The risk of early-age thermal cracking using concrete with high GGBS content: A parametric study of two concrete mixes for a wall-to-slab section
Examensarbete för masterexamen
Structural engineering and building technology (MPSEB), MSc
Carbon reduced concrete, with highest possible cement reduction and incorporation of supplementary cementitious materials (SCM), is one necessary measure to reduce the carbon footprint of the built environment. In Sweden, one of the most used SCMs is slag, a by-product from steel production. Although the availability of slag is limited, it is a temporary solution for reduction of carbon emissions while research of alternative SCMs is being conducted. However, incorporation of slag provoke different hydration reactions, resulting in physical and chemical changes in early-age properties. As current production methods are developed for concrete with ordinary Portland cement (OPC), the different early-age behaviour is of concern. The aim of this thesis is to compare the early performance of two concrete mixes on a principal case of wall-to-slab, one traditional with OPC and one with 50 % cement replacement by slag, in this thesis named S50. By FE analysis, thermal stresses were calculated and the risk of early-age thermal cracking of the two concrete mixes evaluated. A parametric study was conducted to study differences in the material behaviour, and the response to changed geometrical and environmental conditions. Thereafter, a study of risk reducing casting and curing measures was performed where the effect individual as well as combined measures were studied. It was shown that the lower heat generation and early-age strength of slag concrete caused problems of reaching adequate strength levels in a studied cold case. Changes in heat and strength development in combination with a changed restraint, both due to increased dimensions, complicated the prediction of the risk of cracking. Furthermore, the slag concrete required different measures compared to the OPC concrete. Due to the slower strength development, hydration accelerating measures are needed for the S50 concrete. However, there is a fine balance between accelerating the strength gain while also maintaining a low crack risk. Casting with stricter deadlines appears to be more difficult for slag concretes in cold climates. Compromises with strict time plans in winter castings or adjusted production plans might be needed to minimize the cement amount and meet the environmental demands.
Early-age thermal cracking, Crack risk analysis, Finite element analysis, GGBS, Wall-to-slab, Crack reducing measures.