Modelling of autogenous shrinkage of concrete based on paste measurements

Abstract

In order to be able to improve concrete modelling based on its constituent, more knowledge is needed about the material behaviour of these constituents. In this research the focus is on the behaviour of hardening concrete, therefore the properties of hardening cement are of most relevance. Information about cement paste behaviour can be used as input for computer models that are able to represent concrete as a heterogeneous material. During the development of these models the output has to be verified with data from actual concrete. In order to be able to perform measurements on autogenous deformation of cement paste a new test setup was developed. The design is based on the Temperature Stress Testing Machine (TSTM), a setup used for experiments on autogenous deformation in concrete. A device available at the Microlab, called the mini tensile testing machine served as a basis to create the mini-TSTM. The device can either measure the free deformations of cement paste or it can measure the build up of stress in the paste if these deformations are restrained. This part of the research is described in [2]. For the modelling of the heterogeneous material the Delft Lattice Model is used. This is based on a lattice of beams to which different material properties can be assigned. The measured paste deformations from the mini-TSTM are imposed on this lattice as a load. The development of the stiffness (Young's modulus) of the cement paste is taken from Hymostruc, a computer model that determines properties of hardening cement. The material properties of the other concrete constituents, in this case the aggregates, are based on assumptions. The properties and the displacements of the cement develop over time, therefore this model runs a calculation for every time step. With the model simulations of free shrinkage and restrained shrinkage tests on concrete are simulated and the results are compared with the experimental outcome. Qualitatively the results look promising. More tests and simulations are needed to exactly define all the relevant parameters in the model.

A mini-TSTM for measuring paste deformations at early ages

“…The paste deformations were measured with a newly developed mini-TSTM apparatus and compared with the full-scale TSTM results. Numerical simulations with the Lattice model [2] also show the contribution of autogenous shrinkage in the early age stress development. In this paper an attempt is made to measure the deformations for both cement paste and concrete and to examine the differences by means of numerical simulations.…”

“…With a Lattice model [2] the deformations measured from the cement paste ( Figure 5) are taken as input to simulate the behaviour of concrete. The measured shrinkage of a cement paste with CEMIII/B (580 m 2 /kg) is used with a w/c ratio of 0.44.…”

“…The stress is calculated over a horizontal cross-section over the middle of the concrete. In the Figure the stresses are calculated for three different relaxation models [1,2]. For sake comparison, the calculated and measured stresses (with TSTM) are shown together.…”

“…In this research, the focus is on the shrinkage behavior of hardening concrete with emphasis on the hardening properties of the cement paste, which is considered to be most relevance in this respect. Information about the cement paste behavior is used as input data for numerical simulations of concrete [2]. The output has will be verified with data from actual concrete tests.…”

Internal Viscoelastic Modulus Associated with Autogenous Shrinkage in Cementitious Materials

“…Recent numerical simulations, using the lattice model for predicting stress development from a mini temperature stress testing machine (TSTM) on small paste specimens, where thermal effects from heat of hydration are minimal, concluded that a poor agreement was obtained between predicted stresses by using three different models for obtaining stress relaxation modulus (Schlangen et al 2006). The first model was based on an assumption of instant relaxation that internal stresses will cause instant deformation of the micro-porous hydration products.…”