Slab cracks

Slab Cracking – Waffle Slab vs Conventional Raft Slab

Slab Cracking

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?

Terminology

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

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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.

Shrinkage Cracking

Concrete Shrinkage Crack
Typical Shrinkage Crack in Concrete

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

Structural engineer - photo of plastic slab cracks
Plastic shrinkage cracks caused by concrete draping over the steel reinforcement.

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!

Pure Shrinkage

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!

Overloaded Slabs

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.

Need More Advice?

Need more advice about cracking in waffle slabs and raft slabs? Contact Us or Get a Quote.

5 thoughts on “Slab Cracking – Waffle Slab vs Conventional Raft Slab”

  1. Hi Matt
    I am building a 90square two story brick vinear home on highly filled soil. About two thirds of the rear sloping block has fill down to 2-3 meters! Ground floor has two 340mm stepping to accomodate the slope of the land. Would a conventional raft slab hold the house better with less cracking as opposed to a waffle slab?
    Regards
    Don

    1. Hi Don. Thanks for asking. You have quite an amount of fill on that site!
      The soil test will tell you the quality of the fill and whether you can found on the fill or need to found through the fill onto the natural soil underneath.
      If the fill is controlled fill and you can build on it, the type of slab won’t affect the amount of slab shrinkage cracking – that’s affected by concrete curing (how the concrete is allowed to dry).
      If you are concerned about uneven settlement of the fill causing cracks in your house, speak to your builder and engineer about whether you need footings that run right through the fill to found into natural soil.
      As for the stepping slab, slab steps shouldn’t rule out the use waffle slabs per se however I haven’t seen waffles used with such large slab steps in my time.
      Best of luck.
      Matt Cornell

  2. Hi,

    I found your article very interesting as I am about to build my first home and am concerned about the use of a waffle slab. Is it true that a waffle slab can fail under pin point pressure, such as a car jack? Is it a good idea to avoid waffles under the garage of the house? And more importantly would you be happy for your own home to be built on a waffle slab? The soil class in my area is between a M/H.

    Regards,

    Elise

    1. Sorry, one other question. Is there much of a price difference between waffle and raft slabs? I understand that raft slabs use more concrete, but what do you think a fair price increase would be to upgrade an average size home to a raft slab. Most house price estimates I’ve received have only included a waffle slab.

    2. Hi Elise
      Glad you liked my article.
      Even on a M/H site class, waffle slabs are perfectly legal so there’s no need to be too worried about using a waffle slab. There are just a few points to remember about good maintaining drainage around your house. These will be called up on your engineer’s drawings.
      I heard just this week again about a waffle slab failing when a car was jacked in the wrong location. This is a very rare story and I don’t hear of it very often.
      The house I live in has a waffle slab – and it is performing no worse than my last house. Houses sometimes move and crack and I guess we’re fairly realistic about this.
      I’m not sure about the price difference for a waffle slab. Most information on the www is that they are cheaper. Also, as you pointed out, the mass market builders tend to favour them. Our smaller builders tend to prefer raft slabs. I guess they use it as a point of difference.
      Good luck. Feel free to check out our forum too. Your questions would be perfectly suited there!
      Matt Cornell

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