Briefing: The effects of power washing on concrete durability

The cleaning of concrete is often undertaken on health and safety grounds, especially in a marine environment when controlling algal biofouling on stepped sea defences. Despite the fact that more local authorities are doing this, there is no published research or guidance on possible durability issues when exposing a marine concrete to regular power washing regimes. Surface analysis of concrete revetment armour at a UK study site was undertaken in order to investigate the effects of power washing over 3 years on durability. Revetment armour units were monitored from casting to placing on site and surface roughness was found to have doubled. Surface hardness and uniformity have also been measured indicating degradation. When casting revetment units upside down, vibration may encourage the larger aggregate to migrate downwards towards the exposed surface once inverted at the site. Large aggregate particles at the surface leave no surface zone, giving no erosion protection. It is at the top surface where the strongest, most impermeable and most wear-resistant concrete is needed. Continual water jet cleaning practices lead to a higher surface roughness, thereby increasing the surface area offering a bioreceptive surface for further, quicker and denser colonisation; these phenomena provoke and encourage each other.