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

with the lower strengths. Most of the conditioning methods utilizing extra wet
and/or drying time intervals did little to improve the results for the slag mixes;
however, under optimum wet plus dry curing periods, the slag mix Durability
Factors did approach that of the OPC mix.
3. Rapid chloride permeability test values were significantly lower for the GGBFS
mixes compared to the OPC mix. In regard to chloride permeability, both GGBFS
mixes are considered to be low, while the OPC mix result was somewhat high.
The literature is almost universal in supporting this result of slag replacement.
Slag replacement reduces permeability from a reduction in pore size, not
necessarily from a reduction in porosity. Because strength is a function of
porosity, this means that it could be expected that permeability and strength may
not correlate well.
4. The OPC control mix had good salt scaling resistance. Both GGBFS types of
mixes exhibited significantly greater salt scaling than the OPC control. Past
studies indicate that lab scaling tests usually show a reduction in scaling
resistance when slag is used as a replacement for PC due to carbonation effects
and testing conditions.
5. Although strength continues to increase after 28 days, qualitative relationships
between different types of mixes are usually established by 28 days of curing.
Recommendations
1) High-slag content concrete mixes should continue to be considered for
future projects, providing that certain conditions are met. First,
specifications should be written to reflect realistic expectations for: a) the
service requirements of the facility, and b) the potential of the mix design
itself.
a. The specifications should address only those parameters that are of
interest. For example, the freeze-thaw durability and air void system
specifications for exterior concrete should be tailored to the
environment and the extent and manner in which the structure will be
exposed (drainage considerations). Whether or not salt scaling
resistance will be required should be considered. The level of strength
necessary, as opposed to comparison to a non-slag mix, should be
ascertained.
b. Careful attention should be paid during mix design to the actual job
materials that will go into the mix. It should be determined up front
what can be accomplished with the specific mix components in relation
to each other in the proportions anticipated. Once the variables are
studied under controlled laboratory conditions, the mix can be applied
under field conditions.
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