Concrete is one of those technologies that has been used for centuries- first by Romans- and then had to invent again centuries later in England. Throughout its evolution in the modern era, it has served to shelter humans, livestock, machine, etc. Concrete has been used as a very efficient building material for its high compressive strength, good durability, and low cost. However, it’s well-known Achilles’ heel is its brittle nature and limited tensile strength. This shortcoming was solved quite handily about a century ago by using reinforcing bars (rebar) of steel in the tension side of concrete structures.
Steel rebar is functionally efficient and relatively inexpensive, so it does a good job in most cases. However, steel rebar has its own weakness: susceptibility to corrosion (oxidation) when exposed to salts, aggressive chemicals, and moisture. As it corrodes, steel rebar swells and increases the tensile load on the concrete, which begins to crack and spall, creating openings that lead to further and faster deterioration of the steel and concrete.
If allowed to progress far enough, it can compromise the structure’s integrity. Numerous coatings and penetrants have been introduced over the decades to help seal out moisture from concrete, and rebar itself has been upgraded with epoxy coatings or the use of stainless steel. But it isn’t always possible to prevent corrosion in the long term. Further, steel rebar’s penchant to conduct electrical and magnetic fields makes it undesirable in concrete specified for certain power-generation, medical/scientific-imaging, nuclear, and electrical/electronic applications.
To tackle all these shortcomings extensive research is being done on new materials like FRP.
Fiber-reinforced polymer (FRP) is a composite material made up of polymer matrix infused with polymers to reinforce it. The fibers used are generally glass, carbon, aramid, or basalt leading to the formation of FRPs like glass fiber reinforced polymer (GFRP), carbon fiber reinforced polymer (CFRP), etc. Polymers matrix is usually made from polymers like epoxy, vinyl ester, or polyester thermosetting plastic.
Different FRPs provide different properties like GFRPs provide improved strength, elasticity, heat resistance. CFRPs and AFRPs (aramid fiber reinforced polymer) provide improved elasticity, tensile strength, compression strength, electrical strength. While FRPs reinforced with inorganic particulate provide superior isotropic shrinkage, abrasion, compression strength.
Glass fiber reinforced polymer (GFRP) are composite materials constituted by glass fiber reinforced polymer matrix. They make a strong case as an alternative to steel frames in structures. Their high durability and lightweight characteristics, compounded by their insulate behavior to electric fields and magnetic fields make them a valid option as structural members. However, still, they are governed by global and/or local buckling. FRP is also used extensively in the aviation and automobile industry outside of the construction dimension.
One of the most killer disadvantages of FRP is that the price of a typical FRP section is very high compared to that of traditional steel components. Roughly, for the GFRP section price would be three times and for AFRP or CFRP section it would be ten times that of the similar steel section. These costs are compounded because of control-issues. Also, this material is relatively new in the Indian market, and therefore its acceptability and skilled labor are also less.
Fiber-reinforced polymer (FRP) is a form of composite material used to make advanced composite materials. In the 1900s, the birth of modern resins with the discovery of plastics and new synthetic resins led to its increased scientific application. It was in 1935 that first Glass fiber, combined with modern synthetic polymer resins led to Glass Fibre Reinforced Polymer (GFRP).
Different types of concrete, including a conventional HSC and slurry, infiltrated fiber concrete (SIFCON) were investigated as a parameter to understand the effect of steel fiber parameters, including fiber shape, fiber aspect ratio, and fiber volume fraction. The results indicate that both steel fiber-reinforced high-strength concrete FRP tubes (SFRHSCFFTs) and SIFCON-filled FRP tubes (SIFCONFFTs) exhibit highly ductile compressive behavior. The results also indicate that the axial stress-strain behavior of CFFTs is influenced by the presence and amount of internal steel fibers, with particularly significant influences noted on the FRP hoop rupture strains and post-peak strength losses. It is found that the fiber volume fraction significantly affects the compressive behavior of CFFTs. Concrete type, fiber shape, and fiber aspect ratio also have some, but the less significant, influence on the behavior of CFFTs. It is observed that the compressive strength and ultimate strain of CFFTs increase with an increase in fiber volume fraction or a decrease in fiber aspect ratio. It is also observed that CFFTs reinforced with hooked end steel fibers exhibit improved compressive behavior compared to the companion CFFTs reinforced with crimped fibers. (Tianyu Xie, Togay Ozbakkaloglu)
The influence of the pull-out load distance (d) from the edge of the specimens on the failure strength of the web-flange junction has been investigated and a new definition for an ”influence zone” is proposed that is found to be dependent on the loaded length, with a maximum value equal to approximately the PFRP member’s depth. This proposed zone was observed in all laboratory tests and its existence was confirmed by the results of FEM numerical analysis. (Luciano Feo, Ayman S. Mosallam, Rosa Penna)
For pultruded GFRP profiles, characteristic ultimate stresses/elastic moduli are compared to design manual minimum values. The former depends on profile size/shape, whereas the latter are shape-/size-independent. Limit state design stresses are shown to be larger than permissible stress design stresses. However, most of the limit state longitudinal design elastic moduli are smaller and all of the transverse design elastic moduli are larger than the permissible stress values. (Geoffrey John Turvey, Yingshun Zhang)
The testing and analysis undertaken on the square GFRP composite tubes and their assemblies indicate in general that composite materials and similar types of assemblies made out of them can be used as an efficient structural component in a number of civil infrastructure applications such as building bridges. The analysis demonstrates that readily available pultruded FRP tubes, assembled together in an appropriate configuration, meet the strength requirement and the other necessary performance criteria. The endurance of GFRP box beams subjected to static flexure loadings and the consequent failure are the principal measures of structural performance.
The deflection and strain graphs for the single tube and double tube assembly shows linear elastic bending and shear behavior up to failure. In the case of the four-layered tube assembly, the deflection and strain graphs are linear elastic for a considerable amount of load beyond which it shows distinct nonlinear characteristics. The deflections and strains in the elastic region are very symmetric.
Damages accumulate gradually as the load increases, indicated by cracking, which is reflected in the deflection and strain graphs. The ultimate failure is the catastrophic breaking of the fibers on the surface of loading and the sides of the tubes or popping out of the tubes from the assembly. (Prakash Kumar, K. Chandrashekhara, Antonio Nanni).
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