Shear strengthening of reinforced concrete beams with high-strength steel wire and engineered cementitious composites

Abstract

Engineered cementitious composite (ECC) is a type of high-performance fibre-reinforced cementitious composite with good ductility and excellent crack control ability. It has attracted increasing attention as a structural repair material in severely corrosive environments. However, the strength improvement is limited when ECC is used alone for shear strengthening of existing reinforced concrete (RC) members, although its shear capacity is much higher than that of other brittle cementitious materials such as cement mortar. This study proposes a novel shear strengthening method for RC beams with both high load-carrying capacity and good durability through the combination of high-strength steel wire and an ECC layer. The shear behaviours of the beams were tested under static loading. The test results showed that the shear strength and the ultimate displacement were significantly improved through shear strengthening. A large number of fine cracks appeared on the ECC layer before the failure of the beams. The load-carrying capacity was reduced by pre-damage owing to the important role of the shear resistance of the concrete with respect to the total shear capacity. The shear strength of the strengthened beams cannot be accurately predicted by the current design code owing to the ignorance of the shear resistance of ECC.

Keywords

engineered cementitious composite (ECC)high-strength steel wire beam shear strengthening crack control

Introduction

Corrosion of steel bars is the main durability issue for reinforced concrete (RC) structures in harsh environments, such as marine and offshore environments, cold areas where salt is used to de-icing concrete roads and mining construction factories (Chess and Green, 2019). The United States would need to invest approximately 3% of the Gross Domestic Product (GDP) to maintain a state of good repair for its infrastructure. The situation in China is similar. In RC structures, the stirrups are more prone to corrosion than the longitudinal reinforcements because of their closer distance to the atmosphere. The corrosion of stirrups not only could lead to brittle shear failure, but also weakens the confinement of the longitudinal reinforcement and core concrete owing to the loss of the cross-sectional area. Therefore, shear strengthening of corroded RC members is inevitable.

In recent decades, high-performance engineered cementitious composites (ECCs) have been developed based on systematic micromechanical design (Li, 2003). Engineered cementitious composite has attracted more and more attentions in engineering practice (Kim et al., 2004; Lepech and Li, 2009; Yuan et al., 2017). Engineered cementitious composite exhibits extremely different tensile behaviour from that of brittle concrete, the former deforms similar to steel and has the pseudo-strain hardening property (Li and Leung, 1992). The ultimate tensile strain can steadily exceed 3%, and the corresponding crack width remains below 100 μm (Li et al., 2001). The ultra-high ductility and durability of ECC enable it to be widely used as a strengthening or repair material. Chen et al. (2018) used high-strength ECC for the flexural repair of concrete structures with severe steel corrosion. The test results verified the feasibility of the proposed highly efficient repair technique. Hung and Chen (2016) proposed an innovative ECC jacketing for retrofitting shear deficient RC member and proved that the ECC jacket with a single layer of bar meshes was the best retrofitting scheme. Wu and Li (2017) proposed a carbon fibre-reinforced polymer (CFRP)-ECC hybrid system consisting of CFRP composites embedded in an ECC matrix to strengthen RC structures. Premature debonding occurred between the concrete surface and CFRP-ECC hybrid system. As a result, the flexural performance of concrete beams has hardly improved. Jiang et al. (2019) studied the bond behaviour of basalt textile meshes in ultra-high ductility cementitious composites by pull-out tests. The pull-out force versus slip curve has an initial large hardening rigidity and a continuous softening curve after peak. Hu et al. (2019) tested the flexural behaviour of corroded RC beams using a hybrid CFRP-ECC strengthening approach in which the damaged concrete around the longitudinal reinforcement was removed and replaced by ECC. In this case, premature debonding between the old concrete and CFRP-ECC hybrid system was prevented and thus the flexural strength was significantly enhanced. Yang et al. (2018, 2020), Zheng et al. (2020) and Guo et al. (2021) employed a fibre-reinforced polymer (FRP) grid-reinforced ECC matrix for flexural or shear strengthening of RC beams, respectively. They found that the strengthening system was not effective until proper surface treatment of the concrete members was carried out, or a reliable anchoring technique was developed.

It can be concluded from the above review that ECC as an adhesive layer can evidently improve the durability of existing structures owing to its excellent crack control ability. However, ECC is eventually a type of cementitious composite with limited tensile or shear strength (4–6 MPa). It needs to be combined with high-tensile-strength materials such as bar meshes or FRP for structural strengthening. The anchoring of FRP or the strengthening system still relies on polymer adhesives or complicate anchorages, whose use has been challenged considering structural fire.

High-strength steel wires are an advisable alternative to FRPs because they are more ductile, less expensive, less sensitive to high temperature and fire and easier to anchor into existing concrete structures compared with FRP (Wei and Wu, 2014). High-strength steel wires are often used in combination with polymer mortar to strengthen RC structures. Previous studies have proven it to be an effective method for flexural, shear and seismic retrofitting (Kim and Kim, 2011; Kim et al., 2007; Nie et al., 2005a, 2005b; Yang, 2012; Yang et al., 2009). However, this strengthening method also has the problem of poor durability under severe corrosion environment due to the poor crack control ability of polymer mortar.

In this study, ECC is attempted to be employed in combination with high-strength steel wire to improve the shear strengths and deformation capacities of existing RC beams. The advantages of the materials are expected to be fully utilized; that is, ECC acts as an adhesive layer to protect the steel wire and stirrups from secondary corrosion, while the high-strength steel wire functions as the main load-carrying component. The success of the first author in the use of this method for flexural strengthening has injected confidence into the present work (Yuan et al., 2020). The shear behaviours of strengthened and unstrengthened RC beams will be tested under static loading. The influences of the reinforcement ratio of the steel wire and the existence of pre-damage on the crack pattern and development, failure mode, load-carrying capacity and ductility of the strengthened beams will be systematically investigated.

Experimental programme

Specimen preparation

In total, five original RC beams were prepared, four of which were strengthened with ECC and high-strength steel wires. Critical parameters, such as the diameter of steel wires (3.05 mm or 3.6 mm), spacing of steel wires (30 mm or 50 mm) and pre-damage applied before strengthening or not were considered. Herein, the pre-damaged beam was preloaded to make the maximum shear crack width of the beam reach 0.2 mm before strengthening. Table 1 presents the specifications of the tested beams. The following naming rules were specified: (1) the first two characters ‘SE’ indicate that the beam was strengthened with steel wire and ECC; (2) the Arabic numerals followed ‘SE’ represent the diameter of the steel wire; (3) the last two Arabic numerals represent the spacing of the steel wires along the shear span of the beam; (4) the character ‘p’ after the Arabic numerals indicate pre-damaged specimen. For example, ‘SE3.6-50’ refers to a beam strengthened by steel wires with a diameter of 3.6 mm and a spacing of 50 mm. All the beams had identical lengths of 1800 mm and cross-sections of 150 mm × 250 mm. In order to achieve the shear failure mode, sufficient longitudinal bars with diameter of 25-mm and 10-mm in the bottom and upper zones, respectively, were arranged. On the other hand, transverse reinforcement only comprised steel bars with a diameter of 6 mm and a spacing of 150 mm. The geometric dimensions and reinforcement layouts of the specimens are presented in Figure 1.

Table 1.

Specifications of beam specimens.

No	Specimen ID	Depth (mm)	Width (mm)	Length (mm)	Longitudinal tensile reinforcement (mm)	Transverse reinforcement (mm)	Steel wire (mm)	Mortar type	W/wo pre-damage
1	RC-0	250	150	1800	2ϕ25	ϕ6@150	—	—	wo
2	SE3.05-30	250	150	1800	2ϕ25	ϕ6@150	ϕ3.05@30	ECC	wo
3	SE3.05-50	250	150	1800	2ϕ25	ϕ6@150	ϕ3.05@50	ECC	wo
4	SE3.6-50	250	150	1800	2ϕ25	ϕ6@150	ϕ3.6@50	ECC	wo
5	SE3.05-30P	250	150	1800	2ϕ25	ϕ6@150	ϕ3.05@30	ECC	w

ECC: engineered cementitious composite.

Figure 1.

Dimensions, reinforcement details, strain gauges and linear variable differential transformer layout of the specimens (unit: mm). (a) Unstrengthened RC beam and (b) strengthened RC beams. RC: reinforced concrete.

The strengthening procedures were as follows: (1) roughened and then cleaned both sides of the concrete beams; (2) anchored the angle steel to the RC beam with expansion bolts; (3) bolted the steel wires to the angle steel, one end of the steel wire was fixed first and the other end was subsequently tensioned and fixed; (4) evenly applied structural glue to the concrete surface to be strengthened to ensure the bond between old concrete and the strengthening layer; (5) cast ECC to the concrete surface in two stages: the first one is used to cover the steel wires, and the second one is expected to reach the predetermined pouring height. The voids and defects in the ECC layer were cleared by repeated compaction treatment during pouring. The details of the strengthening procedure are shown in Figure 2.

Figure 2.

Details of developed steel wires and strengthening procedures. (a) Image of the steel wire installation and (b) schematic view of the steel wire installation.

Material properties

Table 2 shows the mixture proportions of concrete and ECC. Greenness ECC with a cement substitution ratio of up to 80% was prepared to improve the material’s environmental sustainability. The fibre type in ECC was selected to be polyvinyl alcohol (PVA) fibre. The fibre volume content was determined to be 2%, and the length and diameter were 12 mm and 39 μm, respectively. Table 3 shows the specific performance indicators of PVA fibre. In order to test the tensile ductility of the ECC material, uniaxial tensile tests were carried out. The size of the specimen is 350 × 50 × 15 mm. The test device is shown in Figure 3. The uniaxial strain was measured by linear variable differential transformers (LVDTs) arranged on both sides of the specimen. The FRP sheet was adhered to both ends of the specimens to prevent being crushed of ECC by the machine. The uniaxial tensile stress–strain curves of ECC are shown in Figure 4. It is found that the tensile strength of ECC exceeds 4.5 MPa, and the ultimate tensile strain exceeds 3%, indicating excellent ductility. At the same time, uniaxial compression tests were performed on concrete and ECC cubes with side length of 100 mm. The test results showed that the compressive strength of ECC and concrete were 41.7 and 34.1 MPa, respectively. Three types of steel bars with diameters of 6 mm, 10 mm and 25 mm were used in the test. The mechanical parameters of steel bars and steel wires are shown together in Table 4.

Table 2.

Mixture proportions.

Matrix designation	Cement	Fly ash	Sand	Coarse aggregate	Water	High-range water-reducing admixture, %	Polyvinyl alcohol (PVA) fibre volume fraction, %
ECC	0.2	0.8	0.2	—	0.22	0.8	2.0
Concrete	1.0	—	1.9	3.2	0.56	0.7	—

ECC: engineered cementitious composite.

Table 3.

Specific performance indices of polyvinyl alcohol fibre.

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

Figure 3.

Uniaxial tensile test setup for engineered cementitious composite.

Figure 4.

Typical tensile stress–strain relationship of engineered cementitious composite.

Table 4.

Material properties of the steel bars and steel wires.

Reinforcement type	Diameter (mm)	Cross-sectional area (mm²)	Elastic modulus E_s (GPa)	Yield strength f_y (MPa)	Ultimate strength f_su (MPa)	Ultimate strain εsu
Steel bar	6	28.26	200	424	572	—
	10	78.50	204	467	585	—
	25	490.63	205	503	610	—
Steel wire	3.05	4.45	140	—	1608	0.0175
Steel wire	3.6	6.16	140	—	1620	0.0187

Test setup

The beams were loaded under static loading. The distance between the two supports was 1500 mm, and the distance between the support and the loading point on the same side was 400 mm. On each side, two groups of three strain gauges with a spacing of 50 mm in each group were attached to the stirrups, 100 mm and 200 mm from the loading point. Three LVDTs were installed at the bottom of the pure bending region to observe the deformations of the beams. Two additional LVDTs were installed on the supports to measure the rotation of the beams during the loading process. The test data of the load sensors, LVDTs and strain gauges were recorded through a data acquisition instrument. The test was terminated as the load dropped to 80% of the peak load. The schematic map of the test setup is shown in Figure 5.

Figure 5.

Test setup. (a) Schematic of static loading test and (b) photograph of static loading test.

Results and discussions

Failure modes and crack patterns

Figure 6 shows the failure modes of each specimen. The unstrengthened RC beam cracked at a shear load of 25.2 kN. When the load reached 47.5 kN, one diagonal crack appeared in the middle height of the beam. This diagonal crack extended to the loading point as well as the support as the load increased. With increasing deformation, the number and width of the cracks in the flexural-shear region increased gradually. The stirrup yielded at a load of 100.4 kN. The specimen failed at a load of 241.6 kN owing to the sudden increase in the width of one of the diagonal cracks. Severe concrete spalling was observed at the ultimate state. The failure process is typical shear failure. Figure 6(a) shows the failure mode of this unstrengthened RC beam.

Figure 6.

Typical failure modes for each type of specimen. (a) RC-0, (b) SE3.05-30, (c) SE3.6-50, (d) SE3.05-50 and (e) SE3.05-30P.

Specimen SE3.05-30 was strengthened with steel wires with a diameter of 3.05 mm and spacing of 30 mm. The first crack appeared at a load of 50.2 kN. The initial diagonal crack at the flexural-shear zone occurred at a load of 67.5 kN. After that, the number of cracks in the flexural-shear zone increased rapidly, while that in the pure bending zone remained almost constant. The stirrup yielded a load of 145.3 kN. When the load increased to 175.4 kN, the specimen reached the ultimate load value manifested by the occurrence of a large crack width at the ECC layer. The concrete in the shear compression zone was also severely crushed. As expected, a typical shear failure was observed. Other than the smaller but wider cracks in the flexural-shear zone of RC-0, multiple fine cracks occurred in the SE-3.05-30 ECC layer. During the entire loading process, no bonding failure between the concrete and ECC was observed, indicating the effectiveness of the surface treatment. The failure mode of SE3.05-30 is shown in Figure 6(b).

Specimens SE3.05-50 and SE3.6-50 showed similar loading phenomena and failure modes to those of SE3.05-30. The former had a lower carrying capacity than the latter owing to the smaller stirrup ratio. The ultimate shear strengths of SE3.05-50 and SE3.6-50 were 155.6 kN and 170.7 kN, respectively. There was also no interfacial debonding between the concrete and the strengthened layer. The failure modes of these two specimens are shown in Figure 6(c) and (d).

Specimen SE3.05-30P had an identical strengthened layout with that of SE3.05-30, but it had been pre-damaged prior to strengthening. The first crack appeared at a shear load of 45.3 kN. When the load reached 65.5 kN, a diagonal crack appeared in the ECC layer and continued to extend to the loading point and support with increasing deformation. After that, the cracks in the pure bending zone barely developed, whereas the number of diagonal cracks in the flexural-shear zone increased rapidly. The growth of the crack width was significantly slower than that of the control RC beam. The stirrup yielded at a load of 135.5 kN. When the load increased to 160.3 kN, a localized crack appeared on the ECC layer and the specimen immediately failed in shear. At this state, the mid-span deflection of the beam was 11.59 mm. Compared with SE3.05-30, the failure process was similar, but the shear resistance was lower owing to the unrecoverable pre-damage of concrete in the flexural-shear zone. The failure mode of SE3.05-30P is shown in Figure 6(e).

The crack maps of the flexural-shear zone of the specimens are shown in Figure 7. It was found that the crack propagation of the unstrengthened RC beam was similar to that of strengthened beam. The crack started from the middle section and expanded to both ends along the 45° direction as the deflection increased. However, the crack patterns of these two kinds of beams were extremely different. At the ultimate state, many fine cracks were observed on the ECC layer of the strengthened beam, while only few shear cracks were found in the concrete layer of the RC beam. These results showed that the use of ECC for shear strengthening can significantly reduce the crack width, which is beneficial for protecting the stirrups as well as the steel wires. Through comparing of the strengthened beams with or without pre-damage, it was found that even through evident cracks occurred on the concrete surface before strengthening, the final crack pattern of the beam with pre-damage kept consistent with that of beams without pre-damage.

Figure 7.

Crack maps of the specimens. (a) RC-0, (b) SE3.05-30, (c) SE3.05-50, (d) SE3.6-50 and (e) SE3.05-30P.

Load versus deformation responses

Figure 8 shows the shear load versus mid-span displacement curves of each specimen. As can be clearly seen, all of the specimens underwent similar trends. The load first increased almost linearly with the stirrup yielding strength, and then the stiffness of the curves slowed down until the ultimate load was obtained. Beyond the peak load, the shear resistance dropped suddenly without obvious forewarning, showing evident brittle shear failure characteristics. The stiffnesses of the strengthened specimens are higher than those of the unstrengthened beams due to the contribution from steel wires and ECC. Figure 8 and Table 5 also present the strength indices of the specimens. The strength improvement increased with increasing steel wire diameter and decreased with increasing spacing. Specimen SE3.05-30 exhibited the greatest increase in strength. The mechanism is simple. An increase in the amount of steel wire means that more high-strength steel wires can participate in providing shear resistance, thus leading to a higher shear capacity of the beam. It was also found that the pre-damage results in a significant reduction of the shear strength improvement of the strengthened beam. The improvements in cracking strength, yielding strength and ultimate strength were 79.8%, 35.0% and 32.7%, respectively, for SE3.05-30P, compared with 99.2%, 44.7% and 45.2% for SE3.05-30. This observation is different from that of pre-damaged flexural strengthening beams. For flexural strengthening, the pre-damage of concrete has little effect on the total flexural strength owing to the negligible contribution of concrete to the tensile load (Yuan et al., 2020). In contrast, the shear resistance of concrete plays an important role in the total shear resistance. Once cracking occurs, the shear resistance of concrete cannot be recovered, and its contribution to the total shear resistance of the strengthened beam is significantly reduced. Therefore, if the beams must be shear strengthened, owing to steel corrosion, not only the deterioration of stirrups but also the pre-damage before strengthening on the shear capacity of the strengthened beams should be taken into account.

Figure 8.

Shear load versus mid-span deflection curves of the specimens.

Table 5.

Strength indexes of the beam specimens.

No	Specimen ID	Cracking shear load V_cr (kN)	Yielding shear load V_y (kN)	Ultimate shear load V_u (kN)	Cracking load improvement (%)	Yielding load improvement (%)	Ultimate load improvement (%)
1	RC-0	25.2	100.4	120.8	—	—	—
2	SE3.05-30	50.2	145.3	175.4	99.2	44.7	45.2
3	SE3.05-50	45.0	130.6	155.6	78.6	30.1	28.9
4	SE3.6-50	30.3	132.5	170.7	20.2	32.0	41.3
5	SE3.05-30P	45.3	135.5	160.3	79.8	35.0	32.7

Strain analysis

Figure 9 shows the shear load versus maximum stirrup strain curves of the specimens. The trend of strain variation in the stirrups of the beams can be divided into three stages. At the beginning, the steel strains fluctuated around the ordinate, indicating that the stirrups were not involved in the shear resistance at this stage. The shear load value at the end of the first stage of the unstrengthened RC beam was approximately 30 kN, compared with the range of 60–80 kN for the strengthened RC beams. This difference is attributed mainly to the fact that the shear resistance is provided by the matrix in the first stage. The higher cracking strength of ECC extended the first stage of the strengthened beams. After cracking, the shear load was transferred to the stirrups, and the load versus maximum strain curves entered the second stage. In this stage, the stirrup strains increased rapidly with increasing shear load. The steel strain values were significantly different. Under the same load level, the increase in the number of steel wires leads to reduction of the stirrup strain value. For instance, at a load level of 100 kN, the maximum stirrup strains of RC-0, SE3.6-50 and SE 3.05-50 were 1810, 1135 and 265 με, respectively. Under the same load level, the load carried by each stirrup decreased as the number of steel wires increased. It was also found that pre-damage had a significant influence on the variations in stirrup strain in this stage. The maximum steel strain of SE3.05-30P is evidently larger than that of SE3.05-30 with the same layout of strengthening materials. As indicated above, the shear resistance of concrete is partially lost due to pre-damage, and the steel stirrups must resist a larger shear force. With increasing shear load, the stirrups yielded and the curves entered the third stage. In this stage, the slope of the curves of shear load versus stirrup strain decreased close to zero; that is, the stirrup strains grew rapidly while the shear resistance remained almost constant.

Figure 9.

Shear load versus maximum stirrup strain curves of specimens.

Crack developments

The variations in the maximum shear crack width at the shear span for each specimen were shown in Figure 10. It can be seen that the variation in the maximum shear crack width on concrete was different from that on the ECC layer. The maximum crack width increased almost linearly as the applied shear force increased and exceeded 0.2 mm at just 51% of the yield load or 46% of the peak load. In contrast, for the ECC layers, the maximum crack width first increased slowly with increasing applied load before the crack width was smaller than 0.2 mm and then increased rapidly afterwards owing to crack localization. The applied loads corresponding to these inflexion points were approximately 100% of the yield loads and 80% of the peak loads. A finer crack width of the ECC layer than that of concrete was achieved by a larger number of cracks. This indicates that under the service loading condition (within 50%–70% of the yield strength), the crack width on the ECC layer can be maintained at a quite low level. This is beneficial to protect the steel wires and prevent secondary corrosion of stirrups in severely corrosive environments.

Figure 10.

Variations of the maximum crack width at shear span for each specimen. (a) Variations in V/Vy and (b) variations in V/Vu.

Ductility analysis

Figure 11 shows the ductility coefficients (μ) of the beams. Herein, the ratio of the displacement corresponding to the peak load to the displacement corresponding to the yield load was defined as the ductility coefficient. It can be clearly observed that the ductility of all the strengthened beams is superior to that of the unstrengthened beam. In fact, the yield displacements are similar. The main reason for the difference in the ductility coefficient is that the ultimate displacement has been greatly improved through shear strengthening, which is also clearly observed in Figure 8. Moreover, pseudo-yielding plateaus were observed for the strengthened beams with an ECC layer; therefore, the failure process was much more ductile than that of the unstrengthened beam. This is attributed to the much more ductile shear deformation behaviour of ECC compared with of concrete (Li et al., 1994; Yuan et al., 2014).

Figure 11.

Histogram of ductility coefficients of specimens.

As illustrated in Figure 11, the ductility coefficient of the pre-damaged beam SE3.05-30P was slightly higher than that of the non-damaged beam SE3.05-30. This is due to the lower yield displacement of SE3.05-30P owing to concrete damage.

Shear strength analysis

In order to study the load-carrying capacity of the strengthened RC beam, the existing design code GB 50367-2013 (2013) was firstly adopted for calculation. In this calculation, perfect bonding between the strengthening layer and RC beams was assumed during the loading process. Furthermore, linear-elastic stress–strain relationship was assumed for steel wires. The predicted shear strength V of a strengthened beam can be expressed as

V = V_{b 0} + V_{b r}

(1)

V_{b r} = \frac{(ψ_{v b} f_{r w} A_{r w} h_{r w})}{s_{r w}}

(2)

In the above equations, V_b0 is the shear strength of the unstrengthened RC beam, V_br is the improved shear strength of the RC beam due to strengthening, f_rw is the tensile strength of steel wire; A_rw is the cross-sectional area of the steel wires. h_rw and s_rw are the vertical height and spacing of the steel wire stirrup, and ψ_vb is the shear strength reduction factor, which indicates the ratio between the actual stress to the strength of the steel wires. For the U-shaped steel wire stirrup, ψ_vb is determined to be 0.5 when the shear span ratio is lower than 1.5, and it is determined to be 0.8 when the shear span ratio is greater than 3. When ψ_vb is an intermediate value, it is determined using a linear interpolation.

The comparisons between the measured shear strengths of the strengthening layer (V_brt) and the predicted results (V_brc) based on the design code were shown in Table 6 and Figure 12. In Table 6, the error is calculated by

E r r o r (%) = \frac{| V_{b r t} - V_{b r c} |}{V_{b r t}} \times 100

(3)

Table 6.

Comparison of calculated shear strengths of the strengthening layer with experimental results.

Specimen ID	V_brc (kN)	V_brt (kN)	V_brc/V_brt	Error (%)
SE3.05-30	46.3	54.6	0.85	15.20
SE3.05-50	27.8	34.8	0.80	20.12
SE3.6-50	38.7	49.9	0.78	22.45
Average error (%)			0.81	18.75
SD (%)				3.02
SE3.05-30P	46.3	39.5	1.17	17.22

Figure 12.

Comparison of shear capacity of strengthened layer between measured and predicted results.

As can be clearly observed, the prediction of the shear strength of the strengthened layer by the design code GB 50367-2013 resulted in a non-negligible error. Because the shear capacity of the beam before strengthening (V_b0) was reduced owing to pre-damage, the calculated shear capacity of the strengthening layer of SE3.05-30P was underestimated and inaccurate, as presented in Table 6. As a result, the shear capacity of SE3.05-30P was not considered. The average error, standard deviation and maximum error were 18.75%, 3.02% and 22.45%, respectively. The shear capacities of the strengthening layers were greatly underestimated. The average ratio of V_brc/V_brt was 0.81. This may be attributed to ignorance of the shear capacity of the ECC. In the design code, the default bonding matrix is a polymer mortar, which is a type of brittle material that loses load-carrying capacity once cracking appears. However, the ECC provides a stable shear load to the beam owing to its fibre-bridging effect until crack localization. Thereby the shear capacity can be improved by replacing the polymer mortar with an ECC material. In this case, the current design code is not suitable for the novel strengthening approach in the present study. The shear capacity of the ECC layer should be considered. Owing to the limited number of specimens in the present study, further studies are needed to improve the formula for the shear strength of RC beams strengthened with ECC and steel wire.

It should be noted that the size effect for the shear strengthening is not investigated in the present study. It is well known that size effect exists in the shear strength of RC beams. Larger beams have a smaller nominal maximum shear strength. Qu et al. (2005) studied the size effect of shear contribution of externally bonded FRP jacket for RC beams and concluded that the direct shear contribution of FRP sheets shows little size effect. Nguyen-Minh and Rovňák (2015) investigated the size effect on shear behaviour of uncracked and pre-cracked RC beams strengthened with FRP jackets. It was found that strengthening degree decreased with increasing beam sizes for uncracked beams. However, effect of beam sizes on repaired beams was eliminated (more or less) due to their pre-cracking. For the shear strengthening method with high-strength steel wire and ECC in the present study, further study is needed on the size effect for the practical strengthening applications.

Conclusions

In this study, a novel shear strengthening approach by using high-strength steel wire and ductile ECC was proposed and tested. The main tested parameters are the diameter and spacing of the steel wire and whether to apply pre-damage to the concrete beam or not before strengthening. The shear performance in terms of failure mode, crack pattern and development, strength and ductility were analysed and compared among each specimen. In summary, the following conclusions can be drawn:

The combination of steel wire and ECC was verified to be an effective method for the shear strengthening of RC beams. The shear capacity and ultimate displacement were greatly improved through shear strengthening. Furthermore, the fine cracks on the ECC layer will have a positive impact on the corrosion resistance of the beams.

The failure mode and crack pattern of the strengthened beam with pre-damage were similar to those of strengthened beam without pre-damage. However, the load-carrying capacity was reduced by pre-damage owing to the important role of the shear resistance of the concrete with respect to the total shear capacity. The ductility was improved owing to the lower yield displacement caused by the concrete damage.

The current design code is conservative in predicting the shear strength of RC beams strengthened with combined ECCs and high-strength steel wires. The average error, standard deviation and maximum error were 18.75%, 3.02% and 22.45%, respectively. The shear capacity of the ECC layer should be considered. Owing to the limited number of specimens in the present study, further studies are needed to improve the formula for the shear strength of RC beams strengthened with ECC and steel wire.

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 study was financially supported by the National Natural Science Foundation of China (Grant No. 52068023).

ORCID iD

Fang Yuan

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