The risk of early-age thermal cracking using concrete with high GGBS content: A parametric study of two concrete mixes for a wall-to-slab section
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Structural engineering and building technology (MPSEB), MSc
Publicerad
2022
Författare
Sundelius, Paulina
Wahlqvist, Emy
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Carbon reduced concrete, with highest possible cement reduction and incorporation
of supplementary cementitious materials (SCM), is one necessary measure to reduce
the carbon footprint of the built environment. In Sweden, one of the most used
SCMs is slag, a by-product from steel production. Although the availability of slag
is limited, it is a temporary solution for reduction of carbon emissions while research
of alternative SCMs is being conducted. However, incorporation of slag provoke different
hydration reactions, resulting in physical and chemical changes in early-age
properties. As current production methods are developed for concrete with ordinary
Portland cement (OPC), the different early-age behaviour is of concern.
The aim of this thesis is to compare the early performance of two concrete mixes
on a principal case of wall-to-slab, one traditional with OPC and one with 50 %
cement replacement by slag, in this thesis named S50. By FE analysis, thermal
stresses were calculated and the risk of early-age thermal cracking of the two concrete
mixes evaluated. A parametric study was conducted to study differences in
the material behaviour, and the response to changed geometrical and environmental
conditions. Thereafter, a study of risk reducing casting and curing measures was
performed where the effect individual as well as combined measures were studied.
It was shown that the lower heat generation and early-age strength of slag concrete
caused problems of reaching adequate strength levels in a studied cold case.
Changes in heat and strength development in combination with a changed restraint,
both due to increased dimensions, complicated the prediction of the risk of cracking.
Furthermore, the slag concrete required different measures compared to the OPC
concrete. Due to the slower strength development, hydration accelerating measures
are needed for the S50 concrete. However, there is a fine balance between accelerating
the strength gain while also maintaining a low crack risk. Casting with stricter
deadlines appears to be more difficult for slag concretes in cold climates. Compromises
with strict time plans in winter castings or adjusted production plans might
be needed to minimize the cement amount and meet the environmental demands.
Beskrivning
Ämne/nyckelord
Early-age thermal cracking, Crack risk analysis, Finite element analysis, GGBS, Wall-to-slab, Crack reducing measures.