High-Performance Composites

NOV 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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6 4 | H I G H - P E R F O R M A N C E C O M P O S I T E S L E A R N M O R E @ w w w. c o m p o s i t e s w o r l d . c o m Read an expanded version of this article at short.compositesworld.com/VWrapRehab. Read about another composite strengthening option in "GFRP rebar replaces steel in parking garage," HPC June 2013 (p. 46) or visit short.compositesworld.com/GlassRebar. some cases, limitations imposed by fre codes. As shown in the drawing (p. 62), V-Wrap material placed beneath a hori- zontal slab increases its tension strength and bending resistance; strips are placed on the slab's bottom surface with the f- ber direction parallel to the slab's steel rebar: "The V-Wrap is mimicking the ac- tion of the existing reinforcing steel, and augmenting its strength," he explains. Fiber sheet placed on a slab's upper surface — over supporting columns, for example — counteract the slab's negative bending moment. For additional beam shear capacity, typically where a beam is connected to a column, V-Wrap is applied to the beam in a "U" shape that mimics the beam's steel shear stirrups. The stir- rups and fber prevent or minimize for- mation of 45° shear cracks that can occur if the beam/column intersection is over- loaded. Finally, carbon fber is exception- al at increasing confnement and ductil- ity in concrete columns. When wrapped horizontally around the column, the fber wrap acts as additional reinforcing ties around the column circumference. Thomas cautions that the effects of strengthening local structural elements must be analyzed carefully to determine their infuence on the structure's global behavior. "Increasing the bending ca- pacity of a structural element may over- stress it in shear or, possibly, affect the surrounding elements, and lead to even bigger problems or even localized fail- ure," Thomas notes, pointing out that carbon fber's effectiveness depends on the original stiffness of the structural element to which it is applied. "Flexural strengthening is most effective on con- crete members that were originally light- ly or moderately reinforced," he explains. Adding carbon fber to an already very stiff element doesn't contribute much, because the element must defect to en- gage the bonded fber. Structural follows guidance from the American Concrete Institute's ACI 440.2R-08, Guide for the Design and Construction of Externally Bonded FRP Strengthening Systems for Strengthening Concrete Structures (2008), which, in es- sence, discourages the use of excessive FRP. "Full-scale testing has shown that more is not better," he notes. "Too many plies create stresses that exceed the con- crete's tensile capacity and can actually cause the FRP to fail by peeling it away from the concrete under high loads. To ensure that the upgraded structure will perform in service as predicted, load tests, both cyclic and monotonic (steadi- ly increasing load), can be performed before and after the strengthening, using hydraulic actuators, to validate that the addition of the carbon fber has achieved the required increase in capacity. And to verify that V-Wrap is bonded securely, Structural conducts pull-off adhesion tests: "The V-Wrap, per ACI 440, has to pass a 200-psi test," adds Thomas. Samples of success Thomas cites a recent job in Miami, Fla., which involved converting a former shopping mall to a control center for a telecommunications company. A ~25 percent increase in load-carrying capac- ity was necessary to handle the 150-lb/ ft 2 (730-kg/m 2 ) loads created by the com- pany's equipment. The structural foor, a one-way slab, is supported by precast, prestressed joists that rest on continu- ous cast-in-place concrete beams with precast soffts (sofft, here, is the under- side of a beam, in tension). "We selected an approach in which multiple plies of carbon strips were bonded to the sides and bottoms of each joist, to increase bending moment capacity," explains Thomas. U-wrap strips anchored the lon- gitudinal strips at each end. Beams were strengthened by placing carbon strips on their undersides to increase bending capacity at mid-span, and four U-wraps were applied for shear. V-Wrap sheets also were placed on top of the slab, on each side of, and centered on, column locations to strengthen the beams. In Salem, Ore., the fve-story Court- house Square complex, built in 2000, housed city government offces, retail outlets, a public transit hub and under- ground parking. But buckling foor tiles, water seepage, windows that no longer opened and cracking walls forced build- ing closure. Thomas says Structural's initial inspection identifed errors in the structural design and construction mis- takes, in columns, shear walls, slabs, footings and masonry cladding. "The building's serious faws demand- ed a complex combination of repairs, including concrete enlargement, bonded overlays (i.e., adding additional concrete to slabs) and V-Wrap," Thomas recalls. The parking garage and offce space columns were ftted with formwork to create capitals using cast-in-place rein- forced concrete, to increase their punch- ing shear capacity (i.e., so that the fat slab would be better supported). V-Wrap came into play as column wraps (see photo, p. 63). Structural works on hundreds of proj- ects each year, but Thomas says it's still a challenge. "Unknowns such as struc- tural conditions and load paths, or the location — or lack of — existing rein- forcement," he says, make each project unique. And as long as there's concrete, there'll be work to do. Cost-conscious strengthening Structural reports that workers (inset) can apply V-Wrap composite sheeting to a concrete slab at a fraction of the cost of new construction. Source: Structural

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