CD Designs Blog
Cracking is a cause of complaint within the concrete industry. Our aim here is to educate the end user when cracking should be a cause for concern. Any, if not all, remedial work done over the ten years we have been in business, has been because of the aesthetics rather than structural failure. Cracking can not be prevented but it can be significantly reduced or controlled when the causes are taken into account and preventative steps are taken.
Public perception of cracking generally is “if it has happened, then the product has failed”. In the industry it is accepted it happens, a crack which is non- structural and less than 3/8 inch in height or separation, and not leaking water, it should be considered acceptable. It is in our best interest to ensure the end user is aware of this and more importantly when it should be of concern.
Before we look at the causes of cracking in concrete we must understand what concrete consists of, because it is the characteristics of these materials which induce cracking. Concrete consists of cement, sand, aggregates and water. When mixed together, the water hydrates the cement and a chemical reaction occurs bonding together the sand and aggregates.
After the concrete has hardened it is recognised that there are a number of factors out of our control, it is due to these that cracks occur. These are known as Physical, Chemical, Thermal and Structural factors.
This is something that all concrete can be affected by, Drying Shrinkage. To make concrete we have to use water to hydrate the cement and initiate the chemical reaction. More often than not more water is used than necessary to hydrate the cement. This is because we have to place the concrete using conventional methods i.e. Barrowing and raking and tamping. We work within the S2 range, which is between 50-90 slumps. (See PIC Specification) Without making concrete workable, it would not be possible to achieve the work we carry out.
Any of the excess water which is not used by the cement, will over time evaporate and work its way out of the concrete. As a result the mass of the concrete will reduce. If the mass reduces the concrete will contract. This type of contraction will create tension within the concrete and can and usually dose cause cracking.
Within our concrete slabs there are usually integral items which are crack inducing. E.g. manholes, walls, drain covers. Tension on corners as shrinkage occurs can and will usually lead to cracks forming. These cracks can occur as early as a few hours after the concrete has been placed or up to a few years.
Given the cause (Drying Shrinkage and Integral Items) and effect (Cracking) we can usually predict where the concrete will contract causing it to crack. As a result we place our crack control joints in these areas. It does not stop the concrete from cracking it helps us control where it cracks.
To make concrete a chemical reaction takes place, this reaction continues the full life of the concrete. The reaction progressively slows down once the concrete has hardened until about the 20 year mark, when it is thought it starts to deteriorate. Within the concrete other chemical reactions can take place which work adversely against the concrete and can be crack inducing. The corrosion of reinforcement is a process where steel in concrete starts to rust. When steel rusts it expands within the concrete and can sheer it apart.
The chemical process we are concerned about as end users is Alkali- Aggregate Reaction. The initial reaction poses no great danger to our concrete, but we as users can aggravate this process which will lead to rapid deterioration of our concrete over night. If salt was to be put on the concrete it can accelerate the process especially with the onset of sub-zero temperatures.(See Salt And Concrete) Since salt is usually used to melt snow/ice it is only used in the winter months when it is at its most destructive. Salt alone does little to damage the concrete, it is the moisture that it attracts that does the damage.
Temperature is an important factor which affects our concrete; it is the extremes of these temperatures which can cause the ageing/cracking. Internal concrete (concrete slabs inside buildings) will be subject to a lower range of temperatures, (between 10degreesC and 20 degrees C) therefore will be a lot less affected by expansion and contraction. Our driveways are subject to a range of temperatures reaching 30 degrees in the summer to -10 degrees in winter. These stresses can and will bring on unpredictable movement in the concrete.
In winter a strong frost for a prolong period of time will cause the concrete to contract. We can see this in the expansion joints we have created because they open up, likewise in summer they reduce in size, this happens all year long to a lot lesser extent. We may notice it most when it is at one extreme in temperature. In the winter months thermal contraction can contribute to shrinkage and cause unpredictable hair line fractures, which we have not planned for with our crack control joints.
We can’t predict the affects of our winters on the concrete, however we can ensure the base we put under our concrete is adequate to support it, therefore any fractures will remain only hair line and cosmetic.
Our concrete can be affected by accidental overloading, by a thing or vehicle. This can cause our concrete to crack so it is important that the usage is defined before the specification is designed. (See our PIC Specification for domestic driveways.)
Creep or settlement cracks is another cause for our concrete to crack. We excavate up to 200mm, the sub-grade soil below that is beyond our control.
To summarise both contractor and end user can play their part in crack prevention and deterioration of our concrete.
The contractor can ensure they work within the accepted slump range and ensure no more water is used in the concrete than necessary within the acceptable slump range. This will reduce any shrinkage within the concrete.
The End User can ensure no salt or chemicals be used or spilled on the drive and maintain the protective sealer.
To protect against any thermal expansion and contraction the contractor can ensure crack control joints are placed in the slab at the earliest possible opportunity after the concrete has hardened.
And finally the end user can ensure the driveway is only used for the purpose it was designed for, and not to over load it.
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21st January 2011