CONSTRUCTION 46

Ever J. Barber0

46.1 INTRODUCTION

Composite materials are used by the construc- tion industry to replace or complement conventional materials such as steel and con- crete. The main reasons for the use of composite materials are corrosion resistance, electromagnetic transparency and weight sav- ings. Frequently, structural engineers take advantage of more than one salient feature of composites to formulate a design that is com- petitive with an alternate design based on conventional materials.

Corrosion resistance is the most important advantage of composites with respect to steel for construction applications. The selection of a composite material usually begins with the selection of a resin that is capable of resisting the attack of a corrosive substance. The corro- sive agent can be anything from spring water to sulfuric acid. Most composite manufactur- ers provide corrosion resistance guides for their products. For example, a table listing the maximum operating temperature of isoph- thalic polyester and vinylester resins as a function of the chemical type and concentra- tion is given by TUFSPAN Technical Data and Design Guide (1991). Chemical resistance of common resins used in pultrusion to various chemical and concentrations as function of operating temperature is given by Pletcher (1991). Fibergrate (1992) supplies a Chemical Resistance Guide for their molded fiberglass

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

and pultruded products listing concentration, operating temperature, and frequency of exposure for a variety of chemicals. Most fiber reinforcements are usually corrosion resistant.

Unlike metals, composites do not produce interference with electromagnetic radiation. The resin system can be selected to obtain very low loss factors, but standard resin systems are adequate for most structural applications. Buildings for electromagnetic interference (EMI) testing must be non-magnetic to avoid attenuation and interference with the phenom- enon that is being measured. All computer equipment, for example, must be tested in an EM1 facility. Imaging equipment such as nuclear magnetic resonance (NMR) in hospi- tals must be mounted in a magnetically free environment. An electromagnetically trans- parent cover for communications equipment allows for the use of less-expensive, non-envi- ronmentally protected electronic hardware and reduced maintenance costs. Antennae structures that do not interfere with the signals being relayed or received by the antennae increase the efficiency of the system.

While weight saving is the main driving force behind the application of composites in aerospace, it is not so critical in construction projects. However, reduction of structural weight can be exploited as a secondary advan- tage to help offset the higher cost of composites as compared to conventional materials. Lightweight structures require less foundation and supporting structure. In the case of bridges, a noncorrosive bridge deck can be built to replace existing steel reinforced

Current applications 983

vice for more than twenty years atest the excel- lent corrosion resistance of fiberglass reinforced composites. The following exam- ples of fiberglass reinforced isopolyester resin applications, described extensively in the excellent review by Adams and Bogner (1993), illustrate the feasibility of constructing com- posite structures and using them for many years. A three mile pipe of diameter 254 mm (10 in) is reported in service since 1971. Fifteen miles of piping, carrying saline water with temperature up to 50°C (112"F), pressure up to lo6 N/m2 (10 bar), and exposed to sunlight has been in operation in Saudi Arabia since 1983 without problems. An old sewage duct was lined in 1971, then inspected in 1991 showing no sign of deterioration. More than 300 000 underground fuel tanks are in use in North America alone. Some of these have been in operation for more than 26 years without problems. Internal or external lining of steel tanks has been common practice for more than twenty years in the oil industry to protect and reinforce the bottom of steel tanks that are cor- roded internally because of corrosive substances in the oil or externally because of contact with soil. Wine tanks have been in operation for more than 20 years without problems. Ducts carrying chlorine gases and sulphur dioxide, in use since 1962, and tanks holding hydrochloric acid, in use since 1964, remain in perfect condition. A chimney exposed to organic chemicals, water vapor, and temperature up to 60"C, in operation since 1968 is reported. Chimneys are important con- struction applications because they are load carrying structures designed for large wind loads. A detailed account of recent applica- tions of composites in construction, classified by the type of construction, follows.

concrete decks that corrode rapidly under the attack from de-icing chemicals. For example, a pultruded deck was used to construct the Wick Wire Run vehicular bridge on public road 26, in Taylor County, West Virginia (com- pleted August 1996). An added advantage of a composite deck would be the weight reduc- tion that supposedly would allow the user (highway department) to re-rate some bridges for a higher live load without major modifica- tions to the existing superstructure. The live load could be increased by approximately the same amount of dead load saved with the use of the composite deck minus adjustments for dynamic effects. Other applications where weight savings are important are cladding of buildings, rehabilitation of chimneys, etc.

46.2 CURRENT APPLICATIONS

Current applications of composites in con- struction can be classified by the major advantage of the composite material that is exploited. The main ones are corrosion resis- tance and magnetic transparency. An alternative classification may be based on the type of construction. Composites are used in the form of structural shapes (similar to steel construction), as reinforcement for concrete, cables, and for rehabilitation of existing struc- tures. Reinforcement of concrete may be in the form of conventional reinforcement, pre- stressed concrete, or post-tensioned structures. Rehabilitation applications include repairing deteriorated structures as well as increasing the load carrying capacity of sound structures to re-rate them for higher load capacity.

Some of the recent applications of compos- ites in construction of civil infrastructure will be described in the next section. Further exam- ples of applications can be found in previous reviews (Barbero and GangaRao, 1991, GangaRao and Barbero, 1991), professional journals ( e g SAMPE Journal, ASCE Journal) and edited books (Mufti, Erki and Jaeger, 1991a, 1991b; Iyer, 1991, Neale and Labossiere, 1992). Applications to pipes and tanks in ser-

46.2.1 REINFORCEMENT OF CONCRETE

Concrete can be reinforced with fiber rein- forced composites, with fibers mixed in the concrete, or by polymers added to the concrete mix. This article will concentrate on the use of

984 Construction

fiber reinforced composites, i.e. a combination of fiber and polymer matrix, to substitute or complement the traditional use of steel rein- forcing bars (rebars) in concrete. The addition of polymer or fibers to cementitious materials, sometimes better classified as a ceramics, bear- ing limited resemblance to regular, low cost concrete, falls beyond the scope of this article. The interested reader may wish to consult Mufty, Erki, and Jaeger (1991b, Chap. 2). Use of fiberglass and carbon fibers mixed directly into concrete has been reported (Mufty, Erki and Jaeger, 1991a). Seibu Construction Co. Ltd. used carbon fibers, produced by Mitsubishi Kasei Co., for the exterior walls of the Kitakyushu Prince Hotel in Japan. Kajima Co. also used carbon fiber reinforced concrete pan- els for the exterior of its head office building in Japan. Polymer concrete is used in a variety of applications, such as in highway parapet walls developed by Morrison Molded Fiber Glass (Fig. 46.1). The installation of the lightweight polymer concrete panels reduces to anchoring the panels to an existing parapet, then pouring concrete into the panel that act as a stay-in- place concrete form. Composite reinforcement of regular, low cost concrete can be done using rebars, grids, pre-stressing tendons and post- tensioning cables.

F 1

Fig. 46.1 Installation of polymer concrete panels as highway parapets. (Courtesy of the Quazite

Rebars

Reinforcing bars (rebars) have been used for reinforcing concrete structures that require magnetic transparency, such as imaging equipment at hospitals (Fig. 46.2). The major expected application is to replace steel rebars in concrete bridge decks exposed to de-icing chemicals. The first use of composite rebars for a vehicular, public bridge is the McKinleyville Bridge, located in West Virginia, USA. Construction was completed August 1, 1996, by Orders Construction Co., of St. Albans, WV. The bridge is a 54 m (177 ft) long concrete deck over steel stringer accommodating two lanes of vehicular traffic. The reinforcement of the concrete is exclusively made of composite rebars with E-glass fibers. It was designed by the West Virginia Department of Transportation - Division of Highways (pro- ject S305-27/4-0.03) and the Constructed Facilities Center at West Virginia University. The project was supported by the Federal Highway Administration, composite rebar

Fig. 46.2 Fiberglass reinforced composite rebars during construction of reinforced concrete struc- ture. (Courtesy of Reynolds, Schlattner, Chetter, and

Division o~MMFG.) Roll Inc.)

Current applications 985

manufacturers, and the Corps of Engineers. E-glass polyester rods are cost competitive

with steel rebars at the present time. These rebars have evolved from smooth pultruded rods to engineered rebars with improved bond strength to concrete. Further improvements in bond strength, tensile strength, and durability are certain to occur in the near future.

E-glass composite tanks have been in ser- vice for over 20 years, in permanent contact with highly acid environments and under con- stant stress. The use of the SPI liner (ASTM D-3299 and D-4097) proved very successful to protect glass fibers from attack by chemicals in storage tanks. Also, composites have been in contact with concrete for many years without deterioration. Investigation of the possible degradation of fiberglass rebars in concrete is under way. The possibility of moisture intake of Aramid fibers may be a problem for rebars expected to be in service for at least 50 years. Carbon fibers have superior properties but their cost limits their potential as a replace- ment for construction steel.

Rebars are placed in the form-work to pro- vide for reinforcement of concrete in the same way as fibers are used to reinforce polymers. Usually, two perpendicular orientations are used, with more rebar area in one direction, according to the requirements of the structure. Placement of rebars requires relatively inex- pensive labor. The grid-like structure necessary to reinforce concrete can be obtained pre- assembled in the form of grids. Grids are rectangular networks of rebars produced with continuous fibers. Although more expensive than rebars, they may be convenient if the labor cost is high or the installation difficult, as in the case of tunnels. Grids can also be pro- duced at or near the construction site by tying rebars into a grid. The light weight of the resulting grid makes transportation to the site and installation simple.

Prestressing

Since concrete has a relatively high compres-

sion strength and, for most practical pur- poses, negligible tensile strength it is advantageous to pre-stress concrete so that a state of compressive stress is created before the actual load is applied. (The American Concrete Institute recommendation ACI 363 R suggests a value for the modulus of rupture of concrete f, = 11.7 cfc)1/2 which is a very low value of tensile strength in relation to the compression strength of concrete, f,.) Then, applied loads only reduce the amount of com- pressive stress without producing tensile stresses. In this way, concrete cracks are pre- vented, which in turn reduces moisture intake and degradation. High strength steel tendons are currently used for pre-stressing concrete. Even though cracks are arrested by the pre- stress, concrete is porous and water and chemicals may reach the prestressing tendons. Composite tendons may replace the steel ten- dons for added durability. Pre-stressed concrete is usually pre-cast at a factory, then transported to the site. The tendons are pre- stressed by a hydraulic jack, the concrete is poured and left to cure. After the concrete is set, the tendons are cut to remove the chucks used to apply the pre-stressing load. Some of the pre-stress is lost because of the compres- sion, creep, and shrinkage of concrete. The higher the elastic modulus of the tendon, the higher the pre-stress loss. For this reason, low modulus glass fiber tendons experience less pre-stress loss than steel or carbon fiber ten- dons. Pre-stress loads are high and induce large strains in the tendons, which may accel- erate degradation if the tendons are exposed to highly alkaline environment (Sen, 1992).

Two important requirements for pre-stress- ing applications are a good bond strength between the tendon and the concrete and availability of an effective temporary anchor- age system. The anchor is not very critical because it is temporary. Therefore, it does not have requirements of non-corrosiveness, cost, size, etc., but it must be able to transfer the load and sustain it until the concrete sets. Anchorage systems are reported by Noritake

986 Construction

et aZ. (1992), Mochida, Tanaka and Yagi (1992), - -'*----

Kakihara et al. (1991), etc. for various types of I

tendons based on glass, aramid, and carbon J reinforcement. Sen (1992) used the anchorage

.

developed by Iyer at South Dakota School of Mines and Technology. Efficient utilization of ,_ composite tendons calls for large pre-stressing forces to be applied. Permanent levels of stress in the tendons should be under the stress rup- ture limit of the composite (see Section 46.3.4). The pre-stress forces applied to composite ten- dons may induce cracking of the polymer matrix. Matrix cracking; may be detrimental to

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the fatigue life of the composite. Also, cracking facilitates the ingress of moisture that may cause degradation of organic fibers or precipi- tate alkaline reaction of glass (see Section 46.3.4).

Pre-stressing has been successfully used by Iyer (1993) in a bridge deck (Fig. 46.3) pre- cast and pre-stressed, then transported to the bridge site. The bridge construction was sponsored by Owens-Corning, AMOCO, South Dakota Office of Economic Development and South Dakota Cement Plant; with the participation of Clark Engineering, Glenn C. Barber and Associates, FMG Engineering, Polygon, Shell, Central Mix, Carlon/Ace Hardware and South Dakota Concrete Products. A 180 mm (7 in) concrete deck was post-tensioned with glass and carbon fiber tendons which were subse- quently grouted with epoxy based mortar. The deck is supported by steel girders 2.6 m (8.5 ft) apart and the bridge spans 9 m (30 ft).

Aramid bars were used in a standard pre- stress pre-cast concrete factory to build concrete barges (Noritake et aZ., 1992). At least 10 concrete bridges have been constructed in Japan with some kind of composite reinforce- ment (Mufti, Erki and Jaeger, 1991a). The Shinmiya bridge is a pre-tensioned concrete bridge built in 1988 in a coastal area. The pre- cast concrete girders were pre-stressed with seven strand, carbon composite tendons pro- duced by Tokyo Rope Manufacturing Co. Epoxy coated steel rebars were used for the

Fig. 46.3 Post-tensioned concrete-deck bridge over steel stringers. (Courtesy of Owens Coming.)

stirrups. The Sumimoto Construction Co. built a demonstration bridge in 1990 in Oyama using three concrete box girders pre-tensioned with aramid composite rods, produced by Teijin Co. Grouted anchorages were used to pre-stress the rods at a permanent stress of 70% of their static tensile strength. While the stirrups were also aramid composite rods, epoxy coated steel rebars were used as shear connectors between the girders and the rein- forced concrete deck, which was reinforced with epoxy coated steel rebars. The deck was post-tensioned with aramid composite rods which were then permanently grouted in the deck. The Birdie bridge was built by Kajima Co. in 1990 at the Ibaragi Prefecture using a variety of composite materials. Carbon fiber composite rods, produced by Mitsubishi Kasei, were used to anchor the abutments. Pre- cast concrete panels reinforced with vinylon short fibers are connected with a grid of car- bon fiber composite cables produced by Tokyo Rope Manufacturing Co. The bridge is pre- stressed with aramid composite flat bars commercialized by Nippon Aramid Limited. Half of the aramid bars are used as pre-ten- sioning tendons and the remaining act as post-tensioning tendons,with a permanent

Current applications 987

stress of one third the static tensile strength.

Post-tensioning

Post-tensioning of concrete with steel or com- posite tendons is performed to induce a state of compressive stress in the concrete similarly to pre-stressing, but post-tensioning is per- formed at the construction site. A hole or some kind of access is left in the concrete to thread the post-tensioning cables through. Tensile force is applied to the tendons against the concrete structure before the structure is loaded by the service loads. The anchorage system is subject to severe requirements. First, it must sustain the tension load for the whole life of the structure, which requires very care- ful design against creep in the anchor. Second, the anchor should be resistant to corrosion as the tendon itself if the non-corrosive proper- ties of the system are the objective of replacing steel tendons. The reduced weight of the ten- dons is not very important because the heavy weight of the concrete structure. While the stress losses of fiberglass tendons are smaller than those of steel cables, it is difficult to jus- tify the use of more expensive and novel composite tendons for this reason alone. Therefore, corrosion resistance is the main objective of using composite tendons. In this case the anchorages should be resistant to cor- rosion. Also, the structure to be post- tensioned is usually made of reinforced con- crete (although the reinforcement is not sufficient to carry all the load). If corrosion is a problem, the conventional reinforcement (not pre-stressed or post-tensioned) may also have to be made of composite material rebars.

Anchors for post-tension applications are still being developed and evaluated. Meisseler and Preis (1989) report on an anchor developed to hold glass reinforced tendons. The anchor is of the potted type, in which the tendons comprising a cable are spread at each end and potted in a steel anchor with some grouting material, usually a polymer. The potted type anchors are based

on the transfer of the axial load in the cable, by shear in the grouting material, to the anchor. Attaching the anchor to the tendons is a labor intensive process. The resin used to bond the tendons to the anchor may creep, leading to loss of pre-stress. This anchor was used to partially post-tension several bridges. Cases have been reported where individual tendons slipped and broke at the anchor dur- ing post-tensioning at the bridge site. Porter and Barnes (1991) report several anchorage systems. To avoid the problems that all pot- ted type anchors have, Ahmed and Plecnik (1989) developed a filament wound cable where continuous fibers are wound around the end eyes of the cable. Each cable must be custom made for the required length, but the problems of potted type anchors are elimi- nated.

The Bachigawa-Minami-Bashi bridge (Koga et al., 1992) uses both pre-stressing and post- tensioning carbon fiber tendons. The Schiessbergstrasse bridge in Germany and the Notch bridge in Austria (Wolf and Miesseler, 1992), use partial post-tensioning with fiber- glass cables. Some of the tendons that form a cable contain sensors (copper wire or fiber optical gauges) to monitor the strain level, integrity of the tendon, and location of even- tual damage. A demonstration bridge was built in 1990 by the Sumimoto Construction Co. in Oyama, Japan, using a single concrete box girder, post-tensioned with aramid com- posite rods produced by Teijin Co. The internal tendons are placed in a parabolic housing in each web and post-tensioned with steel grouted anchorages to a permanent stress of 25% of the static tensile strength of the ten- dons. The external tendons are placed at the bottom of the box girder and post-tensioned, with grouted anchorages built with composite casings for added corrosion protection, to a permanent stress of 10% of the static strength. Besides the composite post-tension cables, the regular reinforcement of the girder uses epoxy coated rebars. Further details are given by Mufti, Erki and Jaeger (1991a).

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Rehabilitation

There is significant interest in using compos- ite materials for rehabilitation and upgrading of existing structures. These structures may have been damaged as a result of corrosion of the steel reinforcement or they may need upgrading to new seismic standards, larger traffic loads and volume, etc. Interest in com- posites is motivated by the ease of installation of the reinforcement, its corrosion resistance (mainly at the bond surface), and the possibility of selecting from a variety of elastic modulii which improves the compati- bility between the reinforcement and the existing structure Meier et al. (1992), Saadatmanesh and Ehsani (1991), and others have demonstrated the feasibility of rehabili- tating concrete, steel, and wood structures by reinforcement with composite plates. Meier et al. report a variety of failure modes that may be encountered in concrete beams reinforced with composite plates. The reinforcement may be a cured composite plate bonded to the beam (Munipalle, 1992), or a room-tem- perature cure prepreg directly applied to the structure. The reinforced plates can be applied with or without pre-stressing. While pre-stressing the composite plate increases the efficiency of the reinforcement, it also complicates the rehabilitation process. Since delamination of the reinforcing element is of major concern, special reinforcement details are used at the ends of the reinforcement. Composite materials can also be bonded to the sides of beams to improve the shear strength. While using prepreg materials, both shear strengthening and resistance to delami- nations can be obtained by partially or completely wrapping the beam with the rein- forcement. Meier et al. (1992) reports on the rehabilitation of two bridges in Switzerland. The Ibach bridge was repaired using carbon fiber reinforced plates bonded to the concrete bridge. The rehabilitation of the timber bridge in Sins involved replacement of the wood pavement with transversely pre-

stressed and glued laminated deck (Davalos and Salim, 1992). Some of the timber beams were reinforced with carbon reinforced epoxy plates. Rostasy, Hankers and Ranish (1992) report on the reinforcement of the Kattenbusch bridge (Germany) using steel and glass reinforced composite plates. The main reason cited for the selection of com- posite plates is the documented corrosion of the steel plates at the bonded interface.

Composite jackets have been proposed to retrofit columns for seismic solicitations. Scale samples (40%) were tested (Priestley, Seible and Fyfe, 1992) and promising results were found. A fiberglass epoxy jacket is built around the hinge region of the column. On an active confinement jacket, hoop stress is induced by pressure grouting the space between the concrete column and the compos- ite jacket with either epoxy or concrete. A passive confinement is obtained when no pressure grouting is used. Passive confine- ment is used in regions of high compression stress, over the region of lap-spliced longitudi- nal bars, or for regions of high shear stress. Significant concrete dilation and consequent longitudinal microcracking is necessary to activate the confinement effect in passively encased concrete. Active confinement has the advantage that the confinement effect is always available.

Chimneys have been rehabilitated in Japan by Mitsubishi Kasei using carbon composite tape and strands (Mufti, Erki and Jaeger, 1991a). The composite tape is applied along the length of the chimney to provide addi- tional bending strength. The strand is wound around the chimney to provide hoop rein- forcement. A fire protection mortar is finally applied to protect the polymer composite from fire, to limit the heat gain, and to avoid degra- dation of the polymer matrix that otherwise would occur if the carbon composite were exposed to the environment.

Current applications 989

46.2.2 STRUCTURAL SHAPES Platforms

Composite structural shapes resemble steel hot- and cold-rolled structural sections. Composite shapes are produced by pultrusion in a variety of sizes and shapes (Creative Pultrusions, 1989; MMFG, 1992). The rein- forcement of choice is E-glass mainly because of low cost. Most shapes are produced with isophthalic polyester and vinylester, but some pultrusions using epoxy, phenolic, and even thermoplastics are available at higher cost. Even for the most common polyester and vinylester, there is a wide variety of resins sys- tems, differentiated by mechanical properties, thermal behavior, and cost. While vinylesters are considered to have superior corrosion resistance and mechanical properties than polyesters, high grade polyesters may match the mechanical properties of vinylesters.

Structural shapes are used primarily because of their excellent resistance to chemical degradation. In the case of electromagnetically transparent structures, structural shapes are routinely used for building construction in very much the same way as steel shapes. However, E-glass reinforced shapes have a much lower modulus of elasticity than steel, which causes design problems when direct replacement of steel shapes by composite shapes is attempted. Specialized sections, with cross sections different from steel shapes, are available for use in building systems with con- struction characteristics (e.g. joist spacing) very similar to steel frame buildings (Composite Technology, 1992). Unlike steel shapes, the mechanical properties of compos- ite structural shapes largely depend on the internal reinforcement, and the thermal and corrosion response varies drastically with the resin system used. Since the reinforcement and the resin systems used are not standard- ized, very different set of properties are possible for identical cross sections, from dif- ferent manufacturers.

Industrial platforms built entirely of pul- truded fiberglass reinforced plastics are widely used because of their corrosion resis- tance. An area of application of special interest is offshore platforms for oil production. Two views, from above and below, of a fiberglass well bay platform are shown in Figs 46.4(a) and (b). Steel well bay platforms corrode quickly in the marine environment. Painting jobs are difficult because sand blasting may release paint into the ocean unless costly pre- cautions, such as scaffolding, are taken. Old paint may be lead-based and cannot be stripped unless it is completely captured and dumped at a hazardous waste site. Well bay platforms are installed after the production wells are in operation. The area is usually clut- tered with equipment, which makes difficult the access of heavy lifting equipment needed for installing heavy steel platforms, and weld- ing cannot be done without shutting down oil production. Fiberglass platforms weigh typi- cally 30% of their steel counterpart, allowing for installation by a smaller crew, in less time, with less demand for space for lifting equip- ment, and reduced transportation costs to the site. The composite platforms are assembled with mechanical connections eliminating the need for welding and they are virtually main- tenance free in the corrosive marine environment, without need of painting. A 6 m x 12 m (20 ft x 40 ft) well bay platform was installed in 1986 on Shell’s Southpass 62 pro- duction platform in the Gulf of Mexico by a crew of four in two days and it is estimated that a similar steel platform would have required five days by a crew of eight because of the lack of room for heavy lift equipment.

Building systems

Complete building systems, including the structural frame, cladding and roofing are available in fiberglass reinforced composites (Composite Technology, 1992). Electromagnetic

990 Construction

Fig. 46.4 Well bay platform in an off-shore oil production facility (a) seen from above, @) seen from below. Composite materials manufactured by MMFG. (Courtesy of MMFG.)

Current applications 991

Fig. 46.6 Fiberglass spire installed atop the 55-story Nations Bank in Atlanta, GA. Comuosite materials

Interference (EM) testing buildings must be built of materials free of magnetic interference, with all metals ruled out and the use of timber limited by the need to use steel connectors. Pultruded structural shapes, fiberglass cladding, and foam core panels offer an attrac- tive alternative since all components can be connected with fiberglass bolts and or glued together to form an electromagnetically trans- parent structure. Innovative combinations of glued laminated timber (GLULAM) with com- posites have also been exploited. A computer testing facility (Fig. 46.5) was built for IBM in Poughkeepsie, NY, by Corflex International Inc. of Warren, OH, and Haines Lundberg Waehler of New York, NY, using pultruded structural shapes from Creative Pultrusions Inc. (1989).

manufactured by MMFG. (Courtes; of MMFG.)

Fig. 46.5 Computer testing facility using pultruded structural shapes and fiberglass reinforced panels. (Courtesy of Creative Pultrusions, Inc.)

The Nations Bank building in Atlanta, GA, shown in Fig. 46.6 features a 11 m (36 ft) tall, all fiberglass spire at its top. Electromagnetic transparency of the composite material used allows the spire to house valuable communi- cation antennae. Light weight, molded-in color, and timely delivery were cited as advan- tages of composite materials for this highly visible application. The spire is mounted 312m (1023ft) above ground, and it is designed to sustain a wind pressure of 550 kPa (BO psi).

Pedestrian bridges

A number of pedestrian bridges have been built with composite materials. Early bridges are reported by Meier (1991), Barber0 and GangaRao (1991) and others. While a few bridges have been built by hand lay-up, the majority are built with pultruded sections. Concrete bridges partially reinforced with composite tendons are discussed in previous sections on concrete reinforcement. In this sec- tion, we concentrate on bridges where most of the materials used in the construction of the bridges are composites. Because of cost, E-glass reinforcement has been used most.

Johansen et al., from E. T. Techtonics of Philadelphia, PA, report on the construction of

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shapes. (Courtesy of E.T. Tcchtonics.)

three bridges of 6.1 m (20 ft), 9.75 m (32 ft), and 15.24m (50 ft) (Fig. 46.7) using their design and construction method, called PRESTEK. Given the low stiffness of E-glass reinforced pultruded composites when com- pared to steel, conventional steel or concrete designs are not efficient when implemented with composites. The PRESTEK system uses a beam-truss geometry of pultruded tubes pre- stressed with aramid or steel cables. The three bridges are a king-post truss, a queen-post truss and a bow-string truss respectively.

Maunsell Structural Plastics in Beckenham, Kent, England, with the participation of GEC Reinforced Plastics (pultrusion manufactur- ing), Scott Bader Co. Ltd (resin supplier), Vetrotex UK (glass reinforcement), Ciba Geigy Plastics (adhesives), Linear Composites (Parafil cable stays), R. O'Rourke and Sons (construction management), and University of Dundee, constructed a pedestrian bridge over the river Tay in Aberfeldy, Scotland. The pultruded deck is cable stayed from two A- frame towers, 17.5 m (57 ft) high, to provide for adequate stiffness despite the low stiffness

of the pultruded glass-polyester deck (Fig. 46.8). The bridge is 113m (370ft) long and 2.23 m (7.3 ft) wide, with a main span of 63 m (206 ft). The deck and towers are constructed with Maunsell's interlocking panels (ACCS System by Designer Composites Technology Ltd), pultruded with 70% volume of fiberglass reinforcement and isophthalic polyester resin, then bonded together with an epoxy adhe- sive. The cable stays are Parafil ropes, a Kevlar fiber core in a polyethylene sheath. The bridge deck was designed to be modular so that every component could be handled by hand. The heaviest module, a 6 m (19.7 ft) by 0.6 m (2 ft) plank weighed only 66 kg (145 lb). The weight of the deck is only 150kg/m (lOOlb/ft) and each tower weighs only 2500 kg (5511 lb), facilitating erection and reducing the cost of the foundation. The com- posite system has class 2 fire resistance rating.

Bridge enclosures

The A19 Tees Viaduct in Middlesbrough, UK, is a steel-concrete bridge with 117 m (383 ft) span

Current applications 993

Fig. 46.8 Pedestrian bridge across the river Tay in Aberfeldy, Scotland, features an all-composite deck cable stayed with Parafil cables from two composite A-frame towers. (Courtesy of Maunsell Structural Plastics Ltd.)

(Head, 1988) where rapid deterioration of the steel plate girders was taking place. Maintenance (e.g. painting) and rehabilitation work are difficult since the viaduct spans over railroad tracks. A composite material enclosure was built in 1989 to prevent further deteriora- tion by isolating the bridge from the environment and to facilitate maintenance and rehabilitation. A floor area of 16000m2 (172 200 ft2) using 250 metric tons (275 US tons) of composite material was created under the bridge by interlocking pultruded panels wrapped around the steel girders of the bridge to create a box. The enclosure system was designed by Maunsell Structural Plastics Ltd, of Beckenham, Kent, UK. The system uses inter- locking panels designed by Maunsell and fabricated by GEC Reinforced Plastics Ltd, while the main contractor was Fairclough Construction Ltd. The enclosure system has been also used in the construction in 1992 of the enclosure to the Bromley South Bridge, seen in

Fig. 46.9, over the railway station. Pultruded plank and connector sections are joined to form a floor system suspended from the steel girders of the bridge. Pleasant appearance was required since the bridge is located in a resi- dential area and over the railway station. Light weight and low maintenance costs were cited as additional advantages of composites for this application.

Cooling towers

The resistance of composite materials to humidity and creep under sustained loads has been demonstrated by their successful appli- cation to cooling towers, in operation for more than twelve years (Fig. 46.10). Cooling towers are permanently loaded with the heavy weight of the ceramic filling used for cooling. These towers are built by Ceramic Cooling Towers of Forth Worth, TX, entirely of com- posites except for the ceramic filling. The

994 Construction

Fig. 46.9 Enclosure of the Bromley South bridge is accomplished with interlocking pultruded panels bonded together into a floor system and suspended from the steel girders of the bridge. (Courtesy of Maunsell Structural Plastics Ltd.)

composite material is in permanent contact with hot water and humid hot air circulating through the tower. Reduced maintenance of the tower is cited as an important advantage of using composites. Light weight of the struc- ture of the tower is an additional advantage when the towers must be located on top of buildings. Construction of large industrial cooling towers motivated the development of the UNILITE Modular System which takes advantage of modular construction to reduce cost. Composites facilitate modular construc- tion because of the variety of shapes that can be easily produced.

Marine construction

Potential applications for composites in the construction and rehabilitation of marine and waterfront constructed facilities include docks, piers, harbors, etc. An all-composite pontoon designed by Maunsell Structural Plastics Ltd, of Beckenham, Kent, UK and dis-

tributed by Designer Composites Technology Ltd, UK, is shown in Fig. 46.11. Finger piers of up to 15 m (49 ft) are built with cellular inter- locking pultruded panels, joined to foam filled flotation units. The surface of the pontoon is covered with a non-slip polyurethane coating which is acid, solvent, and heat resistant.

46.3 DESIGN CONSIDERATIONS

Current composite design practice emphasizes simultaneous design of the structure (beams, plates, frames, etc.) and the material (compos- ite) for optimum performance. Design practice for composites used in construction differs from aerospace applications in the sense that standardization of components is required because of cost and safety. Therefore, design is divided into member design, usually per- formed by the manufacturer, and system design, carried out by the structural engineer. Codes of practice do not yet exist for structural design with composites, but they are being

Design considerations 995 . .. . . . ._ ... .

k

Fig. 46.10 All-composite cooling towers like this, developed and manufactured by Ceramic Cooling Tower, have been in operation for more than twelve years.

developed. Some methods of analysis and design recently developed are described in this section.

46.3.1 BEAMS

Beams are the most common structural com- ponent in civil engineering applications. Both deflection and strength are equally important in the design of composite beams. Composite beams are thin-walled and composed of an assembly of flat panels. Most beams are pris- matic but they can have taper. They are produced by pultrusion, filament winding, hand lay-up, automated lay-up, etc.

Deflection of composite beams has two

deflections are controlled by the bending stiff- ness D, equivalent to EI for steel beams. Since composite beams have different values for modulus of elasticity at various points in the cross section, it is not possible to completely define the stiffness with respect to both axis of bending by the product of the modulus and the corresponding moment of inertia (€1 and €Ixx ) Instead, two bending stiffnesses < and Dx are defined, with respect to the strong axis and weak axis respectively. Shear deforma- tions are neglected for steel beams because the shear stiffness of steel is high (G/E = 0.4) while for composites is low ( G / E < 0.1). The shear stiffnesses of a composite beam with respect to the two axis of bending are denoted F y and Fx. The values of the bending, shear, and axial stiffness (Dy, K F ~ , A,) can be obtained from tables supplied by the beam manufacturer, obtained experimentally (Bank, 1989), or com- puted if detailed information about the constituent materials is known (Barbero, 1998). Maximum deflections can be computed using the formulae in Fig. 46.12. Experimental values of shear stiffness usually refer to the product of K F ~ where K is the shear correction factor.

c

. -+-----

Fig. 46.11 The pontoon pier shown is built of inter- locking pultruded panels and foam filled flotation devices. A polyurethane coating protects the surface from acid, solvents, and heat. (Courtesy of

components, bending and shear. Bending Maunsell Structural Plastics Ltd.)

996 Construction

W

n I

4 5 W L + 1 WLZ s%= 384% 8 K F ~

Fig. 46.12 Center deflection of composite beams including bending and shear effects.

Design considerations 997

The sigruficance of the shear deflection with respect to the bending deflection varies with the span, the larger the span the lesser the influ- ence of shear. Sometimes, properties of beams are reported without distinction between the bending and shear components, using an apparent value of modulus Eapp = Dy/ l yy ,where D,, is the bending stiffness with respect to the strong axis. The bending stiffness with respect to the weak axis Dx cannot be accurately obtained as Dx = Eap Ixx. The apparent stiffness Eapp is then used in &e classical deflection for- mulas (for steel) that do not account for shear deformations. The reported values are usually based on three-point bending tests, performed at the factory with a specific span, which is sel- dom reported. The results of using the classical (steel) deflection formulas for spans or loading other than that of the test are only approximate.

The main modes of failure of beams in bending are: (a) compression crushing of the compression flange; (b) local buckling of the compression flange; (c) tensile rupture of the tension flange; (d) shear failure of the web; and (e) web buckling. Since each part (panel) of the cross section can be built with different materials, the failure mode can be controlled by design. Local buckling modes can be elimi- nated by increasing the thickness and choosing the fiber orientations properly (Barber0 and Raftoyiannis, 1993). The com- pression strength of composites is lower than the tensile strength. Therefore, a symmetric section is not the most efficient cross section. Symmetric sections fail in the compression flange first. Tensile failure may occur in unsymmetric sections, when the compression flange works with composite action with a deck, etc. Shear failure is less likely to occur in sections with multiple webs. Incorporation of off-axis fibers (cloth, mats, etc.) increases the shear strength.

46.3.2 COLUMNS

Columns are structural members subjected primarily to compression forces acting along

the length of the member. Lateral forces (e.g. wind forces) and bending moments (e.g. eccentric loading) are considered secondary forces, which are dealt with separately. Column performance is limited by one of two failure mechanisms, crushing and buckling. Crushing is the failure of the material because of excessive compressive stress, similar to yield of steel. Buckling is more frequent in composite columns because thin-walled sec- tions are preferred. A thin-walled section may experience at least three different types of buckling, which are described next.

Long and slender columns fail in a global sense when the axial load reaches a critical value P,, For load values lower that the criti- cal load the column remains straight. When the load reaches critical value, the column experiences sudden lateral deflection. The axial stiffness after buckling is much lower than the stiffness before buckling. Therefore, the lateral deflections are quite large and they usually precipitate another mode of failure like crushing of parts of the cross section, lead- ing to collapse.

The concept of slenderness allows us to compare members of different cross sections and column lengths for their tendency to buckle. For a composite column, the slender- ness is defined as

= L&) (46.1)

where P, is the local buckling load, D is the bending stiffness, and Le is the effective length of the column, which is used to account for different end conditions (Gere and Timoshenko, 1990, p. 589).

Short and stubby columns, which have a low Slenderness value, are less likely to buckle in a global mode as described previously. However, individual parts (flange or web) of the cross section may buckle locally. Local buckling is very likely to occur in composite columns because they are commonly thin walled. The compressive stress required to trig- ger local buckling increases with the thickness

998 Construction

of the cross section and the local stiffness of the material. The narrower the flange or web under consideration, the higher the local buck- ling load P,. The wavelength of local buckling is independent of the length of the column for columns of practical lengths.

Flange buckling of open section members is triggered by pure compression. It occurs during compression of columns and on the compres- sion flange of beams in bending. Web buckling is initiated by shear and it occurs during bend- ing of beams. Web buckling of open section columns is not common because most open sec- tions have wide flanges that buckle first. There is of course no distinction between flanges and webs in closed section columns. Unlike steel structural shapes, composite closed sections are easier to produce and structurally more effi- cient than open section members.

Local buckling can be prevented by choos- ing the section geometry and material properties. Global buckling can be prevented by a combination of section geometry, material properties and bracing. If all buckling modes are prevented, the strength of the member is limited by the crushing strength of the mater- ial itself, which plays a role similar to the yield strength of metals. The crushing strength is a material property which is independent of the thickness of the flange or web and the geome- try of the section. As for any composite property, it depends of the constituents (fiber and resin) and the arrangements of the fibers inside the material (orientation, fiber volume fraction, stacking sequence). The crushing strength is usually determined experimentally but the main factors that influence its value can be highlighted by predictive equations (Tomblin, 1994; Barbero, 1998).

A slender column buckles in a global (Euler) mode. A not-so-slender column may fail in a local buckling mode. A thick-walled stubby column may fail due to crushing. Columns with a slenderness ratio less than 0.5 fail in a local buckling mode. For slenderness larger than 1.5 the mode is purely global (Euler). Columns with slenderness between

0.5 and 1.5 show some type of interactive phe- nomenon. The interaction occurs between the local mode, the global mode, and crushing. Interaction results in lower buckling loads than those predicted by any of the modes act- ing alone.

There are many situations of practical inter- est for which the buckling loads required to produce two or more failure modes (Euler, local, crushing) may be very close. In this case the failure modes interact. That is, the proxim- ity of the stresses to more than one mode of failure causes the structure to fail at a lower stress value that predicted by either of the modes involved should they be acting sepa- rately. Euler and local modes interact to give an overall strength deterioration. Interaction must be taken into account because the strength values predicted by either isolated mode fit are not conservative. The failure load of a column (Po) taking into account local, global, and interaction phenomena simultane- ously can be obtained from the following design equation (Barbero and Tomblin, 1993)

The column properties needed to use this design equation are: the local buckling load PL; the interaction constant c; and the bending stiffness D, which along with the length of the column enter in the computation of the slen- derness 1 (46.1). All these properties can be determined experimentally or predicted ana- lytically. The bending stiffness D and the local buckling load P,, can be predicted analytically, while an analytical study of the interaction phenomenon is presented by Raftoyiannis (1993). The design equation does not have any safety factor included. Typical properties of wide-flange pultruded structural shapes are given in Table 46.1 where the interaction con- stant is c = 0.84 for all sections reported.The length at which maximum interaction occurs is denoted by L‘.

Design considerations 999

Table 46.1 Column properties obtain the Euler buckling load P,,, as the

Section PL (kN) D (kN cm2) L x (cm) 102 x 102 x 6.4 223.25 6094.67 105.9 152 x 152 x 6.4 175.12 20954.42 221.5

Testing of short columns is performed to iden- tify the local buckling load P, in the column design (46.2). Buckling of the flanges is seldom a sudden phenomenon as described by the theory because of the imperfections of the material. Therefore, flange lateral deflections are observed from the onset of the test and they grow as the load is applied. The buckling load P,, to be used in the design corresponds to the asymptote of the hyperbolic curve of load vs. deflection. Since it is not practical to conduct the test up to the large deflections needed to realize the asymptotic value of load, a data reduction technique (Tomblin, 1991) is used.

Columns having slenderness larger than 1.5 will buckle in a global mode (Euler). The Euler buckling load is controlled by the bending stiffness D. A column with pinned-pinned end conditions is subjected to an axial load (prefer- ably under axial displacement control). Pinned end conditions are the only conditions that can be achieved with any degree of certainty. A 100% degree of fixity required by a clamped end condition is not achievable on composite columns because of the difficulties associated with connecting composites. Data from pinned-pinned columns can be used for other end conditions by using the effective length concept (46.1). Weak axis tests are simpler to perform, since strong axis tests require the use of lateral support. Because of the imperfec- tions in the material and loading fixture, the load deflection plot has an hyperbolic shape, the buckling load being given by the asymp- tote of the plot. A data reduction technique known as the Southwell method is used to

inverse of the slope in the A/€' vi: A plot, where A is the lateral deflection and P is the load.

Interaction testing can be performed with the same setup described for global (Euler) testing. The objective is to determine the inter- action constant c in (46.2). Once two or more modes of failure interact, the Southwell method cannot be used. Therefore, only the maximum collapse load is reported for tests performed on columns having slenderness values between 0.5 and 1.5. The collapse load is lower than the critical load that would occur should any of the modes involved act isolated from the others, as the experimental data clearly indicates. The test is conducted with a column length that exhibits maximum interac- tion (L*) , which occurs for a column slenderness 1 = 1. The interaction constant is computed as c = (q + s - l)/qs, where q = P/P,, s = P/P,,, P is the collapse load, P, is the local buckling load (predicted or previously mea- sured on short columns) and P,, is the Euler buckling load (predicted or previously mea- sured.)

46.3.3 REINFORCEMENT OF CONCRETE

Reinforcement of Portland cement concrete can be accomplished with composite reinforc- ing bars (rebars) instead of steel bars to minimize the corrosion of steel and cracking of concrete caused by the expansion of the cor- roding steel rebars. Composite rebars should have good bond with concrete and adequate corrosion resistance. Pultruded rebars are the most common and least expensive alternative for the reinforcement (not pre-stressed) of con- crete. Pultruded rebars have an angle overwrap and/or a sand coating to improve the bond with concrete. Fiberglass rebars, aramid rebars, seven-wire carbon cables, grids, and even gratings have been used in research studies as reinforcements of concrete.

Fiberglass rebars have lower modulus of elasticity than steel rebars, causing larger

1000 Construction

deflections than in steel reinforced concrete beams. Typically, the deflection of reinforced concrete (RC) beams is controlled by two fac- tors: the amount of concrete that remains un-cracked (essentially in compression); and the modulus of elasticity of the reinforcement. The extension of cracking in the tension side of RC beams depends largely on the strength and uniformity of the bond between the rebar and concrete. Good bond translates into uniformly spaced cracks in the tension side of the beams. Therefore, a larger area of concrete remains un-cracked, thus contributing to the bending stiffness and limiting deflections. Fiber rein- forced composite rebars have a linear stress-strain behavior up to failure, thus they experience a brittle behavior when compared to steel rebars that have a yield plateau (i.e. plasticity) before they rupture. Brittle behavior is not convenient from a safety point of view because of possible catastrophic RC member failure. However, concrete reinforced with fiberglass rebars experiences very large deflec- tions up to failure, larger than comparable steel reinforced beams in their plastic regime, because of a combination of low modulus of elasticity and high tensile strength of the fiber- glass rebars.

No code or standard regulating the design of RC beams, reinforced with fiberglass com- posite reinforcing bars (rebars) is in place at the time of this publication. Faza and GangaRao (1993) suggest that it is possible to proceed along the lines of the American Concrete Institute guidelines ACI 318-89 com- plemented by the recommendations ACI 363 R, properly modified to account for the prop- erties of the fiberglass rebars.

46.3.4 ENVIRONMENTAL EFFECTS

Dead loads (e.g. the weight of the structure) will be applied to the composite material for the life of the structure. Sustained loads induce two major effects in composites, creep and stress rupture. Creep and relaxation are two altema- tive descriptions of the same phenomenon.

Creep is observed as increasing elongation under constant load. Relaxation is the reduction of stress over time for a constant elongation. Permanent levels of stress in the composite should be under the stress rupture limit (also called static fatigue) of the composite. Martine (1993) reported stress rupture of E-glass rein- forced composite at 10 000 h at 58% of the initial strength. Glaser, Moore and Chiao (1983) report survival of S-glass reinforced epoxy specimen to a 10 year sustained load test: 90% of the spec- imens survived at 50% of static strength; 98% survived a 40% loading; and 100% survived a 35% loading. To account for stress rupture and other factors in the design of fully overwrapped pressure vessels for compressed natural gas, stress ratios (burst over service fiber stress) of 3.5 for fiberglass, 3 for aramid, and 2.25 for car- bon reinforced composite were proposed by the American National Standards Institute (AGA NGV2 1992). These are indications that the sta- tic strength of fiber reinforcements cannot be utilized for long duration loads like those encountered in construction applications.

Glass fibers, if unprotected, deteriorate when exposed to an alkaline environment. Therefore, glass reinforced composites rely on the protection provided by the resin to resist the attack of alkaline environments. Alkaline degradation is of particular concern in con- crete reinforced with fiberglass composites since the concrete mix is alkaline. While con- crete alkalinity is well documented at the time concrete is poured, only now are studies being conducted to investigate the level of alkalinity of concrete with time. Water is needed to establish a link between the glass fibers and the alkaline concrete environment. If water is present, the link can be established through cracks in the resin. Permeability of the poly- mer matrix resulting from voids or moisture diffusivity of the polymer are being consid- ered as possible additional mechanisms that may place the alkaline concrete environment in contact with the glass fibers.

S2-glass reinforced Shell Epon 9310 com- posite rods produced by pultrusion were used

References 1001

by Sen, Issa, and Mariscal (1992) to pre-stress concrete piles. Seven 3 mm (0.125 in) diameter rods were twisted with one turn every 30 cm (12 in) to create a seven wire strand. The strands were used to pre-stress concrete piles to be used in marine environment. To simulate marine environment, the piles were subjected to wet and dry seven-day cycles in a 15% sodium chloride solution, then tested to failure in bending. To facilitate the moisture ingress, four out of eight glass composite reinforced piles were pre-cracked at the mid-span prior to testing. The bending strength of specimens subjected to the wetting cycles, specially those pre-cracked, reduced significantly over time. From the pre-stressing force applied, it is pre- dicted that 1.6% strain was induced in the rod, while larger values of strain may have been induced during the precracking of the beams. Additionally, radial stresses produced by the twist of the rods in the strand and by the mois- ture ingress in the resin may have caused cracking. Although the neat resin failure strain is reported at 4%, the strain applied to the composite rods may have produced cracking of the matrix, thus facilitating the ingress of moisture carrying the alkaline solution to the glass fibers. Scanning electron microscopy micro-photographs clearly show degradation of the glass fibers along with some cracking of the matrix. An additional pile was fabricated with pultruded fiberglass rods for which an additional coating resin was added to provide a smooth finish. This pile performed signifi- cantly better than the other four after a nine month exposure, perhaps because of the added protection of the finishing resin. It is concluded that the alkalinity of the environ- ment (eg. concrete), the availability of water or other solvent, and the protection provided by the resin must be evaluated for each spe- cific application.

46.4 CONCLUSIONS

Composite materials have been used for many structures that have performed well over

many years of service, mostly in adverse con- ditions. Composites are being used extensively for applications where the advan- tages significantly justify their higher initial cost. Composites have been successful when the design and manufacturing of a product was performed by a single company or group of companies, using the integrated design approach typical of the aerospace industry. The use of composites in the traditional civil engineering environment, where individuals create unique structures from standard com- ponents, has been slow, with the exception of markets where corrosion resistance offsets higher costs and less than optimum perfor- mance. Large markets have not developed partly because of lack of design codes and specifications and lack of awareness of com- posites advantages by structural engineers. A number of factors, including the decline of military industry and the need for rehabilita- tion of USA infrastructure, have produced a flurry of activity in this area, which undoubt- edly will produce significant progress in the application of composites in construction.

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ACKNOWLEDGEMENTS

My sincere gratitude to Prof. G. Turvey for his help researching applications in Europe, to all the con- tributors of information for this article, and to West Virginia University for the support of this project.