Structural Engineers
Are waffle slabs able to satisfactorily protect Australian homes from slab heave and foundation movement?
This question is for you, structural engineers and geotechnical engineers and foundation specialists of Australia.
If you are applying AS2870 to waffle slab designs please comment below. We want to hear from you.
A soil test on your property is used to help structural engineers get some information about the soil on your block. The information in a soil test is used by a structural engineer to design the footings or foundations for your new house or house extension.
Structural engineers need to know if there are any problems with the soil on your block so that they can design your house footings correctly for those site conditions.
The main soil problems are:
Soft soil
Loose soil
Overly wet soil
Clay soil (reactive clays)
Of these, the soil problem that has been getting the worst press lately is reactive clay soils resulting in Slab Heave.
Reactive clays change in volume substantially when they absorb and release moisture.
The change in volume results in the ground surface moving up and down and this can cause damage to a house.
Reactive clays are classed as follows:
Class “M”: a moderately reactive clay. We see this site classification about 35% of the time.
Ground movement isn’t ‘too bad’ and house slabs can easily be designed for this soil.
Ground surface can move vertically between 20mm and 40mm between wet and dry conditions (seasons).
Class M soils will move more than this if subjected to extreme moisture conditions so good drainage around a dwelling is essential.
Class “H”: a highly reactive clay. We see this site class around 20% of the time.
H class soils have been broken into two groups. H2 soils are more reactive than H1 soils so the footings for a H2 site are going to need to be stiffer than if you have a Class H1 or Class M soil.
In general, ground movement on Class H1 and H2 sites is a bit more serious than Class M and Class S sites. Your engineer will need to be a bit more careful with the design and you’ll need to ensure and maintain good drainage around your home.
The footing and slab code starts introducing special requirements for drainage and for protecting pipes from the movement that is likely to occur.
Class “E”: an extremely reactive clay. We see this site classification about 2% of the time.
Very special care needs to be taken with the footing design and extra precautions are needed by the builder and homeowner (for the life of the building) to protect the house from slab heave.
This is a life-long soil condition and future homeowners need to be aware of the limits and disclaimers in the footing design so that they don’t contribute to uneven movement and damage to the house.
Firm sandy sites are classified as Class “S”. These are my favourite sites.
They are nice and easy to design.
There is no reactive clay movement to worry about. the soil is relatively stable and firm and slab on ground houses only need slab thickenings under external walls and internal concentrated load points.
We see Class “S” sites about 15% of the time.
Soft soils, loose soils, wet soils and other problem soils are classed as:
Class “P” problem sites.
We traditionally saw Class P sites about 28% of the time. Recently we’ve seen Class P about 75% of the time. This is because trees have so much influence on the performance of a foundation that soil testers have been forced to remind design engineers when trees are nearby.
For the other causes of Class P soils, usually, the soil test has information that describes the problem with the soil.
You’ll need an engineer to design your footings to solve these problems. Some of the common “Problems” are:
Soil tests for house sites are carried out in Australia to comply with Australian Standard AS2870.
You don’t need to be familiar with this standard unless you are a builder, an engineer or a certifier.
However, the disclaimers and information that is on the soil test and on the engineer’s plans are often derived from the rules in AS2870.
So you need to read these rules, understand them, and pass the information on to the next homeowner. The soil under your house is there for the life of your building.
This is what you should do when you need to arrange a soil test for a new house or a building extension:
Particularly on reactive clay sites, the influence of tree roots on a house can be devastating. that is because tree roots affect the amount of moisture in the soil wherever the tree roots occur. The fact that tree roots do not affect a whole house the same way creates issues with uneven foundation movement which can lead to internal and external damage in a house.
The soil test’s estimate of the normal site reactivity generally does not include the effect of tree roots – so the estimate of reactive soil movement under-estimates the amount of uneven foundation movement possible if trees are close to the building.
Some soil tests do include an estimate of the effect that tree roots will have on the site classification. They call this effect yt.
Remember that tree roots can increase the uneven soil moisture profile on your allotment. Large trees or a group of trees can have quite a large effect on the performance of a foundation.
Consult your soil tester and structural engineer to determine what precautions you can take to protect your home from the effect of trees on your site classification.
When we are investigating movement and damage in a fairly new dwelling we like to get an independent assessment of the soil conditions on the site – even if there is a soil test already.
The site classification industry is pretty good but soil tests only examine the soil in one or two spots on your allotment. If the soil conditions are variable then the soil test may not tell the whole story.
An independent soil test gives us a clue as to the accuracy of the original soil test and whether we should be expecting more or less foundation movement than originally reported.
If you are investigating movement in an existing dwelling, I really recommend getting an independent soil test as part of the investigation.
In small towns, this is especially difficult but it is almost always worth it.
The information in a soil test includes the soil reactivity, information on the load-carrying capacity of the soil (bearing capacity of the soil), the density of the soil (whether the soil is loose or tightly packed) and the amount of moisture in the soil.
A structural engineer will use this information when designing the footings for a house, a commercial building or an industrial building to ensure the soil is not overloaded by the building (so that the imposed loads do not exceed the load-carrying capacity of the soil) and so that the stiffness of the foundation system is capable of moderating the uneven movement potential of the site. For example, double brick walls are a lot less tolerant of uneven movement than single brick or plasterboard walls.
Find a soil tester in Mackay, Queensland.
Find a soil tester in Brisbane, Queensland.
Or use one of our recommended soil testers.
By Matt Cornell
For Cornell Engineers – Structural Engineers
A Structural Engineering Blog
Slab heave occurs in a house concrete slab when the ground moves more than your slab can stand. Waffle slabs and raft slabs are both intended to moderate this ground movement to acceptable limits.
The slab doesn’t normally crack, but if the movement in the slab is more than the brickwork and/or plasterboard walls can tolerate: they crack.
Which is better? A waffle slab or a raft slab?
The quick answer:
A Raft Slab
Why?
Waffle slabs are legal and are their design is covered by AS2870 so engineers are entitled to specify them. However, their use comes with a lot of provisions that relate to site drainage that must be observed for the life of the building.
Your best protection is to read the engineers plans and observe the rules. If you haven’t built yet, find out what the rules are and whether they will restrict your plans for gardens, trees, lawns and swimming pools.
Waffle slabs are a lot less tolerant of poor drainage:
Signs of slab heave include:
You may have realised that we prefer raft slabs at Cornell Engineers. Want some reasons to pick a raft slab over a waffle slab? Check out this post.
Want to know more? Here’s the link to our video on slab heave.
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.
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).
There are three types of cracks concrete that structural engineers that normally investigate. Plastic shrinkage cracks, plastic settlement cracks and structural overload 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 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 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.
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
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.
Cracks in concrete are very common. So, when is slab cracking a cause for concern?
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 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.
These cracks require further assessment and should be referred to the builder then the building registration board, and/or a structural engineer:
Issues to be investigated should include rising damp, termite proofing, ground movement and compliance of the poured slab with the specification.
Now. Which residential slab is more likely to crack? The waffle slab or the conventional raft slab?
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.
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!
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!
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.
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.
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:
Call Cornell Engineers for advice on cracks in your slab and the best rectification method.
For those unfamiliar with some of the terms, refer to our page of Structural Engineering Terminology
Need more advice about cracking in waffle slabs and raft slabs? Contact Us or Get a Quote.