ASR IN CONCRETE

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29 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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ASR IN CONCRETE

The Nuts and Bolts that
Cause the Damage

INTRODUCTION


Durability is far more important to long
performing concrete than strength.


Durability can be broken up into
different issues:


Freeze/Thaw Cycles


Salt and/or De
-
Icer Exposure


Sulfate Exposure


ASR

Damages from ASR


Many problems related to ASR have
been attributed to other mechanisms.


A survey of the United States and
Canada taken in 1999 showed that only
about 20% of the states recognized
that they had ASR problems.


In New Mexico, damages from ASR
were so extreme that corrosion never
had a chance to become an issue.

History of ASR


First recognized in early ’40’s by Stanton
in California pavements as the specific
damaging mechanism


found that problems were less noticeable if
alkali levels in cement were less than 0.6%;


Suitable additions of pozzolanic material
prevented excessive expansions.


The “Bad” Cations


ASR gel is formed when the positively
charged “cations” from the available
alkalis combine with the hydroxide ions.


Sodium and Potassium, which are the
normal constituents of the available
alkalis, form expansive gels.


When the relative humidity of the pore
space exceeds 80%, the reaction will
proceed (This is most of the time).

“Bad Guy” Aggregates


Opal


Colorless to pale gray or brown.


Chalcedony


a fibrous, micro
-
crystalline
form of silica.


Quartz


Microcrystalline and highly
fractured quartz in cherts, quartzite,
gneisses and strained quartz. Coarse,
megascopically
-
crystalline quartz is
normally not reactive.

“Bad Guy” Aggregates


Cristobalite


Normally found in small
square crystals or aggregates in the
cavities of obsidian, rhyolite, andesite and
basalt. Has also been found as a
component of some slag materials.


Tridymite


small euhedral (well formed)
crystals as cavity linings in volcanic rocks
such as obsidian, rhyolite, andesite and as
a porous crystalline aggregate.

The Next Steps


ASR gel forms at the surface of the
aggregates, and begins to in
-
fill around
and through the aggregate particles.


After the developing gel has filled all
available space in and around the
aggregate, any additional gel fills air
voids within the cement paste.

The “Gel that Broke the
Concrete’s Back”


When the gel gets wet, any sodium or
potassium rich gel swells. The swelling
can not be accommodated within the
aggregate and/or paste structure.


Tension cracks are created in the
concrete.


As the cracks grow, more water gets in,
more gel forms, and more expansion
takes place.

Fighting the ASR Menace


In most locations, it is not cost
-
effective
to prohibit the use of reactive
aggregates.


Current research indicates that with
proper mix modifications, the risk of
ASR can be minimized or even
eliminated.