Guide to Formwork for Concrete Chapter 6

ACI STANDARD CHAPTER 6—SPECIAL STRUCTURES 6.1—Discussion Formwork for all structures should be designed, constructed, and maintained in accordance with recommendations in Chapters 1 to 4. This section deals with the additional requirements for formwork for several special classes of work. ACI 344R contains information on design and construction of circular prestressed-concrete structures. 6.2—Bridges and viaducts, including high piers 6.2.1 Discussion—The construction and removal of formwork should be planned in advance. Forms and supports should be sufficiently rigid to ensure that the finished structure will fulfill its intended structural function and that exposed concrete finishes will present a pleasing appearance to the public. 6.2.2 Shoring and centering—Recommended practice in Sections 3.5 and 3.7 for erection and removal should be followed. In continuous structures, support should not be released in any span until the first and second adjoining spans on each side have reached the specified strength. For post-tensioned bridges, the shore design should consider the resulting redistribution of loads on the shores similar to the effects discussed in Section 3.8.7. 6.2.3 Forms—Forms can be of any of a large number of materials but most commonly are wood or metal. They should be built mortar-tight of sound material strong enough to prevent distortion during placing and curing of the concrete. 6.3—Structures designed for composite action 6.3.1 Recommendations—Structures or members that are designed so that the concrete acts compositely with other materials or with other parts of the structure present special forming problems that should be anticipated in the design of the structure. Requirements for shoring or other deflection control of the formwork should be clearly presented by the engineer/architect in the specifications. Where successive placements are to act compositely in the completed structure, deflection control becomes extremely critical. Shoring, with or without cambering portions of the structure during placement and curing of the concrete, should be analyzed separately for the effects of dead load of newly placed concrete and for the effect of other construction loads that can be imposed before the concrete attains its design strength. 6.3.2 Design—Formwork members and shores should be designed to limit deflections to a practical minimum consistent with the structural member being constructed. Where camber is specified for previously installed components of the structure, allowance should be made for the resultant preloading of the shores before application of the dead load of concrete. In members constructed in several successive placements, such as box-girder structures, formwork components should be sized, positioned, supported, or all three to minimize progressive increases in deflection of the structure that would excessively preload the reinforcing steel or other portions of the composite member. In multistory work where shoring of composite members is required, consideration should be given to the number of stories of shores necessary, in conjunction with the speed of construction and concrete strengths, to minimize deflections due to successive loadings. Distinction should be made in such analyses for shores posted to relatively unyielding support, such as foundations, instead of to structures or members already in elastic support (refer to Section 3.8). Composite construction can have beams of relatively light cross sections that are fully adequate when construction is complete. During construction, these beams may not be laterally supported by the formwork, thus leaving them with a high slenderness ratio and reduced beam strength. The engineer/architect should alert the contractor to this problem in general notes on the structural plans or in notes on applicable plans when this condition exists. The formwork engineer/ contractor should be alert to this possibility and provide shoring or lateral support where needed. 6.3.3 Erection—Construction, erection of formwork, or both for composite construction follows basic recommendations contained in Chapter 3. Shoring of members that will act compositely with the concrete to be placed should be done with great care to ensure sufficient bearing, rigidity, and tightness to prevent settlement or deflections beyond allowable limits. Wedges, shims, and jacks should be provided to permit adjustment if required before or during concreting, as well as to permit removal without jarring or impacting the completed construction. Provision should be made for readily checking the accuracy of position and grade during placement. Even though adjustment of forms can be possible during or after placing, it is not recommended. Any required adjustment should be made before the initial set of the concrete. Where camber is required, a distinction should be made between that part which is an allowance for settlement or deflection of formwork or shoring and that which is provided for design loadings. The former should generally be the responsibility of the formwork engineer/contractor who designs the forms and supports unless such camber is stipulated by the engineer/architect. Measurement of camber provided for structural design loadings should be made after hardening of the concrete but before removal of the supports (also refer to Section 1.4.5). This is because the structural deflection occurring upon removal of the supports is a function of the structural design and cannot be controlled by the contractor. 6.3.4 Removal—In addition to meeting the provisions of Section 3.7, forms, supports, or both should be removed only after tests and specified curing operations indicate to the satisfaction of the engineer/architect that the most recently GUIDE TO FORMWORK FOR CONCRETE placed concrete has attained the strength required to develop composite action, and then only after approval of the engineer/architect. The sequence of such removal should be approved by the engineer/architect. 6.4—Folded plates, thin shells, and long-span roof structures 6.4.1 Discussion—For long-span and space structures requiring a complex, three-dimensional design analysis and presenting three-dimensional problems in formwork design, erection, and removal, formwork planning should be done by formwork engineers having the necessary special qualifications and experience. These formwork engineers should consult and cooperate with the engineer/architect to make sure that the resulting surfaces will conform to his or her design. 6.4.2—Design • The engineer/architect should specify limiting values and directions of the reactive forces when the falsework is supported by the permanent structure. • When applicable, the engineer/architect should include a decentering sequence plan with the bidding documents as a basis for the design of the forming and support system to be used by the contractor. • Lateral loads—In determining the lateral forces acting on the formwork, the wind load should be calculated on the basis of a minimum of 15 lb/ft2 (0.72 kPa) of projected vertical area as specified for wall forms in Section 2.2.3. For structures such as domes, negative forces due to suction created by the wind on the leeward side of the structure should be considered. • Analysis—The provisions of Sections 2.1.1 and 2.3 should be adhered to in formwork planning. Assumed design loads should be shown on the formwork drawings. Complete stress analyses should be prepared by competent structural engineers, and the maximum and minimum values of stress, including reversal of stress, should be shown for each member for the most severe loading conditions. Consideration should be given to unsymmetrical or eccentric loadings that might occur during concrete placement and during erection, decentering, or moving of travelers. The vertical or lateral deflection of the moving forms or travelers, as well as the stability under various loads, should be investigated to confirm that the formwork will function satisfactorily and that the concrete tolerances will be met. Particular care should be taken in the design and detailing of individual members and connections. Where truss systems are used, connections should be designed to keep eccentricities as small as possible to minimize deflections or distortions. Because the weight of the formwork can be equal to or greater than the design live load of the structure, form details should be designed to avoid the formwork hanging up and overloading the structure during decentering. Due to the special shapes involved, tolerances based on functions of these shapes should be specified by the engineer/ architect in the bidding documents. 347-23 6.4.3 Drawings—When required, the formwork engineer/ contractor should submit detailed drawings of the formwork for approval of the engineer/architect. These drawings should show the proposed concrete placing sequence and the resulting loads. To ensure that the structure can assume its deflected shape without damage, the decentering or handling sequence of the formwork should be shown on the drawings. The formwork design, drawings, and procedures should comply with federal and local safety laws, as well as the contract documents. Deflection of these structures can cause binding between the form and the concrete during decentering. Formwork drawings and form details should be planned to prevent binding and facilitate stripping of forms. Drawings should show such details as type of inserts and joints in sheathing where spreading of the form can result in the form becoming keyed into the concrete. 6.4.4 Approval—The engineer/architect should review the design and drawings for the formwork and the procedures for construction to ensure the structural integrity of the permanent structure. The engineer/architect should approve in writing the loads imposed by the formwork, the sequence of the concrete placing operations, and the timing and procedures of decentering and stripping. 6.4.5 Construction—In planning and erecting formwork, provisions should be made for adequate means of adjustment during placing where necessary. Telltales should be installed to check alignment and grade during placement. Where the forming system is based on a certain placing sequence, that sequence should be clearly defined and adhered to in the field. 6.4.6 Removal of formwork—Formwork should be removed and decentered in accordance with the procedure and sequence specified on the form drawings or on the contract documents. Decentering methods used should be planned to prevent any concentrated reaction on any part of the permanent structure. Due to the large deflections and the high dead load-to-live load ratio common to this type of structure, decentering and form removal should not be permitted until specified tests demonstrate that the concrete strength and the modulus of elasticity specified in contract documents have been reached. Moduli of elasticity can determine time of decentering, although required compressive strengths may already have been attained. Decentering should begin at points of maximum deflection and progress toward points of minimum deflection, with the decentering of edge members proceeding simultaneously with the adjoining shell. 6.5—Mass concrete structures 6.5.1 Discussion—ACI 116R defines mass concrete as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.” Mass concrete occurs in heavy civil engineering construction, such as in gravity dams, arch dams, gravity-retaining walls, lock walls, powerplant structures, and large building foundations (ACI 207.1R). Special provisions are usually made to control the tempera- 347-24 ACI STANDARD ture rise in the mass by the use of cement or cementitious material combinations possessing low or moderate heatgenerating characteristics, by postcooling (cooling the fresh concrete) or by placing sequence. Heat rise in mass concrete is most often controlled by replacement of cement with pozzolans, particularly fly ash. Formwork for mass concrete falls into two distinct categories, namely, low and high lift. Low-lift formwork, for heights of 5 to 10 ft (1.5 to 3 m), usually consists of multiuse steel cantilever form units that incorporate their own work platforms and, on occasion, lifting devices. High-lift formwork is comparable with the single-use wood forms used extensively for structural concrete. 6.5.2 Lateral pressure of concrete—The lateral pressure formulas for concrete placed in walls can be used for mass concrete (refer to Section 2.2.2). The formwork engineer needs to carefully review the concrete mixture proportion to determine the appropriate formula from Section 2.2.2. Concrete additives or cement substitutes can improve heat generation characteristics, but the same materials can increase concrete set time and increase lateral pressures. Consideration should be given to placing sequence in the determination of pressure. Frequently, concrete is layered in such a way that the fresh concrete rate of placement locally is substantially greater than the average rate of placement. Local lateral pressures can be greater than would be estimated on the basis of the average rate of placement. In addition, the use of large concrete buckets with rapid discharge of concrete can cause high impact loads near the forms. 6.5.3 Design consideration—Mass concrete forming can require special form tie and anchor design. 6.5.3.1—Forming sloping surfaces requires ties or anchors to resist pressure forces that are perpendicular to the face of the form. Using horizontal ties will leave the vertical component of pressure untied. Vertical (hold down) anchors are required. 6.5.3.2—Forms tied or anchored to a rock face require particular care. Often, rock anchors are placed before the forms are erected. This requires the form designer to accommodate tie and anchor misalignment. Rock anchors should be checked to ensure that the anchor can resist the tie forces. 6.5.3.3—Bending and welding of high tensile steel tie rods should not be permitted without the approval of the tie manufacturer. Any approved welding should be by a certified welder using approved written welding procedures. 6.5.3.4—The capacity of anchors and form ties embedded in previously placed concrete is dependent on the strength of the concrete, which is very low at early ages. The embedded strength should be sufficient to sustain design loadings from the new placement and initial bolting stresses. 6.5.4 Tolerances—Refer to Section 3.3 and ACI 117. 6.6—Underground structures 6.6.1 Discussion—Underground structures differ from corresponding surface installations in that the construction takes place inside an excavation instead of in the open, providing unique problems in handling and supporting formwork and in the associated concrete placing. As a result, four factors usually make the design of formwork for underground structures entirely different than for their above-ground counterparts. First, concrete to fill otherwise inaccessible areas can be placed pneumatically or by positive displacement pump and pipeline. Second, rock is sometimes used as a form backing, permitting the use of rock anchors and tie rods instead of external bracing and shores. Third, the limits of the excavation demand special handling equipment that adds particular emphasis to the removal and reuse of forms. Fourth, rock surfaces can sometimes be used for attaching hoisting devices. When placement is done by pneumatic or positive displacement pump and pipeline methods, the plastic concrete is forced under pressure into a void, such as the crown of a tunnel lining. For more information on the pumping process, refer to ACI 304.2R. 6.6.2—Design loads 6.6.2.1 Vertical loads—Vertical and construction loads assumed in the design of formwork for underground structures are similar to those for surface structures, with the exception of unusual vertical loads occurring near the crown of arch or tunnel forms and flotation or buoyancy effect beneath tunnel forms. In placing concrete in the crowns of tunnel forms, pressures up to 3000 lb/ft2 (144 kPa) have been induced in areas of overbreak and near vertical bulkheads from concrete placed pneumatically or by positive displacement pump. Overbreak is the excess removal of rock or other escavated material above the forms beyond the required tunnel lining thickness. Until more definite recommendations can be made, the magnitude and distribution of pressure should be determined by the formwork engineer. The assumed pressure should not be less than 1500 lb/ft2 (72 kPa) acting normally to the form plus the dead weight of the concrete placed pneumatically or by pump. 6.6.2.2 Lateral loads—For shafts and exterior walls against rock, the values listed in Section 2.2.2 should apply. When the shaft form relies on the single shear value of embedded anchors in the previous placement as a means of support, the minimum time lapse between successive placements (or minimum concrete strength) and maximum allowable loading additional to the dead weight of the form should be specified. For arch forms and portions of tunnel forms above the maximum horizontal dimension or spring line of the form, the pressure should be compatible with the pressures discussed under vertical loads in Section 6.6.2.1. 6.6.3 Drawings—In addition to the provisions of Chapters 1, 2, and 3, the following data should be included on the drawings for specialized formwork and formwork for tunnels: • All pressure diagrams used in the design of the form, including diagrams for uplift, for unbalanced lateral or vertical loads, for pressurized concrete, or for any other load applicable to the particular installation; • Recommended method of supplemental strutting or bracing to be employed in areas where form pressures can exceed those listed due to abnormal conditions; • Handling diagrams and procedures showing the proposed method of handling the form during erection or installation for concrete placement plus the method of bracing and anchorage during normal operation; GUIDE TO FORMWORK FOR CONCRETE • Concrete placement method and, for tunnel arch forms, whether the design is based on the unit or bulkhead system of concrete placement or the continuously advancing slope method; and • The capacity and working pressure of the pump and the size, length, and maximum embedment of the discharge line when placement by pumping is anticipated. 6.6.4 Construction—The two basic methods of placing a tunnel arch entail problems in the construction of the formwork that require special provisions to permit proper reuse. These two basic methods are commonly known as the bulkhead method and the continuously advancing slope method. The former is used exclusively where poor ground conditions exist, requiring the lining to be placed concurrently with tunnel driving operations. It is also used when some factor, such as the size of the tunnel, the introduction of reinforcing steel, or the location of construction joints, precludes the advancing slope method. The advancing slope method, a continuous method of placement, is usually preferred for tunnels driven through competent rock, ranging between 10 and 25 ft (3 and 8 m) in diameter and at least 1 mi (1.6 km) in length. The arch form for the bulkhead method is usually fabricated into a single unit between 50 and 150 ft (15 and 45 m) long, which is stripped, moved ahead, and re-erected using screw jacks or hydraulic rams. These are permanently attached to the form and supporting traveling gantry. The arch form for the continuously advancing slope method usually consists of eight or more sections that range between 15 and 30 ft (5 and 9 m) in length. These are successively stripped or collapsed, telescoped through the other sections, and re-erected using a form traveler. Although the minimum stripping time for tunnel arch forms is usually established on the basis of experience, it can be safely predetermined by tests. At the start of a tunnel arch concreting operation, the recommended minimum stripping time is 12 h for exposed surfaces and 8 h for construction joints. If the specifications provide for a reduced minimum stripping time based on site experience, such reductions should be in time increments of 30 min or less and should be established by laboratory tests and visual inspection and surface scratching of sample areas exposed by opening the form access covers. Arch forms should not be stripped prematurely when unvented groundwater seepage could become trapped between the rock surface and the concrete lining. 6.6.5 Materials—The choice of materials for underground formwork is typically predicated on the shape, degree of reuse and mobility of the form, and the magnitude of pump or pneumatic pressures to which it is subjected. Usually, tunnel and shaft forms are made of steel or a composite of wood and steel. Experience is important in the design and fabrication of a satisfactory tunnel form due to the nature of the pressures developed by the concrete, placing techniques, and the high degree of mobility required. When reuse is not a factor, plywood and tongue-andgroove lumber are sometimes used for exposed surface finishes. High humidity in underground construction alleviates normal shrinkage and warping. 347-25