Composite materials have gained acceptance among civil engineers as reinforcements in new concrete construction, both as replacements for traditional steel rebar and grid and, more recently, as fibrous reinforcements in the concrete mix. Equally important, however, is the ever-growing application of composites in structural repair and rehabilitation of existing infrastructure — from buildings to bridges and beyond.
"Composites have perhaps found maximum current use in civil engineering as materials for rapid and cost-effective rehabilitation of deteriorating and under-strength concrete components," says Dr. Vistasp Karbhari, well-known professor of structural engineering at the University of California, San Diego, and a proponent of composite materials in infrastructure. High strength and stiffness, low weight, corrosion resistance, ease of installation and the fact that installation can be rapidly effected without traffic disruption — all of these advantages make composites attractive. On the other hand, cautions Karbhari, composites are not universally applicable to every situation, and key issues still need to be addressed, including fire resistance, smoke and toxicity and long-term durability. Further, repeatable and standardized properties need to be achieved at a reasonable cost, and a complete set of codes and standards has yet to be established.
Strides are being made in all of those areas, says John Busel, chairman of the American Concrete Institute's (ACI) Committee 440. Tasked with developing specifications for fiber-reinforced polymer (FRP) composites used to reinforce or repair concrete structures, the Committee already has issued several (see below). The committee's goal is to provide the designer or engineer information that captures the long-term performance of composites subjected to various environmental conditions, including moisture, chemicals, alkalinity, extreme temperature, UV as well as creep and relaxation, to provide a basis for specification of composites, says Busel.
Meanwhile, repair and rehabilitation strategies already in use are taking advantage of composites' diversity and flexibility, in applications that range as widely as practical infrastructure rehabilitation, homeland security applications and seismic upgrades.
|The American Concrete Institute's (ACI) Committee 440 has developed the following specifications for fiber-reinforced polymer (FRP) composites used to reinforce or repair concrete structures: ACI 440.2R-02 - Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures was released in 2002. Several months ago, a guide providing model test methods for short- and long-term testing of composite materials was released (ACI 440.3R-04 - Guide Test Methods for Fiber-Reinforced Polymers [FRP] for Reinforcing for Strengthening Concrete Structures). Further, Committee 440 is developing a state-of-the-art document that summarizes and addresses the durability of FRP used in conjunction with concrete.|
CULVERT AND PIPE LINERS
Composites have found a unique niche for structural renovation of large-diameter underground pipes. Single- or multi-piece composite liners up to 2.5m/8 ft in diameter or more are being installed inside large-diameter circular and noncircular sewer mains and culverts — a pipe within a pipe — in situations where leak repair, corrosion and chemical resistance and improved hydraulic capacity are required, and where the host pipe is too large for a cured-in-place pipe (CIPP) application. Once installed in the host pipe and held in place with grout pumped into the annular gap, the liners provide a time saving structural repair option that can be implemented without excavation. Liquiforce (Romulus, Mich.) is a specialist in the installation of culvert and trunk sewer liners, with nearly 6,096m/20,000 ft of installed product, says vice president Kim Lewis. The company previously represented several composite liner manufacturers, but now has a license to install products supplied by SABAS Pipeline Systems LLC (Wilmington, Del. and Romford, Essex, U.K.), which employs a unique, patented closed-mold manufacturing method to make the large circular or elliptical forms. According to Lewis, finite element modeling is employed to predict all loading conditions, including those imposed during the installation phase.
SABAS' Lee Skinner explains that liner thickness, modulus and strength is determined by the specific design criteria for each application. "The basis of composite liners comes from the work of the Water Research Centre in the U.K. during the 1970s and '80s, which resulted in the Sewer Rehabilitation Manual," says Skinner. That document, now in its fourth edition, provides liner performance criteria. A "partially deteriorated" Type II SRM design, for instance, assumes that the host pipe, while corroded or damaged, still is capable of maintaining current soil and traffic loads. The liner acts as a "flexible structure" that will prevent further degradation while resisting both the loads induced during installation and the long-term hydrostatic loads imposed by groundwater pressure. In contrast, when significant deterioration is observed in the sewer or culvert (defined as visible large fractures or significant deflection), or there is an anticipated increase in ground or traffic loading, the liner is designed in a "fully deteriorated" condition.
With composites, says Skinner, it also is possible to provide a "Type I" rigid structure whereby the liner, the annular grout and the host pipe act together in "composite" action. This type of rigid design can be used to resist ground and traffic loadings, but to achieve the composite action, careful attention needs to be paid to preparation of the host pipe and the liner to ensure a good shear bond with the grout material. "If there is groundwater present, the liner should be designed as a Type II to account for hydrostatic pressure," he adds.
A composite liner, made in multiple segments that are jacked or pulled into the large host culvert or sewer pipe, invariably reduces the pipe cross-section slightly, which can impact hydraulic capacity — that is, how much water the pipe can carry. But, because the composite liner is smooth with a much lower coefficient of friction than the host pipe (e.g., corrugated steel, concrete or brick), water transport is more efficient so even with a smaller diameter flow hydraulic capacity can be increased.
To make its one-piece liner segments, SABAS starts with a resin-rich corrosion barrier consisting of chopped strand mat wet out with polyester resin, applied to a 1.8m to 2.5m (6-ft to 8-ft) long tubular fiberglass mandrel of the same diameter as the liner's inside dimension — ranging from about 1.2m/4 ft to 2.5m/8 ft or more. This step is followed by a combination woven E-glass fabric with stitchbonded discontinuous fibers, made to SABAS' specification, which is placed dry over the corrosion barrier to provide a reinforcing "skin." At this point, a matching cylindrical female mold, loaded with the same glass fabric skin, is placed over the mandrel. A "polymer mortar" — a mix of sand and resin — is then injected at about 10 psi pressure to a predetermined thickness between the glass skins in the two molds, to form a sandwich structure. A vacuum also is pulled on the mold cavity to facilitate the injection process, allowing the resin to be drawn completely into the skins. The fabric skins filter out the fine sand from the polymer mortar, allowing for complete resin wet out. The glass-to-resin ratio in the skins is high, about 55:45, providing excellent mechanical properties, says Skinner.
SABAS' fiberglass molds are produced from plugs machined with computer-controlled equipment, which allows high accuracy and repeatability. "Because of the very low injection pressures involved, low-cost tools can be used, yet our accuracy is good and production is much faster than hand layup, with repeatable quality," Skinner maintains. "We have the flexibility to design parts in varying thicknesses and can make one-piece or multipiece parts, depending on the project needs."
Typical part thickness varies from 15 mm to 40 mm (0.6 inch to 1.6 inches) but can be up to 100 mm/4 inches or more, if needed because the relatively high sand-to-resin ratio helps control exotherm. The liner sections are bonded together in the field after jacking them into place or pulling them into the host pipe on a wheeled loading trolley. SABAS has patented its "groove-and-groove" joint, explains Skinner. "A traditional tongue-in-groove joint tends to create interlaminar shear loads, which are hard to resist with a sandwich construction and can lead to breakage," he says. "Our method is essentially a lap joint with uniaxial glass-reinforced epoxy strips added on both sides of the lap, which increases the radial thickness of the joint for more strength." During installation, the joints are injected with a flexible filled epoxy, formulated by SABAS, which forms a watertight seal.
The same joint design is used for connecting two-part liners, which have a separate upper arch or crown that joins the lower half or invert. SABAS liners are currently fabricated in India and shipped to the U.S. and other markets. "We expect a U.S. manufacturing facility will be in place in the Midwest within 12 months, as well as an additional plant in Canada within the same period," says Skinner. "We're continuing our R&D into other fields of application and expect to take our matched mold method to other markets and products."
Liquiforce's Lewis reports that the company is currently working on relining a 1.85m/72-inch round sewer liner in Cleveland, Ohio, involving the installation of 440 linear ft of 1.7m/66-inch SABAS liner through multiple bends. Another recent project was a large-diameter culvert relining for the Erie County Department of Public Works, Division of Highways, in western New York. A composite liner was specified when severe rust was discovered in an existing corrugated metal arch culvert under a five-lane collector street carrying 38,000 vehicles per day, says Carl Dimmig, Erie County's supervisor of bridge inspection. The loss of the culvert's structural integrity and strength required a Type I rigid design, with the liner designed to take significant load. Buried utilities nearby would have made full excavation and replacement very difficult, explains Dimmig. "We've used fiberglass liners in other locations previously, so we were comfortable with the technology," he notes. "Digging and replacing the culvert wasn't an option." For the Erie County project, Liquiforce worked with another liner manufacturer, Channeline Sewer Systems Ltd. (Dartford, Kent, U.K.), which manufactured the project's 13 liner sections, using hand layup methods. Each section was made in two parts (an upper and lower half) with tongue-in-groove joints. Parts were manufactured in the U.K. and shipped to the project site, says Lewis.
WATER PIPE REHABILITATION
In many parts of the world, potable water is transported through large-diameter prestressed concrete cylinder pipe (PCCP), a highly engineered high-performance pipe that can withstand relatively high pressure (more than 1,380 kPa/200 psi) and installation depths of 8m/26 ft or more. The pipe consists of five elements: 1) an innermost layer of high-strength concrete, 2) a steel liner, 3) a high-strength concrete core, 4) a layer of closely spaced, helically wound steel prestressing strands that are securely anchored at the end of each 6.2m/20-ft pipe section, sometimes called a "spool," and 5) an outer shell of dense concrete mortar. Because the prestressing wires are in tension as they are wound around the pipes, they effectively subject the surrounding concrete to constant compression as they try to relax, which greatly increases the concrete's performance. Some of the PCCP in use is now more than 25 years old, and if a segment fails, says Karbhari, the results can be catastrophic. "When a single section of the Mojave Siphon pipeline failed during the 1990s, substantial disruption of service occurred," he says. "It took 26 days and more than $500,000 to excavate the joint and effect a repair using conventional methods of repair."
Karbhari and colleague David Lee have developed a technique for rapid and cost-effective repair of PCCP using composites. Their experimental approach was recently tested at the University of California, San Diego as part of a program funded by the California Department of Water Resources. Ameron International (Houston, Texas), which manufacturers both PCCP and composite pipe, provided sections of 2.44m/8-ft diameter pipe for the tests. The tested pipe had a rated burst pressure of 2,965 kPa/430 psi. Sections of the pipe were sliced into narrow rings for the experiments.
Two different composite repair layups were tried on the ring slices: wet layup of 0.3m/12-inch wide unidirectional stitch bonded carbon fiber fabric (680 g/m2 or 20 oz/yd2), impregnated with either epoxy or vinyl ester; and adhesive bonding of 0.15m/6-inch wide, precured E-glass/vinyl ester strips supplied by Hexcel (Dublin, Calif.). The test segments were sandblasted and scored, and both materials were layed up in the hoop direction on separate segments and compacted with rollers and squeegees. Each strip was overlapped and joints were staggered over the pipe circumference. "We used both the wet layup method and the pre-cured strips, to see if a prefabricated product would reduce field variability in repair mechanical properties," says Karbhari. In the field, workers, who enter through manholes, would perform the repairs from inside the pipe.
With the patches in place, three types of tests were conducted. The first involved subjecting the pipe segments to simulated internal pressure, by placing hydraulic jacks inside the pipe test segment, bearing outward against cast reinforced concrete "loading shoes" that fit inside the pipe circumference. Next, external loading was applied, simulating strength in a buried condition, in accordance with ASTM C497-98 procedures; that is, the pipe segments were placed in a flat, vise-type load frame with pressure exerted on both sides by a hydraulic jack. Finally, a full-scale PCCP section was placed upright, sealed at each end with reinforced concrete bulkheads made watertight with gaskets and polymer grout. The bulkheads were post-tensioned with DWYDAG tie rods, and then the sealed section was filled with water and pressurized. For the full-scale test, the composite repair (carbon fiber/epoxy applied in wet layup) was instrumented with strain gauges. All three tests stressed the pipes to failure.
"We assumed that the FRP repair would work in concert with the steel liner, without benefit of the prestress, as if corrosion had caused failure of the wire," explains Karbhari. "Because the steel liner yields at fairly low loads, the FRP liner would have to provide at least 82 percent of the PCCP's pressure capacity." The results showed that composite-repaired specimens, without the prestressing effect, were actually stronger than the original as-built pipe — a glass/vinyl ester repaired segment using prefabricated strips adhesively bonded with Sikadur 30 adhesive from Sika Corp. (Lyndhurst, N.J.) demonstrated a burst pressure 131 percent higher than the as-built control.
According to Karbhari, both types of repair materials worked well, with the prefabricated strips applied with adhesive, providing a higher level of uniformity in properties. The number of needed layers can be readily calculated based on the pipe's pressure rating and appropriate factors of safety. "This is a rapid and cost-effective alternative," he sums up. "Repairs can be done in a matter of a few hours, for less than half the cost of conventional options, with no heavy equipment or digging needed."
Using the repair techniques tested by the University, three emergency PCCP repairs have been successfully concluded by Fyfe Co. LLC (San Diego, Calif.) for the San Diego County Water Authority, says Fyfe's general manager Scott Arnold. "We've used wet layup carbon in the actual field repairs," says Arnold. "But glass or a combination of carbon and glass works just as well, although more layers are needed." Other recent repairs include the Allen-McCullough water pipeline owned by the Metropolitan Water District of Southern California. California is leading the country in its use of composite repair for PCCP, says Arnold, although Fyfe also has completed repairs for municipal water districts in Waukegan, Ill., Norfolk, Va. and Tempe, Ariz.
Composites also are being proposed as alternative materials to brick, clay tiles and concrete blocks traditionally used to make "infill walls" for buildings in earthquake-prone areas. These panels, connecting a building's steel structural elements and hidden inside interior walls, were originally intended to help strengthen and stiffen a structure during an earthquake. However, the unintended result of the increased stiffness may be greater damage to the building's structure, not less, because the infill walls tend to concentrate loads rather than dissipate them. "Even if the structure of a hospital building survives an earthquake, failure of architectural, mechanical or electrical nonstructural components can render the building unusable," says Dr. Amjad Aref of the University of Buffalo, Department of Civil, Structural and Environmental Engineering. Aref has been developing an innovative method to "harmonize" or tie together the performance of structural and nonstructural building components to improve the entire building's resiliency level during and after an earthquake. The research is being conducted thanks to a grant from the Multidisciplinary Center for Earthquake Engineering Research (MCEER), established by the National Science Foundation and headquartered at the University of Buffalo.
The focus of his research is to introduce low-cost composite panels that, under seismic loads, will deform and fail in shear, leaving the steel framework undamaged. Aref and his research team have come up with a basic sandwich panel construction that attaches either to vertical structural columns or to floor beams. The sandwich concept has E-glass/vinyl ester laminate skins, and a core with alternating layers of low-cost polypropylene honeycomb supplied by Nida-Core Corp. (Port St. Lucie, Fla.) and a rubbery visco-elastic material supplied by 3M (St. Paul, Minn.). Attachments are basic steel angles, which are bolted to the panels and welded to the steel structural members. The panels have been extensively tested in the laboratory, by simulating earthquake accelerations using hydraulic actuators.
The composite infill panels have three times the energy dissipation capacity of steel, reports Wooyoung Jung, Aref's student. Under load, the panels fail in shear and buckle, while damping the seismic energy and minimizing floor accelerations. "The panels are intended to be destroyed," explains Aref. "They absorb the movement energy and save the building's structure — and in turn the building's nonstructural elements as well." Added benefits are that composite panels are so light that they do not significantly contribute to the structure's gravity load and they are easily installed. No actual installations have been completed as of yet, but Aref foresees wide application in the future for this low-cost means of seismic upgrade both for existing buildings and new construction.
Another option for composite repair and rehab of concrete is a relatively new reinforcement material called Hardwire. Hardwire LLC (Pocomoke City, Md.) manufactures steel-reinforced composites that utilize thin "cords" of high-tensile twisted steel wire. The number of cords can be varied to produce specific product performance and can be combined with any type of resin in a flat fabric or unidirectional tape. Tensile strength of the wire is as much as 3,200 MPa/450 ksi, which approaches that of E-glass (3,445 MPa/500 ksi) and standard modulus carbon fiber (3,600 MPa/522 ksi) but at a lower price. The twisted cords' geometry helps create a good mechanical bond between the steel and the resin material, as well as the surface to which the material is bonded. The material has been used to repair buildings, bridges and floors as well as for blast hardening and military armor.
Structural Preservation Systems (SPS, Hanover, Md.), a well-known concrete repair contractor with wide experience in traditional composites, is an equity partner in Hardwire and has been using the material in structural rehabilitation projects. A recent project was the Hippodrome Theatre, in Baltimore, Md., built in 1914 as a vaudeville house. Because of its prime location in an area slated for urban renewal, city officials selected the structure for renovation and preservation. SPS was contracted to repair the concrete slab within the theater's main seating area and balcony. When the stairs leading to the balcony were found to be structurally deficient, a combination of both carbon fiber/epoxy and Hardwire repairs were implemented, says the company's Tarek Alkhrdaji. Carbon fiber/epoxy strips were applied to the bottom surface of each stair tread via wet layup, to increase the tread slab's bending capacity. To support the stair risers and guard against cracking due to movement under load, Hardwire strips were bonded to the backside of each riser with an epoxy adhesive.
JUST THE BEGINNING
There are many more repair approaches and success stories, and not enough pages to describe them — from ISIS Canada's innovative research into
repair strategies, to the dozens of concrete repair contractors, like Ace Restoration & Waterproofing Inc. (Fullerton, Calif.), which recently completed a seismic upgrade of an Intel Corp. manufacturing facility using carbon and E-glass on the building's floors. The applications will continue to expand, and as they do, CT will continue to cover this growing market for composites.