Guide to Formwork for Concrete Chapter 5

GUIDE TO FORMWORK FOR CONCRETE CHAPTER 5—ARCHITECTURAL CONCRETE 5.1—Introduction 5.1.1 Objective—The general requirements for formwork presented in preceding chapters for the most part also apply to architectural concrete. Additional information is available in ACI 301 and ACI 303R. This chapter identifies and emphasizes additional factors that can have a critical influence on formwork for cast-in-place architectural concrete. Tilt-up and precast architectural concrete are not considered here. Concrete receiving coatings or plasters that hide the surface color and texture is not considered architectural. 5.1.2 Definition—ACI Committee 303 defines architectural concrete as concrete that is exposed as an interior or exterior surface in the completed structure, contributes to its visual character, and is specifically designated as such in the contract documents. Particular care should be taken in the selection of materials, design, and construction of the formwork, and placing and consolidation of the concrete to eliminate bulges, offsets, or other unsightly features in the finished surface and to maintain the integrity of the surface texture or configuration. The character of the concrete surface to be produced should also be considered when the form materials are selected. Special attention should be given to closure techniques, concealment of joints in formwork materials, and to the sealing of forms to make them watertight. 5.1.3 Factors in addition to formwork—Many factors other than formwork affect the architectural effects achieved in concrete surfaces. They start at the design stage and carry through to the completed project. Factors affecting the concrete can also include the mixture proportions or aggregate, the method of placing the concrete, the consolidation technique, and the curing procedure. Chemicals can have an effect on the final product, whether used as additives in the mixture; applied directly to the concrete, such as curing compounds; or applied indirectly, such as form release agents. Even after the structure is completed, weather and air pollution will affect the appearance of the concrete. These and other influencing factors should be identified and their effects evaluated during the initial design stages. The single most important factor for the success of an architectural concrete job is good workmanship. 5.1.4 Uniform construction procedures—Architectural concrete should have a uniform color and surface finish. The best way for the contractor to achieve this uniformity is to be consistent in all construction practices. Forming materials should be kept the same, and release agents should be applied uniformly and consistently. Placement and consolidation of the concrete should be standardized so that uniform density is achieved. Stripping and curing sequences should be kept constant throughout the work to control color variations. 347-19 5.2—Role of the architect 5.2.1 Preplanning—Much architectural concrete is also structural, but surface quality generally desired for architectural concrete is higher than what is typically satisfactory for structural concrete, and is more costly. The architect can use the latest information available in the art of forming and concrete technology during the design process to keep plans in line with the budget for the structure. However, intricacies and irregularities can raise the budget to a point that outweighs the architectural concrete’s aesthetic contributions. The architect can make form reuse possible by standardizing building elements, such as columns, beams, and windows, and by making uninterrupted form areas the same size wherever possible to facilitate the use of standard form gangs or modules. The increased size of these uninterrupted areas will contribute to forming economy. A prebid conference with qualified contractors will bring out many practical considerations before the design is finalized. 5.2.2 Contract documents and advance approvals—The architect should prepare contract documents that fully instruct the bidder as to the location and desired appearance of architectural surfaces, as well as other specific requirements listed in Sections 5.2.3 to 5.2.7. On major work, this is frequently achieved by specifying a preconstruction mockup prepared and finished by the contractor for approval by the architect, using proposed form materials, jointing techniques, and form surface treatments, such as wetting, oiling, or lacquering. Once such a mockup has been completed to the satisfaction of the architect, it remains at the site for the duration of the work as a standard with which the rest of the work should comply. Design reference samples, which are smaller specimens of concrete with the proposed surface appearance, may also be created for approval by the architect. Small samples like these, kept at the job site for reference, are not as good as a full-scale mockup but can be helpful. The samples should be large enough to adequately represent the surface of the concrete. If the samples are to be used as a basis for acceptance, several should be made to represent the variation that can occur in the finish. In the absence of physical mockups or reference samples, it can be helpful to specify viewing conditions under which the concrete surfaces will be evaluated for compliance with the specifications. 5.2.3 Tolerances—The architect should specify dimensional tolerances considered essential to successful execution of the design. ACI 117 can be consulted, but the architect should realize that the tolerances therein are for concrete construction in general, and more restrictive tolerances can be required for architectural work. No numerical limits are suggested herein because the texture, lighting, and configuration of surfaces will all have an influence. ACI Committee 347R notes, however, that concrete construction tolerances of 1/2 those called for in ACI 117 are considered the achievable limit. 347-20 ACI STANDARD 5.2.4 Camber—The contractor should camber formwork to compensate for deflection of the formwork during concrete placement. The architect should, however, specify any additional camber required to compensate for structural deflection or optical sag (the illusion that a perfectly horizontal long-span member is sagging). The architect should be aware that horizontal members are checked for compliance with tolerances and camber before the removal of the forms and shores. 5.2.5 Joints and details—Location, number, and details of items such as openings, contraction joints, construction joints, and expansion joints should be shown on the design plans or the architect should specify a review of the proposed location of all of these details as shown on the formwork drawings. (Some guidance on joint locations can be found in ACI 224R, 303R, and 332R.) Because it is impossible to disguise the presence of joints in the form face, it is important for their positions to be predetermined and, if possible, planned as part of the architectural effect. The architect can plan joint locations between surface areas on a scale and module suitable to the size of available materials and prevailing construction practices. If this is not aesthetically satisfactory, dummy joints can be introduced to give a smaller pattern. Actual joints between sheathing materials can be masked by means of rustication strips (splayed fillets) attached to the form face. Rustication strips at horizontal and vertical construction joints can also create crisp edges accented by shadow lines instead of the potential ragged edge of a construction joint left exposed to full view. Special care should be taken during placement and vibration to minimize bugholes and honeycombing that form when air is trapped beneath horizontal rustications. Sometimes construction joints in beams can be concealed above the support columns and joints in floors above their supporting beams instead of in the more customary regions of low shear. 5.2.6 Ties and inserts—Form ties and accompanying tie holes are an almost inescapable part of wall surfaces. Architects frequently integrate tie holes into the visual design quality of the surface. If this is planned and any effects or materials other than those provided in Section 5.3.4 are desired, they should be clearly specified as to both location and type. Where tie holes are to be patched or filled, the architect should specify the treatment desired, unless it has been shown on the preconstruction mockup. 5.2.7 Cover over reinforcing steel—Adequate cover over reinforcement, as required by codes, is needed for protection of steel and long-term durability of the concrete. Reinforcement that is properly located is important in the control of surface cracking. For positive assurance of maintaining required cover, the architect can specify appropriate side form spacers as defined in Section 4.3.4. There is no advantage in specifying more cover than required by code because excessive cover can contribute to increased cracking. The architect should specify sufficient cover to allow for any reduction that will result from the incorporation of grooves or indented details and from surface treatments, such as aggregate exposure and tooling. The maximum thickness of any material to be removed should be added to basic required cover. 5.3—Materials and accessories 5.3.1 Sheathing or form facing—Architectural concrete form sheathing should be of appropriate quality to maintain the uniformity of concrete surfaces through multiple uses and to control deflection within appropriate limits. Plywood, steel, glass fiber-reinforced plastic, and aluminum can all be suitable as sheathing or facing materials. Select the grade or class of material needed for pressure, framing, and deflection requirements. Be sure that the chosen material meets the specification requirements for the concrete surface texture. Procedures for controlling the rusting of steel should be carefully followed. 5.3.2 Structural framing—Form facing can be supported with lumber, steel, or aluminum members straight and rigid enough to meet the architectural specifications. 5.3.3 Form liners—A form liner is a material attached to the inside face of the form to alter or improve surface texture or quality of the concrete. It is not required structurally. Wood, rigid plastic, elastomeric materials, and glass fiberreinforced plastics are all suitable liner materials when carefully detailed and fabricated. Plastics should be handled and assembled with care to avoid distortion caused by daily temperature cycles at the job site. 5.3.4 Form ties—Form-tie assemblies for architectural concrete should permit tightening of forms and leave no metal closer to the surface than 1-1/2 in. (38 mm) for steel ties and 1 in. (25 mm) for stainless steel ties. The ties should not be fitted with lugs, cones, washers, or other devices that will leave depressions in the concrete less than the diameter of the device, unless specified. Ties should be tight fitting or tie holes in the form should be sealed to prevent leakage at the holes in the form. If textured surfaces are to be formed, ties should be carefully evaluated as to fit, pattern, grout leakage, and aesthetics. 5.3.5 Side form spacers—Side form spacers, as defined in Section 4.3.4, are particularly important in architectural concrete to maintain adequate cover over reinforcing steel and to prevent development of rust streaking on concrete surfaces. Plastic, plastic-protected, rubber-tipped, or other noncorroding spacers should be attached to the reinforcing bar so that they do not become dislodged during concrete placement and vibration. The number and location of the side form spacers should be adequate for job conditions. 5.4—Design 5.4.1 Special considerations—The general procedure will follow the principles outlined in Chapter 2. The formwork engineer/contractor, however, will frequently have limitations imposed by the architectural design. Some of these considerations are: tie spacing and size, form facing preferences, location and special treatment of form joints, special tolerances, and use of admixtures. Because these factors can influence form design, they should be fully reviewed at the beginning of the form design process. 5.4.2 Lateral pressure of concrete—Architectural concrete can be subjected to external vibration, revibration, set retardants, high-range water-reducing admixtures, and slumps greater than those assumed for determining the lateral pressure as noted in Section 2.2.2. Particular care should be exercised in these cases to design the forms for the increased lateral pressures resulting from sources noted in Section 2.2.2. GUIDE TO FORMWORK FOR CONCRETE 5.4.3 Structural considerations—Because deflections in the contact surface of the formwork reflect directly in finished surfaces under varying light conditions, forms for architectural concrete should be designed carefully to minimize deflections. In most cases, deflections govern design rather than bending (flexural stress) or horizontal shear. Deflections of sheathing, studs, and wales should be designed so that the finished surface meets the architectural specifications. Limiting these deflections to l/400, where l is the clear span between supports, is satisfactory for most architectural formwork. Forms bow with reuse; therefore, more bulging will be reflected in the surface formed after several uses. This effect should be considered when designing forms. When tie size and spacing are limited by the architect, the formwork engineer/contractor may have to reverse the usual procedure to arrive at a balanced form design. Given the capacity of the available tie and the area it supports, the formwork engineer/contractor can find the allowable pressure, design supporting members, and establish a rate of concrete placing. Where wood forms are used, stress-graded lumber (or equivalent) free of twists and warps should be used for structural members. Form material should be sized and positioned to prevent deflections detrimental to the surfaces formed. Joints of sheathing materials should be backed with structural members to prevent offsets. 5.4.4 Tie and reanchor design—Tie layout should be planned. If the holes are to be exposed as part of the architectural concrete, tie placement should be symmetrical with the member formed. If tie holes are not to be exposed, ties should be located at rustication marks, control joints, or other points where the visual effect will be minimized. Externally braced forms can be used instead of any of the aforementioned methods to avoid objectionable blemishes in the finished surface. Externally braced forms, however, can be more difficult and more costly to build. Consideration should be given to reanchoring forms in preceding or adjacent placements to achieve a tight fit and prevent grout leakage at these points. Ties should be located as close as possible to the construction joint to facilitate reanchoring the form to adjacent placements. 5.4.5 Joints and details—In architectural concrete, joints should, where feasible, be located at the junction of the formwork panels. At contraction or construction joints, rustication strips should be provided and fastened to the face of forms. Corners should be carefully detailed to prevent grout leakage. Sharp corners should, wherever possible, be eliminated by the use of chamfer strips except when prohibited by project specifications. 5.4.6 Tolerances—The formwork engineer/contractor should check for dimensional tolerances specified by the architect that can have a bearing on the design of the forms. If no special tolerances are given, the formwork engineer/ contractor can use ACI 117 tolerances for structural concrete. 5.5—Construction 5.5.1 General—Forms should be carefully built to resist the pressures to which they will be subjected and to limit deflections to a practicable minimum within the tolerances specified. Joints in structural members should be kept to a minimum and, where necessary, should be suitably spliced or otherwise constructed to maintain continuity. 347-21 Pour pockets for vibrating or placing concrete should be planned to facilitate careful placement and consolidation of the concrete to prevent segregation, honeycomb, sanding, or cold joints in the concrete. The location of pour pockets should be coordinated with the architect. Attachment of inserts, rustication strips, and ornamental reliefs should be planned so that forms can be removed without exerting pressure on these attachments. Where special forming systems are specified by the engineer of the project for structural purposes (such as one- and two-way joist systems) in areas that are considered architectural, the architect and engineer should coordinate their requirements to be sure the architectural effect is consistent with the forming method and material specified. Forms that will be reused should be carefully inspected after each use to ensure that they have not become damaged, distorted, disassembled, or otherwise unable to perform as designed. 5.5.2 Sheathing and jointing—Contact surfaces of the formwork should be carefully installed to produce neat and symmetrical joint patterns, unless otherwise specified. Joints should be either vertical or horizontal and, where possible, should be staggered to maintain structural continuity. Nailing should be done with care using hammers with smooth and well-dressed heads to prevent marring of the form surfaces. When required, box nails should be used on the contact surface and should be placed in a neat pattern. Wherever possible, sheathing or panel joints should be positioned at rustication strips or other embedded features that can conceal or minimize the joint. Construction joints should be formed with a grade strip attached to the form to define a clean straight line on the joint of the formed surface. Formwork should be tightened at a construction joint before the next placement to prevent seepage of water between the form and previously placed concrete surfaces. Architectural concrete forms should be designed to resist water leakage and avoid discoloration. One method to prevent water loss from the concrete at the joints between sections of the formwork and at construction joints is to attach a gasket of flexible material to the edge of each panel. The gasket is compressed when the formwork is assembled or placed against the existing concrete. Caulk, tape, joint compound, or combinations of these can be used to seal joints. In all cases, unsupported joints between sheathing sheets should be backed by framing. Water-tight forms require more care during vibration to remove entrapped air that can cause bug holes. Textured surfaces on multilift construction should be separated with rustication strips or broad reveals because accumulation of construction tolerances, random textures, or both, prevent texture matching. Furthermore, the grout seal between the bottom of a textured liner and the top of the previous placement is impractical without the rustication strip. 5.5.3 Cleaning, coating, and release agents—Form coatings or releasing agents should be applied before reinforcing steel is placed and should be applied carefully to avoid contacting adjacent construction joints or reinforcing. No form coating should be used unless it can be demonstrated not to stain the concrete or impair the adhesion of paints or other surface treatments. 347-22 ACI STANDARD Form sealers should be tested to ensure that they will not adversely affect the texture of the form lining material. Ties that are to be pulled from the wall should be coated with nonstaining bond breaker or encased in sleeves to facilitate removal. Forms should be carefully cleaned and repaired between uses to prevent deterioration of the quality of surface formed. Film or splatter of hardened concrete should be thoroughly removed. 5.5.4 Ornamental liners and detail—Ornamental concrete is usually formed by elastomeric molds or wood, plastic, or plaster waste molds. Members making up wood molds should be kerfed on the back wherever such members can become wedged between projections in the ornament. Molds should be constructed so that joints will not be opened by slight movement or swelling of the wood. Joints in the molds should be made inconspicuous by pointing. The molds should be carefully set in the forms and securely held in position to reproduce the design shown on the plans. Where wood forms adjoin molds, the wood should be neatly fitted to the profile of the mold and all joints should be carefully pointed. The molds and the adjacent wood forms should be detailed so that the wood forms can be stripped without disturbing the molds. The edge of the mold or pattern strip should be tapered to a slight draft to permit removing the detail material without damaging the concrete. Special provisions should be made for early form removal, retardation, or both when sandblasting, wire brushing, or other treatments are required. Form liners should be attached securely with fasteners or glue recommended by the manufacturer. The form behind the liner should hold the fasteners. The surfaces should be cleaned and dried thoroughly so that the glue will bond. Do not use glue at temperatures lower than those recommended by the manufacturer. 5.6—Form removal 5.6.1 Avoiding damage—When concrete surfaces are to be left as cast, it is important not to damage or scar the concrete face during stripping. Forms should be supported so that they do not fall back or against the architectural surface. The use of pry bars and other stripping tools should be strictly supervised. In no case should pry bars be placed directly against the concrete. Even the use of wood or plastic wedges does not ensure that damage will not occur. Once formwork is removed, the architectural surfaces should be protected from continuing construction operations. 5.6.2 Concrete strength—It is desirable for architectural concrete to have a higher compressive strength than normal for stripping. This can be accomplished by adjusting the mixture proportions or leaving forms in place longer. If concrete is not strong enough to overcome the adhesion between the form surface and the concrete, concrete can scale or spall. Therefore, a good quality surface might require the forms to stay in place longer. The longer the forms stay in place, however, the darker the concrete will become. The engineer/architect should specify what concrete strength is required before stripping can take place. 5.6.3 Uniformity—To ensure surface quality, uniformity in stripping time and curing practices are essential. Where the objective is to produce as consistent an appearance as possible, it is beneficial to protect the concrete by leaving the formwork in place somewhat longer than normal. Early exposure of concrete to the air affects the manner in which the surface dries. The ambient conditions can influence the eventual color of the concrete. 5.6.4 Avoiding thermal shock—Cold-weather concreting requires that special attention be paid to the sudden temperature change of concrete. To avoid thermal shock and consequent crazing of the concrete surface, the change in temperature of the concrete should be controlled within the limits outlined in ACI 303R. This can be accomplished by heating the work area, leaving the forms in place to contain the heat of hydration, or by insulating the concrete after the forms have been removed (refer to ACI 306R). Positive steps should be taken to inspect, record, and document the procedures used to cure the concrete.