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.

Five Questions with Matt Cornell

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Matthew Cornell
0419 518 869

Hi. I’m Matthew Cornell. I’m a structural engineer with 20+ years structural engineering experience in residential, industrial and small commercial jobs in Queensland. These are my 5 questions.

1. How did you become an engineer?

At school I aimed high and worked hard. I was interested in science, building things, electronics and computers. Spurred on and supported by my parents, I achieved a good Year 12 result and went to James Cook University to study engineering. At that time I didn’t know I wanted to be an engineer especially – it just seemed like a good mix of science and fun.

2. Where do you live and why?

I live in Brisbane now but I grew up west of Mackay, Queensland in a sugar producing town called Marian. Mackay was right for me growing up. It’s a great place to go to school and has a lot of spirit. I moved around after university but ended up back in Mackay for 10 years until 2013. Brisbane is an awesome place. There’s so much to do on weekends. There’s a wealth of great people with the same friendly atmosphere I grew up with in Mackay. We chose to live on the north side of the river because of its proximity to the airport. I think Brisbane has a lot of potential and I’m excited to have an office there.

3. What do you like about being an engineer?

I like helping people achieve their property goals. Property is a great wealth building tool but it can also be a great nest egg. New homes and renovations are my favourite types of project. I have great clients that appreciate the service we offer. Working with these people is the thing I like best about my job.

4. What is your greatest challenge?

As a business owner, my greatest challenge is taking time off. As the only engineer in the company, our clients love to know they have direct access to me my phone and email. We have projects in design stage and construction stage constantly and I try to be always available if unexpected conditions arise on site. That makes it hard to go away fro any length of time. I have a great wife and she understands that I have to answer my phone on holidays – and that makes it ok.

5. Where to from here?

My business has always been about being in the right place to provide a service to people who need help with structural engineering. The Queensland market place has changed since we started 10 years ago and we change with it. We’ll always be here for Queenslanders. Our experience and local knowledge means that we’re still a great choice.

 

CQ Rescue

For many years Cornell Engineers was a proud supporter of CQ Rescue. RACQ CQ Rescue  is a community helicopter rescue service with an operating base in Mackay.

Anyone who travels the highway north,south or west of Mackay knows the value CQ Rescue provides – a superfast trip to hospital with an intensive care paramedic the whole way should you need it.

It’s a great rescue service staffed by caring professionals. As a Community Helicopter Provider (CHP), RACQ CQ Rescue relies heavily upon the community to provide funding for operational purposes. These funds are sourced through sponsorships, business donations, workplace giving programs, annual appeals, bequests, and fundraising events.

Cornell Engineers urges you to join in support of this awesome service.CQ Rescue

Five Questions with Glen Place

Learn about Building Designer Glen Place

My name is Glen Place. My business Glen Place T/A Place Designs has been in business as Place Designs since 01/07/1980. I have a Building Design & Builder: Open Licence (QBCC Lic 12080) and our office is in Badila Court, Mount Pleasant, Mackay. This is my website http://www.placedesigns.com.au and these are my 5 questions:

1. What sort of projects do you prefer?

I prefer new Residential and Commercial projects as they are more challenging and rewarding. However all project are treated equally as they have real meaning for our clients. While our business is focused towards the residential market, we also provide comprehensive services to commercial and industrial development and business renovations including villa units, commercial projects, retirement villages and hospital staff accommodation.

2. How do you add value to projects?

I provide the client with sustainable options moving towards a more energy efficient and healthy building. This helps to increase the longevity of the building. I believe I have the ability to identify, assess and recommend design features of buildings which promote sustainable and energy efficiency. I’m also proud of my ability to analyse complex technical issues associated with building design and materials and communication effectively in a succinct, accurate and coherent fashion.

The design philosophy at Place Designs is a combination of your needs and desires and the contribution of our dynamic design team. Clean, simple, elegant building lines dominate our design thoughts for exteriors and efficient and functional flow is our focus for internal layouts. We aim to employ modern building techniques and innovative building products to present a design that is functional.

 3. What is a future trend you would like to see develop?

I would like to see buildings become more accessible to all people. I would like the see the use of more healthy and durable building materials being used in buildings. I would like building to be designed suit the climate and be more aesthetic within their surroundings.

4. Describe a recent project where you had a positive influence.

This current market this is a difficult one, but a recent project where we met the brief and designed a great house was Mackay Harbour. It’s presently under construction by Mackay builder Barry Green.

5. How does Cornell Engineers add value to your product?

Cornell Engineers are attentive to our needs and are quick to understand our vision and assists us achieve our goals. The staff at Cornell Engineers work with us as a collaborative team and don’t carry out their designs in isolation. Cornell Engineers will work with us to provide the best outcome for our client. 

 

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!

Residential Structural Engineers