1 Introduction
Previous studies of rectangular beams have shown that fiber-reinforced polymer (FRP) wraps of the full cross section improve the capacity of the section. The challenge of applying an FRP wrap to a beam with slab and the benefits of such an upgrade have not been assessed. This technology has potential application to highway bridges constructed in accordance with American Concrete Institute (ACI) codes of the 1950s and 1960s where these bridges may have less shear capacity than flexural capacity, or where added load capacity is required. A proven repair method for this reinforced concrete (R/C) application may provide a cost-effective solution for military installations and Corps of Engineers civil works facilities, as well as civilian departments of transportation.
Shear repair of reinforced concrete beams using externally bonded materials is not a new concept. For many years, sheets of steel were applied to the tensile face of damaged beams. The steel was effective in increasing both the shear and flexural capacities of the member, but there have been two major disadvantages to this method. First, bonding the steel to the beam is quite difficult in the field due to its bulk. Second, the new plate is susceptible to corrosion, which can cause loss of the adhesive bond.
An innovative method of beam shear repair involves the use of FRP externally bonded to the faces of the member where shear capacity is deficient. Several schemes are available: FRP plates bonded to the sides, strips of FRP material bonded to the sides, or a jacket (wrap) placed along the shear span. FRP addresses the traditional material weaknesses of steel discussed above: it is not susceptible to corrosion and is relatively conducive to field prepping and hand lay-up. There have been several studies investigating the use of externally bonded FRP sheets to improve strength and stiffness of R/C beams, but most of these have dealt with improving beam flexural strength. Only a few studies have specifically addressed shear.
Al-Sulaimani et al. (1994) tested simply supported R/C beams with fiberglass in all three configurations (plates, strips, and wrap) under four-point loading. The specimens were 6 in. x 6 in. in cross section and 49.2 in. in length. Compression and tension reinforcement as well as web stirrups were present. These beams were pre-damaged before retrofit and were designed to fail in shear (the stirrups served mostly to confine the flexural reinforcement). The researchers determined that fiberglass plates and strips bonded to the sides of the beams produced a modest (25-30%) increase in shear capacity. This repair technique, however, did not provide enough of an improvement to prevent a shear mode of failure. Also, the fiberglass plates and strips peeled off. Beams fitted with a fiberglass wrap, however, nearly doubled the beams' shear capacity, and this increase was sufficient to produce a flexural (i.e., not shear) mode of failure.
Chajes et al. (May 1995) investigated R/C beams with aramid, glass, and graphite wraps loaded in four-point bending. These specimens were structural tees in cross section having a 7.5 in. depth, 5.5 in. wide flange, 2.5 in. thick web, and 48 in. length. These beams were completely lacking in shear reinforcement but contained enough flexural reinforcement (only tension steel) such that a shear failure would occur. While all beams experienced an increase in ultimate capacity they still failed in shear. The glass and graphite wraps were torn along the diagonal crack. The aramid wrap allowed the failed beams to carry some load, however. It is important to note that the purpose of this experimentation was not to force flexural failure, but to determine the effectiveness of the system to increase shear capacity in specimens that were designed to fail in shear. Therefore, the FRP wrap was shown to be effective for shear repair. Chajes et al. published another paper (1995) where the beams were designed to fail in flexure. The only shear reinforcement would be provided by the FRP wrap. In that experiment the beams developed sufficient shear capacity and failed in flexure, as designed.
Based on the results of the studies cited above, it is known that composite wraps are potentially very effective in shear rehabilitation of reinforced concrete. Both research groups concluded that there was a need for full-scale testing of this technology. In 1997 the U.S. Army Construction Engineering Research Labora-tories (CERL) conducted such testing as part of a broader investigation of concrete repair technologies funded and executed under the Army Corps of Engineers Construction Productivity Advancement Research (CPAR) program. The results of this testing were published as an appendix to the final CPAR report (Marshall et al., February 1998), but they are presented here on their own to reach engineers and materials scientists interested in the specific problem of composite-based repair techniques to improve the shear performance of existing R/C structures.
The objective of this study was to test the effectiveness of FRP-based repair techniques on full-scale prestressed high-strength concrete joists fabricated with insufficient shear reinforcement.
Four prestressed high-strength concrete tee-beams (joists) with integral web openings were tested. Two of the joists were used as control specimens. One control joist had insufficient shear reinforcement and one was properly reinforced, designated HJ-6 and HJ-7, respectively. The other two joists were repaired (HJ-4) or upgraded (HJ-3) with FRP to improve their shear performance. Performance criteria were specified for the two joists to be repaired. HJ-3 and HJ-4 were wrapped on three sides, along the outer 8 ft of each end, with an FRP composite system called TYFO S FibrwrapTM. Standard structural engineering practice for shear designs was used to determine the wrap thickness. Calculations were based on controlling shear crack widths to maintain aggregate interlock and proper shear transfer through the concrete.
Technical details about test specimen fabrication, repair material properties and specifications, shear reinforcement techniques, and testing procedures are presented in Chapter 3.
A version of the material presented here was included as an appendix to CERL Technical Report TR 98/47 (Marshall et al., February 1998); that report presented the current topic within a much broader concrete repair context.
A number of FRP composite systems are already on the market for the repair, strengthening, or seismic upgrade of unreinforced or lightly reinforced masonry structures, and new products are regularly becoming available. The FRP/URM Project Team of the Composites Institute Market Development Alliance (CI/MDA) is establishing a database of contacts for companies that market these types of structural enhancement systems. For further information contact Manager, Market Development, Composites Institute, 600 Mamaroneck Ave., Harrison, NY 10528-1632 (914-381-1253, x256 voice; 914-381-1253 fax).
U.S. standard units of measure are used throughout this report. A table of conversion factors for Standard International (SI) units is provided below.
SI conversion factors | ||
1 in. |
= |
2.54 cm |
1 ft |
= |
0.305 m |
1 yd |
= |
0.9144 m |
1 sq in. |
= |
6.452 cm2 |
1 sq ft |
= |
0.093 m2 |
1 sq yd |
= |
0.836 m2 |
1 cu in. |
= |
16.39 cm3 |
1 cu ft |
= |
0.028 m3 |
1 cu yd |
= |
0.764 m3 |
1 gal |
= |
3.78 L |
1 lb |
= |
0.453 kg |
1 kip |
= |
453 kg |
1 psi |
= |
6.89 kPa |
_F |
= |
(_C x 1.8) + 32 |
