Introduction

Concrete repair has been the core activity of the business since 1955. Reinforced concrete is a very durable construction material but it needs to be maintained like most other products if it is to achieve its design life.

Apart from physical damage to the concrete the main reason for deterioration is corrosion of the reinforcement.

Steel reinforcement in the highly alkaline environment provided by the concrete remains in a passive state because a stable oxide film on the steel surface prevents corrosion activity. However this oxide film becomes unstable if the concrete carbonates and it can also be broken down when there are sufficiently high levels of chloride.

Carbonation of the concrete occurs when carbon dioxide in the atmosphere dissolves into the concrete pore water to form carbonic acid. This reduces the alkalinity of the concrete and eventually when the process reaches the reinforcement it destabilises the oxide film. The process is time dependent and the rate is a function of concrete quality.

High chloride ion levels at the reinforcement can locally disrupt the protective oxide film and initiate pitting corrosion. The chlorides may have been added to the concrete when cast in the form of calcium chloride, which was used as an accelerator up until 1970 or unwashed marine aggregate may have been used. Alternatively chlorides from an external source such as marine environment or de-icing salts may enter the concrete over a period of time. Before repairing a reinforced concrete structure it is important to undertake a detailed survey to identify the reasons for the deterioration. CRL Surveys, a division of Concrete Repairs Limited (CRL), is able to provide a comprehensive service to inspect and report on concrete deterioration.

The Condition Survey

The purpose of the survey is to identify the mechanism and extent of the deterioration. The survey report should also include repair recommendations and possibly budget costs for the various repair options.

A simple flow chart such as the one outlined in Fig. 1 will normally produce sufficient information to diagnose the problem.

Chloride levels between 0.4 and 1.0 percent by weight of cement at the reinforcement present a medium risk of corrosion and above 1.0 percent there is a high risk. It is important to sample the concrete at different depths to provide a chloride concentration profile through the concrete. This will determine the chloride levels at the reinforcement and also indicate the source of the chlorides i.e. ingress or cast-in.

The depth of carbonation should be compared to the concrete cover to the reinforcement to assess whether the steel is in carbonated concrete and therefore likely to corrode.

Corrosion is an electrochemical process where the anodic areas are the corrosion sites and the cathodic areas are the passive areas of steel. By measuring the electrical potential of the reinforcement using a half cell a potential map can be prepared which indicates where corrosion is likely to occur. This procedure provides an indication of corrosion activity but it should be remembered that the results will vary through the year depending on climatic conditions and other variables. A more sophisticated procedure called linear polarisation resistance can be used to measure the rate of corrosion but this is a slower procedure and more expensive.

The condition survey is an important procedure, which should not be overlooked. The investment in a quality survey may save the client significant sums of money by identifying the most economic and effective method of repairing the concrete.

Specification and Procurement

The specification for the repairs is based on the results of the survey and the client requirements. CRL is an approved contractor for all the leading material suppliers in the UK so is able to advise on the most suitable for a particular project.

For traditional repair techniques the repair material suppliers can provide standard outline specifications which can be used in the contract documents.

The standard method of measurement published by the Concrete Repair Association is usually used as a basis for the bill of quantities.

Traditional Repair Techniques

(1): On Site Survey
With traditional repair systems only the cracked and spalling areas of concrete are repaired. These are identified by close visual inspection and hammer testing of the concrete.

(2): Concrete Breakout
The defective concrete is normally repaired using electric or pneumatic breakers to completely expose the corroding reinforcement. Saw cutting around the perimeter will avoid any feather edges to the repair area.

An alternative method of breakout is to use high-pressure water, which is called hydrodemolition. This removes the concrete and also cleans the steel at the same time. It is particularly effective for high volume repair areas or in environmentally sensitive situations where noise and vibration may be a problem. However it is also a dangerous procedure requiring very careful control at all times.

(3): Concrete Reinstatement
The exposed steel is cleaned, and treated and then the concrete is reinstated in accordance with the manufacturers instructions. The method of reinstatement can be either hand placed mortars, shuttered repairs, or sprayed concrete. Hand placed mortars are typically used for building repairs where the repair areas are small.

Larger repairs can be shuttered and re-cast using a flowable concrete. Alternatively they can be sprayed back using either a wet or dry gunite process.

It is important to remember that no matter how good the material. the repair is only as good as the operative on site who is applying it. The operative should be trained and experienced in the use of the particular material. CRL keeps training records for all its site employees and has its own "School of Excellence" which is an apprenticeship scheme for new employees to the industry.

(4): Surface Coatings
The corrosion of reinforcement requires both oxygen and water to support the activity. The application of a good protective coating will slow down any corrosion and reduce further carbonation or chloride contamination.

To ensure a good bond the concrete substrate needs to be correctly prepared. Existing paint surfaces will need to be removed by grit blasting and the exposed surface prepared using a thin coat levelling mortar. Unpainted surfaces are cleaned by high pressure water jetting or possibly light grit blasting.

The protective coatings can be applied by brush, roller or spray depending on the manufacturer’s instructions. To ensure an adequate and uniform coverage alternative coats should be different colours and a wet film gauge may be used to monitor film thickness.

Corrosion Control

Traditional repair techniques are perfectly adequate when there is no risk of further corrosion due to carbonation or chlorides.

For long term durability in high corrosion risk areas it is important to consider using a corrosion control system. There are currently two generic types of corrosion prevention and these are corrosion inhibitors and electrochemical repair systems. For further information refer to the section on Corrosion Control.