Flexural strengthening of reinforced concrete beams or slabs using ultra-high performance concrete (UHPC): A state of the art review

Engineering Structures 205 (2020) 110035

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Flexural strengthening of reinforced concrete beams or slabs using ultrahigh performance concrete (UHPC): A state of the art review

T

⁎

Yanping Zhua,b, Yang Zhangb, , Husam H. Husseinc, Genda Chena a

Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, MO 65401, United States Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha 410082, China c Department of Civil Engineering, Stocker Center 102, Ohio University, Athens, OH 45701-2979, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultra-high performance concrete UHPC Flexural strengthening Reinforced concrete RC

The use of ultra-high performance concrete (UHPC) to strength existing reinforced concrete (RC) structures in flexure has been explored in recent decades. As UHPC developed in different countries performed different properties, the effectiveness of RC structures strengthened with UHPC varies. Moreover, the lacking of code provisions restricts the wide application of this novel strengthening technology. It is necessary to review experimental studies for the guidance of code elaboration. In this research, the state of research on the flexural strengthening of RC beams or slabs with UHPC was presented. From the technical literature review, an experimental database was established. In order to examine the effectiveness of strengthening schemes with UHPC, size effect, and mechanical properties of RC beams or slabs, pre-damage degree for RC, strengthening configuration, characteristics of UHPC layer, curing conditions for UHPC and interfacial preparation for concrete substrate were discussed. In the literature review presented, different failure modes of UHPC-RC composite members under flexure were also identified. Then, analytical and numerical models developed in the literature to reproduce structural response and to predict cracking and ultimate capacity of strengthened beams or slabs with UHPC were summarized, and a comparison between these models was presented. The experimental evidence showed that UHPC could be used to increase the flexural strength of RC beams or slabs. Also, a cost analysis comparing with other strengthening techniques (such as CFRP) was presented. Finally, some future work is recommended to complement this promising strengthening technique for existing RC structures under flexure.

1. Introduction Reinforced concrete structures (RC) have been widely used in the world. However, the maintenance of them needs numerous financial investments [1]. They experience aging and function degradation, resulting in insufficient serviceability, sustainability, and durability due to external environment change and high mechanical loading [2,3]. Therefore, it is urgent that most deteriorated RC structures need strengthening to extend service life and to reduce maintenance costs. In the past years, various materials and techniques have been exploited for the rehabilitation of existing RC structures. Concrete or steel jacketing technique [4–6], fiber-reinforced polymer (FRP) [7] wrapping, external prestressing [8], and near-surface mounted, and fiber-reinforced composite materials [9–14] are commonly available retrofitting schemes. Although these techniques can effectively achieve strengthening targets and improve strength and durability, some drawbacks in these

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techniques are conceived, such as, increase the weight of structures in concrete jacketing, corrosion, fire resistance and debonding in steel plate technique and aging of adhesion materials at the interface in strengthening of FRP [15–17]. Recently, a more advanced cement-based material, namely ultrahigh performance concrete (UHPC), has been developed and gained attention to strength flexural RC structures [18,19]. The raw materials of UHPC consisted of cement, quartz powder, fly ash, silica fume, quartz sand, superplasticizers, and steel fibers [18]. It exhibits high compressive and tensile strengths (up to 150 MPa and 8 MPa, respectively) far greater than fiber-reinforced strength concrete [20]. Due to the high compactness and density of the UHPC matrix, excellent physical performance like low permeability can resist the ingress of detrimental substances, resulting in the improvement of durability [3]. After thermal treatment, UHPC shrinkage, mainly affecting its application on structures due to different ages, can become zero [21]. High ductility,

Corresponding author. E-mail addresses: [email protected] (Y. Zhu), [email protected] (Y. Zhang).

https://doi.org/10.1016/j.engstruct.2019.110035 Received 1 July 2019; Received in revised form 21 November 2019; Accepted 1 December 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.

Engineering Structures 205 (2020) 110035

Y. Zhu, et al.

fracture energy (up to 40 kJ/m2), and energy dissipation capacity are also outstanding in UHPC [22]. Another attractive feature is the strainhardening behavior of UHPC in tension because of the additive of steel fibers (1.5–3% by volume) [23]. Moreover, excellent rheological properties of UHPC allow easy casting in the fresh state. Overall, UHPC has shown excellent mechanical and physical performance. Because of these properties, using UHPC to strengthen or repair RC structures has proven to be an effective and promising technique compared to traditional techniques (such as steel plates and FRP [24]). The advantages of this technique not only include significant improvement of capacity and durability of strengthened RC structures but also ensure the possibility of quick construction by prefabrication and assembly, minimal change in section size, and minimum service disruption in traffic. Numerous experimental and numerical research has been conducted in this area [30–45] with a growing application using UHPC for repair. However, as UHPC developed in different countries has performed different characteristics, exhibiting different effectiveness in strengthening existing RC structures, so it is necessary to summarize recent advancement in this intervention technology. Moreover, code provision for the design of strengthening using UHPC is not available. Until now, for the flexural strengthening of RC beams or slabs by UHPC, only one code for internal use was developed for the structure design of UHPC in [25]. Also, the theoretical and analytical models were extended and deduced from the conventional reinforced concrete. It is urgent to review existing calculation methods in the literature for the theoretical reference at practical application. Also, future research direction or trend for the development of strengthening by UHPC needs to be suggested. For example, combining with other technology (like acoustic emission) and concepts (like structural health monitoring) is an interesting topic. In this research, the state of research on the flexural strengthening of RC beams or slabs with UHPC was presented. From the technical literature review, the experimental database was established. In order to examine the effectiveness of strengthening schemes with UHPC, size effect, and mechanical properties of RC beams or slabs, pre-damage degree for RC, strengthening configuration, characteristics of UHPC layer, curing conditions for UHPC and interfacial preparation for concrete substrate were discussed. Different failure modes of UHPC-RC composite members under flexure were also discussed. Then, analytical and numerical models developed in the literature to reproduce structural response and to predict cracking and ultimate capacity of strengthened beams or slabs with UHPC were summarized. Also, a cost analysis comparing with other strengthening techniques (such as CFRP) was presented. Finally, some future work is recommended to complement this promising strengthening technique for existing RC structures under flexure.

It should be noted that the tensile strength of UHPC was obtained by splitting strength test [26] or flexural test [27] while compressive strength of UHPC was obtained by cylinder specimens [28] or cubic specimens [29]. Moreover, experimental results related to cracking/ ultimate moment and failure mode were also reviewed. Fig. 1 shows the typical four types of strengthening configuration, namely C-sided, Tsided, 2-sided, and 3-sided. The C-sided configuration means that UHPC is located in the compressive zone. The T-sided configuration means that UHPC is located in the tension zone. The 2-sided configuration means that UHPC is located on the left and right sides of the crosssection. The 3-sided configuration is a combination of the T-sided and 2-sided configurations. It should be noted that the main reason for using a 2-sided configuration is to provide shear strengthening. Also, this configuration should be less flexural strengthening efficient than the 3sided configuration [30]. However, the flexural strengthening efficiency of the 2-sided was higher than the T-sided configuration when sandblasting concrete substrate surfaces and casting UHPC in-situ around the beams were used, as shown in Table 1. The efficiency between them was comparable for bonding prefabricated UHPC strips to the RC beams using epoxy adhesive [30]. In order to compare the effectiveness of strengthening by UHPC in different literature, the cracking, yielding, and ultimate moments obtained by experiments were normalized by Eq. (1) compared with unstrengthened (control) beams or slabs.

M Percentage Increase in Flexural Strength (%) = ⎛ − 1⎞ × 100 M ⎝ con ⎠ ⎜

⎟

(1) where M represents cracking moment (Mcr ), yielding moment (My ), and ultimate moment (Mu ) of strengthened UHPC-RC composite members, respectively. Mcon represents corresponding moments of unstrengthened beams or slabs. 3. Evaluation of the database In this section, the database in Table 1 was evaluated to determine the distribution of the data in terms of mechanical properties, steel reinforcement, and damage degree of the concrete substrate along with strengthening configuration and the characteristics of UHPC overlay. The collected data was subdivided for evaluation based on the failure mode. As some information on the increase in cracking moment of UHPC-RC composite members lacked while all reviewed literature reported an increase in the ultimate moment. In the following sections, the increase in the ultimate moment is regarded as a primary discussed variable. However, failure modes are strictly related to the strengthening configuration. For example, the UHPC layer crushing cannot occur in the T-sided configuration. Therefore, the discussion of the failure modes observed is carried out for each layout first based on the database in Table 1. Then, a parameter analysis is carried out isolating each parameter in an attempt to study its effect on the beam capacity in each layout. In other words, the impact of each parameter on the beam capacity is analyzed, distinguishing between the different layouts. Also, in each layout, the effect of each parameter on the beam capacity is investigated, distinguishing among the various failure modes.

2. Experimental database Using UHPC to strengthen RC beams or slabs was first proposed in 2007s [1]. It has been widely studied for a strengthening of RC flexure members in recent five years. Until now, seventeen experimental studies on the flexural strengthening of RC members with UHPC have been reported in the technical literature. The experimental database is summarized and established in Table 1, including the descriptions of the RC beams or slabs prior to strengthening, the characteristics of the UHPC overlay and experimental results of UHPC-RC composite systems. In Table 1, ninety-five beams and slabs are included in the database. Size effect of RC, NSC compressive strength ( fc'), longitudinal reinforcement ratio ( ρsl ) in RC and yield strength ( f y ) of steel reinforcement and pre-damage degree for RC were discussed. Also, for application of UHPC for strengthening, the layout of UHPC, curing conditions for UHPC, thickness, elastic modulus, tensile strength, compressive strength, and reinforcement ratio in UHPC, steel fibers content and interfacial preparation for concrete substrate was comprehensively summarized.

3.1. Failure mode Five main types of failure mode, namely concrete substrate crushing (CC), UHPC layer crushing (UC), the rupture of steel reinforcement in UHPC or RC (R), debonding between UHPC and NSC layers (DB) and flexure failure were reported. It should be noted that flexure failure occurred in some reviewed literature was used for the description of the failure of strengthened members in flexure, which was distinct from the other four specific failure modes. Based on the authors’ description, it seems that the flexure failure should represent a process of failure that 2

Specimen name

ST_UHPFR_CS ST_UHPFR_TS ST_UHPFR_3SJ

BU-20 BU-40 BU-60 BL-20 BL-40 BL-60

U1 U2 UB1 UB2

RA1 RA2 RA3 RA6 RB1 RB2 RB3 RB5 RB6 RB7 RB8 RC1 RC2 RC3

NR3(3) NR5(3) R5(3) R10(3)

B2 B3 B4 B5 B6 B7

RC-SB-BOTSJ RC-SB-2SJ RC-SB-3SJ RC-EP-BOTSJ RC-EP-2SJ RC-EP-3SJ

LSA-U(2) LSA-RC(2)

Ref.

[31]-S

[32]-R

[33]-S

[34]-S

[35]-S

[36]-S

[30]-S

[37]-S

Table 1 Experimental Database.

3

400 400

140 140 140 140 140 140

100 100 100 100 100 100

300 300 300 300

100 100 100 100 100 100 100 100 100 100 100 100 100 100

150 150 150 150

250 250 250 250 250 250

150 150 150

b(mm)

180 180

230 230 230 230 230 230

200 200 200 200 200 200

150 150 150 150

200 200 200 200 200 200 200 200 200 200 200 200 200 200

200 200 200 200

380 360 340 380 360 340

250 250 250

h(mm)

Cross-section

35 35

54 54 54 54 54 54

30 30 30 30 30 30

51 51 51 51

35 35 35 35 35 35 35 35 35 35 35 35 35 35

30.9 30.9 30.9 30.9

29.7 29.7 29.7 29.7 29.7 29.7

39.5 39.5 39.5

Concrete fc′ (MPa)

0.47 0.47

0.488 0.488 0.488 0.488 0.488 0.488

1.3 1.3 1.3 1.3 1.3 1.3

0.753 0.753 0.753 0.753

0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785 0.785

0.754 0.754 0.754 0.754

0.397 0.397 0.397 0.397 0.397 0.397

0.603 0.603 0.603

537 537

590 590 590 590 590 590

415 415 415 415 415 415

500 500 500 500

415 415 415 415 415 415 415 415 415 415 415 415 415 415

500 500 500 500

386 386 386 386 386 386

500 500 500

T-sided T-sided

T-sided 2-sided 3-sided T-sided 2-sided 3-sided

T-sided T-sided T-sided T-sided T-sided T-sided

T-sided T-sided T-sided T-sided

T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided

T-sided T-sided T-sided T-sided

C-sided C-sided C-sided T-sided T-sided T-sided

C-sided T-sided 3-sided

layout

ρsl(%) fy(MPa)

UHPC

Internal reinforcement

50 50

30 30 30 30 30 30

20 20 20 20 20 20

30 50 50 100

10 10 10 10 10 5 8 10 12 15 10 10 10 10

50 50 50 50

20 40 60 20 40 60

50 50 50

hu(mm)

45 45

7.5 7.5

17 17 17 17 17 17

– – – – – –

– – – – – – 46 46 46 46 46 46

11 11 11 11

20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7 20.7

– – – – – – – – – – – – – – 48 48 48 48

11.5 11.5 11.5 11.5

10.1 10.1 10.1 10.1 10.1 10.1

12 12 12

ft(MPa)

51 51 51 51

34.6 34.6 34.6 34.6 34.6 34.6

57.5 57.5 57.5

E(GPa)

NR NR

128 128 128 128 128 128

196 196 142 142 144 144

168 168 168 168

122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5

136.5 136.5 136.5 136.5

156.3 156.3 156.3 156.3 156.3 156.3

164 164 164

fc(MPa)

0 0.34

0 0 0 0 0 0

0 0 0 0 0 0

0 0 2 2

0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 2.09 2.09

0 0 0 0 0 0

0 0 0

ρu(%)

3 3

2 2 2 2 2 2

2 2 2 2 2 2

NR NR NR NR

2 2 2 2 2 2 2 2 2 2 2 2 2 2

3 3 3 3

NR NR NR NR NR NR

3 3 3

SF(%)

Rough Rough

SB SB SB EA EA EA

Rough Rough Rough Rough Rough Rough

Rough Rough Rough Rough

Rough Rough Rough Rough Rough Rough Rough Rough Rough Rough Rough Rough Rough Rough

Rough Rough Rough Rough

+ + + + + +

+ + + + + + + + + + + + + +

aggregate aggregate aggregate aggregate aggregate aggregate

Rough Rough Rough

IP

EA EA EA EA EA EA

EA EA EA EA EA EA EA EA EA EA EA EA EA EA

16 46 89 8 36 85

– – – – – – – –

106 156 463 194 175 494 – –

Flexure R

CC U/CC UC CC U/CC UC

Flexure Flexure Flexure Flexure Flexure Flexure

DB DB Flexure Flexure

CC CC CC CC CC CC CC CC CC CC CC CC CC CC

CC + DB CC CC CC

UC R – CC R R

Flexure Flexure Flexure

Failure mode

(continued on next page)

7.5 147.5

25.8 30.8 9.2 10.5 18 19.3

15 10 400 100

1 0 0 1 0 0 0 8.5 13 19.6 22.3 6 3 8

– – – – – –

– – – –

– – – – – – – – – –

– – – – – – – – – – – – – –

60 40 188 225

– – – – – – – – – – – – – – – – – –

20 25 15 0 22 32

– – – – – – 17 28 32 135 181 195

0 3 87 93

28 31 178

29 29 167

– – –

Mu(%)

My(%)

Mcr(%)

Results

Y. Zhu, et al.

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4

RE-20 RE-32 RE-50 RE-100 OV-25 OV-25a OV-50 OV-50a

N-UC H-UC-1 H-UC-2

BEAM-2 BEAM-3 BEAM-4 BEAM-5

BEAM-2 BEAM-3 BEAM-4 BEAM-5 BEAM-6 BEAM-7

R/C RC strengthened without steel R/C strengthened with steel R/C repaired with steel

UN1 UN1.5 UN2 UH1 UH1.5 UH2

A1 A2 A3 B1 B2 B3 C1 C2 C3

[38]-S

[39]-S

[40]-S

[41]-S

[42]-S

[43]-R

[44,45]-S

100 100 100 100 100 100 100 100 100

152 152 152 152 152 152

300 300 300 300

150 150 150 150 150 150

150 150 150 150

2000 2000 2000

300 300 300 300 300 300 300 300

b(mm)

200 200 200 200 200 200 200 200 200

101 101 101 101 101 101

500 500 500 500

250 250 250 250 250 250

250 250 250 250

280 280 280

80 68 50 100 100 100 100 100

h(mm)

Cross-section

35 35 35 35 35 35 35 35 35

51 51 51 70 70 70

22 22 22 22

20.6 20.4 20.3 20.5 20.4 20.2

20.4 20.4 20.4 20.4

60.2 60.2 60.2

33 33 33 33 33 33 33 33

Concrete fc′ (MPa)

0.57 0.57 0.57 0.9 0.9 0.9 1.3 1.3 1.3

0 0 0 0 0 0

0.3 0 0.3 0.3

0.905 0.905 0.905 0.905 0.905 0.905

0.905 0.905 0.905 0.905

0.729 0.729 0.729

2.356 0 0 0 1.885 1.885 1.885 1.885

415 415 415 415 415 415 415 415 415

T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided

T-sided T-sided T-sided T-sided T-sided T-sided

– – – – – – 20 20 20 20 20 20 20 20 20

51 51 51 51 51 51

– 40 40 40

– 3-sided 3-sided 3-sided

560 560 560 560

30 30 30 30

50 50 50

50 50 50 50 50 50

T-sided T-sided T-sided T-sided

T-sided T-sided C-sided

20 32 50 100 25 25 50 50

hu(mm)

T-sided T-sided T-sided T-sided T-sided/U T/C-sided

470 470 470 470 470 470

470 470 470 470

400 400 400

501 – – – 501 501 501 501

T-sided T-sided T-sided T-sided T-sided T-sided T-sided T-sided

layout

ρsl(%) fy(MPa)

UHPC

Internal reinforcement

11.2 14.8 15.5 13 15.6 16.3 22.6 22.6 22.6 22.6 22.6 22.6 22.6 22.6 22.6

– – – – – – – – –

– 12 12 12

– – – – – –

– 44 44 44

41 41 41 41 41 41

40 40 40 40

– – – – 45 45 45 45 45 45

22.4 29.5 29.5

27.4 27.4 27.4 27.4 27.4 27.4 27.4 27.4

– – – – – – – – 41.6 43.3 43.3

ft(MPa)

E(GPa)

170 170 170 170 170 170 170 170 170

160 170 168 182 170 192

– 177 177 177

204 204 204 204 204 204

204 204 204 204

130 140 140

168 168 168 168 168 168 168 168

fc(MPa)

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0

– 0 0 0

0 0 3.35 3.35 3.35 3.35

0 0 3.35 3.35

4.16 4.16 4.16

0 5.89 3.77 1.88 0 7.54 0 3.77

ρu(%)

2 2 2 2 2 2 2 2 2

1 1.5 2 1 1.5 2

– 2.5 2.5 2.5

3 3 3 3 3 3

3 3 3 3

3.5 3.5 3.5

3 3 3 3 3 3 3 3

SF(%)

Rough Rough Rough Rough Rough Rough Rough Rough Rough

No No No No No No

– SB SB SB

+ + + + + + + + +

EA EA EA EA EA EA EA EA EA

Rough + EA ST Rough + EA ST Rough + EA ST

Rough + EA ST Rough + EA ST

Rough + ST Rough + ST Rough + ST

Rough Rough Rough Rough Rough Rough Rough Rough

IP

0 24 35 0 13 18 0 11 15

– – – – – –

– – – –

253 83 349 197 311 243

119 36 293 185

113 152 0

– – – – – – – –

Mcr(%)

Results

– – – – – – – – –

– – – – – –

0 23.5 34.8 0 11.6 18.5 0 10.6 15.1

23 67 90 45 54 75

– 35 116 92

57 32 116 107 154 208

– – – – – – – – – –

16 11 125 73

98 85 27

– – – – – – –

0 0 0 85 20 28 28 56

Mu(%)

– – – – – – – –

My(%)

CC CC CC CC CC CC CC CC CC

Flexure Flexure Flexure Flexure Flexure Flexure

– CC R CC

R R DB CC R R

CC R DB CC

CC CC R

DB CC CC Flexure Shear (DB) Shear Shear (DB) Shear

Failure mode

Note: R-repair, denotes that the depth of cross-section remains unchanged after repairing, S-strengthen, denotes that the depth of cross-section increased after strengthening, C-compression side refers to Fig. 1(a), T-tensile side refers to Fig. 1(b), 2-sided refers to Fig. 1(c), 3-sided refers to Fig. 1(d) with two sides and one tensile side. NR-denotes not reported. UC-UHPC crushing, R-rebar fracture, CC-concrete crushing, DB-debonding at the interface. EA-epoxy adhesion. SB-sandblasting. Bracket behind some specimen name represents the amounts of specimen tested. b and h are cross-section width and depth of unstrengthened (control) beams or slabs, respectively. hu, E, ft, fc and ρu represent the thickness of the UHPC layer, the elastic modulus of UHPC, tensile strength, the compressive strength of UHPC, and reinforcement ratio in the UHPC layer, respectively. SF represents steel fiber content, and IP represents interface preparation. Mcr, My, and Mu represent a cracking moment, yield moment, and ultimate moment, respectively.

Specimen name

Ref.

Table 1 (continued)

Y. Zhu, et al.

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Fig. 1. Four different strengthening configurations: (a) C-sided, (b) T-sided, (c) 2-sided, and (d) 3-sided.

Fig. 2. effect of compressive strength of NSC on the beam capacity in the T-sided configuration.

occurs similarly with typical RC flexural members. This means that tensile steel reinforcement yielded and concrete in the compressive zone crushed with a significant deflection in the middle and wider cracks in the tension zone in concrete. It should be noted that R failure mode includes the rupture of UHPC layers [34,35]. These special cases of the rupture of UHPC layers occurred because of mechanically anchored UHPC layers without embedded steel reinforcement. When this failure occurred with the main crack thoroughly propagated along with the depth of the UHPC layer,

the increase in ultimate strength was higher than other failure modes. The UC failure was reported in [30] and [32], where UHPC was located in the compressive zone (C-sided) along with the 2-sided and 3-sided strengthening configurations. The DB failure mode was observed in [33,35,38,44,45]. In [33,35,38], the specimens without steel reinforcement inside UHPC failed in debonding between the UHPC layer and the RC substrate. Therefore, further studies can be conducted to verify if the debonding failure is related to the steel reinforcement inside UHPC. It should be highlighted that in [44] and [45], the 5

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Fig. 3. effect of reinforcement ratio in RC on the beam capacity in the T-sided configuration.

3.2. Strengthening configuration

mechanism for DB at failure was due to the poor surface preparation (low roughness) of the concrete substrate. Also, the DB failure mode only occurred in the T-sided configuration, and few cracks firstly initiated at the NSC side near the interface. As it can be seen in Table 1, many specimens failed in CC failure mode because these specimens were strengthened with UHPC at the tension face, resulting in the NSC substrate crushing in the compression zone. Also, many closely distributed cracks were developed in the UHPC layers because of the strain hardening behavior of UHPC. These localized macrocracks were different from the cracks in the RC beam under flexure, leading to a different force transfer mechanism between concrete and steel reinforcement in the strengthened beams [35]. It is expected that the strengthening of UHPC can thoroughly exploit the potential of the tensile strength of UHPC and the compressive strength of NSC with failure governed by CC failure mode. It should be noted that when UC occurs in the 3-sided configuration, the behavior becomes more brittle due to lower neutral axis and depth of compressive zone, leading to higher concrete compressive strains. Overall, in the C-sided configuration, the failure modes include flexure failure, UC, and R. In the T-sided configuration, the failure modes include flexure failure, CC, R, and DB. In the 2-sided configuration, the failure modes include UC and CC. In the 3-sided configuration, the failure modes include UC, CC, and R.

Six types of strengthening configuration, namely T-sided, C-sided, 2sided, 3-sided, T-sided-U, and T&C-sided, were reviewed in the literature. The last two special strengthening configurations were found in [41]. T&C-sided means that UHPC was applied in compression and tension zone together to avoid NSC crushing at the compression zone. Multiple crack propagation was obtained in the UHPC layer, and reinforcements in the tension layer were ruptured consecutively. T-sidedU was a kind of application, combining UHPC with two-layered Ushaped carbon fiber reinforced polymer (CFRP) at the free end of simple support strengthened beams in order to avoid debonding failure at the end with strengthening by UHPC only (BEAM-6). Existing steel fibers on the UHPC layer surface was abraded carefully to hinder their possible effect on CFRP strips. This application was developed based on BEAM-4 failure due to the delamination of the layer in [41]. BEAM-4 was reinforced with steel bars, and the interface between UHPC and RC was connected by epoxy. Comparing with BEAM-4, the experimental results showed the first flexural crack load, and the yield load increased in BEAM-6. Cracks were intensified at the midsection of the specimen, and the main crack propagated and localized at the layer. The layer could not delaminate from the free ends and behaved like a rigid plate because of gluing to the tension face. The longitudinal reinforcements in 6

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Fig. 4. effect of yield strength of steel reinforcements in RC on the beam capacity in the T-sided configuration.

tensile strength, ductility, and strain-hardening behavior, compared to conventional concrete, making it more suitable for strengthening or repairing with UHPC at the tensile side. Usually, the deterioration or deficiency occurred in existing RC structures was cracking due to the low tensile strength of NSC, while UHPC can overcome this intrinsic defect in NSC. In Table 1, it can be seen that an increase in ultimate strength ranges from 0% to 150% (except for one increase in 400%) in the T-sided strengthening configuration. The increase in ultimate strength with 3-sided strengthening configuration is also in the range of 0% to 150% approximately. More tested specimens have been conducted with T-sided configuration than other strengthening configurations. Furthermore, the research on UHPC applied in the compression zone (C-sided), and the 2-sided zone is still lacking while they have a similar increase effect on the ultimate strength of about 50%.

the layer were ruptured consecutively. Consequently, the ultimate strength was significantly increased. In [46], a new CFRP-UHPC system for strengthening RC T-beams without the need to reach the tension side was proposed. The concept of the proposed technique is that using a high compressive strength of the UHPC overlay will push the neutral axis up, making CFRP act under tension. This will allow CFRP to work as additional tension reinforcement and increase the moment capacity of the beam. However, the proposed strengthening technique showed an increase of moment capacity only by 9.2%. The limitation in the moment capacity increase was due to the change of failure mode to be governed by shear failure instead of flexure failure. Combining [47,48], it is concluded that this technique is efficient with slabs, and shallow, like ribbed T-beams, and medium depth T-beams. The efficiency decreases as the depth of the beam increases due to the limited moment arm of CFRP. The researches on combination UHPC with CFRP or other external bonded techniques was limited, which may become new insight into this cementitious material (UHPC) application in engineering structures in the future. Because this innovative cement-based material exhibits a higher cost compared to NSC, so it might not be used in the whole structure and only be used for partial substitution of critical components. From Table 1, it is found that more than 90% of studies were performed on the UHPC-RC composite members under flexure with UHPC located at tension face. This may be because UHPC shows higher

3.3. Size effect of RC beams or slabs In [31], the RC cross-sectional size was 150 mm × 250 mm, and the beam length was 2200 mm. Also, the UHPC layer thickness was 50 mm. In [40,41], the RC cross-sectional size was the same as that in [31], and the beam length was 3200 mm. The authors used 30 mm [40] and 50 mm [41] thickness of precast UHPC layers to strengthen the RC beams with the same size. Comparing [31] and [41], the effect of varying beam lengths on the flexural capacity was found. In [31], the 7

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Fig. 5. Percentage increase of Mu versus damage degree in the concrete substrate.

flexural capacity of ST_UHPFR_TS without steel reinforcements in the UHPC layer increases by 31%. In [41], the flexural capacity of BEAM-2 and BEAM-3 increases by 57% and 32%, respectively. It should be noted that these beams have different bond connections at the interface. In [31,33], the RC beams had the same lengths and widths, but different depths. Also, the U1, U2, and ST_UHPFR_TS had the same interface connections and overlay thicknesses. Therefore, as the depth increases from 200 mm in [33] to 250 mm in [31], the increment of ultimate capacity increases from 1.3% to 31%. This is an exciting improvement as the UHPC overlay has the same tensile strength of about 12 MPa and no steel reinforcement. Therefore, this improvement might be attributed to the effect of different cross-sectional depths. In [34,36], the RC beams had the same sizes, while the overlay thickness was different. In [40,41], the RC beams had the same sizes, but different overlay thicknesses were used. These discussions can refer to the effect of UHPC layer thickness on the flexural capacity.

moments increase by 25% and 27% with compressive strength 29.7 MPa and 60.2 MPa, respectively. It is found that the failure mode is not determined by the compressive strength from this limited data. Also, the increase in the ultimate moment ranges from 20% to 30% regardless of the different failure modes. This is because the effect of the compressive strength of RC on the beam capacity was reduced as UHPC was attached to the compression zone. The beam capacity might depend on the tensile performance of RC. In the 2-sided configuration [30], ultimate moments in two types of the strengthened beams increase by 46% and 36% with NSC compressive strength of 54 MPa when UC and CC coincide. This little difference might be due to different treatments for concrete beam surfaces [30]. In the 3-sided configuration, the ultimate moment increases by 178% when flexure failure [31] occurred with NSC compressive strength of 39.5 MPa. In UC [30], the ultimate moment rises by 92%, with a compressive strength of 54 MPa. In CC [42], the ultimate moment of the strengthened beam without steel increases by 35%, and the ultimate moment of the repaired beam increases by 92%, both having a compressive strength of 22 MPa. In R [42], the ultimate moment of the strengthened beam with steel increases by 116% with a compressive strength of 22 MPa. In the T-sided configuration, four types of failure modes are observed in Table 1. In the different failure modes, the effect of compressive strength of NSC on the beam capacity is shown in Fig. 2, where the increase inMu , relative to the corresponding unstrengthened

3.4. Mechanical properties of RC beams or slabs 3.4.1. Compressive strength of RC beams Limited data in the C-sided configuration [31,32,39] is reported. Three different failure modes are listed in Table 1. In flexural failure [31], the ultimate moment increases by 28% with compressive strength 39.5 MPa. In UC failure [32], the ultimate moment increases by 20% with compressive strength 29.7 MPa. In R failure [32,39], ultimate 8

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Fig. 6. effect of elastic modulus of UHPC on the beam capacity in the T-sided configuration.

3.4.2. Reinforcement ratio in RC beams In the C-sided configuration [31,32,39], in flexural failure [31], ultimate moment increases by 28% with reinforcement ratio of 0.603%. In UC [32], the ultimate moment increases by 20%, with a reinforcement ratio of 0.397%. In R [32,39], ultimate moments increase by 25% and 27% with reinforcement ratio of 0.397% and 0.729%, respectively. In the 2-sided configuration [30], the ultimate moment increases by 40% with a reinforcement ratio of 0.488% when UC and CC coincid. In the 3-sided configuration, the ultimate moment increases by 178% when flexure failure [31] occurs with a reinforcement ratio of 0.603%. In UC [30], the ultimate moment increases by 92%, with a reinforcement ratio of 0.488%. In CC [42], the ultimate moment of the strengthened beam without steel (0%) increases by 35%. In R [42], the ultimate moment with a reinforcement ratio of 0.3% increases by 116%. This indicates increasing the reinforcement ratio might change the failure mode and increase the ultimate strength. In the T-sided configuration, Fig. 3 shows the increase in Mu , relative to the corresponding unstrengthened (control) beams or slabs as a function of the reinforcement ratio ( ρsl ) in RC. The reinforcement ratio is calculated by Eq. (2)

(control) beams or slabs is as a function of compressive strength ( fc') of NSC. In Fig. 2, the HV area is cover with the high values of Mu due to the higher depth of the UHPC layer, higher UHPC tensile and compressive strengths, and higher steel area in the UHPC layer compared to LV area. LV area is cover with the low values of Mu due to the lower depth of the UHPC layer, lower UHPC tensile and compressive strengths, and zero steel area in the UHPC layer compared to the HV area. The following figures in the present study also include the LV and HV areas. It is clearly found that the compressive strength of NSC might not determine the failure modes in the T-sided configuration. The increased range for the ultimate moment varies from 0% to 250%, with the maximum increase in [35] with flexure failure. Overall, almost two-thirds of tests were conducted on RC beams or slabs with compressive strength ranging from 20 MPa to 40 MPa because many existing RC structures were constructed by low-strength class concrete. Other research focused on the compressive strength of concrete substrate in the range of 50–60 MPa. It is noted that at low compressive strength (about 20 MPa), the failure mode of the rupture of steel reinforcement was the most possible to occur while the strengthened beams or slabs with the compressive strength of concrete substrate about 55 MPa exhibited concrete crushing (CC). It is interesting to note that the higher compressive strength of the concrete substrate does not improve the ultimate moment much more compared to the lower compressive strength of NSC, as shown in Fig. 2.

ρsl = Asl / bh

(2)

where Asl is the longitudinal steel reinforcement; b, h are cross-section width and depth of unstrengthened (control) beams or slabs, respectively. In Fig. 3, 95% of tested specimens has less than 1% of the 9

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Fig. 7. effect of tensile strength of UHPC on the beam capacity in the T-sided configuration.

3.5. Pre-damage degree for RC

reinforcement ratio in RC beams or slabs. This range also ensures that the unstrengthened (control) beams or slabs are under reinforced, making the experiments more representative. In the range from 0% to 1%, the overall trend of the ultimate moment is increased with the increase of reinforcement ratio. One study [38] focuses on a higher reinforcement ratio (about 2%) in RC, and the debonding between UHPC and NSC layers at failure is observed (Fig. 3(c)). Some studies without longitudinal steel reinforcement inside the RC are conducted, and flexural failure is observed (Fig. 3(a)).

In the C-sided configuration, one literature [39] reported that the RC slab was preloaded, and the maximum width was 0.04 mm at the highest load. The ultimate capacity increased by 27%. In the 2-sided configuration, the intact RC beams were strengthened with the UHPC layer. In the 3-sided configuration, one literature [42] reported that the RC beam was initially damaged up to the yielding of the longitudinal reinforcement and then repaired. Comparing with the intact RC beam strengthened by UHPC, its ultimate capacity decreases by 24% [42]. It is found pre-damage in the RC beam might affect the improvement of the ultimate capacity. In the T-sided configuration, Fig. 5 shows the pre-damage degree for the RC beam or slab before strengthening of UHPC. It can be seen that seven types of pre-damage degree were reviewed; namely no pre-damage (No), flexural cracks with 0.2 mm width and 0.4 mm width induced by preloading, yielded longitudinal steel reinforcement in RC, and subjecting 70%, 80%, and 90% of ultimate load of the control beam or slab. Pre-damage degree induced can better capture the actual circumstance of existing RC structures in the field. Therefore, with consideration of the pre-damage degree, the flexural behaviors of RC structures with the strengthening of UHPC can be closer to the actual behavior in the field. However, many studies in the literature were performed on virgin and intact RC structures, as can be seen in Fig. 5.

3.4.3. Yield strength of steel reinforcement in RC beams For the C-sided configuration, the 2-sided configuration, and the 3sided configuration, discussions for the effect of yield strength of steel reinforcements in RC are similar to those for the compressive strength of NSC substrate and reinforcement ratio in RC. In the T-sided configuration, Fig. 4 shows the increase in Mu , relative to the corresponding unstrengthened (control) beams or slabs as a function of the yield strength of steel reinforcement ( f y ) used in RC. In Fig. 4, most of the studies were performed on strengthened beams or slabs with a yield strength of steel reinforcement used in RC ranging from 400 MPa to 500 MPa. Additionally, other studies with steel reinforcement used in RC having higher and lower yield strength than about 450 MPa produced a lower ultimate moment in UHPC-RC composite members from Fig. 4. 10

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Fig. 8. effect of compressive strength of UHPC on the beam capacity in the T-sided configuration.

the increase in ultimate strength. In Fig. 6, combining with Table 1, the elastic modulus was mostly distributed in the range of 44–46 GPa. The distribution of the increase in ultimate strength was similar to the normal distribution in statistics. The most considerable improvement was presented in the vicinity of 45 GPa, while the least improvement was presented at the edge. For T-sided strengthening configuration, Figs. 7 and 8 show that the increase in ultimate strength varies with tensile and compressive strengths of UHPC, respectively. Both of them show a more extensive scattering with tensile strength ranging from 5 MPa to 40 MPa and compressive strength ranging from 120 MPa to 210 MPa. It should be highlighted that the tensile strength (including direct tensile strength and flexural strength) was obtained by direct tensile test or flexural test, as mentioned before. Specifically, the tensile strength of more than 20 MPa was obtained by conducting the flexural test while the remaining was obtained using direct tensile strength. The effect of this difference on the prediction of the structural response of UHPC-RC composite members was discussed in detail in [35]. From Fig. 7, it can be seen that more tests have been conducted with a tensile strength at about 10 MPa. Also, at about 10 MPa of tensile strength, the maximum increase in ultimate strength was more substantial than that with other tensile strengths. In Fig. 8, the minimum compressive strength was higher than 120 MPa, which agrees well with the minimum requirement of UHPC compressive strength in AFGC [49]. From Table 1, UHPC crushing (UC) occurred in composite members in [30] with a lower compressive strength of UHPC as expected. Also, with

As fewer experiments on damaged RC beams/slabs, it may be not possible to conclude the effect of pre-damage degree on ultimate strength although with introducing damage in RC beams/slabs, the increases in first cracking strength, ultimate strength and stiffness [34] seems to be less than undamaged counterparts in Fig. 5. From this view, it is essential to continue further research in damaged beams/slabs strengthened using UHPC. 3.6. Characteristics of UHPC overlay Following discussions are related to the increase in ultimate strength versus aspects of UHPC overlay, including the elastic modulus (E ), the tensile ( ft ) and compressive ( fc ) strengths, fiber fraction of volume (SF), and thickness layer (hu ) along with steel reinforcement ratio ( ρu ) inside the UHPC layer. These parameters are essential and can directly influence the effectiveness of the strengthening of UHPC. However, the properties of UHPC (E , ft , fc and SF ) reviewed in the literature shows scattering to a certain degree. This is because, in the world, different research institutes developed different mix proportions and characteristics of UHPC, leading to different strengthening effectiveness on existing RC structures. However, the overall aim of previous research on UHPC was similar, including improvement of UHPC performance and application of UHPC, such as protection of existing RC structures. Figs. 6–8 mainly show the influence of mechanical performance on 11

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Fig. 9. effect of UHPC thickness on the beam capacity in the T-sided configuration.

Fig. 11 gives the increase in ultimate strength versus with reinforcement ratio in UHPC in the T-sided configuration. The reinforcement ratio is calculated using Eq. (2) with different cross-section width and height. The steel reinforcement mesh was embedded in UHPC in order to stabilize the tensile performance, which was proposed in [50]. However, in Fig. 11, most studies on the strengthening of UHPC were not reinforced with steel rebar inside UHPC (zero reinforcement ratio). For reinforced UHPC, the reinforcement ratio was mainly in the vicinity of 3.5%, which was significantly higher than the reinforcement ratio in RC mentioned before. This is because the cross-section area of the UHPC layer was lower than the unstrengthened RC beams/slabs area. However, from Fig. 11, with an increase in reinforcement ratio (from 0% to 3.5%) in UHPC, the ultimate strength was not also increased, possibly due to the limited sample quantitative with UHPC reinforced with steel mesh. However, it can be seen from Fig. 11 that reinforced UHPC has a tendency to change other failure modes in plain UHPC without reinforcement to the crushing of concrete and delamination of the UHPC layer with reinforcement ratio increase. This was found in [41], without steel reinforcement in the UHPC layer, localized macrocrack formed in the UHPC layer, which was totally broke regardless of interface treatments. With steel reinforcement embedded in the UHPC layer, the concrete was crushing with mechanical anchorages, while the UHPC layer was delaminated with the epoxy. However, it needs more tests to verify this trend. In [35], the authors compared the flexural behavior of strengthened beams with and

higher compressive strength such as 205 MPa in [40,41], the increase in ultimate strength was significant. For the T-sided strengthening configuration, the increase in ultimate strength versus UHPC thickness and steel fiber content in UHPC was presented in Figs. 9 and 10. It should be noted that both figures depict a similar trend. The thicker UHPC layer or higher steel fiber content increases the ultimate strength of tested specimens. Based on existing experimental studies, as summarized in Table 1, the most preferred thickness of UHPC was 50 mm. The highest increase was also observed in the 50 mm thickness UHPC layer with other optimal parameters. Also, in [35], a 50 mm thick UHPC layer was recommended for use due to softening behavior that occurred in the thicker UHPC layer. This was explained in [35], as UHPC layer thickness increased, UHPC tensile strength decreased because fewer fibers were observed near the UHPC top surface and the strength reduction is more pronounced in the thicker UHPC layer. Similarly, the most preferred steel fiber content was 3%. However, studies on the UHPC thickness of more than 50 mm or the steel fiber content more than 3% are limited, and the trend for changes in ultimate strength is unknown, as shown in Figs. 9 and 10. Therefore, the optimal UHPC thickness and steel fibers content might be 50 mm and 3%, respectively, based on the existing data. Moreover, From Figs. 9 and 10, the maximum increase in ultimate strength corresponds to the rupture of steel reinforcement at failure. This is because the rupture of steel reinforcement occurred needs more external energy than other failure modes. 12

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Fig. 10. effect of steel fiber content on the beam capacity in the T-sided configuration.

without steel reinforcement in the UHPC layer. More closely distributed macrocracks and obvious strain hardening performance were emphasized in the steel-reinforced UHPC layer. This is because of the tensionstiffening effect introduced by embedded steel reinforcements. Also, in [38], the authors also verified that the addition of steel reinforcements to the UHPC layer resulted in a significant increase in the load-carrying capacity. Additionally, the diameter of the steel reinforcement used in the UHPC overlay was reviewed. Steel reinforcement mesh with a 10 mm diameter was used in the UHPC overlay in [39]. Two rebars with a 10 mm diameter were used in the UHPC overlay in [33]. Three rebars with a 8 mm diameter were used in UHPC overlay in [40] and [41]. In [35], the diameter of the steel reinforcement used was not reported, while the overall area of them was provided. From this review, it can be seen that usually, the steel reinforcement with a smaller diameter was used in the UHPC overlay.

strength and split tensile strength for UHPC were 122.5 MPa and 20.7 MPa, respectively. In [36], the specimens were subjected to a curing temperature of 100 °C for 18 h in a steam curing chamber and allowed to attain the atmosphere temperature for another 4 h. In summary, it is found these curing conditions for UHPC are used for the material test specimens only. For the effect of curing type on the behavior of the strengthened beam or slab, the flexural behaviors of normal temperature cured and high temperature cured strengthened slabs were compared in [39]. The researchers found although hightemperature curing improves the flexural capacity of the strengthened slab, the effect of the pre-damage degree in the RC slab on the stiffness of the strengthened slab was higher than that of curing conditions on UHPC overlay. Until now, limited data related to the effect of the curing type on the flexural behavior of the strengthened beam or slab is reported.

3.7. Curing conditions for UHPC

3.8. Interfacial preparation

In [31], the material specimens were cured in a steam curing tank at 90 °C for 3 days. The strength achieved after 3 days in the steam tank (90 °C) was the same with the strength achieved 3 months under normal curing conditions. In [34], the material specimens were placed in an autoclave at 90 °C for 2 days and then in an oven at 200 °C for 1 day. The specimens were air-cooled for 6 h. The average compressive

Fig. 12 shows the interfacial preparation for the RC substrate before strengthening of UHPC. Seven different types of interfacial preparation were reviewed, namely no preparation, rough preparation, aggregate, rough + EA (epoxy adhesion), SB (sandblasting), EA (epoxy adhesion), rough + ST (studs), and ST (studs) only. It is crucial to conduct interfacial preparation according to previous experiments using 13

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Fig. 11. effect of reinforcement ratio in UHPC on the beam capacity in the T-sided configuration.

surface were compared. For a surface with rough + EA, mechanical abrasion was used for roughening of the concrete surface, and then 2.0 mm thickness epoxy adhesive was applied to the concrete surface. Another method to improve the connection between UHPC overly and concrete substrate was using studs (ST). Mechanical anchorages of ST would pass the holes reserved in prefabricated UHPC overlay. Comparing experimental results with rough + EA and ST in [40] and [41], using mechanical anchorages (ST) can better exploit the tensile potential of UHPC. More cracks were developed in the RC beam comparing the beam with rough + EA with only one crack and the delamination of UHPC caused by breaking the concrete cover, which was also observed in beam U1 [33]. Moreover, the ST beam has the main crack in the UHPC layer at the failure. Sandblasting (SB) and epoxy adhesion (EA) were used individually in [30]. The concrete surface was sandblasted with an average depth of 2 mm. Comparing SB and EA effective methods, no significant difference for flexural testing of strengthened beams was observed. However, the SB interface showed an overall better performance in terms of load–deflection behavior as shown in [30]. SB interface roughness was also used in [42]. Surface with rough + ST (studs) was developed by the authors in [39]. STs were used with partial long embedded into the concrete substrate of damaged members and remaining STs covered by the UHPC layer. This examined method can achieve high bond and avoid delamination between the UHPC and RC layers. Overall, from Fig. 12, interfacial

conventional concrete for strengthening [31]. For no preparation for the concrete substrate in [43], the reason for no preparation was due to casting UHPC and NSC at the same time, and this process can form a good bond. For rough preparation, different methods were used to reach the desired roughness level, and different standards were used to evaluate the roughness. In [31] and [33], an air chipping hammer and a pistol grip needle scaler were used for the roughening of the concrete surfaces, respectively. Both average roughness levels of 2–3 mm were evaluated by the sand patch test [51,52]. In [53], the contact surface of the RC substrate was roughened using a high-pressure water jet. In [54], a pneumatic hammer was employed to demolish a 15 mm cover, and the internal surface kept moist prior to casting UHPC. In [38], chisel and hammer were used for roughening, and the surface of the substrate kept moisture for 10 min, then wipe dry with a cloth. The aggregate preparation in [32] represents that the surface of the concrete substrate with a chemical retarder sprayed during casting was washed out at 24 h after concrete casting using a high-pressure water jet to expose the aggregate. Rough + EA (epoxy adhesion) represents a combination of roughening and epoxy adhesion, which was reported in [34,36,40,41,44,45]. The thickness of epoxy adhesion in [34] was 5 mm. An angle grinder was used to create a grid of grooves approximately 3 mm deep with spacing of about 100 mm on the tension surface of the damaged RC beams, which was reported in [29,44,45]. However, in [40] and [41], two methods for the preparation of the substrate 14

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Fig. 12. effect of interfacial preparation on the beam capacity.

4.1. Analytical models

preparation with roughening has been widely studied, and experimental results showed that the rough surface of the concrete substrate could provide the desired bond. Therefore, the roughening concrete substrate surface is suggested in the future application of UHPC strengthening as other methods increase the time cost. In addition, the roughening surface of the concrete substrate can obtain a higher increase in ultimate strength, as shown in Fig. 12. However, quantitative evaluation of the roughness degree for the interface needs development. Roughness degree should be classified accurately based on the improved efficiency to ease the application of the UHPC Layer to achieve the desired improvement of the bond strength.

Generally, the analytical models for bending were extended from the design of reinforced concrete. Two main assumptions were commonly used: (1) Bernoulli hypothesis; plane sections remain plane; (2) Monolithic behavior; no sliding occurred at the interface. It should be noted that different approaches were investigated to determine UHPC tensile behavior used in the calculation in [35], and different tensile behavior of UHPC performed a different structural response of UHPCRC composite members. Three concepts were discussed to obtain the failure load. One is using strain distribution to deduce formula in [32,35,30,39]. Firstly, the strain at the top fiber of concrete was assumed. Then, the stresses were determined based on strain distribution and material laws. Through the iteration process, normal force equilibrium can be achieved. Finally, the moment can be calculated by this equilibrium. In [30], for calculation of the failure load in 3-sided strengthening configuration, the authors used an equivalent rectangular stress block for UHPC in compression while a bilinear stress-strain curve was assumed for UHPC in tension (elastic and softening stages). This was different from the assumption in [35] where before the maximum strain, the material was considered as a bilinear continuum; beyond that, the bilinear softening law was defined as the change of stress versus an opening width for a fictitious crack. Another concept developed in [39] was based on failure mode to determine the failure load. This method was different from the former

4. Analytical models and numerical studies Based on failure mode, some analytical models were proposed for calculation of the cracking moment and ultimate moment of UHPC-RC composite members under flexure after the experiments have been done in literature [32,35,30,38,39,44,45]. Also, FE studies were conducted to understand better the flexural performance of composite members strengthened by UHPC. The comparison of analytical or numerical and experimental results in terms of the cracking moment and the ultimate moment was summarized in Table 2. Generally, both in analytical models and FE modeling, relatively fewer studies on the cracking moment was discussed, compared to the ultimate moment in the literature. 15

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Table 2 Comparison of Analytical/FE and Experimental Results. Ref.

Specimen name

Analytical models Mcr, Mcr,

ana/ exp

Mu, Mu,

FE studies ana/ exp

Mcr, Mcr,

FE/ exp

Mu, Mu,

FE/ exp

[32]-R

BU-20 BU-40 BU-60 BL-20 BL-40 BL-60

0.96 0.9 0.88 1.17 1.04 1.02

0.96 0.92 0.99 1.18 1.19 1.31

1.29 0.94 1.06 0.97 1.06 1.2

1 0.95 1.08 1.18 1.25 1.31

[33]-S

U1 U2 UB1 UB2

– – – –

– – – –

1.5 1.5 1.01 1.01

1.06 1.06 0.99 0.99

[34]

RA1 RA3 RA6 RB1 RB2 RB6 RC1 RC2 RC3

– – – – – – – – –

– – – – – – – – –

– – – – – – – – –

1.02 1.02 1.02 1.05 1.05 1.05 0.95 0.95 0.95

[35]

NR3(3) NR5(3) R5(3) R10(3)

0.7 0.7 0.90* 0.90*

0.95 0.95 0.95* 0.97*

– – – –

– – – –

RC-SB-BOTSJ RC-SB-2SJ RC-SB-3SJ RC-EP-BOTSJ RC-EP-2SJ RC-EP-3SJ

– – – – – –

0.95 1.11 1.07 0.88 1.03 1.04

– – – – – –

0.97 0.95 0.8 0.96 1.15 0.83

[38]

RE-20 RE-32 RE-50 RE-100 OV-25 OV-25a OV-50 OV-50a

– – – – – – – –

1.07** 0.80** 1.00** 0.99** – – – –

– – – – – – – –

0.95*** 1.02*** 1.06*** 1.03*** 1.03*** 0.95*** 0.94*** 1.00***

[39]

N-UC H-UC-1 H-UC-2

1.05 1.15 –

1.07 0.97 0.98

– – –

– – –

[42]

R/C jacket strengthened

–

–

1

1.12

[44,45]

A2 B2 C2

– – –

0.98 1.1 1.15

– – –

– – –

[30]

Fig. 13. Uniaxial compressive stress-strain curve for UHPC [32].

crack widths needs development to understand further and interpreter this experimental phenomenon. 4.2. FE models FE studies were conducted using commercial software [59–63] such as MSC/Marc in [32], ATENA in [33], ABAQUS in [30,34], LS-DYNA in [56] and DIANA 9 in [42]. Generally, results in terms of structural behavior and failure mode obtained from FE modeling agreed well with experimental observations with variations ranging from 0.94 to 1.50 for cracking moment and ranging from 0.83 to 1.31 for the ultimate moment with a certain degree of variability as listed in Table 2. In this table, the ‘ana’, ‘exp’ and ‘FE’ denote analytical, experimental and FE modeling results, respectively; Mcr and Mu represent cracking moment and ultimate moment, respectively. In the next two subsections, two critical aspects during modeling were reviewed in the literature, which might affect and determine the accuracy of structural response in FE models. 4.2.1. Compressive and tensile stress-strain relationship for UHPC Safdar [32] used a tri-linear model in accordance with AFGC [49] to simulate the compressive stress-strain relationship, as shown in Fig. 13. Tri-linear phases include linear-elastic stress rise, plastic plateau, and stress decrease. The tensile stress-strain relationship was modeled by bilinear softening and bilinear hardening, as shown in Fig. 14. In addition, the effect of reduced cracking strength and ultimate tensile strength of UHPC on structural response was discussed in the parameter study in [32]. Paschalis [33] also used strain hardening and softening phases to simulate UHPC in tension (Fig. 15), while for UHPC in compression, only ascending compressive branch and linearly descending were adopted. Moreover, UHPC overlay thickness and amounts of reinforcement of the UHPC layer were discussed in parameter analysis [33]. Murthy [34] and Al-Osta [30] used the concrete damage plasticity (CDP) model for simulation of nonlinear behavior of UHPC in compression and tension, as shown in Fig. 16. From the abovementioned constitutive laws for UHPC in compression and tension (Figs. 13–16), the overall shapes used in different literature are similar because of the high strength and strain hardening behavior of UHPC and are entirely different from the NSC. However, the maximum tensile and compressive strengths, as well as ultimate strains, are different because different

Note: * denotes that the analytical results were obtained from [39]; ** denotes that the analytical results were obtained from [55]; *** denotes that the FE results were obtained from [56].

because experimental observations at the failure determined the ultimate state. In [39], the calculation of ultimate flexural capacity at the ultimate limit state was based on the yield of steel reinforcement, concrete crushing, and yield of UHPC in tension. The calculation of cracking moment at the cracking state did not consider the previously cracked concrete substrate contribution. Also, the third method based on non-linear fracture theory was used in [44,45], which gave a satisfactory prediction of the experimental response of UHPC-RC composite members. Actually, from crack development processes for the strengthened beams in the literature, the first crack usually occurred in the RC beam (intact or damage) near the interface. This crack might influence the stress distribution at the crack tip region, leading to an effect on the cracking of the UHPC layer. The theory for the calculation of carrying the load capacity of the strengthened beam at different

16

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Fig. 14. Tensile stress-strain curve for UHPC (a) hardening stage (b) softening stage [32,34,35].

parameter (unconfined compressive concrete strength) is required as an input. G. Martinola [42] used the tensile behavior of UHPC with multilinear total strain rotating the crack model in Diana 9. 4.2.2. Interface modeling between UHPC overlay and RC substrate In [32], a perfect bond between UHPC and concrete was assumed due to the concrete surface with exposure of aggregate. In [33], the interface was modeled by 2-D contact elements with a friction coefficient equal to 1 and cohesion equal to 1.8 MPa. In [34] and [30], a perfect bond was assumed between UHPC and concrete. Hor Yin [55,56] developed equivalent beam elements to simulate the interfacial bond characteristics of UHPC-concrete composite members accurately. The material characteristics of an equivalent beam element were identified. A perfect bond was also chosen in [42], although smaller displacement in the numerical model was expected due to a bond loss in experimental results. Fig. 15. Tensile function for UHPC adopted in [33].

5. Cost analysis UHPC materials are developed in various institutes. Also, Hor Yin [56] used concrete damage model Mat-72r3 in LS-DYNA for simulation of UHPC behavior. The advantages of this model are that only a single

In [57], the authors reported that 1 m2 UHPC price increases 15$ every 1 cm thick in China. This price is slightly higher than that in [58],

Fig. 16. (a) Uniaxial compression behavior and (b) uniaxial tensile behavior [30]. 17

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with 11.3$ in Saudi Arabic. Also, in [58], the cost of different strengthening materials to achieve the given capacity enhancement was compared. The prices for RC, SCC, and CFRP are 3.9$, 4.6$, and 7.3$. All these prices are slightly lower than that of UHPC. In North America, however, considering the durability of UHPC, the cost of the strengthened structures with UHPC in the whole service life will be lower due to its lower maintenance cost.

(2)

6. Conclusions This research provides a comprehensive review of the existing studies on the flexural strengthening of RC beams or slabs with UHPC. Based on the evaluations of the experimental database, failure mode, analytical models, and FE studies presented in this paper, some conclusions can be drawn as follows:

(3)

(1) The experimental evidence showed that UHPC could be used to increase the flexural strength of RC beams or slabs. For the beams or slabs included in the database, the increase in flexural strength varied from 0% to 400%. The most significant increase in flexural strength was achieved by beams that were strengthened by reinforced UHPC at the tensile side. (2) The strengthening configuration of UHPC located at the tensile side was extensively investigated due to the advantages of strain hardening behavior of UHPC under tension. (3) Most specimens were strengthened with UHPC at tension face, resulting in the concrete substrate in a compression zone. Therefore, it is concluded that strengthening of UHPC can thoroughly exploit the potential of the tensile strength of UHPC and compressive strength of NSC with failure governed by concrete crushing. (4) For rectangular RC beams/slabs strengthened by UHPC, the ultimate strength increased with longitudinal reinforcement ratio in RC increase while the ultimate strength did not show an increase with NSC compressive strength increase and yield strength of steel reinforcement increase. Damage degree in RC prior to strengthening reduced the effectiveness of strengthening of UHPC from limited available experimental data. (5) Ultimate strength showed a more considerable increase with 45 GPa of elastic modulus of UHPC, while an increase in ultimate strength versus tensile and compressive strengths showed a wider scattering. The optimal UHPC thickness and steel fibers content might be 50 mm and 3%, respectively. Moreover, reinforced UHPC has a tendency to change other failure modes to the rapture of steel reinforcement inside the UHPC layer with the reinforcement ratio increase. (6) The roughening surface of the concrete substrate can enhance the bond strength between UHPC overlay and concrete substrate, leading to a higher increase in ultimate strength. Also, the roughening concrete substrate surface is suggested in the future application of UHPC strengthening as other methods increase the time cost of substrate surface preparation. (7) Three concepts based on strain distribution, failure mode, and nonlinear fracture theory, were used for calculation of flexural capacity of UHPC-RC composite members. All of them give a satisfactory prediction in cracking and ultimate strengths. (8) Results obtained from FE modeling agreed well with experimental observations with variations ranging from 0.94 to 1.50 for cracking moment and ranging from 0.83 to 1.31 for the ultimate moment with a certain degree of variability.

(4)

(5)

(6)

(7)

(8)

(9)

(10)

different ages, shrinkage effect in UHPC overlay should be included for analysis of the flexural behavior of UHPC-RC composite members to obtain more accurate results. The effect of the steel reinforcement ratio in the UHPC layer on the failure mode (debonding) of the strengthened beam/slab should be further investigated. Also, the effect of curing conditions for the UHPC layer on the flexural performance of the strengthened beam/slab should be investigated. Furthermore, the impact of the UHPC layer thickness (more than 50 mm) and fiber volume content of 3% on the flexural capacity should be further validated. The previous study on strengthening existing RC structures by UHPC was conducted at unloading status. Further research can focus on strengthening existing RC structures under sustained loading, which can represent actual circumstances in the field because the best way to conduct strengthening in the field is not to disrupt the traffic. The previous study focused on static flexural loading. Very fewer study on cyclic loading is conducted. Additional work is needed to explore the response of UHPC-RC composite structures under cyclic loading (such as fatigue and seismic loading). Most of the experimental studies have focused on strengthening uncracked and virgin RC beams/slabs. Additional work is needed to study damaged RC structures strengthened using UHPC. Quantitative evaluation of roughness degree for the concrete substrate needs development. Roughness degree should be classified accurately based on the improved efficiency to ease the application of the UHPC layer to achieve desired bond strength between UHPC overlay and concrete substrate. Additional studies are needed to consider the effect of slip between UHPC and RC layers on calculations of cracking and ultimate strengths in analytical and numerical models as the bond between them is not perfect. The theory for the calculation of carrying the load capacity of the strengthened beam at different crack widths in the RC beam needs to be investigated further to understand and interpreter the effect of the first crack initiated in the RC beam on the stress distribution in the crack tip region. Additional investigations on flexural strengthening with a combination of UHPC and other composite materials, such as FRP, are needed to explore because of the high cost of UHPC. The response of these composite members and optimization of these strengthening configurations should be identified. Design provisions for existing RC structures strengthened using UHPC need development.

Acknowledgments Financial support to complete this study was provided in part by the U.S. Department of Transportation under the auspices of Mid-America Transportation Center at the University of Nebraska, Lincoln, under Subaward No. 00059709. The findings and opinions expressed in this paper are those of the authors only and do not reflect the views of the sponsors. This research was also made possible with the support of the National Natural Science Foundation of China (Grant 51578226 and 51778221). Declaration of Competing Interest None.

7. Future recommendations

References

In order to better understand the flexural behavior of existing RC beams with UHPC overlay, future work was recommended as follows:

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