Concrete quantities are generally taken off in measures of cubic yards, as this is the way the material supply companies charge for it. For estimating concrete quantities, be sure to include:
When estimating structural concrete, pay particular attention to requirements for concrete additives, curing methods and surface treatments. Special consideration for climate conditions as well as hot or cold weather conditions must be included in your estimate. Also include requirements for concrete placing and finishing methods and equipment.
For accurate concrete estimating, the estimator must consider each of the following major components individually: formwork, reinforcing steel, ready-mix concrete, placement of the concrete mix, finishing of the surface, and curing.
Formwork - A primary cost for cast-in-place concrete is forming. Most jobs today are constructed with prefabricated forms. The selection of the forms best suited for the job and the total square feet of forms required for efficient concrete forming and placing are key elements in estimating concrete construction. Enough forms must be available for erection to make efficient use of the concrete placing equipment and crew. read more.
Reinforcing Steel - When estimating the amount of reinforcing steel, either bar or mesh, if no lap specifications are given, add 10% to your quantities for lapping, splices, and waste. read more.
Concrete Placement - When estimating the placement of concrete by direct chute, the forms available generally determine the volume of concrete placed per hour. The more you have available, the more concrete can be placed in the course of a day. When placing concrete by methods involving a bucket and crane, pumping system, conveyor belt system or other mechanical system, set up enough forms to keep the above systems productive for the entire day. Usually the cost for the use of the equipment for a full day will be charged to you even if you only use it for part of a day.
Cold Weather Pouring of Footings - When placing concrete in cold weather, it may not always be necessary to use heating devices to keep concrete warm while it cures. In many cases, insulating blankets and straw are all that are needed. Each situation must be evaluated individually, a worthwhile exercise in view of the considerable expense that can be saved.
Sequencing the Pour - During the estimating phase of the project, whatever method or sequence you envision for placing concrete should be documented thoroughly. This will allow those in the field to know how you arrived at the estimated costs and which methods they should use (or try to out-do, in terms of cost effectiveness). Another reason for detailed documentation is that during the documentation process, it may become evident that you cannot place the concrete as you planned or that you can do it differently, more efficiently, or at less cost.
Finishing Concrete - Rule of thumb for finishing concrete: allow 1,000 S.F. of slab per each cement finisher.
Drilling Anchor Bolts - Consider drilling anchor bolts into the concrete where design permits to save time, manpower, and materials on layout and templates during the pour.
Bracing & Shoring - Included within costs for forms-in-place are all the necessary bracing and shoring.
It cannot be stressed enough to carefully check all the plans and specifications. Concrete often appears on drawings other than structural drawings, including mechanical and electrical drawings for equipment pads and grouting requirements on steel drawings. Assuming all concrete requirements are indicated on the structural and architectural drawings can be a costly error.
When estimating quantities of concrete for floor slabs or walls, do not bother to deduct small areas (two square feet or so) unless there are a large number of these areas, as this can take up more estimating time than the areas are worth. Also remember that you will be adding approximately 3% to the total volume for waste, thereby making these small areas even less significant.
Always obtain concrete prices from suppliers near the job site. A volume discount can often be negotiated, depending upon competition in the area. remember to add for waste, particularily for slabs and footings on grade.
When estimating the amount of concrete compression testing that will be necessary for a project, figure on a minimum of one test per pour on smaller pours and a minimum of one test for each fifty yards of concrete placed. Each test should consist of taking a set of three cylinders minimum.
Read more concrete estimating tips.
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