Bestämning av utvecklad tryckhållfasthet för platta betongkonstruktioner i ett tidigt skede

Sammanfattning: Concrete is a building material that is normally cast according recipes, where a larger proportion of cement in the mixture results in a faster strength development and increased ultimate strength. The compressive strength, which can be considered as the most important property of concrete, is usually measured by a standardized procedure, where a test specimen is subjected to a progressively increasing load until it breaks. The most common is that the concrete's final strength is measured after 28 days, but the concrete's strength development continues as long as there is access to both unhydrated cement and water. Initially, this process is strongly dependent on the temperature of the concrete, where a higher temperature results in faster hardening, but gradually becomes more diffusion dependent. After 28 days, the reaction is so slow that the concrete is normally considered to have reached its final strength. When casting constructions where demolding strength needs to be taken into account, the concrete's current strenght must be determined before the mold can be removed. This can be done in several ways. A common method is to cast test specimens that are kept in the same environment as the structure, and test them regularly until the desired strength is achieved. A potential problem with this method is that the strength development in the test specimen is different than in larger elements due to different ratios between the bodies' volume and area. This means that the heat developed in the concrete when the cement hydrates is transferred to the surroundings faster in a test specimen compared to a full-sized element. This results in the strength developing more slowly in the specimen. In a factory environment, it is common for the concrete recipe to be adapted so that the demoulding strength can be achieved after a desired, relatively short time. This often results in more cement being used than is necessary for the desired final strength. In this degree project, temperature and strength development has been studied at an early stage for full-sized flat concrete elements in a concrete industry and compared with the corresponding standardized cubic test specimens. The results show a substantial difference in developed strength, which should mean that the cement content can ultimately be reduced.