How to Repair Concrete
Concrete in one form or another has been in use for several thousand years, but when a fellow known as Joseph Aspdin burned limestone and clay together to make Portland cement in 1824 and then blended these with several aggregates along with water we saw the beginning of building on a truly industrial scale. And so we find today that very few structures are made that don’t contain some form of concrete. Most of these structures will also contain metal bars and is known as reinforced concrete.
Concrete has been widely used to make truly inspiring architectural structures such as the Hoover dam, the Panama Canal and the ancient Roman Pantheon dome, which is completely unreinforced. Many structures require the use of accelerators, which speed up the hardening of the concrete, or retarders, which slow the hardening down on large or difficult pours. Plasticizers increase the workability of the concrete, and air entrainments add tiny air bubbles to reduce the possibility of damage during a freeze thaw cycle.
Because the boundaries of construction are being constantly pushed and stretched it can be a challenge to get it exactly right and a whole industry has evolved around the need to repair structures that develop problems.
A major cause of concrete failure is known as spalling. This effect is usually caused by the combination of poor installation and environmental factors which cause the concrete to stress. Steel reinforcement bars that have an insufficient covering of concrete will start to rust this process of oxidisation causes expansion and the concrete is liable to crack & spall.
Spalling usually shows itself in the form of flaky, pitted or broken concrete and at its best will be cosmetic in nature, but can also result in potentially catastrophic structural damage. Because of this it is vital that spalling is addressed when it first appears.
Prevention is better than cure so it is better to address the problem of spalling before the concrete is poured. Each project is unique but there are procedures that can be followed on every job.
- Make sure the concrete is mixed with the right amount of water
- Keep the mix as dry as possible as high water content can weaken the mix
- Allow enough time for the mix to cure properly and handle it carefully whilst curing
- If required, protect the concrete from the elements whilst curing
- The concrete can also be sealed permanently as a protection
- Make sure steel reinforcement bars have a minimum of 50mm of concrete cover
If spalling occurs in concrete it is important to act quickly as the longer it is left the higher the risk of serious damage. When attempting to repair concrete it is best to adopt a systematic approach to the problem. This will result in answering the right questions at the right time and using the correct products.
- Determine the cause of the damage
- Assess the extent of the damage
- Evaluate the need to repair
- Choose the repair method
- Prepare the old concrete for repair
- Apply the repair method
- Cure the repair properly
There are many manufactures that provide specialist materials for repairing concrete. Always make sure you use a quality supplier that has a technical department who will be able to advise on any particular project.
If the work is decorative it may involve the addition of a new facade to the concrete. Repairing cracks will require a certain amount of precision as the crack will need to be widened initially to create a keyed surface. Once widened the crack will need to be cleaned thoroughly making sure that small dust particles are removed as these may prevent the compound from bonding. A repair compound will have been blended to allow the mixture to enter the crack easily and set rapidly and will contain bonding agents to help improve adhesion. If however the cracking is due to a structural problem this will need to be diagnosed and repaired prior to filling cracks.
Where it is not possible to widen a hairline crack then an epoxy resin is often injected under pressure to fill the crack completely. The resins are designed to be used in cold and damp conditions and epoxy putty can also be applied under water, and patching mortars are ideal where the surface has pittedor larger holes have appeared. Depending on the surface it may be advisable to seal the repairs, either with suitable high performance paint or a resin based sealer.