Early age cracking tendency of low-heat concretes containing fly-ash and blast furnace slag

Title: Early age cracking tendency of low-heat concretes containing fly-ash and blast furnace slag
Author(s): Ø. Bjøntegaard
Paper category : conference
Book title: 2nd International RILEM Symposium on Advances in Concrete through Science and Engineering
Editor(s): J. Marchand, B. Bissonnette, R. Gagné, M. Jolin and F. Paradis
Print-ISBN: 2-35158-003-6
e-ISBN: 2351580028
Publisher: RILEM Publications SARL
Publication year: 2006
Nb references: 11
Language: English

Abstract: The paper discusses the effect of fly-ash (FA) on the cracking tendency during the hardening phase due to external restraint. The tested concretes had water-to-binder (w/b) ratio of 0.40 and properties such as hydration heat, coefficient of thermal expansion, autogenous deformation mechanical properties and creep have been investigated and reported. Results from semi-adiabatic tests in the Temperature-Stress testing machine and 1D stress calculations show that the FA concrete (35% FA of binder weight in combination with ordinary Portland cement (OPC, corresponds to CEM I according to EN197) has 25% lower cracking tendency than the reference concrete made only with OPC. The test conditions and evaluation of cracking risk are directly relevant for a 1 m thick concrete structure subjected to external restraint.
Any practically relevant mix-design with FA added directly during the mixing process should however be done with the use of efficiency factors according to the European Standard “EN 206-1”. This will require lower w/b-ratios for FA-concretes (or BFS concretes) than a reference without FA (or BFS), which probably will lead both to higher binder contents (i.e. more hydration heat) and possibly more autogenous shrinkage. Hence, a practically relevant comparison using a lower w/b-ratio for the given FA-addition (35% FA requires w/b=0.34) would likely be less favourable than what is found in the present study using w/b=0.4.
Hence, since hydration heat and autogenous shrinkage, as well as all the other involved concrete properties, are altered when changing concrete proportions it is very difficult, if possible at all, to extrapolate results. Any parameter study on FA (or BFS) relevant for practical concreting should therefore be made in accordance with the requirements from the standard. In this regard it is notable that the efficiency factors are 1.0 for FA and BFS if the materials are a part of a certified cement (cement type II and III). For cracking tendency it appears then more favourable to use CEM II and III cements than using the same amount of FA or BFS added separately during mixing in combination with, for instance, OPC.

Online publication: 2006-08-02
Classification: 3.2 Theme 2: From Fresh to Hardened Concrete
Publication type : full_text
Public price (Euros): 0.00
doi: 10.1617/2351580028.051