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
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Organisations
Publications
El Khoury K
(2024)
Cracking in Edge Restrained Concrete Walls
El Khoury K
(2023)
Experimental assessment of crack prediction methods in international design codes for edge restrained walls
in Structures
El Khoury K.
(2024)
Restraint-Induced Cracking in Edge-Restrained Walls: Validation of Numerical Model and Parametric Study
in fib Symposium
Elwakeel A
(2022)
Assessment of cracking performance in edge restrained RC walls
in Structural Concrete
Elwakeel A
(2022)
INDUCED CRACKING IN EDGE RESTRAINED WALLS - FEA PARAMETRIC STUDY
Elwakeel A.
(2022)
INDUCED CRACKING IN EDGE RESTRAINED WALLS - FEA PARAMETRIC STUDY
in fib Symposium
Shehzad M
(2023)
Predicting the influence of restraint on reinforced concrete panels using finite element models developed from experimental data
in Mechanics of Advanced Materials and Structures
| Description | Twelve full scale edge restrained walls were constructed at Imperial College. Six of the walls cracked within 1 day of casting. With the exception of wall 1 which did not crack, the remaining four of the eleven walls tested at Imperial College cracked between 7 and 20 days from casting. A key experimental finding was 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 the tested walls, 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 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 first 12 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 ongoing theoretical research is focussed on this. The tests show that the crack pattern in edge restrained walls is significantly influenced by wall geometry as well as provided reinforcement. The tests show that a well defined crack pattern can develop in edge restrained walls when no horizontal reinforcement is provided. The addition of horizontal reinforcement both reduces crack width and causes additional cracks to form. Good estimates of the observed behaviour was obtained in 2-D numerical simulations carried out with the nonlinear finite element code ADAPTIC. We 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 findings were periodically reported back to the industrial partners who at their discretion can make use of the findings in their design practice. The research findings have been and are continuing to be disseminated in conference and journal articles. To date, three journal, six conference papers and one PhD thesis have been published as listed in this Research Fish record. Notably, a paper was published in Structures (https://doi.org/10.1016/j.istruc.2023.06.087) which compares 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 |
| Description | fib DISC |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Professional Practitioners |
| Results and Impact | The key research findings were presented at the fib UK DISC conference - Protection of concrete structures and buildings for the future. The conference was held online with attendees mainly from industry. |
| Year(s) Of Engagement Activity | 2023 |
| URL | https://www.concrete.org.uk/events.asp?cmd=moreinfo&ID=3823 |