Cracks in concrete. It is one of the reasons some people opt to not use concrete as a choice of product when landscaping their home’s surrounds. Cracks in concrete are unsightly and sometimes can develop into a trip hazard. Why does concrete crack? Does all concrete crack? Can we stop it from cracking? Read on to find out the answers.
Why Does Concrete Crack?
The simplest answer to this can be summed up in one word – movement. Although concrete is a solid product when it goes hard its weakness is in its strength to span across areas. To counter this, steel reinforcement is used to increase the tensile strength of the concrete. Using steel in concrete lets us do many things we otherwise couldn’t do, such as spanning large areas with minimal cracking and even suspending concrete above the ground.
We may not see it, but concrete is forever moving. Concrete will shrink from the loss of water content that takes place when it is first laid. It expands and shrinks with the rising and falling of temperature and changes in the moisture level of the ground below a slab can also move the concrete. Other factors such as nearby or underground root systems from plants or heavy vehicles driving across slabs can contribute to movement in slabs.
Does All Concrete Crack?
With any of the factors mentioned above, concrete will inevitably crack at its weakest points – it is a fact. It is a normal and natural property of concrete to do so.
Can We Stop It From Cracking?
Although we can never prevent concrete from cracking, we can do a few things to minimise and control the cracking.
1. Maintain Slab Thickness – Cracks usually occur at the weakest points in the slab. If a slab varies in its thickness it means it will most likely crack at its thinnest part. To gain more control over where the cracks occur, slab thickness should be constant.
2. Slow Down the Curing Process – Concrete cures better and cracks less if the curing process is allowed to go on as long as it needs to. High temperatures contribute to uncontrolled cracking because the curing process happens too quickly.
If you are unable to wait for ideal environment circumstances products like aliphatic alcohol and curing agents can be used to help prolong the curing process. Hosing down or ‘flooding’ a slab also helps it to cure slower which will reduce uncontrolled cracking.
3. Use Expansion Foam – Concrete slabs that will be located hard between two existing structures, slabs or walls need room to expand when temperatures rise. Use of expansion foam to separate the existing structure(s) from the new slab will ensure there is room for slab expansion and will decrease the chances of uncontrolled cracking.
4. Avoid Laying Slabs Close to Trees – Root systems of trees will change and grow greater over time and have the power to work their way below your concrete slabs and can create some horribly large cracks in some cases.
If you are going to have garden beds near your concrete surrounds avoid planting plants that will have large root systems in the future. Try to keep new slabs at least 300 mm away from existing plant trunks and do not lay a new slab over already existing root systems.
You may need to dig a thickened edge beam below the slab to prevent some existing root systems from undermining the slab in the future. Ask your prospective concreter for their opinion if you have any concerns about this.
5. Reinforce Internal Corners and/or Existing Adjacent Structure Intrusions – Internal corner points and corners where existing adjacent structures create a hard ‘intrusion’ next to the slab are most likely going to create cracks as time goes by.
Placing additional steel reinforcement across these points can help prevent uncontrolled cracks, however I personally recommend instead to create a controlled joint at these points wherever possible.
6. Create Controlled Joints to Force Cracking at Specific Points – As mentioned in point number 5, cracking can be controlled to an extent by creating control joints.
By purposely weakening the slab at specific points we can create a controlled crack. This is usually done either by saw cutting a straight, neat cut (a ‘sawn’ or ‘saw cut’ joint) in the slab within 24 hours of its being laid or by running a jointing tool to create control joints during the pouring process.
Control joint locations should be decided prior to pouring the slab so that the reinforcing steel can be weakened locally along the joint line. This will help increase the chance that the slab will crack below the neat control joint.
An experienced concreter will be able to decide where control joints should be placed to help minimise uncontrolled cracking.
Although we can aim to prevent cracking in concrete slabs by using the above techniques, almost all experienced concreters will tell you “concrete has a mind of its own” and sometimes it will crack where it wants to and not where we’d like it to.