by Katie Daniel | September 9, 2015 12:26 pm
by Michael Schaeffer
Just as important as any other element in a construction project, the selection of the right formwork greatly affects the project’s schedule, labour requirements, quality, and total cost.
Over the years, formwork moulds have evolved from traditional job-built timber to pre-engineered systems composed of a combination of steel, aluminum, manufactured timber, plywood, and plastics. The traditional formwork relied on skilled craftsmen and the result was often inconsistent safety features, slow onsite construction rates, as well as large levels of waste. Today’s formwork moulds are mostly modular and provide increased accuracy as well as minimized waste. These advancements have led to increased jobsite production and safety, with less labour, while producing a better finished product.
Modern formwork moulds need to be capable of shaping and supporting the concrete until it cures, but they also must withstand all of the weight—concrete, labourers, and equipment—placed on them without distortion. Moulds should be designed so they can be easily handled by hand or by equipment while achieving the required number of reuses. Further, their face material must consistently produce the contractor’s desired concrete finish, whether smooth, textured, or with exposed aggregate.
Less than 20 years ago, a dozen major formwork systems were readily available in North America. However, in the short period since, many more European forming companies have entered the North American market, more than doubling the number of systems available. This increase in competition is pushing innovations to a rate previously unseen in the industry. Thirty-year-old systems that have enjoyed wide use and popularity are being supplanted by new, modern systems that offer greater productivity and a higher-quality product.
Traditionally, the most prevalent system in use for hand-set wall-forming are steel-framed, wood-faced panels that require consumable ties at 0.6 m (2 ft) on-centre (o.c.) and 10 connections per 1 m2 (10.8 sf). These are being replaced with larger, two-person handset systems that require less labour and eliminate consumable purchases with the use of reusable taper ties.
Gang-forming has completely changed over the past 10 years. Older systems of steel-framed wood or steel-faced panels with double-channel stiffbacks that connect with bolts/pins have been overtaken by clamp connection forms with wood or plastic form faces that provide tremendous labour savings in assembly and use. These standard systems assemble and reconfigure very quickly to meet changing structure dimension while also providing a consistent concrete finish. With each new innovation, come new and better safety features.
Selecting the right formwork
As conditions vary for each individual project, there is no simple formula for choosing the right formwork supplier or system.
Formwork typically accounts for 40 to 60 per cent of the total cost of a project’s concrete frame. These percentages include the expense of material and labour, with the largest cost going toward labour. It is important to analyze labour costs thoroughly, as this is the bigger number and its reduction has a much greater impact on bottom line costs.
To help determine the most efficient solution for a project, the contractor typically evaluates several forming systems. As available and capable labour shortages continue in the construction marketplace, it has become even more important to select the right forming system. Simply stated, the choice is often between an inexpensive forming material that is labour-intensive or one that costs more upfront, but provides high productivity. Other factors to be evaluated when choosing a formwork system include:
Another consideration is whether to purchase or rent a system. This decision should be based on the duration of the project and the overall strategy of the construction company. Typically, if a form system has to be rented for more than eight to 10 months, purchasing the system might be more economical. However, along with the purchase of a system, there are additional costs such as maintenance and storage. Some formwork companies, however, offer services for customers who purchase their equipment.
The quality of the product also must be considered in the decision-making process. Steel-framed wall formwork with standard plywood facing requires more maintenance and repairs throughout the life of the form than hot-dipped galvanized steel frames with specially manufactured plywood designed for longer life.
Hydroelectric plant case study
As more unusual designs and projects appear, new formwork challenges are faced. At Muskrat Falls (also shown on the cover), a complete formwork solution package was needed to accommodate the build of the second-largest hydroelectric facility in Canada. The project is located on the lower Churchill River, about 25 km (15 ½ mi) west of Happy Valley-Goose Bay, Labrador. It consists of a powerhouse with four turbines, three dam structures, six spillway piers, separation and retaining walls, and a north and south service bay.
For a huge project such as this, with its many cast-in-place concrete requirements, the formwork manufacturer needed to train and supervise onsite for the use of its systems. Additionally, high standards were built into the design of the formwork solutions, with a zero-tolerance for any lack of site safety. To aid in the project flow and save on jobsite space, the formwork was pre-assembled before being brought to the site. This included pre-assembly of large area formwork, dam platforms, and custom curved formwork.
In one area of Muskrat Falls—the six spillway piers—the structure has true vertical concrete surfaces. Every spillway consists of a bullnose on the upstream side with a large concrete block overhang on the downstream side. Every spillway also includes eight block-outs required for the installation of mechanical gates. This design needed a solution for the symmetrical bullnose to reduce risk of deflection and movement during pouring. The formwork element also needed to be re-used 14 times without any problems. The use of custom radial steel channels instead of wooden gussets was the solution to form the shape of the bullnose.
Another challenge on this job was the separation wall between the spillway and powerhouse structure. The large wall was realized in seven 1.5-m (5-ft) tall monolith pours. It was difficult to perform the monolith pours against the existing structure of the Centre Transition Dam. Custom large-area formwork panels were used to allow and fit the transition between the separation wall and the Centre Transition Dam.
The innovations on the Muskrat Fall project included design and fabrication of a custom off-set bracket to install and support large area formwork off a short starter pour when dam formwork could not yet be installed. An additional innovation was the design and fabrication of a tie-loop anchor to tie down formwork diagonally to vertical rebar, to avoid long ties going through the entire structure to the opposite formwork element.
Tie-less formwork solutions were requested to increase productivity onsite for all the mass concrete pours, reduce the amount of metal embedments and list items in the concrete, and ultimately reduce labour and produce cost savings.
Formwork for bigger sewers
In the eastern end of the Greater Toronto Area (GTA) the cities of Markham and Pickering are experiencing rapid growth. A trunk sewer was required to accommodate the additional sanitary sewer flows projected from the continued future growth.
The goal of this sewer project was to carry 100 per cent of the projected 2036 design sanitary sewers flows. The new Southeast Collector (SeC) trunk sewer is approximately 14 km (8 ½ mi) in length. The sewer was to be constructed at depths of 5 m (16 ft) to more than 40 m (130 ft) below the ground surface.
A major challenge of this project was to ensure the pipeline stayed within the Newmarket Till soil stratum, which is more conducive material for tunnel construction. To do so, a substantial drop in elevation of the pipeline over its length was needed. The use of an earth pressure balance machine as a tunnel-boring machine allowed for different modes of operation to accommodate the changing soils conditions.
Strabag, the contractor, had extremely tight project deadlines and needed high-quality custom built formwork panels ready-to-use onsite. Custom benching (i.e. half-round channel) formwork was supplied for 13 ventilation shafts in total, ranging in depth between 10 and 60 m (30 and 200 ft). The custom formwork panels allowed Strabag to pour the U-shaped benching at the bottom of each ventilation shaft connecting the newly drilled tunnel openings with each other.
These custom formwork panels were based on large area formwork, including wooden gussets and custom splices. Each custom formwork element was highly complex and different in shape and size. Every single formwork element was hand-built.
To meet the city’s requirement for the final concrete finish, the formliners were epoxy-coated during the assembly process and then shipped to the site ready to use. This was an alternative to the use of a controlled-permeability, polypropylene-based formliner. The achieved concrete finish was of a comparable high standard, and was therefore accepted by the client.
Flexible solutions for Ontario’s first cable-stay bridge
When finished in 2017, the Nipigon River Bridge will be the Ontario’s first cable-stay bridge. The $106-million, four-lane cable-stay bridge is part of the Highway 11/17 corridor east of Thunder Bay—a key part of the TransCanada highway. The final design will consist of three towers with cables supporting the bridge deck and a separate sidewalk for pedestrians.
For the first step, the contractor—BOT Ferrovial Nipigon Joint Venture—had to pour the critical substructure components including the piers, footings, and abutments. In this design, the entire cantilevered superstructure will be constructed without the use of in-water structures.
Single-sided walls were provided using a complete system, which consists of lightweight panels that are easy to handle, so they can be erected very quickly by hand, rather than relying on a crane. A framed formwork system was used to form the concrete foundations. Pre-assembled large area formwork system panels were supplied for the north and centre tower walls, which can be erected onsite as needed. The advantage of this formwork is the shape, size, tie-hole pattern, and form-facing of the elements can be adapted to suit any requirement. Shaft formwork with a shaft platform was used to form the hollow interior of the towers.
The biggest challenge for this project was the complicated geometry of the cross beam. The formwork engineers designed pre-assembled large area formwork panels for the interior and exterior walls of the cross beam. That was followed by a high capacity, fast shoring system, with a height of approximately 18 m (60 ft), which was used to shore the cross beam. The shoring frames are designed for large shoring-heights and high loads. The design provided the contractor with a flat working deck
2 m (6 ft) below the sloping cross beam formwork.
Forming a relationship
To consider formwork in the overall design of an innovative project, it is important for the structural engineer to have a good handle on the standard products available in the marketplace. This can be accomplished by contacting the major formwork suppliers, which should be more than eager to provide assistance to educate the industry on forming products and how the structure’s design affects custom products needed and overall costs.
Formwork suppliers need to be contacted during the project’s very early design stages. This can allow for as much information as possible to be incorporated into the bid documents, which provides a more accurate cost to the owner. Formwork suppliers can advise on sizing structural concrete members to meet standard form dimensions or can suggest when custom designs or parts are necessary. As major form suppliers are typically involved in a large number of projects in a wide variety of construction markets, they can draw on their resources to suggest formwork means and methods.
It is also important to ask concrete forming contractors to be involved early in the design stage of the structure. After all, they are ultimately the people who will be physically constructing it and can provide a tremendous amount of knowledge as to the most practical and economical means and methods.
There are many considerations to take into account at the design phase—and considering formwork is of prime importance—especially when an unusual design may involve custom designed parts. With the ability to pinpoint formwork designs to accommodate the needs of the project, planning ahead ensures the success of an economical concrete construction project.
Michael Schaeffer is vice-president at Doka North America, and has been in the design/construction industry for more than 20 years. His career in formwork has also included front line sales, operations, and branch management, but Schaeffer’s current responsibilities include national sales management, training, and product development. He can be reached at email@example.com.
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