Keep it simple when it comes to concrete. Concrete is cement, aggregate and water. The quality, characteristics and quantity of each depends on the requirements of where and how it’s being used. For cement in structural concrete, Portland type I, II or III can be used for regular or high-early strength concrete. The aggregates vary in material, size and color. The proportions of each of these elements, the sizes of the aggregates and the selection of additives must be determined to fit the job. The prices of cement and concrete is updated quarterly in RSMeans’ Change Notice. Additives affect concrete mix in the following ways:
- Accelerators are ideal for cool weather and early stripping.
- Retarders are recommended for warm weather and allowing more time for placing and finishing.
- Air entrainment agents reduce affects of temperature change.
- Calcium chloride helps prevent freezing during curing.
- Fly ash can be added for waterproofing and easier placement.
- Acrylic Polymer Emulsions improve adhesive bonding to existing surfaces.
- Plasticizers reduce water and increase workability.
The first step in structural concrete construction is formwork. From footings, walls and columns to slabs, each requires its own special forms. Concrete contractors frequently have their own forms that they maintain and reuse many times. A general contractor might rent prefabricated forms for a particular contract. Some of the factors that determine the square footage of forms to rent are:
- The time balance between the forming crew and the concrete placing crew. How many cubic yards can the placing crew efficiently place in one day?
- Curing time: How soon can the forms be stripped and cleaned for reuse?
- Project schedule: How soon must all the concrete formwork be finished?
The estimated cost for erecting and stripping formwork for a typical 8′ high foundation wall using prefabricated plywood forms and a crew of four carpenters and one laborer is $4.84 per SFCA. The estimated cost for placing concrete for this 12″-thick wall using a direct chute with a crew of four laborers, a cement finisher and a couple of vibrators is $26.50 per CY.
Reinforcing is a major factor in almost all concrete construction. There are a great variety of accessories to support rebar within concrete structures. The rebar itself can be galvanized or epoxy-coated to prevent rusting. There are also a number of other reinforcing options, such as welded wire fabric, steel fibers, glass fibers, cellulose fibers and many more synthetic fibers. Old Roman structures reveal that they once used horsehair for reinforcing concrete.
Once you have a well-designed and erected concrete foundation wall in place, you’re ready to erect any new structure.