All exposed concrete surfaces require
some type of finishing. Basically, finishing consists of the patch up
work after the removal of forms and the dressing up of the surface by
troweling, sandblasting, and other methods.
Patch-up work may include patching
voids and stone pockets, removing fins, and patching chips. Except
for some floor slabs (on grade), there is always a certain amount of
this type of work on exposed surfaces.
It varies considerably from job to job
and can be kept to a minimum with good quality concrete, with the use
of forms that are tight and in good repair, and with careful
workmanship, especially in stripping the forms.
This may be included with the form
stripping costs, or it may be a separate item. As a separate item, it
is much easier to get cost figures and keep a cost control on the
particular item rather than “bury” it with stripping costs.
Small patches are usually made with a
cement-sand grout mix of 1:2; be certain that the type of cement
(even the brand name) is the same used in the pour because different
cements are varying shades of gray. The labor hours required will
depend on the type of surface, the number of blemishes, and the
quality of the patch job required. Scaffolding will be required for
work above 6 feet.
The finishes required on the concrete
surfaces will vary throughout the project. The finishes are included
in the specifications and finish schedules; sections and details
should also be checked.
Finishes commonly required for floors
include hand or machine troweled, carborundum rubbing (machine or
hand), wood float, broom, floor hardeners, and sealers. Walls and
ceilings may also be troweled, but they often receive decorative
surfaces such as bush hammered, exposed aggregate, rubbed,
sandblasted, ground, and lightly sanded.
Finishes such as troweled, ground,
sanded, wood float, broom finishes, and bush hammered require no
materials to get the desired finish, but require only labor and
equipment. Exposed aggregate finishes may be of two types. In the
first type, a retarder is used on the form liner, and then the
retarder is sprayed off and the surface is cleaned.
This finish requires the purchase of a
retarding agent, spray equipment to coat the liner, and a hose with
water and brushes to clean the surface. (These must be added to
materials costs.) The second method is to spray or trowel an exposed
aggregate finish on the concrete; it may be a two- or three-coat
process, and both materials and equipment are required.
For best results, it is recommended
that only experienced technicians place this finish. Subcontractors
should price this application by the square foot. Rubbed finishes,
either with burlap and grout or with float, require both materials
and labor hours plus a few hand tools. The burlap and grout rubbed
finish requires less material and more labor than the float finish. A
mixer may be required to mix the grout.
Sandblasting requires equipment, labor,
and the grit to sandblast the surface. It may be a light, medium, or
heavy sandblasting job, with best results usually occurring with
green to partially cured concrete.
Bush hammering, a surface finish
technique, is done to expose portions of the aggregates and concrete.
It may be done by hand, with chisel and hammer, or with pneumatic
hammers. The hammers are commonly used, but hand chiseling is not
uncommon. Obviously, hand chiseling will raise the cost of finishing
considerably.
Other surface finishes may also be
encountered. For each finish, analyze thoroughly the operations
involved,material and equipment required, and labor hours needed to
do the work.
The finishing of concrete surfaces is
estimated by the square foot, except bases, curbs, and sills, which
are estimated by the linear foot. Since various finishes will be
required throughout, keep the takeoff for each one separately.
Materials for most operations (except
exposed aggregate or other coating) will cost only 10 to 20 percent
of labor. The equipment required will depend on the type of finishing
done. Trowels (hand and machine), floats, burlap, sandblasting
equipment, sprayers, small mixers, scaffolding, and small hand tools
must be included with the costs of their respective items of
finishing.
Estimating Concrete Finishing.
Areas to be finished may be taken from
other concrete calculations, either for the actual concrete required
or for the square footage of forms required.
Roof and floor slabs, and slabs on
grade, pavements, and sidewalk areas can most easily be taken from
the actual concrete required. Be careful to separate each area
requiring a different finish. Footing, column, walls, beam, and
girder areas are most commonly found in the form calculations.
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