Measurement and modelling of young concrete properties

Abstract: The main aim of this thesis is to refine models for strength and heat development of the young concrete, and evaluate if developed models at Luleå University of Technology (LTU) for creep, autogenous shrinkage and thermal dilation needs refinement. These are of importance in hardening control and in crack control of a concrete structure.Strength development is one of the most important properties in concrete to consider when analysing concrete structures. The need of actions on site is different at various stages of hardening, from the fresh concrete to the hardened concrete. This thesis defines a model analysing maturity and associated strength growth within three important time periods. The model can be applied separately within each of these periods depending on test data available. Known is that the temperature plays an important role on the strength development of concrete structures. Not so well known is that, if the concrete temperature remains high, strength reduction at later ages often occurs compared to hardening at lower temperature. Both these phenomena have been implemented in the model for strength growth and the functionality of the model is demonstrated by evaluation of laboratory tests for five concrete mixes and two types of cement.As heat of hydration affects the temperature levels and several other properties of concrete structures it is important to document the parameters accurately. In the traditional semi-adiabat (TSA) the measured heat energy originates from the reaction between cement and water. This energy warms up the concrete sample and all the ambient materials of the equipment. In order to model these energies, the TSA setup is transformed into an associated sphere. A refined model using a correction factor is introduced, which accounts for energies lost to the TSA setup materials. Results show that the effect of this factor cannot be disregarded. An increased insulation amount gives a decreased cooling factor but an increased need for correction.Creep at constant temperature, both for moist sealed specimens and drying out conditions, are evaluated. The Linear Logarithmic creep Model (LLM) is shown to work well for basic creep, i.e. creep at moist sealed conditions. But, for creep during simultaneous moist drying, there is a need to adjust the current model or develop a new model to account for the increase of creep due to ongoing drying.Autogenous shrinkage and thermal dilation (free deformation) are evaluated from tests. For the free deformation of young concrete at variable temperature the existing models are satisfactory in modelling the thermal dilation, but in some cases the autogenous shrinkage cannot be adequately described. A new or refined model is needed that works for autogeneous shrinkage even at more complex temperature variations.