Over the ages, as we have evolved, so has our engineering and researching skill sets. Civil engineering being the very origin of engineering, has also witnessed many transformations and breakthroughs in the construction technology thereby leading us to the successful current times. Even today, we are constantly innovating, researching and developing technology in pursuit of a sustainable future. Throughout this evolution, researchers and engineers have found themselves in constant search for new and better materials to optimally manage the performance-cost tradeoff in the construction sector.
Many new raw materials have been discovered and many ground-breaking composites have been developed, of which not all but some have proved to be a phenomenal success. Carbon fiber is one of these materials, which is usually used in combination with other materials to form a composite. The properties of carbon fibers, such as high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion, makes them one of the most popular materials in civil engineering. Possessing strength up to five times that of steel and being one-third its weight, we might as well call it ‘the superhero’ of the material world.
Carbon Fibers: Composites and properties
Each carbon filament thread is a bundle of many thousand carbon filaments. A single such filament is a thin tube with a diameter of 5–8 micrometers and consists almost exclusively of carbon. Made of pure carbon in form of graphite, they have low density and a negative coefficient of longitudinal thermal expansion. Carbon fibers are produced by the PAN (polyacrylnitrile) or the pitch method. The PAN method separates a chain of carbon atoms from PAN through heating and oxidation while the pitch method pulls out graphite threads through a nozzle from hot fluid pitch. Construction composites that are most commonly reinforced with carbon fibers are the class of materials known as carbon fiber reinforced polymers (CFRP), also known as carbon fiber reinforced plastic. To make a carbon fiber sheet, carbon fiber fabric is saturated or infused with epoxy resins and heated at high temperatures. Shaped pieces are made by layering several pieces of fabric over a mold, saturating them with resin and heating it until the resin has infused through all layers. The polymer used in CFRP is most often epoxy, but other polymers, such as polyester, vinyl ester or nylon, are also sometimes used. Non-polymer materials can also be used as the matrix for carbon fibers.
Properties of CFRP depend on the layouts of the carbon fiber and the proportion of the carbon fibers relative to the polymer. Their properties differ so much from that of their matrix material, that a relationship is barely discernible any more. CFRP materials are distinguished by their extremely high strength and rigidity. Exceptional durability, high resistance to corrosion, low density, excellent damping properties and a high resistance to impacts combined with exactly modifiable thermal expansion to complement the complex characteristics profile. To be specific, it has a very high modulus of elasticity exceeding that of steel; high tensile strength, which may reach 1000 ksi (7 GPa); low density: 114 lb/ft³ (1800 kg/m³) and high chemical inertness. The main disadvantage of carbon fibers is their catastrophic mode of failure since the carbon fibers are brittle in nature. Also their relative cost is a big drawback since carbon fiber is a high quality material that comes with a price to match.
Applications in Civil engineering
Civil and structural engineering applications differ from many of the other applications in several ways. The loads are generally higher, with the forces to be resisted of the order of tens, if not hundreds, of tones. The loads are often of long duration (years not hours) and long term stiffness (including both elastic stiffness and creeps), this often being the governing criteria. To be effective, the fiber yarn needs to aggregate into a sufficiently large unit to apply a significant force. The application fields of CFRP include bridges, buildings, tunnels, chimneys and others like electric poles, box culverts, among others. Out of these, applications in bridges and buildings occupy the majority of the whole market. Recently, more applications could be found in repairing tunnel lining. Studied in an academic context as to its potential benefits in construction, it has also proved itself as cost-effective in a number of field applications like strengthening concrete, masonry, steel, cast iron, and timber structures. Its use in industry can be either for retrofitting to strengthen an existing structure or as an alternative reinforcing (or pre-stressing material) instead of steel from the outset of a project. The bridge piers, girder, plates and building beams, columns and plates are the most common to be strengthened.
Bridge and Chimney retrofitting applications have witnessed great success owing to the use of carbon fiber reinforced polymer (CFRP) composite technology. Approximately, a little less than a quarter of the world’s bridges are being classified as either functionally obsolete or structurally deficient. That does not mean that the deficient bridges are unsafe; they are classified administratively to indicate they require some form of maintenance or major rehabilitation to restore them to their original condition or to their original load carrying capacity. Retro-fitting is popular in many instances as the cost of replacing the deficient structure can greatly exceed its strengthening using CFRP. Typically, a concrete bridge deck has a 25 to 40 years life span. Old concrete bridge decks that were reinforced with unprotected steel reinforcement are deteriorating rapidly. The CFRP composite deck systems have the potential to fill the need of bridge deck replacement and extend the service life of existing structures. The advantages of a CFRP induced deck are – lightweight, high strength and high performance, chemical and corrosion resistant, easy construction and handling, rapid project delivery, and in most cases, high quality shop fabrication.
Wrapping around bridge sections can also enhance the ductility of the section, greatly increasing the resistance to collapse under earthquake loading. Such ‘seismic retrofit’ is the major application in earthquake-prone areas, since it is much more economic than alternative methods. The fiber wrap systems are also being used to repair deteriorated concrete piers, pier caps, concrete arch, and damaged beams.
Bonded concrete repair using CFRP laminates, rods and wet lay-up fabrics is also a very popular repair technique. The surface mounted composites have been used in numerous concrete bridge strengthening and repair applications. This technique is cost effective, easily to design, install and inspect. Compo-sites here are applied to the soffit of existing concrete decks.
Carbon fiber’s chimney repair applications are also very important. The aged chimneys require some repair and strengthening to continue their safe operation today. CFRP chimney liners been in service up to 20 years have proven CFRP survival at high temperature, resistance to chemicals, structural reliability, low life-cycle cost, and low maintenance. Beam, Column and Slab strengthening is another very important application of Carbon Fiber composites. In flexural reinforced concrete members, the addition of carbon fibers improves the modulus of rupture (bending strength). Also, there is evidence that carbon fibers can be effective replacements for shear steel stirrups commonly used in RCC beams and other structural elements such as shear keys and corbels.
Two techniques can be adopted to strengthen the beams. First one is to paste CFRP plates to the bottom (generally the tension face) of a beam. This increases the strength of beam, deflection capacity of beam and stiffness (load required to make unit deflection). Alternatively, CFRP strips can be pasted in ‘U’ shape around the sides and bottom of a beam, resulting in higher shear resistance.
Columns in building can be wrapped with CFRP for achieving higher strength. The technique works by restraining the lateral expansion of the column. Slabs may be strengthened by pasting CFRP strips at their bottom (tension face). This will result in better performance, since the tensile resistance of slabs is supplemented by the tensile strength of CFRP. In the case of beams and slabs, the effectiveness of CFRP strengthening depends on the perfor-mance of the resin chosen for bonding.
Pre-stressing carbon fiber reinforced polymers
By means of pre-stressing capacity, the stiffness of strengthened structures can be improved greatly for the sake of delaying the onset of cracking, reducing the deflection, relieving the strain in internal reinforcement, etc. In the early stage of CFRP pre-stressing technique, CFRP plate was often chosen as pre-stressing material to strengthen the structures owing to its fixed shape, but this fixed shape often affects the bonding quality greatly because of the uneven-ness of the concrete surface. Not just CFRP plate, even the CFRP sheet is similar in nature, because it must be impregnated by resin before its regidification and only then it can be tensioned and bonded to the structural surfaces. Also, external pre-stressed cables have proved to be a good alternative for steel cables with good durability and first rate behavior in creep and relaxation. These pre-stressing techniques also have a few disadvan-tages and are thus undergoing research for betterment.
Challenges and future for carbon fibers in construction industry
One big restriction on making extensive use of carbon fibers in the infrastructure technology is their considerably high cost. While prices have dropped significantly in the past five years, demand has not increased enough to increase the supply substantially. As a result, prices will likely remain the same for the near future. One way to manage this drawback is by making use of FRP hybrid systems. Since FRP composite materials have a higher initial cost, hybrid FRP systems that combine the high stiffness and/or high compression strength of conventional materials have proven to be effective. For example, to reduce the cost, aramid FRP may be used in a component where the tensile stresses are high and carbon FRP may be used in the section undergoing compressive stresses, as both are excellent in respective properties. Similarly, combi-nation of glass FRP and other such types, depending on the structure requirement, might prove cost effective.
Using carbon fiber reinforcement bars may also become a reality in near future, as it undergoes intense research worldwide. Most countries including India still haven’t properly developed the design and construction specifications for carbon fiber integration in the current technology.
There is enormous scope of use of carbon fibers in India, because of seismically deficient buildings, long coast line and long monsoon season pressing the use of non-corrosive CFRP. Traditional materials, such as wood, are in short supply. There are very few examples of CFRP application for retrofitting before Gujarat earthquake (2001) and it was only after this earth-quake that the technique started gaining attention in India. However, the same is not to the extent warranted by the existing potential of the carbon fibers. As the material is still considered relatively new in this part of the world, most of the works have been carried out in accordance with the limited available guidelines and published literature. At this juncture, there is a need of Government-Industry-Institute partnership to exploit full potential of CFRP so that the technology can be popularly used to breathe new life into failing structures through expeditious, cost effective design and construction.