27 July 2016
All concrete deteriorates over time. The rate at which concrete deteriorates is determined by the quality of the concrete and the environment to which the concrete is subjected. In this short article, Mick Flounders, Contracts Director at CSC Services explains the process of carbonation and how concrete is repaired and protected against future corrosion.
Across both the UK water and power sectors, the process of carbonation in untreated concrete is nearly impossible to avoid. Carbonation is the reaction of carbon dioxide in the environment with calcium hydroxide in cement. This forms calcium carbonate which initially hardens 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 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.
Water and wastewater treatment plants provide a severe environment for concrete. Abrasion, chemical attack and freeze-thaw can significantly reduce the service life of a concrete tank. For this reason, it is wise to protect concrete from new, before chemicals, sulphates and acids have the chance to get inside the concrete and cause damage.
Repair of Concrete
To repair concrete CSC Services would 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 (corrosion protection system) might be used before a specialist 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.
The coating used to protect a concrete tank would be based on what the tank contained. It is necessary to differentiate between water and wastewater tanks as the latter contains water that is usually much more aggressive in terms of water chemistry which will corrode concrete more quickly. Coatings used on clean water tanks need to be DWI approved.
Some of the options available include the following:
CSC Services has access to various cementitious coatings designed as a stand-alone coating that will provide waterproofing to tanks. Modern day cementitious coatings can be applied in thicknesses ranging from 1mm giving the equivalent of 100mm of new concrete cover. 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) in potable water retaining structures. Cementitious coatings can be applied by steel float, brush and spray.
Cementitious coatings are also used as fairing coats or sealer coats for old or damaged concrete. The fairing coat is used to fill out blow holes or voids in the surface of the concrete and in some cases to fill out the concrete returning it back to its original profile before a protective coating is applied.
Polyurea protective coating
Polyurea is a protective coating that is highly resistant to hydrogen sulphide and hydrocarbons commonly found in wastewater tanks. It is a highly elastic waterproof coating resistant to chemicals and abrasion.
Polyurea cures in seconds. It has rapid re-use times. Because of their elasticity, polyurea coatings are ideal for substrates such as concrete, which tend to crack. As an elastomer, these coatings have the ability to stretch and bridge gaps. Due to this flexibility, polyurea coatings are not limited by extremes of temperature, while rigid coatings tend to crack as the substrate expands or contracts under these conditions.
Polyurea can be built to any thickness in one coat so costs associated with application are reduced. Polyurea coatings also offer excellent adhesion. They are regularly tested at up to 2000 psi, compared with conventional epoxy coatings at 600 to 800 psi. Aliphatic polyurea coatings have exceptional UV light resistance, so they will withstand weathering and exposure to sunlight when applied to exterior surfaces.
Clarifying agents, chemicals used for PH correction and disinfection in clean water tank structures also have damaging effects on concrete. Polyurea is pending DWI approval which will significantly open up opportunities to protect clean water tanks in the near future.