SupportinCgolumns Deterioratioonf 20th century concretebuildings Brian Morton looks at causes of, and solutions to, structural problems with concrete buildings. In recent years the deterioration of 20th century reinforced concrete buildings has become a very significant problem. In simplistic terms the problem is associated with the corrosion of reinforcement which can be classified into two types: General corrosion associated with carbonation Pitting corrosion associated with chloride ions The visual signs of serious concrete deterioration are cracks, spalls and rust stains. It is important to determine the cause of deterioration and the likely consequences before deciding on the type and scale of remedial works. The durability of a concrete structure is affected by original design faults, poor detailing, the use of unsuitable materials, shortfalls in workmanship and lack of routine maintenance. These inadequacies accelerate penetration of the concrete by atmospheric carbon dioxide and water borne chloride ions from the environment. Serious breakdown occurs when concrete is permeable or concrete cover to reinforcement is deficient. Permeable concrete is caused by high water/cement ratios, low cement contents, inadequate curing and poor compaction. Lack of an effective waterproof membrane, blocked surface water drainage and leaking expansion joints are all means by which water reaches the surfaces of roof slabs, columns and beams. Concrete with a high alkali content, reactive aggregates and sufficient moisture also puts a structure at risk from alkali- silica reaction. Detail shot of window surround to Water Tower building in Suffolk showing spoiling of the concrete after corrosion of reinforcing rods. Frost damage and leaching are probably the most common causes of surface deterioration. Frost damage usually starts as scaling and may be followed by loosening of the surface aggregates, and is most prevalent on horizontal surfaces. Frost damage on vertical surfaces is often associated with spalling and progressive cracking. Poorly compacted concrete of poor quality and concrete containing shrinkable aggre28 gates are readily saturated and become vulnerable. Leaching is caused by water migrating through the permeable concrete reducing both its alkalinity and its strength, as it leaches lime from the concrete and deposits it on the surface. Evaporation of the moisture leaves unsightly deposits on the surface. Leaching is frequently associated with construction joints although these are not essential for its occurrence. Fine cracks may occur in tensile zones of reinforced concrete members under normal design loading conditions because concrete can only sustain a relatively small tensile stress before cracking develops. However, cracking may also form for a variety of reasons, eg structural deficiency, settlement or alkali-silicareaction. Thermal and shrinkage cracks can be associated with high cement content and high strength concrete. Carbonation is a chemical reaction between atmospheric carbon dioxide and hydrated cement compounds which causes a reduction in the alkalinity of the concrete. The rate of carbonation is dependent on the permeability and moisture content of the concrete. General corrosion is characterised by superficial corrosion products being generated over large areas of the reinforcement causing cracking and spalling of the concrete. Chloride attack is caused by the minisc;ule and highly mobile free chloride ion penetrating the concrete through water present in the pour structure. Through electrochemical action pitting corrosion occurs to the reinforcement. Dealing specifically with the affects of these chemical reactions, generally known as 'concrete cancer', is a very serious problem which is generally not discovered until there are significant issues of further protection and repair if the building is to be saved. The cracking generally commences in localised areas. If an investigation is carried out at this time and the extent of carbonation is determined by spraying a chemical indicator on a fractured surface, then it is possible to reduce the effect on the structure by surface treatment of the rusting reinforcement and Context65March2000
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