electrical lines must be located and protected.
• Access — Can the concrete truck get to the pour site? Do we need a concrete pump?

Step 2: Excavation

We excavate to the required depth, typically:

• House slabs: 300-400mm below finished level
• Shed slabs: 200-300mm below finished level
• Driveways: 250-350mm below finished level

The excavated area must be level and compacted. We use a plate compactor to achieve the required density — typically 95% Standard Proctor Density for residential work.

Step 3: Sub-Base

A proper sub-base is critical for Brisbane's clay soils:

1. Road base (crushed rock) is spread to a minimum depth of 100mm
2. The road base is compacted in layers to create a stable, level platform
3. A vapour barrier (plastic membrane) is laid over the road base to prevent moisture rising through the slab

Step 4: Formwork

Formwork creates the shape of your slab. We use:

• Timber formwork for straight edges
• Flexible formwork for curves
• Steel pins to hold the formwork at the correct height

Every edge is checked with a laser level to ensure the slab will be perfectly flat (or have the correct fall for drainage).

Step 5: Reinforcement

Steel reinforcement prevents cracking and holds the slab together:

• SL82 mesh is standard for residential slabs (8mm bars at 200mm centres)
• SL92 mesh for driveways and heavier loads
• Trench mesh in edge beams and footings
• Bar chairs lift the mesh to the correct position within the slab

The mesh must be at the right height — too low and it does nothing, too high and it can rust.

Step 6: Final Check

Before we call the concrete truck, we do a final check:

• All formwork is level and secure
• Mesh is at the correct height on bar chairs
• Vapour barrier is intact with no tears
• Any penetrations (plumbing, electrical) are in place
• The site is clean and ready

Why This Matters

We've seen countless jobs where another concreter has skipped steps — no road base, mesh sitting on the ground, no vapour barrier. The result is always the same: cracking within 12 months, sinking corners, and moisture problems.

Proper preparation costs a little more upfront but saves thousands in repairs. Every slab we pour at Concrete Concepts Group follows these steps — no shortcuts, no exceptions.

Ready to discuss your project? Get a free quote or call Jarrod on 0424 698 986.