5 Things to Look for When Inspecting a Suspended Concrete Slab

If you’re a structural engineer and you’ve just been asked to inspect a suspended concrete slab before it is poured, now is definitely a good time to go through the things you should be looking for when you carry out your inspection.

Here are my 5 most important things to check when inspecting a suspended concrete slab:

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This is a BIG House

Cornell Engineers was engaged by Brisbane Builder Rigoli Constructions to design the footings and slab for this massive house at Cedar Creek.

With 657m2 of single level slab on ground including 300m2 of verandah, this house is BIG!

Cornell Engineers designed the raft slab on ground for this house.

Matt Cornell, director of Cornell Engineers explains:

There are special considerations a structural engineer has to take when designing a very large residential slab on ground. The decision to not have a control joint in the middle of the slab meant that this large slab had to be constructed and cured appropriately to keep shrinkage cracking to a minimum.

Once the slab was down, the builders did a great job of moist curing the slab which meant that the concrete had plenty of time to gain strength before the coverings were removed. This meant that shrinkage cracks were kept to a minimum.

Now that the roof is on this big house, the plasterboard linings and internal fitout can commence.

We’re looking forward to seeing this house finished. It’s going to be great!

Need a big house?

Need a big house for your family? Somewhere o hang out in a quiet spot in the country? Need a big slab? Contact Cornell Engineers to see how we can help.

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What Volume House Builders Don’t Want You To Know

The number one secret volume house builders don’t want you to know is that you are entitled to a certain standard of construction and finish in your home.

So picture this. You design and build your dream house with a volume builder. It all goes fairly smoothly. You move in. Three months later you see a crack or two in the walls.

“How bad does a crack have to be before it is a problem?”, you wonder.

The mass market builder supervisor says, “Don’t worry! Houses settle and move all the time. Wait 12 months and then we’ll come and do some repairs.”

But you do worry. You look at the crack as you walk past every morning. You see it every night. You worry some more. You show the crack to your friends. You hear horror stories about new houses and cracks. Is your house safe? Will you be able to sell it? Is my house falling down???

Ok. Take a breath.

Yes. Some level of cracking in a house is normal.

So now I’m going to reveal a secret which shouldn’t be a secret.

Queensland’s QBCC and Victoria’s VBA have both published a guide to what is an acceptable crack, deflection, sag and bow.

Check out the manual for your area and next time you complain to your site supervisor or warranty manager – arm yourself with what is acceptable and what sort of damage should be ringing alarm bells.

Here you go:
https://www.vba.vic.gov.au/__data/assets/pdf_file/0008/102896/Guide-to-Standards-and-Tolerances-2015.pdf

https://www.qbcc.qld.gov.au/sites/default/files/Standards_and_Tolerances_Guide_0.pdf

Designing for Polished Concrete

Polished concrete floors look great. They’re a hard-wearing, durable and attractive floor finish that can add value to your home. But not every concrete floor is suitable for polishing because cracks in polished concrete floors are hard to hide.

If you’re building or extending your home and you’d like to use polished concrete, then this is something your engineer needs to know during the design process so that the appropriate precautions can be built into the structural engineering drawings.

What is Polished Concrete?

Polished concrete is any of several decorative concrete surface finishes that leave the concrete surface exposed. Polished concrete used to be grey concrete that was coated with a wax product and then polished. These days, polished concrete includes burnished (glossy steel trowelled)finishes, honed (finely ground and polished ) finishes exposing coloured aggregates, chemical stains or dry-shake pigments.

How long does polished concrete last?

Polished concrete surfaces are easy to maintain and with regular cleaning should last a lifetime.

Regular cleaning of polished concrete floors ensures that the floor does not become slippery caused by surface contamination.

A surface warranty of between 10 and 20 years is not uncommon for residential polished concrete floors.

Five tips for Engineering Polished Concrete Floors

A good polished concrete slab finish starts with good engineering design. Here are our five tips for a great polished concrete slab.

1. Use a higher grade concrete

A higher grade of concrete strength is a great way to improve the finish of a polished concrete slab. A higher strength concrete has better curing characteristics, cracks less but is harder to work and place.

If you normally use N20 for your slabs, consider using N32 or even N40 concrete for your polished concrete slabs. The difference in price is minor compared to the improved concrete characteristics.

2. Use a higher grade of mesh

Concrete cracks. You know that. A heavier slab reinforcing mesh in will help keep those cracks evenly distributed and will keep the cracks so fine that you can barely see them.

If you normally use SL72 mesh in your concrete slabs, use two layers of SL82 or better still SL81 mesh.

3. Concentrate on Curing

Concrete curing is THE most important aspect of a good polished concrete surface. As the poured concrete dries out it shrinks. If the concrete shrinks before the concrete has gained strength it cracks. Curing allows the concrete to dry out slowly enough that the cracks distribute evenly through the concrete as micro cracks.

I recommend curing your polished concrete slabs by covering it with plastic for seven days. Only lift the plastic in this time to moisten the surface more before replacing the plastic again.

4. Keep the Cover Consistent

The depth of the slab reinforcement in the slab is commonly known as reinforcement cover. For a great polished concrete result, the reinforcement has to be not too deep and not too shallow. The reinforcement has to be well supported on bar chairs and positioned according to the design engineer’s specification which will take into account the environment and the particular finish you are looking for.

For instance, bar chairs at 800 x 800 centres that support the mesh reinforcement about 35mm from the top of the concrete slab would be appropriate for most polished concrete slabs.

5. Vibrate Vertically

Compaction of concrete during placement is a must and polished concrete needs very careful vibration. Insert the poker vibrator vertically and at a regular spacing to ensure the concrete is evenly consolidated and free from entrapped air.

Laying a vibrator down in a slab during concrete placement could cause uneven distribution of in the stones in a honed concrete surface so careful and consistent vertical vibration is a must.

Find Out More About Polished Concrete

Find out more about polished concrete floors with CCAA.

Check out this handy guide by BGC Concrete.