Page 87 - Machinery Component Maintenance

72 Machinery Component Maintenance and Repair

Proper Grout Mixing is Important2

Epoxy grouts must be properly mixed if adequate strength is to be
maintained at operating temperatures. The strength of epoxy grouts is the
result of dense cross-linkage between resin and hardener molecules.
Dense cross-linkage cannot occur in either resin-rich or resin-poor areas.
Poorly mixed grout, which may appear to be strong at room temperature,
can soften and creep under load at temperatures in the operating range.
Epoxy grouts are three-component products. They have an epoxy
resin, a hardener, and a graded aggregate. The resin and hardener serve
as an adhesive in the mortar while the aggregate serves as a filler to re-
duce costs. The addition of an aggregate will lower the coefficient of
thermal expansion of the mortar to more closely approach that of con-
crete and steel. Aggregates also serve as heat sinks to absorb the heat
released by curing, and thereby, allow thicker pours.
Both resin and hardener molecules are surface-active, which means
that either is capable of clinging to a surface. That is why it is so critical
that the resin and hardener be premixed for a minimum of three minutes
before adding aggregate. Use of a paint mixer for premixing these adhe-
sive components is preferred over the stick-and-bucket method because it
provides more thorough mixing and will not usually whip air into the
mix.
The aggregate used in preparing an epoxy grout mortar is a key factor
in minimizing the loss of load bearing area caused by the rising of en-
trapped air after grout placement. Aggregate quality is also a key in
minimizing the potential for run-away curing, edge lifting of the grout on
foundation corners, loss of bond to the machinery base and stress crack-
ing of the grout.
Most aggregates have about 25-30 percent voids regardless of particle
sizes or gradation. The liquid components of an epoxy grout have a den-
sity of about 9 lbs per gallon while the aggregate exhibits a bulk density
of about 14-16 lbs per gallon. The particle density is much higher. Be-
cause of this difference in densities, the aggregate falls to the bottom of
the mix and is not immediately wetted. When the liquid and aggregate are
blended together, air that was present in the aggregate as well as air intro-
duced into the mortar during mixing has a tendency to rise. The rate at
which air bubbles rise is governed by both the size of the bubble as well
as the viscosity of the mortar. At any given viscosity, the rise rate in-
creases as the size of the bubble increases; therefore, it is important to
keep the size of the bubbles as small as possible. The size of the bubbles
is determined by the space between aggregate particles.