It may be possible to avoid external restraint by the provision of movement joints, but in most structures it is necessary to cast subsequent pours against hardened concrete in order to satisfy the requirements of continuity in the structural design. The risk of thermal cracking is particularly great in canti- levered retaining walls for reservoirs, basements, bridge abutments, etc. When a vertical section of a wall is being cast, the restraint provided by the base of the wall and by adjoining sections is considerable. Cracks are likely to occur unless precautions are taken. The spacing of joints and the sequence and timing of concrete pours must be carefully planned. If the whole concrete mass is insulated so as to eliminate internal restraint, there will be no cracking provided also there is no external restraint. When the latter is not possible, the risk of cracking can be minimised in several ways by considering the conditions for cracking to occur, namely:
Thus, the induced tensile stress will be minimised by reducing the terms of equations 3.10 and 3.12 namely: ·
The coefficient of thermal expansion can be reduced by selecting the concrete mix ingredients carefully. For example, lightweight aggregate has a lower thermal coefficient as well as a lower effective modulus (more creep). Other types of aggregate (notably limestone) have low coefficients of thermal expansion (BS 8110: Part 2, 1985). ·
The difference between peak and ambient temperature …Tp ÿ Ta† is reduced by cooling the mix ingredients. For example, when the ambient temperature is around 20 ëC, cooling the mixing ingredients to approximately 7 ëC will reduce the peak temperature, Tp, by a corresponding amount. Compared with cooling the cement and aggregate, it is often more convenient and effective to cool the mixing water with the addition of ice, which uses the heat from the other ingredients to provide the latent heat of fusion. ·
The peak temperature can also be minimised by choosing a low-heat type of cement, such as blended cement with fly ash (pulverised fuel ash) or ground granulated blastfurnace slag, which will reduce the rate of temperature rise as well as the peak temperature. · The restraint factor, R, can be reduced by insulating formwork for thick sections (>500 mm) and by having longer formwork striking times. Timber formwork provides better insulation than steel or glass reinforced plastic.
