In this paper, the strengthening of two-way slabs using fiber-reinforced polymers (FRPs) is evaluated experimentally. Two different types of FRP materials were ...
ACI STRUCTURAL JOURNAL
TECHNICAL PAPER
Title no. 101-S64
Fiber-Reinforced Polymer Strengthening of Two-Way Slabs by U. Ebead and H. Marzouk In this paper, the strengthening of two-way slabs using fiber-reinforced polymers (FRPs) is evaluated experimentally. Two different types of FRP materials were evaluated: carbon FRP strips and glass FRP laminates. The dominating failure mode for two-way slab, flexural, or punching shear is based on the slab steel reinforcement ratio. The reinforcement ratios were chosen to serve the purpose of demarcating the two possible modes of failure. The tested specimens were classified according to the purpose of strengthening into specimens strengthened in flexure and specimens strengthened in punching shear. Specimens strengthened in flexure had two steel reinforcement ratios: 0.35 and 0.5%. Results show that the flexural capacity of two-way slabs can increase to an average of 35.5% over that of the reference (unstrengthened) specimen. An increase of the initial stiffness was achieved for flexural specimens; however, an apparent decrease in the overall ductility was evident. FRP materials can be used to increase the flexural capacity of two-way slabs. However, an average decrease in the values of the energy absorption of approximately 30% for flexural strengthening specimens was observed. Specimens strengthened for punching shear have an original slab reinforcement ratio of 1.0%. A strengthening technique that combines the use of carbon FRP strips and steel bolts increases the strength of the slab by 9.0%. An analytical model for the analysis of FRP strengthening of two-way slabs under flexure or punching shear is introduced. Keywords: concrete; fibers; reinforcement; shear; slab; strength.
INTRODUCTION Extensive applications of the fiber-reinforced polymer (FRP) materials as new construction materials have been recently accomplished. FRP materials are lightweight, highstrength, noncorrosive, and nonmagnetic materials. By virtue of these advantages, there is a wide range of recent, current, and potential applications of these materials that covers both new and existing structures. Among different types of FRP materials, carbon fiber-reinforced polymers (CFRPs) and glass fiber-reinforced polymers (GFRPs) are used extensively in the structural engineering field. FRP materials have been used for strengthening reinforced concrete beams, columns, and one-way slabs. The flexural capacity of concrete beams can be increased by bonding FRP sheets, strips, or laminates to the tension side (Ritchie et al. 1991; Al-Sulaimani et al. 1994; Chaallal, Nollet, and Perraton 1998; GangaRao and Vijay 1998). In addition, the shear strength of concrete beams can be increased by gluing FRP laminates to the concrete web at locations of high shear stresses (Triantafillou 1998; Norris and Saadatmanesh 1997). Many research works have dealt with the debonding of FRP sheets to concrete beams (Meier et al. 1993; Arduini et al. 1994). In this regard, some mechanical and finite element models have been developed to provide design guidelines and to investigate theoretically possible modes of failure of FRP-strengthened beams based on experimental data 650
(Malek, Saadatmanesh, and Ehsani 1998; Triantafillou 1998; Nitereka and Neale 1999). Several research programs have been conducted on column strengthening. Concrete-wrapped columns with GFRP laminates showed a considerable enhancement on the column-carrying capacity (Jin, Saadatmanesh, and Ehsani 1994; Soudki and Green 1996). The ease of handling FRP materials provides the means to the extension of their applications for strengthening other structural elements. Very little research has been conducted on the strengthening of reinforced concrete slabs, especially two-way slabs using FRP materials. Some research works dealt with the strengthening of one-way slabs using FRP materials in which slabs were treated in a very similar way to beams (Karbhari et al. 1994; Kikukawa et al. 1998). Two-way slabs with low or medium reinforcement ratios tend to fail in flexure rather than in punching shear. For twoway slabs that have reinforcement ratios of 1.0% and more, the mode of failure tends to be the punching shear type of failure (Marzouk and Hussein 1991). Using FRP materials to enhance two-way slabs in flexure is very desirable from the applicability point of view due to the ease of handling and installing FRP materials. FRP materials are not subject to either corrosion or rust in the long term. The use of FRP materials for strengthening of flexural members can lead to a decrease of the overall structural member ductility, causing a more brittle failure. For punching shear strengthening, an effective strengthening technique for a two-way slab system was developed in an earlier investigation by the authors (Ebead and Marzouk 2002a,b). The technique uses a combination of horizontal steel plates and vertical steel bolts. This technique was very efficient in strengthening two-way slabs in punching shear. The strengthening steel plates were extended twice as the slab depth around the column to act as a drop panel. A minimum of eight 19 mm bolts were required to transfer the horizontal forces induced between the steel plates and concrete. In addition, tightened steel bolts confine strengthened concrete between the steel plates. A similar strengthening technique is adopted in this study by replacing steel plates by the CFRP strips. RESEARCH SIGNIFICANCE The strengthening of two-way slabs using FRP materials is presented. The behavior of two-way slabs strengthened in flexure is discussed. CFRP strips and GFRP laminates can be used to increase the flexural capacity of two-way slabs to an average of 36% over that of the reference (unstrengthened) ACI Structural Journal, V. 101, No. 5, September-October 2004. MS No. 03-114 received March 12, 2003, and reviewed under Institute publication policies. Copyright © 2004, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the JulyAugust 2005 ACI Structural Journal if the discussion is received by March 1, 2005.
ACI Structural Journal/September-October 2004
U. Ebead is a postdoctoral researcher at Sherbrooke University, Quebec, Canada and an assistant professor at Helwan University, Egypt. He received his PhD from Memorial University of Newfoundland, St. John’s, Newfoundland, Canada. His research interests include the use of fiber-reinforced materials for concrete structure strengthening and finite element modeling of fiber-reinforced polymer-strengthened structures. ACI member H. Marzouk is the Chair of the Civil Engineering Discipline at Memorial University of Newfoundland. He received his MSc and PhD from the University of Saskatchewan, Saskatoon, Canada. He is a member of ACI Committees 209, Creep and Shrinkage in Concrete, and 213, Lightweight Aggregate and Concrete. His research interests include structural and material properties of high-strength concrete.
Table 1—Mixture proportions for 1 m3 concrete
Table 2—Compressive strength of concrete Cylinder compressive Specimen title strength, MPa
Strengthening material
Reinforcement Preload, ratio kN
Ref-0.35% Ref-0.5%
30.0 35.0
— —
0.35% 0.5%
— —
Ref-1.0% GFRP-F-0.35%
36.0 29.0
— GFRP laminates
1.0% 0.35%
— 125
GFRP-F-0.5%
38.0
GFRP laminates
0.5%
165
CFRP-F-0.35%
35.0
CFRP strips
0.35%
125
CFRP-F-0.5%
34.0
CFRP strips
0.35%
165
1.0%
210
1.0%
210
1.0%
210
Gravel Sand
1160 kg 690 kg
CFRP1-S-1.0%
30.0
Cement Water
350 kg 175 L
CFRP2-S-1.0%
29.0
CFRP strips + steel bolts CFRP strips + steel bolts
Water-cement ratio High-range water-reducing admixture
0.5 440 mL
Steel-1.0%*
34.0
Steel plates + steel bolts
Air entrainment agent
68.3 mL
*
Specimen A3—Ebead and Marzouk (2002a).
Table 3—Properties of steel reinforcement bars specimen. An increase of the initial stiffness was achieved for flexural specimens; however, an apparent decrease in the overall ductility was evident. An implementation of the ACI 440.2R (ACI Committee 440 2002) is presented for the purpose of verification against experimental results. The estimated ultimate load capacity using the ACI Code is in an accepted level of agreement with the experimental results. EXPERIMENTAL PROGRAM Materials The concrete mixture was designed for an average target cylinder compressive strength of 35 MPa after 28 days. The mixture proportion and the compressive strength are summarized in Table 1 and 2. The steel reinforcement bars were CSA Grade 400 deformed bars. The actual yield strength of the steel reinforcement ranged from 435 to 450 MPa. Three different diameters were used: 10, 15, and 20 mm. The properties of the utilized steel reinforcement are summarized in Table 3. ASTM A 325, 19 mm diameter steel bolts were used for the punching-shear-strengthening specimens. Unidirectional GFRP laminates and CFRP strips were used for strengthening. The thicknesses of the GFRP laminates and CFRP strips were 1.0 and 1.2 mm, respectively. Two different types of two-component adhesive epoxy resins were used with each type of the FRPs as per the manufacturer’s specifications. Epoxy resins were used for the CFRP strips and the GFRP laminates, respectively. The properties of epoxy resins and FRP materials provided by the manufacturers are listed in Table 4 and 5, respectively. Test slabs The tested specimens were square with a 1900 mm side length and 150 mm thicknesses. The test specimens were simply supported along the four edges with corners free to lift and were centrally loaded through the column stub. A layout of the tested slabs is shown in Fig. 1, which also shows reinforcement details of the tested specimens. The selection of the reinforcement ratio of a slab was based on previous studies on slabs of the same dimensions and tested in the same laboratory at the Memorial University of Newfoundland. The failure mode of slabs with reinforcement ratios less than or equal to 0.5% is normally a flexural mode. On ACI Structural Journal/September-October 2004
Bar no. 10 mm
Diameter, Area, mm mm2 11.3 100
Yield strain 0.00235
Mean yield stress, MPa 450
Mean Modulus of ultimate, elasticity E, GPa MPa 660 191
15 mm
16.0
200
0.0025
435
670
193
20 mm
19.5
300
0.0026
440
665
195
Table 4—Properties of one layer of fiber-reinforced polymer materials
FRP
Fiber Thick- Tensile Elastic volume Fiber Weight, ness, strength, modulus, Elongation fraction density GPa at break, % g/m2 content g/cm3 MPa mm
CFRP strips
1.2
2800
170
>1.7
2240
68%
1.5
GFRP laminates
1.0
600
26.13
2.24
913
50 to 80%
2.54
Table 5—Properties of epoxy adhesive Property
Epoxy for strips
Epoxy for laminates
Tensile strength, MPa Elongation at break, %
24.8 1.00
72.4 4.8
Elastic modulus, GPa
4.5
3.1
the other hand, two-way slabs with reinforcement ratios of 1.0% or more are likely to fail due to a punching shear mode (Marzouk and Hussein 1991). Based on this observation, two different reinforcement ratios were chosen to investigate the effectiveness of the flexural strengthening technique: 0.35 and 0.5%. In addition, specimens with reinforcement ratios of 1.0% were used to evaluate the punching shear strengthening. Three unstrengthened specimens were used as reference specimens. These specimens are Ref-0.35%, Ref-0.5%, and Ref-1.0% of reinforcement ratios of 0.35, 0.5, and 1.0%, respectively. Specimens CFRP-F-0.35% and CFRP-F-0.5% had steel reinforcement ratios of 0.35 and 0.5%, respectively, strengthened with CFRP strips. Similarly, specimens GFRP-F0.35% and GFRP-F-0.5% had reinforcement ratios 0.35 and 0.5%, respectively, and were strengthened using GFRP laminates. Specimens CFRP-F-0.35%, CFRP-F-0.5%, GFRP-F-0.35%, and GFRP-F-0.5% will be referred to as flexural strengthening specimens. Specimens CFRP1-S-1.0% 651
worth mentioning that for the punching-shear strengthening, the column stubs were extended on both sides.
Fig. 1—Concrete dimensions and reinforcement details of slabs.
Fig. 2—Steel gages arrangement of tested slabs. and CFRP2-S-1.0% had reinforcement ratios of 1.0% and were strengthened using different geometrical arrangements of CFRP strips and steel bolts. Specimens CFRP1-S-1.0% and CFRP2-S-1.0% will be referred to as punching-shearstrengthening specimens. A minimum concrete cover of 25 mm was maintained for all specimens at compression and the tension sides. Column stubs were square of 250 mm side-dimension and were located at the slab center. The column stubs were extended on the compression side at a distance 850 mm from the concrete surface to allow for the application of the load. Figure 2 shows the locations of steel reinforcement strain gages for specimens of different reinforcement ratios. It is 652
Test setup and instrumentation The specimens were tested using a large reaction steel frame. A 10-ton capacity crane was used to lift and install the specimens vertically inside the frame. Rubber pieces were placed between the back surface of the tested slabs and the supporting edges of the frame. A hydraulic actuator facing the specimen was used to apply a uniform central load through the column stub. A load cell was used to measure the load using four calibrated electrical resistance strain gages fixed to the inner cylinder of the load cell. The actuator had a maximum load capacity of 700 kN and a maximum stroke of 150 mm. Linear variable displacement transformers (LVDTs) were built in the front actuator to measure the central deflection of slabs. The central loads were applied using displacement control to avoid the uncontrolled failure at the maximum loads. The displacement rate for the actuator was 0.25 mm/min. A displacement function of the ramp type was applied through a computerized function generator. Electrical resistance strain gages, 8 mm in length, having a resistance of 120 ± 0.3% and a gage factor equal to 2.070 ± 0.5% were used to measure the steel reinforcement strains at locations shown in Fig. 2. The LVDTs and the electrical strain gages were connected through a master panel to a data acquisition system. The analog electrical signals of loads, deflections, and steel strains were converted through the data acquisition system to digital signals and were stored in digital computer files. Equally spaced dial gages were placed along the width of the specimens to measure the deflection profiles of the specimens during the application of load. The positions of the dial gages are shown in Fig. 1. Load application and testing procedure The unstrengthened reference specimens—Ref-0.35%, Ref-0.5%, and Ref-1.0%—were loaded centrally through the column stub until failure to estimate the ultimate loadcarrying capacity. The ultimate load-carrying capacity of the reference specimens was 250, 330, and 420 kN, respectively. Fifty percent of the ultimate load-carrying capacity of the reference specimens was used as an initial loading for the specimens prior to strengthening. Hence, the specimens with reinforcement ratios of 0.35, 0.5, and 1.0% to be strengthened were loaded prior to strengthening with initial loads of 125, 165, and 210 kN as initial loading. Fifty percent of the load represents a level of load on a building in field where strengthening may be required. The applied loads were completely released to represent a state of shoring two-way slabs in the field prior to strengthening. Afterward, the specimens were removed from the loading frame for strengthening according to the strengthening procedure detailed as follows. After 1 week of curing, the specimens were relocated at the loading frame and were subjected to the central load until failure. Table 2 summarizes the values of the preload of each specimen. Strengthening procedure The concrete surface to be strengthened was roughened carefully using a vibrating hammer to improve the bond characteristics between concrete and the CFRP strips and GFRP laminates. Dust and fine materials caused from the roughening process were removed carefully from the ACI Structural Journal/September-October 2004
Fig. 3—Details of flexural-strengthening specimens. concrete surfaces. In addition, for CFRP strips, a special solvent was used to remove all grease, waxes, foreign particles, and other bond-inhibiting materials from the bonded surface as specified by the manufacturer. The two-part epoxy resin was applied on both the concrete surfaces and the strengthening materials. Afterward, the FRP strengthening materials were bonded to the concrete surface according to the type of strengthening. Flexural strengthening specimens The strengthening material was located at the tension side of the slab and was extended to a location 50 mm before the support. Two 300 mm-width layers of GFRP laminates were bonded to the slab surface in both directions of specimens GFRP-F-0.35% and GFRP-F-0.5%, as shown in Fig. 3. Specimens CFRP-F-0.35% and CFRP-F-0.5% were strengthened using three adjacent CFRP strips of 100 mm width each so that the strengthened width is 300 mm. Additional transverse layers of CFRP strips were bonded at the end of the FRP materials to improve the end anchorage of the FRP strips or laminates with concrete surface. The anchorage layers were 100 mm wide and 500 mm long. Figure 3 shows details of the flexural strengthening specimens. Punching-shear-strengthening specimens The size and configuration of the strengthening materials were based on a similar successful strengthening technique using steel for two-way slabs. That technique employed steel plates and vertical steel bolts to strengthen a two-way slab system. Details of this technique are shown in Fig. 4. The two-way slab strengthening technique is based on a previous three-dimensional finite element stress analysis study of the shear and bending stress distribution in the slab to column connections (Marzouk and Jiang 1996). The numerical study was supported by experimental investigation (Marzouk and Jiang 1997). Based on the recommendation of the two aforementioned studies, the strengthening material was extended around the column to a distance of twice the concrete slab’s depth. The strengthening material was placed on both sides of the slab. Holes were predrilled all the way through the slab thickness and eight 19-mm diameter bolts were installed. Steel bolts were inserted in the slab to provide vertical shear reinforcement and to achieve full interaction between the strengthening material and concrete. The bolts were distributed so that four equi-spaced bolts were inserted ACI Structural Journal/September-October 2004
Fig. 4—Details of steel plates’ strengthening technique (Ebead and Marzouk 2002a).
Fig. 5—Details of punching-shear-strengthening specimens. on the inner circumference and four others were inserted on the outer circumference. Using a calibrated torque wrench, the nuts of the bolts were subjected to a specified torque equal to 441 kN.mm. Details of the punching-shear strengthening specimens are shown in Fig. 5. TEST RESULTS AND DISCUSSION Crack load and deflection Cracks of all specimens prior to strengthening were traced as the load was applied and the first crack load values were recorded. Specimens with a reinforcement ratio of 0.35% indicated the lowest first crack loads of 73, 70, and 68 kN for specimens Ref-0.35%, CFRP-F-0.35%, and GFRP-F-0.35%, respectively. The first crack loads of 84, 80, and 83 kN were 653
Table 6—Ultimate capacities and deflection characteristics of tested slabs Title Ref-0.35%
Cracking load Pcr , Deflection at cracking Ultimate Deflection at ultimate Energy absorption load δcr , mm load Pu, kN load δu, mm kN Ψ, kN.mm 73 7.00 250 42.01 9346
Stiffness K, kN/mm 8.42
Failure mode Flexure
Ref-0.5% Ref-1.0%
84 89
6.25 4.85
330 420
35.57 24.50
9445 5950
12.54 20.08
Flexure Punching shear
CFRP-F-0.35% GFRP-F-0.35%
70 68
7.25 7.69
361 345
18.08 27.72
7821 4597
15.54 24.42
Flexure Flexure
CFRP-F-0.5% GFRP-F-0.5%
80 83
6.03 6.35
450 415
21.03 26.71
6686 7475
26.76 23.15
Flexure Flexure
CFRP1-S-1.0% CFRP2-S-1.0%
103 96
5.02 4.59
491 425
27.71 24.51
10,090 7501
26.10 17.68
Punching shear Punching shear
Steel-1.0%*
85
4.80
645
28.00
8862
35.90
Flexure
*
Specimen A3 (Ebead and Marzouk 2002a).
Fig. 6—Load-deflection strengthening specimens.
relationships
of
flexural
recorded for the specimens with reinforcement ratios of 0.5% for Ref-0.5%, CFRP-F-0.5%, and GFRP-F-0.5%, respectively. The highest first crack load was observed for specimens with a reinforcement ratio of 1.0%. The first crack loads were 89, 103, and 96 kN for specimens Ref-1.0%, CFRP1-S-1.0%, and CFRP2-S-1.0%, respectively. The use of CFRP and GFRP increased the equivalent reinforcement ratio slightly compared with the reference specimens. In the mean time, the associated deflection to the first crack load is decreased as the reinforcement ratio is increased. First crack loads Pcr and the associated deflection values δcr for all specimens prior to strengthening are shown in Table 6. Load-deflection relationships The central load-deflection relationship was recorded using the data acquisition system. In addition, the deflection profile at nine different positions along each slab’s width was measured using dial gages. The variation of the deflection values against 654
the load was largely dependent on the reinforcement ratio. For the reference specimens (Ref-0.35%, Ref-0.5%, and Ref-1.0%), the deflection value decreased as the reinforcement ratio increased. As shown in Fig. 6, the deflection at the ultimate load was decreased from 42.01 to 24.50 mm as the reinforcement ratio was increased from 0.35 to 1.0%. For the flexural strengthening specimens (GFRP-F-0.35%, CFRP-F-0.5%, GFRP-F-0.35%, and CFRP-F-0.5%), the slope of the load-deflection curve was higher than that of the corresponding reference specimens. Moreover, the average deflection at the ultimate load of the flexural strengthening specimens was approximately 0.61 that of the corresponding reference specimens. In general, flexural strengthening specimens experienced smaller deformation compared to the corresponding reference specimens due to the effect of the FRP materials on the overall behavior of the slabs. Regarding the punching-shear-strengthening specimens (CFRP1-S-1.0% and CFRP2-S-1.0%), a slight change in the slope of the load-deflection relationship was noticed compared with the reference specimen, Ref-1.0%. Figure 6 shows the load-deflection relationships for the flexural strengthening specimens, including the associated reference specimens. In addition, Fig. 7 shows the load-deflection relationships for the punching-shear strengthening specimens. Figure 7 also includes the load-deflections relationships for the reference specimen and Specimen Steel-1.0% represented from previous research on strengthening using steel plates for comparison. Specimen Steel-1.0% is referred to as A3 in the previous research (Ebead and Marzouk 2002 a,b). It is clear that steel strengthening leads to a stiffer initial behavior of slabs compared with CFRP strips strengthening. Table 6 summarizes the deflection values δu associated with the ultimate load for all specimens. The deflection profile for the flexural strengthened specimens is shown in Fig. 8. Ultimate load-carrying capacity The ultimate load-carrying capacity will be referred to as the load capacity. The flexural strengthening specimen showed higher load capacity than that of the corresponding reference specimens. Specimens CFRP-F-0.35% and GFRP-F-0.35% showed increases of 44.4 and 38%, respectively, in the load capacity over that of the reference specimen, Ref-0.35%. Moreover, Specimens CFRP-F-0.5% and GFRP-F-0.5% showed increases of 36.4 and 25.8%, respectively, in the load capacity over that of the reference specimen, Ref-0.5%. The load capacity of the corresponding reference specimens was influenced by the reinforcement ACI Structural Journal/September-October 2004
ratio that is in accordance with previous research (Marzouk and Hussein 1991). The load capacity of specimen Ref-0.5% was 1.32 times that of Specimen Ref-0.35%. However, the average increase of the punching-shear-strengthening specimens, CFRP1-S-1.0% and CFRP2-S-1.0% gained over the associated reference specimen, Ref-1.0%, was only 9%. Referring to a previous study, the steel-strengthened specimen gained an increase of 31.36 and 51.76% over that of Specimens CFRP1-S-1.0% and CFRP2-S-1.0%, respectively. Table 6 summarizes the load capacity Pu of all specimens. Stiffness characteristics The stiffness of a slab at any loading point is the slope of the load-deflection curve at that point. The initial stiffness K was evaluated numerically as the slope of the load-deflection curve within the first 5 mm deflection. This is an approximation
made to avoid the misleading initial readings when there is a relaxation of the load actuator. The flexural strengthening specimens showed higher initial stiffness over that of the reference specimens. The average initial stiffness of Specimens CFRP-F-0.35% and GFRP-F-0.35% was approximately 2.37 times that of the reference specimen, Ref-0.35%. Moreover, the average initial stiffness of Specimens CFRP-F-0.5% and GFRP-F-0.5% was approximately 1.99 times that of the reference specimen, Ref-0.5%. Punching-shear-strengthening specimens, CFRP1-S-1.0% and CFRP2-S-1.0%, gained an average increase in the initial stiffness of 9% over that of the reference specimen, Ref-1.0%. The specimen strengthened using steel plates, Steel-1.0%, gained an average increase in the initial stiffness of 70% over the average of that of the punching-shear-strengthening specimens (Ebead and Marzouk 2002a). Table 6 shows the initial stiffness K values of all specimens. Energy absorption characteristics The energy absorption is the area under the load-deflection curve for a tested specimen. This area was evaluated numerically based on the available values of load and the corresponding values of deflection. At the maximum load, it was clearly noticed that the strengthening technique contributed to a decrease in the energy absorption of the flexural strengthening specimens. An average decrease in the values of the energy absorption of approximately 30% for flexural strengthening specimens was observed. An average increase of approximately 31%, however, was recorded in the case of punching-shearstrengthening specimens. Values of the energy absorption Ψ for each slab are summarized in Table 6.
Fig. 7—Load-deflection relationships for punchingshear-strengthening specimens.
Steel reinforcement strains Measurements were made to determine the steel strain distribution at selected radii from the centers of the slabs. The locations of the strain gages shown in Fig. 2 were chosen to track the variation of the steel strain with the distance from the center of the panel. Figure 9 shows the main reinforcement strain gages distribution for specimens subjected to central load with different reinforcement ratios at Location 1 of Fig. 2.
Fig. 8—Deflection profiles of flexural strengthening specimens. ACI Structural Journal/September-October 2004
655
Fig. 11—Typical layout of GFRP flexural strengthened specimen at failure.
Fig. 9—Load-steel reinforcement strain relationships for tested specimens.
Fig. 10—Typical layout of flexural failure of unstrengthened specimens. Location 1 is 170 mm from the center of the slab. Figure 9 combines the steel strain distribution for all specimens at this location. The recorded steel strain indicated that for CFRP strip specimens, the steel reached the yield strain at the failure load. For the specimen strengthened with GFRP, however, the steel strain at failure load was approximately four times the yield strain. As shown in Fig. 9(a), a stiffer behavior was noticed for the flexural-strengthened specimens compared with the associated unstrengthened specimens. In addition, specimens 656
with reinforcement ratios of 0.5% showed stiffer behavior compared with those with 0.35% reinforcement ratios for the same strengthening material. The punching-shear-strengthened specimens showed lower stiffness compared with the reference unstrengthened specimens, as shown in Fig. 9(b). The lower stiffness can be explained due to the stress concentration effect around the bolts locations. In addition, there was not enough confinement to enhance the behavior of these specimens. Failure characteristics For reference specimens, Ref-0.35% and Ref-0.5%, failure mode was classified as flexural-ductile. Flexural reinforcement yielded and the two specimens showed relatively large deflection values before reaching the ultimate load. Specimen Ref-1.0% showed a more brittle failure due to punching shear mode of failure. Figure 10 shows the failure of the reference specimen, Ref-0.5%, showing the typical flexural failure mode for unstrengthened specimens. The typical flexural failure modes of GFRP and CFRP flexural strengthened specimens are shown in Fig. 11 and 12, respectively. It is evident that the FRP materials contributed to an increase of the capacity until the bond between the FRP material and concrete failed. Debonding cracks appeared at a late stage of loading that resulted in a separation of the strengthening materials. These cracks were located along the edges of the strengthening material length. This indicates that end anchoring severed to a certain extent, preventing a premature bond failure at the cutoff end of FRP materials. After the appearance of these cracks, the specimens failed due to accelerated concrete flexural failure after the FRP debonded from the slabs without rupture of the FRP material. Punching-shear-strengthening specimens failed in a punching shear mode of failure as that of the corresponding reference specimen, Ref-1.0%. A local failure at one of the outer diameter bolt locations occurred at late stages of the application of load followed by a sudden punching shear failure of concrete, as shown in Figure 13. It is important to point out that when steel plates were used for the same ACI Structural Journal/September-October 2004
strengthening technique, the punching failure was eliminated and transferred to a more ductile flexure failure. Numerical code evaluation and load-carrying capacity A simplified method for the code evaluation of the loadcarrying capacity for a two-way slab strengthened with steel plates and bolts was suggested previously (Ebead and Marzouk 2002a). This method is based on the analysis of two-way slab recommended by Rankin and Long (1987). This approach is based on the following S P flex = 8M b --------- – 0.172 l – c
(1)
Equation (1) is based on the virtual work done by the action of the yield lines. The value of Mb in Eq. (1) is the radial moment capacity of the slabs. For the original concrete specimens, ACI 318-99 is used to evaluate Mb. The same equation can be used for the analysis of strengthened twoway slab using approximated evaluations of some of the parameters of Eq. (1) according to the type of the strengthening. Punching-shear-strengthened slabs CFRP strips’ contribution in increasing the flexural capacity Mb is limited due to the discontinuity of CFRP strips. The experimental results indicated that the punching failure was initiated on the compression face of the slab at a distance equal to half the slab depth, as shown in Fig. 13, and not at the usual column face as for the case for un-strengthened slabs. The value of c in Eq. (1) for the strengthened slabs can be taken as the side length of the column plus the slab depth. This is an approximated estimation of the contribution of the strengthening system. Hence ceq = c + d
(2)
where d is the slab depth. For this case, Mb1 is the unstrengthened capacity of the slab. According to ACI 318-99, Mb1 is evaluated according to the following expression 2 ( ρ – ρ′ ) M b1 = bd ( ρ – ρ′ )f y 1 – 0.59 ------------------- f y fc ′
(3)
A comparison between the experimental results and theoretical estimation in terms of the ultimate load-carrying capacity is shown in Table 7. An accepted level of agreement is reached between the experimental results and the suggested theoretical estimation. Flexural strengthened slabs In this case, the contribution of the strengthening materials is taken into account when evaluating the radial moment capacity Mb
ACI Structural Journal/September-October 2004
Fig. 13—Failure of punching-shear-strengthening specimens.
Table 7—Comparison with theoretical evaluation Specimen
Pexp
Ptheo
Ptheo /Pexp
CFRP-F-0.35% GFRP-F-0.35%
361 345
332 323
0.92 0.94
CFRP-F-0.5% GFRP-F-0.5%
450 415
420 411
0.93 0.99
CFRP1-S-1.0% CFRP2-S-1.0%
491 425
491 491
1.00 1.16
where Mb2 is the contribution of the strengthening material and is evaluated according to the following equation, assuming full bonds between FRP and concrete
+ ρ′f y d ( d – d′ )
M b = M b1 + M b2
Fig. 12—Typical layout of CFRP flexural strengthened specimen at failure.
(4)
w FRP M b2 = E FRF t FRP ε FRP h – a--- ---------- 2 ηl
(5)
Equation (5) is based on the FRP section analysis as recommended by ACI 440.2R (ACI Committee 440 2002). The term wFRP /ηl is introduced for the two-way slab equation to take into account the ratio between the width of the strengthening material and the slab width l. The factor η is the strengthening efficiency factor and is taken as 0.75 for two-way slabs as recommended by Ebead (2002). The strain in FRP strips layer can be evaluated as 657
h h ε FRP = --- – 1 ε cu + --- ε s d d
(6)
The distance of the neutral axis from the top of slab a can be calculated as follows d a = 0.8 ------------------ ε cu ε cu + ε y
(7)
The test results indicated that, for CFRP strips, it could be assumed that at failure, concrete reached the ultimate strain and the steel reached the yield strain. For the specimen strengthened with GFRP, however, the concrete strain reached the ultimate value and the steel strain at failure reached four times the yield strain. Once the FRP strain and the location of neutral axis are determined, the strengthened moment contribution to the slab can be evaluated from Eq. (5). For the evaluation of the total load capacity, Eq. (1) is used and replacing the value of c by the FRP width wFRP. Hence, in the case of FRP flexural strengthening S P flex = 8M b -------------------- – 0.172 l – w FRP
(8)
The implementation of the prescribed analytical method showed a good agreement with the experimental results as shown in Table 7. SUMMARY AND CONCLUSIONS The following conclusions were drawn for the strengthened two-way slabs using GFRP laminates and CFRP strips: 1. Flexural strengthening specimens using CFRP strips showed an average gain in the load capacity of approximately 40% over that of the reference (unstrengthened) specimens; 2. Flexural strengthening specimens using GFRP laminates showed an average gain in the load capacity of approximately 31% over that of the reference specimens; 3. The flexural strengthening specimens showed a stiffer behavior than that of the reference specimens. A decrease in ductility and energy absorption was recorded, however, due to the brittle nature of the strengthening of the FRP materials. The average energy absorption of the strengthened specimens using CFRP strips and GFRP laminates were 0.77 and 0.64, respectively, of that of the reference specimens; 4. For the suggested flexural strengthening technique, debonding between FRP materials and concrete was the main cause of failure. Slabs failed soon after debonding occurred due to exceeding flexural capacity. No FRP tensile rupture was observed; 5. The test results of the CFRP strips used for punchingshear-strengthening specimens indicated a small average increase within 9% over the unstrengthened specimens. In addition, the strengthened specimens failed under the undesirable sudden punching shear failure mode; and 6. The recommended theoretical analysis used for FRP strengthening of two-way slabs showed a good agreement with experimental test results. ACKNOWLEDGMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) for providing the funds for the project. Sincere thanks are due to the Technical Staff of the Structural Engineering Laboratory
658
of Memorial University of Newfoundland for their assistance during the preparation of the specimens and during testing. Sincere thanks are extended to Sika Canada Inc. for supplying the fiber reinforcement plastic materials and the epoxies, and to Capital Ready Mix Ltd., Newfoundland, for providing the concrete for this project.
NOTATION a c cequ d
= = = =
d′
=
Ec EFRP fc′ fy h K l lp Mb Mb1 Pcr Pflex Pu tFRP wFRP δcr δu Ψ ρ ρ′ η
= = = = = = = = = = = = = = = = = = = = =
distance from top of slab to neutral axis, mm side length of square column, mm equivalent side length due to strengthening, mm distance from compression face to center of tension reinforcement, mm distance from compression face to center of compression reinforcement, mm modulus of elasticity of concrete, MPa the modulus of elasticity of FRP materials, MPa compressive strength of concrete, MPa yield stress of the slab reinforcement, MPa overall slab thickness, mm initial stiffness of specimen, kN/mm side length of square slab, mm length of strengthening steel plates, mm radial moment of resistance of strengthened section, N.mm/mm radial moment of resistance of unstrengthened section, N.mm/mm first crack load of slab before strengthening, kN flexural load-carrying capacity, kN ultimate load of specimen, kN total thickness of FRP material, mm width of FRP materials, mm deflection at slab center at first crack load, mm deflection at slab center at ultimate load, mm energy absorption of specimen, kN/mm tension reinforcement ratio of slab compression reinforcement ratio of slab effective strengthening width coefficient
REFERENCES ACI Committee 318, 1999, “Building Code Requirements for Structural Concrete (ACI 318-99) and Commentary (318R-99),” American Concrete Institute, Farmington Hills, Mich., 1999, 391 pp. ACI Committee 440, 2002, “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (ACI 440.2R-02),” American Concrete Institute, Farmington Hills, Mich., 45 pp. Al-Sulaimani, G. J.; Sherif, A.; Basunbul, I. A.; Blunch, M. H.; and Ghalab, B. N., 1994, “Shear Repair for Reinforced Concrete by Fiberglass Plate Bonding,” ACI Structural Journal, V. 91, No. 4, July-Aug., pp. 458-464. Arduini, M.; D’Amrisi, A.; and Tommaso, A. D., 1994, “Shear Failure of Concrete Beams Reinforced with FRP Plates,” Infrastructure: A New Materials and Methods of Repair, Proceedings of the Third Materials Engineering Conference, San Diego, Calif., pp. 123-130. Chaallal, O.; Nollet, M.; and Perraton, D., 1998, “Strengthening of Reinforced Concrete Beams with Externally Bonded Reinforced Plastic Plates: Design Guidelines for Shear and Flexure,” Canadian Journal for Civil Engineering, V. 25, pp. 692-704. Ebead, U., 2002, “Strengthening of Reinforced Concrete Two-Way Slabs,” PhD Thesis, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada, 245 pp. Ebead, U., and Marzouk, H., 2002a, “Strengthening of Two-Way Slabs Using Steel Plates,” ACI Structural Journal, V. 99, No. 1, Jan.-Feb., pp. 23-31. Ebead, U., and Marzouk, H., 2002b, “Strengthening of Two-Way Slabs Subjected to Moment and Cyclic Loading,” ACI Structural Journal, V. 99, No. 4, July-Aug., pp. 435-444. GangaRao, H. V. S., and Vijay, P. V., 1998, “Bending Behavior of Concrete Beams Wrapped with Carbon Fabric,” Journal of Structural Engineering, V. 124, No. 1, pp. 3-10. Jin, L.; Saadatmanesh, H.; and Ehsani, M. R., 1994, “Seismic Retrofit of Existing Reinforced Concrete Columns By Glass-Fibre Composites,” Infrastructure: A New Materials and Methods of Repair, Proceedings of the Third Materials Engineering Conference, San Diego, Calif., pp. 758-763. Karbhari, V. M.; Seible, F.; Seim, W.; and Vasquez, A., 1994, “Strengthening of Concrete Slabs,” Infrastructure: A New Materials and Methods of Repair, Proceedings of the Third Materials Engineering Conference, San Diego, Calif., pp. 1163-1172. Kikukawa, K.; Mutoh, K.; Ohya, H.; Ohyama, Y.; and Tanaka, H., 1998, “Flexural Reinforcement of Concrete Floor Slabs by Carbon Fiber Textiles,” Composite Interfaces, V. 5, No. 5, pp. 469-478. Malek, M. A.; Saadatmanesh, H.; and Ehsani, M. R., 1998, “Prediction
ACI Structural Journal/September-October 2004
of Failure Load of R/C Beams Strengthened with FRP Plate Due to Stress Concentration at the Plate End,” ACI Structural Journal, V. 95, No. 2, Mar.Apr., pp. 142-152. Marzouk, H.; Emam, M.; and Hilal, M. S., 1996, “Effect of HighStrength Concrete Columns on the Behavior of Slab-Column Connections,” ACI Structural Journal, V. 93, No. 5, Sept.-Oct., pp. 545-554. Marzouk, H., and Hussein, A., 1991, “Experimental Investigation on the Behavior of High-Strength Concrete Slabs,” ACI Structural Journal, V. 88, No. 6, Nov.-Dec., pp. 701-713. Marzouk, H., and Jiang, D., 1996, “Finite Element Evaluation of Shear Enhancement for High-Strength Concrete Plates,” ACI Structural Journal, V. 93, No. 6, Nov.-Dec., pp. 667-673. Marzouk, H., and Jiang, D., 1997 “Experimental Investigation on Shear Enhancement Types for High-Strength Concrete Plates,” ACI Structural Journal, V. 94, No. 1, Jan.-Feb., pp. 49-58. Meier, U.; Deuring, M.; Meier, H.; and Schwegler, G., 1993, “CFRP Bonded Sheets,” Fibre-Reinforced Plastic (FRP) Reinforcement for Concrete Structures: Properties and Applications, A. Nanni, ed., CH-8600, Elsevier Science Publishers B. V., Duebendof, Switzerland, pp. 423-434. Nitereka, C., and Neale, K. W., 1999, “Analysis of Reinforced Concrete Beams Strengthened in Flexure with Composite Laminates,” Canadian Journal for Civil Engineering, V. 26, pp. 646-654.
ACI Structural Journal/September-October 2004
Norris, T., and Saadatmanesh, H., 1997, “Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets,” Journal of Structural Engineering, ASCE, V. 123, No. 7, July, pp. 903-911. Osman, M.; Marzouk, H.; and Helmy, S., 2000, “Behavior of HighStrength Lightweight Concrete Slabs under Punching Loads,” ACI Structural Journal, V. 97, No. 3, May-June, pp. 492-498. Rankin, G. I. B., and Long, A. E., 1987, “Predicting the Punching Strength of Conventional Slab-Column Specimen,” Proceedings, Part 1: Design and Construction, Institution of Civil Engineers, V. 82, pp. 327-346. Ritchie, P. A.; Thomas, D. A.; Lu, L.-W.; and Connelly, G. M., 1991, “External Reinforcement of Concrete Beams Using Fiber Reinforced Plastics,” ACI Structural Journal, V. 88, No. 4, July-Aug., pp. 490-500. Soudki, K. A., and Green, M. F., 1996, “Performance of CFRP Retrofitted Concrete Columns at Low Temperature,” Advanced Composite Materials in Bridges and Structures, Proceedings, M. M. Le-Badri, ed., Elsevier Science Publishers B.V., Montreal, Quebec, Canada, pp. 427-434. Triantafillou, T. C., 1998, “Shear Strengthening of Reinforced Concrete Beams Using Epoxy-Bonded FRP Composites,” ACI Structural Journal, V. 95, No. 2, Mar.-Apr., pp. 107-115.
659
