Material Testing

HAC differs from Portland cement, being composed of calcium aluminates rather than calcium silicates. Its rapid strength development made HAC popular for precast concrete in the UK during the 1960s. Mineralogical ‘conversion’ however, sometimes caused catastrophic reductions in concrete strength and increased vulnerability to chemical degradation. Three UK roof collapses in the mid-1970s led to widespread inspection and monitoring of HAC concrete units, exhaustive research and curtailment of HAC use for structural purposes. A large stock of UK buildings containing HAC concrete remains, in which the HAC is now usually highly converted. Whilst the probability of sudden collapse is now perhaps remote, there is continuing concern over long-term durability, particularly where carbonation has occurred to the depth of steel reinforcement. Analysis of samples for HAC should be carried out by a UKAS accredited laboratory in accordance with BRE IS 15/74.

Steel embedded in good quality concrete is protected by the high alkalinity pore water, which, in the presence of oxygen passivates the steel. The loss of alkalinity due to carbonation of the concrete or the penetration of chloride ions arising from marine or de-icing salts can destroy the passive film. In the presence of moisture and oxygen corrosion of the steel occurs. A characteristic feature of the corrosion of steel within concrete is the formation of macro cells; that is the coexistence of two areas of different electric potential appearing on the same steel surface. This forms a short-circuited galvanic cell, with the corroding area as the anode and the passive area the cathode. The current flow in the concrete is accompanied by an electric field which can be measured at the concrete surface, resulting in equipotential lines that allow the location of the most active corrosion to be identified by the most negative potentials. This is the basis for potential mapping which has become a technique applied to the inspection of reinforced concrete structures and detailed within ASTM C876–09. According to the ASTM method, corrosion can only be identified with 95% certainty at potentials more negative than -350 mV. Experience has shown, however, that passive structures tend to show values more positive than -200 mV and often positive potentials. Potentials more negative than -200 mV may be an indicator of the onset of corrosion.

The removal of core samples for laboratory analysis allows a wide range of definitive data to be obtained, the aggregate type, size, positioning and condition may be crucial to the overall repair recommendations. Similarly, the level of compaction and water cement ratio play an integral part of the overall assessment procedure. In addition, the use of an electron microscope allows the cement paste composition to be analysed in depth, with the effects of carbonation and chloride migration being able to be plotted precisely against reinforcement depth. The effects of heat transfer through cover concrete as a result of fire are clearly identifiable using core samples and laboratory analysis, where site based procedures are less concise. The overall compressive strength of the concrete may also be determined by crushing the sample in accordance with Compressive strength–BS EN 12504-1 and BS EN 12390.