Patent Application
20100319171
Much research has been conducted to date to study the behavior of reinforced concrete (RC) beams strengthened with prestressed fiber reinforced polymer (FRP) sheets and plates. By inducing prestressing the FRP system can be used more efficiently, leading to an increase in the flexural capacity and serviceability of the strengthened beams (Triantafillou et al., 1992; Char et al., 1994; Quantrill and Hollaway, 1998; El-Hacha et al., 2001a; 2001b; El-Hacha et al., 2003). Literature review shows that strengthening of RC beams with prestressed FRP sheets and plates can be grouped under three installation methods.
The first installation method consists of an indirect method for achieving prestress in the FRP system. Saadatmannesh and Ehsani (1991) have investigated an indirect method for prestressing glass FRP (GFRP) plates. Stressing in the GFRP system was achieved by initially cambering the beam upwards with the use of hydraulic jacks followed by bonding the plates to the underside of beams. After the epoxy adhesive was properly cured the cambering system was released; thereby, inducing prestressing in the GFRP plates. Some of the disadvantages associated with this method are that it is labor intensive, only low levels of prestressing can be induced in the plates, not easy to achieve the desired level of prestressing in the plates or sheets, and the reacting floor or foundation must be capable of sustaining the applied vertical loads.
Researchers have also investigated two other methods consisting of directly or indirectly applying the prestress to the FRP system. These next two methods include three phases to achieve the desired level of prestressing. First, the stress is applied with a power operated hydraulic jack or similar device and the prestressing in the FRP system must be controlled with either strain gages or load cells. Second, the sheets or plates are bonded to the concrete surface with an epoxy adhesive or simply anchored to the beam itself. Finally, after the epoxy adhesive has properly cured, the sheets are cut near the ends and the prestressing device is removed.
In the second method or designated as the direct method, the FRP sheets or plates are first anchored at one end (dead end) and then tensioned from the other end (live end) using a power operated hydraulic jack (Wight et al., 2001; Saeki et al., 1997; El-Hacha et al., 2001b). According to this method the FRP sheets or plates must be anchored to the beam itself at either end. The dead end is first anchored before stressing and the live end is subsequently anchored after the stress is applied to the FRP system. In many instances these anchors serve as a permanent anchorage to the FRP system leading to a costly solution due to the high costs associated with fabricating the specialized prestressing anchors and plates. To achieve a cost effective solution these anchorages can be optionally removed for usage in further applications. If left in place, permanent steel anchors are likely to be exposed to significant weathering or galvanic corrosion due to contact with the carbon fibers. Also, the anchors may need to be removed for aesthetics reasons, leading to potential debonding of the prestressing sheets or plates. In order to avoid premature debonding it may be necessary to install U-wraps before removal of the anchors and plates. However, because of the presence of these anchors and plates, the U-wraps must be placed away from the ends of the prestressed FRP system. Other disadvantages that can be associated with this method are that it tends to be laborious, and the beam surface must be properly treated (ACI 546, 1996) prior to drilling for installation of the anchors.
A third method consists of first bonding the end of FRP sheets or plates to steel plates or other devices, which are then tensioned against an external reacting frame (Triantafillou et al., 1992; Char et al., 1994; Garden et al., 1998; Quantrill and Hollaway, 1998). In many of the systems proposed in the literature the prestressing release was carried out by cutting the sheets in specified unbonded regions. In many of these systems the release method was carried out under high strain rates leading to premature debonding and further accentuating the need for end anchorages. In addition, many of the systems proposed in the literature require the use of specialized equipment.
The new innovative external mechanical device was invented for prestressing FRP sheets, which follows within this third method. The invention can overcome some of the disadvantages outlined for each of the three methods. An attractive feature of the device is that the prestressing was achieved with a manual torque wrench without the need for using power operated hydraulic jacks or any other type of sophisticated equipment. In typical prestressing applications, transfer of the prestressing is achieved under high strain rates, which increases the propensity for end debonding at low prestressing levels (Pornpongsaroj and Pimanmas, 2003). This issue can be mitigated by the proposed device because the prestressing release is achieved under low strain rates. For higher prestressing levels in which debonding of the CFRP sheets cannot be prevented solely by controlling the strain rate at transfer, U-wraps can be easily installed at the ends of the prestressing FRP system.
A simple mechanical device was invented for prestressing of carbon fiber reinforced polymer (CFRP) sheets that can overcome on some of the disadvantages of currently used prestressing systems. Significant features of this device are that the CFRP sheets are directly anchored to the mechanical device itself, the prestressing forces are applied with a manual torque wrench without the need for power operated hydraulic jacks, and the prestressing transfer is accomplished under slow strain rates. Experimental investigation clearly corroborates that the device was efficient in applying prestressing to the CFRP sheets and prestress losses during stressing were maintained at a minimum.
As shown in
The system with one CFRP sheet under prestressing is shown in
Referring to
The first step in the assembly operation consisted of impregnating the CFRP sheets to their full length with an epoxy resin, similar to the process used to prepare FRP sheets for tension tests (ACI, 2004). The epoxy resin was a mixture of two components, which works as a matrix to protect the fibers and transfer the stresses between the adjoining fibers (Karbhari, 2001). After the epoxy resin has cured, the CFRP sheets were bonded to the removable steel plates (part A) using the same epoxy resin (see
The next step consisted of fixing the removable steel plates and bonded CFRP sheets to the welded steel plates (part B) by tightening four steel nuts in the anchorage regions (
Design of the Anchorage Region Steel Plates: A total of 3 bond tests were performed according to the test setup shown in
As shown in
The computed average strain gage data obtained from the three tests and from the strain gages installed along the length of the CFRP sheets is shown in
where tf and Ef are the thickness and the elastic modulus of the CFRP sheets, respectively, and Δε and Δx are the variations in strain and distances between the strain gages, respectively. Based on material properties the results presented in Table 4 and Eq. (1) the computed average bond strength was 2.20 MPa (314 psi). This bond strength was then used to size the plates necessary to develop the required bonding surface area. Finally, the length of the plates in the longitudinal direction was based the relation
where bf is the width of the CFRP sheet, λcr is the creep rupture stress limit in FRP composites where for carbon fibers this is limited by ACI 440 (2002) at 0.55, ffu and Af are the tensile strength and area of the CFRP sheets, and μave is the average bond strength determined from the tests shown in
Stressing the CFRP Sheets: After the anchorage regions were created, the desired prestress level was achieved by alternately tightening the steel nuts (part E) in the loading region (see
Leveling of the CFRP sheets was easily controlled in the transverse and longitudinal direction, namely across the length of the steel strips (part C) and CFRP sheets, respectively, by using a carpenter leveler. Leveling is necessary to ensure a uniform and planar surface that is free of significant twisting and warped edges before bonding of the prestressed CFRP sheets to the RC beam.
The removable steel plates and steel strips were fabricated with a slight rounding at the corners to decrease any stress concentrations in the CFRP sheets due to the change in the sheets direction and to prevent damage to the CFRP sheets during prestressing.
In order to simplify usage of the device it is advantageous to relate the vertical displacement, ΔH (see
The theoretical prestress was derived based on the geometric relations of the prestressing system and the deformed CFRP sheets. As shown in
where σ1 is the prestress in the diagonal AB1 and C1D segments, Ef is the elastic modulus of the CFRP sheets, and θ is the angle between the original segment AB and the deformed segment AB1. Finally, based on Eqs. (3) and (4), the normalized prestress, σ2/ffu is
where dimensions L1 and L2 are measured directly from the device, and ffu is the ultimate tensile strength of the CFRP sheets (see
In this research program the vertical displacement was measured by using LVDT's for added precision and continuous reading and subsequently correlated to the prestress in the CFRP sheet, σ2, by Eq. (5). In field conditions a Vernier caliper can be used to measure ΔH and by using design charts one can easily estimate σ2. A Vernier caliber is a standard measuring devise used to get high precision readings. For example,
In this figure different geometric relations were considered as a function of length L2, and L1 was kept constant at 457 mm (18 in.). It is clear that as length L2 increases so does the required vertical uplift and for very long sheets, say L2 greater than 11 m (36 ft) the vertical uplift is within 230 mm (9 in.). Although not investigated in this program, future research should concentrate on developing design charts that can be used to design the diameter of the threaded rods as a function of the desired prestress level and length of the CFRP sheet, L2. These charts will be very much like
After the CFRP sheets were prestressed, the next step consisted of bonding the prestressed CFRP sheet to the RC beam, as shown in
Prestressing Transfer: Transfer of the prestressing was carried out by slowly releasing the threaded rods (part D) in the loading region. This process was accomplished by alternately completing 2 full turns in all four steel nuts (part E). The total time taken to release the prestress in all beams was nearly 5 minutes. For the beam prestressed with 40% (see Table 3) this corresponds to a rate of nearly 180 N/sec (40 lbs/sec). For the other beams the release was performed at a slower rate and the rates are reported in Table 2. After release of the CFRP sheets was achieved, the sheets were cut close to the steel strips and the mechanical device was removed and cleaned for further applications. Properties for the materials used in this research are shown in Table 4. The CFRP sheets used were 0.165 mm (0.0065 in.) thick and 203 mm (8 in.) wide leading to a total reinforcement area of 33.55 mm2 (0.052 in.2).
Test Matrix: Of a total of eight RC beams investigated in this research program, six RC beams were retrofitted with prestressed CFRP sheets that were stressed using the device developed in this research. The other two beams were used as the control specimens and consisted of one unstrengthened beam and one strengthened beam with CFRP sheets, but without prestressed. The remaining beams were strengthened with CFRP sheets that were prestressed to 15%, 30% and 40% of the tensile strength of the CFRP sheets. The RC beams were then tested under a four-point bending system. Test results show that the device was efficient in prestressing the CFRP sheets to the specified stress levels, initial prestress losses were negligible, and prestress losses after transfer were within 10% of the initial prestress. Furthermore, test results clearly showed that the beams strengthened with the prestressed CFRP sheets achieved a higher yielding and ultimate loading.
Creep-Rupture Limits: According to ACI-440 (2002) after consideration of a long-term environmental factor, the sustain stress limit for CFRP composites is ffs=0.55 ffu, where ffu is the design strength. Therefore, in the retrofit of RC beams using prestressed CFRP sheets consideration was given to the creep-rupture limit because in these applications the sheets are continuously subjected to high sustained stresses after prestressing. As such, the levels of prestressing investigated in this research are below the permissible limit of 55%.
Field Application Setup: It is recognized that these laboratory conditions do not match exact field conditions for strengthening in positive moment regions, in which the prestressing apparatus needs to be “hanging” from the RC beams. This is to some extent, more complex than the simpler “from the top” procedure shown in
A potential field application on the underside of a beam is shown in detail in
