Early Thermal Cracking To Eurocode

Abstract:The design of concrete members subject to early age thermal cracking in the UK has previously been carried out to BD281 for bridges and BS80072 for water retaining structures. These standards have now been replaced by the respective parts of BS EN 1992. CIRIA C6603 ‘ Early Age Thermal Crack Control in Concrete’ was published in February 2007 to accommodate changes that had occurred in the UK in relation to the wider range of cements and combinations being used; the development of high performance concretes; and the introduction of the Structural Eurocodes. The use of CIRIA C660 in combination with BS EN 1992 represents a significant step forward in the design methodology for early thermal cracking in the UK.The principal objective of CIRIA C660 is to support both designers and constructors by providing procedures to limit the extent of cracking to an acceptable level and, where appropriate, to avoid cracking altogether. The report and the associated spreadsheets provide a means for estimating all of the input variables for the design process, including thermal effects and profiles, shrinkage strains, restraint factors and concrete properties, and they set out processes for minimising the risk of cracking and the control of crack widths. These processes are aligned with the Eurocode requirements, extended for use in the UK.

This paper focuses on critical changes in design for early-age thermal cracking in the UK resulting from the introduction of BS EN 1992. These changes relate specifically to the way in which the reinforcement ratio is estimated for calculating the minimum steel ratio and the crack spacing and width.BS EN 1992 does not fully address design for early-age thermal cracking.

Dubbs Process

CIRIA C660 has been developed to complement the BS EN 1992 design process and provide estimated crack widths that reflect more reliably those observed in practice, so that a robust and serviceable design may be achieved. In the absence of these recommendations, BS EN 1992 could be interpreted to lead to a significant increase in minimum reinforcement, particularly in sections thicker than about 800mm. Conversely, in some other situations the use of BS EN 1992without CIRIA C660 could lead to significantly less reinforcement than required by previous standards BS 8007 or BD28 for controlling crack widths. The use of CIRIA C660 is therefore recommended in the UK for the design of early thermal cracking effects in combination with BS EN 1992.