A sprayed cementitious repair is designed to give concrete water treatment tanks a 20-year lease of life.
Wessex Water has recently undertaken a major concrete repair project at its Ashford water treatment works (WTW) near Cannington in Somerset.
The repair was needed because concrete water tanks had deteriorated over time due to chemical attack from the treatment process. The reinforced concrete substrate was showing extensive signs of erosion, resulting in 20mm losses of concrete in some areas.
Materials supplier Flexcrete Technologies and coatings systems contractor Ultimate Coatings collaborated to provide a system designed to last at least 20 years before the first maintenance is required. The solution involved applying around 24t of materials.
In 1988, a 2mm thick film of Cementitious Coating 851 was applied to a concrete slice and sealed in a chloride ion diffusion cell - it is still going strong 8,750 days later
Before the repair could start, the contractor removed the weak concrete using high-pressure jet washing. The whole surface was then over coated with two 1mm spray-applied coats of Flexcrete’s Cementitious Coating 851.
This two-component, waterborne cementitious modified polymer coating has been developed to create an effective barrier against the effects of aggressive acid gases, moisture and chlorides. It also has greatly enhanced chemical resistance.
It forms a hard, highly alkaline coating that protects and waterproofs the concrete, and has been designed to resist positive and negative pressure under a 100m head. Importantly for Wessex Water, the material has been approved for use in contact with drinking water.
It is also CE marked in accordance with BS EN 1504 Part 2, the pan-European standard for concrete repair, and approved by the British Board of Agrément.
Flexcrete claims the 2mm coating of its Cementitious Coating 851 will provide the equivalent cover to 100mm of good quality concrete.
It says that, being cement based, the material chemically reacts with the existing concrete substrate to form an integral part of the structure. It will also have a design life to match that of the concrete to which it is applied.
The supplier has evidence that the product will resist chloride penetration for at least 24 years. In 1988, a 2mm thick film of Cementitious Coating 851 was applied to a concrete slice and sealed in a chloride ion diffusion cell in a laboratory at the Vinci Construction Technology Centre - and it is still going strong over 8,750 days later.
“Tests recently carried out show that the barrier properties of 851 are such that a steady state of flux of chloride ions through the coated concrete has not been attained after a test period spanning 24 years, whereas the control concrete achieved this in just 28 days,” says Flexicrete.
The application of the coating at Ashford WTW will prevent further chemical attack to the concrete.
During the project the slabs of the tanks were also cleaned and primed using Flexcrete’s Polymer Admixture 850, to prevent any out-gassing, before a single 2mm spray coat of the 851 cementitious coating was applied.
Ashford WTW is one of Wessex Water’s showcase facilities, and incorporates an education centre for visits from local schoolchildren.