110 CHAPTER TEN
Labor. The time required for finishing the concrete can
be estimated from the information provided in Figure 10.30.
From this table, the productivity rate for machine troweling
concrete is 1 labor hour per 100 sf. Using the locally prevail-
ing wage rate of $13.25 per hour for concrete finishers, the
labor costs can be determined.
10–7 Curing
Proper curingis an important factor in obtaining good con-
crete. The concrete cures with the chemical combinations
between the cement and water. This process is referred to as
hydration.Hydration (chemical combination) requires time,
favorable temperatures, and moisture; the period during
which the concrete is subjected to favorable temperature and
moisture conditions is referred to as the curing period.The
specified curing period usually ranges from 3 to 14 days.
Curing is the final step in the production of good concrete.
Moisture can be retained by the following methods:
leaving the forms in place, sprinkling, ponding, spray mists,
moisture retention covers, and seal coats. Sometimes combi-
nations of methods may be used, and the forms left on the
sides and bottom of the concrete while moisture retention
covers or a seal coat is placed over the top. If the forms are
left in place in dry weather, wetting of the forms may be
required because they have a tendency to dry out; heat may
be required in cold weather.
Sprinkling fine sprays of water on the concrete helps
keep it moist, but caution is advised. If the surface dries out
between sprinklings, it will have a tendency to craze or crack.
Temperature must also be maintained on the concrete.
With cold weather construction, it may be necessary to begin
with heated materials and provide a heated enclosure.
Although the hydration of cement causes a certain amount
of heat (referred to as the heat of hydration) and is of some
help, additional heat is usually required. The forms may be
heated by the use of steam, and the enclosed space itself may
be heated by steam pipes or unit heaters of the natural gas,
liquefied petroleum gas, or kerosene type. The number
required varies for the size of the heater used. Heaters are
available from about 50,000 to 300,000 BTUH. Since many
unions require that an operator work only with the heaters,
local union rules should be checked. With operators, this
heating cost is as much as 20 times the cost per cubic yard as
without operators, depending on the number of operators
required.
During mild weather (40°F to 50°F), simply heating the
mixing water may be sufficient for the placement of the con-
crete, but if temperatures are expected to drop, it may be
necessary to apply some external heat. Specifications usually
call for a temperature of 60°F to 70°F for the first three days
or above 50°F for the first five days.
Labor cost52 hours$13.25 per hour$689Labor hours52 hoursLabor hours51 .6 hundred sf1 labor hour per 100 sfHot weather concreting also has its considerations.
High temperatures affect the strength and other qualities of
the concrete. To keep the temperature down, it may be nec-
essary to wet the aggregate stockpiles with cool water and
chill the mixing water. Wood forms should be watered to
avoid the absorption of the moisture, needed for hydration,
by the forms.
A continuous spray mist is one of the best methods of
keeping an exposed surface moist. There is no tendency for
the surface to craze or crack with a continuous spray mist.
Special equipment can be set up to maintain the spray mist so
that it will not require an excessive number of labor hours.
Moisture-retaining covers such as burlap are also used.
They should thoroughly cover the exposed surfaces and be
kept moist continuously, because they dry out (sometimes
rapidly). Straw and canvas are also used as moisture-retaining
covers. Burlap and canvas may be reused on many jobs, thus
spreading out their initial cost considerably.
Watertight covers may be used on horizontal areas such
as floors. Materials employed are usually papers and polyeth-
ylene films. Seams should be overlapped and taped, and the
papers used should be nonstaining. Often these materials may
be reused several times with careful handling and storage.
Sealing compounds are usually applied immediately
after the concrete surface has been finished. Both the one-
and two-coat applications are colorless, black, or white. If
other materials must be bonded to the concrete surface at a
later date, they must bond to the type of compound used.
Estimating the cost of curing means that first a determina-
tion must be made as to what type of curing will be required
and in what type of weather the concrete will be poured. Many
large projects have concrete poured throughout the year and
thus estimators must consider the problems involved in cold,
mild, and hot weather. Smaller projects have a tendency to fall
into one or perhaps two of the seasons, but there is a general
tendency to avoid the very coldest of weather.
If a temporary enclosure is required (perhaps of wood
and polyethylene film), the size and shape of the enclosure
must be determined, a material takeoff made, and the num-
ber of labor hours determined. For simple enclosures, two
workers can erect 100 sf in 30 to 60 minutes, once the mate-
rials are assembled in one place. If the wood may be reused,
only a portion of the material cost is charged to this portion
of the work. The enclosure must be erected and possibly
moved during the construction phases and taken down
afterward—and each step costs money.
Heating of water and aggregate raises the cost of the
concrete by 3 to 10 percent, depending on how much heat-
ing is needed and whether ready-mix or job mix concrete is
being used. Most ready-mix plants already have the heating
facilities and equipment, with the cost spread over their
entire production. Job mixing involves the purchase and
installation of equipment on the job site as well as its opera-
tion in terms of fuel, labor hours, and upkeep.
The cost of portable heaters used to heat the space is
usually based on the number of cubic yards of space to be
heated. One worker (not an operator) can service the heaters