Sensitivity of strength and durability properties of blended cement concrete to changes in water/binder ratio and binder content
Date
2019
Authors
Du Preez, Morne Phillip
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Abstract
There are numerous studies that assess the effects of changes in the water/binder (w/b)
ratio and binder content on strength and durability properties of concrete. However, limited
information is available on the extent to which changes in w/b ratio and binder content
affects concrete strength and durability. This study focused on the sensitivity of concrete to
changes in these parameters to further the understanding of the material by determining the
importance of the parameters on the resultant concrete properties and providing practical
guidelines for the mix design process.
Concrete test specimens were made using two w/b ratios (0.45 and 0.65), two binder
contents (350 and 450kg/m³) and three binder types PC (plain Portland Cement), 70/30
PC/fly ash (FA) and 50/50 PC/ground-granulated blastfurnace slag (GGBS). One parameter
was varied at a time. Compressive strength tests were conducted at 3, 7, 14, 28 and 56 days
and durability index tests at 28 and 56 days.
The results indicate that increases in w/b ratio lead to lower compressive strengths and
increased permeability, sorptivity and chloride conductivity. Increases in binder content have
beneficial effects on compressive strength; however, this trend does not continue indefinitely
and is related to the paste content range, aggregate content and grading. Furthermore,
binder content increases proved to be detrimental to concrete durability by decreasing the
oxygen permeability (OPI) and increasing the water sorptivity (WSI) and chloride conductivity
(CCI) indices. In general, for all the binder types investigated, both the strength and
durability properties were more sensitive to changes in w/b ratio than binder content.
The compressive strength of FA concretes was the most affected by a change in w/b ratio.
The sensitivity hierarchy of compressive strength of PC and GGBS concretes to a change in
w/b ratio varied with age. For the three different binder types, sensitivity of compressive
strength to changes in w/b ratio generally decreased with age. The extent to which a
change in binder content affects compressive strength varies with age and no clear binder
type hierarchy is evident. The w/b ratio therefore remains the controlling factor of
compressive strength.
For the parameters tested, the OPI of FA concretes exhibited the highest sensitivity to
changes in w/b ratio followed by GGBS and PC concretes respectively. Moreover, GGBS
concretes were the most sensitive to changes in binder content followed by PC and FA
concretes. As with compressive strength, sensitivity of OPI to changes in both w/b ratio and
binder content decreases with age. The sensitivity variance of OPI between changes in w/b
ratio and binder content is considered to be minimal, less than 5.0%, and therefore both of
these parameters need to be carefully considered in the mix design process when assessing
permeability requirements.
The results also indicate that the sensitivity of WSI to changes in w/b ratio follows a similar
pattern to compressive strength and decreases in the following order: FA > PC > GGBS.
Furthermore, this sensitivity increases with age apart from one FA specimen. The sensitivity
trends for binder content variations are not well defined; however, they generally decrease
with age. W/b ratios and paste contents therefore need to be kept as low as possible in
concrete mixes.
CCI sensitivity to changes in w/b ratio decreases in the following order: PC > FA > GGBS.
The sensitivity to changes in binder content generally decreased as follows: GGBS > PC
FA. The extent to which a change in w/b ratio and binder content affects CCI generally
decreases with age. These findings reiterated the need to control the paste volume.
Adopting the highest replacement levels of GGBS leads to the lowest chloride conductivity.
However, these concrete mixes exhibit the highest sensitivity to changes in paste content
Description
A research report submitted to the Faculty of Engineering and the Built Environment,
University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for
the degree of Master of Science in Engineering.
Johannesburg, 2019
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Citation
du Preez, Morné Philip. 2019. Sensitivity of strength and durability properties of blended cement concrete to changes in water/ binder ratio and binder content, University of the Witwatersrand, https://hdl.handle.net/10539/28070