Comparison of the estimated crack opening width at an early stage of concrete hardening

Introduction. The calculation of the crack opening width is one of the requirements for the operational reliability of reinforced concrete structures. Cracks in reinforced concrete elements occur both under operating loads and during the initial period of strength gain due to the low tensile strength of concrete and the nature of its operation in general. The increasing speed of construction requires taking into account the thermally stressed state of not only massive, but also conventional structures at an early age of concrete, including calculating the crack opening width at the strength gain stage to ensure the design and operational characteristics of structures and assess the possibility of the loading of the structures whose concrete has not yet reached its operational characteristics. Aim. The study is aimed at analyzing the need to take into account the features of concrete at an early age (the ratio of strength and modulus of deformation differs from the design age) when calculating the crack opening width. Materials and methods. The paper analyzes the calculation methods laid down in the regulatory documents. Calculations of crack opening widths for reinforced concrete sections with different percentages of reinforcement have been performed. A comparison of the calculation methods and the obtained values of the crack opening width according to various regulatory documents for concrete in the early stages of hardening was performed, as well as an assessment of the effect of changes in the modulus of elasticity on the cracking process. The analysis was performed taking into account both the standard modulus of elasticity of concrete (28 days) and its reduced value on day 7. Experimental research data was used to verify the results. Results. It was found that a decrease in the modulus of elasticity of concrete in the early stages of hardening (up to 24 %) leads to an increase in the estimated crack width (up to 11 %) with identical strength. Conclusions. The results of the study emphasize the need to take into account the changes in the modulus of elasticity of concrete in the early stages of hardening when designing the structures. This will make it possible to more accurately predict the crack opening width, including temperature and shrinkage, and will increase the reliability of reinforced concrete structures during operation.