Guide to Formwork for Concrete Chapter 7

347-26 ACI STANDARD CHAPTER 7—SPECIAL METHODS OF CONSTRUCTION 7.1—Recommendations The applicable provisions of Chapters 2, 3, and 4 also apply to the work covered in this chapter. 7.2—Preplaced-aggregate concrete 7.2.1 Discussion—Preplaced-aggregate concrete is made by injecting (intruding) mortar into the voids of a preplaced mass of clean, graded aggregate. For normal construction, the preplaced aggregates are vibrated thoroughly into forms and around reinforcing and then wetted and kept wet until the injection of mortar into the voids is completed. In underwater construction, the mortar displaces the water and fills the voids. In both types of construction, this process can create concrete with a high content of coarse aggregate. The injected mortar contains water, fine sand, portland cement, pozzolan, and a chemical admixture designed to increase the penetration and pumpability of the mortar. The structural coarse aggregate is similar to coarse aggregate for conventional concrete. It is well washed and graded from 1/2 in. (13 mm) to the largest size practicable. After compaction in the forms, it usually has a void content ranging from 35 to 45%. Refer to ACI 304.1R. 7.2.2 Design considerations—Due to the method of placement, the lateral pressures on formwork are considerably different from those developed for conventional concrete as given in Section 2.2.2. The formwork engineer/contractor should be alerted to the unique problems created by preplaced-aggregate, by mass placings where heat of hydration and drying shrinkage are critical, and by differential pressures in the form structure when mortar injection varies greatly from one form face to another. For additional information, refer to ACI 359, ACI 207.1R, and ACI SP-34. Because of the pressure created during aggregate packing and mortar pumping, forms that mortar is injected through should be anchored and braced far more securely than for ordinary concrete. Particular attention should be paid to uplift pressures created in battered forms. Provision should be made to prohibit even the slightest uplift of the form. Injection pipes spaced 5 to 6 ft (1.5 to 1.8 m) apart, penetrating the face of the form, require that the form be checked for structural integrity as well as a means of plugging or shutting off the openings when the injection pipes are removed. Some of these problems are reduced where mortar can be injected vertically in open top forms. Forms, ties, and bracing should be designed for the sum of: a) The lateral pressure of the coarse aggregate as determined from the equivalent fluid lateral pressure of the dry aggregate using the Rankine or Coulomb theories for granular materials; or a reliable bin action theory (refer to theories and references presented in ACI 313 and ACI 313R); and b) The lateral pressure of the injected mortar; as an equivalent fluid the mortar normally weighs 130 lb/ft3 (21 kN/m3), but can weigh as much as 200 lb/ft3 (32 kN/m3) for highdensity mortars. The time required for the initial set of the fluidized mortar (from 1 to 2 h) and the rate of rise should be ascertained. The maximum height of fluid to be assumed in determining the lateral pressure of the mortar is the product of the rate of rise (ft/h) and the time of initial set in hours. The lateral pressure for the design of formwork at any point is the sum of the pressures determined from Steps (a) and (b) for the given height. 7.2.3 Construction—In addition to the provisions of Chapter 3, the forms should be mortar-tight and effectively vented because preplaced-aggregate concrete entails forcing mortar into the voids around the coarse aggregate. Where increased lateral pressures are expected, the workmanship and details of formwork should be of better quality than formwork for conventional concrete. 7.2.4 Materials for formwork—For unexposed surfaces, mortar-tight forms of steel or plywood are acceptable. Absorptive form linings are not recommended because they permit the coarse aggregate to indent the lining and form an irregular surface. Form linings, such as hardboard on common sheathing, are not successful because they do not transmit the external form vibration normally used for ensuring a void-free finished surface. Where external vibration is used, added strength is needed in the form. 7.3—Slipforms 7.3.1 Discussion—Refer to ACI 313 for silo construction. Slipforming is a quasicontinuous forming process in which a special form assembly slips or moves in the appropriate direction leaving the formed concrete in place. The process is, in some ways, similar to an extrusion process. Plastic concrete is placed in the forms, and the forms can be thought of as moving dies to shape the concrete. The rate of movement of the forms is regulated so the forms leave the concrete only after it is stiff enough to retain its shape while supporting its own weight and the lateral forces caused by wind and equipment. Formwork of this type can be used for vertical structures, such as silos, storage bins, building cores, bearing wall buildings, piers, chimneys, shaft linings, communication and observation towers, nuclear shield walls, and similar structures. Horizontal slipforming lends itself to concrete structures, such as tunnel linings, water conduits, drainage channels, precast elements, canal linings, highway median barriers, pavements, curbs, shoulder barriers, and retaining walls. Vertical slipforms, concreted while rising, are usually moved in small increments by jacks that propel themselves on smooth steel rods or tubing embedded in or attached to the hardened concrete. Horizontal slipforms generally move on a rail system, tractor treads, wheels, and other similar means resting on a shaped berm. Working and storage decks and finisher’s scaffolding are attached to and carried by the moving formwork. The vertical or horizontal movement of forms can be a continuous process or a planned sequence of finite placements. Slipforms used on structures such as tunnels and shafts should comply with the applicable provisions of Section 6.6. Slipforms used on mass concrete structures, such as dams, should comply with the applicable provisions of Section 6.5. GUIDE TO FORMWORK FOR CONCRETE 7.3.2—Vertical slipforms 7.3.2.1—A vertical slipform system has five main components: sheathing, wales, yokes, jacks and jackrods, and working or storage decks and scaffolding. The sheathing or vertical forms can be wood staves, plywood, metal, glass fiber-reinforced plastic, wood, or a combination of these materials. The function of the sheathing is to contain and shape the concrete. Wales have three main functions: • Support and hold the sheathing in place; • Transmit the lifting force from the yokes to the sheathing and to the other elements of the form; and • Provide support for various platforms and scaffolding. Yokes support the wales at regular intervals with their legs, transmit the lifting forces from the jacks to the wales, and resist the lateral force of plastic concrete within the form. The jacks, installed on the yoke’s beams, climb up the jackrods and provide the force needed to raise the entire slipform system. Various platforms, decks, and scaffolding complete the slipform system. They provide space for storing concrete, reinforcing steel, and embedments, and serve as a working area for placing and finishing. 7.3.2.2 Design and construction considerations—Slipforms should be designed by engineers familiar with slipform construction. Construction of the slipform and slipping should be carried out under the immediate supervision of a person experienced in slipform work. Drawings should be prepared by a slipform engineer employed by the contractor. The drawings must show the jack layout, formwork, working decks, and scaffolds. A developed elevation of the structure should be prepared, showing the location of all openings and embedments. The slipform engineer should be experienced in the use of the exact brand of equipment to be used by the contractor because there is significant variation in equipment between manufacturers. 7.3.2.3 Vertical loads—In addition to dead loads, live loads assumed for the design of decks should not be less than the following: Sheathing and joists.................................. 75 lb/ft2 (3.6 kPa) or concentrated buggy wheel loads, whichever is greater Beams, trusses, and wales ........................ 50 lb/ft2 (2.4 kPa) Light-duty finishers’ scaffolding .............. 25 lb/ft 2 (1.2 kPa) 7.3.2.4 Lateral pressure of concrete—The lateral pressure of fresh concrete to be used in designing forms, bracing, and wales can be calculated as follows. Inch-pound version: 6000R p = c1 + ---------------T where c1 = 100; p = lateral pressure, lb/ft2; R = rate of concrete placement, ft/h; and T = temperature of concrete in the forms, °F. 347-27 SI version: 524R p = c1 + -------------------T + 17.8 where c1 = 4.8; p = lateral pressure, kPa; R = rate of concrete placement, m/h; and T = temperature of concrete in the forms, °C. c1 = 100 lb/ft2 (4.8 kPa) is justified because vibration is slight in slipform work because the concrete is placed in shallow layers of 6 to 10 in. (150 to 250 mm) with no revibration. For some applications, such as gas-tight or containment structures, additional vibration can be required to achieve maximum density of the concrete. In such cases, the value of c1 should be increased to 150 lb/ft2 (7.2 kPa). 7.3.2.5 Tolerances—Prescribed tolerances for slipform construction of building elements are listed in ACI 117. 7.3.2.6 Sliding operation—The maximum rate of slide should be limited by the rate for which the forms are designed. In addition, both maximum and minimum rates of slide should be determined by an experienced slipform supervisor to accommodate changes in weather, concrete slump, initial set of concrete, workability, and the many exigencies that arise during a slide and cannot be accurately predicted beforehand. A person experienced in slipform construction should be present on the deck at all times during the slide operation. During the initial placing of the concrete in the slipform, the placing rate should not exceed that for which the form was designed. Ideally, concrete should be placed in approximately 6 to 8 in. (150 to 200 mm) lifts throughout the slipform operation. The level of hardened concrete in the form should be checked frequently by the use of a probing rod to establish safe lifting rates. Forms should be leveled before they are filled and should be maintained level unless otherwise required for out-of-tolerance corrections. Care should be taken to prevent drifting of the forms from alignment or designed dimensions and to prevent torsional movement. Experience has shown that a plumb line, optical plummet, laser, or combination of these used in conjunction with a water level system is effective in maintaining the form on line and grade and for positioning openings and embedded items. The alignment and plumbness of a structure should be checked at least once during every 4 h that the slide is in operation and preferably every 2 h. In work that is done in separate intermittent slipping operations, a check of alignment and plumbness should be made at the beginning of each slipping operation. More frequent readings should be taken on single tall structures with relatively small plan sections, as the form system in these structures tends to twist and go out of plumb more readily. Sufficient checks of plumbness should be provided to readily detect and evaluate movements of the form for all slipformed structures so that appropriate adjustment can be made in sufficient time by experienced personnel. 347-28 ACI STANDARD Detailed records of both vertical and lateral form movements should be maintained throughout the slipform operation. 7.3.3 Horizontal slipforms—The general provisions of Section 2.1.4 should be met and the formwork engineer/ contractor should submit drawings of the slipform for review and approval by the engineer/architect. These drawings should show the handling diagrams, the placing procedure, and the provisions for ensuring attainment of the required concrete surfaces. 7.4—Permanent forms 7.4.1 Discussion—Permanent forms, or stay-in-place forms, are forms left in place that may or may not become an integral part of the structural frame. These forms can be rigid —such as metal deck, precast concrete, wood, plastics, and various types of fiberboard—or the flexible type—such as reinforced, water-repellent, corrugated paper, or wire mesh with waterproof paper backing. When the permanent form is used as a deck form, it is supported from the main structural frame with or without an intermediate system of temporary supports. If temporary supports are required under, or to provide structural stability for, the structural frame members to support the weight of the fresh concrete without causing excessive deflection or member instability, such information should be specified by the engineer/architect. 7.4.2 Design considerations—If the stay-in-place form is not covered in the contract specifications because it has no function in the finished structure, the form manufacturer’s specifications should be used; the manufacturer’s recommended practice should be followed for size, span, fastenings, and other special features pertinent to this type of form, such as being water repellent and protected against chemical attack from wet concrete; and the minimum requirements of Chapters 2 and 3 should be followed. Particular care should be taken in the design of such forms by the formwork engineer/ contractor to minimize distortion or deformation of the form or supporting members under the construction loads. The engineer/architect who specifies the use of permanent rigid forms should consider in the structural analysis both the construction dead and live loads on the form as well as the structure’s stability during construction, in addition to consideration of the form’s performance in the finished structure. When metal deck to become an integral part of the structure is used as a permanent form, its shape, depth, gage, coating, physical dimensions, properties, and intermediate temporary support should be as called for in contract documents. If structural continuity is assumed in the design of the form, the engineer of the permanent structure should specify the required number of permanent supports over which the form material should be continuous. 7.4.3—Installation 7.4.3.1 Shop drawings—The formwork engineer/ contractor should submit fully detailed shop drawings for all permanent deck forms to the engineer/architect for review, approval, or both. Shop drawings should show all form thicknesses, metal gauges, physical dimensions and properties, accessories, finishes, methods of attachment to the various classes of the work, and temporary shoring requirements. 7.4.3.2 Fastenings—The permanent deck form should be properly fastened to supporting members and to adjacent sections of deck form and properly lapped, in accordance with manufacturers’ recommendations, to provide a tight joint that will prevent loss of mortar during the placement of concrete. Where required, end closures for corrugated or fluted forms should be provided, together with fill pieces where a tight fit is required. To prevent buckling, allow for expansion of metal deck forms after fastening and before concrete placement. Flexible types of forms (those that depend for lateral stiffness on supporting members) should be drawn tight for proper installation. Adequate temporary bracing or anchors should be provided in the plane of the top chord of the supporting members to prevent lateral buckling and rotation of these supports and to maintain the required tension in the flexible form. Paper or metal forms used to form voids in concrete construction should be properly placed and anchored to the reinforcement and to side or deck forms with wire ties or other approved methods to prevent displacement or flotation during placing of concrete. Water should be prevented from entering voids. Where water intrusion is possible, weep holes should be provided to reduce its entrapment. 7.4.4 Deflections—The vertical and lateral deflections of the permanent form between supports under the load of fresh concrete should be investigated by the engineer/architect. Temporary supports, such as shoring and stringers, should be specified, if necessary, to keep deflection within desired tolerances. 7.5—Forms for prestressed concrete construction 7.5.1 Discussion—The engineer/architect should indicate in the contract documents any special requirements for prestressed concrete construction. It may be necessary to provide appropriate means of lowering or removing the formwork before full prestress is applied to prevent damage due to upward deflection of resilient formwork. Pretensioning or post-tensioning of strands, cables, or rods can be done with or without side forms of the member in place, in accordance with Section 7.5.2. Bottom forms and supporting shores or falsework should remain in place until the member is capable of supporting its dead load and anticipated construction loads, as well as any formwork carried by the member. The concreting sequence for certain structures should also be planned so that concrete is not subjected to bending stress caused by deflection of the formwork. 7.5.2—Design 7.5.2.1—Where the side forms cannot be conveniently removed from the bottom or soffit form after concrete has set, such forms should be designed with slip joints or with added panel and connection strength for additional axial or bending loads that can be superimposed on them during the prestressing operation. 7.5.2.2—Side forms that remain in place during the transfer of prestressing force should be designed to allow for GUIDE TO FORMWORK FOR CONCRETE vertical and horizontal movements of the cast member during the prestressing operation. The form should be designed to minimize the restraint to elastic shortening in the prestressing operation. For example, small components or wrecking strips should be planned that can be removed or destroyed to relieve load on side forms as well as to eliminate their restraint during prestressing. In all cases, the restraint to shrinkage of concrete should be kept to a minimum, and the deflections of members due to prestressing force and the elastic deformation of forms or falsework should be considered in the design and removal of the forms. 7.5.2.3—For reasons of safety, when using posttensioned, cast-in-place elevated slabs, the contractor should be careful to ensure that supporting shores do not fall out due to lifting of the slab during tensioning. For large structures where the dead load of the member remains on the formwork during prestressing, displacement of the dead load toward end supports should be considered in design of the forms and shoring, including sills or other foundation support. 7.5.3 Construction accessories—Hold-down or pushdown devices for deflected cables or strands should be provided in the casting bed or forms. All openings, offsets, brackets, and all other items required in the concrete work should be provided for in the formwork. Bearing plates, anchorage assemblies, prestressing steel, conduits, tube enclosures, and lifting devices shown or specified to be set in concrete should be accurately located with formwork templates and anchored to remain within the tolerances given on contract documents. Quality and strength of these accessories should be as specified. 7.5.4 Tolerances—Prescribed ranges of tolerances for job site precast and plant manufactured precast-prestressed concrete members are given in ACI 117 and the PCI report7.1 on tolerances. 7.5.5 Special provisions for curing and for safety of workers—Where necessary to allow early reuse of forms, provisions should be made to use accelerated curing processes such as steam curing, vacuum processing, or other approved methods. Safety shields should be provided at end anchorages of prestressing beds or where necessary for the protection of workmen or equipment against possible breakage of prestressing strands, cables, or other assemblies during prestressing or casting operation. 7.6—Forms for site precasting 7.6.1 Discussion—Forms for site precasting are used for precast concrete items that can be either load- or nonloadbearing members for structural or architectural uses. 7.6.2 Construction—Exterior braces only should be used when exposed metal or filled-in pockets resulting from the use of metal ties would present an objectionable appearance. To ensure uniformity of appearance in the cast members or units, particularly in adjacent units where differences in texture, color, or both would be visible, care should be taken that the contact surfaces of forms or form liners are of uniform quality and texture. 347-29 Form oil or retardant coatings (nonstaining, if required) should be applied uniformly and in accordance with manufacturers’ recommendations for this particular class of work. 7.6.3 Accessories—It is particularly important in this class of work that positive and rigid devices be used to ensure proper location of reinforcement. All openings, cutouts, offsets, inserts, lift rings, and connection devices required to be set in concrete should be accurately located and securely anchored in the formwork. The finished surfaces of members should be free of lift rings and other erection items where it will be exposed, interfere with the proper placing of precast members or other materials, or be subject to corrosion. Such items should be removed so that no remaining metal will be subject to corrosion. The quality and strength of these accessories should be as required by the contract documents, but the lifting devices or other accessories not called for in the contract documents are the responsibility of the contractor. 7.6.4 Tolerances—Prescribed tolerances for precastconcrete construction are listed in ACI 117. 7.6.5 Removal of forms—Precast members or units should be removed from forms only after the concrete has reached a specified strength, as determined by the field-cured test cylinders or beams and job history of concrete curing. Where required to allow early reuse of forms, provisions can be made to use accelerated curing processes, such as steam curing, or other approved methods. Methods of lifting precast units from forms should be approved by the engineer/architect. 7.7—Use of precast concrete for forms 7.7.1 Discussion—Precast concrete panels or molds have been used as forms for cast-in-place and precast concrete, either as permanent forms, integrated forms, or as removable, reusable forms. They have been used for both structural and architectural concrete, designed as structurally composite with the cast-in-place material or to provide a desired quality of outer surface and, in some cases, to serve both of these purposes. Concrete form units can be plain, reinforced, or prestressed, and either cast in the factory or at the job site. The most common use of precast concrete form units has been for elevated slabs acting compositely with topping concrete, as in bridge and commercial or institutional construction. Precast units are also common as ground holding systems in tunneling and as stay-in-place forms for rehabilitation of navigation lock walls. 7.7.2—Design 7.7.2.1 Responsibility for design—Where the integrated form is to act compositely with the structure concrete, the form panel should be designed by the engineer/architect who should also indicate what additional external support is required for the permanent forms. For permanent forms intended to achieve a desired architectural effect, the engineer/ architect can specify surface finish and desired minimum thickness of architectural material. Design and layout of temporary forms and supporting systems should normally be the responsibility of the formwork engineer/contractor. 347-30 ACI STANDARD 7.7.2.2 Connections—Connection details should be planned to overcome problems of mating precast members to each other and to the existing or cast-in-place structure. 7.7.2.3 Bonding concrete form to concrete structure— Effective bond between precast form unit and the concrete structure is essential and can be achieved by: 1) special treatment, such as grooving or roughening the form face in contact with the structure concrete; 2) use of anchoring devices extending across the interface between form panel and structure concrete; 3) a combination of 1) and 2); and 4) use of paint-on or spray-on bonding chemicals. Lifting hooks in a form unit can be designed to serve also as anchors or shear connectors. 7.7.2.4 Code requirements—Precast concrete forms used in composite design with cast-in-place concrete in buildings should be designed in accordance with ACI 318. 7.7.3—During and after concreting 7.7.3.1 Vibration—Thorough consolidation of site-cast concrete is required to prevent voids that would interrupt the bond of the form to structure concrete, but sufficient care should be used to prevent damage of concrete panels by contact with vibrators. 7.7.3.2 Protection of architectural finish—Care should be taken to avoid spilling fresh concrete on exposed surfaces, and any spilled or leaked concrete should be thoroughly removed before it has hardened. After concreting, protection of precast architectural concrete form facings may need to be considered. 7.8—Forms for concrete placed under water 7.8.1 Discussion—There are two basic approaches to the problem of placing concrete under water: the concrete can be mixed in the conventional manner and then placed by special methods, or the preplaced aggregate method can be used. In the first approach, placement can be made by either pump, underwater bucket, or tremie. The tremie is a steel pipe, suspended vertically in the water, with a hopper attached to the upper end above the water surface. The lower end of the pipe, with an ejectable plug, extends to the bottom of the area to be concreted. This pipe is charged with concrete from the surface. Once the pipe is filled with concrete, it is kept full and its bottom should be kept immersed in the fresh concrete. In the second approach, the forms are filled with coarse aggregate, which is then grouted so that the voids around the aggregate are filled as discussed in Section 7.2. The grout is introduced at the bottom and the water is displaced upward as the grout rises. 7.8.2—Underwater bucket and tremie 7.8.2.1 Design—Forms for underwater concreting are designed with the same considerations as other forms covered in Section 2.2, except that the density of the submerged concrete can be reduced by the weight of the water displaced. Because of large local pressures that can develop due to the head of concrete in the tremie, the location of the tremie and possible resulting loads on the form should be evaluated by experienced personnel. Some designers have ignored the effects of submergence because this results in a practical design that is sturdy enough to withstand the extra rigors of underwater conditions. In tidal zones, forms should be designed for the lowest possible water level. Changes in construction schedules can transform a planned submerged placement to one made above water, thus losing the offsetting water pressure. 7.8.2.2 Construction—Underwater forms should be built on the surface in large units because final positioning and fitting when done under water by divers is slow and costly. For this reason, foundations should be kept simple in shape, and forms should be free of complex bracing and connection details. Through-ties, which could interfere with the concrete placing, should be avoided. Forces imposed on preassembled forms during lifting should be considered in the form design. Forms should be carefully fitted and secured to adjacent materials or construction to avoid loss of mortar under pressure developed. If there is any water current flow past the form, small openings in the form should be avoided as they will permit washing or scouring of the fresh concrete. When it is intended to permit concrete to overflow the form and screed it off to grade, it is essential that the form is positioned to the proper grade and is detailed so that the overflow will not interfere with the proposed method and devices for stripping. Forms should be well detailed, and such details should be scrupulously followed so that divers employed to remove the form can visualize and plan their work before descending. Multiuse forms can have special devices for positioning forms from above water and special stripping devices, such as hydraulic jacks, that permit releasing the form from the surface. 7.8.3—Preplaced aggregate 7.8.3.1 Design—The formwork should be designed with the same considerations as mentioned previously in Section 7.2.2. 7.8.3.2 Construction—It is important to ensure that silt is excluded from the forms because silt chokes the voids in the aggregate and interferes with the flow of grout. Silt, if left adhering to the aggregate, can reduce the bond between the aggregate and the grout. The inspection of the forms before concrete placement should verify that the perimeters of the forms are effectively sealed against the leakage of grout or the intrusion of silt or other fines.