Material: Recycled
Glass
Issue: When glass cullet is indiscriminately combined with Portland Cement, chemical
reactions can occur which may
reduce the strength of the concrete.
These include sugar contamination
and an alkali-silica reaction.
To minimize the adverse
effects of these reactions, ingredients
and methods used in standard PCC
production should be modified. Best Practice: An adverse result of using glass cullet in Portland
Cement Concrete (PCC) is the alkali-silica
reaction (ASR). PCC producers have known ASR for many years
because it can also occur when
using vitreous (glassy) rock or
slag with high amorphous silica
content.
The silica from the cullet
or aggregate can combine with
the alkalis in Portland Cement
to form a siliceous gel.
This gel will then absorb
water and expand, causing structural
weaknesses in the concrete. In
addition, when mixed in PCC, sugar
can cause an unpredictable increase
in setting time and a decrease
in the ultimate strength. A number of methods have been
proposed to minimize sugar residue
contamination and ASR, or to reduce
their adverse affects.
Modifications to the standard
production of PCC include the
following:
·
Cleaning
of glass aggregate
·
Use
of low alkali cement
·
Use
of low alkali pozzolan or set
retarder
·
Use
of an air entrainment system
·
Reduction
of moisture content
·
Increasing
surface area to volume ratio of
glass aggregate Visual inspection of otherwise
clean glass cullet may not reveal
the presence of sugar residue
from previous food contents.
Therefore, all cullet should
be washed prior to its use as
aggregate in PCC. If possible, a high temperature wash should
be used to expedite the removal
of sugar from the cullet. The glass can then be air-dried to minimize
the addition of any uncalculated
moisture to the PCC mixture. Reducing the reactants present
in the PCC mixture can minimize
the alkali-silica reaction. Because the cement is the source of the alkalis
(Na2O + K2O),
low alkali Portland Cement should
be used.
The American Society for
Testing Materials defines low
alkali cement as consisting of
less than 0.60% by weight of alkalis.
The amount of alkalis can
also be reduced by replacing up
to 25% of the Portland Cement
with a low alkali siliceous pozzolan,
such as ASTM Class F fly ash. Alternately, a set retarder
can be used in place of the pozzolan
to slow the curing time of the
PCC. This allows more time for the ASR gel to form
and expand before the concrete
hardens, resulting in less internal
stress. It should be noted that using the set retarder
in conjunction with a pozzolan
will not increase the overall
resistance to ASR.
However, a set retarder
can be used in conjunction with
an air entrainment system, which incorporates
evenly dispersed microscopic pockets
of air in the PCC. When ASR occurs, these air pockets allow additional
space for the gel to expand, thereby
lessening the potential for structural
weakness. The impact of ASR can also
be alleviated by minimizing the
amount of moisture in the PCC
mixture, which will decrease the
expansion of any gel produced. However, using less water will also cause an increase in viscosity,
thereby decreasing the “slump.”
The resulting mixture may
be difficult to pour. If a lower viscosity is needed, a High Range
Water Reducing (HRWR) admixture
can be added. Research shows that increasing
the surface area to volume ratio
of the glass will reduce the effects
of the reaction.
This can be accomplished
by using glass with smaller particles. Finer particles tend to accelerate the reaction, which may allow
the gel to expand before the concrete
hardens.
Research has suggested
that up to 20% of the total aggregate
should be replaced by glass 16
mesh and finer particles. Air drying the poured PCC for
several weeks to months, as opposed
to any accelerated drying methods,
will also minimize the ASR.
Air-drying appears to lock
the alkali in a solid state, and
future rewetting the concrete
only brings a portion of the alkali
back into solution. In addition, some research
suggests that a higher proportion
of green glass in the cullet source
may also suppress the ASR.
This may be due to the
higher concentration of chromium
oxide in green glass, although
adding this chemical directly
to the PCC mix does not appear
to affect the reaction. Implementation: Glass has been used in aesthetic
applications in PCC for many years. However, research is not
yet conclusive enough to recommend
the use of glass in structural
concrete.
Therefore, these strategies
should only be used with Portland
Cement in areas where unpredictable
final strength will not endanger
the public. Benefits: The methods outlined in this best practice will help
minimize adverse effects encountered
when using recycled glass as aggregate
in non-structural Portland Cement
Concrete. Application Sites: Glass suppliers, concrete fabrication facilities,
and construction sites. Contact: For more information about
this Best Practice, contact CWC,
mailto:info@cwc.org. References: Eliminating or Minimizing Alkali-Silica Reactivity. (SHRP-C-343), Strategic Highway Research Program, National
Research Council, May 1993. Gillot, J.E., and Wang, H.;
Improved Control of Alkali-Silica Reaction
by Combined Use of Admixtures.
Department of Civil
Engineering, The University of
Calgary, Calgary, Alberta, Canada,
1993. Manual of Concrete Practice, Chemical Admixtures for Concrete. (212-3R), American Concrete Inst. Nelson, B., Students Turn Glass to Concrete
for Science Prize,
Office of Public Affairs,
Columbia University, New York,
NY, 1996. Shin, C. J., S&EE, Inc., Bellevue, WA Standard Specification for
Portland Cement.
(ASTM C 150-00), American Society for Testing and Materials, 2000. Issue
Date / Update: November 1996 |
