Carbon fibre is not new. In fact, it’s used everywhere from consumer sports equipment products like hockey sticks and golf clubs to sophisticated applications such as race cars and aircraft.
More recently, carbon fibre has been prophesied as the next new thing in modern construction, a possible replacement for steel in many applications. Used in place of steel, carbon fibre reinforcement polymers (CFRP) can extend a structure’s life, reduce structural weight, and reduce GHG and carbon emissions, all at a price comparable to steel.
Carbon fibres are made from polyacrylonitrile drawn into long fibres heated to high levels without contact with oxygen, resulting in thin, strong crystalline filaments twisted together like yarn.
The material’s attraction is both its immense tensile strength and its strength-to-weight advantage over steel. Furthermore, carbon fibre neither corrodes nor degrades and can be formed into almost any required shape.
Beyond new structures, carbon fibre also has an important role to play in the restoration and strengthening of many existing ones. When applied in combination with epoxy resins, carbon fibre can work wonders as a secondary structural strengthening material, either in the form of a wrap or as reinforcement plates.
During the recent Buildings Week, Bruce Hudson and Gregor Dolenc, members of the technical sales team at Sika Canada Inc., explained the challenges facing the repair or reinforcement of structural elements supporting older bridges and buildings that can be met using carbon fibre.
As an example, Hudson outlined the challenges presented by a bridge passing over an environmentally sensitive portion of the Etobicoke Creek in the west end of Toronto. Nearby road expansion had increased loads on the bridge since its construction in the 1950s.
”The structure was showing some signs of fatigue and stress cracking,” said Hudson.
Studies concluded that closing the bridge would cause major traffic disruptions if topside upgrade repairs were undertaken. Increasing the amount of concrete on the underside was considered but dismissed due to environmental concerns related to the creek below.
Instead, the application of carbon fibre strips was chosen.
First, testing the tensile strength of the existing concrete was undertaken to determine the correct concrete surface profile.
“The engineer then mapped out where these strips were to be located,” explained Hudson.
To ensure full contact, a two-part adhesive was applied both to the existing concrete and to 18-metre-long carbon fibre strips, using a jig to give a slightly domed shape along the centreline. The strips were placed in position by four site workers. Rollers then applied pressure the full length of the strips to ensure bonding. Overall, it was a remarkably straight-forward process.
This process was essentially repeated for remedial work on elevated sections of Toronto’s infamous Gardiner Expressway. Here, concrete chunks caused by the expansion of corroded steel rebar had fallen from precast girder boxes onto traffic lanes running underneath. Although involving a lengthier four-month process including comprehensive surface preparation, a total of six kilometres of the company’s CarboDur 100-millimetre-wide strip plates were installed with minimal traffic disruption of the city’s main east-west downtown artery.
Similar techniques have been used to reinforce columns in underground parking areas and to provide seismic reinforcement to structures of all sorts, including historic monuments located in sensitive areas.
Carbon fibre technology has also been able to solve some interesting architectural design challenges for buildings undergoing major interior renovations.
Hudson explained that a high-end Toronto retailer wanted a “wow” factor for customers entering their store. That required a redesign of the structure at its entry point including the removal of portions of the second floor concrete slab. Structural integrity was retained with the installation of multiple carbon fibre bands on the undersides and topsides of the remaining slab areas, anchored at each end. The overall profile required the removal of only ¼ inch of concrete.
By giving new life to existing structures, carbon fibre technology can reduce major disruptions during restorations and avoid the carbon emissions created by their demolition and replacement.
John Bleasby is a Coldwater, Ont.-based freelance writer. Send comments and Inside Innovation column ideas to email@example.com.