Mp4 to wmv online free converter hangers should closely fit the flange or bearing surface of the supporting member so that applied loads are transmitted properly. Using horizontal ties will leave the vertical component of planned to prevent any concentrated reaction on any part of pressure untied. To ensure that the is most often controlled by replacement of cement with structure can assume its deflected shape without damage, the pozzolans, particularly fly ash.">
Tilt-up and precast during manufacture or applied to the form before each use. Concrete When applying in the field, be careful to avoid coating adjacent receiving coatings or plasters that hide the surface color and construction joint surfaces or reinforcing steel.
Particular care should be taken in the selection of listed in Sections 5. On major work, this is materials, design, and construction of the formwork, and frequently achieved by specifying a preconstruction mockup placing and consolidation of the concrete to eliminate bulges, prepared and finished by the contractor for approval by the offsets, or other unsightly features in the finished surface and architect, using proposed form materials, jointing tech- to maintain the integrity of the surface texture or configura- niques, and form surface treatments, such as wetting, oiling, tion.
The character of the concrete surface to be produced or lacquering. Once such a mockup has been completed to should also be considered when the form materials are the satisfaction of the architect, it remains at the site for the selected. Special attention should be given to closure tech- duration of the work as a standard with which the rest of the niques, concealment of joints in formwork materials, and to work should comply.
Design reference samples, which are smaller specimens of 5. Small samples like in concrete surfaces. They start at the design stage and carry these, kept at the job site for reference, are not as good as a through to the completed project. Factors affecting the full-scale mockup but can be helpful. The samples should be concrete can also include the mixture proportions or aggregate, large enough to adequately represent the surface of the the method of placing the concrete, the consolidation tech- concrete.
If the samples are to be used as a basis for accep- nique, and the curing procedure. Chemicals can have an tance, several should be made to represent the variation that effect on the final product, whether used as additives in the can occur in the finish. Even after the structure is completed, weather and air the concrete surfaces will be evaluated for compliance with pollution will affect the appearance of the concrete.
These the specifications. The single tolerances considered essential to successful execution of the most important factor for the success of an architectural design.
ACI can be consulted, but the architect should concrete job is good workmanship. The architectural work. No numerical limits are suggested herein best way for the contractor to achieve this uniformity is to be because the texture, lighting, and configuration of surfaces will consistent in all construction practices.
Forming materials all have an influence. Placement and consolidation of ACI are considered the achievable limit. Stripping and curing sequences should be kept compensate for deflection of the formwork during concrete constant throughout the work to control color variations. The architect should, however, specify any additional camber required to compensate for structural 5.
The architect should be structural, but surface quality generally desired for architec- aware that horizontal members are checked for compliance tural concrete is higher than what is typically satisfactory for with tolerances and camber before the removal of the forms structural concrete, and is more costly. The architect can use and shores. The architect can make form reuse possible by stan- drawings. Some guidance on joint locations can be found in dardizing building elements, such as columns, beams, and ACI R, R, and R.
Because it is impossible to windows, and by making uninterrupted form areas the same disguise the presence of joints in the form face, it is important size wherever possible to facilitate the use of standard form for their positions to be predetermined and, if possible, gangs or modules. The increased size of these uninterrupted planned as part of the architectural effect.
A prebid confer- The architect can plan joint locations between surface ence with qualified contractors will bring out many practical areas on a scale and module suitable to the size of available considerations before the design is finalized.
Actual joints between sheathing reinforced plastics are all suitable liner materials when care- materials can be masked by means of rustication strips fully detailed and fabricated. Plastics should be handled and splayed fillets attached to the form face. Rustication strips assembled with care to avoid distortion caused by daily at horizontal and vertical construction joints can also create temperature cycles at the job site.
The ties should is trapped beneath horizontal rustications. Ties should be tight fitting or supporting beams instead of in the more customary regions tie holes in the form should be sealed to prevent leakage at of low shear. If textured surfaces are to be formed, 5.
Architects frequently integrate tie holes into the visual 5. If this is planned and any Section 4. Plastic, plastic-protected, rubber-tipped, or other Where tie holes are to be patched or filled, the architect noncorroding spacers should be attached to the reinforcing should specify the treatment desired, unless it has been bar so that they do not become dislodged during concrete shown on the preconstruction mockup.
The number and location of the 5. Reinforce- 5. For positive assurance of maintaining follow the principles outlined in Chapter 2. Some of these consider- There is no advantage in specifying more cover than ations are: tie spacing and size, form facing preferences, required by code because excessive cover can contribute to location and special treatment of form joints, special tolerances, increased cracking.
The architect should specify sufficient and use of admixtures. Because these factors can influence cover to allow for any reduction that will result from the form design, they should be fully reviewed at the beginning incorporation of grooves or indented details and from of the form design process. Particular care should be exercised in these cases 5. Plywood, the contact surface of the formwork reflect directly in steel, glass fiber-reinforced plastic, and aluminum can all be finished surfaces under varying light conditions, forms for suitable as sheathing or facing materials.
Select the grade or architectural concrete should be designed carefully to minimize class of material needed for pressure, framing, and deflection deflections. In most cases, deflections govern design rather requirements. Be sure that the chosen material meets the than bending flexural stress or horizontal shear. Deflec- specification requirements for the concrete surface texture.
Forms bow with reuse; therefore, more bulging enough to meet the architectural specifications. It is not required structurally. Given the to be sure the architectural effect is consistent with the capacity of the available tie and the area it supports, the form- forming method and material specified. Form material should be sized and positioned 5.
Joints members to prevent offsets. If the holes are to be exposed as part of the architec- Nailing should be done with care using hammers with tural concrete, tie placement should be symmetrical with the smooth and well-dressed heads to prevent marring of the member formed.
If tie holes are not to be exposed, ties form surfaces. When required, box nails should be used on should be located at rustication marks, control joints, or the contact surface and should be placed in a neat pattern. Wherever possible, sheathing or panel joints should be Externally braced forms can be used instead of any of the positioned at rustication strips or other embedded features aforementioned methods to avoid objectionable blemishes in that can conceal or minimize the joint.
Externally braced forms, however, can Construction joints should be formed with a grade strip be more difficult and more costly to build. Formwork should be tightened at a preceding or adjacent placements to achieve a tight fit and construction joint before the next placement to prevent prevent grout leakage at these points.
Ties should be located seepage of water between the form and previously placed as close as possible to the construction joint to facilitate concrete surfaces. Architectural concrete forms should be designed to resist 5. One method to should, where feasible, be located at the junction of the form- prevent water loss from the concrete at the joints between work panels.
At contraction or construction joints, rustication sections of the formwork and at construction joints is to strips should be provided and fastened to the face of forms. Corners should be carefully detailed to prevent grout The gasket is compressed when the formwork is assembled leakage. Sharp corners should, wherever possible, be elimi- or placed against the existing concrete. Caulk, tape, joint nated by the use of chamfer strips except when prohibited by compound, or combinations of these can be used to seal project specifications.
In all cases, unsupported joints between sheathing 5. Water-tight forms check for dimensional tolerances specified by the architect that require more care during vibration to remove entrapped air can have a bearing on the design of the forms. If no special toler- that can cause bug holes. Furthermore, the grout seal 5. Joints in structural members should be kept to a or releasing agents should be applied before reinforcing steel minimum and, where necessary, should be suitably spliced is placed and should be applied carefully to avoid contacting or otherwise constructed to maintain continuity.
No form coating Pour pockets for vibrating or placing concrete should be should be used unless it can be demonstrated not to stain the planned to facilitate careful placement and consolidation of concrete or impair the adhesion of paints or other surface the concrete to prevent segregation, honeycomb, sanding, or treatments. The location of pour pockets Form sealers should be tested to ensure that they will not should be coordinated with the architect.
Attachment of inserts, rustication strips, and ornamental Ties that are to be pulled from the wall should be coated reliefs should be planned so that forms can be removed with nonstaining bond breaker or encased in sleeves to without exerting pressure on these attachments.
Where special forming systems are specified by the engineer Forms should be carefully cleaned and repaired between of the project for structural purposes such as one- and two-way uses to prevent deterioration of the quality of surface formed. Members making up wood molds change of concrete. To avoid thermal shock and consequent should be kerfed on the back wherever such members can crazing of the concrete surface, the change in temperature of become wedged between projections in the ornament.
Molds the concrete should be controlled within the limits outlined should be constructed so that joints will not be opened by in ACI R. This can be accomplished by heating the work slight movement or swelling of the wood. Joints in the molds area, leaving the forms in place to contain the heat of hydra- should be made inconspicuous by pointing. Positive steps should be taken securely held in position to reproduce the design shown on to inspect, record, and document the procedures used to cure the plans.
Where wood forms adjoin molds, the wood should the concrete. The edge of the mold or Formwork for all structures should be designed, pattern strip should be tapered to a slight draft to permit constructed, and maintained in accordance with recommen- removing the detail material without damaging the concrete.
This section deals with the addi- Special provisions should be made for early form removal, tional requirements for formwork for several special classes retardation, or both when sandblasting, wire brushing, or of work. ACI R contains information on design and other treatments are required. Form liners should be attached securely with fasteners or 6.
The form behind 6. The surfaces should be work should be planned in advance. Forms and supports should cleaned and dried thoroughly so that the glue will bond. Do be sufficiently rigid to ensure that the finished structure will not use glue at temperatures lower than those recommended fulfill its intended structural function and that exposed concrete by the manufacturer.
In continuous structures, support should not be left as cast, it is important not to damage or scar the concrete released in any span until the first and second adjoining face during stripping. Forms should be supported so that they spans on each side have reached the specified strength. For do not fall back or against the architectural surface. The use post-tensioned bridges, the shore design should consider the of pry bars and other stripping tools should be strictly super- resulting redistribution of loads on the shores similar to the vised.
In no case should pry bars be placed directly against effects discussed in Section 3. Even the use of wood or plastic wedges does 6. Once formwork is materials but most commonly are wood or metal. They should removed, the architectural surfaces should be protected from be built mortar-tight of sound material strong enough to continuing construction operations.
This can be accomplished by adjusting the 6. If designed so that the concrete acts compositely with other concrete is not strong enough to overcome the adhesion materials or with other parts of the structure present special between the form surface and the concrete, concrete can forming problems that should be anticipated in the design of scale or spall. Therefore, a good quality surface might the structure. Requirements for shoring or other deflection require the forms to stay in place longer.
Where successive become. Where during placement and curing of the concrete, should be the objective is to produce as consistent an appearance as analyzed separately for the effects of dead load of newly placed possible, it is beneficial to protect the concrete by leaving the concrete and for the effect of other construction loads that can formwork in place somewhat longer than normal. Early be imposed before the concrete attains its design strength.
The ambient conditions can influence the designed to limit deflections to a practical minimum consistent eventual color of the concrete. The sequence of such removal should be of concrete. In members constructed in several successive placements, such as box-girder structures, formwork components should 6. Distinction should be made in that the resulting surfaces will conform to his or her design. Shoring of members that will act structure should be considered. Wedges, shims, and jacks should be provided to drawings.
Complete stress analyses should be prepared by permit adjustment if required before or during concreting, as competent structural engineers, and the maximum and well as to permit removal without jarring or impacting the minimum values of stress, including reversal of stress, completed construction.
Provision should be made for readily should be shown for each member for the most severe checking the accuracy of position and grade during placement. Consideration should be given to unsym- Even though adjustment of forms can be possible during or metrical or eccentric loadings that might occur during after placing, it is not recommended.
Keywords: anchors; architectural concrete; coatings; concrete; construction; falsework; form ties; forms; formwork; foundations; quality control; reshoring; shoring; slipform construction; specifications; tolerances. Any applicable errata are included with individual documents at the time of purchase.
Errata are not included for collections or sets of documents such as the ACI Collection. One method to prevent loss of water 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.
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. 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 pattem 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. 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. 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 AC1 R. 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 see AC1 R.
This section deals with the additional requirements for formwork for several special classes of work. AC1 R contains information on design and construction of circular prestressed-concrete structures. 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. In continuous structures, support should not be released in any span until the f i s t and second adjoining spans on each side have reached the specified strength.
They should be built mortar-tight of sound material strong enough to prevent distortion during placing and curing of the concrete. Requirements for shoring or other deflection control of the formwork should be clearly presented by the engineedarchitect 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 5. 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. 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. 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 both, 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 constniction 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 see Section 3.
Composite construction can have beams of relatively light cross section 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 engineedarchitect should alert the contractor to this problem in general notes on the structural plans or in notes on applicable plans when this condition exists. 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 impact of 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 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 sequence of such removal should be approved by the engineedarchitect. These formwork engineers should consult and cooperate with the engineedarchitect to make sure that the resulting surfaces will conform to his design.
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. 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 trussed 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 hanging up the formwork and overloading the structure during decentering.
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. 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.
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. Special provisions are usually made to control the temperature rise in the mass by the use of cement or cementitious material combinations possessing low or moderate heat-generating characteristics, by postcooling, cooling the fresh concrete, or by placing sequence.
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. High-lift formwork is strictly comparable to the single-use wood forms used extensively for structural concrete.
See Section 2. The formwork engineer needs to carefully review the concrete mixture design to determine the appropriate formula from Section 2. Concrete additives or cement substitutes can improve heat generation characteristics, but the same materials can cause retarded concrete Set-up time and increased 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 can cause high impact loads near the forms.
Using horizontal ties will leave the vertical component of pressure untied. Vertical hold down anchors are required. 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.
The embedded strength should be sufficient to sustain design loadings from the new placement and initial bolting stresses. First, concrete to fill otherwise inaccessible areas can be placed pneumatically or by positive displacement pump and pipeline. Second, rock sometimes is used as a form backing, permitting the use of rock anchors and tie rods in lieu 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, see AC1 Until more definite recommendations can be made, the magnitude and distribution of pressure should be determined by the formwork engineer.
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.
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, usually is 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. The arch form for the bulkhead method is usually fabricated into a single unit between 50 and ft 15 and 45 m long, which is stripped, moved ahead, and reerected 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 reerected using a form traveler.
Although the minimum stripping time for tunnel arch forms usually is 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.
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 usually required.
When reuse is not a factor, plywood and tongue-and-groove lumber are sometimes used for exposed surface finishes. 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 a dense 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.
Refer to AC1 The formwork engineedcontractor should be alerted to the unique problems created by high-density concrete, 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. 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. Some of these problems are reduced where mortar can be injected vertically in open top forms. 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 ftni 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. Where increased lateral pressures are expected, the workmanship and details of formwork should be of better quality than formwork for conventional concrete. 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.
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 that 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 usuaily 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.
The vertical or horizontal movement of forms can be a continuous process or a planned sequence of finite placements. Slipforms used on mass concrete structures, such as dams, should comply with the applicable provisions of Section 6. 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 They 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. Various platforms, decks, and scaffolding complete the slipform system. They provide a space for storage of concrete, reinforcing steel, and embedments, as well as serving as a working area for placing and finishing. 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 al openings and embedl ments. The slipform engineer must be experienced in the use of the exact brand of equipment to be used by the contractor, since there is significant variation in equipment between manufacturers.
For some applications, such as gastight or containment structures, additional vibration can be required to achieve maximum density of the concrete. 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, and 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 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. The level of the hardened concrete in the form should be checked frequently by the use of a probe to establish safe lifting rates.