Chapter 6 Concrete 111
Proper curing improves the strength, watertightness, and weather
resistance of concrete and requires time, temperatures above freezing,
and moisture. Figure 6-7 shows how time and moisture affect strength
of concrete. Concrete is kept moist by sprinkling with water, by applying
curing compounds to the surface, or by covering the concrete with sheets of
plastic or other materials. In cold weather, fresh concrete must be kept from
freezing. This is done by covering the completed job and providing supple-
mental heat.
Working with Concrete
Concrete work requires a sequence of six steps: preparing the subgrade
and form, estimating volume, obtaining concrete, placing, finishing, and
curing. The sequence is important for creating a quality concrete product.
Preparing Subgrade and Forms
Subgrade is a leveled soil surface that provides uniform support for
concrete slabs and footings. The subgrade must be placed at the correct
elevation. The subgrade should be compacted and moist. A moist subgrade
does not absorb water from fresh concrete.
subgrade: The
leveled surface on
which footings and
some concrete floors
and sidewalks are
placed.
Percent
of
28
day
moist
cure
concrete
150%
125%
100%
75%
50%
25%
0%
3 7 28 90 180
Days cured
Moist cure
In air after 7 days covered
In air after 3 days covered
In air entire time
Figure 6-7. Twenty-eight days is generally considered sufficient time for concrete
to reach full strength. Notice that concrete continues to gain strength even after
180 days, if moisture is present.