Understanding the cracking behaviour of reinforced concrete elements subjected to the restraint of imposed strains

Lead Research Organisation: Imperial College London
Department Name: Civil & Environmental Engineering

Abstract

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.
 
Description To date nine full scale edge restrained walls have been constructed at Imperial College. Four of the walls cracked within 1 day of casting. With the exception of wall 1 which did not crack, the remaining four of the nine walls tested at Imperial College cracked between 7 and 20 days from casting. The main experimental findings to date are that appropriate selection of formwork and thermal insulation is essential if magnification of the imposed strains is desired for experimental investigations in the laboratory. In walls tested to date, experimentally derived values of restraint factor were found to be similar at first cracking to values calculated, allowing for creep, using standard industry guidance provided in CIRIA report C766 (2018). The early age temperature profile in the walls was found to be significantly affected by heat flow into the base. The effect of this is beneficial since it reduces the restrained strain near the bottom of the wall where restraint is greatest. The tests to date suggest that the risk of cracking in edge restrained walls can be reasonably assessed by comparing experimentally derived restrained strains with the concrete tensile strain capacity calculated using the CIRIA C766 recommendations which allow for creep. The measured long term crack widths are significantly greater in the walls which cracked at early age rather than later. Comparison with crack widths calculated with EN 1992 and CIRIA Report C766 shows that EN 1992 gives the best estimate of crack widths in walls which cracked at early age. Measured crack widths exceed crack widths calculated with C766 in five of the first12 walls tested at Imperial College and University of Leeds. This is potentially concerning since crack widths in structures like basements need to be controlled to specified limits to minimise water ingress. This finding, which is consistent with field observations by others suggests that the CIRIA Guidance on the calculation of crack width may need refinement. Our current experimental and theoretical research is focussed on this. We have also developed a procedure for monitoring long-term deformation and cracking in the walls using digital image correlation. To this end, the DIC system and the wall were enclosed in a blackout 'tent' to eliminate all external light sources. A constant level of illumination was provided inside the tent. The cameras were mounted on a solid 75mm square section of invar-36 to minimise thermal instability. Finally, image quality was improved by combining consecutive series of 16 images.
Exploitation Route The research is ongoing so it is premature for the outcomes to date to be put to use by others. However, the research findings are regularly reported back to the industrial partners who at their discretion can make use of the findings in their design practice. The outcomes findings are being disseminated in conference and journal articles as the research develops. To date, several papers have been published. A paper was presented at the fib Symposium held in Lisbon in June 2021. This paper was selected for publication, in an extended form, in a special themed issue of the fib journal Structural Concrete. The paper is available online (https://doi.org/10.1002/suco.202100688). Two further conference papers, jointly authored with Leeds, have been published (SEMC-University of Cape Town 2022 (Experimental assessment of crack width estimations in international design codes) and fib Congress Oslo 2022: Induced cracking in edge restrained walls - FEA parametric study). Other papers are under review or in preparation including one comparing measured crack widths with those predicted by CIRIA Report C766 which is widely used by industry. The finding that CIRIA Report C766 tends to underestimate crack widths in the tested walls is potentially significant and may have implications for practice and future revisions of CIRIA report 766.
Sectors Construction