Guide to Formwork for Concrete Chapter 4

ACI STANDARD • • • • CHAPTER 4—MATERIALS 4.1—General The selection of materials suitable for formwork should be based on the price, safety during construction, and the quality required in the finished product. Approval of formwork materials by the engineer/architect, if required by the contract documents, should be based on how the quality of materials affects the quality of finished work. Where the concrete surface appearance is critical, the engineer/architect should give special notice and make provision for preconstruction mockups. Refer to Chapter 5 for architectural concrete provisions. 4.2—Properties of materials 4.2.1 General—Formwork for Concrete1.3 describes the formwork materials commonly used in the United States and provides extensive related data for form design. Useful specification and design information is also available from manufacturers and suppliers. Table 4.1 indicates specific sources of design and specification data for formwork materials. This tabulated information should not be interpreted to exclude the use of any other materials that can meet quality and safety requirements established for the finished work. 4.2.2 Sheathing—Sheathing is the supporting layer of formwork closest to the concrete. It can be in direct contact with the concrete or separated from it by a form liner. Sheathing consists of wood, plywood, metal, or other materials capable of transferring the load of the concrete to supporting members, such as joists or studs. Liners are made of wood, plastic, metal, cloth, or other materials selected to alter or enhance the surface of the finished concrete. In selecting and using sheathing and lining materials, important considerations are: • Strength; • Stiffness; • Release; • Reuse and cost per use; • Surface characteristics imparted to the concrete, such as wood grain transfer, decorative patterns, gloss, or paintability; • Absorptiveness or ability to drain excess water from the concrete surface; • Resistance to mechanical damage, such as from vibrators and abrasion from slipforming; • Workability for cutting, drilling, and attaching fasteners; • Adaptability to weather and extreme field conditions, temperature, and moisture; and • Weight and ease of handling. 4.2.3 Structural supports—Structural support systems carry the dead and live loads that have been transferred through the sheathing. Important considerations are: • Strength; • Stiffness; • Dimensional accuracy and stability; Workability for cutting, drilling, and attaching fasteners; Weight; Cost and durability; and Flexibility to accommodate varied contours and shapes. 4.3—Accessories 4.3.1 Form ties—A form tie is a tensile unit used to hold concrete forms against the active pressure of freshly placed plastic concrete. In general, it consists of an inside tensile member and an external holding device, both made to specifications of various manufacturers. These manufacturers also publish recommended working loads on the ties for use in form design. There are two basic types of tie rods: the onepiece prefabricated rod or band type and the threaded internal disconnecting type. Their suggested working loads range from 1000 to more than 50,000 lb (4.4 to more than 220 kN). 4.3.2 Form anchors—Form anchors are devices used to secure formwork to previously placed concrete of adequate strength. The devices normally are embedded in the concrete during placement. The actual load-carrying capacity of the anchors depends on their shape and material, the strength and type of concrete in which they are embedded, the area of contact between concrete and anchor, and the depth of embedment and location in the member. Manufacturers publish design data and test information to assist in the selection of proper form anchor devices. 4.3.3 Form hangers—Form hangers are devices used to suspend formwork loads from structural steel, precast concrete, or other members. 4.3.4 Side form spacers—A side form spacer is a device that maintains the desired distance between a vertical form and reinforcing bars. Both factory-made and job-site fabricated devices have been successfully used. Advantages and disadvantages of the several types are explained in References 1.3, 4.1, and 4.2. 4.3.5—Recommendations 4.3.5.1—The recommended factors of safety for ties, anchors, and hangers are given in Section 2.4. 4.3.5.2—The rod- or band-type form tie, with a supplemental provision for spreading the forms and a holding device engaging the exterior of the form, is the common type used for light construction. The threaded internal disconnecting type of tie (also called through tie) is more often used for formwork on heavy construction, such as heavy foundations, bridges, power houses, locks, dams, and architectural concrete. Removable portions of all ties should be of a type that can be readily removed without damage to the concrete and that leaves the smallest practicable holes to be filled. Removable portions of the tie should be removed unless the contract documents permit their remaining in place. A minimum specification for form ties should require that the bearing area of external holding devices be adequate to prevent excessive bearing stress in form lumber. 4.3.5.3—Form hangers should support the dead load of forms, weight of concrete, and construction and impact loads. Form hangers should be symmetrically arranged on the supporting member and loaded, through proper sequencing of GUIDE TO FORMWORK FOR CONCRETE 347-17 Table 4.1—Form materials with data sources for design and specification Materials Principal uses Data sources “American Softwood Lumber Standard,” PS 20-94 Wood Handbook, Reference 4.3 Sawn lumber Form framing, sheathing, and shoring Manual for Wood Frame Construction, Reference 4.4 National Design Specification for Wood Construction, ANSI/AF&PA NDS-1997, Reference 4.7 Timber Construction Manual, Reference 4.6 Structural Design in Wood, Reference 4.5 Engineered Wood Products, Reference 4.21 Engineered wood* Form framing and shoring “Code for Engineering Design in Wood,” (Canada) CAN3-086 “Engineering Design in Wood (Limit States Design),” CAN/CSA-096.1-94 “Construction and Industrial Plywood,” PSI-95 Plywood Form sheathing and panels Panel framing and bracing Heavy forms and falsework APA Plywood Design Specification, Reference 4.8 APA Concrete Forming, Reference 4.20 Specification for Structural Steel Buildings—Allowable Stress Design and Plastic Design, Reference 4.9 Specification for Design of Cold Formed Steel Structural Members, Reference 4.10 Forms for One-Way Joist Construction, ANSI A48.1 Column and joist forms Steel Recommended Industry Practice for Concrete Joist Construction, part of Reference 4.1 Stay-in-place deck forms ASTM A 446 (galvanized steel) Shoring Recommended Safety Requirements for Shoring Concrete Formwork, Reference 4.19 Steel joists used as horizontal shoring Expanded metal bulkheads, single-sided forms Aluminum† Forms for Two-Way Concrete Joist Construction, ANSI A48.2 Form panels and form framing members Horizontal and vertical shoring and bracing Recommended Horizontal Shoring Beam Erection Procedure, Reference 4.18 Standard Specification and Load Tables for Open Web Steel Joists, Reference 4.17 Expand Your Forming Options, Reference 4.16 Aluminum Construction Manual, Reference 4.11 Mat Formed Wood Particle Board, ANSI A208.1 Reconstituted wood panel products‡ Form liners and sheathing Hardboard Concrete Form Liners, LLB-810a Performance Standard for Wood-Based Structural Use Panels, PS2-92 Insulation materials Wood fiber or glass fiber Stay-in-place form liners or sheathing Other commercial products Cold-weather protection for fresh concrete Fiber or laminated paper pressed tubes or forms ASTM C 532 (insulating form board) Column and beam forms Void forms for slabs, beams, girders and precast piles Internal and under-slab void forms Corrugated cardboard Void forms in beams and girders (normally used with internal “egg-crate” stiffeners) A Study of Cardboard Voids for Prestressed Concrete Box Slabs, Reference 4.12 Note: Manufacturers’ recommendations, when supported by test data and field experience, are a primary source for many form materials. In addition, the handbooks, standards, specifications, and other data sources cited herein are listed in more detail in Formwork for Concrete and in the references for Chapter 4 and Chapter 8 of this document. Be sure to check cautionary footnotes for engineered wood, aluminum, and panel products made of reconstituted wood. *Structural composite lumber products are proprietary and unique to a particular manufacturer. They cannot be interchanged because industry-wide common grades have not been established to serve as a basis for equivalence. †Should be readily weldable and protected against galvanic action at the point of contact with steel. If used as a facing material in contact with fresh concrete, should be nonreactive to concrete or concrete containing calcium chloride. ‡Check surface reaction with wet concrete. 347-18 ACI STANDARD Table 4.1 (cont.)—Form materials with data sources for design and specification Materials Concrete Principal uses Data sources Stay-in-place forms Building Code Requirements for Structural Concrete and Commentary, ACI 318 Molds for precast units Precast Concrete Units Used as Form for Cast-in-Place Concrete, ACI 347.1R Ready-made column forms Glass fiber-reinforced plastic Domes and pans for concrete joist construction Custom-made forms for special architectural effects Using Glass-Fiber Reinforced Forms, Reference 4.13 Nonmetallic Form Ties, Reference 4.14 Form ties Cellular plastics Other plastics, including ABS, polypropylene, polyethylene, polyvinyl chloride, polyurethane Rubber and rubberized or architectural fabrics Form lining and insulation Cellular Plastics in Construction, Reference 4.15 Stay-in-place wall forms Insulating Concrete Forms Association Form liners, both rigid and flexible, for decorative concrete Plastic Form Liners, Reference 4.22 Chamfer and rustication formers Form lining and void forms Inflatable forms for dome and culvert construction Form ties, anchors, and hangers Hold formwork secure against loads and pressures from concrete and construction activities Side form spacers Maintain correct distance between reinforcement and form to provide specified concrete cover for steel Plaster Waste molds for architectural concrete Release agents and protective form coatings Help preserve form facing and facilitate release Monolithic Dome Institute Safety factors recommended in Section 2.4 Also refer to Reference 4.14 Side Form Spacers, Reference 4.2 Choosing and Using a Form Release Agent, Reference 4.23 Note: Manufacturers’ recommendations, when supported by test data and field experience, are a primary source for many form materials. In addition, the handbooks, standards, specifications, and other data sources cited herein are listed in more detail in Formwork for Concrete and in the references for Chapter 4 and Chapter 8 of this document. Be sure to check cautionary footnotes for engineered wood, aluminum, and panel products made of reconstituted wood. the concrete placement, to minimize twisting or rotation of the hanger or supporting members. Form hangers should closely fit the flange or bearing surface of the supporting member so that applied loads are transmitted properly. 4.3.5.4—Where the concrete surface is exposed and appearance is important, the proper type of form tie or hanger will not leave exposed metal at the surface. Otherwise, noncorrosive materials should be used when tie holes are left unpatched, exposing the tie to the elements. 4.4—Form coatings and release agents 4.4.1 Coatings—Form coatings or sealers are usually applied in liquid form to contact surfaces either during manufacture or in the field to serve one or more of the following purposes: • Alter the texture of the contact surface; • Improve the durability of the contact surface; • Facilitate release from concrete during stripping; or • Seal the contact surface from intrusion of moisture. 4.4.2 Release agents—Form release agents are applied to the form contact surfaces to prevent bond and thus facilitate stripping. They can be applied permanently to form materials during manufacture or applied to the form before each use. When applying in the field, be careful to avoid coating adjacent construction joint surfaces or reinforcing steel. 4.4.3 Manufacturers’ recommendations—Manufacturers’ recommendations should be followed in the use of coatings, sealers, and release agents, but independent investigation of their performance is recommended before use. When concrete surface color is critical, effects of the coating, sealing, and release agents should be evaluated. Where surface treatments such as paint, tile adhesive, sealers, or other coatings are to be applied to formed concrete surfaces, be sure that adhesion of such surface treatments will not be impaired or prevented by use of the coating, sealers, or release agent. Also, consider bonding requirements of subsequent concrete placements.