StableConcrete

Stable Concrete for Machine Bases

In a general sense, "concrete" refers to a combination of aggregate (stone, gravel, sand, etc) and a binder (portland cement, epoxy, etc) which hardens into a solid mass. Everyday construction concrete, PCC (Portland Cement Concrete), is based on portland cement. "Polymer concrete" uses polymer resin (e.g. epoxy), instead of cement, to hold the aggregate together.

Concrete is an attractive material for machine tool frames because

• it is stiff
• it absorbs vibration well
• it is low cost

Starting in the 1980s, polymer concrete has become quite popular in construction of precision machine tools, first in Europe and now worldwide. The first commercially important concrete for this purpose was the epoxy-granite composite Granitan.

Polymer concretes have much better dimensional stability than PCC. Slocum declares PCC unsatisfactory for precision machinery due to:

• reaction shrinkage from cement hydration
• shrinkage due loss of excess nonstoichiometric water, which leaves conduits for humidity-induced expansion or contraction, and
• non-elastic dimensional changes (e.g. creep and microcracking in the inherent brittle/porous structure).

PCC shrinkage during cure is 0.04 - 0.1%, so it is wise to wait several weeks for the bulk of this curing shrinkage to complete (although concrete typically continues to shrink, at a slower rate, for months or years).

In "latex concrete", which is PCC with a substantial addition of acrylic or SBR polymer emulsion, most of the pores are filled with polymer; this tends to reduce shrinkage, make the concrete more stable against moisture-related effects, and make it less brittle. Using acrylic latex concrete for thin-shell roofs is described here. Rheomix 141, a styrene-butadiene copolymer, has also been recommended for that application. It seems likely that one of these admixtures would considerably improve the stability of a cast-concrete machine base.

GVCS applications

• Multimachine
• Steam Engine