The Ultimate Guide to Using the Correct Concrete Mix for Building Houses

When it comes to building houses, getting the concrete mix right is non-negotiable. At Unitech SA, we know from decades of experience that the correct mix underpins every safe, durable, and cost-effective structure. A poorly chosen mix can mean cracks, moisture problems, or even a compromised foundation. The right mix, on the other hand, ensures strength, longevity, and peace of mind for the homeowner.

This guide shares how we at Unitech SA select and deliver the correct concrete mix for residential foundations, slabs, and structural elements—tailored to Gauteng’s conditions.

Why the Right Mix Matters

We’ve seen first-hand the costly impact of the wrong ratios. Too much water, poor aggregate grading, or inadequate curing can weaken the entire structure. For us, getting it right means:

• Load-bearing safety: transferring the full weight of the house evenly into the soil.

• Durability: resisting cracking, moisture ingress, and long-term wear.

• Compliance: meeting South African standards (SANS 50197-1) and local municipal requirements.

When we handle your build, we specify the exact mix strength and curing method based on soil tests, climate, and structural design—not just generic ratios.

Our Approach to Cement Types

South Africa follows the CEM classification system under SANS standards. What many call “ordinary Portland cement” is sold here as CEM I.

At Unitech SA we typically work with:

• CEM I 42,5N or 42,5R – general foundations, slabs, and house footings.

• CEM I 52,5N – for projects requiring high early strength.

• Blended cements (CEM II, CEM III) – when improved workability or sustainability is needed.

We match the cement type not only to strength, but also to site environment—for example, blended cements where sulphate resistance is helpful.

Concrete Mixes We Use

Standard Ready-Mix Concrete

We prefer ready-mix for most residential projects because it guarantees consistent quality. Our suppliers batch to spec, we test every delivery, and we control slump for proper placement.

What you get with our ready-mix:

• Uniform strength and composition.
• Time savings on site.
• Fewer labour risks compared to hand mixing.

In Gauteng, for residential house foundations, we generally specify 25 MPa–30 MPa at 28 days, depending on soil conditions. For poor soils or double-storey loads, we move to 35 MPa and adjust reinforcement.

Lightweight Concrete

For floors and roofs where load reduction matters, we use lightweight concrete mixes. They help reduce dead loads and improve thermal performance, while still meeting strength requirements.

Factors We Consider Before Specifying a Mix

When you build with Unitech SA, we don’t guess the mix. We engineer it around three main areas:

1. Environmental conditions – Johannesburg’s hot summers mean we allow for faster drying. We adjust with retarders or curing methods to prevent early cracking.

2. Soil conditions – Expansive clays or high water tables require stronger mixes (30–35 MPa) and detailed drainage solutions.

3. Reinforcement strategy – Steel rebar, mesh, or fibres are selected based on structural loads. We always combine reinforcement with the right mix to maximise tensile strength.

Testing and Quality Control on Our Sites

Every load is checked, because concrete is only as good as its control.

• Slump tests: On site, we test to ensure workability sits in the 75–100 mm range—ideal for house foundations and slabs.

• Cube tests: We cast test cubes and send them for lab curing and crushing at 7 and 28 days. This confirms the concrete is reaching the design strength (25–35 MPa as specified).

• Moist curing: Our teams ensure 7 days minimum curing, often with curing blankets or continuous dampening, so the concrete achieves its full potential.

Why Choose Unitech SA?

Unlike general builders who may rely on rules of thumb, we design every concrete mix for the actual conditions of your site. Our engineers and site managers:

• Assess soil type and drainage before specifying strength.
• Match cement type and additives to your environment.
• Test every load for compliance.
• Deliver a foundation that keeps your investment safe.

In short, your house rests on more than concrete—it rests on our expertise.

What strength concrete do you use for house foundations in Johannesburg?

At Unitech SA we typically use 25–30 MPa concrete for standard foundations. For weak soils or heavier structures, we specify 35 MPa or higher.

What slump range do you target?

We control slump between 75–100 mm—workable enough for placement, stiff enough to avoid segregation.

How long do you cure house foundations?

We cure for at least 7 days using blankets, dampening, or curing compounds. This prevents early cracking and ensures strength gain.

Do you still use a 1:3:6 ratio?

Not anymore. We specify by MPa strength class and supplier batch ticket, not old ratios. This guarantees consistency and compliance with SANS.

What type of cement do you use?

We generally use CEM I 42,5N or 42,5R, which is the South African equivalent of ordinary Portland cement. For high-strength needs we use CEM I 52,5N.

See our full portfolio of home renovations and building projects here.