The age old question has been raised again this week. Should you go with a waffle slab or stick to a conventional raft footing and slab when building a new house?
For those unfamiliar with the terms, refer to our page of Structural Engineering Terminology
Today I’d like to address just the issue of slab cracking.
Waffle Slab vs Raft Slab
Which slab is more likely to crack? The waffle slab or the conventional raft footing?
Check out this post: Why does concrete crack?
The Good News
Concrete cracks are very common. How do you know if the cracks in your slab are bad cracks?
The good news is that a crack in a house slab doesn’t mean the concrete has failed.
Concrete does crack. It’s rare to see a new house slab that doesn’t have some cracks in it.
Hairline cracks generally won’t 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.
Of the slabs that I have inspected, the majority exhibited shrinkage cracking. (What is shrinkage cracking?) Either no curing was used, it was a hot day and the bleed water evaporated or the slab was over-worked and the bleed water was pushed away during screeding.
The same cracks will occur regardless of whether is is waffle slab or conventional slab. So no clear winner yet.
Plastic Shrinkage Cracks
The other slab cracking that we see in residential slabs, but less frequently, is parallel lines following the mesh at around 20 centimetre centres in both directions.
This is caused by poor
compaction of wet concrete and the concrete slumping over the mesh reinforcement. Sometimes, in hot weather, it is also caused by the concrete drying out quicker around the hot mesh.
Again this cracking can occur 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!
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 don’t 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 freely because there is less restraint by the 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!
Concrete slabs will crack when they are overloaded. The steel reinforcement in concrete 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’s 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.
Who’s the Winner?
So, are waffle slabs less likely to crack than conventional raft slabs? My opinion is a reserved yes. The problems that cause cracks in slabs affect both slab types, but 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.
When to Worry
The Australian Standard AS2870-2011 gives advice on when slabs cracks are bad enough to cause concern – and often this is when you’ll need an engineer to help solve the problem.
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.