J. Cent. South Univ. (2019) 26: 2609-2622

DOI: https://doi.org/ 10.1007/s11771-019-4198-0

Flexural behavior of reinforced concrete beams with high performance fiber reinforced cementitious composites

SIVA Chidambaram R¹, PANKAJ Agarwal²

1. CSIR-Central Building Research Institute, Roorkee 247667, India;

2. Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: This article presents an experimental study on the flexural performance of reinforced concrete (RC) beams with fiber reinforced cementitious composites (FRCC) and hybrid fiber reinforced cementitious composites (HFRCC) in the hinge portion. Beam specimens with moderate confinement were used in the study and tested under monotonic loading. Seven diverse types of FRCC including hybrid composites using fibers in different profiles and in different volumes are employed in this study. Companion specimens such as cylindrical specimens and prism specimens are also used to study the physical properties of composites employed. The moment-curvature, stiffness behavior, ductility, crack pattern and modified flexural damage ratio are the main factors considered in this study to observe the efficacy of the employed hybrid composites. The experimental outputs demonstrate the improved post yield behavior with less rate of stiffness degradation and better damage tolerance capacity than conventional technique.

Key words: reinforced concrete beams; fiber reinforced composites; flexural behavior; flexural damage ratio

Cite this article as: SIVA Chidambaram R, PANKAJ Agarwal. Flexural behavior of reinforced concrete beams with high performance fiber reinforced cementitious composites [J]. Journal of Central South University, 2019, 26(9): 2609-2622. DOI: https://doi.org/ 10.1007/s11771-019-4198-0.

1 Introduction

The shear carrying capacity of the beam column joint decides the earthquake resistant behavior of the structure. The joint should have sufficient shear resistance capacity to transfer the forces from beam members to the foundation without collapse. Thus plastic hinge formation in the beam region instead of joint region is encouraged with respect to strong column weak beam concept. Proper design and detailing will allow the formation of plastic hinges in the beam and provide more ductility to the structure. The detailing in the form of closely spaced stirrups in the potential hinge region improves elemental stability and allows large rotation during earthquake. But the closely spaced stirrups lead to steel congestion and construction difficulty and the poor tensile nature of conventional concrete limits the ductility. The use of optimum percentage of fiber in concrete provides better post peak strain behavior under compression, tension and bending compared to conventional concrete. The use of steel fiber reinforced concrete (SFRC) improves the shear carrying capacity of reinforced concrete (RC) beams with optimum amount of stirrups instead of closely spaced stirrups and also enhances concrete structural capacity [1-5].

Flexural rigidity of a beam increases when steel fiber volume and compressive strength of composite increase [6]. In SFRC, the volumetric content of fiber, fiber dispersion, fiber profile and fiber orientation decide the performance level [7, 8]. The presence of coarse aggregate significantly affects even fiber dispersion in practice, also the higher volume fibers in concrete create balling problem and affect the concreting work. High volume fibers and absence of coarse aggregate in fiber reinforced cementitious composites (FRCCs) offer better ductile performance than SFRC. The enhanced fiber dispersion due to the absence of coarse aggregate improves the fiber crack bridging action and provides tensile strain hardening behavior which is difficult to achieve in conventional and SFRC. The exercise of high performance fiber reinforced cementitious composites (HPFRCCs), overcomes the limitations of SFRC and also offers better toughness property. The presence of higher volume fibers and mineral admixtures improves stress– strain and toughness behavior of the composites and provides better post peak strain behavior. The cracking mechanism of HPFRCC allows shear crack distribution prior to critical failure which improves the ductility [9]. Different types of FRCC such as Ductal, engineered cementitious composites (ECC), Ducon, slurry infiltrated fiber reinforced concrete (SIFCON), etc, are classified under this HPFRCC category. The performances of these composites are different and depend on the materials used. In SIFCON preparation, steel fibers varying from 4% to 15% in volume have been used, whereas in ECC preparation synthetic fibers varying from 2% to 3% are preferred. In both the composites, additions of fine mineral admixtures and chemical admixtures are desired to fill the pores and to improve the strength without increasing the water content. In FRCC the volumetric ratio of fiber, orientation of fiber and its dispersion decide the synergetic action of the composite [10, 11]. The flexural and shear carrying capacity of RC elements have been enhanced with HPFRCC which is comparatively better than SFRC and also able to retard the post elastic degradation in stiffness of structural member even without the special confining reinforcement. In particular, composite with higher volume steel fiber increases the flexural strength than the lesser volume composite but affects the ductility. The use of HPFRC in RC elements offers superior hybrid action in resistance to load and deformation [12]. Fiber orientation in FRCC plays an important role upon cracking in the RC elements [13]. Higher volume of fiber in the composites significantly affects the workability and requires higher water binder ratio [14, 15]. Thus, the fiber hybridization which is the combination of metallic and synthetic fibers in concrete offers better resistance without increasing the volume of fibers. The fiber hybridization helps to utilize the benefit of different fiber properties in the composite to improve the certain performance compared to uni-fiber composites. The synergetic effect of macro and micro fibers in the composites enhances the strength and ductility more than mono fiber systems do [16].

1.1 Early studies

Numerous experimental and analytical studies have been conducted to study the mechanical properties of ECC prepared with different percentages of poly vinyl alcohol (PVA) fiber and polyethylene (PE) fibers under different kinds of loading such as compression, tension and bending. The post peak stain behavior of ECC under tension shows that the composite action of fiber with mineral admixture offers the strain hardening. The observed post peak strain behavior improves the ductility and better post peak strength retention capacity compared to fiber reinforced concrete [17-19]. The durability of the RC flexural member has been improved and restricted the crack growth under service load using strain hardening composites. Also, a new design was proposed by MAALEJ et al [20]. HIROSHI et al [21] studied the seismic response of RC beam with ECC and the test results showed improved ductility and 1.5% tension strain capacity. The cyclic testing of precast coupling beam with cementitious composite prepared with twisted steel fiber and PE fiber shows better shear resistance without closely spaced transverse steel reinforcement and also shows slower rate stiffness degradation [22]. MAALEJ et al [23] stated that while using ECC in RC beams the de-bonding was delayed and deflection behavior was improved. The application of HPFRCC for RC beams strengthening has been studied and observed remarkable improvement in strength and stiffness [24]. FARHAT et al [25] examined the effect of HPFRCC in flexural and shear strengthening. Test results show increased failure load and also flaw tolerance under repeated flexural cyclic loading.RAYMOND et al [26] studied the seismic behavior of HPFRCC for RC coupling beams and showed that the HPFRCC can be used in structural element due to susceptibility to seismic events and its energy dissipating capacity.

PATODI et al [27] conducted a comparative study on the use of steel and Recron 3S fiber in the preparation of ECC and its influence on the composite physical properties. The experimental results show that the ECC with Recron 3S shows better response under tension and impact loading compared to ECC with steel fiber. These composites have been employed in the joint region of moment resisting frame and observed improved inelastic behavior compared to ECC with steel fiber. MAALEJ et al [28] conducted a review on potential application of hybrid ECC on various structural applications and suggested that the fiber hybridization significantly improves the behavior of the structure. SAMEER et al [29] investigated the influence of PVA micro fibers enabled composite on the performance of concrete beams and observed enhanced strength and crack resistance behavior. GIOVANNI et al [30] examined the application of HPFRC in strengthening of RC beams and observed enhanced strength and stiffness. KIM et al [31] experimentally investigated the application of strain-hardening cement-based composite in flexure dominant RC beam repairing work and observed dense minor cracks without concrete spalling and enhanced moment with delayed reinforcement yielding. TAYEH et al [32] conducted a detailed review on ultra high performance fiber concrete (UHPFC) and critically concluded that the UHPFC offers superior mechanical properties, better uni-axial strength and low permeable nature and it can be used in structures requiring higher shear resistance and in restoration of structures exposed to severe environment. HOU et al [33] examined the shear resistance of RC beams with ultra high toughness cementitious composite (UHTCC) beams and concluded that the beams with UHTCC have better shear resistance, show steady crack propagation and also multiple cracking behaviors in shear. It can be provided as a replacement for minimum transverse be reinforcement. ZHANG et al [34] examined the effectiveness of PVA-ECC in two different volumes on static and cyclic behavior of RC beams under bending and observed that the crack bridging effect of fiber is affected under cyclic loading than static loading. LAMPROPOULOS et al [35] used the ultra high performance fibre reinforced Cementitions concrete (UHPFRCC) as a strengthening material in RC beams and observed superior performance and improved efficiency over other strengthening techniques. ALI et al [36] examined the influence of engineered cementitious composite with shape memory alloy fiber and PVA fiber hybridization under tensile and impact loading and observed that the randomly distributed SMA fibers in optimum amount improved the ECC impact and tensile performance. HEMMATI et al [37] studied the performance of RC frame with HPFRCC in the plastic hinge region and observed increased load carrying capacity and ductility ratio with improved plastic hinge characteristics. ABRISHAMBAF et al [38] investigated the tension properties of UHPFRCC and showed that fiber orientation had mere positive influence on tensile strength and hardening than its volumetric ratio. YOO et al [39] investigated the flexural behavior of UHPC with fiber hybridization using different length fiber and observed the hybridization of long and short fiber affects the peak load carrying capacity and energy dissipation property compared to composite with long and medium fiber hybridization.SMARZEWSKI et al [40] studied the tensile and fracture behavior of UHPC with steel and poly propylene fiber hybridization and observed that the use of fiber hybridization increases the tensile stress and strain and also increases the fracture toughness property. The micro structure investigation shows numerous pores in the composite due to the poor bond strength between poly propylene fiber surface and the composite matrix. HE et al [41] used Portland cement and asphalt emulsion as binder in the ECC preparation and examined its property under bending. The test results revealed that the use of 10% asphalt emulsion increases the hardening behavior whereas the higher percentage of asphalt severely damages the composites behaviour.PAKRAVAN et al [42] studied the influence of PVA and poly propylene fiber hybridization under tension and observed improved ductility and also noticed that the low modulus fiber combination decreases the tensile strength capacity. But the hybridization of higher modulus fiber with lower modulus fiber decreases the cost of composites and shows better energy dissipation capacity. WANG et al [43] used UHTCC as an external layer to the RC beams and examined its flexural behavior.The test results show that the external layer of UHTCC enhances the resistance to applied load and stiffness retention capacity. Also noticed that the use of UHTCC at the tension zone restricts the macro cracks formation and disperse into dense fine cracks.

1.2 Research significance

In RC elements the FRCCs have been used as strengthening material, as a replacement for conventional material in the plastic hinge location of frames. Fiber hybridizations between synthetic and steel fiber, steel fiber and steel fiber have been done and better performance than mono fiber enabled composites has been observed. However, there are lesser studies on fiber hybridization using different profiles on the performance of RC beams in the potential hinge region. This study mainly focuses on the use of fiber hybridization using different profiles in different volumes on the flexural behavior of RC beams. Hybrid composites are used in the plastic hinge location of beam without closely spaced stirrups and tested under single point loading. This paper presents the experimental outcome on the physical properties of the composites and the deflection behavior of RC beams with different hybrid cementitious composites (HCC) in hinge location. These cementitious composites are prepared using polypropylene fiber and steel fibers such as hooked end, crimpled and brass coated steel fiber varying from 2% to 3% in volume. The moment-curvature behavior, post yield stiffness behavior, modified flexural damage ratio and energy absorption are the parameters used to evaluate the performance of the experimental test results obtained.

2 Experimental program

The composite and conventional concrete have been prepared using ordinary Portland cement (OPC) 43 grade having specific gravity 3.14; fine aggregate having specific gravity of 2.65 and locally available coarse aggregate having a maximum nominal size of 12.5 mm and specific gravity of 2.70. Super plasticiser (Conplast SP 430) was used in different volumes as mentioned in Table 1 to increase the workability. Mechanical properties of different concrete composites are examined under compression, split tension and bending using standard cylindrical and prism specimens.

2.1 Compression and split tension behavior of HPFRCC

The axial stress-strain behavior and split tensile behavior of different composites are studied on cylindrical specimen with a standard size of 100 mm×200 mm. Engineered cementitious composites (ECC) and hybrid fiber cementitious composites (HFCC) using different metallic fibers and synthetic fibers are used in preparation of composites. The detailed cementitious mix ratio by mass basis is presented in Table 1. There are two types of composites namely ECC and HCC using steel and polypropylene fiber hybridization. The hooked end, crimpled steel fiber having a tensile strength of 1100 MPa with an aspect ratio of 60 (d 0.60 mm× 35 mm) and brass coated steel fiber of d 0.15mm× 7 mm is used in the composite preparation. The HFCC with hooked end fiber, crimpled fiber and brass coated fibers are named as HECC, CECC and BECC, respectively. Standard cylindrical samples have been prepared and tested under uni-axial compression and the corresponding axial deformation has been measured using linear variable differential transducer (LVDT) having a gauge length of 100 mm. Table 2 presents the volume fractions of fibers and the composite details. Figure 1 shows the fibers used in the study.

Figure 2 shows the axial stress-strain curve of all compression specimens and the summarised test results are presented in Table 3. The conventional concrete specimen CC1 shows an average compressive strength of 27 MPa with abrupt post peak strength degradation which shows the brittle nature of plain concrete. All HPFRCC specimens possess better post peak strain behavior with low stiffness than conventional concrete because of presence of high volume synthetic fiber.In specimen CC2 (ECC), the observed compressive strength is 25 MPa with ultimate strain of 0.02 which is nearly 1.5 times higher than conventional concrete. The compressive strength of HECC composites i.e. CC3 and CC 4 is also considerably low but initial stiffness is higher than synthetic fiber in specimen CC2 due to effective crack bridging property of metallic fiber.

Table 1 Cementitious materials mix proportions of conventional concrete and HPFRCC (Mass fraction, %)

Table 2 Mix volume fractions (%) of different fibers in HPFRCC preparation

Figure 1 Materials used in this study:

There BECC composite (CC5 and CC6) shows enhanced compressive stress and better post peak strain behavior. The stress-strain behaviour of higher volume brass coated steel fiber specimen CC5, exhibits a better response as compared to lesser volume specimen CC6 of the same type of fiber because of the metallic fiber resistance property to the applied force. The macro level crack bridging effect of metallic fiber confines the concrete and restricts the cracks formation and spalling of concrete. It can be understood from the failure pattern of compression specimen shown in Figure 3. The compressive behavior of CECC composite specimens CC7 and CC8 is further improved due to the additional anchorage of crimpled fiber than other fibers. In HPFRCC specimens, a volumetric enlargement with multiple crack formations is evidenced without spalling and crushing of concrete, as shown in Figure 3. The HECC composite holds the cracked portion together and increases the post peak strain property while BECC composites impede the crack growth and the specimen shows higher volumetric enlargement. The additional anchorage of crimpled steel fiber in CECC composite increases the strength and also restricts the volumetric enlargement.

Figure 2 Stress-strain behavior of compression specimens

Table 3 Compression and flexural test results

The split tensile strength to compressive strength ratio differs with respect to composites. The split tensile strength of conventional concrete specimen CC1 is approximately 12.5%, while that of ECC composite specimen CC2 is 17% of its compressive strength. The hybridization of steel and synthetic fiber composites shows different tensile behavior with respect to their anchorage. The tensile strength of HECC and CECC composite ranges from 19% to 20% of its compressive strength, but the inadequate anchorage and lesser length of BECC have only 12% of its compressive strength.

2.2 Flexural behavior of composite specimens under two-point loading

The flexural behavior of different ECC composites is evaluated using prism of size 100 mm×100 mm×500 mm under two-point static loading. The corresponding deflection is measured using LVDT at midpoint as shown in Figure 4. All the prism specimens were tested under load control at a speed of 50 N/s and the inbuilt data acquisition was used to record the load and deflection data acquired during testing. Figure 5 shows the flexural behavior of prism specimens.

Figure 3 Failure patterns of different composites:

Figure 4 Test setup of prism specimens (Unit: mm)

Figure 5 Flexural stress-deflection curve

The average flexural strength (f_s) to compressive strength (c_s) ratio (f_s/c_s) of conventional concrete is 14.5%. The average f_s/c_s of HPFRCC composite specimens is about 20% with an improved and stable post yield deformation.The type of fibers, volume and anchorage of fibers play a vital role in post peak behavior of HPFRCC composite specimen. Hybrid composite CECC possesses higher flexural strength due to the synergetic effect of crimpled fiber anchorage with synthetic fiber. The crimpled fiber anchorage effectively transfers the stress across the cracks and allows the specimen to deform gradually that can be observed from the post peak deflection curve of CC7 and CC8 in Figure 5. The flexural strength of CECC composite is nearly 1.8 times greater than conventional concrete. The post yield degradation of BECC composite specimen has considerably higher rate than other hybrid composites due to its anchorage deficiency in crack bridging ability. The flexure energy of HPFRCC specimens is calculated using the area of load- deflection curves and the estimated values are shown in Figure 6. The energy dissipation capacity of HPFRCC composite is significantly higher than conventional concrete. The optimum amount of steel fiber along with synthetic fiber in cementitious composites may effectively improve the post-yield deformation with enhanced energy dissipation capacity.

Figure 6 Flexural energy dissipation

3 Flexure behaviour of RC beam specimens with HPFRCC

Eight sets of RC beam specimens composed of different HPFRCC composites at the mid region with total length of 1.2 m are tested under monotonic loading. Cross sectional detail of beam specimen remains the same in all the specimens but different composites in the mid span region. The details of HPFRCC composite are given in Table 4.

Table 4 Detailed configuration of RC beam specimen with different HPFRCC

Figure 7 shows the test setup. All the specimens were tested under load control in flexural testing machine with a test speed of 50 N/s.The clear span length of beam specimens is 1050 mm. The applied load is measured using inbuilt data acquisition and corresponding deflection is acquired using LVDT in three different locations.

Figure 7 Detailed configuration of RC beam specimen with test set up

3.1 Moment-curvature behavior of RC beams

Figure 8 shows the moment-curvature curve of tested beam specimens and the summarized test results are presented in Table 5.

The formation of early cracks and its inclined propagation restricts the post yield behavior of conventional specimen SB0. The inelastic response of beam specimen with different HPFRCC is significantly improved and more stable than conventional specimen. The yield value of HPFRCC beams is comparatively higher than the conventional specimen. The composites in the hinge region resist the applied load and reduce the rate of crack propagation by bridging the cracks.

Figure 8 Moment-curvature relationship of RC beams

Table 5 Test results of RC beams with different composites

The HPFRCC specimens experience ductile behavior even though with inclined cracks. The compressive strength of conventional concrete is higher than the ECC but the specimen SB 1 shows 13% higher yield moment compared to conventional beam specimen. The first noticed crack moments (f_cm) of specimen SB 1 and SB 0 are 13 and 10 kN·m, respectively while in HECC composite specimens SB2 and SB3 they are 17 and 16 kN·m, respectively. The absence of coarse aggregate, high volume fiber and blended dispersion restricts the crack formation, hinders its growth and allows the tension reinforcement to yield without significant loss in strength. In particular the hybridization of metallic and synthetic fiber acts as micro reinforcement in resisting the cracks growth and enables multi level crack bridging mechanism. The synthetic fiber bridges the micro cracks whereas the metallic fiber bridges the macro level cracks. Thus, the HPFRCC beam specimens manifest enhanced ductile response than the conventional specimen.

The beam specimens (SB4 and SB5) with BECC encounter a sudden loss in moment after attaining the peak load. The uniform mixing of brass coated steel fiber increases the composite compressive strength but the lesser length and poor anchorage profile has lesser resistance in hindering the crack growth. However, higher volume of synthetic fiber in SB5 shows improved post yield response than SB4 but has lesser yield stiffness.It manifests the importance of fiber anchorage capacity in crack bridging. The anchorage of crimpled fiber in the composite results 1.7 times increase in f_cm of specimens SB6 and SB7 than conventional specimen. In all the hybrid composite beams the first type (SB2, SB4 and SB6) composites have higher strength and stiffness compared to the second type of beam specimens (SB3, SB5 and SB7). However, the second type composites have better ductile behavior than the first type composites. This is possible because of the volumetric difference of metallic fiber and synthetic fiber. The higher volume of metallic fiber increases the crack resistance and stiffness and higher volume of synthetic fiber increases ductility. This is the synergetic advantage of HFCC in providing the combined performance.

3.2 Stiffness and strength degradation

The post elastic strength (F_deg) and stiffness (K_deg) degradation are calculated using the following Eqs. (1) and (2) and the estimated values are presented in terms of degradation over post yield rotation as shown in Figure 9.

                        (1)

                       (2)

where F is the maximum load of each cycle, kN; F_y is yield load, kN; K is stiffness, kN/mm; K_y is yield stiffness, kN/mm.

The nonlinear behavior of a specimen is evaluated using its rate of variation in the percentage of post yield degradation in the inelastic range. The rate of degradation shows ductile/brittle behavior of the specimens. The lower rate stands for the ductile and the higher rate of degradation stands for brittle response of structure. The HPFRCC enabled beam specimens exhibit steady and better strength and stiffness retention. There is no sudden strength degradation between the rotation 0.02 and 0.08 radian except in beam specimen SB 4. The conventional specimen has 90% degradation at 0.06 radian but the same is observed at 0.08 radian to 0.09 radian with HPFRCC specimens. This steady rate of degradation manifests the ductile performance of the beam specimens. The stiffness degradation of all composite specimens shows similar kind of response, irrespective of fiber volume.

Figure 9 Stiffness and strength degradation response of RC specimens

3.3 Energy dissipation and ductility

The curvature ductility and cumulative energy dissipation are calculated to estimate the percentage of the ductility enhancement. The curvature ductility is calculated using Eq. (3). This is the ratio between the curvature of section at ultimate point (f_u) and the curvature of section when longitudinal reinforcement reaches the elastic limit (f_e). The energy dissipation is calculated on the basis of area enclosed by the load-deflection curve. Figure 10 shows the ductility comparison and Figure 11 shows the comparison of cumulative energy dissipation of all beam specimens.

                                  (3)

Figure 10 Comparison of curvature ductility

Figure 11 Energy dissipation of beam specimens

A 40%-85% increase in ductility is observed with fiber reinforced composite beam specimens than the specimen SB0. The composites with higher volume of synthetic fiber have higher ductility over composite with lesser volume of synthetic fiber. The specimen SB0 marks the least ductile behavior in terms of ductility and energy dissipation compared to HPFRCC specimens as shown in Figure 10. Figure 11 shows that the HPFRCC specimen energy dissipation is more consistent than the control specimen SB0. It shows that the fiber profile and its volumetric percentage govern ductile property. Figure 10 clearly shows the difference in ductility ratio of HFCC specimens. In particular the composites SB2, SB4 and SB6 show lesser ductility compared to SB3, SB5 and SB7 due to the volumetric content of metallic fibers used. Higher volume of synthetic fiber provides micro level crack resistance and shows enhanced ductility compared to the hybrid composite having higher volume metallic fiber.

3.4 Failure pattern and damage index

Figure 12 shows the failure pattern of beam specimens. The first crack load of specimen SB 0 is 20 kN after initiating the vertical cracks in mid span. As the load increases, the cracks become inclined and widen as shown in Figure 12(a). The conventional beam specimen exhibits flexural-shear failure while the HPFRCC composite enabled beam specimens exhibit flexural failure. In specimen SB1, very dense flexural cracks are noticed in the composite region. In other hybrid composite enabled specimens, initially few vertical cracks are formed. This shows that the metallic fiber in the composites effectively bridges the cracks and restricts their growth and allows the primary crack to propagate at very slow rate towards loading direction. Figure 13 clearly demonstrates the crack bridging mechanism of different composites.Figure 13(a) shows the absence of effective crack bridging mechanism and gradual opening of cracks in synthetic fiber based composites (SB1).Figure 13(b) shows the effectiveness of steel fiber with proper anchorage restricting the crack propagation in different stages. The lesser length of brass coated steel fiber without anchorage (SB6) fails in restricting the cracks growth as shown in Figure 13(c). In HPFRCC specimens, inclined shear cracks are also noticed but the growth of primary vertical cracks leads the specimen to fail in flexure.

Modified flexural damage ratio (MFDR, R_mfd) [44] as shown in Eq. (4) is used to quantify the damage ratio. MFDR is the ratio of the secant stiffness at the onset of failure (M_m/f_m) and the minimum secant stiffness (M_x/f_x) reached so far.

                        (4)

The MFDR values vary from 0 to 1, whereas “0” indicates no damage and “1” indicates the onset of failure of the member. The calculated MFDR of each specimen is plotted versus ductility as shown in Figure 14. The conventional specimen completely collapsed at μ=5 whereas HPFRCC beam specimens fail at much higher ductility levels. The measured ductility at collapse stage in HPFRCC beam specimen is nearly two times higher than conventional concrete beam specimen. A higher volume synthetic fiber based composite shows high damage tolerance capacity over the lesser volume synthetic fiber, possibly because of the fiber profile and its composite action in resisting the applied load and deflection. The HFCC effectively resists multiple crack growth and improves the moment carrying capacity compared to the ECC specimens which allows the reinforcement to yield at lesser deflection compared to ECC specimen. Thus, the difference in damage ratio between ECC and HFCC is observed.

Figure 12 Failure pattern of RC beam specimens under bending

Figure 13 Cracking pattern of different composites:

Figure 14 Modified flexural damage ratio vs ductility

4 Conclusions

The use of HPFRCC in the RC beams potential hinge region shows enhanced strength and post yield behavior without sudden loss in strength. The performance of hybrid composites varies with respect to the fiber crack bridging property that transfers the stress across the cracks. The stiffness retention and damage tolerance capacity show that the specimens with HPFRCC in the hinge region have significant influence on resisting the load and improve the post yield behavior. Based on the above discussions the following conclusions are drawn.

1) The hybrid composites have improved mechanical properties and damage tolerance capacity than conventional concrete.The axial stress-strain behavior shows 100% higher failure strain over conventional concrete as well as failure pattern authenticates the crack resistance and damage tolerance of the hybrid composites. The flexural strength of HPFRCC specimens is nearly 20% of its compressive strength. The post peak behaviour and its rate of degradation show the efficacy of the hybrid fiber in the ductility enhancement.

2) The HPFRCC is able to enhance the moment capacity as well as to retard the yielding of tension reinforcement due to crack bridging property of fibers. The tensile behavior of HPFRCC and the anchorage of hooked end fiber and crimpled fiber in the hybrid composites effectively bridges the macro level cracks and restricts its growth, improves the first crack moment and yield moment effectively.

3) The HPFRCC in hinge region of beam specimen shows low rate of strength and stiffness degradation. The ECC and HFCC composite beam specimens show 80% of post peak strength retention between the rotation 0.02 and 0.07 radian. Also 60% to 90% stiffness loss occurred between the post yield rotation 0.02 to 0.08 radian. The stiffness degradation rate is steady and slow in the post elastic region which proves that the hybrid effect of different profile fiber can hold better post elastic stiffness without sudden loss.

4) The crack pattern of ECC and HFCC specimens are diverse in nature. The ECC specimen shows multiple fine cracks in the hinge region whereas the HFCC specimens show single primary crack growth with less number of fine cracks. In both cases, the fibers offered crack bridging effect but the presence of metallic fiber and its anchorage profile in HFCC provides better resistance to macro level crack compared to ECC. Thus, the HPFRCC (ECC/HFCC) composite offers crack growth resistance and reduced rate of crack propagation at micro and macro level of the hinge region. This slower rate of crack propagation increases the post peak deformation without loss in strength.

5) The damage index of different composite specimens manifests that the composite with higher volume synthetic fiber has better damage tolerance capacity. The damage level of HPFRCC is much lower than the conventional beam specimen. The HPFRCC specimen shows 1.5-2.5 times more damage resistant compared to conventional specimen.

6) The use of HPFRCC in the hinge region of the beam specimens increases the dissipation of energy which increases the ductility level of the specimens. The HFCC composite can be preferred with respect to the specific requirement such as strength, and stiffness enhancement can be done using higher volume of metallic fiber where the ductility enhancement can be achieved using higher volume of synthetic fiber in the composites. This is the synergetic advantage of HFCC. Thus, the ECC and hybrid composites is recommended as an alternate to the conventional concrete technique in the potential hinge location due to its superiority in terms of strength, ductility and damage tolerance capacity compared to the conventional technique.

Acknowledgements

Authors thank the support of Reliance Industries and Bakaert Industries, India for providing fiber for the experimental work.

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(Edited by FANG Jing-hua)

中文导读

高性能纤维增强水泥基复合材料钢筋混凝土梁的抗弯性能

摘要：本文对纤维增强水泥基复合材料(FRCC)和混和纤维增强水泥基复合材料(HFRCC)在铰链部分的抗弯性能进行了试验研究。采用中等约束的梁试件，在单调荷载作用下进行试验。本研究采用了7种不同类型的FRCC，包括混和纤维在不同截面、不同体积下的复合材料。同时采用柱状试样和棱镜试样等伴随试样，研究复合材料的物理性能。弯矩-曲率、刚度特性、延性、裂纹形态和修正的弯曲损伤率是研究混和复合材料使用效果中的主要因素。实验结果表明，改进后材料的屈服性能、刚度退化率较低，损伤容限能力优于常规技术的。

关键词：钢筋混凝土梁；纤维增强复合材料；弯曲性能；弯曲损伤率

Received date: 2018-06-12; Accepted date: 2019-02-01

Corresponding author: SIVA Chidambaram R, PhD, Assistant Professor; Tel: +91-13322-283223; E-mail: krsinelastic@gmail.com; ORCID: 0000-0002-6243-2744