118 Section 2 Construction Materials
Final finishing, called troweling, is done with steel trowels, brooms,
and other tools. Steel hand trowels are used to produce a smooth finish. A
broom is used to produce a slip-resistant finish for sidewalks and roadways,
Figure 6-18.
Curing
Curing, the hardening of concrete, requires moisture. Concrete that
dries too quickly will not reach full strength. Covering finished concrete
with plastic sheets or periodically misting the surface with water is essential
to proper curing. Another alternative is to spray the surface with a water-
proof coating called curing compound to prevent water in the concrete from
evaporating too quickly.
In general, concrete should be allowed to cure as long as practical before
being put into service. For example, concrete made with Type I Portland
cement needs at least seven days of controlled curing.
Remove forms early to allow patching as soon as possible. However, the
concrete must not be too fragile when forms are removed.
Testing Concrete
Two types of tests are commonly used when working with concrete.
Slump tests measure the consistency of batches of concrete, indicating
the workability of the concrete as it goes into the form. Compression tests
measure the strength of cured concrete.
Slump tests are done by carefully filling and consolidating concrete in a
slump cone. Once the concrete is consolidated, the top is leveled off and the
slump cone is carefully removed from the concrete. The difference in height
between the slumped concrete and the cone is measured, Figure 6-19. This
measurement is the slump of the batch of concrete. If the slump is less than
1″, water needs to be added to the mix to make the concrete more workable.
If the slump is greater than 5″, aggregate and Portland cement need to be
added to improve the quality of the cured concrete.
troweling: Process
that produces a
dense, smooth, hard
surface.
curing: A chemical
process that causes
concrete to become a
solid.
slump tests: Measure
the consistency of
batches of concrete.
slump cone: A
device that holds
concrete for a slump
test.
Figure 6-18. A broom finish produces a skid-resistant surface.