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F. Papworth and P. Bamforth
There are three stages in the design life where cracking can occur, the plastic stage due to changes in concrete while it is hardening, early age due to changes while concrete cures in the hardened state and long term load induced cracks. Designers generally follow Australian Code requirements for maximum allowable strain in the reinforcement and assumes that will take care of early age strains, and in most concrete it does. However, where fine crack widths are required, or where concrete with a high heat output is used, code requirements alone may be insufficient to control crack widths adequately. Designers also often leave the plastic crack control entirely to the contractor. Unfortunately that may not be the best approach as specifications for mix design have a high impact on the requirements for plastic crack control. This paper considers requirements for all three stages from a design perspective to highlight where additional guidance might be given in Australian codes.
F. Papworth and P. Bamforth
In the UK, BS8007 has provided the basis for the design for early-age thermal cracking. This is to be replaced by EN1992-3 and in conjunction with the replacement of the general design code, BS8110 by EN1992-1-1 this has led to significant changes in some aspects of design. BS8007 was supported by CIRIA 91 which provided background to the design method and data for use in the design process. This has been updated and replaced by CIRIA C660 which brings the design into line with the Euro-codes and provides current information required to support the design process. The significant changes are described and their implications are discussed. AS3600 and AS3735 crack control provisions are based on the principles of BS 8110 and may need revision to take account of the changes outlined herein.
F. Papworth, Reuben Barnes, Michael Fox, R. Munn
The reaction of cement with water is exothermic. As the heat can not immediately escape it builds up in the body of the concrete. Over the first 36hours the cement reacts rapidly and the concrete temperature quickly rises. Subsequently, after the concrete has gained considerable stiffness, the concrete cools. Depending on the thickness of the concrete, the ambient conditions and any insulation, it takes the centre of the pour from 24hrs-4weeks to cool back to ambient. The early temperature history has dramatic effects on the concrete properties,some positive some potentially negative. On the negative side high concrete temperatures can induce microcracking and a chemical change to the cement hydrates. Possible outcomes are 30% strength reduction, early thermal cracking and Delayed Ettringite Formation (DEF). This is discussed in section 1.
On the positive side the high temperature leads to rapid hydration & high early age strengths enabling early stripping, prestress application and curing cessation. By predicting and measuring insitu temperatures the contractor can plan for and verify high early age concrete strengths using the maturity or matched cure methods. This is discussed in section 2.
Insitu concrete temperatures and resultant strains can be predicted mathematically and reinforcement can be designed to distribute any cracks to an acceptable level. However, the designer is generally unaware of the proposed construction scenario. Furthermore Australian Standards only provide a general reinforcement ratio based on required degree of crack control. There are no Australian Standards for designing based on actual insitu temperatures, restraint and allowable crack width. Methods the designer and contractor can use to predict insitu temperatures are given in section 3 and a crack widths design process is described in section 4.
Project examples of the methods described are given in each section.
F. Papworth, P. Chen, P. Trinder, A. Peak
This paper summarises the factors controlling concrete’s heat of hydration, the likely problems caused by the heat generated and the available solutions using blended cements. The heat of hydration of various silica fume and slag cement blends are compared theoretically and experimentally. Results show that under certain circumstances an OPC/silica fume concrete is likely to out performs high slag blends in thick slabs and that a slag/silica fume triple blend offers the best performance at all thicknesses. Use of blended cements to reduce temperature rise on five contracts is outlined to show the use and limitations of the model.