Concrete Slabs: Thickness, MPa, Reinforcement

Slab thickness, reinforcement and concrete strength work together as a system. Changing one without adjusting the others is how slabs crack under load or deflect under span.

Concrete Slabs: Thickness, MPa, Reinforcement — practical guidance from the team running concrete slab work on the Garden Route. Below is how the three variables interact, what the common thicknesses and mixes are actually for, and the detail that decides whether a slab cracks in its first summer.

The three variables, and why they move together

A concrete slab has to do two things: carry the load imposed on it, and stay flat while it does. Three things decide whether it manages that — how thick it is, how strong the concrete is, and where the steel sits inside it. They are not independent choices.

Thickness governs stiffness and load spread. Double the thickness of a ground slab and you roughly quadruple its ability to bridge a soft spot in the subgrade. This is why the answer to a poor subgrade is usually a thicker slab or a better base, not a stronger mix.

Strength, given in MPa, is the compressive strength of the concrete at 28 days. It governs how much load the concrete can take in compression, how well it resists abrasion, and — importantly on the coast — how dense and impermeable it is. A 30 MPa mix is not just stronger than a 20 MPa mix; it is tighter, and it lets less salt-laden moisture through to the steel.

Reinforcement does almost nothing for a slab in compression. Its job is tension: holding the slab together where it wants to bend or shrink apart. Mesh in a ground slab controls shrinkage cracking and keeps any crack that does form tight. Bars in a suspended slab carry the actual bending load, and their position within the depth is what makes them work.

Get the relationship wrong in either direction and it shows. A thin slab with a rich mix still deflects. A thick slab with the mesh lying on the ground does nothing that a thick slab without mesh would not have done.

The three slab types you will actually encounter

Ask what types of slabs exist and you get a textbook list of a dozen. On a Garden Route residential or light commercial job, three cover almost everything.

Ground-bearing slabs sit directly on prepared fill and transfer load straight into the ground. Garage floors, house floors, patios, workshop floors. The ground carries the load; the slab spreads it. The critical work is under the slab, not in it.

Suspended slabs span between supports — walls, beams or columns — and carry their load in bending. First floors, roof slabs, slabs over basements. These are engineered, always, and the reinforcement design is the whole job.

Rib-and-block slabs are the common suspended alternative in South African residential work: precast concrete ribs with hollow blocks between them and a structural topping poured over. They are lighter, need less propping and go up faster than a solid cast-in-place slab, which is why so many local first floors use them.

Slab type	Load path	Typical use	Design input needed
Ground-bearing	Straight into the subgrade	Floors, garages, patios, workshops	Soil assessment; engineer if loads are heavy
Suspended, cast in place	Bending, into walls or beams	First floors, roof slabs, over basements	Always engineered
Rib-and-block	Bending, via precast ribs	Residential first floors	Supplier design plus engineer sign-off

How thick, and where the common numbers come from

There is no single correct slab thickness, but there are conventions, and they exist for reasons worth understanding.

For a domestic floor on decent ground, 100 mm with mesh is the long-standing default and it works. Move to a driveway or a garage taking a vehicle and 125 to 150 mm is the sensible range. A workshop floor with a bakkie, a hoist or racking on it starts at 150 mm and goes up from there depending on the point loads. A patio carrying nothing but furniture can sit at 85 to 100 mm quite happily.

What pushes those numbers up is rarely the load on top. It is the ground underneath. Poor, variable or made-up ground means the slab has to bridge, and bridging is a thickness problem. On a site where the fill has not been properly compacted in layers, adding 50 mm of concrete is a cheaper fix than a callback in two years.

Suspended slabs are a different conversation entirely. Thickness there is a function of span, load and support conditions, and it is decided by an engineer, not a rule of thumb. If someone quotes you a suspended slab thickness before anyone has looked at the span, that is a warning about how the rest of the job will run.

Mesh, bars, and the mistake that undoes both

Reinforcement in a ground slab is usually welded mesh — commonly a 193 or 245 reference for domestic work, heavier where loads warrant it. Its purpose is crack control. Concrete shrinks as it cures, and shrinkage cracks are not a defect so much as a certainty. Mesh does not prevent them; it holds them closed so they stay hairline instead of opening into something that lets water and salt through.

The single most common failure we see is mesh lying on the subgrade. Steel at the bottom of a slab that is cracking from the top does nothing at all. Mesh needs to sit in the upper third of a ground slab, held there on proper chairs or spacers, and it needs to stay there while the concrete is placed. Hooking it up with a rake as the pour goes past is not a method, and it does not work.

Cover matters just as much, and more so here than inland. On the Garden Route the air carries salt, and salt is what corrodes reinforcement. Corroding steel expands, and expanding steel spalls the concrete off the face. Adequate cover to the steel, and a dense enough mix that chlorides move slowly through it, is the whole defence.

Laps are the third detail. Mesh sheets have to overlap enough to transfer force across the joint. A butted sheet edge is a designed-in crack line.

What goes under the slab decides more than the slab does

By the time concrete arrives on site, most of the outcome has already been decided. The layers below the slab are where slabs are won and lost.

The subgrade has to be stripped of topsoil and organic material, then made up in compacted layers rather than dumped and levelled. Fill placed in one deep lift will consolidate unevenly under the slab, and the slab will then span across the soft patches whether it was designed to or not.

Above the fill goes a compacted stone or G5 base, giving the slab a uniform, free-draining bed. Then the damp-proof membrane, lapped and taped, turned up at the edges and not punctured. On low-lying erven around Wilderness, Sedgefield and the Great Brak flats, where the winter water table sits high, the membrane and the base thickness are doing real work, not ticking a box.

This is also where the question of whether to waterproof a slab gets answered. A properly installed DPM under a ground slab is the waterproofing. Coating the top of a slab afterwards to stop damp coming up is treating a symptom, and it usually just moves the problem to the wall junction.

Joints, curing and the coastal problem

Concrete moves. Joints are where you decide in advance where that movement shows up, instead of letting the slab decide for you.

Saw-cut or formed control joints at sensible centres, cut early enough to work, give shrinkage somewhere to go. Isolation joints keep the slab separate from columns, walls and anything else that will move independently. Skipping joints on a large pour to save a day is how you get a random crack across the middle of a garage floor.

Curing is the other half. Concrete gains strength by hydrating, and hydration needs water and time. On a windy George afternoon, the surface of a fresh slab can dry long before the concrete below it has finished, which leaves a weak, dusty, crazed top layer over sound concrete. Keeping the surface wet or covered for the first days is the cheapest quality control available on any concrete job, and it is the one most often skipped.

Coastal exposure sharpens all of it. Salt air, high humidity, strong drying winds and a big day-to-night temperature swing between the coast and the Outeniqua slopes all work against a young slab. We plan pours around wind and heat here for the same reason we would plan around rain elsewhere.

Where the spec comes from, and when an engineer is not optional

SANS 10400 sets the performance requirements for structural elements, and a slab that forms part of the structure of a building has to satisfy them. In practice that means one of two routes: a deemed-to-satisfy solution within the standard prescriptive limits, or a rational design by a competent person.

You are into rational design territory whenever the slab is suspended, whenever it carries an unusual load, whenever it sits on questionable ground, and whenever it forms part of a retaining structure. If a slab is holding anything up other than itself and the floor finish, get an engineer.

It is worth being blunt about why. An engineer’s fee on a residential slab is a small fraction of the cost of the concrete, and a fraction of a fraction of the cost of remediating a slab that deflects or cracks structurally. The cases where people regret paying for a design are rare. The cases where people regret not paying for one are not.

How a slab gets priced, and what moves the number

A slab quote is not a rate per square metre multiplied by an area, though plenty are presented that way. The concrete itself is often less than half of it.

What sits inside the price: site clearance and excavation, imported fill and compaction, the stone base, the DPM, edge formwork, reinforcement supply and fixing, the concrete supply, placing and finishing labour, joint cutting, and curing. On a difficult access site, getting the truck close enough to discharge — or the pump you need because it cannot — can be one of the larger single line items.

What moves the number most is rarely the slab specification. It is ground conditions, access, and how much fill has to be imported. Two identical slabs on two erven a street apart can differ substantially for that reason alone, which is why we would rather walk the site than quote off a plan.

If a quote arrives as a single square-metre rate with no breakdown, ask what is in it and what is excluded. The exclusions are where the variations come from later.

Frequently asked questions

How thick should a concrete slab be for a floor?

For a domestic floor on sound, well-prepared ground, 100 mm with correctly positioned mesh is the usual specification. Garages and driveways typically move to 125–150 mm because of vehicle loads, and workshop or light industrial floors start at 150 mm. Poor or variable ground pushes all of those numbers up, because the slab then has to bridge soft spots rather than simply spread load.

What is the standard thickness of a floor slab in South Africa?

There is no single legislated thickness. SANS 10400 sets performance requirements rather than a fixed dimension, so the answer comes from the load and the ground. In practice 100 mm is the residential convention for a ground-bearing floor, and anything suspended is designed by an engineer to suit its span.

What size mesh for a 150 mm concrete slab?

Mesh reference is selected for the loading and the crack-control requirement, not the slab depth alone, so a 150 mm slab can carry anything from a light domestic mesh to substantially heavier reinforcement. What matters as much as the reference is position: the mesh must sit in the upper third of the slab on proper chairs, with adequate laps between sheets. Mesh dragged up during the pour, or lying on the subgrade, contributes essentially nothing.

What are the three types of slabs?

In residential and light commercial work: ground-bearing slabs that sit on prepared fill and transfer load into the ground; suspended slabs cast in place that span between supports and carry load in bending; and rib-and-block slabs, which use precast ribs with infill blocks and a structural topping and are the common choice for South African first floors.

What MPa concrete do I need for a slab?

Domestic ground-bearing slabs commonly use 20–25 MPa, with 25–30 MPa typical where there is vehicle traffic, abrasion or coastal exposure. Suspended and structural slabs are specified by the engineer. On the coast the case for the higher strength is not just load — a denser mix is more impermeable, which slows chloride ingress to the reinforcement.

Should you waterproof a concrete slab?

A ground-bearing slab is protected by the damp-proof membrane installed underneath it before the pour, correctly lapped and turned up at the edges. That is the waterproofing, and it has to be right first time because it cannot be redone later. Coating the top surface afterwards to stop rising damp is treating the symptom, and it usually pushes the moisture out at the wall junction instead. Suspended slabs exposed to weather — a roof slab or a balcony — are a separate case and do need a designed waterproofing system over them.

Why does a new concrete slab crack?

Almost always shrinkage. Concrete loses volume as it cures, and if it cannot shrink freely it cracks. Reinforcement and control joints do not prevent that shrinkage, they manage where it shows and keep the cracks tight. Cracking becomes a defect rather than a characteristic when the joints were omitted or cut too late, when the surface dried out before curing was complete, or when the mesh was in the wrong place. Wide, offset or progressively opening cracks are different, and they need looking at.

Related reading

• Concrete MPa, plain
• What a concrete slab costs per m²
• Concrete curing in coastal climates
• Foundation types for George soils
• Concrete slabs in George