ACI-334.3R-2005.pdf

ACI 334.3R-05 became effective September 16, 2005. Copyright © 2005, American Concrete Institute. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom. Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer. 334.3R-1 Construction of Concrete Shells Using Inflated Forms Reported by Joint ACI-ASCE Committee 334 ACI 334.3R-05 This report provides information on the construction of structural concrete shells using an inflated form. Major facets of the construction process are covered, including foundations, inflation, monitoring, and backup systems. Other aspects, such as the geometric variations of inflated forms, thickness of polyurethane foam, and mixture proportions for shotcrete, are also considered. Keywords: dome; fabric; inflation; polyurethane foam; reinforcement; shotcrete; thin shell. CONTENTS Chapter 1General, p. 334.3R-2 1.1Introduction 1.2Scope 1.3History 1.4Methods 1.5Definitions 1.6Preconstruction 1.7Work schedule Chapter 2Foundations, p. 334.3R-4 2.1General 2.2Concrete 2.3Soil conditions 2.4Reinforcement material 2.5Placement of reinforcement 2.6Placement of anchors 2.7Concrete placement 2.8Foundation dowels 2.9Uplift prevention Chapter 3Inflated forms, p. 334.3R-6 3.1General 3.2Inflated form material and manufacturing 3.3Field layout 3.4Form protection 3.5Initial stretching 3.6Inflation 3.7Construction tolerances 3.8Air pressure maintenance 3.9Collapse prevention 3.10Miscellaneous connections 3.11Fabric form repair 3.12Polyurethane foam (when used) 3.13Preparation 3.14Foam application 3.15Construction hazards Chapter 4Shotcrete dome, p. 334.3R-9 4.1General 4.2Reinforcement material and size 4.3Clear spacing between bars 4.4Splices John F. AbelFrederick L. CrandallLuis F. MeyerWilliam C. Schnobrich David P. BillingtonPhillip L. GouldJohn C. MillerBarry South Arthur J. Boyt, Jr.Takashi HaraThomas E. Nelson, Jr.Jason P. South James L. ByrneMichael D. HunterJohn K. ParsonsArnold Wilson John V. ChristiansenMark A. KetchumDale A. PearceyL. Brent Wright Matthew S. ChurchSamaan G. LadkanyRyan E. Poole David B. South Chair 334.3R-2ACI COMMITTEE REPORT 4.5Cover 4.6Preliminary reinforcement mat (premat) 4.7Shell reinforcement 4.8Preconstruction shotcrete tests 4.9Shotcrete compression tests 4.10Proportions and materials 4.11Field practice 4.12Nozzle operator qualifications 4.13Shotcrete operation 4.14Discharge time 4.15Joints 4.16Multi-pass technique 4.17Curing 4.18Shotcrete placement tolerance 4.19Shotcrete damage 4.20Completion Chapter 5References, p. 334.3R-13 5.1Referenced standards and reports 5.2Cited references CHAPTER 1GENERAL 1.1Introduction (Fig. 1.1) 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 100 years, both from research and practical experience. In the past 40 to 50 years, the use of inflated forms 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. 1.2Scope (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. 1.3History (Fig. 1.3) Since the early 1940s, several methods of construction using inflatable forms have been used. These methods include shotcrete applied to the form exterior, and foam and shot- crete applied to the form interior. In 1942, Wallace Neff received a patent on a system where the form was inflated to the shape of the structure, and then the reinforcing bar and shotcrete were placed on the exterior of the form (Neff 1942). Dante Bini later developed and received a patent on a system where the reinforcement and concrete were placed on the exterior of the form before it was inflated. It was then raised by air pressure to form the dome (Fig. 1.4) (Bini 1986). In 1972, Lloyd Turner received a patent on a process in which the inflated form was sprayed with foam on the inside to a desired thickness creating a self-supporting foam dome Fig. 1.1Faith Chapel Christian Center, Birmingham, Ala.: 280 ft (85.35 m) diameter and 72 ft (22 m) tall that includes a 3200-seat sanctuary, classrooms, and an administration building. Fig. 1.2Price City Works Complex, Price, Utah. Four domes: 130 x 43 ft (40 x 13.1 m) fire station; 130 x 43 ft (40 x 13.1 m) storage facility; 130 x 43 ft (40 x 13.1 m) mainte- nance shop; and 90 x 40 ft (27 x 12.2 m) office and adminis- tration building. Fig. 1.3U.S. Borax and Chemical Co., Boron, Calif.: two 20,000 ton (18,000 tonne) borax storage domes, 150 x 79 ft (45.7 x 24.1 m). CONSTRUCTION OF CONCRETE SHELLS USING INFLATED FORMS334.3R-3 (Turner 1972). The patent was later reissued with 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 Turners (South 1979). Their method differed in that the structure was self supporting only after the shotcrete 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 Crenosphere, the technique patented by David South for the construction of thin shell domes of diameters larger than 300 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 rein- forcement. 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 steel 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. 1.4Methods (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 2000 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 construc- tion. 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 shotcrete. Although each method has unique construction chal- lenges, 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 manufacturingshape, size, fabric, and fabrication; Foundation detailsanchor system, uplift prevention, layout, and form tension; Air pressurebackup system, monitoring, and collapse prevention; and Applied loadslive loads and dead loads. 1.5Definitions basketthe personnel aerial lift platform that raises workers to work on the dome. dead loadsthe fixed weight of a structure plus any fixed loads such as attached equipment, bridges, supports, head houses, platforms, catwalks, ceilings, and conveyors resting or hanging from the structures. embedsanchor bolts, inserts, pipe sleeves, pipes, conduits, reinforcement, wiring, flashing, instruments, and other devices encased in the concrete. Fig. 1.4Construction of Bini shell.Fig. 1.5Construction of Turner shell. Fig. 1.6Construction of South shell. Fig. 1.7“Eye of the Storm,” Sullivans Island, S.C.: prolate ellipse residence80 ft (24.4 m) long, 57 ft (17.4 m) wide, and 34 ft (10.4 m) tall. 334.3R-4ACI COMMITTEE REPORT inflatorthe fan or blower assembly. manometerthe pressure gauge for measuring the air pressure within the inflated form. preliminary reinforcement mat (premat)a grid of No. 3 or 4 (No. 10 or 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. reboundaggregate and cement paste that ricochets off the surface during the application of shotcrete because of collision with the hard surface, reinforcement, or other aggregate particles. shear keya longitudinal notch in the footing that acts as a mechanical shear connector between the dome shell and the footing. shotcrete (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 ACI 506R for more information on shotcrete. 1.6Preconstruction All-weather road access to the site should be provided for the constructors 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 preconstruction 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.) 1.7Work 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 shotcrete, 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 require- ments. Job site cooperation 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 2FOUNDATIONS 2.1General 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 inflated form, and the air seal to prevent the pressurized air from escaping. The footing ring can also be used as a tension ring to resist the horizontal thrust of internal loading. 2.2Concrete Certification that the concrete meets ASTM C 94 should accompany each concrete delivery. Concrete properties and handling should conform to ACI 301 and the following: Minimum 28-day compressive strength of 3000 psi (210 MPa); Maximum coarse aggregate size of 1 in. (25 mm); Air entrainment of 6.5 ± 1.5% (these are higher levels than in ACI 214R); No added calcium chloride; Water-cement ratio of 0.55 or less; and Slump of 2 in. (50 mm) minimum to 8 in. (200 mm) maximum at the point of discharge. If placed on aggressive soils, greater strength or chemical resistance can be achieved by adjusting the mixture propor- tions. For instance, the use of sulfate-resistant cement may be required. 2.3Soil conditions When soil conditions allow, excavating a trench to the required dimensions and placing the concrete and reinforcing bars in the trench is acceptable. If trenches are not practical, then wood or metal forms can be used. The top of the footing Fig. 2.1Dome edge constructions (Billington 1982). Fig. 2.2A typical anchoring system. CONSTRUCTION OF CONCRETE SHELLS USING INFLATED FORMS334.3R-5 should be formed and finished to final grade and geometry for proper inflated form attachment. Because the dome is light and the shell is monolithic, it is generally tolerant of differential settlement. Spread footings are normally used, but if the soils do not have adequate bearing capacity, pilings can be used to support the dome. The ring beam and the dome must always be integral. If piles or columns are used, the ring beam can be