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The size of the UK concrete repair sector is estimated to exceed 3% of the entire construction industry. Unplanned maintenance and refurbishment work forms a significant proportion of this. In many cases concrete repair is done in an unstructured way, often resulting in inappropriate repairs that require further, premature repair.

Steel rebar embedded in concrete is prevented from corroding in most circumstances by the formation of a protective passive layer or film on the steel surface, which is maintained by the highly alkaline environment of concrete. However, if the concrete becomes contaminated by chemicals that disrupt the protective layer or reduce the concrete’s alkalinity, the steel is prone to corrosion if moisture and oxygen are present. This leads to cracking and spalling of the concrete covering, requiring repairs.

Types of corrosion

Corrosion is caused within concrete from carbonation and chloride contamination, either individually or through a combination of both. Carbonation is caused by the alkaline elements reacting with acidic gases, usually the carbon dioxide that is present in the atmosphere. This reduces the alkalinity of the concrete so the protective layer on the steel breaks down. Chlorination is a process where there is an ingress of chloride ions from a variety of sources, including de-icing salts, marine environments and contaminated water resulting in more localised corrosion.

Types of repair

The choice of repair option should be determined by the extent of the damage, and the repair strategy should be supported by the most up-to-date and relevant information available. Remember, the earlier a problem is spotted, the greater the chance that it can be treated with a simple and cost-effective repair. Furthermore, having a good maintenance regime that involves checking the structure regularly for cracks and rust staining can reduce the extent of the damage.

Repair strategies can be divided into patch repairs, which are considered here, or electrochemical methods that are used to more generally limit or prevent further damage to structures affected by reinforcement corrosion.

BS EN 1504 covers aspects relating to the design and execution of repair and remediation treatments. Patch repairs are the primary method of repair and are versatile, cost effective and can be used for both small isolated repairs (hand applied) and large areas (sprayed concrete and flowable mortars). Patch repair requirements are less onerous when combined with electrochemical treatments. The main techniques for applying a patch repair are:

Hand applied

Carbonation on a building facade usually requires relatively small, isolated repairs and so lends itself to hand-applied patch repairs. The mortar or concrete is mixed then worked around and behind the reinforcement by hand in layers. The thickness of each layer and the application varies depending upon the material used and the orientation of the surface. A typical procedure is to apply layers of 25-50 mm thick on vertical work and

20-30 mm thick for overhead areas. Care should be taken when applying additional layers to ensure the previous mortar has gained sufficient strength, but has not set. If the following layer is delayed, the surface can be scoured and dampened with water before the next layer is applied, or a bonding bridge applied. The repair can be finished with a trowel using the surrounding concrete as a guide.

Recast

This technique is usually most suitable for large-volume repairs or where large areas of dense reinforcement are present. Access for vibration to compact the concrete is often a problem so flowable grouts and self-compacting proprietary micro-concretes have been developed to minimise the vibration required. Where bonding agents are used, the formwork and pouring sequence have to be carefully designed so they can be rapidly positioned before the bonding agent dries. The concrete or mortar has to be carefully placed to avoid the entrapment of air.

Sprayed concrete

Sprayed concrete can offer several advantages, including quick construction as formwork is reduced or eliminated, plus it can be rapidly placed. It also offers a good bond to the substrate. Sprayed concrete has been successfully used in many applications, including bridge soffits, beams, parapets and abutments; steel and reinforced concrete framed buildings; cooling towers; industrial chimneys; tunnels; water-retaining structures; jetties and sea walls.