Ground
Granulated Blast Furnace Slag (GGBFS) is a recyclable material
created when the molten slag from melted iron ore is quenched
rapidly and then ground into a powder. This material has cemetitious
properties and has been used as a replacement for cement for over
100 years. Recently, Wisconsin has begun using it in some of its
highway projects. Wisconsin has experienced several problems with
GGBFS, which include slow strength gain and decreased surface
quality. Countering these problems, GGBFS concrete has higher
late strength and lower permeability. This project investigates
these GGBFS characteristics and has several objectives.
The
first objective is to monitor the variability of GGBFS sources.
Since slag is produced as a by-product of iron production, the
material varies based on the type of iron production. Because
it is a waste product, the final characteristics of the slag are
not of major concern to the iron producers. As a result, the variability
in GGBFS can be significant. To research this variability, monthly
samples will be collected from a local vendor, and the physical
and chemical characteristics of each sample will be recorded.
The
second objective is to study the strength gain and air void development
of different mixtures. Generally, GGBFS concretes develop strength
more slowly than Ordinary Portland Cement (OPC) concretes, but
will exhibit higher strength after one year. GGBFS concretes are
also less permeable, but they can develop abnormal air void systems.
The performance of GGBFS concretes depends not only on the amount
of GGBFS used, but also on the chemical composition of the other
materials in the mixture. Three different GGBFS replacement levels
will be studied in this part of the project: 0%, 30%, and 50%.
Thirty percent is the most commonly used replacement level. This
means that of the cemetitious material in the concrete, 30% is
GGBFS, and 70% is OPC. The project will investigate the performance
versus a 100% OPC concrete (0% GGBFS), and will also determine
whether a 50% level is a reasonable replacement level. Along with
the GGBFS replacement level variable, four different cement brands
and two coarse aggregate types will be used to determine its performance
with different materials. There will also be mixtures created
and cured in 40o F conditions to determine the performance of
GGBFS concrete at lower temperatures.
The
third objective is to determine the surface wearing (scaling)
resistance of different concrete mixtures. GGBFS concretes have
proved to be susceptible to surface scaling. The reason for this
poor performance characteristic is not yet known and will be investigated
in this project. Previous studies have shown that carbonation
in the concrete's top layer could be a major cause of the decreased
scaling resistance in GGBFS concrete. Twelve mixtures will be
tested for scaling resistance. Again, 0%, 30% and 50% GGBFS replacement
levels and two coarse aggregates will be tested. Two cement brands
will be studied. For each mix, there will be four different curing
conditions for the sample blocks. In addition, two curing methods
to reduce carbonation in the concrete will be investigated at
the 30% and 50% replacement levels. The blocks will be ponded
with a salt water solution and subjected to cyclic freeze-thaw
conditions for 60 to 100 days. After every five cycles, the amount
of scaling will be measured.
Under
these objectives, the goals of the project are the following:
determine the sensitivity of GGBFS with different materials and
conditions, predict strength development and scaling resistance
depending on the replacement level, and propose application guidelines
for using GGBFS in Wisconsin concrete paving projects.
