Strength and Durability of a 70% Ground Granulated Blast Furnace ...

A statistical analysis was performed to determine which of the five variables was
significant to the relationship between achieved strengths of the slag and OPC mixes. The
results showed that percent slag and slag fineness were the only two significant variables.
However, in most of the studies in the data set, w/cm and admixtures were held constant
between the slag and OPC mixes. Additionally, the literature indicates that the effect of
fineness becomes less important at ages later than 28 days.
As noted above, at any given slag replacement, other factors apparently affect the percent
strength. However, it is difficult to separate percent slag replacement from amount of slag
per volume of concrete. For high slag content mixes, looking at all the mixes containing
Grade 120 slag, several things were examined. Fig. 47 shows the effect of cement content
on percent strength. It appears that on the average, somewhere around 130 kg/m 3 [215
lbs/cy] OPC (with slag) may be required to achieve parity with an OPC (zero slag) mix,
although as little as 60 kg/m 3 [100 lbs/cy] OPC might be successfully used to reach
minimum specified strength. It is hypothesized that below this value, there is insufficient
activator and hydroxide produced to completely activate the slag, which will result in
strengths being lower than 100% OPC mixes. The MoDOT mix only contained 112
kg/m 3 [189 lbs/ cy]. However, the minimum required cement content may go up or down
depending on the activity of the slag, the characteristics of the cement, and, of course, the
amount of slag present. In a practical sense, if prevention of thermal cracking in mass
concrete is the primary objective, and is being met while still meeting a minimum
compressive strength specification, then achieving 100% parity with a zero slag mix may
not be necessary. An increase in cement content may defeat the purpose of using the slag
in mass concreting, which is to reduce thermal cracking. Thus, it seems that there is a
limiting percent of slag replacement that is
practical.
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