Page 131 - Machinery Component Maintenance

114 Machinery Component Maintenance and Repair

moisture on the underside of the plastic the next day, the concrete is not
ready for the placement of epoxy grout. Repeat the test until there is no
moisture. Unprotected cured concrete will absorb moisture from rain, so
give it the moisture test also.
During the placement of concrete for the pump foundation, samples of
the concrete should be taken to make slump and compressive strength
tests. During a routine compressive strength test for a 600 HP pump, a
3,OOO-psi concrete mix, which had passed the slump test earlier, cracked
at 1,400 psi. The foundation was chipped out and the job started over at
the expense of the concrete supplier. The concrete mix had been in the
truck too long and additional water had been added to pass the slump test.
This re-tempering of the concrete made it much weaker.
Surface preparation of the new concrete can begin after curing by chip-
ping away the laitance with a light-duty pneumatic hammer and a sharp,
pointed chisel. Jackhammers may crack the foundation and should not be
used. At least the top 1/2 in. of concrete should be chipped until all lait-
ance is removed and the aggregate is exposed (see Figure 3-29). Founda-
tion edges should be chamfered at least 2 to 4 in. at a 45" angle in order

Figure 3-29. All laitance must be chipped off the surface of the foundation so as to expose
the aggregate.