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Advances in Civil Engineering
Volume 2012 (2012), Article ID 635176, 20 pages
http://dx.doi.org/10.1155/2012/635176
Research Article

Shear Strengthening of RC Beams Using Sprayed Glass Fiber Reinforced Polymer

1Department of Civil Engineering, Islamic Azad University, Ahar Branch, Ahar 5451116714, Iran
2Department of Civil Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4

Received 11 December 2011; Accepted 5 February 2012

Academic Editor: Dawn E. Lehman

Copyright © 2012 Sayed Mohamad Soleimani and Nemkumar Banthia. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The effectiveness of externally bonded sprayed glass fiber reinforced polymer (Sprayed GFRP) in shear strengthening of RC beams under quasi-static loading is investigated. Different techniques were utilized to enhance the bond between concrete and Sprayed GFRP, involving the use of through bolts and nuts paired with concrete surface preparation through sandblasting and through the use of a pneumatic chisel prior to Sprayed GFRP application. It was found that roughening the concrete surface using a pneumatic chisel and using through bolts and nuts were the most effective techniques. Also, Sprayed GFRP applied on 3 sides (U-shaped) was found to be more effective than 2-sided Sprayed GFRP in shear strengthening. Sprayed GFRP increased the shear load-carrying capacity and energy absorption capacities of RC beams. It was found that the load-carrying capacity of strengthened RC beams was related to an effective strain of applied Sprayed GFRP. This strain was related to Sprayed GFRP configuration and the technique used to enhance the concrete-FRP bond. Finally, an equation was proposed to calculate the contribution of Sprayed GFRP in the shear strength of an RC beam.

1. Introduction

Many concrete structures such as bridges that are in use today have exceeded their design life. In the USA alone, about 25% of their 600,000 bridges are either structurally deficient or functionally obsolete [1]. On the other hand, code requirements have been changed, the shear requirements have become more stringent for concrete girders and especially for bridges, and allowable traffic loads have been increased. Some elements of these structures have also been weakened due to corrosion of steel rebars containing longitudinal (tension and compression) and vertical (shear) reinforcements. Therefore, rehabilitation and strengthening of these concrete structures is one of the priorities for engineers today. In fact, this new challenge necessitates a close collaboration between structural and materials experts. Advantages of FRP composites have encouraged researchers around the world to focus on the externally bonded FRP composites for strengthening of concrete slabs, columns, and beams.

This paper deals with shear strengthening of RC beams using sprayed glass fiber reinforced polymer (GFRP) composites. This technique and relevant materials properties (glass fiber and polymer) have been discussed in detail elsewhere [2]. This technique, as compared to externally bonded FRP fabrics and laminates, is quite new for strengthening of RC structures. Hence, a limited number of publications are available with respect to this technique. On the other hand, externally bonded FRP including glass, carbon, and aramid (e.g., Kevlar) fibers have been studied for flexural and shear strengthening of RC beams and strengthening of RC columns extensively. As a result, new guidelines are available to design concrete structural elements strengthened with externally applied FRP such as the American ACI 440.2R-02 [3], Canadian CSA-S802-02 [4], ISIS design manual [5], and European fib-TG9.3-01 [6].

Two major failure modes for RC beams strengthened in shear using externally bonded FRP have been reported: FRP has peeled off at the concrete-FRP interface (FRP debonding) and FRP has fractured in tension. Due to stress concentrations at debonded areas or at the corners, FRP fracture in tension may occur at a stress lower than the FRP tensile strength. Clearly, shear capacity of RC members strengthened in shear with externally bonded FRP depends on the mode of failure.

RC beams with deficiency in their shear strength (i.e., expected to fail in shear) were retrofitted using Sprayed GFRP. Different thicknesses and schemes were used, and their effectiveness was evaluated under quasi-static loading.

2. Beam Design and Testing Procedure

A total of 29 RC beams were cast to investigate the shear strengthening using Sprayed GFRP under quasi-static loading. These beams contained flexural reinforcement, but none or less than the required stirrups. The total length of these beams was 1 m, and they were tested over an 800 mm span. Load configuration and cross-sectional details are shown in Figure 1.

635176.fig.001
Figure 1: Load configuration and cross-sectional details of RC beams.

The parameters needed for calculating the load-carrying capacity of the beam shown in Figure 1 are tabulated in Table 1. Since not enough shear reinforcement was provided, the maximum strength of the beam would be governed by the shear strength of concrete as well as the shear strength provided by the steel stirrups where applicable. Calculations show that if resistance factors are not considered, the capacity of this beam under quasi-static loading is 131 kN if enough reinforcement is provided for shear. At this point, tension reinforcement would start yielding. It is also worth noting that the beam was designed to produce a typical shear failure mode since not enough stirrups were provided and the shear strength of the concrete was far below the flexural strength of the beam. The RC beam with no stirrups and with stirrups (Φ4.75 at 160 mm) is predicted to have a capacity of about 80 kN and 100.2 kN, respectively.

tab1
Table 1: Properties of RC beams.

In quasi-static loading conditions, all of the beams were tested in 3-point loading using a Baldwin 400 kips universal testing machine. Three linear variable differential transformers (LVDTs) were used to capture the deflection at the midspan as well as two more points along the beam as shown in Figure 1. The test setup for quasi-static loading is shown in Figure 2. Applied load and deflections were constantly monitored and recorded using a data acquisition system based on a PC.

635176.fig.002
Figure 2: Beam test setup under quasi-static loading.

3. Specimen Preparation

All specimens were identical in dimensions. Casting was done on a vibrating table to ensure proper consolidation of the concrete. Specimens were demolded after one day and immersed in lime-saturated water. At the age of 28 days, the beams were removed from the curing tank and set out to dry under normal laboratory conditions. A minimum of one week of drying was allowed prior to any testing, surface preparation, or spraying.

Surface preparation is the key to successful strengthening using externally bonded FRP. The surface must be dry, clean, and free of oil, debris, and loose materials. Different techniques were used for surface treatment before applying Sprayed GFRP; they are discussed later.

4. Retrofit Schemes

Different configurations can be used for shear strengthening of RC beams using externally bonded GFRP. In general, the number of surfaces around the beam and the thickness of strengthening materials are of greatest interest. Throughout this investigation, different retrofit schemes with different thicknesses (with and without mechanical fasteners) were studied.

In FRP wrap systems, FRPs are bonded on the lateral faces of the beam with the fibers perpendicular or inclined to the longitudinal axis of the beam. The FRPs can also be placed on both lateral faces in a continuous way underneath the beam web resembling U-shaped external stirrups. The performance of the U-shaped bands can be further improved by adding additional longitudinal FRP strips over the ends of the U-shaped bands.

Sprayed GFRP was applied either on both lateral faces or on three faces excluding the top (i.e., compression face). Boyd [7] reported a difficulty involving the inability of the fibers to stay in place when bent around sharp corners during the retrofit process. To overcome this problem and to avoid possible failure of the FRP due to stress concentrations at the corners of the beam section, when sprayed or fabric GFRP was applied on three sides of the beam, the corners of the beam section were rounded to a radius of 35 mm. This was also recommended by ISIS Canada [5].

Different thicknesses of Sprayed GFRP were applied and studied in this project. For surface preparation, different techniques such as sandblasting, epoxy glue, and hammering the surface were investigated. Through bolts and nuts were also tried with emphasis on concrete-GFRP bond strength enhancement.

5. Results and Discussion

A total of 29 RC beams were tested under quasi-static loading. Beam designations and details are tabulated in Table 2. The following notations are used for beam designations:C: control,NS: no stirrups,S: stirrups (Φ4.75 at 160 mm),SS: stirrups (3Φ4.75 at 50 mm),B2: sprayed GFRP on 2 lateral sides of the beam,B3: sprayed GFRP on 3 sides of the beam,SB: sandblasted (i.e., concrete surface),EP: epoxy was used before spraying the GFRP (i.e., primer and putty, Wabo MBrace system, details about this system is available in [2]),4B: 4 through bolts,6B: 6 through bolts,6H: 6 through holes.

tab2
Table 2: RC beams designations and details.
5.1. Control Beams with No GFRP

Six beams were tested under quasi-static loading without the GFRP coating. Results are reported here and will be used later as bench marks for comparison.

5.1.1. Control Beam with No GFRP and No Stirrups

One beam (beam C-NS in Table 2) was tested under quasi-static loading with no stirrups and no GFRP. The result of this test is shown in Figure 3. A typical shear failure was observed in this beam with a crack of about 45°. This shear crack became flatter at the load point as shown in Figure 3. Load carrying capacity was in good agreement with the predicted value.

635176.fig.003
Figure 3: Load versus midspan deflection of control RC beam C-NS.
5.1.2. Control Beams with No GFRP and Stirrups at 160 mm

Two beams (beams C-S-1 and C-S-2 in Table 2) were tested under quasi-static loading with no GFRP and Φ4.75 stirrups at 160 mm. The results of these tests are shown in Figures 4 and 5. The presence of stirrups produced multiple cracks as compared to one large crack in the RC beam with no stirrups (compare Figure 3 with Figures 4 and 5). Load-carrying capacity was about 10% less than the expected value.

635176.fig.004
Figure 4: Load versus midspan deflection of control RC beam C-S-1.
635176.fig.005
Figure 5: Load versus midspan deflection of control RC beam C-S-2.
5.1.3. Control Beam with No GFRP and Stirrups at 50 mm

One beam (beam C-SS in Table 2) was tested under quasi-static loading with no GFRP and 3Φ4.75 stirrups at 50 mm. The result of this test is shown in Figure 6. Flexural and shear cracks were observed during the test and the beam ultimately failed in shear after reaching its flexural capacity. Since the amount of tension reinforcement (600 mm2) was about 2.7% of the concrete cross-sectional area (150 mm × 150 mm), undeformed reinforcing bars for shear (i.e., 3Φ4.75 at 50 mm stirrups) were not too effective in capturing shear cracks after yielding tension reinforcement. As a result, when tension reinforcement started yielding, the shear cracks propagated towards the concrete compression zone. Failure took place when the shear cracks entered the concrete compression region, which also showed some crushing. This can be seen in the pictures illustrated in Figure 6.

635176.fig.006
Figure 6: Load versus midspan deflection of control RC beam C-SS.
5.1.4. Control Beam with No GFRP, Stirrups at 160 mm and 6 through Holes

One beam (beam C-S-6H in Table 2) was tested under quasi-static loading with Φ4.75 stirrups at 160 mm, no GFRP, and 6 through holes with a diameter of 12.5 mm (1/2 in.). The location of these holes is illustrated in Figure 7, and the result of this test is shown in Figure 8.

635176.fig.007
Figure 7: Cross-sectional details of RC beam C-S-6H.
635176.fig.008
Figure 8: Load versus midspan deflection of control RC beam C-S-6H.

The purpose of this test was to determine the magnitude by which the load-carrying capacity of the beam would decrease if through holes were created for GFRP bond enhancement. It was observed that only 4% of the load-carrying capacity of this beam was lost due to the presence of the through holes. The load-carrying capacity of beam C-S-6H was 87.7 kN which was about 3.9 kN less than that of beams C-S-1 and C-S-2.

5.1.5. Control Beam with No GFRP, No Stirrups, and 6 through Bolts and Nuts

One beam (beam C-NS-6B in Table 2) was tested under quasi-static loading with no stirrups, no GFRP, and 6 through bolts and nuts. The location of these bolts and their details are illustrated in Figure 9, and the result of this test is shown in Figure 10.

635176.fig.009
Figure 9: Cross-sectional details of RC beam C-NS-6B.
635176.fig.0010
Figure 10: Load versus midspan deflection of control RC beam C-NS-6B.

A torque of 67.8 N·m (50 lb·ft) was applied to tighten the nuts on both sides of the beam, as shown in Figure 9. This torque was kept constant during experimentation and was applied to all beams containing through bolts and nuts.

The purpose of this test was to determine the benefits, if any, of these bolts in increasing the shear capacity of the beam. According to the obtained results, the use of these bolts and nuts overcame the weakness of having through holes in the RC beam. In effect, the shear capacity of the RC beam maintained its original capacity (i.e., with no through holes). It was also noticed that the applied torque provided more confinement for concrete, and, as a result, more energy was used up during the beam’s failure compared to beam C-NS with no bolts.

5.2. Sprayed GFRP on Two Sides

Nineteen beams in total were strengthened by Sprayed GFRP on their lateral sides. Different techniques were used to evaluate the effectiveness of Sprayed GFRP in the shear strengthening of RC beams. In the following sections, these techniques will be discussed, and the results will be compared. Each result will also be compared with its corresponding control specimen as described in Section 5.1.1.

5.2.1. Beams with No Mechanical Fasteners

Nine beams were tested with Sprayed GFRP applied to their lateral sides and no mechanical fasteners were used. The purpose of these tests was to find the best type of concrete surface to create a stronger GFRP-concrete bond. Three different techniques were employed as follows.(1)The concrete surface was sandblasted and then washed by a high-pressure washer. The beam was left for a couple of days in the laboratory environment to make sure that the surface was completely dried before applying the Sprayed GFRP.(2)The concrete surface was roughened using a small pneumatic concrete chisel. This technique provided a rougher surface than sandblasting. Then, the concrete surface was washed using a high-pressure washer and dried before Sprayed GFRP application.(3)The concrete surface was sandblasted and then washed by a high-pressure washer. After the surface dried, primer and putty (Wabo MBrace-surface preparation for fabric GFRP system) were applied to the concrete surface prior to Sprayed GFRP application.

Figure 11 shows the prepared surface before Sprayed GFRP application using pneumatic concrete chisel. This pneumatic tool weighs around 1.7 kg with a stroke speed of 2600 min−1, rated air pressure of 0.59 MPa, and rated air consumption of about 3 m3/min.

635176.fig.0011
Figure 11: Surface preparation using pneumatic concrete chisel.

One beam (beam B2-NS-SB) was tested while Sprayed GFRP was applied after preparing the surface using the sandblast technique. The beam contained no stirrups, and its details can be found in Table 2. Figure 12 shows the test result of this beam while the test result of its control beam (beam C-NS) is also included.

635176.fig.0012
Figure 12: Load versus midspan deflection of RC beam B2-NS-SB.

It is clear that sandblasting technique was not an effective way to enhance the Sprayed GFRP-concrete bond. This bond failed before having any contribution to the enhancement of shear strength of this RC beam. As a result, the load-carrying capacity was unchanged due to premature bond failure as shown in Figure 12.

Two beams (beam B2-NS-EP and beam B2-S-EP) were tested while Sprayed GFRP was applied over the cured primer and putty (Wabo MBrace). The purpose of these tests was to identify the effectiveness of this technique in providing a better Sprayed GFRP-concrete bond. Figure 13 shows the test result of beam B2-NS-EP (beam with no stirrups; details are tabulated in Table 2). The test result of its control beam (beam C-NS) is also included in Figure 13 for comparison.

635176.fig.0013
Figure 13: Load versus midspan deflection of RC beam B2-NS-EP.

The test result of beam B2-S-EP (beam with Φ4.75 stirrups at 160 mm with tabulated details in Table 2) is shown in Figure 14 while the test result of its control beam (beam C-S-2) is also included in the same figure.

635176.fig.0014
Figure 14: Load versus midspan deflection of RC beam B2-S-EP.

From these test results, one can conclude that the Sprayed GFRP-concrete bond showed an improvement by introducing an intermediate layer of Wabo MBrace primer and putty compared to the sandblasting technique. Load-carrying capacity of these beams increased in proportion with the cross-sectional area of the applied Sprayed GFRP on the lateral sides of the RC beam.

Six beams (beam B2-NS and beams B2-S-1, B2-S-2, B2-S-3, B2-S-4, and B2-S-5) were tested while Sprayed GFRP was applied on the lateral sides of the beam over a roughened surface using the pneumatic concrete chisel. The purpose of these tests was to identify the effectiveness of this technique in providing a better Sprayed GFRP-concrete bond. Figure 15 shows the test result of beam B2-NS (beam with no stirrups; details are tabulated in Table 2). The test result of its control beam (beam C-NS) is also included in Figure 15 for comparison. Test results of beams B2-S-1, B2-S-2, B2-S-3, B2-S-4, and B2-S-5 (beams with Φ4.75 stirrups at 160 mm with tabulated details in Table 2) are shown in Figures 16, 17, 18, 19, and 20 while the test result of their control beam (beam C-S-2) is also included in each figure.

635176.fig.0015
Figure 15: Load versus midspan deflection of RC beam B2-NS.
635176.fig.0016
Figure 16: Load versus midspan deflection of RC beam B2-S-1.
635176.fig.0017
Figure 17: Load versus midspan deflection of RC beam B2-S-2.
635176.fig.0018
Figure 18: Load versus midspan deflection of RC beam B2-S-3.
635176.fig.0019
Figure 19: Load versus midspan deflection of RC beam B2-S-4.
635176.fig.0020
Figure 20: Load versus midspan deflection of RC beam B2-S-5.

Roughening the concrete surface using a pneumatic chisel, as shown in Figures 15, 16, 17, 18, 19, and 20, appears to be a promising technique in enhancing the bond between concrete and GFRP. It was also noticed that load carrying capacity was proportional to the cross-sectional area of GFRP material to a certain point, beyond which increasing this area did not increase the load-carrying capacity.

Figures 21(a)21(e) show crack development in beam B2-S-1 under 3-point quasi-static loading, and Figure 21(f) shows the strong bond between GFRP and concrete, which was clearly greater than tensile/shear strength of concrete and concrete-rebar bond strength. It is worth mentioning that all Sprayed GFRP plates were cut at the midspan of the beam (both cases: Sprayed GFRP on 2 lateral sides and on 3 sides) to make sure that the GFRP contribution only in shear strengthening would be measured. It is obvious that since Sprayed GFRP consists of randomly distributed chopped fibers, unlike unidirectional FRP fabrics, any portion of this composite material underneath the neutral axis of the RC beam will increase the flexural capacity of the beam.

fig21
Figure 21: Beam B2-S-1. (a) to (e) Crack development under 3-point loading; (f) strong sprayed GFRP-concrete bond.

By cutting the cured Sprayed GFRP at the midspan and underneath the neutral axis, the contribution of this composite material toward flexural strengthening is minimized, and, therefore, the shear strengthening benefits of Sprayed GFRP can be calculated and formulated based on its geometry and properties.

5.2.2. Using through Bolts and Nuts as Mechanical Fasteners

Ten beams were tested using through bolts and nuts as mechanical fasteners to overcome the premature failure, if any, due to FRP debonding and to observe FRP rupture at the beam’s failure. There were either 4 or 6 bolts as mechanical fasteners, and the test results of these two groups of tests are discussed in this section.

Using 4 Through Bolts as Mechanical Fasteners
Six beams were tested using 4 bolts: 3 beams with no stirrups and 100 mm width Sprayed FRP on their lateral sides and 3 beams with Φ4.75 stirrups at 160 mm and 150 mm width Sprayed FRP on their lateral sides. Cross-sectional details and bolt locations are shown in Figure 22.
Load versus midspan deflection curves of beams B2-4B-NS-1, B2-4B-NS-2, and B2-4B-NS-3 with their control specimen (beam C-NS-6B) are reported in Figures 23, 24, and 25. Figures 26, 27, and 28 show load versus midspan deflection curves for beams B2-4B-S-1, B2-4B-S-2, and B2-4B-S-3 along with their control specimen (beam C-S-6H).
From the pictures illustrated in Figures 23, 24, 25, 26, 27, and 28, one can conclude that the presence of through bolts as mechanical fasteners can certainly prevent premature GFRP debonding failure.

fig22
Figure 22: Cross-sectional details of RC beams; (a) B2-4B-NS-1 to B2-4B-NS-3; (b) B2-4B-S-1 to B2-4B-S-3.
635176.fig.0023
Figure 23: Load versus midspan deflection of RC beam B2-4B-NS-1.
635176.fig.0024
Figure 24: Load versus midspan deflection of RC beam B2-4B-NS-2.
635176.fig.0025
Figure 25: Load versus midspan deflection of RC beam B2-4B-NS-3.
635176.fig.0026
Figure 26: Load versus midspan deflection of RC beam B2-4B-S-1.
635176.fig.0027
Figure 27: Load versus midspan deflection of RC beam B2-4B-S-2.
635176.fig.0028
Figure 28: Load versus midspan deflection of RC beam B2-4B-S-3.

Using 6 through Bolts as Mechanical Fasteners
Four beams were tested using 6 bolts: 3 beams with no stirrups and 100 mm width Sprayed FRP on their lateral sides and one beam with Φ4.75 stirrups at 160 mm spacing and 100 mm width Sprayed FRP on its lateral sides. Cross-sectional details and bolt locations are shown in Figure 29.
Load versus midspan deflection curves of beams B2-6B-NS-1, B2-6B-NS-2, and B2-6B-NS-3 with their control specimen’s test result (beam C-NS-6B) are reported in Figures 30, 31, and 32. Figure 33 shows load versus midspan deflection curve for beam B2-6B-S-1 while its control specimen’s load-deflection response (beam C-S-6H) is also included.
Again, from the pictures in Figures 30, 31, 32, and 33, one can conclude that the presence of through bolts as mechanical fasteners can certainly prevent premature GFRP debonding failure. In all cases GFRP rupture was observed. Depending on GFRP thickness, this rupture can occur before (i.e., at the same time of) or after shear failure of RC beam. Contribution of GFRP in shear strengthening, which was proportional to its cross-sectional area to a certain point, will be addressed later.

fig29
Figure 29: Cross-sectional details of RC beams; (a) B2-6B-NS-1 to B2-6B-NS-3; (b) B2-6B-S-1.
635176.fig.0030
Figure 30: Load versus midspan deflection of RC beam B2-6B-NS-1.
635176.fig.0031
Figure 31: Load versus midspan deflection of RC beam B2-6B-NS-2.
635176.fig.0032
Figure 32: Load versus midspan deflection of RC beam B2-6B-NS-3.
635176.fig.0033
Figure 33: Load versus midspan deflection of RC beam B2-6B-S-1.
5.3. Sprayed GFRP on Three Sides

Four beams, all with Φ4.75 stirrups at 160 mm, were strengthened using Sprayed GFRP on their 3 sides (i.e., U-shaped). As mentioned earlier, since shear strengthening was the primary focus of this research, the GFRP was cut at the midspan of the beam underneath the neutral axis of the beam’s cross-section to minimize its contribution in flexural strengthening (see the top right picture in Figure 36 for example). In this way, the contribution of GFRP to the shear strength of an RC beam, if any, would be explored. Load versus midspan deflection curves are shown in Figures 34, 35, 36, and 37 for beams B3-S-1 to B3-S-4, respectively. To show the benefits of this technique, the test result of beam C-S-2 (control beam) is also included in each figure. Notice that beams B3-S-3 and B3-S-4 showed significant increases in their load-carrying capacities, and a clear tension-steel yielding was observed in these two beams. In all 4 beams, the mode of failure was changed from shear to flexure.

635176.fig.0034
Figure 34: Load versus midspan deflection of RC beam B3-S-1.
635176.fig.0035
Figure 35: Load versus midspan deflection of RC beam B3-S-2.
635176.fig.0036
Figure 36: Load versus midspan deflection of RC beam B3-S-3.
635176.fig.0037
Figure 37: Load versus midspan deflection of RC beam B3-S-4.

6. Energy Evaluation

Peak loads and absorbed energy up to 10 mm, 15 mm, and 20 mm midspan deflections of the tested RC beams are provided in Table 3. Based on the information provided in Table 3, one can draw the following conclusions.(1)Although using primer and putty (Wabo MBrace) as an intermediate layer between concrete and Sprayed GFRP (beams B2-NS-EP and B2-S-EP) increased the load-carrying capacity, the energy absorption capacity was not increased as much as the load carrying capacity (it even decreased for beam B2-NS-EP).(2)Roughening the concrete surface using a pneumatic concrete chisel was an effective way to increase the concrete-FRP bond. This, in turn, increased the energy absorption capacity of strengthened beams as well.(3)Using through bolts and nuts effectively increased both the load-carrying capacity and the energy absorption capacity in strengthened beams. Either sandblasting or roughening the concrete surface by a chisel can be employed when this type of mechanical fastener is used.(4)U-shaped Sprayed GFRP was the most promising way to gain the maximum possible benefits in shear strengthening from these advanced materials. Tension steel yielding was observed in a flexural failure type in beams B3-S-3 and B3-S-4. The confinement provided by U-shaped Sprayed GFRP also effectively increased the energy absorption capacity of these strengthened beams. As a result, it should always be recommended to apply the U-shaped Sprayed GFRP configuration for shear strengthening, where possible.(5)The presence of steel stirrups was effective in increasing the load-carrying and energy absorption capacities of strengthened RC beams. This is a benefit because, in practice, RC beams contain steel stirrups, and adding Sprayed GFRP as external shear reinforcement can more effectively increase the beams’ performance under large loads compared to those with no stirrups.

tab3
Table 3: Peak loads and area under the load versus midspan deflection curves of RC beams.

7. Modeling and Proposed Equation

In all tests performed in this study, the Sprayed GFRP fracture occurred after the peak load (shear capacity) was reached. This shows that after a certain strain is placed on Sprayed GFRP, which would clearly be less than its strain at rupture, there would be no contribution of the FRP to the shear strength of RC beams.

If we consider a single shear crack in an RC beam with a 45° angle with respect to the horizontal axis, the horizontal projection of the crack can be taken as , which is shown in Figure 38.

635176.fig.0038
Figure 38: Depth of FRP Shear Reinforcement.

Therefore, for Sprayed GFRP applied continuously on both sides of an RC beam with a thickness of on each side and modulus of elasticity of , the product of will give the shear resisted by the Sprayed GFRP.

Strengthened beams can be divided into four groups:(1)Sprayed GFRP on two sides with mechanical fasteners,(2)Sprayed GFRP on two sides with epoxy interlayer,(3)Sprayed GFRP on 3 sides (U-shaped),(4)Sprayed GFRP on two sides with no mechanical fasteners or epoxy interlayer.

The shear contribution of Sprayed GFRP for different beams tested in this study as well as the product of are tabulated in Table 4.

tab4
Table 4: Product of for different configurations of sprayed GFRP.

Contribution of Sprayed GFRP to shear strength (i.e., column in Table 4) versus product (i.e., column in Table 4) is drawn in Figures 39 and 40. Figure 39 shows the results for RC beams strengthened by Sprayed GFRP on three sides, two sides with mechanical fasteners, and two sides with epoxy, while Figure 40 demonstrates the results for those strengthened on two sides with no mechanical fasteners and no epoxy.

635176.fig.0039
Figure 39: Contribution of Sprayed GFRP in shear strength versus for RC beams strengthened by Sprayed GFRP on three sides, two sides with mechanical fasteners, and two sides with epoxy.
635176.fig.0040
Figure 40: Contribution of Sprayed GFRP in shear strength versus for RC beams strengthened by Sprayed GFRP on two sides with no mechanical fasteners and no epoxy.

From the first set of specimens shown in Figure 39, a value of 0.003 will be achieved for , while a value of 0.002 is attained for from Figure 40.

Based on the results reported in Figures 39 and 40, the following equation is proposed to calculate the contribution of Sprayed GFRP composites in the shear strength of RC beams: where, = contribution of Sprayed GFRP in shear strength of RC beam [N], = average thickness of the Sprayed GFRP [mm], = depth of FRP shear reinforcement as shown in Figure 38 [mm], = modulus of elasticity of FRP composite, = {0.002, for side bonding to the web when no mechanical fasteners/epoxy are used, 0.003, for side bonding to the web when mechanical fasteners are used, 0.003, for side bonding to the web when an interlayer of epoxy is used, and 0.003, for continuous U-shaped around the bottom of the web}.

The validity of this equation is shown in Table 5. It is clear that the calculated values for are very close to their experimental values. The proposed equation (1) is very similar to Equation (11.5) of CSA S-806-02 [4]. As a result, this proposed equation can easily be implemented in the Canadian Standard Code for shear-strengthening design using Sprayed GFRP composites.

tab5
Table 5: Validity of proposed equation to calculate the contribution of sprayed GFRP in shear strength of RC beam.

8. Conclusions

There are some important things that should be mentioned here.

In Sprayed GFRP application, since U-shaped wrapping will always be applied continuously in practice, the spacing of FRP shear reinforcement (i.e., ) has been left out of the proposed equation. This makes the proposed equation simple to apply.

CSA S-806-02 [4] restricts the summation of shear resistance provided by steel stirrups () and FRP composite to a certain value (Clause 11.3.2.2 Equation (11.2)) as follows: where it = factor to account for low-density concrete, = resistance factor of concrete, = specified compressive strength of concrete [MPa], = width of the web of a beam [mm], and = distance from extreme compression fiber to centroid of tension reinforcement [mm].

It is equally important to keep this restriction in mind while designing shear strengthened RC beams using Sprayed GFRP. In other words, when the Sprayed GFRP coating exceeds a certain thickness, equation (2) will rightly put an upper limit for FRP contribution in the shear strength of an RC beam.

While is either 0.002 or 0.004 for fabric FRP (Equation (11.5) of CSA-S806-02) and 0.002 or 0.003 for Sprayed GFRP (1), in shear strengthening of RC beams there is no major benefit in using ultra-high-strength fabric FRP, and Sprayed GFRP with its strain at rupture of 0.63% can be considered a more economical product compared to fabric FRP with a strain to rupture of about 2.1% (i.e., 5 to 10 times more than 0.004 and 0.002, resp.). It is necessary to mention that all these limits are actually derived from FRP-concrete bond limitations.

It is worth noting that , effective strain of FRP in (1), is governed by the compressive strength of concrete. can be assumed as a maximum strain of GFRP at which the integrity of concrete and secure activation of the aggregate interlock mechanism are maintained. Since in this study the compressive strength of concrete was constant, the relationship between the effective strain of Sprayed GFRP and the compressive strength of concrete could not be established. In general, if we consider a relationship such as the one proposed by ISIS Canada (Equation 2.40) for wrap GFRP, the following equation (or an equation with similar format) can be used to predict the effective strain of Sprayed GFRP for an RC beam with a different concrete compressive strength: where it = effective strain of Sprayed GFRP corresponding to compressive strength of concrete used in RC beam and = compressive strength of concrete in RC beam (MPa).

Note that resistance factor of FRP composites, , has not been introduced into the proposed equation (1). In CSA S806-02 a value of 0.75 is recommended as the resistance factor of FRP composites, and this value can also be applied in (1) as a safety factor.

Implementing into (1), it can be written as: where it is the resistance factor for Sprayed GFRP composite, and a value of 0.75, based on CSA S806-02 [4], is recommended.

References

  1. U.S. Census Bureau Statistical Abstract of the United States, “Table 1090: Bridge Inventory—Total Deficient and Obsolete: 1996 to 2010, and by State 2010,” Transportation, vol. 2012, p. 685, 2012.
  2. S. M. Soleimani, Sprayed glass fiber reinforced polymers in shear strengthening and enhancement of impact resistance of reinforced concrete beams, Ph.D. thesis, The University of British Columbia, Vancouver, BC, Canada, 2006.
  3. ACI Committee 440, “Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures,” ACI 440.2R-08, American Concrete Institute, Farmington Hills, Mich, USA, 2008.
  4. CSA-S806-02 (R2007), “Design and construction of building components with fibre-reinforced polymer,” Tech. Rep., Canadian Standards Association, Rexdale, Ontario, Canada, 2007.
  5. “Strengthening Reinforced Concrete Structures with Externally-Bonded Fiber Reinforced Polymers,” September 2001, ISIS Canada Design Manual no. 4.
  6. fib-TG9.3, “Design and Use of Externally Bonded Fiber Polymer Reinforcement (FRP EBR) for Reinforced Concrete Structures,” Bulletin 14, July 2001.
  7. A. J. Boyd, Rehabilitation of reinforced concrete beams with sprayed glass fiber reinforced polymers, Ph.D. thesis, The University of British Columbia, Vancouver, BC, Canada, 2000.