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Reinforced and prestressed concrete are structural materials. Two major failures (Wood, undated, accessed July 2010; Wood, 2003, 2008) following inadequate repair contracts have highlighted the importance of structural consideration of both deterioration and repair. When damage or deterioration has occurred necessitating a repair, the changes to the structure before, during and after the repair need to be a primary consideration. This will throw up a range of questions about the structure and how the actual build quality relates to the designers’ intentions, current standards and how deterioration has altered this.

The IStructE publication ‘Appraisal of existing structures’ (IStructE, 2010) deals comprehensively with the process of structural appraisal. If

Figure 3.1 De la Concorde overpass after collapse, five dead. The deteriorated badly repaired half joint and lower part of the cantilever can be seen hanging down.

reinforcement drawings are available, they will need to be validated with checks on reinforcement size, location and cover. If they are not available, it is essential that they are recreated from surveys. This is very expensive and underlines the value to owners of maintaining full records of construction, modifications and repairs.

The structural reinforcement cover surveys are complementary to and should be integrated with the investigations to evaluate cover in considering corrosion risk. However, missing bars and top reinforcement cover greater than specified will go unremarked in a corrosion survey, but they may indicate serious structural deficiencies even before deterioration is considered.

When damage or deterioration becomes obvious to the owner, the first questions relate to overall safety and the usually more immediate risk to the public from falling spalls. So, structural engineering input (Wood, 2006) to the investigation and subsequent repairs is essential. The amount of deterioration concrete structures can suffer before remedial measures are necessary is small. Codes for concrete design include no margin for deterioration and modern design to minimise costs has reduced strength reserves and robustness.

Focusing the investigation on the areas where there is structural risk from vulnerable details and/or the worst deterioration usually enables a better and more cost-effective diagnosis to be made than random sampling. Figure 3.2 shows inadequate bearing seating of precast beams below a leaking car park joint.

Careful evaluation of the causes of cracking is an essential procedure in the appraisal of a concrete structure. Cracking arises from:

• Structural tensile strains from flexure and shear

• Early age ‘non-structural’ effects, thermal, shrinkage, etc

• Long-term ‘non-structural’ conditions corrosion (Concrete Society, 1992), AAR (BRE, 2007, IStructE, 1992, 2010), etc.

Relating crack widths to causes and monitoring changes in crack widths are powerful diagnostic tools. Any area where tensile strains exceed about 150 microstrain will crack and the crack orientation indicates the principal stress direction. Compressive stresses suppress cracking. Cracking relates to the combination of all structural and ‘non-structural’ effects. For example, the axial compression in a column suppresses map cracking from AAR so only vertical cracking develops. Cracking provides a channel for the ingress of chlorides and carbonation.

Sets of cores taken during an investigation which contain cut reinforcement immediately raise alarm bells for the structural engineer. Not only is it extremely difficult to structurally reconnect a cut reinforcing bar, but the structural damage from cutting a bar may be far greater than that from the deterioration being investigated. All coring proposals need to be checked by a structural engineer and coring should be preceded by a cover survey of

Figure 3.2 Inadequate seating of precast beams below a leaking car park joint (Wood, 2009).

Figure 3.3 Ferroscan and 50 mm core sampling with pull-off tests of repairs on Tuckton Bridge (from Wood, 2008).

the area to ensure that the core goes between reinforcing bars, not through one. The core size should be minimised to that which is essential for tests, typically 50 mm or 70 mm.

Once the reinforcement layout and visual signs of corrosion at spall locations are apparent, a sensible estimate, guided by selective opening up, needs to be made of the extent of corroding bars which are not yet spalling the cover. Corrosion of a bar near the surface will cause a spall, but if the bar depth is more than twice its diameter there will be a very severe loss of section before a spall can develop. With closely spaced deep bars delamination of an area can develop, rather than individual spalls above bars.

One needs to consider the likely condition of steel in hidden concrete behind cladding and in joints where carbonation and/or chloride ingress will have developed. Periodic moisture from condensation, driving rain and/ or due to failing sealants can then accelerate corrosion. Some cladding has hidden fixings of mild steel, which will corrode, or galvanised steel which last only a little longer or of stainless steel which is good, as long as there is no bimetallic corrosion.

When concrete is saturated, which limits oxygen availability, and there is a high chloride concentration, corrosion develops as ‘black rust’ without the expansion to produce spalling. This makes it difficult to detect. Structurally ‘black rust’ is important as it tends to cut bars locally, (Figure 3.4), especially the bends on the shear links and column stirrups which rapidly reduces the beam or column strength.

Figure 3.4 Local pitting corrosion of links in a half joint, from chlorides in saturated concrete.

Half-cell surveys help identify the areas of active corrosion on that day and corrosion rate measurements give an instant number, but they give no indication of the severity of reinforcement section loss. Both work best when concrete is damp. They can help locate areas for selective opening up to answer the structural question: ‘How much loss of section and bond strength at laps has occurred over the life of the structure?’

Selective opening up can give a better indication of current damage and the likely extent of extra cutting out for repairs to effectively control corrosion. Where conditions are right cathodic protection (CP) can reduce the need for cutting out. However before applying CP the safety of the already corroded structure must be checked.

It is an unusual repair contract which does not reveal that deterioration is more extensive than was apparent from the initial investigation. There is a strong case for having a trial repair contract to enable more extensive and deeper destructive investigation and evaluation of the effectiveness of repairs before a large-scale contract is let.

Effective repair requires cutting out to a sound substrate and to clear all carbonated and chloride contaminated material around the reinforcement (Figure 3.5). This cutting out inevitably further weakens the structure and the effects of this must be evaluated before the contract is let, including the potential consequences of more extensive cutting out, if that is found to be necessary as work progresses.

Figure 3.5 Structurally damaging corrosion which will be substantially increased by cutting out to clear chlorides.

The cost of access and disruption associated with repairs makes it important to be aware of the extent to which further deterioration may develop over the next 20 years and its structural consequences (Wood, 2009). Tuckton Bridge, built in 1905, is a good example of progressive cycles of repair (Wood et al., 2008). Often deterioration has been aggravated by bad local details creating ponding or channelling moisture and salt into joints. Upgraded drainage and providing shelter to slow deterioration should be considered in the prediction of future deterioration.

Extrapolation from the statistical analysis of cover depth related to chloride and/or carbonation profiles, including the data from the deeper investigation in the trial repair contract, should enable future trends to be predicted. The likely cost of these future repairs, as well as the currently proposed repairs, relative to the value of the structure must be considered before embarking on extensive remedial work. The poor performance of many concrete repairs, with 50% failures reported within 10 years (Matthews and Morlidge, 2006), needs to be considered in the prediction.

Some concrete structures are highly stressed with thin members with little margin for deterioration. Corrosion in exposed columns creates particular difficulties as this can involve cutting out behind all stirrups and main bars leaving just a central core of uncontained concrete, which makes removal and full recasting a more reliable option than patching. With precast construction, the joints may be badly constructed and poorly toleranced and they are particularly vulnerable to deterioration.

Other forms of concrete structure, sometimes for architectural effect, use large low-stressed members which can be cut into with little detrimental effect. An evaluation of stress levels and reinforcement configuration in a member will give a preliminary indication of the need for achieving a structural repair. For cosmetic reasons and/or corrosion control the repair material does not necessarily need to reinstate the load-carrying role of the original concrete, but it is important that it stays safely attached.

Where the reinforcement is corroded the structural role of the bars needs to be reconsidered relative to their current condition. Appraisal may have shown inadequacies in the original design relative to current requirements for shear strength, robustness and/or impact resistance. It may be appropriate, and in other cases essential, to remedy these shortcoming as part of the overall remedial works.

Large concrete members often contain reinforcement for early age crack control. If these bars with low covers are corroding, it may be appropriate, subject to a structural check, to cut some of them out to facilitate repair.

The three golden rules for design also apply to concrete repair. One must balance the three key objectives of:

• Fitness for purpose, including restoration of strength and control of further deterioration

• Cost effectiveness, including minimising disruption costs to the users of the structure

• Delighting the eye, either restoring to match the original concrete or providing an overall configuration and finishes to suit the continuing use.

Cutting out and recasting concrete to be structurally effective is difficult, time consuming and expensive. Sometimes it is simpler to provide a ‘belt and braces’ structural bypass, minimising the cutting out and leaving the original material to continue to deteriorate. In other circumstances, the complete removal of the element and recasting of the whole will be simpler, cheaper and structurally more effective then patching. There can be no standard rules: the remedial strategy and the detailing must be developed on a case- by-case basis.