1 December 2016
Over the years CSC Services have developed specialist knowledge to identify, plan and implement the correct approach for individual concrete repair requirements. Mark Lemon, Managing Director of CSC Services takes a closer look at concrete and why protecting it with a specialist coating should always be done by an experienced concrete repair contractor.
In untreated concrete, the process of carbonation is nearly impossible to avoid once there is exposure to the elements. Carbon dioxide will slowly penetrate the surface of the concrete reacting to moisture and calcium hydroxide found in concrete. This forms calcium carbonate which initially hardens the concrete raising the compressive strength. Alkalinity then reduces in the concrete and it starts to breakdown. Depending on the permeability, concrete carbonation may advance at a rate of 1 – 5mm per year. As carbon dioxide travels deeper into the concrete it can reach steel reinforcement if embedded in the structure. As the concrete around the steel is broken down by carbonation the corrosive effects of the air and water rust the steel causing it to expand, spall and crack. This in turn causes a high rate of failure to the surrounding area.
A specialist concrete repair contractor should always cut out the area around the spalled steel, removing concrete back to sound substrate. The exposed steel would then be cleaned and prepared back to bright steel before treatment to protect it in the future. Depending on the scope of works, cathodic protection might be used before a concrete repair mortar is used to fill out the repair area. The surface area of the concrete structure can then be prepared by abrasive or water blasting and a fairing coat applied to bring the structure back to its original depth. Finally a specialist coating can be applied.
To be able to apply the correct specialist coating to concrete correctly it is important to understand the wide range of properties in concrete. It is quite porous and uneven in texture, with differences in density from one square inch to the next.
Concrete can be a simple mixture of Portland cement, lime, aggregate, and water. It can also contain fillers selected for specific structural properties and resins selected for specific chemical -resistant properties, along with various additives selected to add specific strengthening properties. Each variation creates a different surface profile to which a coating would be installed.
Concrete is also naturally alkaline so any coating used will need to be able to penetrate and bond with the alkaline surface of the concrete. Oil-based or alkyd coatings are unsuitable because the alkali in the concrete will react with the natural or synthetic oils in the coating, causing the creation of a soap-like film (saponification) that leads to delamination of the coating from the concrete.
An established contractor such as CSC Services will work with a number of coatings manufacturers and be able to offer independent advice on the best coating to meet the repair requirement.
CSC Services has access to various cementitious coatings designed as a stand-alone coating that will provide waterproofing to internal and external concrete and steel structures. Modern day cementitious coatings can be applied in thicknesses ranging from 1mm upwards. This aids in helping against water ingress and carbonation. There are also various cementitious coating products which are approved by the DWI (Drinking water inspectorate) for use in potable water retaining structures. Cementitious coatings can be applied by steel float, brush and spray.
All too often coatings will fail due to incorrect substrate surface preparation. Dirt, grease, dust and other surface latents will form a barrier between the substrate and coating to be applied which means there is no mechanical bond present. Surfaces generally need to be grit blasted or hydro blasted to form a mechanical key. Correct substrate surface preparation of concrete involves correct application of pore fillers and fairing coats to address blow holes or honey combing in concrete surfaces. Fairing coats need to be applied correctly to leave the required surface profile.
CSC Services always recommends using a specialist contractor to advise on coatings for specific environments. Some products have high abrasion resistance, tensile strengths and high elongation properties. Using a high tensile strength coating with no elongation properties on a substrate where movement may occur will result in cracks and failure in the coating. A lack of specialist knowledge may result in incorrect mixing of products. Most industrial products are two pack systems. If they aren’t mixed correctly they will not cure correctly.
Using a specialist contractor will ensure the correct application method of a product at the correct application temperature. Various products require certain environmental conditions. Dew points, temperature, humidity, and air flow need to be considered to avoid condensation causing a failure in the curing process.
Curing time is also a key consideration. Coating products require a certain amount of time to chemically bond as specified in the technical data. Not leaving sufficient time may mean the product fails. After curing, the product should be fully cleaned and inspected for fails. A scheme of programmed appropriate after-care and inspections should be scheduled.
CSC Services will always advocate the importance of having a good maintenance regime that involves checking a structure regularly for cracks and rust staining. This can reduce the extent of damage. The earlier a problem is spotted, the greater the chance that it can be treated with a simple and cost-effective repair.
For more information about the full range of services that CSC Services provide along with recent examples of work undertaken visit www.csc-services.co.uk