Page 96 - Machinery Component Maintenance

Machinery Foundations and Grouting 79

collectively as the “gel.” It also contains crystals of calcium hydroxide,
unhydrated cement, impurities, and water-filled spaces called capillary
pores. The gel water is held firmly and cannot move into the capillaries,
so it is not available for hydration of any unhydrated cement. Hydration
can take place only in water within the capillaries. If the capillary pores
are interconnected after the cement paste has cured, the concrete will be
permeable. The absence of interconnected capillaries is due to a combi-
nation of suitable water to cement ratio and sufficiently long moist curing
time. At least seven uninterrupted days of moist curing time are required
for machinery foundations. Even test cylinders of concrete taken at the
jobsite from the pours are often allowed to cure under water for twenty-
eight days before testing.
Concrete which has not been allowed to cure properly, even though
ingredients are properly mixed in the correct ratio, may be weak and fria-
ble or it may be only slightly under ultimate strength, depending upon the
humidity and ambient temperature present when curing. Improperly
cured concrete will also be permeable and therefore less resistant to deg-
radation from lubricating oils or other materials that may be present.
An illustration of hairline cracks caused by shrinkage of concrete dur-
ing curing can be seen in Figure 3-2 and Figure 3-3. Figure 3-2 is a pho-
tograph of the cambered surface of an airport runway which has been
grooved with a diamond saw to facilitate draining of rain water in an at-
tempt to reduce hydroplaning of aircraft in wet weather. In this photo-
graph a 50% solution of epoxy grout liquid (without aggregate) in ace-
tone was poured on the surface of the runway. Note the degree of
penetration into the concrete between furrows as the solution drains
away. In the photograph of Figure 3-3 the highly volatile solvent has all
but evaporated from the surface, exposing the wetted crack openings like
a fingerprint. Before wetting with the solution, cracks were invisible to
the naked eye. This condition exists in most concrete machinery founda-
tions and is caused by water loss from the capillary pores in the concrete
while curing. This water loss causes shrinkage which would not be expe-
rienced if the concrete had been immersed in water for 28 days like the
samples from each pour that are usually sent to the laboratory for testing.
While such shrinkage cracks do not constitute structural failure in ma-
chinery foundations, they do provide a path for the penetration of lubri-
cating oils into the foundation. One interesting fact was that cored con-
crete samples from this runway typically had 6,000 psi compressivc
strength.
It is good construction practice to seal the surface of a foundation with
a good quality epoxy paint as soon as the forms are removed. This seal-
ing of the foundation accomplishes two objectives. First, it seals in water
and encourages more complete curing of the concrete, and second, it pre-