e t Economical Reinforced oConcrete Construction N l a c i n h c e T Engineering Technical Note ETN-C-1-10
Introduction
Experience has shown that the initial cost of reinforced concrete structures can be reduced through planning and detailing in such a way as to minimize the expenses related to the materials and the construction activities associated with formwork, reinforcement, and concrete. Frequently, these cost-reducing techniques are not obvious to the designer. For example, formwork costs are generally 40 to 60 percent of the completed reinforced concrete structure. Material costs for concrete and reinforcement are on the order of 10 to 30 percent. The labor cost percentage for placing the concrete and the reinforcement is the remainder. This technical note addresses many of the areas that have shown to result in overall cost savings.
Formwork
Select one framing system and use it throughout the structure. structure . For each framing system used, a separate forming system will be necessary.. This means additional costs associatnecessary ed with the formwork and its mobilization, as well as a learning curve for the construction personnel will be incurred. As a result, experience has shown that it is difcult to economically justify the use of more than one framing system. The exception usually occurs on large structures when usage changes over the building height, such as a high-rise with a multiple oor parking garage in the lower oors and residences in the upper oors. Arrange and organize structural members to fully utilize structural capacity . The thickness of oor slabs may be governed by the rerequired re rating specied by the building code, so span the slab as far as practical (considering deection) with the minimum amount of reinforce reinforce-ment. Another basic structural member frequently underutilized is the concrete wall. Concrete walls normally have to be of a minimum thickness and provided with a minimum amount of reinforce-
ment. These walls, whether they are exterior walls or interior partitions, can carry signicant axial loads, provide lateral load resistance to the overall structure as shear walls, or act as transfer girders when the column layout is changed. Use concrete members that provide architectural interest and nishes. The extra cost associated with the formwork and related architectural details for the desired effect will more than offset the additional expense if a separate nish is eliminated. Panelized concrete structures are routinely constructed with numerous architectural nishes. Similarly, Similarly, concrete oors can be stained and sealed rather than carpeted or tiled. Other examples include the use of at plate oors as exposed ceilings and exposed interior walls that may utilize self-consolidating concrete along with high quality formwork. Orient one-way structural members to span the same direction throughout the entire structure. structure. Experience has shown that structures that are detailed with the one-way structural members oriented in the same direction throughout the entire structure tend to be constructed most efciently, because there is less confusion and fewer mistakes than similar structures with multiple framing directions.
Use modular formwork. Traditionally, modular forms have been used to form large areas of walls or oors where the forms can be moved in large sections and reused many times (often 10 to 20 times). The use of proprietary forming syssystems have become more common and are being used to cast smaller members. Similarly, Similarly, proprieproprietary forming systems are increasingly being used in more customized applications such as slip formed elevator and stairway shafts and curved exterior walls. Generally, more intricate shapes can be justied even though additional costs are incurred when the forms are used multiple times. A unique shape or nish may cost more but can be justied if architectural interest is provided.
Arrange columns in a regular pattern. Arrange columns in a regular pattern throughout each oor level of the structure as well as vertically. This practice enforces consistency on the other structural members which will dictate that the formwork and reinforcement layout are also consistent. Formwork will be reused and setup will become repetitive and efcient. This repetitiveness and efciency will also carry over to all aspects related to the reinforcing bars. Use a consistent column size. Experience has shown that it is more efcient to use the same size column throughout a structure’s height. Rather than varying the column size, it is more efcient to vary the number of bars, and vary the concrete compressive strength. This approach of using consistent column size will result in less variation in column forms, fewer variations in the slab and beam forms, and standardization of the steel reinforcement. Use oor framing system with the minimum depth. The depth of the oor is normally governed by deection (serviceability) considerations. However, by minimizing the oor depth, and thereby the oor-to-oor height, the cost of a structure is minimized because the overall height of the building is minimized. In certain areas where zoning limits the building height, additional oor(s) may be included. Other areas of cost savings include the exterior façade; interior partition walls; vertical runs of plumbing, electrical, mechanical, and elevators; stairs; formwork height; and shoring. Use beams and joists of the same depth. While using beams and joists of the same depth throughout a structure may seem wasteful, just the increased cost of shoring—not counting a separate set of forms—will exceed the minor additional material costs for the concrete and reinforcing bars. Using the same depth allows all the formwork to be the same and assists in reducing the interferences between the structure and mechanical systems. Another similar technique is to use relatively wide, shallow beams that can be up to four to ve times wide as deep. The quantity of steel reinforcement would then be varied rather than the depth of the beam. If this does not work, consider limiting the beams to two depths throughout the structure. Use only one, one-way joist pan size throughout the structure. The cost of material saved from optimizing pan sizes is likely to be insignicant when compared to additional shipping and handling the various pan sizes. Also, the overall oor depth is established by the deepest pan or deepest beam and, as such, no building height-related savings will be possible. Use available, standard joist form sizes. Specify available standard sizes, for one-way and two-way joists (wafe slabs). Nonstandard sizes have to be specially
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fabricated and the entire cost may need to be charged to the specic project rather than amortized over several projects. Contact potential suppliers for form size availability. Use standard shaped forms. Avoid shapes that have to be either fabricated by the form supplier or customized by carpenters in the eld. Similarly, inordinate eld fabrication costs can be incurred when the forms have to be modied for tapering or haunches. Use beams that are 2-inches wider, on each side, than the column. A beam made intentionally wider than the column will allow the outermost horizontal bars in the beam to pass by the vertical longitudinal column bars with minimal interference. Another advantage is that it is easier to position the beam form if placed on top of the column form than connecting the beam form to the side of the column form. Use the same oor-to-oor heights throughout the structure. If changes in oor-to-oor heights are necessary, reduce the heights in the upper stories. Cutting the column form down in length is easier than adding to it. Use a at plate oor system for spans up to 25 feet. A at plate is the most economical oor system because the formwork and related construction are all based off the same at formwork. The at plate is recognized as being the least expensive, fastest, and shallowest framing method available. Use various techniques to increase shear capacity at the column slab connection. Issues concerning shear capacity at the column slab connection are frequently resolved by increasing the oor slab thickness, use of a drop panel, increase in the column size (perimeter), puddling higher strength column concrete around the column, increasing the oor slab’s concrete strength, or by use of shear stud rails. Column capitals should only be used as a last resort. Shear stud rails are recognized as an effective way to increase shear capacity around the column perimeter and have recently been recognized in ACI 318-08. Alternatively, puddling of high-strength concrete around the column perimeter a set distance during column concrete placements is frequently used. The higher-strength column concrete is then integrated with the concrete placement for the oor slab to avoid cold joints and potential load transfer issues. Similarly, stud shear rails and puddling can be used together. See Figures 1 and 2 for examples of these construction techniques. Use drop panels for increased shear capacity. Rather than using a tapered column capital, a drop panel can be formed around columns to increase the shear capacity. See Figure 3.
Economical Reinforced Concrete Construction [ETN-C-1-10]
Column concrete placed in the nearby slab section.
Use standard lumber dimensions for drop panel thickness. Actual lumber dimensions are 1½, 3½, 5½, and 7¼ inches for nominal sizes 2x2, 2x4, 2x6, 2x8, respectively. Assuming a form thickness of ¾ inches, the resulting drop panel height would be 2¼, 4¼, 6¼ and 8 inches, respectively. See Figure 4.
Figure 1 – Concrete “puddling” around the columns to enhance punching shear capacity (Photo courtesy of Skidmore, Owings and Merrill LLP)
Standard Lumber Dimensions and Drop Panel Height Lumber Size Nominal
Actual
Drop Panel Height*
2x
1 ½″
2¼″
4x
3 ½″
4¼″
6x
5 ½″
6¼″
8x
7 ¼″
8″
* ¾-inch form sheathing
Figure 4 – Drop panel height for economy
Figure 2 – Shear studs at an exterior column used to increase punching shear capacity (Photo courtesy of Skidmore, Owings and Merrill LLP)
Specify the time when forms may be stripped for self-supporting members and strength for others. Members such as columns and walls can be stripped based on time, such as 12 hours after concrete placement. For oors, beams, and slabs, stripping timecan be specied by referring to a specic percentage of compressive strength gain (e.g., 75 percent of 28-day compressive strength). However, these members have to be reshored until compressive strength has been achieved to minimize deection. Appropriate stripping specications will minimize the quantity of formwork required and result in a decrease in formwork cost. Use high-early strength concrete. By using highearly strength concrete, the time necessary to wait for stripping the forms can be minimized. If the structure is large enough, the faster cycle time may allow for a signicantly faster overall construction time, possibly avoiding seasons with unfavorable weather.
Figure 3 – Means of increasing shear capacity in at slabs
Use predetermined construction joints. Ideally, the contractor and designer should coordinate the locations and details for construction joints. Properly located construction joints will allow the contractor to efciently sequence the concrete placement. Similarly, the joint itself can be located where it is hidden in the completed structure or detailed in such a way that it becomes part of the nish.
CRSI Technical Note
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Consider allowing the use of dowel bar mechanical splices at construction joints. At most construction joints, the most common detail is to simply extend the bars through the joint and lap splice with the bars in the subsequent pour, without damaging the formwork. As required, an alternate detail the contractor could use is a dowel bar mechanical splice shown in Figure 5. This splice type contains a ange that can be nailed to the form. After the forms are stripped, the adjoining reinforcing bar can be screwed onto the coupler. The mechanical splice eliminates the bar or dowel penetration through the forms, and can extend the life of the forms if cutting holes is the only other option. See Figure 5.
Figure 5 – Dowel bar mechanical splice
Consider the formwork. Provide designs that retain the general integrity of formwork. Where possible, minimize modications to the formwork for recesses, protrusions, haunches, stubs, and the like to extend the service life of the forms. Alternately, architectural formwork may have a single-use or multiple re-use depending on the member(s) being formed; higher costs can be anticipated for the greater level of detail necessitated with this type of formwork.
Reinforcement Use Grade 60 bars. ASTM A615 Grade 60 bars are the most widely used and inventoried reinforcing bar. Grade 40 may require 50 percent more steel than Grade 60. ASTM A615 Grade 75 or 80 bars are readily available, but not normally inventoried by fabricators. Grade 75 or 80 bars are available at competitive prices only on mill orders in lots ranging from 25 to 75 tons per bar size. However, smaller quantities may be obtained from warehouses. If a minimum mill order of one bar size can be used for column vertical bars in a tall structure, Grade 75 or 80 may lower the cost of the column vertical reinforcement by decreasing the congestion and reducing the number of crossties.
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Use the largest bar size possible. Placing costs and fabrication costs are minimized by using the largest practical bar sizes while still meeting the design requirements. In most cases, it takes as much time to fabricate and place one small bar as it does one large bar. However, a greater quantity of smaller bars may be required for crack control reinforcement or other serviceability issues. Consider using #14 and #18 longitudinal column bars. Rather than using many smaller longitudinal bars in a heavily loaded column, using a fewer number of #14 or #18 bars may be warranted. The large bars reduce steel congestion, placing cost, fabrication cost, and cost of splices. However, because a crane may be required to place the heavier bars, consideration should be given to whether a crane will be on the job site for other reasons. Also, large bars almost always have to be mechanically spliced, which is another design consideration. Use columns with tied transverse reinforcement rather than spiral. Individual ties are generally preferred for eld fabricated columns in non-seismic areas. Reasons include the weight per foot of spirals is two to three times as much as comparable column ties, difculty of consistently fabricating spirals with the identical diameter and pitch, and handling and placing spirals in the eld. For shop fabricated columns, there is a trend towards automated fusion welding machines in which the spirals are wound onto pre-placed longitudinal bars and welded together to make a cage. In some applications there are signicant reasons to use spirals. These advantages may be sufcient to off set the increased costs. Structural advantages of spirals over tied transverse reinforcement include an approximate 20 percent increase in axial capacity, and a one quarter decrease in development length of bars enclosed with spirals. Spiral reinforcement provides enhanced connement, thus it is a preferred lateral reinforcing type for round members in seismic applications. Use straight bars wherever possible. Fabricating and placing straight bars is faster and easier than bent bars. Use ACI standard bar bend types. See standard bar shapes and bends provided in ACI 315, Details and Detailing of Concrete Reinforcement . Nonstandard bends disrupt shop routine and cost more to fabricate. Use bars in one plane. It is highly recommended that reinforcing bars be designed so their bends are located in one geometric plane. Bars with bends in two or three planes, i.e. having x, y, and z components, are difcult and expensive to fabricate. Moreover, these bars are difcult to hold to proper eld tolerances because adjustment in one direction can impact the tolerance in the remaining one or two directions.
Economical Reinforced Concrete Construction [ETN-C-1-10]
Use beams of sufcient width for ease of bar and concrete placement. Concrete cover, stirrup thickness, radius of stirrup, actual bar diameter, bar lap, number of bars, and aggregate size all contribute to the minimum beam width. Actual bar size is larger than the nominal size. For example, even though bar sizes are designated as #4, #5, #6, the actual dimensions are 9/16, 11/16, and 7/8 inches, respectively. Table 1 shows overall bar diameters for all bar sizes. When bars are spaced too close together, not only is placement of the bars difcult but placement of the concrete is hindered as well. Table 1 – Overall reinforcing bar diameter
Elevation of Concrete Wall
Figure 6 – Using stock length of bars cut and spliced in the eld
lengths to provide savings in fabrication, handling, and placing since sorting is reduced. The minor added cost associated with the extra material used due to variable length laps is more than offset with the savings due to reduced labor while cutting and sorting the reinforcing bar.
Bar Size #3 #4 #5 #6 #7 #8 #9 #10 #11 #14 #18
[#10] [#13] [#16] [#19] [#22] [#25] [#29] [#32] [#36] [#43] [#57]
Approximate Diameter Outside Deformations, in. [mm] 7/16 9/16 11/16 7/8 1 1-1/8 1-1/4 1-7/16 1-5/8 1-7/8 2-1/2
[11] [14] [18] [22] [25] [28] [32] [36] [40] [48] [63]
Use repetitive bar sizes and lengths. Standard reinforcing bar length is 60 feet. However, some fabri cators stock shorter bar lengths. As a general rule, the longest available (and possible) bar lengths should be used to reduce fabrication and placing costs. Minimize the bar sizes specied in the design. This minimizes the sizes handled in the shop and placed in the eld. Also, lap lengths can exceed the minimum length required by code so that a minimum of bar lengths can be fabricated and placed in the eld. Use stock length bars. In some situations reinforcing bars can be sheared to a xed length, while in other situations, stock length bars can be cut and spliced in the eld for trapezoidal or irregular-shaped walls and slabs. See Figure 6. Use a minimum number of different bar
Use lap splices. Where possible, bars should be lap spliced. Also, specify a consistent lap splice length for a given bar size. Lap splices should be considered a minimum length and providing a longer lap is generally not detrimental, unless congestion is an issue. Use mechanical splices. For columns with #14 and #18 bars, and for #11 and smaller bars where congestion is an issue, use mechanical splices. Use compression mechanical splices where possible. When a mechanical splice is needed and the bar is always in compression, use a compression mechanical splice. A compression mechanical splice is considered to be the fastest mechanical splice to install in the eld. Provide a 4 to 6-inch gap to place concrete when bars are closely spaced. On heavily-reinforced beams such as transfer girders, where the spacing between bars is relatively close, provide a gap. Sometimes bars may be bundled to provide this gap. Experience has shown that 4-inch slump concrete with ¾ inch aggregate will not ow easily through a 2-inch space between bars. Similarly, vibrator heads are 2 to 3 inches in width and sometimes become entangled in the reinforcement. Use intermediate single leg ties. Closed, intermediate column ties (hoops) are difcult to place and align around the interior, longitudinal column bars. Single leg ties with a 135° hook at one end, and a 90° hook at the other end are easier to place in the column interior. These ties are known as “dog” or “stick” ties, and conform to CRSI typical bar bend Type T9. Eliminating unnecessary intermediate ties also serves to simplify concrete placement in the column, as illustrated in Figure 7 (next page).
CRSI Technical Note
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• Long-term mechanical properties; • Durability in severe environments; • High early strength; • Toughness; • Volume stability. These properties are usually achieved with special admixtures, which alter the plastic properties and workability of the concrete during placement, making it less viscous (more uid) than conventional concrete. Longterm strength properties are usually unaffected. (a)
Self-consolidating concrete (SCC) is a special type o f high-performance concrete. It is dened by the National Ready-Mix Concrete Association (NRMCA) as a highly owable, non-segregating concrete that can ow into place, ll the formwork, and encapsulate the reinforcement without any mechanical consolidation. In general, SCC is concrete made with conventional concrete materials and, in most cases, with a viscosity-modifying admixture (VMA). SCC is also useful in applications where high quality surface nishes are desired without bugholes or honeycombing. Increased form pressures may be generated when SCC is used, necessitating possible changes in formwork design.
T9 tie (b)
Figure 7 – (a) An interior hoop tie is difcult to place, whereas a (b) column with intermediate T9 ties is preferred
Concrete Use low-strength concrete. 4,000 psi compressive strength concrete is usually sufcient. Exceptions are for columns in high-rises and oor systems in which there are shear capacity issues. Columns, shear walls, and joints may require higher strength concrete to enhance their axial and exural capacity. Use high-performance concrete where placement and consolidation is expected to be difcult. Highperformance concrete is dened within ACI as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. These requirements could potentially include the following enhancements: • Ease of placement and consolidation without affecting strength;
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ACI Committee Report 237 indicates that SCC provides the following features, which are equally applicable to general high-performance concrete: • It is great at replicating architectural form features; • Free fall into the formwork can be greater than the conventional limit of 5 ft.; • Less screeding operations are required to ensure at surfaces (self-leveling characteristic); • It facilitates accelerated construction, through higher rate of casting or placing and shorter construction duration; • It facilitates and expedites the lling of highly re inforced sections and complex formwork while ensuring good construction quality which may lead to increased productivity, reduces the labor requirement and cost, or both; • Improved exibility in spreading placing points during casting. This can reduce the need for frequent movement of transit trucks and the need to move the pump lines to place concrete (possible reduction in the number of pumps, pump operators, and so on). This greater exibility in scheduling construction activities and procuring the required resources results in both time and resource savings.
Economical Reinforced Concrete Construction [ETN-C-1-10]
SCC may be evaluated in the eld using a standard slump test; however, the slump cone is often inverted. Instead of measuring the distance between the top of the cone and the top of the sample, the puddle of concrete is measured in 2 orthogonal directions to determine the spread diameter. Standard slump measurements of highly owable concrete is practically irrelevant, as it has no measureable slump (>> 12 in.). Other mixture evaluation tests include the J-ring test, used to evaluate the ow and segregation characteristics of high-performance concrete as shown in Figure 8.
Use high-strength concrete in columns. Highstrength concrete can be justied in columns if the use of higher strength concrete reduces the amount of longitudinal reinforcement. Similarly, column sizes can be reduced or just use one column size on a project. Specify the same strength concrete in all columns of a story, to minimize mistakes. Specify few mix designs. On most projects only two strengths of concrete are necessary, a normal mix (4,000 to 5,000 psi) and a high-strength mix (8,000 psi or greater). Some projects may necessitate three. Limit coarse aggregate size to ¾ inch. As a matter of practice limit the coarse aggregate to ¾ inch since the minimum clear bar spacing is normally 1 inch and ACI 318 Code requires a clear distance of four-thirds the aggregate size.
Figure 8 – J-ring test apparatus for high- performance concrete (Photo courtesy of Skidmore, Owings and Merrill LLP)
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Contributors: Adapted from an article by Russell S. Fling with updates and revisions by CRSI Staff. Keywords: Concrete, Economy, Formwork, Reinforcement Reference: Concrete Reinforcing Steel Institute-CRSI [2009], “Economical Reinforced Concrete Construction,” CRSI Technical Note ETN-C-1-10 , Concrete Reinforcing Steel Institute, Schaumburg, Illinois, 7 pp. Historical: Formerly Engineering Data Report No. 30 Note: This publication is intended for the use of professionals competent to evaluate the signi cance and limitations of its contents and who will accept responsibility for the application of the material it contains. The Concrete Reinforcing Steel Institute reports the foregoing material as a matter of information and, therefore, disclaims any and all responsibility for application of the stated principles or for the accuracy of the sources other than material developed by the Institute.
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