Eigenstresses in Hardening Concrete

High performance concrete containing microsilica is often used when high durability is required. This is due to very low permeability of cement paste containing microsilica. However, observations of increased microcracking in concrete containing microsilica. The reason for increased cracking sensitivity of concrete containing microsilica raise questions about the long term durability of high performance concrete containing microsilica. The reason for increased cracking sensitivity of concrete containing microsilica may be due to a combination of increased autogenous shrinkage and brittleness of this type of concrete.

This thesis investigates the development of eigenstresses inside and around aggregates in hardening concrete. Influence of increased autogenous shrinkage taking place in cement paste containing microsilica is studied.

A material model for hardening cement paste is set up. The material model is based on experiments determining several material properties as a function of time. Calculations of the development of eigenstresses are carried out by use of mathematical solutions for a single inhomogeneity embedded in a matrix undergoing shrinkage. A sensor is developed to measure directly these eigenstress. By use of the sensor it is possible to measure directly the eigenstresses developing in a spherical aggregate taking aging and viscoelastic effects into account.

Results from measurement and calculations are of the same order of magnitude. The difference in results from calculations and measurements is assumed to be due to insufficient knowledge of the viscoelastic/plastic behaviour in the very young cement paste. Results from calculations as well as measurements show a strong increase in eigenstress development for cement paste containing microsilica. The effect is mainly seen after 24 hours where eigenstresses in cement paste without microsilica show decreasing stresses due to the influence of viscoelastic effect being stronger than the influence of shrinkage. Cement paste containing 20% microsilica shows continuous increasing stresses also after an age of 25 hours. Thus, in the case of cement paste containing 20% microsilica the effect of shrinkage id dominating in relation to the viscoelastic effect.

The risk of matrix cracking around aggregates in high-shrinkage concrete is assessed by simplifying the geometry of an aggregate into a two dimensional problem without neighboring aggregates. A granite cylinder embedded in a disc of neat cement paste containing 20% microsilica is investigated experimentally. An estimation of the critical age at which a crack of a certain length will propagate is carried out by combing measurements of eigenstresses with two fracture mechanical models. The models are verified by comparing calculated results with results from experimental observations.

The investigation carried out in this work shows that the risk of cracking is larger in concrete containing microsilica. This conclusion is based on the fact that eigenstresses developing in aggregates when cast in cement paste containing 20% microsilica are about 4 times larger compared to cement paste without microsilica. Furthermore, cement paste containing 20% microsilica show increased brittleness and this increased cracking sensitivity.