Permanent Formworks (Precast)

 

  Rod McClelland,

Current Practice Sheet No. 12

PERM A NENT   FORMWORK   FOR  C COMPOSITE  B B RIDGE  D DECK S Introduction Permanent formwork, unlike traditional formwork, is left in place for the life of the element it is supporting. This system has been used successfully in many areas of concrete construction; on composite bridge decks it  provides a permanent permanent soffit between the main support support beams. There are two types of permanent formwork used in composite decks:

  Structurally participating, which is designed to provide the temporary

o

support for the wet concrete and construction loads and then become  part of the permanent works contributing to the strength of the completed element.

  Structurally non-participating, which is designed solely to support the

o

wet concrete and the construction loads.

Consultant (formerly Alfred McAlpine Capital Projects) 

 

The twelfth in a series of Current Practice Sheets prepared by the Concrete  Bridge Development Group

Structurally participating formwork The most common system consists of a precast concrete unit, generally 300mm wide, which incorporates a lattice girder (see Figure 1). The girder reinforcement provides strength to the unit in its temporary condition and also contributes to the strength of the finished deck. The units are normally designed to provide a maximum clear span of 3.65m. During erection, the ends of the units are bedded onto the main beams using a bituminised compressible strip, or other as approved by the supplier. Sealing between the units is also essential using either a fillet of mortar, adhesive tape or a proprietary sealant. Figure 2 shows the units being placed using a purpose-made frame to speed up the erection process. Rates of 2.5m2/per hour have been achieved for placing and sealing these units.

non-partici pating Structurally non-partici There are two systems generally used for non-participating formwork, namely glass fibre reinforced plastic (GRP) and glass fibre reinforced cement (GRC). Panels made from GRP have metal bar stiffeners encapsulated in the material to improve the stiffness (see Figure 3) and are designed to cater for spans up to 4m. Advice and information regarding the maximum spans and the predicted deflections is provided by the manufacturer. The length and width of the unit is limited by weight, to enable two men to manhandle a  panel into position. The joints between panels and main beams should be sealed in accordance with the manufacturer’s instructions as recommendations can vary depending on the type of main beam material used, i.e. concrete or steel. Placing rates of 10-20m2 per man hour have been achieved using this system.

Figure 1: Lattice girders in place.

Bene Benefits fits of permanent permanent for mwork The use of permanent formwork has the following benefits:

  eliminates the need for falsework

o

  reduces the need for site skill levels

o

  increases the potential for standardisation

o

  speeds up fixing time and eliminates stripping

o

  allows early access for following operations.

o

Specific benefits for the construction of bridge decks include:

  Provision of a safe working platform early in the deck construction

o

 process, which is a major advantage when the bridge is spanning a rail track, river or live carriageway.

  Elimination of the need to strip formwork in difficult and confined

o

spaces when working at height.

Figure 2: Purpose-made lifting frame for lattice girder units.

 

  GRC panels are available in the form of flat sheets for spans up to approximately 800mm and profiled sheets to cater for spans up to 1.2m. An example of the profiled sheet is shown in Figure 4. Where greater spans are required using these profiles, a temporary support system can be used. However, the introduction of a support system will obviously reduce many of the benefits gained by using the permanent formwork.

The GRC panel, although cement-based, is made from a combination of materials that provide a dense surface with good resistance to the ingress of salts and should not normally require further protection.

Panels capable of larger spans without temporary support can be manufactured in GRC but they are of multi-skin construction. However, the Highways Agency does not currently permit the use of this type of panel on  bridge decks because because of problems with delam delamination. ination.

Highways Agency r equireme equirements nts Advice note BA 36/90(1) gives guidance on the use of permanent formwork in the construction of bridge decks. Information on the use and limitations of the various types are provided and examples of the systems are shown. The advice note also stresses the importance of durability when considering the type of material to be used. Figure 4: GRC panels. (Courtesy of BCM GRC Ltd)

Durability The precast concrete system has to satisfy Highways Agency requirements that any cement based materials that are likely to be affected by de-icing salts are to be protected in accordance with BD 43/03 (2). The advice note recommends that a hydrophobic pore-lining impregnant should be applied to the surface of the concrete and currently monomeric alkyl (isobutyl) trialkoxy silane is recommended. The silane is sprayed onto the underside of the deck and the process can be slow and in some cases difficult to apply. In order to eliminate this spray process, trials have been carried out using a corrosion inhibitor that is included in the concrete mix. These inhibitors would be too expensive to use in thick concrete sections but are ideal for thin precast formwork panels.

emarks rks Concluding r ema There is no doubt that the use of permanent formwork on composite bridge decks can provide major benefits in the form of speed, economy and health and safety. Studies have shown that, taking into account all the relevant factors between  precast concrete and GRP on the larger spans, the total costs are similar. Therefore, the choice could depend on the particular benefits of each system, e.g., is a crane available, is erection time critical, can the reinforcement be simplified, is need a concrete soffit preferred, are minimum  panel joints preferred, do panels to be cut on on site, etc. The span of the GRC panel is limited and therefore the costs cannot be compared with the other two systems on longer spans. However, GRC should be considered for spans up to 1.2m and may be the most economical solution where close spaced beams are used.

References: 1. HIGHWAYS AGENCY. BA 36/90: The use of permanent formwork . 1991. 2. HIGHWAYS AGENCY. BD 43/03: The impregnation of reinforced and prestressed concrete highway structures using hydrophobic pore-lining impregnation. 2003

Further Rea Reading ding CONCRETE BRIDGE DEVELOPMENT GROUP. Fast construction of concrete bridges. Technical Guide No 5, 2005.

Figure 3: GRP panels with steel beams.

The GRP panel is impermeable and as long as the ends and joints are sealed  properly the ingress of harmful de-icing salts should be prevented. The application of a silane type material should not be necessary for this system.

CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION.  Bridges – design for improved buildability. Report 155, 1996. CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION.  Bridge detailing guide. Report C543, 2001. CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION. Permanent  formwork in construction. Report C558, 2001. BRITISH STANDARDS INSTITUTION. BS 5400: Steel, concrete and composite bridges, Part 5: Code of practice for the design of composite bridges. 2005.