Flexural behaviour of ECC and ECC–concrete composite beams reinforced with hybrid FRP and steel bars

Abstract

Owing to the good ductility of steel and high strength and excellent corrosion resistance of fibre-reinforced polymer (FRP), concrete beams reinforced with hybrid steel and FRP bars exhibit better ductility than FRP-reinforced concrete beams as well as higher load-carrying capacities and better corrosion resistance than steel-reinforced concrete beams. However, the inherent brittleness of concrete in tension results in steel corrosion because of wide cracks and accelerated fracture of FRP reinforcement because of crack-induced stress concentration. This study investigated the effects of ultra-high ductile engineered cementitious composites (ECCs) on the flexural behaviour of hybrid steel and FRP-reinforced beams. Six hybrid-reinforced beams with various reinforcement ratios, matrix types and ECC pouring positions were tested in four-point bending. The flexural behaviours of the beams in terms of failure modes, crack patterns and developments, load versus deformation relationships and ductility are discussed herein in detail. We observed that substituting ECC with concrete results in a higher load-carrying capacity and better ductility of the hybrid reinforced beams owing to the excellent characteristics of ECC materials. When a layer of ECC is poured in the tension zone, the average crack width and crack spacing along the beam decrease; therefore, the longitudinal reinforcements can be adequately protected.

Keywords

crack control engineered cementitious composite experiments fibre-reinforced polymer flexural behaviour hybrid reinforced beam

Introduction

Steel corrosion has become one of the main reasons for the insufficient durability of corrosive reinforced concrete (RC) structures such as underground, underwater and marine engineering structures. The replacement of non-corrosive fibre-reinforced polymer (FRP) bars with steel bars in engineering structures is one of the most effective methods of solving the problems of concrete expansion, concrete cover spalling off, and structural strength reduction caused by steel corrosion (Abedini and Zhang, 2021; American Concrete Institute (ACI), 2015; Benmokrane and Masmoudi, 1996; Dong et al., 2018; GangaRao et al., 2006; Monaldo et al., 2019; Parandoush and Lin, 2017; Quagliarini et al., 2016; Sun et al.,2020; Vijay and GangaRao, 2001). FRP-reinforced concrete members have the advantages of high flexural strength and good durability, but the two inherent limitations of FRP bars, namely, low elastic modulus and poor ductility, limit their application in engineering structures (Abdalla, 2002; El-Nemr et al., 2018; GangaRao et al., 2006; Yuan and Wu, 2019; Zhang et al., 2021). The combined use of FRP and steel bars, in which FRP bars are placed at the corners of concrete members that are susceptible to corrosion, achieves a good corrosion resistance, high post-yield stiffness and small residual deformation (Aiello and Ombres, 2002; Bakis et al., 2001; Ge et al., 2015; Jo et al., 2004; Lau and Pam, 2010; Nanni et al., 1994; Qin et al., 2017; Qu et al., 2009; Yuan and Chen, 2018). However, owing to the tension brittle inherent in concrete, cracks with noticeable crack widths frequently appear in the RC member. In this scenario, the FRP bars are in the state of stress concentration, and the tensile fracture process will be accelerated. In addition, an excessively large crack width will also cause the steel bars to corrode, even when they are located deeper in the concrete.

In recent years, a type of ultra-high ductile fibre-reinforced cementitious composites, called engineered cementitious composites (ECCs), has been rapidly developed and widely used in engineering structures (Kim et al., 2004; Lepech and Li, 2009). ECC is a new type of construction material designed based on microstructure and micromechanics, with ultra-high toughness and multiple cracking properties (Li, 2003; Li and Leung, 1992). ECC and concrete have similar tensile strengths (4–6 MPa) and compressive strengths (30–80 MPa), but they behave very differently when in tension. The ultimate tensile strain of ECC can reach 3%–5%, which is two orders of magnitude higher than that of concrete. Before strain localisation, the crack width can be effectively controlled below 100 μm, and the crack spacing is 3–6 mm in the saturated state (Li et al., 2001).In compression, the uniaxial compressive strain at peak stress is approximately two times that of concrete with the same strength grade, which significantly increases the ductility of the flexural members (Yuan and Chen, 2019; Zhou et al., 2015). Research has shown that the compatible deformation between steel bars and ECC reduces the interface bonding stress and effectively prevents the occurrence of longitudinal splitting cracks and matrix spalling (Fischer and Li, 2002a). Both steel-reinforced ECC beams (Fischer and Li, 2002b; Yuan et al., 2014) and FRP-reinforced ECC beams (Li and Wang, 2002; Yuan et al., 2013) exhibited higher flexural strength, superior crack control ability, higher ductility and better damage tolerance than the corresponding concrete beams. The excellent crack control ability and good durability of ECC have enabled its application in the tension zone surrounding the longitudinal reinforcements to form reinforced ECC–concrete composite beams (Ge et al., 2018, 2019; Maalej and Li, 1995; Zhang et al., 2006). Results indicated that the flexural strength and deformation ability increased slightly, but the crack width before the yielding of steel reinforcement was only 20% of that in conventional RC beams. With these unique characteristics, ECC is considered to solve the previously mentioned limitations of hybrid FRP and steel-reinforced concrete beams associated with the brittle cracking behaviour of concrete. Although steel- or FRP-reinforced ECC or ECC–concrete composite beams have been extensively studied, relevant research on hybrid steel- and FRP-reinforced ECC or ECC–concrete beams is scarce. Therefore, further studies on the flexural behaviours of these types of beams must be conducted.

This paper proposes a new type of ECC beam reinforced with hybrid FRP and steel bars. The FRP bars are arranged at the corners, and the steel bars are placed deeper in the beam members. Owing to the high cost of ECC, this study investigated the hybrid-reinforced ECC–concrete composite beams with ECC only used to substitute concrete in the tension zone was investigated to achieve a higher cost/performance ratio. The effects of reinforcement ratios, matrix types, and ECC replacement positions on the flexural behaviours of the beams in terms of failure modes, crack patterns, crack developments, moment–deflection responses, and ductility were systematically investigated.

Experimental program

Specimen preparation

In total, six beams, including one FRP–steel hybrid-reinforced concrete beam, three hybrid-reinforced ECC beams and two hybrid-reinforced ECC–concrete composite beams, were designed and tested in this study. For the ECC–concrete composite beam members, ECC was cast only in the tension or compression zone with a height of 90 mm, and the other areas were cast with concrete. Concrete was poured first, and ECC was poured after the concrete initially set. Penetration of fresh concrete into the ECC layer can hence be prevented. In addition, transverse grooves (10 mm depth ×50 mm width × 140 mm length) on the ECC layer were made at every 100 mm to enhance bonding between concrete and ECC. The mixture proportions of ECC and concrete are listed in Table 1. For the ECC material, the volume fraction of polyvinyl alcohol (PVA) fibre was 2%, and the specific indices of the fibre are presented in Table 2. The cross-sectional dimensions of all test specimens were b × d = 150 mm × 300 mm.The beam length (L) was 2200 mm. Figure 1 shows the geometric dimensions of the specimens. The main parameters considered were the longitudinal reinforcement ratio (0.70%, 1.01% or 1.79%), matrix types (ECC, concrete or ECC–concrete composite), ECC pouring position (tension zone or compression zone).Two pairs of steel and FRP bars with diameters of 10, 12 or 16 mm were used as the main tensile reinforcement. The FRP bars were placed at the bottom layer that were closer to the concrete cover owing to their high corrosion resistance. Two steel bars with the same size as the longitudinal tension bars were employed in the corners at the compression side to anchor the stirrups and hold them in place in the mould. The tested beams were reinforced with 8- and 12-mm-diameter bars in the upper and bottom zones, respectively. The shear reinforcement was accomplished using 8-mm-diameter deformed bars with a spacing of 100 mm. The reinforcement layouts of the specimens are shown in Figure 1. The specific parameters of the specimens are listed in Table 3. The naming rules of the specimens are as follows: (1) The first two characters ’HR’ indicate hybrid steel and FRP reinforcement; (2) the characters after ‘R’ indicate the matrix type, among which ‘C’, ‘E’ and ‘EC’ mean concrete, ECC and ECC–concrete, respectively; (3) the Arabic numerals after the hyphen indicate the diameter of the longitudinal bars; (4) for the composite beams, the last letter, ‘T’ or ‘C’, represents the 90-mm layer ECC pouring in the tension zone or the compression zone, respectively. For example, ‘HRE-16’ means a hybrid steel and FRP-reinforced ECC beam with longitudinal bars’ diameter of 16 mm.

Table 1.

Mixture proportions.

Matrix designation	Cement	Fly ash	Sand	Coarse aggregate	Water	High-range water-reducing admixture, %	PVA fibre volume fraction, %
ECC	0.2	0.8	0.2	–	0.22	0.8	2.0
Concrete	1.0	–	1.9	3.7	0.54	–	–

Table 2.

Specific performance indices of PVA fibre.

Length (mm)	Diameter (µm)	Tensile strength (MPa)	Elongation (%)	Elastic modulus (GPa)	Density (g/cm³)
12	39	1620	7	42.8	1.3

Table 3.

Information on the beam specimens.

No.	Specimen ID	Depth (mm)	Width (mm)	Length (mm)	Longitudinal tensile steel reinforcement (mm)	Longitudinal tensile FRP reinforcement (mm)	Transverse reinforcement (mm)	Matrix type
1	HRC-16	300	150	2200	2ϕ16	2ϕ16	ϕ8@100	Concrete
2	HRE-10	300	150	2200	2ϕ10	2ϕ10	ϕ8@100	ECC
3	HRE-12	300	150	2200	2ϕ12	2ϕ12	ϕ8@100	ECC
4	HRE-16	300	150	2200	2ϕ16	2ϕ16	ϕ8@100	ECC
5	HREC-T	300	150	2200	2ϕ16	2ϕ16	ϕ8@100	ECC–concrete (90-mm ECC layer in the tension zone)
6	HREC-C	300	150	2200	2ϕ16	2ϕ16	ϕ8@100	ECC–concrete (90-mm ECC layer in the compression zone)

Figure 1.

Dimensions, reinforcement details, strain gauges and LVDT layout of the specimens (unit: mm): (a) HRE-10, HRE-12 or HRE-16, (b) HRC-16, (c) HREC-C and (d) HREC-T.

Material properties

To measure the tensile properties of the ECC, uniaxial tensile tests on specimens with dimensions of 350 mm × 50 mm × 15 mm were conducted. The typical tensile stress–strain curves of the ECC are shown in Figure 2. The measured ECC tensile strength exceeded 5 MPa and the ultimate tensile strain approached 4%, demonstrating excellent tensile ductility. In addition, a batch of ECC and concrete cubes with dimensions of 10 mm × 100 mm × 100 mm were prepared and tested in uniaxial compression. The testing time was synchronised with the beam specimens. The measured cubic compressive strengths (f_cu) of ECC and concrete were 42.1 and 49.4 MPa, respectively. Simultaneously, uniaxial tensile tests were performed on both CFRP and steel bars. The mechanical properties of different types of bars are listed in Table 4.

Table 4.

Material properties of the steel and FRP bars.

Reinforcement type	Diameter (mm)	Elastic modulus E_s (GPa)	Yield strength f_y (MPa)	Ultimate strength f_su (MPa)
Steel bar	8	193	366	524
	10	204	467	585
	12	205	521	628
	16	198	528	635
FRP bar	10	116	–	1740
	12	115	–	1680
	16	112	–	1560

Figure 2.

Typical tensile stress–strain relationship of ECC.

Measurement layouts and test setup

All beams were subjected to four-point bending. The distance between the load point and the support was 800 mm. Three linear variable differential transformers (LVDTs) were arranged in a pure bending zone to observe the displacement and curvature of the beams. A group of strain gauges with an interval of 50 mm was attached to the mid-span concrete/ECC surface to measure the strain distributions along the beam height. Strain gauges were also attached to the steel and FRP bars at the pure bending region with a spacing of 100 mm. Additionally, an instrument to measure crack width was used throughout the experiment. The measurement layout of the specimens is shown in Figure 1. The loading was applied using a hydraulic jack. The applied load was distributed to the three points of the beam through the distribution beam. Steel shims were placed below the support and the loading point to prevent stress concentration. The specific layout of the loading device is shown in Figure 3. Before the formal loading, the specimens were preloaded and unloaded to eliminate the gap between the machine and the test specimen. The load was first applied at an interval of 10 kN and maintained for 3 min at each interval to observe and record the test phenomenon. When the predicted ultimate load value was approached, the load interval was changed to 5 kN until failure occurred. The loading was terminated when the load decreased to 80% of its peak value. All of the test data were collected using a TDS-530 data acquisition instrument.

Figure 3.

Schematic setup of the four-point bending test: (a) picture of static loading test and (b) schematic view of static loading test.

Results and discussion

Failure modes and crack patterns

For specimen HRC-16, the initial crack appeared in the pure bending zone at a load of 38 kN, and it rapidly extended to the concrete compression zone as the load increased. With the continuous increase in applied load, the number of cracks in the pure bending zone continued to increase, and the width of the cracks gradually increased. At a load of 130.5 kN, the steel bar yielded and the corresponding mid-span deflection was 5.64 mm. Thereafter, the crack width increased rapidly with an increase in deflection, but the number of cracks remained almost constant. At 249 kN, the member underwent initial concrete crushing in the compression zone, and the load-carrying capacity began to decrease. The mid-span deflection at the peak load was 62.29 mm. The beam specimen finally failed through flexural compression owing to severe concrete crushing. The final failure mode of the specimen is shown in Figure 4(a).

Figure 4.

Typical failure modes for each type of specimen: (a) HRC-16, (b) HRE-16, (c) HRE-10, (d) HRE-12, (e) HREC-T and (f) HREC-C.

Specimen HRE-16 had an identical reinforcement layout and geometric dimension as HRC-16, but its matrix was different. For HRE-16, the initial crack occurred at a load of 40.5 kN and the corresponding mid-span deflection was 1.62 mm. As the load increased, several fine cracks appeared in both the pure bending and bending-shear regions, which extended to the compression zone as the beam deflection increased. The beam yielded at a load of 130.8 kN and deflection of 9.22 mm. As the load continued to increase, the number of cracks increased, while the width of the cracks remained almost unchanged. At 295 kN, the ECC compression zone was noticeably crushed, and the ultimate state was attained. The mid-span deflection was 83.51 mm. In the ultimate state, hundreds of fine cracks appeared along the beam. However, for HRC-16, only approximately 15 bending and shear cracks with noticeable widths appeared. This indicated that substituting concrete with ECC can effectively reduce the crack width and increase the durability of the beam member. The specimen HRE-16 eventually failed through the ECC crushing in the compression zone. The failure mode of HRE-16 is shown in Figure 4(b).

The geometric dimension and matrix type of specimen HRE-10 were the same as those of HRE-16, but the reinforcement ratio was lower. For HRE-10, the initial crack appeared in the pure bending zone at a load of 29.5 kN. The crack propagation before the peak load was similar to that of HRE-16. Specimen HRE-10 yielded at a load of 60 kN and a mid-span deflection of 5.01 mm. At a load of 115.8 kN, the FRP reinforcement fractured with aloud ‘bang’. Thereafter, the load abruptly decreased and the crack width developed rapidly. The beam failed through the tensile fracture of the FRP reinforcement. The final failure mode of HRE-10 is shown in Figure 4(c).

The reinforcement ratio of the ECC specimen HRE-12 was higher than that of HRE-10 but lower than that of HRE-16. As a result, the load-carrying capacity of HRE-12 was between that of HRE-10 and HRE-16. The yield load and peak load of HRE-12 were 80.3 and 175.5 kN, respectively, and the corresponding deflections were 5.83 and 63.14 mm, respectively. Owing to the lower reinforcement ratio, specimen HRE-12 finally failed through the tensile fracture of the FRP bars. The ECC compression zone was also slightly crushed. The final failure mode of HRE-12 is shown in Figure 4(d).

The ECC–concrete composite beam HREC-T had the same reinforcement layout and geometric dimension as HRC-16, but the 90-mm concrete layer in the tension zone was replaced by ECC. For HREC-T, cracks first appeared in the mid-span zone under a load of 40 kN.As the external load increased, several fine cracks appeared in the ECC layer, but the number of cracks in the concrete layer was very limited. An interesting observation was that the wide cracks in the concrete layer were distributed into hair-like filigrees in the ECC layer. The specimen yielded at a load of 55. 9 kN and a deflection of 6.6 mm. At 259.5 kN, concrete crushing occurred in the compression zone and the specimen underwent compression failure. The final failure mode of the HREC-T is shown in Figure 4(e).

In contrast to specimen HREC-T, the composite beam HREC-C employed a 90-mm layer ECC in the compression zone. The initial cracks appeared under a load of 39 kN. As the external load increased, an increasing number of flexural and shear cracks appeared along the beam span. The composite beam HREC-C yielded at a load of 121.5 kN and deflection of 7.41 mm. Beyond yielding, the crack number remained almost constant, but the crack width increased significantly with increasing deflection. When the load became 250 kN, a main shear crack was formed under the loading point of the beam. Specimen HREC-C finally failed through ECC crushing in the compression zone (Figure 4(f)). Because ECC had better compressive deformation ability than concrete, the failure process was gentler and matrix spalling was avoided compared with the concrete beam HRC-16.

Figure 5 shows the crack map of each specimen. The ECC position was observed to have a significant effect on the crack pattern of the specimens. For the specimen HRC-16 without ECC, wide and few tensile cracks were distributed along the beam. In the compression zone, the concrete crushing zone was large and conspicuous because of severe concrete spalling. Nevertheless, for the ECC specimen HRE-16, several tensile cracks with fine crack widths were distributed uniformly. Moreover, the crushing zone of the ECC beam was smaller than that of the concrete beam. The reduction in the bar diameter of the ECC beam had almost no effect on the tensile crack distribution but a significant effect on the failure map of the compression zone. The compression zone remained intact for specimens with bar diameters of 10 and 12 mm as the failure, as they were governed by the tensile fracture mode. For the composite beams, the ECC used in the tension zone significantly reduced the crack width, while the ECC used in the compression zone delayed the compression failure process.

Figure 5.

Crack maps of the specimens: (a) HRC-16, (b) HRE-16, (c) HRE-10, (d) HRE-12, (e) HREC-T and (f) HREC-C.

Load versus deformation responses

Figure 6 shows the moment versus mid-span deflection curves of the specimens. All of the beams had a similar shape to the moment-deflection curve. The moment–deflection curve can be divided into three stages: elastic, cracking and post-yielding stages. The moment increased almost linearly as a function of deflection before matrix cracking. Thereafter, the concrete or ECC cracked, and the slope of the curve continuously decreased with an increase in the applied load. Inflexion points occurred at the exact moments of steel yielding, and the slope of the curve continued to decrease. Nevertheless, the post-yielding stiffness was still apparent owing to the use of linear elastic FRP reinforcement. Finally, the moment decreased rapidly owing to the tensile failure manifested by FRP rupture or compression failure manifested by matrix compression.

Figure 6.

Moment versus mid-span deflection curves of the specimens.

The moment versus deflection curves also exhibited slight differences among the beams. Before steel yielding, the curve slope increased with the reinforcement ratio. For beams with the same reinforcement layout, the beams with concrete in the compression zone (HRC-16 and HREC-T) had higher stiffness than those with ECC in the compression zone (HRE-16 and HREC-C). This was attributed to the higher elastic modulus of concrete compared with that of ECC. Beyond yielding, the beams with ECC in the tension zone (HRE-16 and HREC-T) exhibited higher stiffness than those with concrete in the tension zone (HRC-16 and HREC-C).After the steel yielded, the concrete entered the softening stage during this period, which contributed slightly to the stiffness of the beams. However, the tensile strain hardening behaviour of the ECC maintained its resistance to the tension load, which had a positive effect on the stiffness of the beams.

Figure 6 and Table 5 also show the ultimate strength and deformation of the beams. Both the load-carrying capacity and ultimate deflection were observed to increase with the reinforcement ratio for the hybrid-reinforced ECC beams. This was determined by the failure modes of the beams. The ECC beams with bar diameters of 10 and 12 mm exhibited tensile failure. The smaller the reinforcement ratio, the faster it fractured. The ECC beam with a bar diameter of 16 mm exhibited flexural compression failure; thus, the failure process was ductile. The ECC position had a significant effect on the loading and deformation capacities of the beams. Compared with the concrete beam HRC-16, the composite beam with ECC in the tension zone (HREC-T) exhibited a slightly higher increase in strength than ECC in the compression zone (HREC-C). This increased strength was attributed to the contribution of ECC material to the moment capacity of the beam section owing to its ability to bear tensile stresses. The deformation capacity had the opposite trend. The composite beam with ECC in the compression zone exhibited higher improvement owing to the superior compressive ductility of ECC to concrete.

Table 5.

Strength indexes of the beam specimens.

No.	Specimen ID	Cracking moment M_cr (kN m)	Δ_cr (mm)	Yielding moment M_y (kN m)	Δ_y (mm)	Ultimate moment M_u (kN m)	Δ_u (mm)
1	HRC-16	15.2	1.37	52.2	5.64	99.6	62.29
2	HRE-10	11.8	2.42	24.0	5.01	46.3	34.19
3	HRE-12	12.6	1.44	32.1	5.83	70.2	63.14
4	HRE-16	16.2	1.62	52.3	9.22	118.0	83.51
5	HREC-T	16.0	1.11	55.9	6.60	103.8	70.45
6	HREC-C	15.6	1.50	48.6	7.41	99.8	75.84

Strain analysis

Figure 7 shows the concrete strain variation along the beam height of all specimens. The strains were observed to be distributed almost linearly along the section at each load level, which meant that the plane cross-section is generally applicable. Figure 7 also shows that the neutral axial moved gradually away from the centroid of the beam cross-section towards the extreme compression fibre as the applied load increased. The area of the compression zone decreased as sectional moment increased.

Figure 7.

Sectional height (y) versus concrete strain (ε_c) relationships of beams: (a) HRC-16, (b) HRE-10, (c) HRE-12, (d) HRE-16, (e) HREC-T and (f) HREC-C.

The moment–longitudinal reinforcement strain curves of the specimens are shown in Figure 8. For the steel reinforcement, the steel strain values exceeded the yield value for all specimens. Beyond yielding, the slope of the moment– steel strain curve decreased, but the post-yielding stiffness was still evident owing to the existence of the FRP bars (Figure 8(a)), which differed from that of beams reinforced with only steel. The growth of steel strains was more pronounced for hybrid-reinforced ECC beams with lower reinforcement ratios. For beams with the same reinforcement ratio, the steel strains of the beams with ECC in the tension zone were lower than those with concrete in the tension zone at the same load level. For example, at a moment of 60 kN m, the steel strain values of HRE-16 and HREC-T were 1571 and 2041 µε, respectively, compared with 3133 and 2431 µε for HRC-16 and HREC-C, respectively. This phenomenon was also observed for FRP reinforcement, as illustrated in Figure 8(b). We can conclude that the employment of ECC in the tension zone can steadily bear part of the tension force and thus delay the reinforcement strain development. For concrete in tension, the load-carrying capacity was lost immediately after the first cracking. In contrast, ECC provided the tension load capacity even when multiple fine cracks appeared, and the load-carrying capacity was not lost until strain localisation occurred.

Figure 8.

Moment versus longitudinal reinforcement strain curves of the specimens: (a) steel strain and (b) FRP strain.

Crack developments

Figure 9 shows the variation in the maximum crack width with increasing moment for each specimen. Note that for the ECC–concrete composite beam with ECC in the tension zone (HREC-T), only the crack width in the ECC layer was measured. The variation trend of the moment versus maximum crack width curves was generally consistent with that of the moment versus mid-span deflection curves. Matrix type and ECC positions significantly affected crack development. For the hybrid reinforced concrete beam HRC-16, the maximum crack width increased almost linearly with the increase in the ratio between the applied load to the peak load (M/M_u) and exceeded 0.5 mm at half of the peak load (Figure 9(b)). In contrast, for the hybrid-reinforced ECC beam HRE-16, the maximum crack width initially increased linearly and then remained almost unchanged as a function of the applied moment until crack localisation appeared. The maximum crack width was maintained below 0.15 mm at 50% of the peak load. For the ECC–concrete composite beams, the composite beam with ECC in the compression zone (HREC-C) exhibited a variation trend similar to that of HRC-16, while the variation trend for the composite beam with ECC in the tension zone (HREC-C) was consistent with that of HRE-16. This indicated that the application of the ECC layer in the tension zone decreased the maximum crack width to an extremely low level. Therefore, the strategic use of ECC to replace concrete surrounding the main tensile reinforcement is preferable to extend the life cycle of RC structures while maintaining the minimum associated increase in material cost.

Figure 9.

Variations in the maximum crack width for each specimen: (a) variations with M/M_y and (b) variations with M/M_u.

The service load is always designed to be 50%–70% of the yield load to consider the serviceability limit state. As a result, the maximum crack width value at the service state should be particularly considered. Figure 9(a) shows the variation in the maximum crack width with the ratio of the applied moment versus yield moment (M/M_y). We observed that the mean value of the maximum crack width for the specimens with ECC in the tension zone (HRE-10, HRE-12, HRE-16 and HREC-T) at 70% of the yield moment was 0.095 mm, compared with 0.360 mm for the beams with concrete in the tension zone (HRC-16 and HREC-C). We inferred from Tsukamoto’s study that the flow of aggressive substances into an RC member can be significantly reduced when the ultimate crack width is below the critical crack width of 0.102 mm ( Tsukamoto, 1990 ). Therefore, we can conclude that, under service load conditions, the ECC layer will prevent the migration of aggressive substances into the concrete or reinforcement.

Ductility analysis

Figure 10 shows a comparison of the ductility of the specimens. The ductility coefficient in this study was defined by the ratio of the ultimate mid-span deflection to the yield deflection (µ = Δ_u/Δ_y). Figure 10 shows that all of the beams exhibited good ductility except for specimen HRE-10, which exhibited tensile failure manifested by the premature fracture of the FRP bars. Among the other five beams, the deformation capacities of HRC-10 and HRE-12 were relatively small, while their ductility were ranked as the top two. This was because of their much smaller yield displacements. A comparison of the two types of ECC–concrete composite beams revealed that the ECC position had a minimal effect on the ductility. Specimen HREC-T had both a larger yield and ultimate deflections compared with HREC-T, resulting in adjacent ductility coefficients.

Figure 10.

Histogram of the ductility coefficients of the specimens.

Conclusions

In this study, a group of hybrid steel and FRP-reinforced beams with various reinforcement ratios, matrix types and ECC positions were tested under four-point bending. The flexural behaviours in terms of failure mode, crack pattern, flexural strength, strain variation, crack propagation and ductility were analysed in detail. The following conclusions were drawn from this study:

The substitution of concrete with ECC has a significant effect on the flexural strength and crack development but a slight effect on the ductility of the hybrid-reinforced beams. The flexural strength was increased by 34% with the use of ECC. More but thinner cracks formed on the tensile face of the ECC beam rather than fewer and wider cracks as in the concrete beam.

The increase in the reinforcement ratio changed the failure mode of the hybrid-reinforced ECC beam from tension to compression.

The use of an ECC layer in the tension zone reduced the maximum crack width of the beam below the level of 0.1 mm under service load conditions, which would prevent the migration of aggressive substances into the concrete or reinforcement and is beneficial to the durability of the hybrid-reinforced beam.

The use of an ECC layer in the compression zone avoided the severe matrix spalling that was observed in the hybrid-reinforced concrete column. The deformation capacity can also be increased owing to the higher compressive ductility of ECC than that of concrete.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research described in this paper was financially supported by the National Natural Science Foundation of China (Grant Nos. 52068023 and 51608199).

ORCID iD

Fang Yuan

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