How to lay a Concrete Slab

Here are 6 steps for successful in-situ concrete slab construction!

Techniques such as concrete curing and cutting of joints, together with a close eye on maintenance are some of the essentials required to cast an in-situ concrete slab. However arguably the most critical steps lie in planning and design.

In situ, also known as cast-in-place means construction carried out at the building site using available raw materials. Here are 6 steps to cast this kind of concrete slab professionally and importantly, to cast a slab that is fit for purpose over the required service life.

Step 1: Planning

Define exactly what you want, answering questions such as:

• What is the slab going to be used for?
• What is its required service life?
• How much foot or industrial traffic is the concrete going to need to handle?
• What finish do you want on the slab surface and if the slab needs to be suspended, what finish is required for both the top and bottom?

Don’t be put off by the many questions needing to be answered. Use them as guidelines to jot down your needs for the concrete slab and share this with your engineer and/or contractor. A clearly thought out brief upfront, shared with your construction partner, can save many construction headaches and possible claims later on.

Tip: Choose your contractor carefully. Don’t only consider cost. Look at previous projects done by your preferred supplier and speak to previous clients to make sure that their workmanship is to a high standard.

Step 2: Design

At this stage, your engineer and contractor should meet with you to discuss the expectations for the service life of the concrete slab. Whether it is going to be a professional industrial floor or ‘stoepie’ for the family home, every slab needs a certain type of design.

The design will be dictated by the answers to questions such as those mentioned under Step 1. Remember that 50-60% of floors don't live up to initial expectations, so it cannot be over emphasised that to build an in-situ concrete slab revolves around clarifying these upfront.

Step 3: Construction

At this point, you will need to assess which cement product to use and whether to use commercially available ready mixed concrete or mix the concrete yourself. If you opt to use commercial concrete, make sure that you brief the supplier with the right specifications i.e. correct concrete strength, workability and finish. If you decide to mix the concrete yourself, follow the guidelines for volume batching which appear on the back of cement bags.

When it comes to concrete pouring, make sure that your concrete is 'workable'. It should not have the texture of a custard-like liquid. Move the concrete with a shovel to fill area to be concreted. Be sure not to finish off the surface by adding cement to 'dry it out' as this will definitely lead to cracking.

Step 4: Curing of concrete

As soon as the concrete is set, curing must start. Curing of concrete is essentially making sure the concrete has sufficient water available so that it can build strength. This can be done by water puddling, putting wet sand over the top of the concrete slab (and ensure it stays wet) or by sealing it off so no moisture evaporates. If concrete is not cured, you will see cracking of the concrete on the slab.

Step 5: Contraction joints

Concrete shrinks when it loses moisture. Be sure to use construction joints, especially when the concrete pour cannot continue and wet concrete must be cast against set concrete. Alternatively, cut joints into the slab using a concrete cutting machine to create contraction joints in the fresh concrete; typically in the first 24 hours.

Cutting of joints should be specified by your consulting engineer when you brief in your expectations for the slab, but if you are doing everything yourself a rule of thumb to follow is for cut depth to be a quarter of the slab thickness e.g. 25mm joint depth in a 100mm slab. You can saw the joints using either an early-cut saw or convention saw. Joint sealing is also needed.

Tip: If commercial flooring is being laid, make sure that no debris falls into the joint because this will result in unsightly ravelling.

Step 6: Maintenance

Keep an eye on any flaws or cracks that develop. Quick maintenance action can save you money and effort in the long run.