Economy of Construction
Achieving overall economy in a reinforced concrete project can be achieved by considering the costs related to formwork, concrete, and reinforcing steel, which are the three main cost components in any project. While the following economical guidelines are effective, they are not meant to be comprehensive; other cost-savings opportunities may be available depending on the project.
Formwork costs on average are approximately 50 percent of the cost of the completed projected. For that reason, it is important to consider the following guidelines at the onset of any project.
Select one framing system and use it throughout the structure wherever possible. Using the same framing system as often as practical throughout the structure has been shown to result in significant cost savings. Forms are reused many times and it is easier for the crew when erecting the forms, resulting in reduced labor costs.
Use standard shaped forms. Rectilinear members are the most cost effective to form. Whenever possible, avoid shapes that have to be either fabricated by the form supplier or customized by carpenters in the field.
Use modular formwork whenever practical. Modular formwork can be used in customized applications such as slip-formed shafts for elevators and stairways and curved exterior walls. The cost of using this type of formwork can usually be justified if it can be reused multiple times in a project.
Use floor framing systems of minimum depth with a constant elevation for the bottom surface of the system. Providing the minimum depth based on code-prescribed serviceability requirements will result in minimum floor-to-floor heights and, thus, an overall reduction in the building height. Overall height reduction translates to a reduction in the costs associated with essentially all the vertical runs in the building (façade; elevators; stairs; interior partition walls; and plumbing, electrical, and mechanical conduit and ductwork). The underside of a reinforced concrete floor or roof should be kept level for maximum economy.
Orient one-way structural members to span in the same direction throughout the entire structure. Structures that have one-way members oriented in the same direction throughout the entire structure tend to be constructed more efficiently than those where multiple framing directions are used. This efficiency is attributed to less confusion and fewer mistakes made in the field because of the overall regularity of the structure.
Arrange columns in a regular pattern. Columns should be arranged in a regular pattern throughout each floor of the structure, if possible, because this helps in achieving consistency in the formwork and reinforcement layout of all the structural members. Installing the formwork is repetitive and efficient and the formwork can be readily reused; this efficiency carries over to all aspects related to the reinforcing bars.
Use a consistent column size. In low-rise buildings, the same column size should be used throughout the entire height of a building as should the same concrete compressive strength. The number of reinforcing bars can change over the height as needed. In taller structures, column size can change over the height along with concrete compressive strengths. The number of changes usually depends on the height of the building, but should be kept to a minimum practical number.
Specify the time when forms may be stripped for self-supporting members and the strength when forms may be stripped for other members. Forms for columns and walls can be stripped based on time after the concrete has been placed (for example, 12 hours). For beams and slabs, forms can be stripped after a specific percentage of concrete compressive strength has been attained (for example, 75% of the specified 28-day compressive strength). Appropriate stripping specifications will minimize the required amount of formwork and will result in lower formwork costs.
Use high-early-strength concrete. The use of high-early-strength concrete enables the formwork to be stripped sooner than conventional concrete. Faster cycle times may allow for a faster overall construction time, which translates to significant overall cost savings.
Use predetermined construction joints. The location for construction joints should be the contractor’s prerogative with input from the engineer of record where required. Properly located construction joints will allow the contractor to sequence concrete placement efficiently.
In-place costs for reinforcing steel constitute approximately 20 percent of the completed structure. The following guidelines are time-tested ways to achieve economy for reinforcing steel.
Use Grade 60 reinforcing bars. ASTM A615 Grade 60 bars are the most widely used and inventoried reinforcing bars and are utilized in many applications. The benefits of using reinforcing bars with a yield strength greater than 60,000 psi are usually realized in high-rise buildings where the high-strength bars are used in the columns primarily at the lower levels.
Use the largest bar size possible. Placing and fabrication costs are minimized by using the largest practical bar sizes that satisfy both strength and serviceability requirements.
Use straight bars wherever possible. Fabricating and placing straight bars is faster and easier than bent bars.
Use ACI standard bar bend types. Specify standard bar shapes and bends. Nonstandard bends disrupt shop routine and are costlier to fabricate.
Use bars in one plane. Wherever possible, reinforcing bars should have bends located in one geometric plane. Bars with bends in two or three planes are difficult and expensive to fabricate.
Use repetitive bar sizes and lengths. The standard length for reinforcing bars is 60 feet. The longest available bar lengths should be used to reduce fabrication and placing costs. The number of bar sizes specified in a particular project should be minimized; this reduces the number of sizes that must be handled in the shop and placed in the field.
Use stock length bars. In the case of irregularly-shaped walls and slabs, it is usually more cost effective to use stock length bars that are cut and spliced in the field in lieu of using individual bars that have been fabricated to a required length. The added cost associated with the extra material used due to variable lap lengths is usually minor and is more than offset by the savings due to reduced labor that would otherwise be required to cut and sort the individual bars.
Use the appropriate splice for a given situation. Wherever possible, bars should be lap spliced, and a consistent lap splice length should be specified for a given bar size. Where congestion is an issue, use mechanical splices.
Provide a 4- to 6-inch gap between bars. A 4-inch slump concrete with ¾ inch aggregate will not flow easily through a 2-inch space between bars. Vibrator heads, which are usually 2 to 3 inches in width, may not fit between the bars or can become entangled in the bars if the space between the bars is too small.
Draw details to scale to ensure that the reinforcing bars will fit within the section. Scaled drawings that show all the reinforcement are essential, especially in narrow beams, slabs with multiple openings, slab-column and beam-column joints, and columns with more than 2% longitudinal reinforcement. It is important to include the overall dimensions of the reinforcing bars, as well as hook dimensions and bend radii, when drawing the scaled details.
In-place costs associated with concrete are about 30 percent. Concrete costs can be reduced by considering the following guidelines.
Use moderate-strength concrete for floor and roof systems. Concrete with a compressive strength of 4,000 to 5,000 psi is usually sufficient for conventionally reinforced floor and roof systems. In low-rise buildings, using these concrete strengths for the columns is typically sufficient as well. Using higher-strength concrete in the lower-story columns of high-rise buildings helps decrease overall column size, thereby increasing usable space.
Limit coarse aggregate size to ¾ inch. Minimum clear bar spacing requirements include 4/3 of the maximum aggregate size. Limiting the coarse aggregate to ¾ in. helps ensure that the concrete can easily flow between reinforcing bars.
Cost-effective Reinforced Concrete Floor Systems Based on Span and Live Load
Numerous types of cast-in-place, reinforced concrete floor systems are available that can be utilized to satisfy virtually any span and loading requirements. For low-rise buildings, the floor system accounts for most of the in-place cost of the structural frame. The cost of the columns and walls and the cost of the lateral-force-resisting system grow linearly and exponentially with building height, respectively, but the cost associated with the floor system is still important.
The following table can be used as a preliminary guide in selecting an economical floor system. The size and geographic location of the project, the availability of skilled labor, and local building code requirements are a few of the factors that can significantly affect overall cost. Each project is unique, and reinforced concrete floor systems other than those that are recommended in the table may be more cost effective.