Concrete hydration temperatures for the design of crack-width reinforcement in concrete water-retaining structures – design values versus in-situ values

摘要

Cracking in concrete can occur due to temperature changes at early ages and exposure to ambient temperature changes in the long term. Design codes and standards allow engineers to design for cracking by quantifying the effects of thermal variations into outputs such as limiting crack widths and reinforcement configurations. Design values given in these codes are however not fully understood by many users, may not be representative of recent developments in concrete materials technology and can potentially result in over-conservative designs. In this paper, concrete hydration temperatures were measured on site using a Thermocouple Data Logger and compared to values used in the project-specific design with the intention of providing a basis on which a database of temperatures representative of mixes commonly used in the South African industry may be compiled. Findings revealed that measured temperature values were around 30 – 50% lower than those given in design codes. Among the reasons identified for this is the fact that readily used design codes for crack-width design of water-retaining concrete structures in South Africa were compiled with data obtained from the use of 42.5 N cements, which may well have been quite different from the now more modern and readily available 52.5 N cements. Furthermore, design codes focus extensively on factors like binder content, binder type and formwork type, while the effects of other factors such as coarse aggregate type, coarse aggregate nominal size and construction sequence (which also play a significant role) are not quantified in the selection process of temperature values. Recommendations for further studies are made which aim to incorporate a wider variety of factors that affect development of thermal properties of concrete. This can allow members of the project team (engineer, contractor) to act during the relevant stages of design/construction of a project to mitigate thermal effects that can incur unwanted cracking.