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Content archived on 2022-12-23

Corrosion of steel in reinforced concrete structures

Objective

A. BACKGROUND

Reinforced concrete has been used successfully in the construction industry since the early years of this century and now it is one of the major building materials. Over the years a large number of reinforced concrete structures has been constructed, for example many of the structures erected during the rebuilding of Europe after the Second World War. We now have a large stock of reinforced commercial buildings, domestic dwellings, marine structures, bridges etc., some of which are beginning to show signs of deterioration, particularly those over 30 years of age.

As time goes on we will be faced with an increasing number of structures which require repair and maintenance. As fashions and requirements change, structures and groups of structures will require remodelling and in some instances replacement. In order to ensure that the new structures are more durable, we need to understand what can be done to ensure that the risk of corrosion of the reinforcing steel is reduced to a minimum.

Before any work is carried out on an existing structure it must be assessed to determine its condition. Work carried out in COST 509 has gone some way in developing means of assessing and monitoring the condition of the steel reinforcing in the concrete and determining the repair requirements. However not all aspects have been investigated particularly those relating to prestressed concrete. Techniques have been developed to determine if corrosion of the embedded steel is occurring and the rate of corrosion can also be estimated. These techniques only give a snapshot of the conditions prevailing at the time of the measurements and continuous monitoring is necessary to collect more data of the corrosion rate in order to make a more reliable prediction.

Having assessed a structure and determined what needs to be done to maintain or repair the structure an engineer needs to know the options for repair and the effectiveness of repair techniques. Work has been done on electrochemical methods of rehabilitation of reinforced concrete in COST 509 but because of the breadth of the subject it was not possible to cover all the possible methods of repair and there are still areas which need evaluating.

Whilst the above two topics deal with existing structures and are applicable to most steel reinforced concrete structures there are treatments which are carried out to structures as a result of change of requirements e.g. to improve the thermal efficiency of a building, to overcome problems which are not associated with reinforcement corrosion such as rain penetration between concrete components or simply that they are no longer considered aesthetically pleasing and the environment needs enhancing. The exterior of a structure such as a block of flats constructed from large precast concrete panels may be overclad for reasons such as these. Over cladding is also used over reinforced concrete structures, which are suffering corrosion. The assumption being made that if the concrete is sheltered from the weather and therefore kept dry, corrosion of the reinforcement will stop or at least be reduced to a tolerable level. The validity of this argument has not been substantiated. As the structural stability of the overclad structure still relies on the original reinforced concrete it is essential that we understand how the concrete behaves beneath the cladding. We need to know if the rate of carbonation of the concrete increases beneath the cladding and if the risk of corrosion of the steel in the concrete decreases.

With the changes of life styles, there is often a need to bring new facilities to existing mature areas such as shopping, leisure and sports facilities. The creation of such facilities is expensive and therefore it is essential that buildings housing such facilities are durable. Methods to ensure the required durability is achieved must be developed.

B. OBJECTIVES AND BENEFITS

The objective of this action is to support the construction industry and owners of steel reinforced concrete buildings and structures by the technical and economic optimisation of the resources used to construct, monitor and maintain steel reinforced concrete buildings and structures. (The annual cost of repair of reinforced concrete structures in Western Europe is in excess of ECU 5 billion. A problem which is likely to be even greater in Eastern Europe).

It is expected that following the final workshop of COST 509, in September 1996, the attendance of technicians from European countries concerned with the service life of concrete structures (designers, owners, and those responsible for maintenance of structures) will encourage a much wider participation of private companies and public agencies. This will help in achieving one of the most important goals of a COST action; to test in the field most of the proposed solutions for a prolonged service life.

To realize this objective the problem will be tackled from four aspects.

Prevention which will be to evaluate the options available to reduce the risk of corrosion of metal reinforcement embedded in concrete and corrosion protective treatments applied to the concrete before conditions have developed where corrosion of the metal reinforcement may occur. The development of such techniques will lead to structures of longer maintenance free lives with a corresponding reduction of cost in use.

The development of Monitoring procedures by which corrosion of the embedded metal can be identified, in particular before corrosion has progressed to a point where there is physical damage to the reinforced concrete. Similarly the development of sensors which can be embedded in the concrete can give advanced warning of a corrosion problem. The availability of such techniques will allow the evaluation of structures when there is a transfer of ownership and allow the prospective new owners to assess the long term costs of taking over a building. The early identification of problems and early action will also result in better planning of maintenance and/or a reduction of repair costs. It is intended that the monitoring studies will be extended to real structures such that advice will be available based on practical experience.

Maintenance techniques will be evaluated to increase their effectiveness and to prolong the life of the maintained structure with the aim of removing the need for continual maintenance and reduce the cost in use of the structure. As with monitoring it is intended that the work will be extended to real structures so that practical advice will be available to the practitioner.

Many older, structurally sound, reinforced concrete buildings have become aesthetically unacceptable or are suffering habitability problems such as condensation, rain penetration and poor thermal efficiency. Such buildings can be revitalized by overcladding. However the Effect of overcladding existing concrete structures on the performance of the reinforced concrete needs to be evaluated to ensure the expected life of the environmentally enhanced structure. The information from this study will be of special practical importance to potential participants of the COST Action "Housing Estate Regeneration (Panel constructed dwellings)" submitted to the Urban Engineering Technical Committee for approval, providing the tools for engineers to assess the reinforced concrete structure of blocks of flats and give guidance on methods of repairing the concrete. Information will be available to enable a decision to be made as to whether an economic repair can be carried out before decisions are made on the upgrading of the flats.

C. SCIENTIFIC PROGRAMME

The scientific programme will be carried out under the four topics outlined above, Prevention, Monitoring, Maintenance and the Effect of overcladding existing concrete structures.

Prevention will look at the various ways of preventing corrosion of metals embedded in concrete in predominantly new construction. Alternative reinforcing steels will be assessed to determine in which conditions they are suitable and to determine under which conditions they are cost effective. In the last round, COST 509 looked at the suitability of epoxy coated steels and has given guidance on their suitability. In this Action it is proposed that stainless steels and galvanized steel are investigated so that the engineer can have a range of corrosion resisting materials from which to make his selection according to the conditions that his structure will be expected to endure and his cost restraints.

The use of inhibitors will be studied to determine if (a) they can provide protection to the embedded metals, (b) they provide protection for an adequate length of time and (c) that they do not lead to reduction of the structural properties of the concrete. Inhibitors can be used either as a material cast into the concrete at the time of manufacture or can be added at a later date either before corrosion has occurred or after corrosion has occurred as part of a repair and maintenance package.

The Monitoring section of this Action will investigate mechanisms by which changes in concrete and prestressing steel can be monitored and develop practical methods by which these techniques can be applied. Sensors which can be embedded in the concrete at time of construction to give advance warning of corrosion problems will also be studied.

Corrosion of steel in concrete is generally associated with either carbonation of the concrete or the ingress of a deleterious material into the concrete. Carbonation is the loss of alkalinity which is the result of the diffusion of carbon dioxide into the concrete and its reaction with the alkaline materials of the cement matrix. At the pH of uncarbonated concrete, greater than about 12,5, plain carbon reinforcing steel is passive. With carbonation the pH of the concrete falls to a level at which the steel is no longer passive and at risk to corrosion. The passive layer of steel in uncarbonated concrete can be disrupted by the presence of deleterious materials such as chloride and the steel can corrode. Monitoring for changes of the properties of the concrete will give advanced warning of the onset of corrosion.

In COST 509 the assessment of electrochemical techniques suitable for monitoring steel in concrete was made. It was apparent from this work that the techniques investigated were not appropriate for the study of prestressed concrete structures and that alternative methods would be needed if prestressed steel was to be monitored.

The Repair technique applied to concrete structures suffering deterioration due to the corrosion of the embedded reinforcement will vary according to the cause of corrosion, the requirements of the owner of the building as well as the cost of the repair. In general terms the most expensive technique is cathodic protection especially if an external electrode and overlay are required. Cathodic protection is normally applied to structures suffering corrosion due to the presence of chloride in the concrete. In this Action it is proposed that techniques for low cost cathodic protection will be investigated and at the same time the suitability of schemes for carbonated concrete will be studied. In COST 509 the effectiveness of desalination of concrete as a repair method was assessed and it was found that the biggest unknown was the speed at which chloride would diffuse into the concrete about the reinforcing steel from the bulk concrete. Such information is essential if the long term effectiveness of this repair technique is to be determined and whether it will be necessary to carry out retreatment at intervals.

The most common method of repair is the replacement of cracked and spalled concrete with patch repair materials. This method has slowly evolved on the basis of trial and error. There has been no systematic assessment of the properties required from repair materials nor has there been a systematic evaluation of the long term performance of the materials used for patch repairs. Patch repairs deal with the local defect but there is no information as to the effect of the change of environment within the patch on the steel in the unrepaired concrete. These are all questions, which need to be answered to ensure effective durable patch repairs. It is intended that the part of the Action will lead to an understanding of the properties that a repair material requires.

New developments using inhibitors to arrest corrosion in existing concrete and as an additive to repair mortars are being introduced and these also need assessing in the same way to ensure that they are effective and that they are used effectively.

The effect of overcladding an existing concrete structure is not understood. The practice of environmentally enhancing structures is becoming widely used. The procedure has been used for structures with widely differing uses such as bridges and high rise blocks of flats. Such cladding is also used to correct defects in structures such as rain penetration or used to improve the thermal efficiency of the structure. The main load bearing structure of these clad buildings is still the reinforced concrete and the concrete must remain sound to ensure the structural stability of the structure. The effect of the overcladding of the concrete must be understood. Does the rate of carbonation increase, does the rate of corrosion decrease or increase, how is the performance of the concrete monitored? This Action is intended to address these questions.

D. ORGANIZATION AND TIMETABLE

This action will be carried out in four distinct groupings relating to Prevention, Monitoring Maintenance and the Effect of overcladding concrete. Each group will have a leader with delegated authority from the Management Committee who shall be in overall control of this Action. The group leaders will be responsible for ensuring that the members of each group collaborate and that regular contact is maintained and arranging group meetings when appropriate. The group leaders will report on progress to the Management Committee at its regular meetings. The Management Committee will report to the Technical Committee. Workshops will be held at yearly intervals to ensure that all collaborators in the Action will have the opportunity to follow the progress of each group and to ensure that there is inter group collaboration. An invitation will be extended to COST Action "Housing Estate Regeneration" within the Urban Engineering Technical Committee's programme to these workshops to provide information transfer between the two Actions.

During each twelve months after the group leaders have been appointed the groups will meet not more than 2 times per year. The timing of the meetings to be determined by the work programme agreed at the first group meetings.

Short Term Scientific missions will be made where appropriate, the Management Committee deciding on the allocation of funds.

E. ECONOMIC DIMENSION

The following COST countries have actively participated in the preparation of the Action or otherwise indicated their interest: Austria, Belgium, Czech Republic, Finland, France, Germany, Ireland, Italy, The Netherlands, Norway, Portugal, Slovenia, Spain, Sweden, Switzerland, United Kingdom.

On the basis of national estimates provided by the representatives of these countries and taking into account the coordination costs to be covered over the COST budget of the European Commission, the overall cost of the activities to be carried out under the Action has been estimated, in 1996 prices, at roughly ECU 6,7 million.

This estimate is valid under the assumption that all the countries mentioned above but no other countries will participate in the Action. Any departure from this will change the total cost accordingly.

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Italy

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