ACI 334 3R:2005 pdf free download.Construction of Concrete Shells Using Inflated Forms.
For centuries, arched and dome-shaped structures have efficiently enclosed large clear-span volumes. The strength of compound-curved surfaces allowed early builders to construct self-supporting thin-shell buildings from a variety of materials. Due to the tremendous amount of time and effort needed to create the desired shapes. construction of these thin-shelled structures sometimes spanned several decades.
Knowledge of the design and construction of thin-shell concrete structures has greatly increased over the past tOO years. both from research and practical experience. In the past 4(11050 years. the use of inflated fbrms has allowed shells to be constructed more economically (South 1990). This new type of construction process presents new challenges and concerns. Safety measures and construction tolerances are addressed in this report for many types of systems using inflatable forms.
1.2—Scope (Fig. 1.2)
This report contains the lessons learned in the construction of thin-shell concrete dome structures using inflated forms. As this method of construction continues to gain popularity. additional research is needed to increase understanding of the behavior of this type of shell so that inflated-form structures continue to meet adequate levels of safety and serviceability. Included are construction procedures. tolerances, and design checks to ensure that the finished structure meets adequate safety and serviceability levels. This document focuses primarily on inflated form thin shells using polyurethane foam as part of the construction process. Many structures are built using fabric forms where the concrete is applied directly to the form either from the outside or the inside. These general guidelines apply to all methods.
1.3—History (Fig. 1.3)
Since the early 1940s. several methods of construction using inflatable forms have been used. These methods include
(Turner 1972). The patent was later reissued ith concrete applied to the interior of the foam (Fig. 1.5).
In 1979. David and Barry South were issued patents on a method similar to that of Turner’s (South 1979). Their method differed in that the structure was self supporting only after the shoicrete was in place (Fig. 1.6) (South 1986).
All patents for the use of inflated forms in construction of thin shells are now in the public domain with one exception:
the CrenosphcretM. the technique patented by David South for the construction of thin shell domes of diameters larger than 3(X) ft (91 m) using a cable net restraint system and ribs.
When the concrete is placed on the outside of the form, the cables will be buried in the concrete and function as reinforcement. When the concrete is placed on the inside of the form, the cables are removed once the structure is solid.
Bridges and arch buildings have been built using inflated forms where inflation forces are restrained by seel hoops placed on the exterior of the inflated form. Some very large dome-type structures have used steel tie-down systems to allow higher inflation pressures.
1.4—Methods (Fig. 1.7)
Inflated-form, thin-wall shotcrete construction has become one of the most common and widely used methods in the construction of domes. The Monolithic Dome Institute estimates over 2(XX) thin shells have been built over the last 30 years using the fabric form method, whereas those built with conventional forming methods are few in number.
Until recently, only a few contractors have possessed the skills and the equipment necessary to undertake this type of construction. As architects and engineers are becoming aware of the advantages of this inflated form method and its use increases, industry design and construction standards are needed.
Shotcrete can be placed on the inflated form from either the outside or inside. Some systems use higher air pressure and the inflated fabric form to support all the loads, whereas others support some construction loads with a reinforcement layer and initial layers of shoicrete.
Although each method has unique construction challenges, they all have many similar characteristics. This report does not distinguish between the different methods or make judgments as to the validity of each. It discusses the construction factors that are common to all of the inflated form methods:
• Inflated form mwzufacturing—shape, size, fabric, and fabrication;
inflator—the fan or blower assembly.
manometer—the pressure gauge for measuring the air pressure within the inflated form.
preliminary reinforcement mat (prLmat)—a grid of No. 3 or 4 (No. lOor 13) bars at approximately 2 ft (0.6 mm) on center, which gives the dome additional stiffness and strength before the first layer of structural reinforcement is placed.
rebound—aggregate and cement paste that ricochets off the surface during the application of shoicrete because of collision with the hard surface. reinforcement, or other aggregate particles.
shear key—a longitudinal notch in the footing that acts as a mechanical shear connector between the dome shell and the footing.
shotcrele for construction of thin shells using inflated forms)—generally a mixture of cement, sand, pea gravel with a maximum aggregate size of 3/8 in. (10 mm), and water projected at high velocity onto a surface. See ACt 506R for more information on shotcreie.
1 .6—Preconstruction
All-weather road access to the site should be provided for the constructor’s personnel and vehicles during construction.
The contract documents should provide the general layout of the dome, including a center point and orientation for doorways. The preconsiruction and construction testing procedures should be agreed upon between the owner and the constructor. (The owner usually provides for all testing either in-house or by use of a testing agency.)
1.7—Work schedule
Most of the work done inside and outside the dome is from baskets. Because only a few people can work out of any single basket, production can be increased by working longer hours or. on larger structures, using more baskets. When spraying foam or shoicrete. schedules are greatly influenced by weather conditions or how much work that can be done at once, so flexibility is important in creating the work schedule. The constructor may work one, two, or three shifts, arranging their work to best fit the project requirements. Job site cooperalion is important to assure a quality. safe, and productive project. as well as to minimize the risk associated with this method of construction (for example. relying on fans to hold up the dome).
CHAPTER 2—FOUNDATIONS
2.1 —General
The dome foundation usually consists of a reinforced concrete ring-beam footing. circular in plan. rectangular in section, and designed for anticipated loadings and soil bearing conditions. The footing usually acts as a tension ring to resist vertical and internal loads. Design considerations include the size of the dome, the occupancy. local building codes, relevant national standards, and soil report (Fig. 2.1) (Billington 1982).
The footing ring beam provides the foundation for the finished structure, anchorage points for the inflated form (Fig. 2.2), the weight to resist the upward pressure of the
a grid over the entire surface of the shell spaced approximately 10 ft (3 m) horizontally and 6 ft (2 rn) vertically.
When the structural reinforcement is placed. the nozzle operators should concentrate on properly embedding and covering the reinforcing bar. Next, (hey should focus on adding sufficient thickness to cover the depth gauges. If smoothness is a project requirement, then the last layers of shotcrete should be sprayed with finer sand mixtures and no coarse aggregate.
Judging the depth of the shotcrete being applied is sometimes difficult. The nozzle operator should check the depth of spray during production. The constructor should verify that a uniform spraying pattern is being followed. Lighting should he adequate to allow the nozzle operator to see the work clearly.
4.14—Discharge time
Normally, the nozzle operator sprays shotcrete that has a slump of 4 (08 in. (100 to 200 mm). Because it usually takes 25 to 45 minutes to unload a truck. the concrete gets stiffer as time goes on. By using high-range water-reducing admix- Lures and retarding admixtures. the time available to discharge a load can he greatly extended. Water can also be added to maintain the same slump as long as the specified strength is maintained and the maximum specified water- cement ratio is not exceeded.
The quality control technician taking samples should be certified as a Concrete Field Technician or Concrete Inspector and have a working knowledge of the relationship between the strength of the shotcrete and the time sitting in the truck.
4.1 5—Joints
Unfinished work should not be allowed to stand for more than 30 minutes. unless edges slope to a thin edge. For structural elements that will be under compression and for construction joints shown on the approved construction documents, square joints arc recommended. Before placing additional material next to previously applied work, sloping and square edges should be clean and damp.
4.16—Multi-pass technique
Close review of existing insulated thin shells constructed by the interior shotcrete method shows that the shells are not stratified but monolithic, and contain almost no cold joints. Thin shells with interior shotcrete application cannot be constructed by applying all of the shotcrete in one layer because the level of air pressure that would be required to hold the full-depth weight of the wet shotcrcte far exceeds how much air pressure the inflated fabric form can hold. Research (Bingham 1997) and experience show that the layering of shotcrete does not cause cold joints in insulated thin shells with interior shoicrete construction.
4.17—Curing
During the curing period. shotcrete should be maintained above 40 °F(4 °C)and in a moist condition. Shotcrete should be kept continuously moist or should be sealed with an approved curing compound as discussed in ACI 506R.
However, most dome construction using the internal method of spraying shotcrete inside an inflated form retains a humid environment and does not require additional moisture for curing. Curing should continue for 7 days after shotcrete is applied, or until the specified compressive strength is obtained. Where concrete is applied to the outside of an inflated foam, proper curing methods must be maintained. These methods may include soaking the concrete with water or applying curing compound. If curing compound is used. it should have the design professional’s explicit agreement.
4.l8—Shotcrete placement tolerance
Shotcrete cannot be placed perfectly. A certain amount of voids or shadowing around the reinforcing bar is always present. Small voids should not be considered a defect. The definition of a small void depends on the thickness of the wall and size of the reinforcing bar, A 0.5 x 0.5 x 0.5 in, (13 x 13 x 13 mm) void in a 2 in. (50mm) wall should be considered as small. As the wall becomes thicker, slightly larger voids should be tolerated. Shadowing behind a reinforcing bar should not be considered excessive if the shotcrete embeds at least 80% of the surface of the bar. Continuous voids indicate poor shoicrete practice and should not be accepted.
Because of the dome shape. the shotcrete shell is largely in compression, hut the compressive forces are small. Usually. the average compressive stress in a shoicrete dome is less than 5(X) psi (3.4 MPa). When shotcrete is placed in tension areas of the dome, the proper embedment of bar splices is the most important consideration. ACI 318. Chapter 19. requires that the length of the overlap of a reinforcing bar at the splice be 1.2 times greater than the lap length in conventional concrete but not less that 18 in. (460 mm). This is because. in a thin shell structure, the bar can he close to a surface. Also, in a shell placed by the shotcrete method, it allows for a degree of shadowing or voids at a splice. Even where the reinforcing bar is properly embedded and is at the specified thickness, the pattern of the reinforcing bar grid can be visible on the inside surface, sometimes called “reinforcing bar ghost lines.”
When a qualified nozzle operator places shotcrete, there should be few defects. If defects are suspected, random cores can be taken. If more than 15% of the cores taken have excessive defects, then additional cores should be taken to decide an actual amount of defective work in the dome.
To ensure that the finished dome will behave as anticipated, the shoicrete thickness should not be less than the specified thickness by more than 10%. nor exceed the specified thickness by more than 25%, so as not to increase the dead load at any specific location.
4.1 9—Shotcrete damage
When poor workmanship is discovered or placement tolerances are exceeded, the designer should be informed. Shotcrete that exhibits sags, sloughs, segregation, honeycombing. sand pockets. or other obvious defects should be removed and replaced while still plastic. Hardened shotcrete defects should be reviewed by the designer to determine whether the structural integrity of the dome is in question. The designer should present the findings to the contractor. The constructor and designer should agree on the method and extent of the repairs.
4.20—Completion
After the final shotcrete is applied, the air pressure should remain constant until the designer determines that the structure is self supporting. In domes with thicker wall sections, the intlators can be turned off after the designer determines enough shotcrete has been placed and sufficient strength gained for the dome to be self supporting, and able to take the load of successive layers of shoicrete. This is generally atier the first mat of structural reinforcement is placed and fully embedded with shotcrete.
When the blowers are turned off and the shotcrete operation is completed. finish work, such as plumbing. painting. and interior framing, can begin.
CHAPTER 5—REFERENCES
5.1—Referenced standards and reports
The standards and reports listed below were the latest editions at the time this document was prepared. Because these documents are revised frequently, the reader is advised to contact the proper sponsoring group if it is desired to refer to the latest version.
American ‘oncre1e !ns(ifufr
21 4R Evaluation of Strength Test Results of Concrete
301 Specifications for Structural Concrete
304R Guide for Measuring, Mixing, Transporting. and Placing Concrete
309R Guide for Consolidation of Concrete
318 Building Code Requirements for Structural Concrete and Commentary
506R Guide to Shoicrete
506.2 Specification for Shotcrete
C 660 Guide to Certification of Shoicrete Nozzlemen
ASTM biternaiwnal
C 94 Standard Specification for Ready-Mixed Concrete
D 751 Standard Test Methods for Coated Fabrics
The preceding publications may be obtained from the following organizations:
American Concrete Institute
P.O. Box 9094
Farmington Hills. Ml 48333-9094
ASTM International
100 Barr Harbor Dr.
West Conshohocken, PA 19428
5.2—Cited references
Billinglon. D. P., 1982, Thin Shell concrete Structures. McGraw-Hill Book Co., Inc.. 373 pp.
Bingharn. J. L.. 1997, “Bond Strength Between Layers of Shoicrete,” MS thesis. Brigham Young University, Provo. Utah, 33 pp.
Bini, D., 1986. “Thin Shell Concrete Domes,” (‘osicrete Inwrnaiional, V. 8. No. I, Jan.. pp. 49-53.
Boyt. J., 1986. “Up. Up and Away.” concrete International, V. 8, No. 1, Jan., pp. 37-40.
Jacobs, S. E.. 1996, “Large Diameter Low Profile Air Forms Using Cable Net Support Systems For Concrete Domes.” MS thesis. Brigham Young University. Provo. Utah, 117 pp.
Neff, W., 1942, Un lied State.c Patent No. 2,270,229. Peterson, G. P.. 1998, “Pull-Out Testing of Cast in Place Epoxy Grouted Reinforcement Sleeves.” MS project. Brigham Young University, Provo. Utah, 20 pp.
South, D. B., 1979. “Building Structure and Method of Making Same.” United States Patent No. 4.155,967 5.
South. D. B.. 1986, “The Past Leads to the Present.” C’oncrete Internaiwnal, V. 8, No. 1 Jan., pp. 54-47.
South, D. B., 1990. “Economics and the Thin Shell Dome.” concrete International, V. 12. No. 8, Aug., pp. 18-20.
South. J. P.. 1996. “Preliminary Analysis and 1)esign of Large Span Air Formed Concrete Domes.” MS thesis. Brigham Young Universäy, Provo. Utah, 118 pp.
Turner, L. S., 1972, “Method of Molding a Building.” United States Patent No. 3,277,219.11.
Wilson. A.. 1986. “Controlling Construction Mishaps.” concrete International. V. 8. No. 1. Jan., pp. 33-36.
Wilson, A., 1990. “Very Large Air-Formed Concrete Shells,” concrete International, V. 12, No. 8, Aug., pp. 2 1-23.