Category Archives: Tips & Tricks

Tips and tricks for building in Queensland.

Structural Engineers Brisbane

If you’re looking for a structural engineer in Brisbane, then I’d like to help you. I’ve been a structural engineer since 1991. I have been running my own business since 2003. I specialise in designing and certifying houses, extensions and renovations.

I can design and certify retaining walls, steel beams, new footings, relocated walls, double storey extensions and more. What’s more – I’m easy to talk to, I love taking your ideas and needs and making them a reality and I really, really like working with Queenlanders – the people AND the buildings!

My company is Cornell Engineers and our Brisbane office is located in Hendra. We happily service and inspect buildings all over Brisbane and throughout Queensland by appointment.

I’m here to help. Call me if you need a structural engineer in Brisbane. Phone 07 3102 2835.

If you are after home renovation or new build ideas and would like to see some of our current projects being built, add us on FaceBook.

Cyclone Damage

Cyclone Damage

We are structural engineers. We can help you if your building was damaged by the cyclone.

We can assist with structural engineering and make-safe work after the cyclone. If it is important that you get the right beam size and repairs done quickly, call us for structural engineering in Ayr, Townsville,  Bowen and Mackay.

The best number to call us on?

07 3102 2835

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

IMG_3487

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.

Wind Speed

Buildings in Australia are designed to resist wind forces by complying with Australian Standards. The “design gust wind speed” for a particular site is determined by considering the height of the building, the location of other buildings and topography on all sides of the building. Houses in exposed locations or on tops of hills are designed for higher wind speeds and consequently higher forces than buildings in built-up areas.

We use the latest version of Australian Standard AS1170.2 to determine the wind speed for your structure. Where the building complies with AS4055, we can classify the wind speed using that standard too.

Call us for a wind assessment for your building today!