Page 88 - Machinery Component Maintenance

Machinery Foundations and Grouting 73

The linear coefficient of thermal expansion of unfilled epoxy grout is
about ten times greater than that of concrete or steel or 6-8 X lop5 in./
in. "E When aggregate is added to form a mortar, the linear coefficient
of thermal expansion is reduced, and the more aggregate added, the
closer it approaches the coefficient of concrete and steel. It is important
that the thermal expansion coefficient of epoxy mortar approach that of
concrete and steel in order to minimize edge lifting on foundation corners
and to minimize stress cracking of the grout when temperatures fall be-
low the curing temperature. The ratio of aggregate to epoxy adhesive in
the mortar should be as high as possible without exceeding the point at
which the mortar becomes permeable. As stated earlier, most commer-
cial epoxy grout mortars have a thermal expansion coefficient of about
1.2-1.4 X lop5 in./in. "E
Most epoxy adhesives cure by exothermic reaction, Le., they release
heat on curing. If an epoxy grout cures too fast, high curing temperatures
are reached and locked-in stresses may be created after heat dissipation.
Aggregate serves as a heat sink. Consequently, it is usually desirable to
have as high an aggregate loading as possible. Because the hydroxide ion
accelerates the curing of epoxy resins and because water contains hy-
droxide ions, it is important that the aggregates used in preparing the
mortar be kiln dried. As little as one ounce of water per cubic foot of
mortar will dramatically increase curing rates. This small amount of
moisture is not detectable by sight or touch. Kiln drying is a common
practice with bagged aggregates. Even low cost blasting sands are kiln
dried.
The viscosity of the mortar is determined by the viscosity of the liquid
(which is determined by temperature), the shape and the amount of ag-
gregate as well as the amount of surface area present in the aggregate.
The greater the surface area the greater the viscosity of the mortar. While
high viscosity in an epoxy mortar is helpful in reducing the rise rate of air
bubbles it also rcduces the fluidity of the mortar. A powder aggregate
would certainly eliminate air rising problems, but unfortunately, a paste
consistency would be reached long before an adequate quantity of aggre-
gate is added to significantly reduce the linear coefficient of the mortar
expansion.
A high aggregate loading can be accomplished in mortar without
eliminating its fluidity and without creating a permeable mortar by care-
ful grading of near-spherical aggregates. Theoretically, the selection of
each particle size should be the largest that will fit in the space beween
particles of the next larger size. The amount of each grade present should
be that which fills these spaces without significantly increasing total vol-
ume of the aggregate. The variation in particle size should not be so great
as to cause classification of the aggregate in the mortar before curing;