508 Falsework and Forms

Documentation Requirements 508 Falsework and Forms

Location

Prior to the erection of the forms for each substructure unit, the inspector should satisfy himself that the Contractor is placing the forms in the correct location. This should be accomplished by available methods that do not require the use of instruments.

Types and Use

Footing concrete may be placed against rock, hard shale, or sheeting. All other concrete must be placed in substantial forms that are designed and constructed so that finished concrete will conform to plan lines and dimensions and will have a satisfactory surface. Forms for exposed surfaces are to be made of acceptable materials that will produce a smooth surface with a minimum number of joints. Acceptable materials include sheet plywood, fabricated metal forms, fabricated metal frames with plywood inserts, or dressed lumber of uniform thickness with a form liner of plywood, hardboard, or sheet metal.

Form lumber that has had many uses, and bent metal forms that will not produce an acceptable surface on concrete when stripped, regardless of finish specified, are to be rejected. Exercise care to obtain as flush a fit as possible at panel joints. When rustication grooves are required, panel joints should, if possible, be made to coincide.

The underside of a deck that cantilevers out from the fascia beam is considered an exposed surface and requires forms with smooth surfaces. The underside of pier caps is considered an exposed surface and forms with smooth surfaces should be used and cut to fit neatly around columns or piles.

The inside of all forms are to be coated with a bond-breaker. If the forms are not so coated and oiling is necessary, it should be done before placing the reinforcing steel or preferably before assembly of the forms.

Design

Forms must be adequately braced and provided with walers and form ties that are properly designed to maintain the proper dimension and alignment for the proposed height and rate of concrete placement. Some suppliers of form ties specify the height of concrete in feet (meters) per hour that can be placed for their design. All form ties and anchor bolts used for form support must be designed for removal of 2 inches (50 mm) in from the exposed surfaces of concrete.

Incidental Work

Moldings for the 3/4-inch (19 mm) beveled edges and rustification grooves must be surfaced on all sides and be of uniform section. The bevel strip should be nailed at sufficient intervals to completely fill a corner or contact the form for the full length. Rustification strips are fastened to the forms in such a manner that the molding will remain in contact with the concrete when the forms are stripped, and will not be removed until the concrete has set sufficiently to avoid damage.

Weep holes through abutments and retaining walls are formed in such a manner as to obtain a smooth circular opening. To form the hole, metal such as downspouts or sonotube may be used and later removed, or noncorrodible rigid plastic pipe may be used and left in place, provided the gradient and inside diameter are in accordance with 508.03.

All scrap wood, dirt, and other foreign material, including ponded water, must be removed from within the forms prior to placing concrete. If the forms are too deep or narrow to permit easy removal of foreign material from the top, a temporary opening should be left at the bottom for removal of foreign material. An opening must also be provided when necessary for inspection. Temporary openings must be made mortar tight after the forms have been cleaned and inspected.

An inspection should be made of the forms for proper fit and holes where leakage of cement paste may occur. Openings must be corrected in such a manner as to close the hole and provide a smooth form surface. Filler strips, plugs, and tin commonly are used to plug such openings. Forms should be watched closely during the placing of the concrete and any leaks must be corrected immediately.

Verification of Dimensions

Before any concrete is placed, form dimensions should be measured for compliance with the plan requirements and approved change orders. Measurements that will result in concrete equal to or greater than plan dimensions are considered verified plan dimensions. The measurements must be checked for compliance with the plan dimensions and then recorded and filed in the project records. A statement that the dimensions have been checked and are in compliance with plan requirements is not acceptable verification. The recording may consist of any of the three following methods:

1.        A tabulation of all the verified plan dimensions for simple shapes

2.        A sketch on an appropriate form showing all of the verified plan dimensions

3.        The plan sheet for the structure unit with the verified dimensions checked thereon

Whatever method is used, the Inspector should date and sign the sheet. If checks are made on different days, dates should indicate the day each check was made. If different inspectors check parts of the measurements, each should initial those checks that he has made.

If measurements are not in compliance, make correction and recheck the dimensions before the concrete may be placed.

 

Description of Falsework (508.02)

Falsework is the system of temporary support of formwork for concrete members. The falsework is to remain in place until the concrete members have attained required strength and are self-supporting. This includes the system of supporting formwork for deck slabs and pier caps.

Falsework Plans

For cast-in-place concrete slab bridges, the Contractor must submit a falsework plan per 501.05.B.3. No superstructure concrete can be placed until the plan is received and the falsework conforms to the submitted plans. The Contractor may substitute elements of equal or greater strength if it does not involve a change in depth that effects elevations. Any other deviations from the accepted plan that the Contractor desires or become necessary due to unforeseen conditions must be covered by submission of a revised plan.

District Review of Falsework Plans

Although ODOT of acceptance of falsework plans in not required, a review should be made at the project to ascertain that the existing conditions shown in the plan are representative of those found in the field.

Falsework Camber

The maximum deflection that is permitted in the falsework of a slab bridge is specified in 508.02. Camber equal to this deflection must be built into the falsework to compensate for falsework deflection. In addition, camber equal to 1/800th of the span must be built into the falsework to compensate for deflection of the slab after falsework is released. Also camber to conform to the vertical curvature of the profile grade must be provided.

If unusual requirements for span of an existing road or channel or restrictions due to vertical clearance exist, contact the Office of Structural Engineering to evaluate acceptable site specific camber requirements.

Falsework Materials

Falsework members must be of the section and length shown on the submitted plans. Members having a greater section modulus may be used; but, if this involved a change in depth and affects elevations, details of modifications should be included on a resubmission of the effected plan.

Steel members such as stringers must be in good condition. They must not show loss of section through rusting, excessive weldments, or holes that would affect their strength.

Timber shall be sound and of the required size. Used timber that shows deterioration and stress cracks may not perform its function and must not be used.

Piling and Posts

Piling must be driven to the bearing called for on the submitted plans. In order to determine the required blow count, it will be necessary for the Contractor to perform dynamic load testing.

Consolidation of Wood

Allowance for consolidation of wood wedges and blocking must be provided. Using rough-cut timber, an allowance of 1/16 inch (2 mm) for each contact surface generally will be necessary.

Independent Support

Where phased construction or adjacent concrete decks are separated by an open joint or closure pour, forms for the cantilevered edges of each slab must be supported independently from the adjacent structure. This is necessary to avoid movement of the forms due to differential deflections during placing of the concrete.

The finishing machine must also be supported by the structure on which the concrete is being placed and independent of any adjacent structure or support. If it is not , the finishing machine will not move with the deck as the concrete is placed and can result in areas where the superstructure concrete is either too thick or too thin.

Closure Pour

Closure pours are normally specified during phased construction when the cross bracing or diaphragms between the phases are not in place prior to the placement of the superstructure concrete. A closure pour is not to eliminate traffic vibration, but to allow differential deflection to take place between the phases when the superstructure concrete is placed. In order to properly place the superstructure concrete, the closure pour should not be waived unless the deadload deflection that occurs when the superstructure concrete is placed is less than inch.

Superimposed Concrete

Prior to placing sidewalks, safety curbs, or other superimposed concrete on the deck of a slab bridge, the falsework must be removed or released, and allowed to deflect.

Removal of Falsework

Falsework may be removed when the conditions tabulated in the table of section 511.17-1 of the Construction And Material Specifications have been met, unless QC/QA is being used. If QC/QA concrete is being used follow SS898. Any piling not removed must be cut off at least to the slope line or rip rap line of the bed of stream.

Documentation Requirements - 508 Falsework and Forms

1.        Received falsework plan submitted per 501.05.B.3 for slab deck bridges

2.        Falsework constructed to approved drawings

3.        Document bearing obtained and number of falsework piling

4.        Number and size of bracing on falsework

5.        Protection during cold weather

6.        Forms oiled prior to steel placement