Cracks that develop in newly-laid concrete can be frustrating. You could be forgiven for thinking there was something wrong with the mix, or how it was laid.
In fact, the main reason that concrete develops cracks is usually down to aftercare.
Whatever the cause, cracked concrete can be a headache, so anything that you can do to reduce the risk is to be welcomed. Here are some suggestions to consider.
Properly cure your concrete
When the average person thinks about laying concrete, they likely believe that it’s a simple case of mixing the concrete, pouring it into the desired position and allowing it to set.
But, in reality, one of the most important parts of the process is curing. It is this that allows it to become a solid and impermeable surface with less chance of cracking.
Why do you need to cure concrete?
Concrete needs to retain a certain amount of moisture while it sets to allow all the ingredients in the mix to create the strongest bond. If it sets too quickly, it will be weakened in places and be unable to stand up to the wear and tear it has to take.
How to cure concrete
There are a variety of different ways to cure concrete. If you have a small amount, then you can regularly spray the surface evenly. This is impractical for larger areas though because you end up wasting large amounts of water.
Another method is ‘ponding’, where a dam wall made of sand is built up around the concrete and then flooded. This gives the concrete a constant source of water, but it means that any other work in the area must be put on hold while the concrete is cured.
A more common approach is to use a plastic wrap to protect against evaporation.
Special compounds can be applied to create a sealed layer or a moisture-retaining cover can be laid on top that that provides a steady source of moisture.
Have everything on hand and be prepared for the weather
Concrete starts to set as soon as it is laid, so it’s important to be organised before you begin. Decide early which curing technique you intend to use, and make sure you have all the tools you’ll need on hand before you begin.
The weather will impact on the speed that concrete sets. If it’s hot, the moisture in the mix will evaporate more quickly than it would if it’s colder.
If you’re pouring concrete when it’s humid, try adding ice cubes to the mix to cool it down. If it’s particularly cold, then the moisture in the mixture can freeze. This will cause the concrete to flake and lose its integrity. If that is the case, use blankets and heaters to prevent freezing.
Control joints and expansion joints
Like most materials, concrete expands and contracts with different levels of heat and moisture.As it is such a strong material, this expansion can allow cracks to develop.
One way to help reduce risk is by using control or expansion joints. A control joint is a continuous vertical joint filled with mortar, but with a bond breaker on one side that prevents tensile stress developing across it.
These control joints widen as concrete masonry shrinks, preventing it from cracking.
They need to placed at regular intervals in large surface areas of concrete, and at key points such as corners.
They can be created while the concrete is being poured by using a tool to create the joint, or cut into the concrete when it has cured enough to prevent chipping. Expansion joints are put in place before the concrete is poured.
These allow the slab to move, reducing stress at places where it makes contact, such as where concrete meets a building or another expanse of concrete. A pliable material is usually used to construct these joints and they should extend to the depth and width of the concrete slab.
Ensure you have a compacted base
Getting the base right for your concrete will reduce the risk of a slab cracking. The base is the layer between the existing soil and the concrete slab. It’s usually made of a crushed stone material, and is what will support the slab over its lifetime, so it needs to properly constructed and compacted if it’s going to do its job.
The dirt beneath your concrete slab is constantly moving, and the base prevents this movement from cracking the concrete. It does this by providing a layer that absorbs the different levels of movement between the soil and the concrete slab.
A base also aids drainage. If a slab is poured directly on soil, which is then washed away, the concrete will sink and crack. A good base allows run off, keeping your concrete supported and crack free.
Using the appropriate water ratio
Concrete doesn’t require much water to achieve maximum strength. More water can make the it easier to handle, but it will also greatly reduce its strength. Shrinkage is one of the main causes of concrete cracking. As concrete hardens, it dries and shrinks due to evaporation of excess water.
The more water in the mix, the more evaporation takes place. This shrinkage literally pulls the concrete apart, leaving cracks.
Getting the right ratio of water is your mix reduces the chances of your concrete developing cracks. Ready-mix concrete is produced at a batching plant using the latest technology to ensure that the right proportion of ingredients goes into the mix.
Steel grid reinforcement
As a brittle material, concrete performs well in compression, but less well when held in tension. Steel grid, or mesh, reduces the internal stresses that occur in concrete caused by changes in temperature.
The steel grid is laid where the concrete is to be poured. It’s then immersed in the concrete mix which, as it begins to dry, grips the reinforcing bars and allows stress to be transmitted between the two materials.
Professionally mixed, properly prepared, and treated concrete will be free of cracks for years to come.
