Experimental study on flexural behavior of damaged reinforced concrete (RC) beam strengthened by toughness-improved ultra-high performance concrete (UHPC) layer

Composites Part B 186 (2020) 107834

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Experimental study on flexural behavior of damaged reinforced concrete (RC) beam strengthened by toughness-improved ultra-high performance concrete (UHPC) layer Yang Zhang a, Xingliang Li a, Yanping Zhu a, b, *, Xudong Shao a a b

Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China Architectural, Environmental and Civil Engineering, Missouri University of Science and Technology, MO, 65401, United States

A R T I C L E I N F O

A B S T R A C T

Keywords: Ultra-high performance concrete (UHPC) Damaged reinforced concrete (RC) beam Strengthening Toughness Flexural behavior Ultimate flexural capacity

The flexural test was conducted to investigate crack resistance, loading bearing capacity, deformation charac­ teristics and failure modes of damaged reinforced concrete (RC) beams strengthened by reinforced ultra-high performance concrete (UHPC) layer (Abbreviated as UC). Also, their mechanical properties were compared with those of the unstrengthened RC beam (Abbreviated as I–C). Additionally, the effects of pre-damage degrees in the RC beams and three strategies for the improvement of toughness of the reinforced UHPC layer on the cracking and flexural performance of UC were discussed. The results showed that UC acted monolithically under flexure without interfacial debonding before typically flexural failure. Compared with I–C, cracking and ultimate loads of UC increased by 1.57–3.32 times and 1.72–2.21 times, respectively. The reinforced UHPC layer effec­ tively suppressed the cracking of the RC beam, making crack width in the RC beam propagate slowly with the load. Moreover, the severer the pre-damage degree of the RC beam, the smaller improvement of the flexural performance of UC. The addition of steel wire mesh, orientation of steel fibers and moderate-temperature steam curing further improved the cracking and flexural performance of UC with the most evident improvement in the addition of steel wire mesh. Finally, the theoretical model and formula were proposed for the calculation of load bearing capacity of UC with consideration of the influence of the pre-damage degrees in the RC beams, and the experimental results verified that the theoretical formula can accurately predict the load bearing capacity of UC.

1. Introduction Reinforced concrete (RC) structures have been widely subjected to external (overloading and cyclic-loading) and internal (corrosion and aging) challenges which make RC structures often encounter damages [1,2] and performance degradations during their operation, resulting in a variety of safety risks and a reduced service life [3,4]. Therefore, it is essential to select economical and durable materials for the rehabilita­ tion and strengthening of the damaged RC structures to extend their service life [5,6]. Recently, an advanced cement-based composite ma­ terial, ultra-high performance concrete (UHPC), has been developed [7, 8]. Compared with normal strength concrete (NSC), UHPC exhibits higher compressive and tensile strengths, and excellent impact and fa­ tigue resistance [9]. Due to a large number of uniformly distributed steel fibers in the matrix [10], tensile toughness of UHPC is significantly improved [11–15]. Moreover, UHPC has low water and gas

permeability, and high durability [16,17]. In particular, microcracking with a crack width less than 0.05 mm, does not affect the permeability and durability of UHPC structures [18]. In addition, after heat curing, UHPC reaches the desired tensile strength (>7 MPa) and compressive strength (>150 MPa) at the early age with low shrinkage and creep coefficients at the late stage [19,20]. Because of these excellent me­ chanical properties of UHPC as abovementioned, using UHPC to strength the damaged RC structures might enhance the mechanical performance and durability of UHPC-RC composite structures [21,22]. Cast-in-place UHPC overlay was first used to strengthen RC struc­ tures in Switzerland and North America [23,24], and gradually became popularized. Brühwiler [25], Habel [26,27], Oesterlee [28], Prabhat [29,30], Safdar [31], Spyridon [32] and Yin [33] carried out experi­ mental studies and numerical analyses on the flexural behavior of RC beams strengthened by UHPC layer. The influence of many parameters on the flexural capacity, stiffness and crack resistance of the

* Corresponding author. Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China. E-mail address: [email protected] (Y. Zhu). https://doi.org/10.1016/j.compositesb.2020.107834 Received 17 August 2019; Received in revised form 6 December 2019; Accepted 29 January 2020 Available online 31 January 2020 1359-8368/© 2020 Elsevier Ltd. All rights reserved.

Y. Zhang et al.

Composites Part B 186 (2020) 107834

strengthened beams was investigated, such as different thicknesses of the UHPC layer, different pre-damage degrees of the RC beam (80%, 90% of ultimate load of the control RC beam), reinforcement ratios and yield strengths of tensile steel embedded in the UHPC layer, tensile properties of UHPC and restrained shrinkage of the UHPC layer. The results indicated that the reinforced UHPC layer significantly restrained the crack development and improved the flexural capacity, stiffness and cracking resistance of the RC beam; good intactness of UHPC-RC com­ posites was proven at the interface without debonding occurred prior to the failure of the composite members; carrying load capacity and stiff­ ness of the strengthened structure increased with the increase in depth of the UHPC layer, reinforcement ratio, and yield strength of steel rein­ forcement in the UHPC layer; the slighter the pre-damage degree of the RC beam, the higher the ultimate bearing capacity of the strengthened beam. Moreover, the early autogenous shrinkage of UHPC was relatively high. However, after normal curing for 90 days, the hydration reaction almost stopped and mechanical properties of UHPC remained un­ changed. Therefore, the time-dependent behavior of the strengthened structure related to the autogenous shrinkage mainly occurred within 90 �rio [35] carried out days after casting of UHPC. Noshiravani [34] and Ma experimental studies to investigate the rotation capacity, stress redis­ tribution and ultimate bearing capacity of the UHPC-RC composite beams under combined flexure and shear forces. The results showed that the reinforced UHPC layer improved the ultimate bearing capacity, ductility and rotation capacity of the composite beams as well as the shear capacity to a certain extent. Tohru [36] and Murthy [37] con­ ducted flexural fatigue tests on the UHPC-RC composite structures. The fatigue failure modes and damage characteristics were obtained. Moreover, a finite element model was proposed to predict S–N curve and load-displacement behavior of the strengthened beam with consider­ ation of the pre-damage degree in the RC beam, fracture behaviors of NSC and UHPC, and elastoplastic behavior of the reinforcing steel. The results showed that the UHPC layer successfully strengthened the damaged RC beam and improved the fatigue resistance of the RC beam. No debonding was found at the interface during the test. In studies by Ming [38] and Al-Osta [39], the flexural properties of test beams with two different reinforcement techniques (by sand blasting RC beams surfaces and casting UHPC in-situ and by bonding prefabricated UHPC strips to the RC using epoxy adhesive) were investigated, and the bond strength test of UHPC and NSC interface with and without epoxy ad­ hesive, and the freeze-thaw test were carried out. The results showed that the UHPC layer restrained the crack development in RC; cracking and failure load as well as stiffness of the RC beams were obviously improved after strengthening of UHPC. Meanwhile, the interface of both techniques exhibited good bonding strength, and the integrity of the combined structure was guaranteed. To sum up, the above research demonstrates that the application of UHPC to strengthen damaged RC structures not only improves the cracking and flexural performance of RC structures, but also enhances the permeability and durability of RC structures as well as extends their service life. The improvement of tensile toughness of UHPC might further enhance the strengthening efficiency and structural durability, but the studies in this area are still insufficient. In addition, in the existing studies, the pre-damage degree of the RC beam is evaluated by the ratio of the preloaded load to the ultimate load. It is difficult to use this method to evaluate pre-damage degrees of RC components in the practical engineering. Also, the calculation models for flexural bearing capacity of UHPC-RC composite beams without consideration of the influence of the pre-damage degree in the RC beam may be unsafe in the case of severe damage in the RC beam. In response to these problems, the flexural capacity, stiffness, crack resistance and interface slip of damaged RC beams strengthened with toughened UHPC are studied by a four-point bending test in the present paper. Compared with the existing research, the significance of this study is as follows: (1) In order to cater to the actual project, the pre-damage degree in the RC beam is evaluated by the crack width (0.2 mm, 0.3 mm and 0.4 mm), and the effects of pre-

damage degrees in the RC beams on cracking and flexural performance of the strengthened beams are investigated; (2) For the improvement of toughness of the UHPC layer, four strategies are investigated, namely surface-roughened steel fibers used in UHPC for the replacement of the commonly used steel fibers, the addition of steel wire mesh in the reinforced UHPC layer, the orientation of surface-roughened steel fibers during casting of UHPC and moderate-temperature steam curing for UHPC. (3) Calculation models for flexural bearing capacity of UHPC-RC composite beams with consideration of the influence of the pre-damage degree in the RC beam are proposed. These models might provide guidance for the application of this emerging technology in the practical reinforcement engineering. 2. Experimental program 2.1. Materials In the experiment, NSC (Class C50) was designed according to the JTG 55–2011 [40]. NSC was made of cement, sand, gravel, water, and water reducer. UHPC was composed of cement, silica fume, fly ash, quartz powder, quartz sand, super plasticizers, and steel fibers with surface roughness treatment. For improving the tensile performance of UHPC, the total volume fraction of steel fibers (3%) included 1% of straight steel fiber with 8 mm in length and 0.12 mm in diameter and 2% of hooked-end steel fiber with 13 mm in length and 0.2 mm in diameter. The elastic modulus and tensile strength of steel fibers were 200 GPa and 2000 MPa, respectively. The water reducing agent, polycarboxylate water reducer, with a volume fraction of 1.5% was added in UHPC, and the water-reducing rate was higher than 30%. The mix proportions of UHPC are shown in Table 1. According to the relevant codes for concrete mechanical properties (GB/T50081-2002 [41] and GB/T31387-2015 [42] for NSC and UHPC, respectively), the tests for material properties were conducted and the material properties of NSC and UHPC were obtained. The compressive strength was obtained by cubic specimens with dimensions of 150 � 150 � 150 mm for NSC and 100 � 100 � 100 mm for UHPC, respec­ tively. Prism specimens with dimensions of 150 � 150 � 300 mm for NSC and 100 � 100 � 300 mm for UHPC were respectively fabricated to obtain elastic modulus. Meanwhile, the initial cracking strength and tensile stress-strain relationship of UHPC cured at normal temperature and toughened by moderate-temperature steam curing and orientation of steel fibers in the process of casting UHPC were obtained by the direct tensile test (Fig. 1). All test specimens for the material performance were prepared from the same batch used for the strengthened specimens. The curing conditions for the material test specimens were also consistent with that for the strengthened specimens. The number of specimens in each group was three for the material performance test. The measured mechanical properties of UHPC and NSC are shown in Table 2. The mechanical properties of steel reinforcement used in the experiment were obtained by the tensile test [43]. The yield strengths of steel bar (HRB400) used in the test beams were 472.5 MPa with 10 mm in diameter and 465 MPa with 16 mm in diameter. Both of them had an Table 1 Mix proportions of UHPC. Ingredient

Amount(kg/m3)

Portland cement Silica fume Fly ash Quartz sand Quartz powder Polycarboxylate water reducer Water Steel fibers (3%Vol.) W/P Ratio

771.2 154.2 77.1 848.4 154.2 20.1 180.5 235.5 0.18

Note: W/P ratio means the weight ratio of water to paste material. 2

Y. Zhang et al.

Composites Part B 186 (2020) 107834

elastic modulus of 198.6 GPa. The yield strength of the steel wire mesh added in the UHPC layer was 275 MPa. 2.2. Test specimens In the present study, thirteen identical RC beams were constructed and used for the evaluation of the flexural performance of RC beams strengthened with UHPC layer. One RC beam was used as a baseline without strengthening, while twelve RC beams were strengthened with the UHPC layer at the tensile side with various parameters. As shown in Fig. 2, the total length of the RC beam was 2300 mm, having a rectan­ gular cross-section with a height of 300 mm and width of 200 mm. The RC beam was reinforced at the tensile side with three longitudinal rib­ bed steel bars with a diameter of 16 mm and a length of 2250 mm. On the contrary, two reinforcements with a diameter of 10 mm at the compressive side of the RC beam were used to support the stirrups. In order to avoid shear failure of the strengthened beams, the stirrups with a diameter of 10 mm was placed along the whole length of the RC beam. Specifically, the stirrups with a spacing of 100 mm were placed at pure bending region, while the spacing became 70 mm near the end of the RC beam. The UHPC layer, having a depth of 50 mm and a width of 200 mm was cast along the whole length of the tensile side of the RC beam. The UHPC layer was reinforced with one layer of steel reinforcement mesh with the longitudinal reinforcement spacing of 53 mm and the trans­ verse reinforcement spacing of 150 mm. All reinforcements in the UHPC layer had a diameter of 10 mm. The diameter of additional steel wire mesh was 3 mm and the size of mesh hole was 25 mm � 25 mm. The details of the test beams with strengthening parameters are given in Table 3. One RC beam was used as a control beam labeled as I–C

Fig. 1. Experimental results of the direct tensile tests. Table 2 Mechanical properties of UHPC and NSC. Materials

Compressive strength (MPa)

First cracking strength (MPa)

Modulus of elasticity (GPa)

N–U H–U N–U–O NSC

145 164 147.5 51.0

8.1 9.3 11.6 /

43.2 46.3 45.0 34.8

Note: H–U, N–U, N–U–O indicate moderate-temperature steam cured UHPC, normal-temperature cured UHPC, fiber-oriented UHPC, respectively.

Fig. 2. Detailed scheme for the test specimen (unit: mm). 3

Y. Zhang et al.

Composites Part B 186 (2020) 107834

for the evaluation of the mechanical properties of the unstrengthened RC beam. The remaining of twelve test beams are divided into six groups according to the test parameters, and the two test beams with the same test parameter are in each group. The test parameters include the curing conditions for UHPC, the pre-damage degrees for the RC beam and the different strategies for the improvement of toughness of the UHPC layer. In Table 3, UC represents the RC beams strengthened with reinforced UHPC layers in the tensile side of the RC beams. H, S and O represent moderate-temperature steam curing for UHPC, placement of steel wire mesh in the UHPC layer and orientation of steel fibers in the process of casting UHPC, respectively. Subscripts 0.2, 0.3 and 0.4 respectively represent different pre-damage degrees in the RC beams, corresponding to the maximum crack widths of 0.2 mm, 0.3 mm and 0.4 mm in the RC beams at the highest preload values. Except for the curing condition of H-UC0.2 with steam curing at 60� C for 72 h, other test beams were cured at normal temperature for 28 days after casting UHPC layer. It should be noted that if UHPC is cured by high-temperature steam with a temper­ ature of 90� C for 48 h in the actual reinforcement project, the temper­ ature gradient distributed along the section height exists in the original RC structures during the high-temperature steam curing. Resulting high temperature stress might lead to further cracking in the original damaged RC structures. Therefore, a moderate temperature of 60� C was used in the steam curing in this study.

UHPC casting. When UHPC flowed through the U-shaped guiding groove, the steel fibers were oriented along the longitudinal direction of the RC beam due to the wall effect (Fig. 3(f)). (5) Curing (Fig. 3 (g)): The surface of the UHPC was covered by plastic wraps, sprayed with water periodically for 48 h, and then demoulded. After that, the test beam was cured at room temperature for 28 days, or cured by steam for 72 h at a temperature of 60� C. 2.4. Test setup and instrumentation Fig. 4 shows the test setup and arrangement of measurements in the test beam. The four-point flexural test was conducted for all test beams loaded by a hydraulic jack. A distributed steel beam was placed in the middle span to achieve pure bending region with a distance of 700 mm. The distance between the two supports of the test beam was 2100 mm. In the initial loading stage, at each load step, the load increment was 8 kN. When the load was close to the cracking load, it became 4 kN per step to obtain the cracking load accurately. Then, the load increment became 10 kN per load step after the RC beam cracked in the composites. When the load increased to 80% of the theoretical peak load, a displacement-controlled loading was conducted with 0.1 mm of deflection in the midspan per step. The test was terminated when the applied load dropped below 80% of the test peak load. Vertical dial gauges were installed on the top at the support and the bottom of the test beam in the middle to measure the deflection. Horizontal dial gauges were installed at the interface between UHPC and RC layers to measure the interface slip. Strain gauges were adhered on the top, bottom, and side surfaces of the test beams as well as the surface of tensile steel bars for measuring their strains. Meanwhile, the extensometers were attached on the bottom surface of UHPC layer and the side surface of RC beam near the interface to record the tensile deformation after the UHPC layer and RC beam cracked.

2.3. Fabrication and preparation of test specimens The procedures for the fabrication of pre-damaged RC beams strengthened by UHPC layer are shown in Fig. 3. (1) The RC beams were cast and cured at laboratory temperature for more than 90 days (Fig. 3 (a)). (2) Preloading (Fig. 3 (b)): In order to simulate the pre-damage in the RC beam, the RC beam was preloaded under flexure before strengthening. The pre-damage degrees were evaluated by the maximum crack widths of 0.2 mm, 0.3 mm and 0.4 mm introduced in the RC beams at the highest loads during preloading. To ensure that the preset crack width accurately reached, on the one hand, a device for measuring the crack width was used and several cracks were monitored at each load step in the process of preloading; on the other hand, when the crack width was close to the target value, the load increment at each load step was reduced as much as possible (i.e., 1 kN). If one of the cracks reached the target value, the loading was stopped immediately. (3) Substrate processing (Fig. 3 (c)): After preloading, the tensile surface of the RC beam was roughened with the macrotexture depths of about 2.5–5.0 mm. The contacting surfaces were cleaned of any debris by highpressure water guns before casting UHPC around it, spraying water for more than 24 h to keep the NSC substrate fully moist to minimize the water loss in the UHPC due to absorption by the unsaturated RC beam. (4) Casting UHPC layer (Fig. 3(d)): After substrate processing, the reinforcement mesh and additional steel wire mesh (if needed) were placed at the top of the RC beam and then UHPC was cast around it. The steel fibers in UHPC were subjected to surface roughness treatment (Fig. 3(e)), i.e. the steel fibers were immersed in the aqueous solution of zinc phosphate and phosphoric acid for 15 min at 85� C, and then dried out. After that, the treated steel fibers were added into the UHPC premix and stirred for 15 min before UHPC was cast. To achieve orientation of steel fibers, the baffles with a groove width of 3 cm were used during

3. Experimental results and discussion 3.1. Failure mode and characteristic load The comparison of crack patterns and failure modes between the intact beam (I–C) and the strengthened beams (UC) is shown in Fig. 5. It can be seen that failure modes for all the test beams were similar with typically flexural failure. Compressive normal strength concrete (NSC) at the top of UC crushed while the wide main cracks appeared on the bottom of the UHPC layer. The UC acted monolithically under bending before the composite beams failed. At failure, some longitudinal cracks locally appeared in the RC side near the interface in the test beams. From Fig. 5, the wider and sparser cracks were observed at the bottom of I–C at failure. In I–C, most of the cracks ran through the whole beam width with crack widths more than 0.2 mm. When I–C was close to the failure, three cracks in the pure bending region in the middle span rapidly widened, and finally became the main cracks. However, for UC at fail­ ure, the cracks on the bottom surface of the UHPC layer were short with a dense distribution, and only one main crack appeared on the UHPC layer. The widths of cracks were small, but a large number of uncon­ nected cracks were observed. The crack development process in the tested beam is presented in Fig. 6. The crack development of I–C is divided into three stages. Stage one: When the load reached about 15.4% of the ultimate load (Pu ), the first visible crack with a width of about 0.02 mm appeared in the tensile side of the pure bending region in the middle span of I–C. Stage two: With the load increase, the number of cracks in the tensile zone increased with their length and width increasing also. When the load was about 46.8%Pu , the main crack with a width of 0.2 mm appeared in the pure bending section. Stage three: As load increased continuously, the width and length of cracks developed rapidly while the number of cracks did not increase any more. Compressive NSC at the top was crushed at Pu . Meanwhile, three main cracks with a width up to 1.2 mm

Table 3 Description of test specimens. Samples

Curing

Material

I–C UC0.2 UC0.3 UC0.4 S-UC0.2 O-UC0.2 H-UC0.2

NC NC NC NC NC NC HC

RC RC-UHPC RC-UHPC RC-UHPC RC-UHPC-Steel wire mesh RC-UHPC-Fiber orientation RC-UHPC

4

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 3. Preparation of RC beams strengthened with UHPC.

Fig. 4. Test setup and measuring point arrangement.

Fig. 5. Failure modes of test beams.

were observed in the tensile zone. The crack development process in UC can be divided into five stages (Fig. 6). Stage one: When the load reached about 20%Pu , the first visible

crack with a width of about 0.01 mm occurred in the bottom surface of UHPC layer in the middle of the pure bending region. However, the flexural cracks in the damaged RC layer did not propagate. Stage two: 5

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 6. Crack patterns of test beams at each stage (Unit: mm).

6

Y. Zhang et al.

Composites Part B 186 (2020) 107834

When the load increased to 23.0%–34.1%Pu , the original preloading cracks in the damaged RC beam started to expand. A vertical visible crack with a width of about 0.02 mm near the loading point was observed in the RC layer. The width of cracks in the UHPC layer almost remained unchanged, but the number of cracks in the UHPC layer increased rapidly with a fine and dense distribution. Stage three: When the load increased to 26.4%–41.8%Pu , the crack width in the UHPC layer reached 0.05 mm. With the load increase continuously, the num­ ber of cracks in the RC layer increased rapidly while their widths did not increase dramatically (0.02 mm–0.03 mm). The cracks in the RC layer rapidly extended upwards to the top of the beam. The cracking position was basically consistent with the crack distribution in the damaged RC beam. Additionally, the crack developed slowly in the UHPC layer with an upward trend, and the number and width of cracks also increased. Stage four: When the load increased to 71.5%–92.8%Pu , about one or two cracks with a width of 0.20 mm appeared in the middle of the UHPC layer. Due to high tensile strength and strain hardening capacity of UHPC after cracking, the further development of cracks in the strengthened beam was delayed. Stage five: When the crack width in the UHPC layer exceeded 0.20 mm, one of the cracks developed as the main crack near the middle span with the width increasing rapidly. However, other cracks still developed slowly until the failure of the test beam. At failure, the maximum crack width in the RC layer was less than 0.3 mm and NSC was crushed in the compressive zone of the strengthened beam. The main crack width in the UHPC layer increased significantly with steel fibers pulled out at the interface of the main crack. However, the width of other cracks in the UHPC layer remained unchanged. More­ over, several longitudinal cracks appeared from the end of the main crack in the RC side along the interface. The characteristic loads of each test beam in bending are summa­ rized in Table 4, where based on Euler-Bernoulli beam theory, the nominal stresses at the moment of cracking of the bottom of the I–C and the UHPC layer of the UC are calculated. The average value of the nominal cracking stresses of two test beams in each group is shown in Fig. 7. It can be seen that compared to the I–C, the cracking and ultimate loads of all strengthened beams (UC) are significantly improved. The cracking load (Pucr ) of the UC is 1.39–3.33 times of that of the I–C and the maximum nominal cracking stress among UC is 10.5 MPa in S-UC0.2. Also, the nominal cracking stress in UC0.4 with the most severe predamage degree reaches 5.01 MPa, which is much higher than that of the I–C (3.85 MPa). The ultimate load (Pu ) of the UC is 71.4%–126.3% higher than that of the I–C. Two reasons might contribute to the improvement of the cracking and ultimate loads of the strengthened beams. One is due to ultra-high tensile strength, toughness and strain hardening behavior of UHPC, and high internal reinforcement ratio. The reinforced UHPC layer with excellent properties can effectively inhibit the crack development of the composite beams and improve their

Fig. 7. Comparison of nominal cracking stresses.

bending resistance. On the other hand, the sectional inertia moment increases due to the increase of the overall section height after strengthening with the UHPC layer. In summary, the pre-damage degree in the RC beam has a great in­ fluence on the bending resistance of the strengthened beams. The higher the pre-damage degree of the RC beam, the lower the cracking and ul­ timate loads of the corresponding strengthened beam. For the strengthened beams with the RC beams preloaded with crack widths of 0.3 mm and 0.4 mm, the cracking stresses are 20.5% and 37.5% lower than that of UC0.2 and the corresponding ultimate loads also decrease by 3.3% and 11.5%, respectively. However, the cracking and ultimate loads of the strengthened beams are further enhanced by placing steel wire mesh and orientation of steel fibers in the UHPC layer for the improvement of toughness. Compared with UC0.2, the cracking and ul­ timate loads increase by 31.1% and 13.0% for S-UC0.2, respectively, and 25.2% and 7.8% for O-UC0.2, respectively. Additionally, the cracking and ultimate loads of H-UC0.2 are 7.4% and 4.8% higher than that of the UC0.2, respectively. This is because the mechanical performance of steam cured UHPC is higher than that of normal-temperature cured UHPC [4]. In addition, the load (Pccr ) at which the crack in the tensile zone of the RC layer in the strengthened beam started to propagate is greatly improved with increase percent between 242% and 447% compared with the I–C. Therefore, it is clear that the crack resistance of the original damaged RC beam is enhanced after strengthened by UHPC. 3.2. Load-crack width relationship The maximum crack width of UHPC or NSC in the tensile zone was monitored at each load step, and the load-crack width curves are shown in Fig. 8. Fig. 8(a) shows that the main crack width in the UHPC layer varies with load. The cracking load of UHPC in the strengthened beam (UC) is much higher than that of the unstrengthened RC beam (I–C), while the development of the crack in the UHPC layer in UC was obvi­ ously slower. The load carried by UC is 2–3 times of that carried by I–C at the same crack width. From Fig. 8(a), at point A, the visible cracks appeared in the UHPC layer of UC. In the AB stage, the number of fine cracks rapidly and continuously increased as the load increase. How­ ever, the crack width of UHPC developed slowly with load at this stage. As UHPC shows good tensile toughness with the occurrence of multiple microcracks, the strengthened beam can obtain higher load bearing capacity with small crack width. After the point B, several adjacent cracks in the UHPC layer connected to form a main crack and the development of crack width in UHPC was obviously accelerated. When the crack width of UHPC exceeds 0.2 mm, the corresponding load rea­ ches about 85–95% of the ultimate load of the strengthened beams. When the crack width of UHPC exceeds 0.3 mm, the strengthened beams

Table 4 Measured characteristic loads (unit: kN). Specimen

Pcr

Pccr

Pu

I–C UC0.2-1 UC0.2-2 UC0.3-1 UC0.3-2 UC0.4-1 UC0.4-2 H-UC0.2-1 H-UC0.2-2 S-UC0.2-1 S-UC0.2-2 O-UC0.2-1 O-UC0.2-2

33.0 85.0 81.5 64.1 68.3 51.9 54.0 88.8 90.2 109.8 108.6 102.7 105.9

– 127.9 125 115.0 116.0 89.1 90.0 130.9 137.9 143.8 148.9 146.0 147.6

214.1 418.0 412.4 400.1 402.7 368.2 367.0 440.3 430.0 463.8 473.9 442.4 453.0

Note: Pcr , Pccr and Pu represent the cracking load of UHPC layer, the cracking load of the RC layer in the strengthened beam and ultimate load of the test beam respectively. 7

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 8. Load-maximum crack width curves: (a) in UHPC; (b) in RC.

are close to the ultimate state, that is, the crack width of UHPC develops rapidly, and the carrying load capacity of the beams is no longer increased. Comparing the load-maximum crack width curves in the strengthened beams with different parameters, the pre-damage degree of the RC beam has a great influence on the crack propagation of the UHPC layer. The more severe pre-damage degree in the RC beam, the quicker the development of crack width in the UHPC layer. However, the addition of steel wire mesh, fiber-oriented treatment or moderatetemperature steam curing, which can improve the toughness of the UHPC layer, further slows down the crack propagation of the UHPC layer during bending. It should be noted that the UHPC crack propa­ gation in S-UC0.2 is the slowest. This indicates that the addition of steel wire mesh is the most effective to restrain the crack propagation of the UHPC layer. From Fig. 8(b), the cracking load of the RC layer in the UC is obvi­ ously higher than that of I–C and the further development of cracks in the RC layer is effectively delayed. This is because the UHPC layer can effectively inhibit the occurrence of cracks in the RC beam due to the constraint of the reinforced UHPC layer with high tensile strength and strain hardening capacity. During the bending test of the strengthened beams, the main crack width in the tensile zone in the RC layer increases linearly with load. The crack propagation in the RC layer does not obviously accelerate before the failure. At the failure of the strengthened beam, the main crack width in the RC layer does not exceed 0.25 mm, while the main crack width in the I–C was more than 0.4 mm. This in­ dicates that the UHPC layer effectively suppressed the crack propagation of the RC layer during the whole loading process. In addition, with the increase in the pre-damage degree in the RC layer, the crack propagation speed in the RC layer of the corresponding strengthened beam is increased. However, the crack propagation speed in the RC layer of the beam strengthened by toughened UHPC layer (S-UC0.2, O-UC0.2 and HUC0.2) is slower than that of the UC0.2. For a clear comparison, the test loads corresponding to crack widths of 0.02 mm, 0.05 mm, 0.1 mm and 0.2 mm in the test beams are listed in Table 5. The varying trends of the loads versus characteristic crack widths are shown in Fig. 9. It can be seen that the cracking loads of the UC are obviously higher than that of I–C. The loads corresponding to crack widths of 0.02 mm, 0.05 mm, 0.1 mm and 0.2 mm are 57%–232%, 149%–404%, 236%–472% and 165%–333% higher than that of the I–C, respectively. Meanwhile, the loads develop linearly with the increase of crack widths from 0.01 mm to 0.2 mm in the UHPC layer. When the crack widths of the UHPC layer reach 0.05 mm, 0.1 mm and 0.2 mm, the averaged loads of the UC are 85.9%, 185.7% and 334.1% higher than the averaged cracking load of the UC, respectively. This indicates that the development of cracks in the UHPC layer did not weaken the car­ rying load capacity of the UC. The UC still have good bending resistance for a long time after the UHPC layer cracked.

Table 5 Cracking loads of the specimens corresponding to different crack widths (unit: kN). Specimen

Pucr =Pccr

P0:05

P0:1

P0:2

RC UC0.2-1 UC0.2-2 UC0.3-1 UC0.3-2 UC0.4-1 UC0.4-2 H-UC0.2-1 H-UC0.2-2 S-UC0.2-1 S-UC0.2-2 O-UC0.2-1 O-UC0.2-2

33.0 85.0 81.5 64.1 68.3 51.9 54.0 88.8 90.2 109.8 108.6 102.7 105.9

39.0 157.0 158.9 130.8 127.1 97.3 99.0 167.0 171.1 193.0 196.7 185.0 186.2

50.0 238.0 233.0 210.9 200.8 172.8 168.0 253.0 254.6 285.6 285.9 264.8 262.3

100.3 353.0 359.1 301.8 315.6 263.2 266.0 382.3 380.3 430.3 433.9 400.0 410.8

Note: P0:05 , P0:1 and P0:2 represent the test loads corresponding to the crack widths of 0.05 mm, 0.1 mm and 0.2 mm, respectively.

Fig. 9. Varying trends of load with characteristic crack width.

3.3. Load-deflection responses and ductility The load-midspan deflection curves of the test beams are shown in Fig. 10. Generally, in the elastic stage, the initial stiffness of the I–C is slightly higher than that of the UC. However, after the I–C cracked, the stiffness of UC gradually became greater than the I–C, and the increase of stiffness is more evident with load increase. This is because for UC, the preloaded damage in the RC layer reduces the overall stiffness. There­ fore, the slopes of load-deflection curves of UC are smaller than that of 8

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 10. Load-deflection curves.

the I–C in the elastic stage. After the I–C enters the stage of crack propagation, the stiffness of I–C decreased rapidly. However, the stiff­ ness of the UC does not decrease significantly prior to the ultimate load and the slopes of their curves are obviously higher than that of I–C at the middle and late loading stage. This is because the toughness of the UHPC layer gradually develops after cracking, and reinforced UHPC layer re­ strains the crack development in the RC layer. Also, the UHPC-RC composite section increases bending inertia moment. Generally, the load-deflection curves of the strengthened beams could be divided into five stages. Stage one: the curve develops linearly without UHPC cracking; Stage two: the slope of the curve decreases slightly, but it remains linear. Multi-cracking in the UHPC layer was observed in this stage. In the above two stages, the curve presents a large linear range with the stiffness of the UC slightly reduced; Stage three: the slope of the curve reduces obviously. The steel reinforcement in the UHPC layer of the test beam starts to yield. The development of deflection and crack width accelerates; Stage four and five: The curves enter the descending branch after the ultimate load reaches; the load decreases and the deflection increases rapidly. It should be noted that the descending branch of load-deflection curve of the I–C was not be tested because the I–C suddenly destroyed with NSC crushing at the ultimate load. However, most of the strengthened beams appear a hor­ izontal branch or a long descending branch after the ultimate load in the load-deflection curves. Comparing the load-deflection curves of the UC with different pa­ rameters, it can be seen that the flexural stiffness of the UC decreases with the increase in the pre-damage degree of the RC beam. When the load is below about 80 kN, the different pre-damage degrees in the RC beams (0.2 mm, 0.3 mm and 0.4 mm) have no obvious effect on the stiffness of the UC because the uncracked UHPC layer or cracked UHPC layer with small crack width restrains the crack propagation in the RC layer. With the increase of load, the UHPC layer begins to enter the crack development stage, and the restraint effect of the UHPC layer is grad­ ually weakened after cracking. Also, the deformation at the pre-cracking position in the RC layer gradually increases. At this time, the stiffness of strengthened beams (UC) starts to behave differently with different predamage degrees. The specific manifestations are as follows: the more severe the pre-damage in the RC beam, the smaller the stiffness of the corresponding strengthened beam; with the increase of load, the more evident reduction in stiffness is observed. In addition, the flexural stiffness of the UC (S-UC0.2, O-UC0.2 and H-UC0.2) with toughness improvement by placement of steel wire mesh, orientation of steel fibers and moderate-temperature steam curing respectively is slightly higher than that of the UC0.2 without any toughened strategies. This is because the improvement of the cracking strength and post-cracking toughness of the UHPC layer produces a stronger restraint on the propagation of

cracks in the damaged RC layer. In the early cracking stage of the UHPC layer, the stiffness of S-UC0.2, O-UC0.2 and H-UC0.2 is basically the same. In the middle and late stage, O-UC0.2 obtains a slightly higher bending stiffness. It is possibly because that the fiber orientation increases the number of steel fibers passing through the crack planes, and the crack development is strongly restrained by the fiber bridging effect, which effectively restricts the bending and tensile deformation caused by the propagation of cracks in the UHPC layer, and thus improves the stiffness of O-UC0.2. The ductility of the strengthened beams reflects the reserved car­ rying load capacity from the yielding load to the ultimate bearing ca­ pacity, and can be quantitatively evaluated by the ductility (μ). According to the literature [44], the ductility for the test specimens is defined as the ratio of deflections at the maximum load and at the yielding load as shown in Eq. (1). � μ ¼ ΔPu ΔPy (1) where ​ μ is the ductility; ΔPu andΔPy are the deflections at the maximum load and at the yielding load, respectively. The calculated ductility of the test beams were given in Table 6. In Table 6, the ductility of damaged RC beams strengthened with UHPC layer decreases compare with the intact RC beam. On the one hand, this is because a significant amount of main cracks formed and developed in the tensile zone of the I–C under bending increases the rotational deformation of the section and the deflection at the ultimate state, thus making the I–C have higher ductility. On the other hand, the strengthened beams have higher reinforcement ratio at the tension re­ gion and UHPC has high tensile toughness and strain hardening Table 6 Ductility of test beams. Specimen

ΔPu(mm)

ΔPy(mm)

μ

γ

I–C UC0.2-1 UC0.2-2 UC0.3-1 UC0.3-2 UC0.4-1 UC0.4-2 H-UC0.2-1 H-UC0.2-2 S-UC0.2-1 S-UC0.2-2 O-UC0.2-1 O-UC0.2-2

30.6 13.2 13.3 15.8 16.2 22.5 25.1 12.9 13.0 12.3 12.1 11.2 11.3

7.9 8.3 8.5 8.9 8.7 9.1 9.2 7.7 7.9 9.3 9.1 7.9 7.8

3.87 1.59 1.56 1.78 1.86 2.47 2.73 1.68 1.65 1.32 1.33 1.42 1.45

1 0.41 0.40 0.46 0.48 0.64 0.70 0.43 0.42 0.34 0.34 0.37 0.37

Note: γ is the ductility index of specimen, i.e., the ratio of μ between the strengthened beam and I–C. 9

Y. Zhang et al.

Composites Part B 186 (2020) 107834

capacity. Also, only one main crack formed in the reinforced UHPC layer after steel reinforcements in the UHPC layer yielded. These reasons cause a significant reduction in the deformation capacity of the com­ posite members. As a result, the strengthened beams exhibit a lower ductility. In addition, as can be seen from the comparison of the ductility among the strengthened beams, the more severe the pre-damage degree in the RC beam, the lower the stiffness of the corresponding strength­ ened beam, the greater the deflection at the ultimate load, and the higher the ductility (μ) of the UC. Moreover, the ductility of moderatetemperature steam cured strengthened beam (H-UC0.2) is slightly higher than that of normal-temperature cured strengthened beam (UC0.2). This might be explained as follows: from Fig. 10, it can be seen that the stiffness of H-UC0.2 before stage II is slightly higher than that of UC0.2 because the material properties of moderate-temperature steam cured UHPC are better than those of UHPC cured at room temperature, that is, the deflection of H-UC0.2 at yielding of the steel reinforcement is slightly smaller than that of UC0.2. However, the deflection of both at the limit state is basically the same, so the ductility of H-UC0.2 is slightly higher than that of UC0.2 based on the definition in equation (1). However, the ductility of S-UC0.2 with steel wire mesh and O-UC0.2 with oriented steel fibers is further reduced, which indicates that increasing the reinforcement ratio in the UHPC layer or orienting the steel fibers in the process of casting UHPC reduces the ductility of the strengthened beams.

This can be explained in two folds. On the one hand, reinforced UHPC layer in the UC carries part of the applied load. Even when the UHPC layer is cracked, UHPC and steel reinforcements inside can still carry part of the applied load. On the other hand, the reinforced UHPC layer restrains the crack development of the RC layer and the force carried by steel reinforcements in the RC layer increases slowly. As a result, the tensile stress in the steel reinforcement in the RC layer decreases, which effectively reduces or controls the crack width of the RC layer and greatly improves the cracking performance of the RC layer. However, for I–C, the obvious internal force redistribution occurs after NSC cracking. The tensile force carried by NSC is transferred to the tensile steel rein­ forcement. Therefore, the tensile strain of the steel reinforcement in the I–C is high. In addition, the pre-damage degree in the RC beam has obvious in­ fluence on the tensile strain of steel reinforcements in the UHPC and RC layers in the strengthened beam. The more severe the pre-damage de­ gree in the RC beam, the more tensile force undertaken by steel re­ inforcements in the UHPC and the RC layers due to the internal force redistribution. As a result, with more severe pre-damage in the RC beam, the tensile strain of steel reinforcements increases more quickly with the load and the cracking resistance of the strengthened beams decreases. However, for the UHPC layer toughened by the addition of steel wire mesh, orientation of steel fibers and moderate-temperature steam curing, the tensile strain of steel reinforcements in the UHPC and RC layers in the strengthened beams (S-UC0.2, O-UC0.2 and H-UC0.2) is smaller than that in UC0.2 under the same load. This is because the cracking strength and post-cracking toughness of UHPC are improved using these strategies compared to the UHPC layer without using any strategies for the improvement of toughness. Among them, the reduction in the tensile stain of steel reinforcements in both the RC and UHPC layers in the S-UC0.2 is the most evident. Overall, the tensile stress of steel reinforcements in the strengthened beam is reduced and resulting cracking resistance of the strengthened beam is improved with the UHPC layer toughened by different strategies. The strain gauges and extensometers mounted at the top surface of the RC layer and the bottom surface of the UHPC layer at the midspan recorded the compressive strain in the NSC and the tensile strain in the UHPC with the load. The corresponding load-strain curves are shown in Fig. 12. From Fig. 12, the compressive strain of NSC on the top of the strengthened beams is obviously smaller than that of the I–C during the whole bending process. When the NSC in the compression zone reaches the ultimate compressive strain, the load carried by the UC is much higher than that of the I–C. Therefore, the bending resistance of the strengthened beam is greatly improved. Generally, under the same load, the higher the pre-damage degree in the RC beam, the higher the strain in the top surface of the RC layer as well as in the bottom surface of the UHPC layer in the strengthened beam. Moreover, under the same load, the compressive strain of NSC and tensile strain of UHPC in the

3.4. Load-strain behaviors The load-strain relationships of tensile steel reinforcements in UHPC and RC layers of the strengthened beam are shown in Fig. 11. Overall, the tensile strain of steel reinforcements in UHPC and RC layers expe­ riences three stages with the load. (1) The elastic stage prior to cracking of UHPC or RC (before point A): the tensile strain of steel reinforcements in UHPC and RC layers increases linearly at this stage. (2) After the cracking of UHPC or RC layer (In the AB stage), the growth rate of tensile steel reinforcement strain begins to accelerate with the load. The slopes of the load-strain curves slightly decrease after point A. However, the steel reinforcements still work in the elastic range, and the strain curves develop linearly because UHPC maintains high tensile strength and toughness after cracking and the force applied on UHPC is slowly transferred to the steel reinforcement due to the bridge capacity of steel fibers. (3) Yielding of the steel reinforcement: when the load exceeds about 80% ultimate load at B point in the curves, a wide main crack forms in the UHPC layer and the longitudinal tensile steel re­ inforcements in UHPC and RC layers begin to yield in sequence. The strain of steel reinforcements increases rapidly and the curve tends to be gentle. Comparing the load-strain curves of the tensile steel reinforce­ ment in Fig. 11(a), the tensile steel reinforcement strain in the RC layer of the UC is obviously lower than that of the I–C at the same load level.

Fig. 11. Load-tensile reinforcement strain curves: (a) in RC beam; (b) in UHPC layer. 10

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 12. Load-concrete strain curves: (a) the top of RC beam; (b) the bottom of UHPC layer.

strengthened beams toughened through the steel wire mesh, orientation of steel fibers and moderate-temperature steam curing are lower than those of the strengthened beams (UC0.2) without any improvement in the toughness. The most evident reduction in the compressive strain of RC and the tensile strain of UHPC is observed in the S-UC with the steel wire mesh. It is worth noting that in most of the strengthened beams, the curves of load-compressive strain at the top of the RC layer show a descending portion after the NSC reaches the ultimate compressive strain (as shown in Fig. 12(a)). Moreover, as shown in Fig. 12(b) after UHPC cracking (A point), the tensile strain at the bottom of UHPC in­ creases linearly without obvious inflection point until the tensile strain of 4000με. This indicates that the reinforced UHPC layer exhibits good tensile strength and toughness after cracking. This also illustrates the reinforced UHPC layer in the strengthened beam is not destroyed in tension before the limit state, and still maintains tensile capacity.

without sudden changes. All the strengthened beams present a small slip less than 0.275 mm at failure. This might be due to a strong bonding strength at the interface between UHPC and RC. Therefore, the small interface slip has little effect on the monolithic composite action of the strengthened beams. Additionally, the pre-damage degree of the RC beam has a significant influence on the interface slip of UHPC-RC composite beams. The more severe the pre-damage degree in the RC beam, the quicker the devel­ opment of the interface slip. Moreover, three strategies for the improvement of toughness of the UHPC layer have certain influence on the interfacial slip of UHPC-RC. The growth rate of interface slip of HUC0.2 cured by moderate-temperature steam is obviously more rapid than that of the normal-temperature cured strengthened beam (UC0.2). This is because during the steam curing, large shrinkage occurring in the cast-in-place UHPC produces additional shear stress at the interface of H-UC0.2, weakening the bonding strength of the interface to a certain extent. However, the interface slips of S-UC0.2 with the addition of steel wire mesh and O-UC0.2 with the orientation of steel fibers, both are obviously smaller than that of UC0.2 without any toughened strategies at the middle and late loading stage. Moreover, the final value of interface slip of S-UC0.2 and O-UC0.2 is less than 0.15 mm. Interestingly, the slip in O-UC0.2 with orientation of steel fibers at the early loading stage (prior to point A) is significantly larger than that of UC0.2, although the interface slip develops slowly at the late loading stage. This might be explained as follows: in order to orient the steel fibers during casting of UHPC, the baffles were used to guide UHPC, and the majority of steel fibers were oriented along the longitudinal direc­ tion. Some steel fibers did not flow into the gap between coarse aggre­ gates on the roughened surface of the RC substrate. This leads to the decrease in the number of steel fibers distributed in the “transition layer” [45] at the interface between UHPC and RC, thus weakening interfacial bonding properties.

3.5. Load-slip curve in the UHPC-RC interface Dial gauges as arranged in Fig. 4 measured the slip at the UHPC-RC interface. Comparing the readings recorded by dial gauges in different locations along the interface, it is found that the maximum slip at the interface occurred near the end of the beam. The load-maximum slip curves are shown in Fig. 13. When the load reaches 7.9%–17.3% of the ultimate load of the UC, the UHPC-RC interface begins to slip. After that, the interface slip of UHPC-RC develops slowly with load. The load-slip curves of the majority of the strengthened beams change smoothly

4. Ultimate flexural capacity calculation of the strengthened beam From experimental observations, it is found that no debonding occurred at the interface of UHPC-RC composite beams before the fail­ ure and the interface slip was small prior to the ultimate load. Therefore, the effect of the interface slip between the UHPC and RC layers on calculation of the ultimate flexural capacity of the strengthened beam is ignored. Moreover, an ideal elastic-plastic bilinear model for the steel reinforcement under tension and compression is adopted as shown in Fig. 14 (a), the yield strength (fy ) of the steel reinforcement remains unchanged without strengthening stage after yielding. Moreover, because reinforced UHPC exhibits high tensile toughness and significant strain hardening characteristics [28,46,47], UHPC performs high

Fig. 13. Load-maximum slip curves. 11

Y. Zhang et al.

Composites Part B 186 (2020) 107834

curve and reloading curve of NSC. In Fig. 15, fc ; ε0 and εcu represent the cube compressive strength, the strain corresponding to fc and the ulti­ mate strain of NSC, respectively; εcp is the residual plastic strain of NSC in compressive zone after unloading; σ ci and εci are the stress and strain at the unloading point, respectively. � �� � � �2 8 εci εci εci ε þ 0:13 <2 0:145 > 0 < ε0 ε0 ε0 (2) εcp ¼ � � � � > : ε εci ε0 0:707 ci 2 þ 0:834 >2

ε0

ε0

According to the equation (2), to obtain the residual plastic strain εcp of NSC in compressive zone of the RC beam after unloading, it is necessary to obtain the compressive strain εci of NSC at the beginning of unloading. The compressive strain εci can be calculated through the strain of tensile steel εsti at the beginning of unloading by geometry relationship as shown in Fig. 16 (b). The strain of tensile steel εsti is calculated by the crack width of NSC in the RC beam under flexure as shown in equation (3) in JTG D62-2004 in Chinese [50].

Fig. 14. Constitutive models of steel bar and UHPC: (a) Tensile and compres­ sive steel reinforcement; (b) UHPC in tension.

carrying load capacity and plastic deformation capacity after cracking, and the ultimate strain εuu can be above 0.5% which exceeds the tensile yield strain of steel. Therefore, a bilinear model considering tensile strain hardening is adopted for modeling UHPC stress-strain relationship in this study as shown in Fig. 14 (b). It is assumed that the ultimate tensile strength is the same as the initial cracking strength (fut ) of UHPC. In Fig. 14(a), εy and εu represent yield strain and ultimate strain of the steel reinforcement, respectively. In Fig. 14(b), εut and εuu represent initial cracking strain and ultimate strain of UHPC, respectively.

εsti ¼

wt ð0:28 þ 10ρÞ C1 C2 C3 ð30 þ dÞ

(3)

where, wt represents maximum crack width of the RC beam at the beginning of unloading, ρ and d represent reinforcement ratio and diameter of longitudinal tensile reinforcement in the RC beam respec­ tively; C1 , C2 and C3 represent the shape coefficient of steel bar surface, the coefficient of the long-term loading effect and the coefficient related to the type of external force applied on the structure, respectively, and in this paper, the values of C1 , C2 and C3 ​ are 1.0, 1.5 and 1.0, respectively. After calculating εsti , the εci and εsci at the unloading point can be obtained by geometric relationship as shown in equation (4). The planesection assumption is adopted in the calculation model as shown in Fig. 16(b) and (c). � � xi εci ¼ εsti hct a1

4.1. Residual strains in NSC and steel reinforcement of damaged RC beam The RC beam was preloaded to produce cracks and introduce dam­ ages. After unloading, the preloaded cracks were not completely closed, which triggered the residual strains in the NSC compressive zone and in the tensile and compressive steel reinforcements in the RC beam. The more severe the pre-damage degree in the RC beam, the larger the re­ sidual strain after unloading as well as the smaller contribution of compressive NSC and steel reinforcements in the RC layer on the flexural capacity of the strengthened beam. As a result, a lower ultimate flexural capacity was obtained. This is manifested by the experimental results listed in Table 4, where the ultimate load of the strengthened beam decreased with the increase of the crack width in the RC beam caused by preloading. Therefore, for the calculation of flexural capacity of the strengthened beam, introducing the residual strain in the damaged RC beam can capture the influence of the pre-damage degree of the RC beam on the flexural capacity of the strengthened beam. In the Code for Design of Concrete Structures (GB50010-2010) in Chinese [48], a stress-strain relationship for NSC under compression is provided for modeling compressive NSC in the RC beam under loading in this study. It is assumed that compressive ultimate strength is equal to the cube compressive strengthðfc Þ. The Berkeley model [49] provides unloading curve of compressive NSC, and the unloading formula is given in equation (2). Fig. 15 presents compressive loading curve, unloading

�

εsci ¼ εsti

xi hct

a2 a1

� (4)

In Fig. 16, the height of NSC compressive zone of the RC beam sec­ tion (xi Þ is obtained according to the axis force equilibrium: Cci þ Csci ¼ Tsi ; � �2 � � Z xi ε 1 εci Cci ¼ σc bdy ¼ fc bxi ci (5) ε0 3 ε0 0 Csci ¼ Es εsci Asc Tsi ¼ Es εsti Ast �

σ c ¼ fc 1

� 1

εci ε0

�2 � ð0 � εc � ε0 Þ

(6) (7)

Equation (7) was the compressive stress-strain relationship of NSC [37]; Asc and Ast are the area of compressive steel and tensile steel in the RC beam, respectively; Es is the elastic modulus of steel. Then, obtained compressive NSC strain (εci Þ at the unloading point is substituted into the equation (2), and the residual strain (εcp Þ of NSC after unloading can be obtained. According to the calculation model in Fig. 16(c), the residual strains of the tensile steel (εstp ) and the compressive steel (εscp ) are calculated by equation (8). � � h a1 εstp ¼ εcp cpt xp

εscp ¼ εcp

� xp

a2 xp

� (8)

In the equation (8), the height of compression zone of the RC beam

Fig. 15. Stress-strain curve of NSC under cyclic loading under compression. 12

Y. Zhang et al.

Composites Part B 186 (2020) 107834

Fig. 16. Analytical model of residual strain: (a) Cross section; (b) Strain and stress distribution at the beginning of unloading (εci , σci ); (c) Residual strain and stress distribution (εcp , σ cp ).

after unloading (xp) can be obtained according to the axis force equi­ librium: Ccp þ Ccsp ¼ Tsp. The principle used is the same as the equations (5) and (6). According to the equations (2), (3) and (8), the residual compressive strain (εcp) of NSC after unloading, the strain of tensile steel (εsti) in the RC beam at the beginning of unloading, and residual strain of tensile steel (εstp) after unloading can be calculated. The comparison between the calculated results and the corresponding test results is shown in Table 7. In the table, C0.2, C0.3 and C0.4 represent the crack widths of 0.2 mm, 0.3 mm and 0.4 mm in the preloaded RC beams, respectively. The experimental values of each group are the average values of preloaded RC beams with the same pre-damage degree. From Table 7, the calcu­ lated values of εsti, εcp and εstp are in good agreement with the experimental results of the test beams, and the deviation between them is in the range of 9.5%–7.9%. This indicates that the theoretical for­ mulas developed can accurately predict the residual strain in the cracked RC beams after unloading.

Fig. 17. Analytical model of ultimate flexural capacity: (a) Cross section; (b) Stress distribution.

4.2. Ultimate flexural capacity of the strengthened beams

From the experimental results, the more severe the pre-damage de­ gree in the RC beam, the larger the residual stress in compressive NSC and steel reinforcements in the cracked RC beam, and the lower the flexural capacity of the strengthened beam. Therefore, the residual stress in the damaged RC beam after unloading affects the flexural resistance of the strengthened beam. When calculating the ultimate flexural ca­ pacity of damaged RC beams strengthened by UHPC, it is necessary to eliminate the residual stresses in the NSC as well as the residual stresses in the compressive and tensile steel reinforcement in the RC beams to obtain their effective strengths (fce , fyce and fyte ). As a result, the residual stress and effective strength of materials are considered for calculation of ultimate flexural capacity of the strengthened beam, which can reflect the influence of the pre-damage degree in the RC layer on the flexural resistance of the strengthened beam. The effective strengths of NSC in the compressive zone and tensile and compressive steel reinforcement in the damaged RC beam are calculated as follows:

According to the simplified plastic theory, the calculation and analysis model of ultimate flexural capacity of strengthened beams are shown in Fig. 17. At the ultimate state, NSC in the compressive zone in the RC layer reaches compressive strength (fc ), and the contribution of cracked NSC in the tensile zone is ignored. Also, the compressive steel in the RC layer, the tensile steel in the UHPC layer and the tensile steel in the RC layer all yield (fy ). It is assumed that the stress uniformly distributed along the height of UHPC layer reaches the cracking strength (fut ) of UHPC at the failure, based on the tensile constitutive model of UHPC as shown in Fig. 14(b). Table 7 Comparisons of εsti, εcp and εstp (unit: με). Item

εsti εcp εstp

Cal. Exp. δ Cal. Exp. δ Cal. Exp. δ

C0.2

C0.3

C0.4

1147.0 1104.2 3.9% 63.3 58.4 8.4% 187.3 203.4 7.9%

1720.5 1638.0 5.0% 106.3 111.2 4.4% 311.6 297.6 4.7%

2294.6 2249.7 2.0% 158.5 144.8 9.5% 465.8 443.8 5.0%

Note: ‘Cal.’ represents the calculated results by formula (2), (3) and (8); ‘Exp.’ Exp Cal represents the experimental results; δ ¼ � 100%. Exp

fce ¼ fc

Ec εcp

fyte ¼ fy

Es εstp

fyce ¼ fy

Es εscp

(9)

where fce , fyte and fyce represent the effective strengths of NSC, tensile and compressive steel reinforcement in the damaged RC beam, 13

Y. Zhang et al.

Composites Part B 186 (2020) 107834

respectively. Ec and Es represent the elastic modulus of NSC and steel respectively. The compressive depth (xn ) of NSC in the strengthened beam is ob­ tained by the equilibrium of the axial forces: Tst þ Tu þ Tsu ¼ Cc þ Csc , as simplified in equation (10). xn ¼

fut bhu þ fyte Ast þ fy Asu fce b

fyce Asc

(10)

where Tst , Tsu , and Tu represent the resultant forces of tensile rein­ forcement in the RC and UHPC layers, and the resultant force of the UHPC layer, respectively; Csc and Cc represent the compressive resultant forces of steel reinforcements and NSC in the RC layer, respectively. Asc , Ast and Asu represent the area of compressive steel reinforcement, tensile steel reinforcement in the RC layer and tensile steel reinforcement in the UHPC layer, respectively. Then, the predicted ultimate flexural capacity Mu of the strengthened beam is calculated by taking moment of forces about the neutral axis location as given below. � � hu Mu ¼ fy Asu ðht au Þ þ fut bhu ht þ fyte Ast ðht hu a1 Þ þ fyce Asc ðxn a2 Þ 2 þ fce b

xn 2 2

Fig. 18. Comparison of Mu from experiment and calculation.

crack width and the strains of steel reinforcements in the UHPC and RC layers with more severe pre-damage degree developed more quickly with the load. (5) The cracking and flexural resistance of the strengthened beams with the UHPC layer toughened by the placement of steel wire mesh, oriented steel fibers and moderate-temperature steam curing were further improved. The most evident improvement in cracking and flexural resistance was observed in the strengthened beam with the addition of steel wire mesh. (6) The equations for the calculation of ultimate flexural capacity of the strengthened beams with consideration of the pre-damage degree in the RC beams were proposed in this study. The calcu­ lated results were in good agreement with the experimental values, indicating that the proposed equations can accurately predict ultimate flexural capacity of damaged RC beams strengthened by reinforced UHPC layer.

(11)

The ultimate flexural capacity of the strengthened beams was calculated using equation (11) and was then compared with the tested values, as shown in Fig. 18. The all groups showed a good agreement between the tested and calculated values, with errors less than 10%, indicating that the equations proposed were feasible to predict the ul­ timate flexural capacity of the strengthened beams within acceptable accuracy. 5. Conclusion Four-point flexural test was conducted on twelve damaged RC beams strengthened by the UHPC layer and one intact RC beam. Their me­ chanical performances were compared. Moreover, the effects of different preloaded damage degrees in the RC beams and three strategies for the improvement of toughness of the UHPC layer on the flexural performance of the strengthened beams were discussed. The following conclusions were drawn:

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

(1) The cracking and flexural performance of the damaged RC beams strengthened by reinforced UHPC layer were significantly improved. Compared with the unstrengthened RC beam, the cracking and ultimate flexural capacity of the strengthened beams increased by 39.4%–233.7% and 71.4%–126.3%, respectively. (2) The typically flexural failure was observed during the test of the strengthened beam (UC). At failure of UC, only one main crack appeared in the UHPC layer, while other cracks were densely distributed with small crack width. Also, most of the cracks did not propagate along the whole depth and width of the UHPC layer. Moreover, no debonding was observed at the interface of UHPC and RC before the failure of UC. (3) The reinforced UHPC layer effectively restrained the further crack development in the RC layer. In the UC, the crack propa­ gation speed in the UHPC and RC layers was relatively slow, and the crack width in the RC layer of UC was less than 0.25 mm at the failure. (4) The cracking and flexural performance of UC were significantly influenced by the pre-damage degree in the RC beam. The more severe the pre-damage degree in the RC beam, the lower the cracking and flexural bearing capacity of UC, and the more obvious reduction in the stiffness at late loading stage. Also, the

This research is sponsored by the National Natural Science Founda­ tion of China (Grant Nos. 51578226, 51778221), Major Research Project of Industrial Technology of Guangzhou (Grant No.201902010019) and Major Program of Science and Technology of Hunan Province (Grant No. 2017SK1010). These supports are gratefully acknowledged. References [1] Tadesse G, Wakjira, Ebead Usama. Internal transverse reinforcement configuration effect of EB/NSE-FRCM shear strengthening of RC deep beams. Compos B Eng 2019;166:758–72. [2] Bodzak Przemysław. Flexural behaviour of concrete beams reinforced with different grade steel and strengthened by CFRP strips. Compos B Eng 2019;167: 411–21. [3] Emmons PH. Concrete repair and maintenance illustrated: problem analysis, repair strategy and techniques. Kingston, MA: R.S. Means Company; 1994. [4] Chen Wensu, Pham Thong M, Henry Sichembe, Chen Li, Hong Hao. Experimental study of flexural behaviour of RC beams strengthened by longitudinal and Ushaped basalt FRP sheet. Compos B Eng 2018;134:114–26. [5] Denari� e E, Brühwiler E. Structural rehabilitations with ultra-high performance fibre reinforced concretes. Int J Restor Build Monument 2006:453–67. Aedificatio. [6] Elsanadedy Hussein M, Abbas Husain, Almusallam Tarek H, Yousef A, Al-Salloum. Organic versus inorganic matrix composites for bond-critical strengthening applications of RC structures – state-of-the-art review. Compos B Eng 2019;174: 1–21. [7] Richard Pierre, Cheyrezy Marcel. Composition of reactive powder concretes. Cement Concr Res 1995;7(25):1501–11.

14

Y. Zhang et al.

Composites Part B 186 (2020) 107834 [29] Prem Prabhat Ranjan, Murthy A Ramachandra. Acoustic emission and flexural behaviour of RC beams strengthened with STUHPC overlay. Construct Build Mater 2016;123(1):481–92. [30] Prabhat PR, Murthy AR, Ramesh G. Flexural behaviour of damaged RC beams strengthened with ultra high performance concrete. Adv Struct Eng 2015:2057–69. [31] Safdar Muhammad, Matsumoto Takashi, Ko Kakuma. Flexural behavior of reinforced concrete beams repaired with ultra-high performance fiber reinforced concrete (UHPFRC). Compos Struct 2016;157:448–60. [32] Spyridon A, Paschalis Andreas P, Lampropoulos, Tsioulou Ourania. Experimental and numerical study of the performance of ultra high performance fiber reinforced concrete for the flexural strengthening of full scale reinforced concrete members. Construct Build Mater 2018;186:351–66. [33] Yin Hor, Teo Wee, Shirai Kazutaka. Experimental investigation on the behaviour of reinforced concrete slabs strengthened with ultra-high performance concrete. Construct Build Mater 2017;155:463–74. [34] Noshiravani Talayeh, Brühwiler Eugen. Rotation capacity and stress redistribution ability of R-UHPFRC-RC composite continuous beams: an experimental investigation. Mater Struct 2013;46:2013–28. [35] Pimentel M� ario, Nunes Sandra. Experimental tests on RC beams reinforced with a UHPFRC layer failing in bending and shear. In: HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials; 2016. [36] Makita Tohru. Bending fatigue test of R-UHPFRC–RC composite beams. Test Report MCS 2013;23. 09.01-part3. [37] Ramachandra Murthy A, Karihaloo BL, Rani P Vindhya, Priya D Shanmuga. Fatigue behavior of damaged RC beams strengthened with ultra high performance fibre reinforced concrete. Int J Fatig 2018;116:659–68. [38] Lee Ming Gin, Wang Yung Chih, Chiu Chui Te. A preliminary study of reactive powder concrete as a new repair material. Construct Build Mater 2007;21:182–9. [39] Al Osta MA, Isa MN, Baluch MH, Rahman MK. Flexural behavior of reinforced concrete beams strengthened with ultra-high performance fiber reinforced concrete. Construct Build Mater 2017;134:279–96. [40] Ministry of Housing, Urban-Rural Development, Beijing China. Specification design of ordinary concrete (JGJ 55-2011). 2011 [in Chinese]. [41] Ministry of Housing, Urban-Rural Development, Beijing China. Standard for test method of mechanical properties on ordinary concrete. GB/T50081-2002); 2002 [in Chinese]. [42] General Administration of Quality Supervision. Inspection and quarantine, beijing, China. Reactive powder concrete (GB/T 31387-2015). 2015 [in Chinese]. [43] Standardization Administration of the People’s Republic of China. Metallic materials-tensile testing-part 1: method of test at room temperature (GB/T228.12010). ISO 6892-1: 2009. MOD; 2010 [in Chinese]. [44] Pam H, Kwan A, Islam M. Flexural strength and ductility of reinforced normal-and high-strength concrete beams. Proc Inst Civ Eng: Struct. Build. 2001;146(4):381–9. [45] Yuan Qun. Study on basic problems in quality evaluation of concrete structure with aging disease and in bonding reinforcement. Master dissertation. Dalian, China: Dalian University of Technology; 2000 [in Chinese]. [46] Tjiptobroto P, Hansen W. Tensile strain hardening and multiple cracking in highperformance cement-based composite containing discontinuous fibres. ACI Mater J 1993;90(1):16–25. [47] Zhang Zhe, Xu-dong SHAO, Wen-guang LI, Zhu Ping, Chen Hong. Axial tensile behavior test of ultra-high performance concrete. China J Highw Transp 2015;28 (8):50–8. in Chinese. [48] Ministry of Housing, Urban-Rural Development, Beijing China. Code for design of concrete structures (GB50010-2010). 2011 [in Chinese]. [49] Blakeley RWG, Park R. Prestressed concrete section with cyclic flexure. ASCE. J Struct Div 1973;99(8):1717–42. [50] Ministry of Transport of the People’s Republic of China. Code for design of highway reinforced concrete and prestressed concrete bridges and culverts (JTG D62-2004). 2004 [in Chinese].

[8] Meng Weina, Khayat Kamal Henri. Improving flexural performance of ultra-highperformance concrete by rheology control of suspending mortar. Compos B Eng 2017;117:26–34. [9] Wu Zemei, Shi Caijun, Khayat Kamal Henri. Investigation of mechanical properties and shrinkage of ultra-high performance concrete: influence of steel fiber content and shape. Compos B Eng 2019;174:1–12. [10] Hung Chung-Chan, He-Sheng Lee Si Nga Chan. Tension-stiffening effect in steelreinforced UHPC composites: constitutive model and effects of steel fibers, loading patterns, and rebar sizes. Compos B Eng 2019;158:269–78. [11] Graybeal B. Material property characterization of ultra-high performance concrete. Rep. No. FHWA-HRT-06-103. Washington, DC: Federal Highway Administration; 2006. [12] Katrin Habel, Paul Gauvreau. Response of ultra-high performance fiber reinforced concrete (UHPFRC) to impact and static loading. Cement Concr Compos 2008;30 (10):938–46. [13] Makita Tohru, Brühwiler Eugen. Tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC). Mater Struct 2014;47:475–91. [14] Ahmad Shamsad, Rasul Mehboob, Adekunle Saheed Kolawole, Al-Dulaijan Salah U, Maslehuddin Mohammed, Ali Syed Imran. Mechanical properties of steel fiberreinforced UHPC mixtures exposed to elevated temperature: effects of exposure duration and fiber content. Compos B Eng 2019;168:291–301. [15] Chun Booki, Yoo Doo-Yeol. Hybrid effect of macro and micro steel fibers on the pullout and tensile behaviors of ultra-high-performance concrete. Compos B Eng 2019;162:344–60. [16] Charron Jean-Philippe, Denari� e Emmanuel, Brühwiler Eugen. Transport properties of water and glycol in an ultra-high performance fiber reinforced concrete (UHPFRC) under high tensile deformation. Cement Concr Res 2008;38:689–98. [17] Graybeal B, Tanesi J. Durability of an ultrahigh-performance concrete. J Mater Civ Eng 2007;19(10):848–54. [18] Rafiee A. Computer modeling and investigation on the steel corrosion in cracked ultra high performance concrete. Structural materials and engineering series No. 21. Kassel, Germany: Kassel University Press GmbH; 2012. [19] Burkart I, Müller HS. Creep and shrinkage characteristics of ultra high strength concrete (UHPC). In: Proceedings of the second international symposium on ultra high performance concrete. Germany: Kassel University Press; 2008. [20] Yang Li, Shi Caijun, Wu Zemei. Mitigation techniques for autogenous shrinkage of ultra-high-performance concrete – a review. Compos B Eng 2019;178:1–12. [21] Brühwiler E. Improving safety and durability of civil structures. In: Damage Assessment and Reconstruction after War or Natural Disaster; 2009. p. 63–93. [22] Zhang Yang, Zhu Yanping, Yeseta Marlyn, Meng Dongliang, Shao Xudong, Qi Dang, Chen Genda. Construct Build Mater 2019;215:347–59. [23] Brühwiler Eugen, Denari� e Emmanuel, Habel Katrin. Ultra-high performance fibre reinforced concrete for advanced rehabilitation of bridges. Lausanne, Switzerland: Ecole Polytechnique F� ed�erale de Lausanne (EPFL) MCS-ENAC, EPFL; 2005. CH1018. [24] Doiron Gaston, Eng P, Eng M. Project manager repair and retrofit markets-ductal. In: Canada. Pier repair/retrofit using examples of completed projects in North America. First International Interactive Symposium on UHPC. Lafarge North America Inc.; 2016. [25] Brühwiler E, Denari� e E. Rehabilitation and strengthening of concrete structures using ultra-high performance fibre concrete reinforced. Struct Eng Int 2013;23: 450–7. [26] Habel Katrin, Denari�e Emmanuel, Brühwiler Eugen. Experimental investigation of composite ultra-high performance fibre reinforced concrete and conventional concrete members. ACI Struct J 2007;104(1):93–101. [27] Habel Katrin, Denaríe Emmanuel, Brühwiler Eugen. Time dependent behavior of elements combining ultra-high performance fiber reinforced concretes (UHPFRC) and reinforced concrete. Mater Struct 2006;39(5):557–69. [28] Oesterlee Cornelius. Structural response of reinforced UHPFRC and RC composite members. EPFL 2010;4848:30–51.

15