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

Machine tool frames made of PCC date at least as far back as the Yeoman lathe, but have never been widely adopted. In contrast, since the 1980s, polymer concrete has become quite popular in construction of precision machine tools, first in Europe and now worldwide [1]. 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).

While polymer concrete resolves these problems, it is much more costly than PCC and is less compatible with the OSE requirements. It is worth revisiting the application of "plain old concrete" to machine frames with its weaknesses in mind.

PCC shrinkage behavior

PCC shrinkage during cure is 0.04 - 0.1%, and shrinkage continues at slower and slower rate for months or years. It is necessary to be patient -- probably waiting 1 to 3 months before considering the concrete dimensionally stable. TBD: do thinner sections stabilize more quickly, and if so, how much?

After the curing process is essentially finished, the concrete remains humidity-sensitive, a bit like wood -- expanding in high humidity and contracting when air is dryer. Concrete also responds to temperature: it has a thermal expansion coefficient close to that of steel. Sealing the surface with paint or similar coating can greatly reduce humidity effects. (This should also reduce carbon dioxide absorption, which causes an additional shrinkage chemical reaction: carbonation.)

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.

Steel reinforcement

PCC is strong in compression but much weaker in tension. The loads on a machine frame will typically put some parts of the frame in tension, and these parts must be reinforced. The simplest support is to cast reinforcing bar (rebar) into the concrete mass. While rebar makes the section strong, it also is almost certain to make it filled with small cracks. This is because the concrete sets and locks to the rebar long before it finishes shrinking.

GVCS applications

• Multimachine
• Steam Engine