Guide to Formwork for Concrete Chapter 3

GUIDE TO FORMWORK FOR CONCRETE 347-9 CHAPTER 3—CONSTRUCTION 3.1—Safety precautions Contractors should follow all state, local, and federal codes, ordinances, and regulations pertaining to forming and shoring. In addition to the very real moral and legal responsibility to maintain safe conditions for workmen and the public, safe construction is, in the final analysis, more economical than any short-term cost savings from cutting corners on safety provisions. Attention to safety is particularly significant in formwork construction that supports the concrete during its plastic state and until the concrete becomes structurally self-sufficient. Following the design criteria contained in this guide is essential for ensuring safe performance of the forms. All structural members and connections should be carefully planned so that a sound determination of loads may be accurately made and stresses calculated. In addition to the adequacy of the formwork, special structures, such as multistory buildings, require consideration of the behavior of newly completed beams and slabs that are used to support formwork and other construction loads. It should be kept in mind that the strength of freshly cast slabs or beams is less than that of a mature slab. Formwork failures can be attributed to substandard materials and equipment, human error, and inadequacy in design. Careful supervision and continuous inspection of formwork during erection, concrete placement, and removal can prevent many accidents. Construction procedures should be planned in advance to ensure the safety of personnel and the integrity of the finished structure. Some of the safety provisions that should be considered are: • Erection of safety signs and barricades to keep unauthorized personnel clear of areas in which erection, concrete placing, or stripping is under way; • Providing experienced form watchers during concrete placement to ensure early recognition of possible form displacement or failure. A supply of extra shores or other material and equipment that might be needed in an emergency should be readily available; • Provision for adequate illumination of the formwork and work area; • Inclusion of lifting points in the design and detailing of all forms that will be crane-handled. This is especially important in flying forms or climbing forms. In the case of wall formwork, consideration should be given to an independent work platform bolted to the previous lift; • Incorporation of scaffolds, working platforms, and guardrails into formwork design and all formwork drawings; • Incorporation of provisions for anchorage of alternative fall protection devices, such as personal fall arrest systems, safety net systems, and positioning device systems; and • A program of field safety inspections of formwork. 3.1.1 Formwork construction deficiencies—Some common construction deficiencies that can lead to formwork failures are: • Failure to inspect formwork during and after concrete 347-10 ACI STANDARD • • Fig. 3.1—Inadequate bearing under mudsill. Fig. 3.2—Uplift of formwork. Connection of shores to joists and stringers should hold shores in place when uplift or torsion occurs. Lacing to reduce the shore slenderness ratio can be required in both directions. placement to detect abnormal deflections or other signs of imminent failure that could be corrected; • Insufficient nailing, bolting, welding, or fastening; • Insufficient or improper lateral bracing; • Failure to comply with manufacturer’s recommendations; • Failure to construct formwork in accordance with the form drawings; • Lack of proper field inspection by qualified persons to ensure that form design has been properly interpreted by form builders; and • Use of damaged or inferior lumber having lower strength than needed. 3.1.1.1 Examples of deficiencies in vertical formwork— Construction deficiencies sometimes found in vertical formwork include: • Failure to control rate of placing concrete vertically without regard to design parameters; • Inadequately tightened or secured form ties or hardware; • Form damage in excavations resulting from embankment failure; • Use of external vibrators on forms not designed for their use; • Deep vibrator penetration of earlier semihardened lifts; • Improper framing of blockouts; • Improperly located or constructed pouring pockets; • Inadequate bulkheads; Improperly anchored top forms on a sloping face; Failure to provide adequate support for lateral pressures on formwork; and • Failure to provide adequate bracing resulting in attempts to plumb forms against concrete pressure force. 3.1.1.2 Examples of deficiencies in horizontal formwork—Construction deficiencies sometimes found in horizontal forms for elevated structures include: • Failure to properly regulate the rate and sequence of placing concrete horizontally to avoid unanticipated loadings on the formwork; • Shoring not plumb, thus inducing lateral loading and reducing vertical load capacity; • Locking devices on metal shoring not locked, inoperative, or missing. Safety nails missing on adjustable twopiece wood shores; • Failure to account for vibration from adjacent moving loads or load carriers; • Inadequately tightened or secured shore hardware or wedges; • Loosening or premature removal of reshores or backshores under floors below; • Premature removal of supports, especially under cantilevered sections; • Inadequate bearing area or unsuitable soil under mudsills (Fig. 3.1); • Mudsills placed on frozen ground subject to thawing; • Connection of shores to joists, stringers, or wales that are inadequate to resist uplift or torsion at joints (refer to Fig. 3.2); • Failure to consider effects of load transfer that can occur during post-tensioning (refer to Section 3.8.7); and • Inadequate shoring and bracing of composite construction. 3.2—Construction practices and workmanship 3.2.1—Fabrication and assembly details 3.2.1.1—Studs, wales, or shores should be properly spliced. 3.2.1.2—Joints or splices in sheathing, plywood panels, and bracing should be staggered. 3.2.1.3—Shores should be installed plumb and with adequate bearing and bracing. 3.2.1.4—Specified size and capacity of form ties or clamps should be used. 3.2.1.5—All form ties or clamps should be installed and properly tightened as specified. All threads should fully engage the nut or coupling. A double nut may be required to develop the full capacity of the tie. 3.2.1.6—Forms should be sufficiently tight to prevent loss of mortar from the concrete. 3.2.1.7—Access holes may be necessary in wall forms or other high, narrow forms to facilitate concrete placement. 3.2.2—Joints in the concrete 3.2.2.1—Contraction joints, expansion joints, control joints, construction joints, and isolation joints should be installed as specified in the contract documents (refer to Fig. 3.3) or as requested by the contractor and approved by the engineer/architect. GUIDE TO FORMWORK FOR CONCRETE 347-11 Fig. 3.3—Forming and shoring restraints at construction joints in supported slabs. 3.2.2.2—Bulkheads for joints should preferably be made by splitting the bulkhead along the lines of reinforcement passing through the bulkhead. By doing this, each portion can be positioned and removed separately. When required on the engineer/architect’s plans, beveled inserts at control joints should be left undisturbed when forms are stripped and removed only after the concrete has been sufficiently cured. Wood strips inserted for architectural treatment should be kerfed to permit swelling without causing pressure on the concrete. 3.2.3 Sloping surfaces—Sloped surfaces steeper than 1.5 horizontal to 1 vertical should be provided with a top form to hold the shape of the concrete during placement, unless it can be demonstrated that the top forms can be omitted. 3.2.4 Inspection—The inspection should be performed by a person certified as an ACI Concrete Construction Inspector or a person having equivalent formwork training and knowledge. 3.2.4.1—Forms should be inspected and checked before the reinforcing steel is placed to confirm that the dimensions and the location of the concrete members will conform to the structural plans. 3.2.4.2—Blockouts, inserts, sleeves, anchors, and other embedded items should be properly identified, positioned, and secured. 3.2.4.3—Formwork should be checked for camber when specified in the contract documents or shown on the formwork drawings. 3.2.5—Cleanup and coatings 3.2.5.1—Forms should be thoroughly cleaned of all dirt, mortar, and foreign matter and coated with a release agent before each use. Where the bottom of the form is inaccessible from within, access panels should be provided to permit thorough removal of extraneous material before placing concrete. If surface appearance is important, forms should not be reused if damage from previous use would cause impairment to concrete surfaces. 3.2.5.2—Form coatings should be applied before placing of reinforcing steel and should not be used in such quantities as to run onto bars or concrete construction joints. 3.2.6—Construction operations on the formwork 3.2.6.1—Building materials, including concrete, should not be dropped or piled on the formwork in such a manner as to damage or overload it. 3.2.6.2—Runways for moving equipment should be provided with struts or legs as required and should be supported directly on the formwork or structural member. They should not bear on or be supported by the reinforcing steel unless special bar supports are provided. The formwork should be suitable for the support of such runways without significant deflections, vibrations, or lateral movements. 3.2.7 Loading new slabs—Overloading of new slabs by temporary material stockpiling or by early application of permanent loads should be avoided. Loads, such as aggregate, lumber, reinforcing steel, masonry, or machinery should not be placed on new construction in such a manner as to damage or overload it. 3.3—Tolerances Tolerance is a permissible variation from lines, grades, or dimensions given in contract documents. Suggested tolerances for concrete structures can be found in ACI 117. The contractor should set and maintain concrete forms, including any specified camber, to ensure completed work is within the tolerance limits. 3.3.1 Recommendations for engineer/architect and contractor—Tolerances should be specified by the engineer/ architect so that the contractor will know precisely what is required and can design and maintain the formwork accordingly. Specifying tolerances more exacting than needed can increase construction costs. Contractors should be required to establish and maintain control points and benchmarks in an undisturbed condition until final completion and acceptance of a project. Both should be adequate for the contractor’s use and for reference to establish tolerances. This requirement can become even more important for the contractor’s protection when tolerances are not specified or shown. The engineer/architect should specify tolerances or require performance appropriate to the type of construction. Specifying tolerances more stringent than commonly obtained for a specific type of construction should be avoided, as this usually results in disputes among the parties involved. For example, specifying permitted irregularities more stringent than those allowed for a Class C surface (Table 3.1) is incompatible with most concrete oneway joist construction techniques. Where a project involves features sensitive to the cumulative effect of tolerances on 347-12 ACI STANDARD Fig. 3.4—Reshore installation. Improper positioning of shore from floor to floor can create bending stresses for which the slab was not designed. Table 3.1—Permitted abrupt or gradual irregularities in formed surfaces as measured within a 5 ft (1.5 m) length with a straightedge Class of surface A B C D 1/8 in. (3 mm) 1/4 in. (6 mm) 1/2 in. (13 mm) 1 in. (25 mm) individual portions, the engineer/architect should anticipate and provide for this effect by setting a cumulative tolerance. Where a particular situation involves several types of generally accepted tolerances on items such as concrete, location of reinforcement, and fabrication of reinforcement, which become mutually incompatible, the engineer/architect should anticipate the difficulty and specify special tolerances or indicate that governs. The project specifications should clearly state that a permitted variation in one part of the construction or in one section of the specifications should not be construed as permitting violation of the more stringent requirements for any other part of the construction or in any other such specification section. The engineer/architect should be responsible for coordinating the tolerances for concrete work with the tolerance requirements of other trades whose work adjoins the concrete construction. For example, the connection detail for a building’s façade should accommodate the tolerance range for the lateral alignment and elevation of the perimeter concrete member. 3.4—Irregularities in formed surfaces This section provides a way of evaluating surface variations due to forming quality but is not intended for evaluation of surface defects, such as bugholes (blowholes) and honeycomb, attributable to placing and consolidation deficiencies. The latter are more fully explained by ACI 309.2R. Allowable irregularities are designated either abrupt or gradual. Offsets and fins resulting from displaced, mismatched, or misplaced forms, sheathing, or liners, or from defects in forming materials are considered abrupt irregularities. Irregularities resulting from warping and similar uniform variations from planeness or true curvature are considered gradual irregularities. Gradual irregularities should be checked with a straightedge for plane surfaces or a shaped template for curved or warped surfaces. In measuring irregularities, the straightedge or template can be placed anywhere on the surface in any direction. Four classes of formed surface are defined in Table 3.1. The engineer/architect should indicate which class is required for the work being specified or indicate other irregularity limits where needed, or the concrete surface tolerances as specified in ACI 301 should be followed. Class A is suggested for surfaces prominently exposed to public view where appearance is of special importance. Class B is intended for coarse-textured, concrete-formed surfaces intended to receive plaster, stucco, or wainscoting. Class C is a general standard for permanently exposed surfaces where other finishes are not specified. Class D is a minimum-quality requirement for surfaces where roughness is not objectionable, usually applied where surfaces will be permanently concealed. Special limits on irregularities can be needed for surfaces continuously exposed to flowing water, drainage, or exposure. If permitted irregularities are different from those given in Table 3.1, they should be specified by the engineer/architect. 3.5—Shoring and centering 3.5.1 Shoring—Shoring should be supported on satisfactory foundations, such as spread footings, mudsills, or piling, as discussed in Section 2.7. Shoring resting on intermediate slabs or other construction already in place need not be located directly above shores or reshores below, unless the slab thickness and the location of its reinforcement are inadequate to take the reversal of stresses and punching shear. The reversal of stresses results from the reversal of bending moments in the slab over the shore or reshore below as shown in Fig. 3.4. Where the conditions are questionable, the shoring location should be approved by the engineer/architect. If reshores do not align with the shores above, then calculate for reversal stresses. Generally, the dead load stresses are sufficient to compensate GUIDE TO FORMWORK FOR CONCRETE for reversal stresses caused by reshores. Reshores should be prevented from falling. All members should be straight and true without twists or bends. Special attention should be given to beam and slab or one- and two-way joist construction to prevent local overloading when a heavily loaded shore rests on the thin slab. Multitier shoring, single-post shoring in two or more tiers, is a dangerous practice and is not recommended. Where a slab load is supported on one side of the beam only (refer to Fig. 2.1), edge beam forms should be carefully planned to prevent tipping of the beam due to unequal loading. Vertical shores should be erected so that they cannot tilt and should have a firm bearing. Inclined shores should be braced securely against slipping or sliding. The bearing ends of shores should be square. Connections of shore heads to other framing should be adequate to prevent the shores from falling out when reversed bending causes upward deflection of the forms (refer to Fig. 3.2). 3.5.2 Centering—When centering is used, lowering is generally accomplished by the use of sand boxes, jacks, or wedges beneath the supporting members. For the special problems associated with the construction of centering for folded plates, thin shells, and long-span roof structures, refer to Section 6.4. 3.5.3 Shoring for composite action between previously erected steel or concrete framing and cast-in-place concrete—Refer to Section 6.3. 3.6—Inspection and adjustment of formwork Helpful information about forms before, during, and after concreting can be found in Reference 1.3 and ACI 311.1R. 3.6.1—Before concreting 3.6.1.1—Telltale devices should be installed on shores or forms to detect formwork movements during concreting. 3.6.1.2—Wedges used for final alignment before concrete placement should be secured in position before the final check. 3.6.1.3—Formwork should be anchored to the shores below so that movement of any part of the formwork system will be prevented during concreting. 3.6.1.4—Additional elevation of formwork should be provided to allow for closure of form joints, settlements of mudsills, shrinkage of lumber, and elastic shortening and dead load deflections of form members. 3.6.1.5—Positive means of adjustment (wedges or jacks) should be provided to permit realignment or readjustment of shores if settlement occurs. 3.6.2 During and after concreting—During and after concreting, but before initial set of the concrete, the elevations, camber, and plumbness of formwork systems should be checked using telltale devices. Formwork should be continuously watched so that any corrective measures found necessary can be promptly made. Form watchers should always work under safe conditions and establish in advance a method of communication with placing crews in case of emergency. 347-13 3.7—Removal of forms and supports 3.7.1 Discussion—Although the contractor is generally responsible for design, construction, and safety of formwork, criteria for removal of forms or shores should be specified by the engineer/architect. 3.7.2—Recommendations 3.7.2.1—The engineer/architect should specify the minimum strength of the concrete to be attained before removal of forms or shores. The strength can be determined by tests on job-cured specimens or on in-place concrete. Other concrete tests or procedures (refer to ACI 228.1R) can be used such as the maturity method, rebound numbers, penetration resistance, or pullout tests, but these methods should be correlated to the actual concrete mixture used in the project, periodically verified by job-cured specimens, and approved by the engineer/architect. The engineer/architect should specify who will make the specimens and who will make the tests. Results of such tests, as well as records of weather conditions and other pertinent information, should be recorded by the contractor. Depending on the circumstances, a minimum elapsed time after concrete placement can be established for removal of the formwork. Determination of the time of form removal should be based on the resulting effect on the concrete.* When forms are stripped there should be no excessive deflection or distortion and no evidence of damage to the concrete due to either removal of support or to the stripping operation (Fig. 3.5). When forms are removed before the specified curing is completed, measures should be taken to continue the curing and provide adequate thermal protection for the concrete. Supporting forms and shores should not be removed from beams, floors, and walls until these structural units are strong enough to carry their own weight and any approved superimposed load. In no case should supporting forms and shores be removed from horizontal members before the concrete has achieved the strength specified by the engineer/architect. As a general rule, the forms for columns and piers can be removed before forms for beams and slabs. Formwork and shoring should be constructed so each can be easily and safely removed without impact or shock and permit the concrete to carry its share of the load gradually and uniformly. 3.7.2.2—The removal of forms, supports, and protective enclosures, and the discontinuance of heating and curing should follow the requirements of the contract documents. When standard beam or cylinder tests are used to determine stripping times, test specimens should be cured under conditions that are not more favorable than the most unfavorable conditions for the concrete the test specimens represent. The curing records can serve as the basis on which the engineer/architect will determine the review or approval of form stripping. 3.7.2.3—Because the minimum stripping time is a function of concrete strength, the preferred method of determining stripping time is using tests of job-cured cylinders or concrete in place. When the contract documents do not *Helpful information on strength development of concrete under varying conditions of temperature and with various admixtures can be found in ACI 305R and ACI 306R. 347-14 ACI STANDARD Fig. 3.5—Stripping sequence for two-way slabs. specify the minimum strength required of concrete at the time of stripping, however, the following elapsed times can be used. The times shown represent a cumulative number of days, or hours, not necessarily consecutive, during which the temperature of the air surrounding the concrete is above 50 °F (10 °C). If high early-strength concrete is used, these periods can be reduced as approved by the engineer/architect. Conversely, if ambient temperatures remain below 50 °F (10 °C), or if retarding agents are used, then these periods should be increased at the discretion of the engineer/architect. Shorter stripping times listed for live load to dead load ratios greater than 1.0 are the result of more reserve strength being available for dead load in absence of live load at time of stripping. Walls* ............................................................................. 12 h Columns* ........................................................................ 12 h Sides of beams and girders* ........................................... 12 h Pan joist forms† 30 in. (760 mm) wide or less ................................... 3 days Over 30 in. (760 mm) wide ..................................... 4 days Structural live Structural live load less load more than structural than structural dead load dead load Arch centers ....................................14 days Joist, beam or girder soffits Under 10 ft (3 m) clear span between structural supports ......... 7 days‡ 7 days 4 days *Where such forms also support formwork for slab or beam soffits, the removal times of the latter should govern. †Of the type that can be removed without disturbing forming or shoring. ‡Where forms can be removed without disturbing shores, use half of values shown but not less than 3 days. 10 to 20 ft (3 to 6 m) clear span between structural supports .......14 days‡ 7 days Over 20 ft (6 m) clear span between structural supports .......21 days‡ 14 days One-way floor slabs Under 10 ft (3 m) clear span between structural supports .........4 days‡ 3 days 10 to 20 ft (3 to 6 m) clear span between structural supports .........7 days‡ 4 days Over 20 ft (6 m) clear span between structural supports .......10 days‡ 7 days Two-way slab systems§........Removal times are contingent on reshores where required, being placed as soon as practicable after stripping operations are complete but not later than the end of the working day in which stripping occurs. Where reshores are required to implement early stripping while minimizing sag or creep (rather than for distribution of superimposed construction loads as covered in Section 3.8), capacity and spacing of such reshores should be designed by the formwork engineer/contractor and reviewed by the engineer/architect. Post-tensioned slab system§........As soon as full posttensioning has been applied. 3.8—Shoring and reshoring of multistory structures 3.8.1 Discussion—The following definitions apply for purposes of this discussion: shores—vertical or inclined support members designed to carry the weight of formwork, concrete, and construction loads. reshores—shores placed snugly under a stripped concrete slab or structural member after the original forms and shores have been removed from a large area. This requires the new ‡Where forms can be removed without disturbing shores, use half of values shown but not less than 3 days. §Refer to Section 3.8 for special conditions affecting number of floors to remain shored or reshored. GUIDE TO FORMWORK FOR CONCRETE slab or structural member to deflect and support its own weight and existing construction loads applied before the installation of the reshores. It is assumed that the reshores carry no load at the time of installation. Afterward, additional construction loads will be distributed among all members connected by reshores. Multistory work represents special conditions, particularly in relation to the removal of forms and shores. Reuse of form material and shores is an obvious economy. Furthermore, the speed of construction in this type of work permits other trades to follow concreting operations from floor to floor as closely as possible. The shoring that supports freshly placed and low-strength early-age concrete, however, is supported by lower floors that were not originally designed specifically for these loads. The loads imposed must not exceed the safe capacity of each floor providing support. For this reason, shoring or reshoring should be provided for a sufficient number of floors to distribute the imposed construction loads to several slab levels without causing excessive stresses, excessive slab deflections, or both.1.3,2.8,2.9,2.10 Reshoring is used to distribute construction loads to the lower floors. In a common method of analysis, while reshoring remains in place at grade level, each level of reshores carries the weight of only the new slab plus other construction live loads. The weight of intermediate slabs is not included because each slab carries its own weight before reshores are put in place. Once the tier of reshores in contact with grade has been removed, the assumption is made that the system of slabs behaves elastically. The slabs interconnected by reshores will deflect, equally during addition or removal of loads. Loads will be distributed among the slabs in proportion to their developed stiffness. The deflection of concrete slabs can be considered elastic, that is, neglecting shrinkage and creep. Caution should also be taken when a wood compressible system is used. Such systems tend to shift most of the imposed construction loads to the upper floors, which have less strength. Addition or removal of loads may be due to construction activity or to removing shores or reshores in the system. Shore loads are determined by equilibrium of forces at each floor level. 3.8.2 Advantages of reshoring—Stripping formwork is more economical if all the material can be removed at the same time and moved from the area before placing reshores. Slabs are allowed to support their own weight, reducing the load in the reshores. Combination of shores and reshores usually requires fewer levels of interconnected slabs, thus freeing more areas for other trades. 3.8.3 Other methods—Other methods of supporting new construction are less widely used and involve leaving the original shores in place or replacing them individually (backshoring and preshoring), which prevents the slab from deflecting and carrying its own weight. These methods are not recommended unless performed under careful supervision by the formwork engineer/contractor and with review by the engineer/architect because excessively high slab and shore stresses can develop. 3.8.4 Design—Refer to Chapter 2. 347-15 3.8.5 Placing reshores—When used in this section, the word shore refers to either reshores or the original shores. Reshoring is one of the most critical operations in formwork; consequently, the procedure should be planned in advance by the formwork engineer/contractor and should be reviewed or approved by the engineer/architect. Operations should be performed so that areas of new construction will not be required to support combined dead and construction loads in excess of their capability, as determined by design load and developed concrete strength at the time of stripping and reshoring. Shores should not be located so as to alter the pattern of stress determined in the structural analysis or induce tensile stresses where reinforcing bars are not provided. Size and number of shores, and bracing, if required, should provide a supporting system capable of carrying any loads that could be imposed on it. Where possible, shores should be located in the same position on each floor so that they will be continuous in their support from floor to floor. When shores above are not directly over shores below, an analysis should be made to determine whether or not detrimental stresses are produced in the slab. This condition seldom occurs in reshoring because the bending stresses normally caused by the offset reshores are not large enough to overcome the stress resulting from the slab carrying its own dead load. Where slabs are designed for light live loads or on long spans where the loads on the shores are heavy, care should be used in placing the shores so that the loads on the shores do not cause excessive punching shear or bending stress in the slab. While reshoring is under way, no construction loads should be permitted on the new construction unless the new construction can safely support the construction loads. When placing reshores, care should be taken not to preload the lower floor and not to remove the normal deflection of the slab above. The reshore is simply a strut and should be tightened only to the extent necessary to achieve good bearing contact without transferring load between upper and lower floors. 3.8.6 Removal of reshoring—Shores should not be removed until the supported slab or member has attained sufficient strength to support itself and all applied loads. Removal operations should be carried out in accordance with a planned sequence so that the structure supported is not subject to impact or loading eccentricities. 3.8.7 Post-tensioning effects on shoring and reshoring— The design and placement of shores and reshores for post-tensioned construction requires more consideration than for normal reinforced concrete. The stressing of posttensioning tendons can cause overloads to occur in shores, reshores, or other temporary supports. The stressing sequence has the greatest effect. When a slab is post-tensioned, the force in the tendon produces a downward load at the beam. If the beam is shored, the shoring should carry this added load. The magnitude of the load can approach the dead load of 1/2 the slab span on both sides of the beam. If the floor slab is tensioned before the supporting beams and girders, a careful 347-16 ACI STANDARD analysis of the load transfer to the beam or girder shores or reshores will be required. Similar load transfer situations occur in post-tensioned bridge construction.