Long-term shear performance of concrete beam with carbon fiber-reinforced composite grid stirrups under sustained loadings and seawater immersion

Abstract

Using carbon fiber-reinforced composite (CFRP) grid stirrups to replace traditional steel stirrups in reinforced concrete (RC) beams can effectively improve the corrosion resistance of structures in marine environments. In this study, the durability of RC beams with CFRP grid stirrups under the coupled effects of sustained loads and immersion in seawater was studied. The sustained loads were set at 24% and 48% of the ultimate bearing capacity of the RC beam, and the specimens were immersed in artificial seawater at room temperature for 90 days, 180 days, and 360 days, respectively. The crack load and shear capacity of the beam were measured at the scheduled time. The initial stiffness of the beam decreased as the immersion time increased, and the higher the sustained load was, the lower the initial stiffness of the conditioned beam. The contribution of the CFRP grid stirrup to the shear load capacity after the cracking load was reached was more significant than that before. The effects of the 48% sustained load on the ultimate tensile strain of the CFRP stirrup were more significant than those for the 24% sustained load. Both immersion and sustained loading improved the cracking load and shear capacity of the beam, and the contribution of sustained loading to the shear capacity decreased as the immersion time and the sustained load level increased. Equations were proposed for predicting the shear strength of RC beams with CFRP grid stirrups under sustained loading and seawater immersion.

Keywords

Carbon fiber-reinforced composite grid stirrup reinforced concrete beam sustained load seawater immersion durability

Introduction

Reinforced concrete (RC) structure services in marine environments are needed to bear the load and damage caused by chloride ions (Su et al., 2021; Lu et al., 2021). The stirrup of RC structures rusts first due to the smaller thickness of the protective layer compared with that of the longitudinal steel bar, and the structural performance degrades prematurely, which seriously affects the service life of the structure (Zhou et al., 2015; Dong et al., 2018). Previous investigations have verified that fiber-reinforced polymer (FRP) composites have excellent mechanical properties and chloride ion resistance and are considered an ideal material to replace steel bars in marine engineering (Yoshitake et al., 2020; Hu et al., 2022b; Tang et al., 2020). FRP composites used in civil engineering mainly include carbon FRP (CFRP), glass FRP (GFRP), and basalt FRP (BFRP). Among these materials, CFRP displays superior mechanical properties, including high specific strength, a high tensile modulus, low thermal conductivity, and high chemical stability. Thus, this material has excellent development potential for applications in civil engineering (Hu et al., 2022a; Si et al., 2023; Isleyen et al., 2021).

Focusing on the service environments for marine engineering structures, durability studies were performed on FRP composites. The long-term performance of FRPs in concrete degraded in strong alkaline environments (Guo et al., 2018; Lu et al., 2020; Xie et al., 2022). Alkalinity is the key factor leading to the performance degradation of FRP (Lu et al., 2020); notably, hydroxyl leads to the decomposition of ester bonds in the resin matrix and the sizing agent on the fiber surface, exposes the defects of the fiber itself, and breaks the silica oxygen bonds of the basalt fiber (Xie et al., 2022; Wang et al., 2017b). Even alkali-silica reactions occur between basalt fiber and concrete aggregates (Lu et al., 2020), which accelerate the performance degradation of BFRP. The interfacial adhesion between the glass fiber and the resin matrix of GFRP is weak, resulting in a decrease in long-term performance (Wang et al., 2017b). Furthermore, chloride ions react with aluminum ions and ferrous ions in basalt fiber, which is unfavorable to the long-term performance of BFRP; however, NaCl is beneficial for resisting the moisture uptake of FRP (Guo et al., 2018). Carbon fibers retain integrity and strong fiber-resin matrix interface bonding properties in corrosive environments, so CFRP has the best durability performance among FRPs. Similar conclusions can also be found in the literature (Wang et al., 2017c; Hu et al., 2022b; Guo et al., 2018).

Generally, externally bonded FRP plates are used to strengthen RC members due to their convenience during use and excellent mechanical properties. However, the durability of these members has also been considered. Tang et al. (Tang et al., 2020) studied the effects of wet−dry cycles under a sustained load on the performance of FRP-strengthened RC beams immersed in 5% NaCl solution, and the results indicated that the beams mainly underwent concrete separation and CFRP–concrete interface debonding. Shrestha et al. (Shrestha et al., 2016) confirmed that the failure modes of CFRP–concrete interfaces changed from concrete cohesion to mode or interface failure as an effect of water immersion. The build-up of barnacles is beneficial the long-term performance of strengthened beams immersed in seawater (Choi et al., 2012). Our previous study showed that the ultimate bearing capacity of RC beams strengthened with prestressed CFRP can be significantly improved, and an inclined U-shaped anchorage changed the failure mode from CFRP debonding to CFRP fracturing (Lu et al., 2021).

FRP bars are commonly used as longitudinal bars and to create stirrups (Yuan et al.). Al-Hamrani and Alnahhal (Al-Hamrani and Alnahhal, 2021) indicated that the shear capacity and stiffnesses of beams with GFRP stirrups showed significant reductions compared to the corresponding values for beams with steel stirrups. The results for beams reinforced with BFRP bars and stirrups indicated that the shear capacity decreased as the span-to-depth ratio increased (Issa et al., 2016) and that the span-to-depth ratio showed a more pronounced influence on the shear performance (Fan et al., 2021). Dong et al. (2018) studied the flexural performance of concrete beams fully reinforced with BFRP bars in seawater–sand areas. The experimental results indicated that cracks were sparser after seawater immersion, the failure mode of the beams changed from concrete crushing to shear failure, and the contribution of the stirrups clearly decreased.

CFRP grids are also currently used for repairing or strengthening RC structures (Guo et al., 2021; Yoshitake et al., 2020; Cabral-Fonseca et al., 2018; Zheng et al., 2021, 2022). Guo et al. (Guo et al., 2021) studied the flexural behavior of RC beams strengthened with CFRP grids, and the shear strengthening effects were obvious, but the beam with two-layer FRP grids did not perform better due to premature debonding. Zheng et al. (Zheng et al., 2021, 2022) studied the mechanical performance of corrosion-damaged beams strengthened with CFRP grids and engineered cementitious composites. The bearing capacities of the corroded beams were sufficiently improved, and the CFRP grids were controlled by the serviceability limit state and not the limit-state ultimate strength (Yost and Schmeckpeper, 2001).

As mentioned above, CFRP exhibits excellent properties, and CFRP grids used for stirrups are convenient for construction and overcome the shortcomings of FRP bar stirrups, for example, premature bond slip failure is likely to occur with at least one side overlap, and the tensile strength of the bent portion is significantly less than that of the straight part (Yuan et al.). The use of CFRP grid stirrups is expected to improve the durability of RC beams under loading conditions in marine environments. However, studies of the durability of RC beams strengthened with CFRP grid stirrups considering the coupled effects of seawater and sustained load are insufficient. Thus, durability tests of RC beams reinforced with CFRP grid stirrups were conducted under sustained loading and seawater immersion. The influence of the sustained load levels on the development of deflection and the shear behavior were discussed.

Experimental program

Raw materials

The cement used in this study was commercial ordinary Portland cement (P.O. 42.5), provided by Shanxi Jinlong Cement Co., Ltd (Xi′an, China). The sand and coarse aggregate (granite) were sourced from a local supplier. The particle size of the coarse aggregate ranged from 5 mm to 30 mm. In accordance with the relevant guidelines (JGJ206-2010; JTJ-2011; GB50010-2010), the mixture proportion by weight of the constituents was determined as: cement (1.0):water (0.40):coarse aggregate (2.71):fine aggregate (1.11). The average 28-days compressive strength of the concrete was 35.6 MPa, obtained with 150 mm concrete cubes. The strength of the concrete conditioned with the tested beam increased from 36.5 MPa to 39.2 MPa due to the further hydration reaction in seawater after 360 days of immersion.

To prevent the normal section failure of the specimens, the longitudinal bar in this experiment was an HRB500 stainless steel ribbed bar with a diameter of 16 mm, and the yield strength and ultimate strength obtained in the laboratory were 578.4 ± 16.6 (standard deviation) MPa and 789.7 ± 6.6 MPa, respectively.

A CFRP grid stirrup was provided by Nanjing Loyalty Composite Material Equipment Manufacture Co., Ltd (Nanjing city, China). The grid size is shown in Figure 1(a). The width of the grid is 50 mm, the edges are 25 mm, the total width is (50 n + 50) mm, and the minimum width is 100 mm. The grid can be produced continuously, and the length can be cut as needed. In this test, the CFRP grid is cut into 100 mm × 200 mm (width × height) stirrups. The width is two intervals, and the height is four intervals, as shown in Figure 1(b). The tensile strength, tensile modulus, and elongation at break of the single-limb CFRP are determined to be 2192.2 MPa, 143.4 GPa, and 1.53%, respectively (Li et al., 2020).

Figure 1.

CFRP grid used in this study: (a) dimensions; (b) photograph.

Details of the concrete beams

According to ACI 440.1R-15 (ACI440.1 R, 2015) and the guidelines in the ‘Technical code for infrastructure application of FRP composites’ in China (GB 50608 2010), the dimensions of the concrete beam were designed to be 1700 mm × 150 mm × 250 mm, and the concrete cover was 25 mm. The longitudinal reinforcement ratio was 2.7%, and the stirrup ratio of the CFRP grid was 0.27%. A shear span of 400 mm and shear-span-to-depth ratio of 2.05 were used, and a four-point bending test was performed. Five stainless steel bars were used as tensile longitudinal bars, and two stainless steel bars were used as compressive longitudinal bars. CFRP grid stirrups with a spacing of 150 mm were applied. The details of the beam are shown in Figure 2.

Figure 2.

Details of the tested beam: (a) schematic diagram and (b) reinforcing mesh.

To verify the effect of the CFRP grid stirrups, it is necessary to ensure that the beam undergoes shear failure during testing. Thus, the following condition must be satisfied.

M_{u} \geq V_{u}

(1)

Where M_u is the ultimate flexural capacity and V_u is the ultimate shear capacity. High-strength longitudinal steel bars were used in this study, and the spacing between CFRP grid stirrups was increased to ensure the shear failure of the beam.

For different values of experimental variables, including sustained load levels and immersion periods, a shear test was performed on each beam. The ultimate bearing capacity of the beam was tested with a 3000 T pressure testing machine, and the ultimate bearing capacity of the test beam was 1%∼2% of the maximum machine load. To improve the measurement accuracy, a 100 T pressure sensor was employed, and the load values in the test were obtained from the 100 T pressure sensor.

RC structures often have cracks in actual projects. To simulate the stress in actual structures, 48%F_u (where F_u is the ultimate bearing capacity of the beam under four-point bending), or approximately 124 kN, was selected for the sustained load, and the pure bending section cracked. Additionally, the lower sustained load level of 24%F_u (62 kN) was selected for comparison to analyze the impact of having no cracks in the beam. The immersion period was set at 90, 180, and 360 days. The specimens used in this study are summarized in Table 1.

Table 1.

Summary of the specimens used in this study.

Specimen	Sustained load level	Seawater immersion period
B-0-0	0	0
B-0-90		90
B-0-180		180
B-0-360		360
B-24%-90	24%F_u (62 kN)	90
B-24%-180	24%F_u (62 kN)	180
B-48%-90	48%F_u (124 kN)	90
B-48%-180	48%F_u (124 kN)	180

Note: 0, 24%, and 48% indicate sustained load levels of 0, 24% and 48% of the ultimate load, respectively. 0, 90, 180 and 360 indicate immersion for 0, 90, 180, and 360 days, respectively.

Sustained load and seawater immersion device

The sustained load and seawater immersion device, as shown in Figure 3(a), was designed by the authors, as shown in Figure 3(a). The frame was made of square steel pipes, the net width was set at 200 mm, and the span was 1500 mm. To resist the deformation caused by sustained loading, two square steel tubes were used on the top of the frame, and a satisfactory result was obtained. The loads were applied by a jack; one end of the jack was supported on the force sensor, the force value was directly read in real time from the force display, and the other end was supported on the distribution girder, which transmitted the load to the concrete beam. There was a certain degree of unloading due to the creep of the concrete beams, and the jack was adjusted daily to ensure that the load was at the predetermined value. A stainless-steel sink was used to realize seawater immersion. The dimensions were approximately 1800 mm × 200 mm × 300 mm, and complete immersion of the beam was achieved. To ensure the accuracy of the test, the net span of the device in the erosion environment was the same as the span in the shear test, as shown in Figure 3(b). The tested beams were conditioned under sustained loads and immersion after 28 days of curing under laboratory conditions.

Figure 3.

Sustained load and seawater immersion device: (a) photographs and (b) schematic diagram.

Immersion media

Artificial seawater was prepared in accordance with ASTM 1141 (ASTM-D1141, 2013), and the chemical composition was 24.53 g of NaCl +4.09 g of Na₂SO₄ + 5.20 g of MgCl₂ + 1.16 g of CaCl₂ + 0.025 g of SrCl₂ + 0.695 g of KCl +0.201 g of NaHCO₃ + 0.101 g of KBr +0.027 g of H₃PO₃ + 0.003 g of NaF in 1 L of deionized water.

Shear performance test for the beam after immersion

After coupled seawater immersion and sustained loading for a scheduled time, a shear test was performed on the concrete beam by a four-point bending test. The equipment was the same as that for the ultimate bearing capacity test, and the shear test was carried out at a loading rate of 2 mm/min by displacement control. The applied load was increased by 5 kN per grade and held for 1 min. The linear variable differential transducer (LVDT) data and crack width (a crack observation instrument with an accuracy of ±0.01 mm was employed for testing) were observed. The positioning of the LVDT is shown in Figure 2(a). White paint was used to cover the beams before testing, and a 50 mm × 50 mm mesh over the beam surface was used to observe the development of cracks. The strains of the stirrup were recorded by strain gauges (Measurement type 120-3AA, see Figure 2(a)), and the strain gauges were provided by Zhejiang Huangyan Testing Apparatus Factory (Taizhou, China). A TDS-530 high-speed static data logger (Tokyo Sokki Kenkyujo Co., Ltd, Tokyo, Japan) was utilized to record the strain and deflection data.

Results and discussion

Crack patterns and failure mode

Figure 4 shows the failure mode and crack distribution for the tested beams. All beams failed due to a shear compression. The first small crack in the pure bending section perpendicular to the bottom of the beam was observed as the applied load increased. The crack extended, and a more cracks were observed in the pure bending section. A small oblique crack suddenly appeared in the middle of the section from the loading point to the connecting line of the support as the load was further applied, and the crack gradually extended from the middle of the connecting line to both sides of the connecting line as the load increased. At the same time, the crack width in the pure bending section continued to increase, but the crack did not expand upwards. The cracks in the shear bending stage continued to expand, the small crack around the main crack became longer, and the crack width became wider with the continuous application of the load. A "bang" sound was heard as the load approached the ultimate bearing capacity, and some concrete partially fell off.

Figure 4.

Failure modes of the tested beam: (a) B-0-0, (b) B-0-90, (c) B-0-180, (d) B-0-360, (e) B-24%-90; (f) B-24%-180, (g) B-48%-90, (h) B-48%-180.

Figure 4(a)–(d) show the crack distribution in the shear zone as a function of immersion time. Compared to those in the control beam (B-0-0), more inclined shear cracks were found for specimens immersed after 90, 180, and 360 days. As the inclined shear crack approached the level of the reinforcement, the CFRP grid started to slip from the concrete. The higher the deformation in the CFRP grid was, the higher the stress concentration leading to crack formation and greater the loss of bond stress at the interface (Issa et al., 2016). Our previous study results indicated that the tensile strength and tensile modulus retention of single-limb CFRPs were 89.8% and 96.4% after 360 days of immersion in seawater (Li et al., 2020), leading to a decrease in resistance to deformation.

Figure 4(e) and (f) show the coupled effects of 24%F_u and immersion time on the failure mode of the beam. Compared with those in the beam without a sustained load (B-0-90, B-0-180, Figures 4(b) and 4(c)), the number of inclined shear cracks decreased for the beam with 24%F_u. The results indicated that sustained loading improved the shear stress redistribution due to the improvements in the force transmission between the CFRP grid and concrete, and immersion for 90 or 180 days displayed limited effects on the failure mode. A large sustained load level reduced the force transmission between the CFRP grid and concrete, and the number of inclined shear cracks increased for the specimens with 48%F_u. As shown in Figure 4(g) and (h), the main inclined cracks were observed in the shear zone. Thus, 48%F_u and seawater immersion reduced the bond between the CFRP grid and concrete (Zhou et al., 2015; Dong et al., 2018; Lu et al., 2020).

Figure 5 shows evidence of the damaged interface of the CFRP-concrete. CFRP grid breakage was observed after the concrete was removed, and a high slip occurred at the longitudinal and transverse nodes.

Figure 5.

Node slip of the CFRP grid stirrup.

Load–Deflection curves

Figure 6 shows the load deflection behaviors for the tested beams. The applied load linearly increased with the midspan deflection prior to the onset of cracking in the pure bending section. At this stage, the initial stiffness (E₀) defined as the load, induced unit deformation. The initial stiffness varied with the immersion duration and the sustained load level. In the case of the conditioned specimens, the initial stiffness decreased after 90 days of immersion, followed by levelling off as the immersion duration increased. Compared to the initial stiffness of the control beam, the initial stiffness of beams immersed after 90, 180, and 360 days was reduced by 8.6%, 22.3% and 22.8%, respectively.

Figure 6.

Mid-span deflection of all specimens.

The coupling of seawater immersion and sustained loading increased the initial stiffness. The initial stiffness of the beams with 24%F_u and immersed for 90 and 180 days increased by 21.3% and 31.7%, respectively, while the initial stiffness of the beam with 48.0%F_u increased by 42.9% and 28.3%, respectively. Thus, sustained loading improved the initial stiffness. The higher the sustained load level was, the less insignificant the effects of sustained loading on the initial stiffness. The improvement of the stiffness was beneficial to the compactness of the concrete under the sustained load, a high sustained load was led to cracking at the bottom of the beam, and the initial stiffness decreased for the specimens after 180 days of immersion (Jia et al., 2022; Gabet et al., 2008).

Once cracks occurred in the pure bending section, the load nonlinearly increased with deflection, and an obvious slope change was observed from the load-deflection curves. In this nonlinear stage, the secant of the curve was lower than that in the previous stage, which was attributed to the crack propagation in the flexural and shear zones of the concrete. Part of the concrete was nonfunctional after cracking, and some of the beam stiffness was lost, resulting in a reduction in the stiffness of the beam. The load corresponding to the change in the stiffness was the cracking load.

The loads associated with the appearance of the first flexural crack and shear crack are listed in Table 2. In the case of specimens immersed in the simulated seawater, both flexural cracks and shear cracks decreased in abundance with an increase in the immersion duration. Compared to that of the control specimens, the crack load of the pure bending section after 90, 180, and 360 days immersion decreased by 19.0%, 38.1% and 42.9%, respectively, and the cracking deflection decreased by 26.7%, 40.0% and 33.3%, respectively. This was attributed to the improvement in the compressive strength of the beam in seawater (increased from 36.5 MPa to 39.2 MPa after 360 days of immersion), mainly due to the further hydration reaction in seawater; however, the brittleness of the concrete increased (Hu et al., 2022b). The higher the compressive strength was, the more brittle the concrete.

Table 2.

Details of the test results.

Specimen	Sustained load level	Immersion period (days)	Initial stiffness (kN/mm)	Cracking load (kN)		Ultimate load (kN)	Failure deflection (mm)
Specimen	Sustained load level	Immersion period (days)	Initial stiffness (kN/mm)	Pure bending Section	Shear bending Section	Ultimate load (kN)	Failure deflection (mm)
B-0-0	0	0	75.5	105	175	259.4	6.7
B-0-90		90	69.0	85	165	285.8	7.1
B-0-180		180	58.7	65	123	280.6	7.8
B-0-360		360	58.3	60	130	285.9	10.1
B-24%-90	24%F_u (62 kN)	90	91.6	95	235	321.4	12.6
B-24%-180	24%F_u (62 kN)	180	99.4	145	155	262.7	5.9
B-48%-90	48%F_u (124 kN)	90	107.9	135	165	322.0	7.0
B-48%-180	48%F_u (124 kN)	180	96.9	125	155	284.1	6.4

In the case of specimens exposed to sustained loading and seawater immersion, the cracking load increased, and the increase in the cracking load was larger than that of the specimens only immersed in seawater. Compared to that of the specimens immersed in seawater, the crack deflection corresponding to the cracking load increased due to the application of the sustained load. In the case of the coupled sustained loading and seawater immersion after 180 days of exposure, the crack deflection of specimens with 24%F_u and 48%F_u increased by 30.0% and 50.0%, respectively.

The failure deflection of the beam increased from 6.7 mm (B-0-0) to 7.1 mm, 7.8 mm, and 10.1 mm after 90, 180, and 360 days of immersion, respectively (see Table 2). The results were in accordance with the failure mode, associated with CFRP and the interface of CFRP-concrete damage (Lu et al., 2020; Pan and Yan, 2021). Regarding the effects of sustained loading, the failure deflection of the beam decreased from 7.8 mm (B-0-180) to 5.9 mm and 6.4 mm for specimens B-24%-180 and B-48%-180, respectively. This may be mainly because the beam previously underwent a certain elastic deformation under the action of sustained loading, therefore the elastic deformation decreased during the loading process, the stiffness increased rapidly, and the failure deflection decreased.

Strain on the stirrup

Figure 7 shows the strain variation in the CFRP grid for the beam under various conditions. The tensile strain on the stirrup did not vary as the shear load capacity increasing before the cracking load was reached. Thus, shear load capacity was mainly supported by the concrete, and the contribution of the stirrup to the shear load capacity was small before the cracking load was reached in the pure bending section. Figure 7(a) shows the effects of seawater immersion on the tensile stain of the CFRP grid stirrup. The ultimate strain on the stirrup increased with the shear load capacity after cracking. Therefore, the contribution of the stirrup to the shear load capacity increased with the propagation of diagonal cracks. Due to the degradation of CFRP, the strain of the stirrup decreased faster as the immersion time increased (Li et al., 2020).

Figure 7.

Effects of immersion (a) and sustained loading (b) on the tensile strain of the stirrup.

Figure 7(b) shows the effects of sustained loading on the tensile strain of the stirrup for the specimens immersed in seawater for 90 or 180 days. The effect of 24%F_u on the tensile strain of the stirrup with loading was insignificant after 90 days of immersion, while the 48%F_u significantly influenced the tensile strain of the stirrup. For 24%F_u, the bond between the CFRP grid and concrete was enhanced, which resulted in an increase in the bond stiffness. The larger the bond stiffness was, the lower the tensile strain on the CFRP grid stirrup under the same applied load. When the sustained load was increased to 48%F_u, the bond between the CFRP grid and concrete deteriorated, which made the material susceptible to sliding, as can be observed for the beams immersed for 90 or 180 days in Figure 7(b).

Evolution of the shear load capacity

Figure 8(a) shows the effects of seawater immersion on the shear load capacity for the beam without sustained loading. The shear load capacity increased with increasing immersion time. Compared to that of the control specimen, the shear load capacity increased by 10.0%, 8.2% and 10.2% for the specimens after 90-, 180- and 360-days immersion, respectively. The increase in the shear load capacity may be attributed to the enhancement of the bond between the CFRP grid and the concrete, increasing the compressive strength of the concrete (Dong et al., 2018; Yuan et al., 2022). The shear load capacity after 180 days of immersion can be attributed to the degradation of the CFRP surface. However, the water absorption and swelling of the CFRP, as well as the chemical reactant deposition at the CFRP-concrete interface after 360 days of immersion, resulted in a slight improvement in the shear performance of the beam (Pan et al., 2023).

Figure 8.

Effects of immersion (a) and sustained loading (b) on the shear load capacity.

Figure 8(b) shows the effects of sustained loading on the shear load capacity at various immersion times. Compared to those for the specimens after 90 days of immersion without sustained load, the 24%F_u and 48%F_u conditions increased the shear load capacity by 12.5% and 12.7%, respectively. Compared to those for the specimens after 180 days of immersion without sustained load, the 24%F_u conditions reduced the shear load capacity by 6.4%, and the 48%F_u conditions increased the shear load capacity by 1.2%. Thus, the sustained load was beneficial for improving the performance of the beam during short-term exposure. However, the degradation of the CFRP and its interface with concrete led to performance degradation as the exposure time increased.

In comparisons of the shear load capacity of the control specimens and the conditioned specimens as listed in Table 2, the coupling of sustained loading and immersion increased the shear load capacity. Compared to the control specimen, B-24%-90 and B-48%-90 displayed shear load capacity increases of 23.9% and 24.1%, respectively, but after 180 days of immersion, B-24%-180 and B-48%-180 displayed increases of only 13.0% and 9.5%, respectively.

Determination of the shear load capacity

The shear load capacity of the FRP-bar-reinforced concrete beam with FRP stirrups, can be expressed as follows:

V_{u} = V_{c} + V_{f}

(2)

Where V_u is the ultimate shear capacity of the FRP-reinforced concrete beam and V_c and V_f are the concrete shear strength and CFRP grid stirrup shear strength, respectively.

According to the code of ACI 440.1R-15 (ACI440.1 R, 2015) and GB 50608-2010 (GB506080, 2010), the shear load capacity of FRP stirrups perpendicular to the axis of the beam can be expressed as follows:

V_{f} = \frac{A_{f v} f_{f v} d}{s}

(3)

Where A_fv is the amount of FRP shear reinforcement with spacing s. f_fv is the tensile strength of FRP for shear design based on, the smallest of the designed tensile strength, strength of the bent portion of FRP stirrups, or stress corresponding to 0.004E_f. d is the distance from fibers under extreme compression to the centroid of the tension reinforcement. Failure of the FRP stirrups was caused by the node slip of the CFRP grid. The maximum tensile strains of the CFRP grid stirrups are listed in Table 3. The shear load capacity of the FRP stirrups was determined with equation (3), and the results are plotted in Figure 9(a).

Table 3.

Prediction of the shear strength of the reinforced beam with CFRP grid stirrups.

Specimen	Elastic modulus of CFRP grid stirrups (GPa)	Maximum tensile strain of CFRP grid stirrups (με)	Predicted CFRP stirrup shear load determined by equation (3) (kN)	Predicted concrete shear load determined by equation (4) (kN)	A	B
B-0-0	143.4	2800	21.4	110.2	1.25	-
B-0-90	145.3	680	5.3	110.2		-
B-0-180	141.1	350	2.6	110.2		-
B-0-360	138.3	500	3.7	110.2		-
B-24%-90	145.3	240	1.9	110.2	-	755
B-24%-180	141.1	360	2.7	110.2	-	755
B-48%-90	145.3	1900	14.7	110.2	-	1012
B-48%-180	141.1	2000	15.0	110.2	-	1012

Note: The elastic modulus of the CFRP grid stirrups was obtained from Biao Li et al. (2020).

Figure 9.

Shear load capacity of (a) CFRP grid stirrups and (b) concrete.

Compared to that of the control specimens, the CFRP stirrup shear strength was reduced by 75%, 88% and 83% after 90-, 180-, and 360-days immersion, respectively. The CFRP stirrup shear strength of specimens at 24% of the ultimate load was reduced by 91% and 87% after 90 and 180 days of immersion, respectively, and the shear load capacity of specimens at 48% of the ultimate load was reduced by 31% and 30% after 90 and 180 days of immersion, respectively. The reduction in the CFRP stirrup shear strength was attributed to the reduction in the maximum tensile strain of the CFRP grid stirrups, as shown in Table 3.

It was previously reported that concrete shear strength calculated based on the ACI 440.1R-15 (ACI440.1 R, 2015) and GB 50608-2010 (GB506080, 2010) codes is conservative. Ahmed H. Ali et al. (Ali et al., 2021) studied a total of 510 FRP-reinforced beams without stirrups. A unified mode was proposed, and it was proven to be more refined and unified than traditional modes. The concrete shear strength was determined as:

V_{c} = 0.35 λ_{s h} {(E_{f} ρ_{f})}^{1 / 4} {(f_{c}^{'})}^{1 / 4} {(\frac{1}{1 + 0.005 d})}^{1 / 2} {(\frac{d}{a})}^{λ_{a}} b d

(4)

Where λ_sh is the cross-section shape factor and equals to one for a rectangular cross section. λ_a is a shear mechanism factor set as one and 1/3 for beams with a/d ≥ 2.5 and a/d < 2.5, respectively. E_f is the tensile modulus of the CFRP grid stirrups, where ρ_f is the fiber-reinforced polymer reinforcement ratio. a is the shear span. d is the effective depth of the beam.

According to the results obtained with equation (4), the concrete shear strength was 110.2 kN, and this value was compared with the test results in Figure 9(b). Figure 9(b) shows that the concrete shear strength of the control specimens was accurately predicted by equation (4), and the shear load capacity of the conditioned specimens with concrete was larger than that predicted by equation (4). This was attributed to the increase in the shear strength of the reinforced concrete beam and the reduction in the CFRP gird stirrup shear strength. This phenomenon was caused by the water absorption of the CFRP grid stirrups, and it resulted in the swelling of the CFRP grid stirrups, followed by a stronger bond between the CFRP grid stirrups and concrete. The conditioned specimens showed a more uniform distribution of diagonal cracks than the control specimens. The concrete shear strength increased by 27%, 27% and 29% after 90 days, 180 days, and 360 days of immersion, respectively. The sustained load deteriorated the interface bond between the CFRP grid stirrups and concrete, and it resulted in the concrete shear strength of specimens with a sustained load in seawater being less than that not in seawater. In the case of the species with a sustained load in seawater, compared to the specimens after 180 days of immersion, those at 24% and 48% of the ultimate load displayed reductions in the shear load capacity of 30% and 36%, respectively.

In the case of the specimens in seawater, Figure 9(b) shows that of the average tested concrete shear strength was 138 kN with a margin of error of 1 kN. To take the contribution of seawater immersion to the concrete shear strength into account, equation (5) was proposed as follows:

V_{c t} = A \times V_{c}

(5)

Where V_ct is the conditioned concrete shear strength. A is a constant parameter and is fitted based on the test results. t is the duration of the test. The value of A is determined to be 1.25, and it is reasonable to take the influence of water immersion into account. Form the study of (Dong et al., 2018), the shear resistance capacity of concrete increased by 1.1 times.

Figure 9(b) shows that the concrete shear strength decreased with increasing of immersion time. Referring to Wang et al. (2017a) and Chen et al. (2006), equation (6) was proposed to determine the concrete shear strength with immersion and sustained loading as

V_{c t} = \frac{1}{\exp ((- t + 90) / B)} \times V_{c}

(6)

Where V_ct is the conditioned concrete shear strength. B is a constant parameter and is fitted based on the test results. t is the duration of the test. The values of B are listed in Table 3.

Figure 10 shows the relationship between the test results and the calculated concrete shear strength determined with equations (5) and (6). In the case of the specimens with a sustained load in seawater, R² = 0.86 and 0.92 indicate that the proposed equations accurately predict the concrete shear strength.

Figure 10.

Comparison of the test results and the predicted results.

Conclusions

An experimental study was conducted to investigate the effects of the combination of sustained loading and seawater immersion on the durability of RC beams with CFRP grid stirrups. Based on the results of this study, the following conclusions were drawn:

(1) All specimens failed due to shear compression. Failure was caused by the node slip of the CFRP grid stirrup. More diagonal cracks were observed in the conditioned beam than in the control specimens.

(2) The initial stiffness of the conditioned beam decreased after immersion. The coupling of seawater immersion and sustained loading increased the initial stiffness, and the initial stiffness for 48%Fu was less than that for 24%Fu.

(3) The crack load decreased with increasing immersion duration. The cracking loads of all specimens under coupled sustained loading and seawater immersion increased, and the extent of the increase was larger than that for the specimens in seawater alone.

(4) The contribution of the CFRP grid stirrup to the shear load capacity was small before the cracking load was reached, and the tensile strain of the CFRP grid stirrup increased after the cracking load was reached. The ultimate tensile strain of the CFRP stirrup decreased with immersion time. The effects of 48%Fu on the ultimate tensile strain of the CFRP stirrup were more significant than those of 24%Fu.

(5) The shear load capacity increased with increasing immersion time. The sustained load increased the shear load capacity, and the contribution of sustained load to the shear loading capacity decreased with increasing immersion time and level of the sustained load.

(6) A prediction equation was proposed to determine the concrete shear strength of reinforced beams with CFRP grid stirrups in seawater. The prediction results roughly match the test results.

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: This work was supported by the National Natural Science Foundation of China (Nos. 51808047, 52078141 and 12032009).

ORCID iD

Jun He

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