Concrete is estimated by the cubic yard
(cy) or by the cubic foot (cf) and then converted into cubic yards.
Concrete quantities are measured in cubic yards as it is the pricing
unit of the ready-mix companies, and most tables and charts available
relate to the cubic yard.
Roof and floor slabs, slabs on grade,
pavements, and sidewalks are most commonly measured and taken off in
length, width, and thickness and converted to cubic feet and cubic
yards (27 cf # 1 cy). Often, irregularly shaped projects are broken
down into smaller areas for more accurate and convenient
manipulation.
When estimating footings, columns,
beams, and girders, their volume is determined by taking the linear
footage of each item times its cross-sectional area. The cubic
footage of the various items may then be tabulated and converted to
cubic yards.
When estimating footings for buildings
with irregular shapes and jogs, the estimator must be careful to
include the corners only once. It is a good practice for the
estimator to highlight on the plans which portions of the footings
have been figured.
When taking measurements, keep in mind
that the footings extend out from the foundation wall; therefore, the
footing length is greater than the wall length.
In estimating quantities, the estimator
makes no deductions for holes smaller than 2 sf or for the space that
reinforcing bars or other miscellaneous accessories take up. Waste
ranges from 5 percent for footings, columns, and beams to 8 percent
for slabs.
The procedure that should be used to
estimate the concrete on a project is as follows:
1. Review the specifications to
determine the requirements for each area in which concrete is used
separately (such as footings, floor slabs, and walkways) and list the
following:
(a) Type of concrete
(b) Strength of concrete
(c) Color of concrete
(d) Any special curing or testing
2. Review the drawings to be certain
that all concrete items shown on the drawings are covered in the
specifications. If not, a call will have to be made to the
architect-engineer so that an addendum can be issued.
3. List each of the concrete items
required on the project.
4. Determine the quantities required
from the working drawings. Footing sizes are checked on the wall
sections and foundation plans. Watch for different size footings
under different walls.
Concrete slab information will most
commonly be found on wall sections, floor plans, and structural
details. Exterior walks and driveways will most likely be identified
on the plot (site) plan and in sections and details.
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