FIBER REINFORCED CEMENT BASED COMPOSITE SHEETS FOR STRUCTURAL RETROFIT

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1 Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds) 2005 International Institute for FRP in Construction FIBER REINFORCED CEMENT BASED COMPOSITE SHEETS FOR STRUCTURAL RETROFIT H.C. Wu and P. Sun Advanced Infrastructure Materials Laboratory, Department of Civil and Environmental Engineering, Wayne State University, USA. ABSTRACT In recent years, advanced fiber reinforced composites have been steadily developed and utilized in a wide spectrum of construction applications. Typical reinforcing fibers include glass and carbon, whereas a typical matrix usually involves polymer-based such as epoxy or vinyl ester. In this paper, we will report findings from our on-going project aiming at developing cement-based composite thin sheet for structural retrofit. Retrofit efficiency of using both epoxy based and cement based thin sheet will be compared and discussed, paying special attention to the types of loading and bonding to concrete substrate. KEYWORDS FRP, FRC, strengthening, retrofit. INTRODUCTION The U.S. has an estimated $20 trillion investment in civil infrastructure systems. Because of aging, overuse, exposure, misuse, and neglect, many of these systems are deteriorating and becoming more vulnerable to catastrophic failure when earthquake or other natural hazards strike. It would be prohibitively costly and disruptive to replace these vast networks. They must instead be renewed in an intelligent manner. It is generally recognized that fiber reinforced polymer (FRP) sheets are one of the most vital materials for repair, strengthening, and rehabilitation of existing structures. Applications involve such as externally bonded composite fabrics or jackets on beams, columns, and bridge decks. Significant improvements in compressive, shear, and flexural behavior of bonded concrete elements are obtained. Typically, increases in strength and failure strain of several times are obtained with external FRP reinforcement (e.g. Nanni et al. 1992; McConnell 1993; Seible and Priestley 1993; Karbhari et al. 1993; Saadatmanesh et al. 1994; Labossiere et al. 1995). FRP composites are typically composed of two major constituents, namely fiber and matrix. Typical reinforcing fibers include glass and carbon, whereas a typical matrix usually involves polymer-based such as epoxy or vinyl ester. In an effective retrofit with external FRP sheets, a layer of dry fiber sheet (usually unidirectional tape) is placed on the top of a coat of polymer resin that will harden to bond the fiber sheet to the concrete structure. When needed, multiple layers of fiber sheets can be sequentially added by repeating the same procedure. The success of this FRP technology is attributed to the high strength and high stiffness of the fibers. On the other hand, the polymer matrix is expensive, unfamiliar to the construction industry and has low temperature resistance hence vulnerable to fire. Furthermore, prior to applying resin coating, concrete surface must be thoroughly cleaned and smoothed, including grinding and patching that are labor intensive and sometimes require complete shutdown of operation of the structure. Instead of using polymeric resins we have been developing innovative cement-based matrix materials for making thin composite sheets. Cement-based materials have many advantages in comparison to polymeric resins. For instance, much less or no surface preparation is needed for good bonding. Additional benefits include much higher fire and vandalism resistance, and user friendly to the construction industry. FIBER REINFORCED CEMENT COMPOSITES Significant progress on developing continuous fiber reinforced cement-based composites has been made recently (Cox 1994, Peled and Bentur 2003, Kruger et al 2003, Reinhardt et al 2003, Curbach and Hegger 2003, Wu and Teng 2002, Wu 2004). For instance, Wu and his co-workers have created very high performance continuous fiber reinforced cementitious composites. In this case, we use the same kind of fiber reinforcement 343

2 (unidirectional tapes or woven) as in a regular FRP sheet, and we use cement materials to replace polymer resin. The preparation procedure is analogous to regular FRP. A fiber tape or fabric is impregnated with cement slurry to form a thin composite sheet. Our current process involves the following steps (1) precut a fabric to designated dimensions, (2) submerge in cement slurry for rapid and full penetration into the fabric, and (3) remove the impregnated fabric from the slurry tank and immediately attach to concrete cylinder or beam specimens. Composite sheet as thin as 2 mm can be made with two layers of unidirectional fiber tapes. Both carbon and glass fibers have been used in our investigation although we are aware of potential durability problem of glass fibers in cement alkaline environment. Selected thin composite samples have been tested according to ASTM C78-75, Standard Test Method of Flexural Strength of Concrete (using simple beam with third-point loading). Very high flexural strength (more than 100 MPa) and excellent ductility have been achieved. Such high strength and high ductility are not the norm of cement materials, but it should not be too surprising since the high strength comes from the carbon fibers and the high ductility is attributed to multiple cracking phenomenon of brittle cement matrix (Li and Wu 1992; Li and Leung 1992). Our current work has demonstrated that thin cement sheets having excellent strength/ductility can be produced within minutes. This fast process is a must for in-situ processibility on job sites and for achieving low cost. Compressive Behavior of Confined Concrete After successful production of thin cement composites, such thin sheets have been employed to retrofit concrete cylinder samples in a preliminary study at Wayne State University (Wu and Teng 2002). The purpose of these tests is to evaluate retrofit effectiveness of the innovative thin Fiber Reinforced Cement (FRC) composite. Cement based matrix developed at Wayne State University and epoxy resin were used separately to make thin CFRC sheets and CFRP sheets. Both CFRC and CFRP composites contain two layers of unidirectional carbon fiber tapes. The average thickness of the CFRC jackets is 3.0 mm, whereas the CFRP between 2-3 mm. These thin composite sheets were then employed to wrap mm by mm (4 inch by 8 inch) concrete cylinders. The bond length of the CFRC samples is 76.2 mm (3 inches) and 50.8 mm (2 inches) for the CFRP. A 38.1 mm (1.5 inches) gap exists between the top of the cylinder and the top of the composite sheets at both ends (see Figure 1). (a) (b) Figure 1: Concrete confined with (a) CFRP composite jacket, (b) CFRC jacket. Concrete cylinders, unconfined and confined with CFRC or CFRP composites were tested using a high-stiffness, high-capacity MTS testing machine following ASTM C39-96, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. This equipment has sufficient capacity and stiffness, which is required for conducting such tests. The machine is also equipped with a sophisticated computer control and data acquisition system. The acquired data including the applied axial load and axial deformation of concrete were recorded automatically. Per ASTM Practice C , steel retaining rings and rubber pads were used without other capping during the tests. 344

3 Test Results The axial stress versus axial strain relationships of the unconfined and confined concrete are shown in Figure 2. The initial portions of the stress-strain responses of the confined specimens essentially followed the curves of the unconfined concrete. The average compressive strength of the unconfined concrete is 54 MPa. The CFRC group shows a compressive strength two times higher than that of the control from 54 MPa to 100 MPa. In addition, the ductility is increased by 3 times from 2 mm to 6 mm. The CFRP sample has the highest compressive strength (105 MPa) and ductility. Nevertheless, the differences between CFRC and CFRP are insignificant. The CFRP jacketed samples showed explosive failure that was triggered by the complete rupture of the CFRP jacket. The CFRC samples also show fiber rupture failure similar to the CFRP sample. Nevertheless, the CFRC samples have a much less violent global failure than the CFRP. Compressive Strength (MPa) 120 Unconfined Confined w/ CFRP Confined w/ CFRC #1 Confined w/ CFRC # Deflection (mm) Figure 2: Compressive behavior of unconfined and confined concrete Flexural Behavior of Concrete Beam Thin FRC sheets have been also employed to retrofit concrete beams for improvement in flexural capacity. Cement based matrix developed at Wayne State University and epoxy resin were used separately to make thin CFRC sheets and CFRP sheets. Both CFRC and CFRP composites contained one layer of 2-D carbon mesh. The thickness of the CFRC and CFRP sheets was between 3-4 mm. (Vf=4.2%). Such thin CFRC or CFRP sheet was attached to the tension-side of concrete beam prior to being tested under 3-point bending (as shown in Figure 3). Figure 3 Flexure testing 345

4 Without retrofit, plain concrete beam showed brittle behavior. When retrofitted with a thin CFRP sheet, flexural strength of the concrete beam increased significantly, as well as ductility (in Figure 4). As shown in Figure 5, several cracks developed gradually in concrete when the loads continued to increase, followed by delamination between the concrete beam and the CFRP sheet, which gradually propagated to the right. Final failure occurred as a result of concrete crushing under the loading pin CFRP 6000 Load (N) CFRC Plain Displacement (mm) Figure 4: Flexural response of plain, CFRC retrofit, CFRP retrofit beams 5(a) 5(b) 346

5 5(c) Figure 5 Various stages when loads continued to increase during flexure testing (a) first crack, (b) more cracks developed, (c) delamination occurred between the concrete beam and the CFRP sheet and gradually propagated to the right, and final failure due to concrete crushing under the loading pin As for the concrete beam retrofitted with the CFRC sheet, there was primarily one single flexural crack, and the final failure was due to rupture of the CFRC sheet. There was no evidence of delamination between the concrete beam and the CFRC sheet (see Figure 6). As shown in Figure 4, the flexural strength of the CFRC beam is much lower than the CFRP beam. The reason is yet to be discovered. CONCLUSIONS Figure 6 Failure mode of the CFRC beam It is confirmed that the compressive and flexural strength of concrete can be significantly improved using external FRC wraps. In addition, the ductility of the retrofitted concrete is increased significantly. The final failure of the confined concrete cylinder is provoked by the onset of rupture of the composite wrap, whereas the final failure of the retrofit beam is also triggered by the rupture of the FRC sheet. It is observed that bonding between concrete and FRC is excellent and no sign of delamination is noted. On the other hand, the CFRP beam shows a progressive delamination. Nevertheless, the CFRP beam has a much higher flexural strength than the CFRC beam. Ductility of both beams is about the same. 347

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