Numerical Analysis of Long Term Effects on Eigenstresses and Micro-cracking in Concrete

Abstract

When designing a concrete structure, a structural engineer often assumes that it is initially free of stresses and cracks. Serviceability limit state analyses are therefore often carried out only taking into account the structure’s self weight and the other imposed loads. As a result, initial tensile stresses and (internal) cracking that can arise from, e.g., partially restrained shrinkage is not accounted for. This might imply that service life predictions can be too optimistic since micro-cracks can promote the penetration of chlorides, with the result of an accelerated corrosion of the reinforcement.

The impact of restraint can be at both the structural level (plane sections must remain plane) and the material level (concrete is a non-homogenous material from components having different shrinkage behaviour and stiffness).

In this contribution the results of the numerical analysis of eigenstresses and micro-cracking due to the shrinkage of the cement paste in concrete are highlighted.

The two-dimensional Delft beam lattice model (BLM) is used to simulate concrete on meso-level as a three-phase material. Stress relaxation in the cement paste is calculated with the activation energy method.

For the comparison of the calculated micro-cracking the results of laboratory tests on concrete are used by which the chloride penetration is measured under different restraint conditions with different levels of micro-cracking.

The results of the numerical analyses with BLM specimens showed that micro-cracking can increase the diffusion coefficient of (uncracked) concrete with a factor 1.2–1.6, depending on the magnitude of stress relaxation.

The residual tensile eigenstresses in concrete reduces due to micro-cracking and stress relaxation till values in the order of 0.5–1.0 MPa which are not taken into account in global calculations and can speed up (unexpected) cracking in the concrete.