Editing Reinforced concrete (section)

==Non-steel reinforcement==
There is considerable overlap between the subjects of non-steel reinforcement and fiber-reinforcement of concrete. The introduction of non-steel reinforcement of concrete is relatively recent; it takes two major forms: non-metallic rebar rods, and non-steel (usually also non-metallic) fibers incorporated into the cement matrix. For example, there is increasing interest in [[Glass fiber reinforced concrete|glass fiber reinforced concrete (GFRC)]] and in various applications of polymer fibers incorporated into concrete. Although currently there is not much suggestion that such materials will replace metal rebar, some of them have major advantages in specific applications, and there also are new applications in which metal rebar simply is not an option. However, the design and application of non-steel reinforcing is fraught with challenges. For one thing, concrete is a highly alkaline environment, in which many materials, including most kinds of glass, have a poor [[service life]]. Also, the behavior of such reinforcing materials differs from the behavior of metals, for instance in terms of shear strength, creep and elasticity.<ref>{{cite book |title=BS EN 1169:1999 Precast concrete products. General rules for factory production control of glass-fiber reinforced cement. |url=https://shop.bsigroup.com/ProductDetail?pid=000000000019974480 |date=15 November 1999 |publisher=British Standards Institute |isbn=0-580-32052-9 |access-date=29 May 2018 |archive-date=12 June 2018 |archive-url=https://web.archive.org/web/20180612113107/https://shop.bsigroup.com/ProductDetail?pid=000000000019974480 |url-status=live }}</ref><ref>{{cite book |title=BS EN 1170-5:1998 Precast concrete products. Test method for glass-fiber reinforced cement. |url=https://shop.bsigroup.com/ProductDetail?pid=000000000001302372 |date=15 March 1998 |publisher=British Standards Institute |isbn=0-580-29202-9 |access-date=29 May 2018 |archive-date=12 June 2018 |archive-url=https://web.archive.org/web/20180612113115/https://shop.bsigroup.com/ProductDetail?pid=000000000001302372 |url-status=live }}</ref>

[[Fiber-reinforced plastic|Fiber-reinforced plastic/polymer]] (FRP) and [[glass-reinforced plastic]] (GRP) consist of fibers of [[polymer]], glass, carbon, aramid or other polymers or high-strength fibers set in a resin matrix to form a rebar rod, or grid, or fiber. These rebars are installed in much the same manner as steel rebars. The cost is higher but, suitably applied, the structures have advantages, in particular a dramatic reduction in problems related to [[corrosion]], either by intrinsic concrete alkalinity or by external corrosive fluids that might penetrate the concrete. These structures can be significantly lighter and usually have a longer [[service life]]. The cost of these materials has dropped dramatically since their widespread adoption in the aerospace industry and by the military.

In particular, FRP rods are useful for structures where the presence of steel would not be acceptable. For example, [[MRI]] machines have huge magnets, and accordingly require [[non-magnetic]] buildings. Again, [[toll booths]] that read radio tags need reinforced concrete that is transparent to [[radio waves]]. Also, where the [[design life]] of the concrete structure is more important than its initial costs, non-steel reinforcing often has its advantages where corrosion of reinforcing steel is a major cause of failure. In such situations corrosion-proof reinforcing can extend a structure's life substantially, for example in the [[intertidal zone]]. FRP rods may also be useful in situations where it is likely that the concrete structure may be compromised in future years, for example the edges of [[Balcony|balconies]] when [[balustrade]]s are replaced, and bathroom floors in multi-story construction where the service life of the floor structure is likely to be many times the service life of the [[waterproofing]] building membrane.

Plastic reinforcement often is [[Strength of materials|stronger]], or at least has a better [[Specific strength|strength to weight ratio]] than reinforcing steels. Also, because it resists corrosion, it does not need a protective [[concrete cover]] as thick as steel reinforcement does (typically 30 to 50&nbsp;mm or more). FRP-reinforced structures therefore can be lighter and last longer. Accordingly, for some applications the [[whole-life cost]] will be price-competitive with steel-reinforced concrete.

The [[material properties]] of FRP or GRP bars differ markedly from steel, so there are differences in the design considerations. FRP or GRP bars have relatively higher tensile strength but lower stiffness, so that [[Deflection (engineering)|deflections]] are likely to be higher than for equivalent steel-reinforced units. Structures with internal FRP reinforcement typically have an [[Deformation (engineering)|elastic deformability]] comparable to the plastic deformability (ductility) of steel reinforced structures. Failure in either case is more likely to occur by compression of the concrete than by rupture of the reinforcement. Deflection is always a major design consideration for reinforced concrete. Deflection limits are set to ensure that crack widths in steel-reinforced concrete are controlled to prevent water, air or other aggressive substances reaching the steel and causing corrosion. For FRP-reinforced concrete, aesthetics and possibly water-tightness will be the limiting criteria for crack width control. FRP rods also have relatively lower compressive strengths than steel rebar, and accordingly require different design approaches for [[reinforced concrete column]]s.

One drawback to the use of FRP reinforcement is their limited fire resistance. Where fire safety is a consideration, structures employing FRP have to maintain their strength and the anchoring of the forces at temperatures to be expected in the event of fire. For purposes of [[fireproofing]], an adequate thickness of cement concrete cover or protective cladding is necessary. The addition of 1&nbsp;kg/m<sup>3</sup> of [[polypropylene]] fibers to concrete has been shown to reduce [[spall]]ing during a simulated fire.<ref name="tunnel">{{cite book |chapter-url= http://www.tunnels.mottmac.com/files/project/3372/Airside_Road_Tunnel.pdf |page=645 |title=Proceedings of the Rapid Excavation & Tunneling Conference, New Orleans, June 2003 |chapter=Chapter 57: The Airside Road Tunnel, Heathrow Airport, England |author= Arthur W. Darby |year=2003 |via= www.tunnels.mottmac.com |url-status=dead |archive-url= https://web.archive.org/web/20060522164623/http://www.tunnels.mottmac.com/files/project/3372/Airside_Road_Tunnel.pdf |archive-date=2006-05-22}}</ref> (The improvement is thought to be due to the formation of pathways out of the bulk of the concrete, allowing steam pressure to dissipate.<ref name="tunnel"/>)

Another problem is the effectiveness of shear reinforcement. FRP [[rebar]] stirrups formed by bending before hardening generally perform relatively poorly in comparison to steel stirrups or to structures with straight fibers. When strained, the zone between the straight and curved regions are subject to strong bending, shear, and longitudinal stresses. Special design techniques are necessary to deal with such problems.

There is growing interest in applying external reinforcement to existing structures using advanced materials such as composite (fiberglass, basalt, carbon) rebar, which can impart exceptional strength. Worldwide, there are a number of brands of composite rebar recognized by different countries, such as Aslan, DACOT, V-rod, and ComBar. The number of projects using composite rebar increases day by day around the world, in countries ranging from USA, Russia, and South Korea to Germany.