Structure and Infrastructure Engineering, 2014 http://dx.doi.org/10.1080/15732479.2014.930497
Enhancement of flexural behaviour of CFRP-strengthened reinforced concrete beams using engineered cementitious composites transition layer Hamdy M. Afefy*, Nesreen Kassem1 and Mohamed Hussein2 Structural Engineering Department, Faculty of Engineering, Tanta University, Tanta, Egypt (Received 22 November 2013; final version received 7 March 2014; accepted 31 March 2014) This paper aimed to develop and evaluate an efficient strengthening method for reinforced concrete beams, based on engineered cementitious composites (ECC) to be applied as a transition layer prior to the application of the carbon fibrereinforced polymer (CFRP) strengthening sheet. The role of the proposed transition layer is to control the cracking of concrete and detain or even avoid premature de-bonding of the strengthening CFRP sheets. As the ability of the transition layer to exhibit a strain hardening behaviour is mainly dependent on the used fibre volumetric ratio, three ECC mixes with three different polypropylene fibre volumetric ratios were used (fibre volumetric ratio of 0.5%, 1% and 1.5%). The experimental results showed that while the used CFRP strengthening sheet can increase the ultimate load by about 28.8% compared with the control un-strengthened beam, this increase can reach about 48.5% by applying the same CFRP sheet to the proposed ECC transition layer that contains a fibre volumetric ratio of 1.5%. Moreover, this layer integrated with the mention ratio of the fibre content enabled the CFRP sheet to be in a complete contact with the strengthened beam without any de-bonding up the rupture of the CFRP sheet at failure. Keywords: crack width; cracks spacing; carbon fibre-reinforced polymers; ductility; engineered cementitious composites; reinforced concrete beams; strengthening
1.
Introduction
The progressively rapid deterioration of infrastructure is becoming a prime problem for both researchers and engineers in construction industry. Engineers are confronted with the continuous challenge of developing new techniques by researchers in order to repair, replace or rehabilitate the existing structures. Deterioration in all types of reinforced concrete (RC) structures is further provoked by load- induced stresses greater than the design stress, excessive concentrations of chlorides in construction materials, by high humidity and temperatures, and by marine environments. The construction industry is urgently in need of non-corrosive materials as alternatives to steel reinforcement or, at least, a proper protection for the main steel and its concrete tension cover. In order to remedy this problem, various solutions ranging from replacement of the structure to strengthening with a variety of techniques have been proposed. In addition, many methods to counter the threat of corrosion in steel reinforcement such as epoxy coatings, cathodic protection, increased concrete cover thickness and polymer concrete have been proposed, but none of these measures has provided a long-term solution. In the current research, the tension zone around the main tension steel of the test beams was replaced by an engineered
*Corresponding author. Email:
[email protected] q 2014 Taylor & Francis
cementitious compositess (ECC) material and then the strengthening carbon fibre-reinforced polymer (CFRP) sheets were bonded to its bottom surface. That was done for the aim of enhancing the strengthened beams cracking behaviour resulting in improved bond characteristics, which in turn increases the strengthening efficiency along with providing protection for the main tension steel, simultaneously. Fibre-reinforced polymer (FRP) materials as a new construction strengthening materials have been developed and used for recent, current and potential applications that cover both new and existing structures. FRP composites have become more popular in recent years due to the reduction in their cost, combined with newer understanding of the versatility and benefits of the material properties. Among different types of FRP materials, a CFRP is used extensively in the structural engineering field. The externally bonded reinforcement (EBR) technique has been more generally applied due to its simple installation procedure. Design guidelines and specifications have also been established well for this system (ACI 440.2R-08). In particular, their practical implementations for strengthening by epoxy bonding are numerous (Benzarti, Freddi, & Fre´mond, 2011; Cromwell, Harries, & Shahrooz, 2011; Jumaat, Rahman, & Rahman,
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H.M. Afefy et al.
2011; Ombres, 2010; Rasheed, Harrison, Peterman, & Alkhrdaji, 2010). Although external strengthening of RC beams with epoxy-bonded CFRP has been established as an effective tool for increasing flexural and/or shear strength, the method still suffers from some drawbacks. Many of these drawbacks are attributed to the characteristics of currently available commercial CFRP-strengthening systems. Even though CFRPs have high strength, they are very brittle. When loaded in tension, FRPs exhibit a linear stress – strain behaviour up to failure, without exhibiting a yield plateau or any indication of an impending failure. As FRPs behave differently than steel, they consequently suffer from a significant loss in beam ductility, particularity when CFRPs are used for flexural strengthening of RC beams (Arduini & Nanni, 1997; Bencardino, Spadea, & Swamy 2002; Lee, Pan, & Ma, 2004; Saadatmanesh & Ehsani, 1991; Spadea, Swamy, & Bencardino, 2001; Toutanji, Zhao, & Zhang, 2006). Over the years, many studies focusing on the behaviour of CFRP-strengthened beams have been conducted to better understand their behaviour under different loading conditions, aiming also to develop the best technique for the application of CFRP fabric sheets and strips. Ehsan, Sobuz, and Sutan (2011) verified that the attachment of CFRP laminates with edge strip plates had substantially influenced the performance of CFRPstrengthened beams. In addition, the important practical issues that were encountered in strengthening of beams with different types and different thicknesses of FRP were addressed (Murali & Pannirselvam, 2011). Hence, a simple method of applying FRP for strengthening the beam with different FRP types with different thicknesses was proposed. In addition to the practical experience for choosing the strengthening configuration, design guidelines for FRP RC structures were stipulated (Pilakoutas, Neocleous, Guadagnini, & Matthys, 2011) ECC is a special category of the new generation of high-performance fibre-reinforced cementitious composites with microstructures tailored according to micromechanics theory, featuring high ductility and durability characteristics (Dhawale & Joshi 2013; Li, 1993, 2012; Li & Li, 2011a, 2011b). ECC’s high tensile ductility, deformation compatibility with existing concrete and self-controlled micro-crack width lead to their superior durability under various mechanical and environmental loading conditions such as fatigue, freezing and thawing, chloride exposure and drying shrinkage (Li & Li, 2008, 2011a, 2011b; Sahmaran, Li, & Li, 2007). Experience of the use of the ECC material in a bridge deck patch repair case showed that high tensile strain capacity is one of the most important properties of a durable repair material to resist typical failures in repaired systems and that such high tensile strain capacity can be achieved economically in ECC materials (Li, 2004).
ECC is a mortar-based composite that is reinforced with short random fibres, such as polyethylene (PE) or polyvinyl alcohol (PVA). The use of PVA fibres has better tensile crack-bridging properties while the use of PE fibres shows a better compressive behaviour (Fischer & Li, 2007). ECC is a micromechanically designed material that uses a micromechanical model to tailor-make the required properties. By using this micromechanical tool, the fibre amount used in the ECC is typically less than 2% by volume. The moderately low fibre content has also made shotcreting ECC viable. Furthermore, the most expensive component of the composite fibres is minimised resulting in ECC that is more acceptable to the highly cost-sensitive construction industry (Dhawale & Joshi, 2013). Therefore, ECC is designed to resist large tensile and shear forces but at the same time has compatible properties, such as compressive strength and thermal expansion, to ordinary concrete based on Portland cement (Li, 2002). Since the introduction of this material two decades ago, ECC has undergone major evolution in both materials development and the range of emerging applications. A variety of experiments have been performed to assess the performance of ECC at the structural level (Fischer & Li, 2003a, 2003b; Fukuyama, Matsuzaki, Nakano, & Sato 1999; Li & Fischer, 2002) for both seismic and non-seismic structural applications. These experiments provide new insights into how the material properties enhance the structural performance of the structure. Under tensile loading, in contrast to normal concrete where a single unstable crack develops into a failure plane, a characteristic of ECC is the development of multiple stable micro-cracks, bridged by fibres (Spagnoli, 2009). As a micro-crack forms, the fibres within the matrix bridge the crack, preventing it from propagating into a failure plane. The multiple microcracking behaviour of ECC is dependent on the fibre crackbridging law and on the degree of heterogeneity in the material. Crack initiation sites are typically at material flaws, which in the majority of cases are bubbles of entrapped air (voids). Crack initiation behaviour is therefore influenced by the size and spatial distribution of voids in the material (Wang & Li, 2004). Final failure occurs when one of the multiple cracks forms a fracture plane. Many researchers have confirmed that applying CFRP laminates to RC beams can increase both the ultimate load carrying capacity and the stiffness of the beams. Further observations proved that the application also delays the cracking moment and mitigates the development of cracks (FIB, 2001). Most of the investigations carried out on RC members strengthened in flexure have indicated that EBR technique cannot fully utilise the strength of the FRP composites. Brittle failure due to premature FRP debonding and concrete cover separation has been the most common type of failure (Teng, Chen, Smith, & Lam 2001). CFRP composites have very desirable high
Structure and Infrastructure Engineering strength, but cannot be utilised properly if there is a poor FRP – concrete bond. Typically, failure of a FRP –concrete bond occurs within the concrete just a few millimetres below the surface (Chen & Teng, 2001; Pan & Leung 2007; Wu, Hu, Wu, & Zheng, 2011; Yuan, Teng, Seracino, Wu, & Yao, 2004). The de-bonding mechanism results in the loss of the composite action between the concrete and FRP laminates and initiates when high interfacial shear and normal stresses exceed the strength of concrete. The current research is undertaken in the aim of enhancing the bond characteristics between the externally bonded CFRP (EB-CFRP) strengthening materials and the substrate concrete beams by replacing the concrete tension zone around the main tension steel by an ECC transition layer with different fibre contents. The polypropylene (PP) fibres were used in the ECC material mix with three different volume ratios in order to accomplish the optimum ratio of fibre that can guarantee the complete composite action between the CFRP sheets and the substrate beams up to failure. That was done in order to be able to utilise the overall strength of the CFRP sheets. At this stage of research, the contribution of the ECC layer in enhancing the ultimate strength was not of prime importance. On the other hand, enforcing the CFRP sheet to use its full tensile capacity was the most important issue. In addition, this layer will provide a good protection for the strengthened beams steel reinforcement against any future attack that leads to corrosion.
2. Experimental work programme 2.1 Test beams For the purposes of this study, six beams were fabricated and cast then tested up to failure. The beams were divided into two groups: the first group contained three beams representing the reference beams, and the second group contained three beams representing the CFRP-strengthened beams where three different fibre ratios were implemented in the ECC transition layer. All beams had the same concrete dimensions, i.e. the total length of the beams was 2200 mm while the centre-to-centre span was 2000 mm. The beams cross section was 150 mm width £ 300 mm total depth. The main steel of the beams was two high tensile steel bars of 12 mm diameter representing a reinforcement ratio of 0.56% that guarantee the tensioncontrolled failure according to the current ECP 203 (2007) and ACI 318-11 (2011) codes. The secondary steel was two high tensile steel bars of 10 mm diameter, while the stirrups were mild steel bars of 8 mm diameter and spaced every 100 mm all over the entire span of the beams. Two beams of group 1 were completely cast with concrete without any ECC transition layer, which represented the control beam (B-C) and the reference CFRP-strengthened beam (B-C-CF). The remaining
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beam of group 1, beam B-C-ECC, was cast with 50 mm depth £ 150 mm width ECC layer where the main tension steel was embedded into the ECC transition layer as illustrated in Figure 1. The second group, group 2, represented the CFRP-strengthened beams. The three beams had the same configuration as that of the reference beam (B-C-ECC) as shown in Figure 1 but the used fibre content ratio was the control parameter where it had three ratios namely: 0.5%, 1% and 1.5% for the three beams. It is worth mentioning that the fibre content of the reference beam, B-C-ECC, was 1%. All test beams were cast at the same time vertically in wooden forms upside down where the tension sides were at the top. Both beam B-C and B-C-CF were cast completely with concrete, while the remaining four beams were cast partially leaving the top 50 mm depth without concrete. Two days after casting, the standard cubes, the concrete prism of 150 mm 3 150 mm 3 700 mm and the sides of the specimens B-C and B-C-CF were stripped from the moulds and covered by plastic sheets, while the sides of partially cast beams were left in position to act as shuttering when ECC transition layers were poured. The upper surface of all specimens were cured by water until the 10th day and then allowed airdrying. After about 4 weeks, the partially cast beams were prepared for ECC pouring where the top surfaces were roughened using chisel to remove slurry cement from external surfaces of coarse aggregates. Before pouring the ECC layers, the contact surfaces of the beams were recleaned with brush and high-pressure air to ensure a clean bonding surface, and then they were adequately damped (Li, 2004). Just before casting the ECC layers, a bonding agent (Addibond 65, provided by CMB Company, Tanta, Egypt) was applied to the concrete surfaces as illustrated in Figure 2. The upper surface of the ECC layers were cured by water for 7 days and then allowed air-drying until the testing day. Four beams (B-C-CF, BS-0.5, BS-1 and BS-1.5) were strengthened using CFRPs, in accordance with ACI 440 (2008) recommendations (ACI 440.2R-08) as shown in Figure 1. Because the adopted strengthened beams are considered as bond-critical application, surface preparation requirements should be based on the intended application of the CFRP system. The concrete surface was prepared with abrasive techniques. The common feature of the four strengthened beams was that all beams were strengthened typically using CFRP sheets of dimensions 150 mm width, 0.13 mm thickness and 1800 mm length extended at the tension side of the beams. In addition, two 100-mm width CFRP anchorage U-shaped sheets were used at both ends of the CFRP sheets and ran up to 250 mm depth at both sides in order to prevent the premature peeling of the sheets at both ends (Bilal, Amal, Hage, & Harajli, 2005; Sarah, James, & Oguzhan, 2008).
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H.M. Afefy et al.
750 mm
300
2
500 mm
750 mm
10 8@ 100 mm
2
12
150
2000 mm 2200 mm
Beam B-C
300 50
2
10 8@ 100 mm
2
12 50 mm thickness ECC layer (1% fiber content) 2000 mm
150 Beam B-C-ECC 100 mm 10
CFRP anchorage sheets
8@ 100 mm 2
100 mm
250
300
2
1600 mm
12 EB-CFRP sheet (150 mm width x 0.13 mm thicness x 1800 length) 2000 mm
150 Beam B-C-CF 100 mm 10 8@ 100 mm 2
12
150 Beams BS-0.5, BS-1, BS-1.5
Figure 1.
2.2
100 mm
250
300 50
2
1600 mm
50 mm thickness ECC layer (0.5%, 1% and 1.5% fiber content) EB-CFRP sheet (150 mm width x 0.13 mm thicness x 1800 length) 2000 mm
Concrete dimensions and reinforcement detailing for all tested beams.
Material properties
The used concrete was made from ordinary Portland cement (type I), natural sand and a mixture of crushed pink limestone types I and II, as the coarse aggregate of maximum size of 14 mm, with a mixture proportion as shown in Table 1 in order to obtain concrete with a target strength of 25 MPa. The actual compressive strength of the used concrete was determined at the testing day as the average of three standard cubes of 150 mm side length as reported in Table 3. ECC layer was made with ingredients typically found in concrete, including ordinary Portland cement, sand in addition to silica fume and super-plasticiser based on poly-
carboxylic ether. The mix proportions of the ECC used as strengthening materials are listed in Table 1. Water-tobinder ratio (W/B) was 0.26, while 10% of the design cement content was replaced by silica fume. High strength PP fibre was selectively chosen, and its volume in mix was varied to be 0.5%, 1% and 1.5%. The diameter and length of the PP fibres were 0.025 and 12 mm, respectively. The tensile strength of the used PP fibres was 626 MPa. PP fibre can decline the brittleness of structure effectively by improving the continuity and uniformity of concrete and reduce the cracking obviously during plastic shrinkage. The target compressive strength of the used ECC was determined to be 60 MPa, while the actual strength for
Structure and Infrastructure Engineering
5
yield strength, ultimate strength and Young’s modulus were 250 MPa, 364 MPa and 205 GPa, respectively. Regarding the used CFRP sheets, carbon fibre fabric SikaWrap 230C/45 and epoxy-based impregnating resin Sikadur-330 were implemented (the materials were provided by SikaEgypt Company, Cairo, Egypt). Table 2 presents the mechanical properties for CFRP fabric sheet along with the epoxy resin as provided by the manufacturer.
2.3
Figure 2. Preparing the contact surfaces just before casting the ECC layers.
each beam was determined based on the compressive test results of three standard cubes of 150 mm side length as reported in Table 3. The tensile behaviour of the used ECC was determined by uniaxial tensile test of nine dumbbellshaped specimens, three specimens for each fibre content percentage (tested cross section: 10 £ 30 mm). Figure 3 shows a sample of the stress – strain relationship measured from the uniaxial tensile tests based on average sense for each fibre content value. On the other hand, the concrete tensile strength of the tested beams was determined by splitting tensile test for three cylinders 150 mm diameter and 300 mm height. The average value was approximately 2.49 MPa. In order to determine the mechanical properties of the used 10 and 12 mm diameter deformed high tensile steel bars, tensile tests were performed on three specimens. The mean value of tensile yield strength, ultimate strength and Young’s modulus were 410 MPa, 631 MPa and 205 GPa, respectively, for 10 mm bar and 396 MPa, 571 MPa and 200 GPa, respectively, for 12 mm bar. For the 8-mm mild steel bars that were used as stirrups, the mean value of tensile
Table 1.
Test set-up and instrumentation
One bay of three-dimensional steel frame as presented in Figure 4 was equipped to carry out the tests. A 100-mm gauge length linear variable differential transducer (LVDT) was used in order to measure the vertical deflection at mid-span point of the test beams, while 10mm strain gauges were used in order to measure the developed strains in the internal reinforcement at the tension side along with the CFRP strain for the CFRPstrengthened beams at the mid-span section. In addition, a Pi-gauge of 100 mm gauge length was used in order to measure the deformation at the concrete tension and compression sides. Hence, the concrete compressive strain can be obtained. The beam was loaded by means of four points loading test. In several steps, the beam was loaded up to failure. The load on the beam was measured by a load cell of 600 kN capacity. A laser level was used to ensure the coincidence of the axes of the beam, load cell and the loading beam before testing. After each loading step, the vertical mid-span deflection, the Pi-gauge readings, the strains in the longitudinal steel and the CFRP sheets were recorded. The loading rate for all beams ranged from 0.05 to 0.07 kN/s. An automatic data logger unit had been used in order to record and store data during the test for load cell, steel strain gauges, CFRP strain gauges, Pi-gauges and LVDT.
3.
Results and discussion
The test results of the reference group included the control beam, the beam provided with an ECC transition layer only and the CFRP-strengthened beams in addition to the strengthened group included CFRP-strengthened beams where the strengthening CFRP sheets were bonded to ECC
Mix proportions of RC substrate beams and the ECC materials having different fibre contents for one cubic meter (kg/m3).
Concrete mix
W/Ba
Cement
Sand
Crushed pink limestone
Water
Silica fume
Super-plasticiser
PP fibre (12 mm)
RC ECC-0.5% ECC-1% ECC-1.5%
0.45 0.26 0.26 0.26
300 1100 1100 1100
650 660 660 660
1300 – – –
126 315 315 315
– 110 110 110
– 25 25 25
– 6 12 18
a
W/B is the water/binder ratio, B ¼ cement þ silica fume.
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H.M. Afefy et al. presented in the sequence regarding mode of failure, load – deflection response, cracking characteristics of the ECC transition layer, ultimate capacity and ductility.
3.1
Figure 3. Stress – strain relationships for uniaxial tensile tests of ECC material with different fibre content.
Table 2. Mechanical properties of the CFRP sheets along with the epoxy resin. Criteria
CFRP sheets
Epoxy
3500 230 1.50 0.13
30 21.40 4.80 –
Tensile strength (MPa) Modulus of elasticity (GPa) Failure strain (%) Thickness (mm)
transition layers having different fibre content as presented in Table 3. Generally, the proposed strengthening technique proved to be efficient both in the reconstruction of the tensile zone around the tension reinforcement of RC beams and in improvement of the CFRP-strengthened beams performance as a whole. Further discussion is
Failure mode
The normal flexural failure is usually observed in underreinforced unstrengthened beams and is characterised by extensive yielding of the internal steel reinforcement in tension, followed by crushing of concrete in the compression zone. This failure occurred in the control beam, B-C, as shown in Figure 5(a). As for beam B-CECC, more flexural cracks were developed where hair cracks were developed in the ECC transition layer and stopped to propagate along the RC beam. Then the cracks widened significantly after the yielding of the steel reinforcement, and finally the concrete crushed in compression side at the mid-span section as illustrated in Figure 5(b). The beam B-C-CF exhibited different kinds of failure mode. The CFRP de-bonding process initiated at a major flexural crack near the mid-span section and then propagated towards the end where the CFRP anchorages located. When this occurred, the CFRP sheet was ruptured, as shown in Figure 5(c), accompanied by peeling-off the concrete cover. The de-bonding of the CFRP sheet occurred due to high local interfacial stress near wide cracks. The widening of the crack was the direct driving force of propagation, and the relative vertical displacement between the two faces of the crack produced the peeling stress on the interface. This peeling stress resulted in the peeling-off failure because the cracked concrete
Load cell
Loading beam 500mm
750 mm
750mm
h =300 mm Pi-gauge
LVDT Pedestal
Pedestal
The beam of the main testing frame
Figure 4.
Test set-up.
Structure and Infrastructure Engineering Table 3.
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Failure characteristics of all beams. Concrete cube strength (MPa)
Cracking load (kN)
Group no.
Beam
RC
ECC
RC
ECC
Py (kN)
Pu (kN)
Dy (mm)
Du (mm)
Maximum steel strain, micro-strain
Group 1
B-C B-C-ECC B-C-CF BS-0.5 BS-1 BS-1.5
24.8 25.3 25.3 26.2 26.2 26.2
– 56.3 – 57.4 57.4 57.4
24.3 36.1 37.4 38.4 39.5 41.3
– 40.4 – 45.5 49.3 53.3
84.1 96.3 108.8 109.3 110.3 107.7
97.2 103.6 125.2 131.9 137.5 140.5
7.7 7.6 7.4 7.9 7.7 5.4
34.1 23.2 20.3 27.1 28.2 31.6
3436 3806 3037 3920 5353 7367
Group 2
Maximum CFRP strain, micro-strain – – 5370 6480 9610 11,789
Py ¼ total vertical load at yielding, Pu ¼ ultimate failure load, Dy ¼ mid-span deflection at yielding, Du ¼ mid-span deflection at complete failure.
(a)
(b)
Control beam, B-C
Figure 5.
(c)
B-C-ECC beam
Failure modes of all beams of group 1.
cover was unable to transmit this force. No failure of the end anchors was observed while the rupture of the CFRP sheet was occurred near the mid-span of the beam. Figure 6 depicts the failure modes of the CFRPstrengthened beams of group 2. It can be seen that changing the fibre content of the ECC transition layer has a great influence on the bond efficiency between the CFRP sheet and the ECC transition layer. For a small fibre content of beam BS-0.5, a complete separation between the CFRP sheet and the beam was observed just after the rupture of CFRP sheet as shown in Figure 6(a). This can be attributed to the poor cracking control provided by the relatively small amount of fibre content used in the ECC layer. Increasing the fibre content to 1% as beam BS-1 showed enhanced bond effect where localised de-bonding was developed around the mid-span of the beam as shown in Figure 6(b). The mode of failure for beam BS-1.5 (fibre content ¼ 1.5%) showed outstanding result of increased bond between the CFRP sheet and the ECC transition layer where no separation between the CFRP-strengthening sheet and the beam was observed even after the complete failure of the beam as illustrated in Figure 6(c).
3.2
B-C-CF beam
Cracking, yielding and ultimate loads
The cracks first appeared in RC beams once the tensile capacity of the concrete was exhausted. In the current research, there are two different materials having
dissimilar tensile capacity where the tensile capacity of the ECC layer is about two times that of the substrate concrete beam. As a consequence, for all the beams having the ECC transition layers, cracks began to appear at substrate beam, and soon later they propagated into the ECC transition layer as illustrated in Table 3. With further increases in acting load, cracks propagated in the tension side penetrating both substrate beam and the ECC transition layer. However, more cracks were noticed in the ECC layer compared with those appeared in the substrate beam where more hair cracks were developed in the ECC layer and stopped to propagate to the substrate beam. The recorded cracking loads for both ECC transition layers along with the substrate beams for all beams are presented in Table 3. It can be noted that providing either an ECC transition layer or EB-CFRP sheets delayed the appearance of cracks in substrate beams by about 48% and 54%, respectively, for beams B-C-ECC and B-C-CF compared with that of control beam, B-C. Furthermore, it can be observed that the combined effect of both ECC transition layer and the EB-CFRP sheets showed more delay of crack appearance as exhibited by beams BS-0.5, BS-1 and BS-1.5. In addition, increasing the fibre content yielded more delay in crack appearance. Regarding the yielding load based on the slope of the load – deflection relationship for the test beams, using an ECC transition layer delayed the yielding load by about 15%, while the application of CFRP sheet on a virgin RC beam delayed the yielding load by about 29% compared
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H.M. Afefy et al. (a)
(b)
BS-0.5 beam
Figure 6.
BS-1 beam
BS-1.5 beam
Failure modes of all beams of group 2.
with that of the control beam, B-C. On the other hand, the combination of the CFRP sheet and the ECC transition layer showed approximately the same yielding load for beams BS-0.5, BS-1 and BS-1.5 compared with that of the beam B-C-CF. However, the corresponding deflections decrease with increasing the fibre content ratio. With respect to the ultimate capacity, providing an ECC transition layer can increase the ultimate loadcarrying capacity of the beam by about 6.5% (compare the ultimate capacities of beams B-C-ECC and B-C), while applying one ply of the EB-CFRP sheet yielded an increase of about 28.8% in the ultimate capacity (compare the ultimate capacities of beams B-C-CF and B-C). The algebraic sum of both increases is 35.3%, while the manifested increase when applying both of them simultaneously, as beam BS-1, is about 41%, which is higher than the algebraic sum of individual increases. That can be attributed to the enhanced composite action attained by bonding the CFRP-strengthening sheet to a transition layer with enhanced cracking behaviour. Increasing the fibre content to 1.5% resulted in increased capacity by about 44.5% as exhibited by beam BS-1 when compared with that of beam B-C. That can be attributed to the developed new mode of failure due to the enhanced composite action between the ECC transition layer and the EB-CFRP sheet.
3.3
(c)
CFRP sheet resulted in decreased vertical deflection and increased instantaneous stiffness at the same vertical load as those of both B-C and B-C-ECC beams. In addition, the exhibited load –deflection plateau beyond the yielding load was shorter than that of the control beam where a sudden drop in the vertical load was observed due to the sudden rupture of the CFRP sheet. When considering the CFRP-strengthened beams of group 2, the manifested load –deflection plateaus after yielding were varied according to fibre content provided in the ECC transition layer. That can be attributed to the different post-peak behaviour due to changing the fibre volumetric ratio. Figure 7 shows that, for all CFRPstrengthened beams (BS-0.5, BS-1 and BS-1.5), both strength and stiffness were enhanced significantly compared with those of the reference beams (B-C and B-C-CF). In addition, the structural performance was enhanced with increased fibre content from 0.5% to 1.5%. Among all CFRP-strengthened beams, beam BS-1.5 showed an enhanced performance from the viewpoints of stiffness, ultimate capacity and the manifested deflection plateau. Increasing the fibre content ratio for beams BS-1 and BS-1.5 showed a remarkable increase in the tangential stiffness after the first cracking of the beams compared with that of the CFRP-strengthened beam without ECC
Load – deflection response
Figure 7 depicts the load –deflection relationship for all beams. Each beam exhibited linear behaviour up to the cracking load, beyond which a rapid change in the slope of the load –deflection curve was observed. By further increasing the loads, the yielding of the internal steel reinforcement occurred in the control beam B-C and then a steady plateau in the load – deflection curve was observed soon after that till the complete failure of the control beam. Beam B-C-ECC showed a similar behaviour as that of the control beam except that its post-cracking stiffness is higher than that of the beam B-C. Providing one ply of
Figure 7. Load – deflection relationship for the test beams.
Structure and Infrastructure Engineering Table 4. Cracking characteristics of the ECC transition layer near failure. Beam B-C-ECC B-C-CF BS-0.5 BS-1 BS-1.5 a
Major crack width (mm)
Average crack spacing (mm)
1 2a 0.42 0.39 0.35
62 100a 55 38 19
The crack width and spacing are for the substrate concrete beam.
transition layer. After yielding of the main tension steel, the combined behaviour of both the CFRP sheet and the ECC transition layer controlled the behaviour. For beam BS-0.5, approaching failure state, the CFRP sheet began to de-bond that showed increased deflection with small increased load. That was due to the strain hardening behaviour of the ECC layer. With further loading, the CFRP sheet was ruptured suddenly resulting in a step drop in the resisting load. For beam BS-1, a similar trend was noticed as manifested by the beam BS-0.5 with higher stiffness and resisting load. That can be attributed to the enhanced bond characteristics between the EB-CFRP sheet and the ECC transition layer which retained the de-bonding and mitigated its propagation of the CFRP sheet compared with that exhibited by the beam BS-0.5. Regarding beam BS-1.5, a different mode of failure was noticed, while the CFRP sheet was in complete bond to the ECC layer up to failure, the ECC transition layer was failed resulting in the separation between the RC substrate beam and the ECC transition layer. That separation enforced the CFRP sheet to work solely in an unbounded manner up to the end of the test.
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transition layers along with those developed in the concrete substrate of beam B-C-CF near the failure state. It is well recognised that the de-bonding of the CFRP sheet is highly dependent on the crack width and spacing, because high local interfacial stress will be developed near wide crack. The relative vertical displacement between the two faces of the crack produces the peeling stress on the interface. This peeling stress could result in the peeling-off failure based on the ability of the cracked concrete cover to transmit this force. This means that reducing the crack width along with better crack distribution along the interface of the CFRP sheets will result in better bond characteristics along the CFRP interface up to failure. Based on the test results of beam B-C-CF, the relatively wide major crack (2 mm) and the large crack spacing (100 mm) resulted in complete de-bonding in the constant bending moment region as illustrated in Figure 5 (c). Increasing the fibre content of the ECC transition layer from 0.5% to 1%, to 1.5% resulted in reduced crack width along with reduced crack spacing that exhibited better bond along the CFRP interface with the ECC transition layer. As a result, the mode of failure had been changed from de-bonding of the CFRP sheet as for beam BS-0.5 to the rupture of the CFRP sheet without de-bonding as manifested by beam BS-1.5. Figure 8 displays the front view of both beams BS-1 and BS-1.5. It can be noticed that increasing the fibre content ratio from 1% to 1.5% showed better crack distribution where the number of cracks was doubled for beam BS-1.5. On the other hand, separation between the ECC transition layer and the substrate beam was happened resulting in premature failure of the beam, BS-1.5. In other words, the ultimate tensile capacity of the ECC transition layer is a significant parameter and has to be increased in order to show better enhancement in the ultimate capacity of the beam due to the EB-CFRP sheet.
3.4 Crack width and crack spacing developed in the ECC transition layer
3.5
Table 4 illustrates the measured major crack width along with the average crack spacing developed in the ECC
Mufti, Newhook, & Tadros (1996) suggested that, in order to determine the ductility of a RC beam strengthened with
Beam BS-1
Figure 8.
Ductility
Beam BS-1.5
Crack distribution along the ECC transition layer for beams BS-1 and BS-1.5.
10
H.M. Afefy et al.
Table 5.
Ductility parameters for test beams. Displacement (mm)
Loading (kN)
Group no.
Beam
Ds
Du
DF
Ps
Pu
SF
PF
Group 1
B-C B-C-ECC B-C-CF BS-0.5 BS-1 BS-1.5
6.1 5.8 5.7 5.9 5.1 4.1
31.1 21.7 14.3 17.5 15.9 9.6
5.10 3.74 2.51 2.97 3.12 2.34
67.3 77.1 87.1 87.4 88.2 86.2
97.2 103.6 125.2 131.9 137.5 140.5
1.45 1.34 1.44 1.51 1.56 1.63
7.40 5.01 3.61 4.48 4.87 3.81
Group 2
Ps ¼ total vertical load at service limit state, Pu ¼ ultimate failure load, Ds ¼ mid-span deflection at service loading, Du ¼ mid-span deflection corresponding to the ultimate load.
FRP, it is necessary to consider a deformability factor (DF) and a strength factor (SF), and then combine these two parameters to define an overall factor called performance factor (PF). The DF can be determined by dividing the deflection at ultimate limit state by the deflection at serviceability limit state corresponding to a concrete compressive strain of about 0.1%. SF is the ratio of the ultimate load to the serviceability load. Table 5 shows the DF, SF and PF for all tested beams. It is empirically defined that the FRP-strengthened members should present a PF larger than 4 to guarantee an adequate ductile behaviour (Garcez, Meneghetti, & Filho 2008). Table 5 also presents that the PF has been increased when an ECC transition layer was implemented solely (beam B-C-ECC) compared with that of the control beam, B-C. On the other hand, the CFRP-strengthened beam, B-C-CF, exhibited lower PF than that of the beam B-C. In addition, its PF is lower than 4. When applied the CFRP sheet on an ECC transition layer, the obtained PFs are in-between those of beams B-C-ECC and B-C-CF. Beam BS-1 manifested the highest PF among all CFRP-strengthened beams. In addition, it showed adequate ductility according to Garcez et al. (2008). The beam BS-1.5 showed higher ductility compared with that of the beam B-C-CF. However, it failed to exhibited adequate ductility according to Garcez et al. (2008). Despite that the beam BS-1.5 exhibited the highest SF among all strengthened beam along with the control beam, its DF showed the lowest one. That can be attributed to the occurrence of local de-bonding between the ECC transition layer and the substrate concrete and the corresponding premature failure of of the ECC transition layer.
4.
Conclusions
Based on the studied hybrid strengthening techniques of RC beams, loading scheme, the thickness of the ECC transition layer, the configuration of the EB-CFRP sheet and according to the used concrete dimensions and
reinforcement detailing, the following conclusions can be drawn: (1) Based on the results of the present experimental programme, it can be concluded that the proposed strengthening technique, even with the possibility of further improvements, as any other technique, proved to be efficient both in the reconstruction of the tensile zone around the tension reinforcement of RC beams and in the improvement of the entire beam performance, particularly in a more efficient exploration of the resistance properties of strengthening with sheets of CFRP. (2) The application of one ply of EB-CFRP sheet as a strengthening material, solely, can enhance the ultimate load-carrying capacity of the RC beams on the expense of their ductility. However, utilising an ECC transition layer in the hybrid strengthening system showed enhanced ultimate load-carrying capacity and ductility of the CFRPstrengthened beams. (3) Compared with the control unstrengthened beam, utilising an ECC transition layer alone can increase the ultimate load-carrying capacity of the strengthened beam by about 6.5%, while applying one ply EB-CFRP sheet yielded an increase of about 28.8% in the ultimate capacity. The algebraic sum of both increases is 35.3%, while the manifested increase when applying the EB-CFRP sheet on the beam having an ECC transition layer, as beam BS-1, is about 41%, which is higher than that the algebraic sum of individual increases. That can be attributed to the enhanced composite action attained by bonding the CFRP strengthening sheet to a transition layer with enhanced cracking behaviour. Also, increasing the fibre content to 1.5% resulted in increased capacity by about 44.5%. (4) Increasing the fibre content in the ECC transition layer results in decreased major crack width along with better crack distribution leading to complete
Structure and Infrastructure Engineering bond between the EB-CFRP-strengthening sheet and the ECC transition layer up to failure as exhibited by beam BS-1.5. Thus, the whole tensile capacity of the CFRP sheet can be transferred to the strengthened beam. As a result, both ultimate capacity and ductility can be significantly improved. (5) The exhibited mode of failure showed that the tensile resistance of the ECC transition layer has to be increased in order to show better enhancement due to EB-CFRP-strengthening sheet. In sum, the bond characteristics between the EB-CFRP and the ECC transition layer are not the only parameter that guarantees the enhanced global behaviour of the studied beam. Hence, more parameters such as the effect of the ingredients of the ECC layer have to be studied such as the type of the fibre and proportions of the ingredients of the mix as a further research.
Notes 1. 2.
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