The pioneering efforts by the American Composites Manufacturers Association (ACMA) paved the way for the acceptance of Fiber Reinforced Polymer (FRP) composites in the International Building Code (IBC) in 2009 as a construction material both interior and exterior, through inclusion of appropriate requirements to allow their use in a manner intended by the code alongside traditional building materials.
It has always been a Herculean task in getting composites specified as a viable alternate material in different market segments – storming the bastion of traditional materials such as steel, aluminum, concrete, timber has never been easy. In building construction, the concept of being heavy has always been synonymous with strength. The fact that designers & architects have generally been conservative in their approach; preferring to err on the side of caution, when it comes to specifying composites for load-bearing structures in civil engineering, is well known. For sure, there has been a sea change over the years on such thinking; aided largely by hours of testing leading to establishment of codes of practice and eventually international standards, to instill confidence that composites are a “light weight-high strength-safe option”.
In this context, the American Concrete Institute (ACI) Committee 440 adopting the new standards in 2008 designed to provide FRP rebar manufacturers with critical tools to aid engineers in specifying composite rebar for structural applications, was a significant step in supplementing the ACI 440. 2R-06 design specification. With assimilation of extensive data over the years, ACI 440 has been progressively updated and the recent ACI 440.1R-15 “Guide for the Design and Construction of Structural Concrete reinforced with FRP Bars” vindicates, in a way, the growing confidence; albeit slowly, in the use of composite rebar by the building construction fraternity.
Concrete has high compressive strength but limited tensile strength. To overcome limitations in tensile, reinforcing bars are used in the tension side of concrete structures. Historically, steel rebar have been used as a cost-effective reinforcement. However, insufficient concrete cover, poor design or workmanship and the presence of large amounts of aggressive agents including environmental factors can lead to cracking of the concrete and corrosion of the steel rebar. When corrosion occurs due to soil and other conditions, the resulting products have a larger volume (2-5 times) than the metal product from which they are originally derived. The concrete cannot withstand the tensile load developed from this volume increase and eventually cracks and spalls, leading to further deterioration of the steel. The combination of ongoing deterioration and loss of reinforcement properties results in significant and expansive outlays for repair and maintenance, endangering the structure itself. Epoxy coated steel rebar is a band-aid economical solution.
FRP rebar (with glass or carbon fiber reinforcement and vinyl ester resin) provide superior tensile properties and built-in corrosion resistance and are economically feasible from a life-cycle cost-benefit analysis perspective. The ACI 440 recommendations are based on principles of equilibrium and compatibility and the law of mixtures of fiber reinforcement and resin. The resin-fiber interface bond characteristics are responsible for transferring the stress from concrete to the reinforcement and developing an equilibrium stress, particularly when the concrete cracks. Service limits in the FRP concrete such as deflection, crack width and crack spacing are directly influenced by the bond properties of the reinforcement in concrete.
FRP rebar are anisotropic in nature – the new design philosophy allows consideration to be given to either composite rupture or concrete crushing as the mechanism that controls failure and is based on limit-state design principles, followed by checking for fatigue endurance, creep rupture endurance and serviceability criteria.
The low modulus of elasticity of FRP vis-a- vis steel is well known. FRP rebar hence need to be supported at a spacing that is two-thirds that of steel rebar. The multifarious advantages of FRP rebar such as reduced weight, lower installation times, flexibility in using conventional concrete in lieu of low-permeability concrete with added corrosion inhibitors are some of the tangible benefits. A major plus is the ability to reduce the superstructure weight in view of the superior load-bearing characteristics of FRP rebar, thereby facilitating lower ‘overall” building cost (including foundation).
Boron-free ECR glass fibers have been known to provide superior reinforcing properties vis-a-vis E-glass fibers in view of the proven superior corrosion resistant characteristics and resistance to leaching of metallic ions of the former.
It is at the drawing board (design) stage that a meeting of minds of the composite engineer, architect and civil engineer needs to happen to enable greater use of FRP rebar in projects that are on the anvil. The ($$) benefit of composites would be obvious when determining the overall building cost and an assessment of the life cycle cost.
For more widespread use of FRP rebar, the myth of “concrete mind-sets” of architects and civil engineers is slowly being blown, as realization dawns on the proven engineering advances in composites, ably supported by the comprehensive ACI 440.1R-15 Guide.
The reality? FRP rebar are the future…. and staking their claim, in waxing confidence as a commercially viable, reliable and “strong” option in building construction.