ABSTRACT
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
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.
