Structural Engineers

Which type of residential slab system should you choose if you want fewer slab cracks in your house slab? Should you choose a waffle slab or a conventional raft footing and slab when building a new house? Does it even matter if your house slab cracks?

Before we get started, we need to understand what concrete is and why it cracks.

What is Concrete?

Concrete is like a cake mixture; except the cake is made with cement, sand, rocks, and water.

When mixed, the water and the cement react with each other to make a glue that binds the sand and rocks together.

The concrete cake mixture is poured from concrete trucks or cement mixers into slab formwork. The mixture is spread out and allowed to dry (cure).

Four Types of Cracks in Concrete

There are three types of cracks concrete that structural engineers that normally investigate. Plastic shrinkage cracks, plastic settlement cracks and structural overload cracks.

Plastic Shrinkage Cracks

These cracks normally ‘appear’ within a couple of days of pouring the concrete.

Actually, they actually within the first few hours of the concrete being poured before the concrete has any strength.

As the concrete mixture dries out the glue starts to harden – but the exposed concrete surfaces dry faster than the rest of the mixture. As the surface dries out it shrinks – as a woollen jumper shrinks in the wash.

The concrete jumper pulls tight on the surface of the concrete (like a shrunken jumper on a a full-size footballer). On the inside, the concrete is still wet. The surface concrete does not have much strength yet so as it pulls tight over the wet concrete it stretches a little bit tight until <CRACK>. The surface pulls apart slightly and a crack has formed.

This type of cracking is called shrinkage cracking because it is caused when the concrete surface shrinks faster than the concrete strength can resist.

Plastic Shrinkage Cracks Look Like Random Lines

Plastic shrinkage cracks can look like random lines across a slab and sometimes they run close to and parallel to a sawn joint or tool joint but that is because the sawn joint was installed in the right location but AFTER the concrete had already cracked.

Of the slabs that I have inspected, the majority exhibited shrinkage cracking.

Plastic shrinkage cracking is more extensive when either no curing was used, the concrete was poured on a hot or windy day or the slab was over-worked, and the bleed water was pushed away during screeding.

Plastic shrinkage cracks can be controlled by adjusting the shape of the concrete to be poured, by applying water sprays to the concrete surface to stop the surface drying out too quickly, by not pouring concrete in hot weather, by applying curing compounds and evaporation retardants (special chemicals that get sprayed on the concrete) and by covering the poured surface with plastic membranes to minimise evaporation of moisture from the surface.

Plastic Settlement Cracks

Plastic settlement cracks tend to occur in more uniform patterns on the surface of a concrete slab. Typically, the cracks occur over the top of the mesh reinforcement (cracks spaced at multiples of 20cm) or along footing beams.

They occur when the wet concrete settles under its own weight and is ‘held up’ by the reinforcing mesh.

They are caused by inadequate compaction of the wet concrete. These cracks can be prevented by ensuring the concrete is adequately compacted and vibrated when it is poured.

Sometimes, in plastic settlement cracks also are caused by concreting in hot weather when the concrete dries out quicker around the hot reinforcement. These cracks can be avoided by not pouring concrete in hot weather or by cooling the concrete reinforcement before pouring the concrete.

Pure Shrinkage Cracks

As the whole body of concrete cures, it also shrinks slightly. If the ends or sides of a concrete slab or beam are restrained by footings, walls, or other structures then the concrete element will go into tension. As soon as the tensile forces exceed the capacity of the drying concrete it will crack to relieve those stresses.

Steel reinforcement is used in concrete to resist these forces. Cracks form but they are held tightly closed by the reinforcement.

For slabs where cracking is intolerable, substantially more reinforcement must be used to control these cracks.

Don’t be fooled into thinking that “low shrinkage concrete” can be used to prevent these shrinkage forces. Low shrinkage concrete does not equal early age crack control https://www.ccaa.com.au/imis_prod/documents/Library%20Documents/CCAA%20Datasheets/NOTE2010.pdf

Structural Overload Cracks

The fourth type of slab cracking occurs when concrete has been overloaded. Steel reinforcement is placed inside the concrete to distribute or to resist the tensile forces that develop in a structural slab.

Structural cracks form when the forces in the concrete are more than the steel and concrete can resist or when the steel reinforcement has been placed in the wrong position.

An example of a structural overload cracks might be observed after a heavy load is dropped on a concrete slab or along a concrete beam that has been loaded beyond it ‘cracking moment’.

Structural overload cracks are an extremely important safety aspect of structural concrete – particularly for suspended concrete slabs and beams. When designed appropriately, concrete should fail slowly enough to give the occupants enough time to identify the cracks and act – either to evacuate the building, to strengthen the building or to condemn the building.

We rarely see structural overload cracks in a slab on ground house slabs, so we’ll ignore this type of cracking for now.

When Is Slab Cracking a Cause for Concern?

Cracks in concrete are very common. So, when is slab cracking a cause for concern?

Hairline Cracks

Hairline cracks generally will not affect the strength of your house slab because they often don’t penetrate right through the concrete. They are often surface cracks and are controlled by proper placement of the slab reinforcement (around 30mm to 40mm below the surface).

Hairline cracks in older slabs tend to fray and might appear wider at the surface but when I have inspected core samples taken through older cracks, once again the crack stops at the reinforcement.

Hairline cracks therefore are not a major problem for most concrete slabs.

Cracks Less Than 2mm Wide

Cracks in concrete ground slabs are more noteworthy if they are up to 2mm wide but they still do not draw a lot of attention from building regulators. Queensland and New South Wales governments recommend monitoring these cracks for 12 months. If the cracks are still no wider than 2mm they are not considered a defect.

Homeowners however may view 2mm cracks in an exposed concrete slab a little bit differently. At 2mm wide, slab cracks are quite noticeable. Several 2mm wide cracks may cause some dissatisfaction in a homeowner so we recommend builders take all precautions to minimise slab cracking.

Several 2mm wide slab cracks could be grounds for further investigation to determine whether the builder has complied with the building contract.

Slab Cracks More than 2mm Wide

These cracks require further assessment and should be referred to the builder then the building registration board, and/or a structural engineer:

• Distinct cracks: around 2mm wide and accompanied by 10mm to 15mm change in offset from a 3m straightedge centred over the defect.
• Wide cracks: 2-4mm cracks and accompanied by 15mm to 25mm change in offset from a 3m straightedge centred over the defect.
• Gaps in slab: 4mm-10mm wide cracks and more than 25mm change in offset from a 3m straightedge centred over the defect.

Issues to be investigated should include rising damp, termite proofing, ground movement and compliance of the poured slab with the specification.

Waffle Slab vs Raft Slab

Now. Which residential slab is more likely to crack? The waffle slab or the conventional raft slab?

Plastic Shrinkage Cracking

The same plastic shrinkage cracks will occur regardless of whether it is waffle slab or conventional slab. The way the slab is cured is the controlling factor in controlling plastic shrinkage cracks.

So, no clear winner yet.

Plastic Settlement Cracking

We saw that plastic settlement cracks are caused by poor compaction of wet concrete and the concrete slumping over the mesh reinforcement.

Again this cracking can occur just as easily on both types of slab if the mesh isn’t cooled or the slab concrete isn’t vibrated. Still no winner in the raft slab vs waffle slab shoot out!

Pure Shrinkage Cracks

The only type of cracking that might be different between waffle slabs compared to conventional slabs would be pure shrinkage caused by the concrete trying to shrink in volume as it cures.

These cracks do not follow the mesh and sometimes start in internal corners. You will also see shrinkage in long, thin slabs where there are no control joints.

In waffle slabs the slab can shrink more freely because there is less restraint by the ground to the slab contracting. In conventional slabs, the edge beams in the ground stop the slab shrinking in overall length. Engineers use heavier mesh in larger house slabs to counter these shrinkage forces. So, waffle slabs just took the lead!

Overloaded Slab Cracks

Concrete slabs will crack when they are overloaded. The steel reinforcement in most concrete ground slabs is there to control the width of cracks under normal conditions. When a slab is overloaded, the steel stretches and cracks become visible.

A stronger slab system can take more load before it cracks. In theory there is no real winner here because waffle slabs and raft slabs are designed for similar loads and will behave similarly when overloaded.

However, raft slabs are cast against the ground whereas waffle slabs are cast onto polystyrene void formers and strips of concrete. The raft slab edges back a point. An overloaded raft slab is less likely to crack because it is cast onto the ground and normally the ground will take more load than a polystyrene void former.

Which Slab is Less Likely to Crack?

So, are waffle slabs less likely to crack than conventional raft slabs? My opinion is a reserved yes. The problems that cause cracks in ground slabs affect both slab types. There should be less shrinkage stresses and fewer cracks in a waffle slab, but a raft slab is less likely to crack if it is overloaded.

A Note on Crack Repair

Once shrinkage cracks and plastic settlement cracks have formed in concrete slabs on ground they are very difficult to repair. So, it is always better to take the effort to prevent them when pouring.

However, if cracks have already appeared, they can be disguised with one of these techniques:

• Filling with a fine-grained cementitious grout.
• Demolition and removal and replacement of the slab section.
• Tiling.
• Spray paving.

Call Cornell Engineers for advice on cracks in your slab and the best rectification method.

Terminology

For those unfamiliar with some of the terms, refer to our page of Structural Engineering Terminology

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