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FA 3.1 Crack free concrete structures

To reduce the risk of early-age cracking, and thus improve the aesthetics and durability

Project manager: Knut O. Kjellsen
Disciplinary responsible:     Professor Terje Kanstad
PhD: Anja Klausen
International advisor: Professor Jan Erik Jonasson, Luleå University of Technology, Sweden

The project collaborated with Luleå University, Sweden.

Cracking and cracking mechanisms of young concrete is of particular importance due to the development of new cementitious compositions, alternative fillers and pozzolanas, new admixtures, alternative curing methods etc. The project wanted to determine the impact of these new developments on the properties of young concretes and its cracking behaviour.

Main topics

Early age cracking calculations

The issue is risk of early age cracking (thermal cracking) in heavy structures where the hydration generated heat may cause large temperature differences. A new special purpose early age concrete calculation program, "CrackTestCOIN", was developed. The use of lowheat concrete is favourable for sustainability reasons. So far it seems like the optimum FA content, is around one third of the total binder content. When higher FA dosages are being used, the negative effect of the loss in tensile strength seems to be larger than the positive effect of reduced hydration heat.

Development of advanced laboratory equipment

To improve the capacity for measurement of autogenous shrinkage and thermal
dilation, a new test rig has been built within the project. This Free Deformation system (FD-system) is intended to measure the concrete's deformation properties both under isothermal and realistic temperature conditions, without losing moisture to the surroundings. The system consists of seven rectangular moulds with dimensions 100×100×580 mm inside.

In addition the Temperature-Stress Testing Machine (TSTM) rig at SINTEF/NTNU had been upgraded and modernized within the project. In the TSTM rig the concrete is exposed to a temperature development and degree of restraint corresponding to the conditions in a given construction. The results can be used to compare different concrete properties or to calibrate crack risk calculations and other design analysis.