Stress strain performance of steel spiral confined recycled aggregate concrete

Journal Pre-proof Stress strain performance of steel spiral confined recycled aggregate concrete Muhammad Junaid Munir, Syed Minhaj Saleem Kazmi, Yu-Fei Wu, Indubhushan Patnaikuni, Yingwu Zhou, Feng Xing PII:

S0958-9465(20)30026-3

DOI:

https://doi.org/10.1016/j.cemconcomp.2020.103535

Reference:

CECO 103535

To appear in:

Cement and Concrete Composites

Received Date: 30 July 2019 Revised Date:

1 October 2019

Accepted Date: 20 January 2020

Please cite this article as: M.J. Munir, S.M. Saleem Kazmi, Y.-F. Wu, I. Patnaikuni, Y. Zhou, F. Xing, Stress strain performance of steel spiral confined recycled aggregate concrete, Cement and Concrete Composites, https://doi.org/10.1016/j.cemconcomp.2020.103535. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Stress strain performance of steel spiral confined recycled aggregate concrete

1 2 3 4 5 6 7 8 9

Muhammad Junaid Munir a, b, †, Syed Minhaj Saleem Kazmi a, b, †, Yu-Fei Wu b, *, Indubhushan Patnaikuni b, Yingwu Zhou a, Feng Xing a a

Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, China b

School of Engineering, RMIT University, 376-392 Swanston St, Melbourne, Victoria-3001, Australia.

10 11

ABSTRACT

12

Improvement of concrete strength owing to confinement by lateral reinforcement is disregarded in

13

the current concrete design practice. The focus of this study is to use the pre-existing lateral

14

reinforcement to enhance the behaviour of recycled aggregate concrete (RAC). The stress strain

15

performance of steel spiral confined concrete specimens with different confinement pressure,

16

recycled aggregates (RA) replacement ratio and target strength is studied. The results show a

17

decrease in compressive strength of concrete with the rise in RA replacement ratio. Around 43, 37

18

and 33% drop in the strength is observed for 100% RA replacement ratio having different target

19

strengths of NAC (i.e., 25, 40 and 60 MPa). However, steel confinement has a positive role to offset

20

the negative effect of RA on strength. The rise in the confinement pressure results in improved

21

ductility and stress strain behaviour of RAC. Due to scant research work related to steel spiral

22

confined RAC, existing models cannot estimate the stress strain performance of steel spiral

23

confined RAC effectively. Therefore, a new model is developed in this study, considering a large

24

experimental program. The newly developed model can be effectively used to predict the stress

25

strain performance of both steel spiral confined normal aggregate concrete and RAC, which

†

Joint first author * Corresponding author; E-mail: [email protected]

1

26

provides guidelines for the design of RAC members. Furthermore, a relationship is also developed

27

to determine the allowable content of RA for given confinement without affecting the concrete

28

strength, which may be used to decide the allowable content of RA in designing the RAC

29

compression members.

30

Keywords: Recycled aggregate concrete, Recycled aggregates, Steel spiral, Confinement, Stress

31

strain

behaviour

2

32

1. Introduction

33

Due to the demolishing of old structures and construction of new buildings, a huge quantity of

34

construction and demolition (C&D) waste is generated every year worldwide. China generates

35

around 0.2 billion tons of C&D waste annually [1]. Similarly, 24 million tons of C&D waste is

36

generated yearly only in São Paulo state of Brazil [2]. Generally, C&D waste largely comprises of

37

waste concrete [3]. Each year, Australia dumps approximately 5 million tons of waste concrete to

38

landfills after recycling around 6 million tons of the waste [4].

39

Concrete being the most extensively used material is mainly comprised of natural resources.

40

Natural coarse aggregates occupy up to 50% of the total concrete volume [5]. Annually, around

41

48000 million tons of natural coarse aggregates are consumed in the manufacturing of concrete and

42

this demand is increasing drastically [6, 7]. The reuse of C&D waste in the form of recycled

43

aggregates (RA) to produce recycled aggregate concrete (RAC) may be a viable option for the

44

conservation of natural resources. Thus, dumping issues of C&D waste into the landfills can be

45

resolved.

46

Owing to the environmental, economic and social significance, various researchers have studied the

47

performance of RAC [8, 9]. Old adhered mortar present on the surface of RA is the main difference

48

between RA and natural aggregates (NA), which results into the inferior properties of RAC than

49

traditional concrete [5, 6, 10]. Therefore, the practical application of RAC is restricted to non-

50

structural purposes such as road bases. Approximately, 20% and 40% drop in compressive strength

51

and elastic modulus was reported for 100% RA replacement ratio in RAC in comparison with

52

normal aggregate concrete (NAC) [11, 12]. Therefore, the behaviour of RAC worsens with the rise

53

in replacement ratio of RA and is the major concern regarding the structural application of RAC.

3

54

Previously, various researchers explored different ways to enhance the behaviour of RAC [13-15].

55

For this reason, either the old adhered mortar of RA is strengthened or removed. To strengthen the

56

adhered mortar, different techniques like a coating of RA with organic, inorganic or cementitious

57

solutions and deposition of calcium carbonate (CaCO3) on the surface of RA were adopted by

58

researchers [16-18]. Similarly, techniques like mechanical grinding, thermal treatment and soaking

59

in acids were adopted to enhance the performance of RAC by removing the old adhered mortar of

60

RA [19, 20]. Although improved behaviour was observed for RAC with treated RA, the overall

61

performance of RAC with treated RA was still observed much lower than NAC [5, 21].

62

Furthermore, treatment techniques of RA may result in higher CO2 emission, increased energy

63

consumption, increased environmental burden through the production of waste fine material [8, 22].

64

Hence, there is still the utmost need to enhance the behaviour of RAC.

65

It has been well reported in the literature that lateral confinement by materials such as steel tubes

66

and fiber-reinforced polymer (FRP) enhances the load-carrying capacity and mechanical

67

performance of NAC [23-25]. Various researchers have also explored the performance of RAC

68

using different confinement techniques [26-30]. Improved strength and deformation capacity were

69

observed by Yang and Han [31] for steel tube confined RAC. Similarly, Xiao et al. and Zhao et al.

70

[32, 33] studied the improved performance of RAC after confining with FRP. Although the

71

improved performance of RAC can be achieved by different confinement techniques, most of the

72

techniques are used for strengthening of the old structure and are considered practically unsuitable

73

for constructing a new structure.

74

Strength and ductility of NAC can also be enhanced by confining with spiral reinforcement [34].

75

Several models are present in the literature to estimate the behaviour of NAC confined with steel

76

spirals. Among different models, Richart et al. model has been adopted widely by different

77

researchers to develop better performing models for steel spiral confined NAC [35, 36]. Mander et 4

78

al. [24, 25] developed the axial stress strain model after testing various full-scale steel spiral

79

confined concrete columns. Razvi and Saatcioglu [37] and Hoshikuma et al. [38] also studied the

80

stress strain performance of steel spiral confined NAC considering different compressive strengths,

81

shapes of the concrete specimens, confinement pressures and properties of transverse

82

reinforcement.

83

2. Research significance

84

For efficient waste management and natural resource conservation, various countries are promoting

85

the utilization of waste materials including C&D waste [39], recycled glass powder [40-44], fly ash

86

[45], waste marble powder [46-48], slag [49], rice husk ash [50-52] and sugarcane bagasse ash [53-

87

55] in the production of novel construction materials. Recently, local administration in China’s

88

main cities has made it obligatory to consume RA in the manufacturing of concrete [39, 56].

89

However, the inferior properties of RAC than NAC has limited the applications of RAC. Moreover,

90

non-eco-friendly and costly techniques are required to enhance the performance of RAC. Although

91

confinement through lateral reinforcement improved the performance of NAC, limited experimental

92

data is present in the literature about the application of steel spirals in improving the behaviour of

93

RAC. The aim of this study is to utilize steel spirals to enhance the performance of RAC. Steel

94

spirals are commonly utilized in the construction of NAC columns. However, the strength

95

improvement of concrete due to confinement by lateral reinforcement is ignored in the current

96

concrete design practice. As steel spirals are already utilized in the concrete columns, improving the

97

performance of RAC by confining through steel spirals can be an easily adoptable and cost-

98

effective solution. It is therefore important to find out whether the inferior performance of RAC can

99

be fully or partially offset by considering the pre-existing transverse reinforcement in concrete

100

compression members. This study aims to analyze the behaviour of steel spiral confined NAC and

101

RAC. For this reason, stress strain behaviour of steel spiral confined concrete specimens having 5

102

different confinement pressure, replacement ratio of RA and target strength is considered. After

103

improving the parameters of top-performing stress strain models for steel spiral confined NAC, a

104

novel model for stress strain behaviour of steel spiral confined NAC and RAC is also developed to

105

reflect the effect of replacement ratio of RA. The newly developed model can be effectively utilized

106

to predict the stress strain performance of both steel spiral confined NAC and RAC, which provides

107

guidelines for the design of RAC members. Furthermore, a relationship is also developed to

108

determine the allowable amount of RA for given confinement without affecting the concrete

109

strength, which may be used to decide the allowable content of RA in designing the RAC

110

compression members.

111

3. Test Program

112

3.1.

113

In this work, RA with a size between 4 – 20 mm were obtained from a local C&D waste recycling

114

plant in Shenzhen, China. Owing to the screening and crushing of various sources of C&D waste at

115

the same recycling plant, age and strength of the parent concrete of RA were unknown. RA mainly

116

included concrete waste with other impurities (like ceramics, bricks, glass and asphalt) less than

117

5%. Based on the guidelines provided by CSIRO, Australia [57], RA used in this study are

118

classified as class 1B with a total amount of impurities less than 30%. Crushed granite was utilized

119

as NA during this work. Figure 1 shows the almost similar grading of both NA and RA satisfying

120

the limits of ASTM C33:2016 [58]. Physical properties of NA and RA are presented in Table 1.

121

Lower bulk density and higher water absorption are observed for RA as compared to NA, which

122

may be related to the adhered mortar [59, 60].

123

All the concrete mixtures (presented in Table 2) were prepared using ordinary Portland cement

124

(type P.II52.5R), river sand and tap water. Before concrete mixing, all the aggregates were first

Materials

6

125

oven-dried at 105oC for 24 hours and then additional water was added in each mixture to consider

126

the water absorption capacity of aggregates. For series A and B, all the concrete mixtures showed a

127

slump of 90-100 mm, whereas the slump of 50-60 mm was observed for all the concrete mixtures of

128

series C.

129

To confine the concrete specimens, 3 mm round steel wire was used as a spiral reinforcement. The

130

properties of the steel wire are presented in Table 3, whereas stress strain performance is depicted

131

in Fig. 2.

132

3.2.

133

For each series, 48 (i.e., 12 unconfined and 36 steel spiral confined) concrete specimens with a

134

diameter of 150 mm and height of 300 mm were cast in accordance with ASTM C192:2016 [61]

135

and 28 days curing was performed. Test variables comprised of confinement pressures, replacement

136

ratios of RA, and target strengths of NAC specimens. Four different replacement ratios of RA (i.e.,

137

0, 20, 50 and 100%) along with three different target strengths of NAC specimens (i.e., 25 MPa, 40

138

MPa and 60 MPa) were considered during the study. To consider three varying confinement levels

139

(i.e., a lower, similar and higher peak stress than NAC specimens), three different pitches (i.e.,

140

center to center spacings) of steel spirals (i.e., 40, 30 and 20 mm, respectively) were selected

141

considering Wu and Wei [62] model. Facilities of local steel manufacturer were utilized to fabricate

142

the steel spirals with an outer diameter of 150 mm to confine the specimens. To examine the pure

143

effect of confinement pressure, no longitudinal reinforcement and concrete cover were considered

144

in this study.

145

To cast the steel spiral confined specimens, concrete was poured after placing the steel spirals in the

146

cylindrical molds. Three specimens were cast and tested for each combination. Table 4 presents the

147

details of concrete specimens. The designations of concrete cylinders show replacement ratios of

Details of specimens

7

148

RA (R0, R20, R50 and R100 indicate 0, 20, 50 and 100% replacement ratio of RA, respectively),

149

pitches of steel spirals (S20, S30 and S40 denote pitch of 20, 30 and 40 mm, respectively) and

150

target strengths of NAC (series A, B and C show the mix design with target strength of 25 MPa, 40

151

MPa and 60 MPa for NAC, respectively). For example, R20-S40-B represents the concrete

152

specimen with 20% of NA replaced by RA, confined by 40 mm pitch of steel spirals and having the

153

target strength of 40 MPa for NAC.

154

3.3.

155

Compression tests were performed on the specimens using displacement control mode with a rate of

156

0.3 mm/min through the MTS machine having a maximum load capacity of 3000 kN. Data

157

acquisition system and four linear variable displacement transducers (LVDTs) mounted at 90o

158

relative to each other on an aluminum frame (designed with a gauge length of 185 mm in the

159

middle of concrete specimens) were used (Fig. 3) to record the load and to measure the axial

160

deformation, respectively. During the test, applied load and deformation were measured to examine

161

the stress strain behaviour of concrete specimens.

162

4. Results and Discussion

163

4.1.

164

Figure 4 shows the failure behaviour of concrete specimens. For unconfined specimens, larger in

165

size and less number of cracks were noticed for RAC than NAC, which may be attributed to the

166

lower energy dissipation capacity and ductility of RAC as compared to NAC [63]. RAC was

167

noticed to be more brittle as compared to NAC owing to the presence of porous adhered mortar and

168

weak bond of RA with the cement paste. As obvious, brittleness was observed higher in series C

169

specimens, due to the higher strength than series A and B specimens.

Test setup and instrumentation

Failure mode

8

170

For confined specimens, steel spirals produced a triaxial stress state on the concrete core through

171

lateral stress and resulted in high ductility and ultimate capacity. This behaviour is observed more

172

noticeable with the increase in confinement pressure. Maximum tensile stress was produced in the

173

middle portion of spiral reinforcement [64]. As the axial load reached the peak load, significant

174

cracks were noticed in the confined specimens. All the confined specimens were tested until the

175

fracture of spiral reinforcement. A loud noise of fracture and sudden crushing of concrete were

176

observed at the failure. All the specimens failed owing to the loss of transverse reinforcement after

177

fracturing of spiral reinforcement at multiple points.

178

4.2.

179

The effect of the replacement ratio of RA on the stress strain behaviour of confined and unconfined

180

specimens is presented in Fig. 5. The axial strain of a specimen was calculated from the average

181

reading of the four vertical LVDTs. For all the specimens, the shape of the stress strain curves

182

changes gradually and becomes flatter with the rise in replacement ratio of RA.

183

For unconfined specimens, a decrease in initial slope (i.e., elastic modulus) and a lower peak are

184

observed with the rise in replacement ratio of RA as shown in Fig. 5(a-c). In the case of series A,

185

unconfined specimens show a decrease in the slope of descending branches of stress strain curves

186

with the rise in RA replacement ratio. This may be related to the lower strength of concrete in series

187

A with the rise in replacement ratio of RA. In case of series B and C specimens, the descending

188

portions of the stress strain curves are observed steeper with the rise in replacement ratio of RA,

189

which may be related to the more brittle behaviour of RAC in comparison with NAC as reported in

190

a past study [65]. Unconfined specimens also show lesser energy absorption capacity through the

191

smaller area under the curve with the rise in replacement ratio of RA.

Axial stress strain performance

9

192

For all the confined specimens, lower stress strain curves are obtained with the rise in RA

193

replacement ratio, as shown in Fig. 5(d-l). Effect of confinement in terms of strength improvement

194

increases with the rise in RA replacement ratio. Furthermore, the effect of confinement is observed

195

more prominent for lower strength concrete such as series A specimens as compared to series B and

196

C specimens. Similar findings are reported in the previous study by Wang et al. [64]. Strength drop

197

owing to an increase in replacement ratio of RA also reduces in the presence of confinement, which

198

is consistent with the observations of Teng et al. [66] for FRP confined RAC.

199

Figure 6 presents the effect of confinement pressure on the stress strain behaviour of concrete

200

specimens. For all the specimens, the increase in peak stress and strain is observed with the rise in

201

confinement pressure, depicting the beneficial role of confinement to enhance the energy

202

dissipation capacity of concrete specimens.

203

4.3.

204

4.3.1. Peak stress

205

Figure 7 shows the effect of confinement pressure and replacement ratio of RA on the average peak

206

stress values of concrete specimens. For all the specimens, the reduction in peak stress is observed

207

with the rise in RA replacement ratio. For example, R100-A, R100-B and R100-C specimens show

208

43%, 37% and 33% decrease in average peak stress than R0-A, R0-B and R0-C specimens,

209

respectively. The difference in strength reduction is related to the difference in concrete mix design

210

for each series [67-69]. Similar behaviour was reported in the previous studies by Pacheco et al.

211

[70] and Xu et al. [71].

212

The rise in confinement pressure results in an increase in average peak stress. All the confined

213

specimens of series A except R100-S40-A show average peak stresses higher in comparison with

214

unconfined R0-A specimens. Similarly, all the confined specimens of series B except R50-S40-B,

Stress strain characteristics

10

215

R100-S40-B and R100-S30-B show average peak stresses higher in comparison with unconfined

216

R0-B specimens. In case of series C, only the peak stresses of R0-S40-C, R0-S30-C, R0-S20-C and

217

R20-S20-C specimens are observed higher than unconfined R0-C specimens, whereas, all the other

218

specimens show peak stresses still lower than unconfined R0-C specimens. By comparing, R100-

219

S40-B with R0-S40-B and R100-S20-B with R0-S20-B, reduction in strength owing to the

220

utilization of RA reduces to 36% and 32% by the confinement provided by 40 mm and 20 mm pitch

221

of spiral reinforcement, respectively. Similar behaviour is observed for series A and C specimens as

222

well, which depicts the positive role of increasing confinement pressure to offset the adverse effect

223

of RA replacement ratio. Zhao et al. [33] also reported a similar trend for FRP confined RAC.

224

4.3.2. Peak strain

225

Figure 8 presents the effect of confinement pressure and replacement ratio of RA on the average

226

peak strain values of concrete specimens. For all the confined and unconfined specimens, a rise in

227

peak strain is observed with the rise in replacement ratio of RA. For example, R0-A specimens

228

show an average peak strain value of 0.0019, which increases to 0.0032 for R100-A specimens.

229

Similarly, R0-S20-C specimens show an average peak strain value of 0.0034, which increases to

230

0.0043 for R100-S20-C specimens. The rise in the peak strain of RAC may be related to the

231

decrease in elastic modulus of RAC as compared to NAC, which results in larger deformation as

232

reported in previous studies [63, 72].

233

The rise in confinement pressure results in an increase in average peak strain. As obvious, peak

234

strains of all the confined specimens in each series are observed higher than respective unconfined

235

NAC specimens. As the effect of confinement in terms of strength improvement is more significant

236

for lower strength concrete, the increase in peak strain is also noticed higher in case of series A

237

specimens than series B and C specimens. 11

238

4.3.3. Ultimate strain

239

Figure 9 shows the effect of confinement pressure and replacement ratio of RA on the average

240

ultimate strain values of concrete specimens. For unconfined specimens, the ultimate strain is taken

241

at 85% of the peak stress as per previous studies [40, 48]. For series A specimens, a rise in ultimate

242

strain is noticed with the rise in RA replacement ratio, which may be related to the production of

243

low strength concrete in series A with the rise in RA replacement ratio. However, no significant

244

effect on the ultimate strain of unconfined specimens is noticed in series B and C with the rise in the

245

replacement ratio of RA. It should be considered that the descending curve of concrete specimens

246

depends on the rigidity of the testing machine. Therefore, the ultimate strain values of unconfined

247

specimens presented in Fig. 9 are for reference only and will not be considered for the modeling.

248

For the confined specimens, the ultimate strain was reported at the first fracture of steel spirals

249

following the previous study by Wei and Wu [73]. The rise in confinement pressure and RA

250

replacement ratio result in an increase in the average ultimate strain. As obvious, ultimate strains of

251

all the confined specimens in each series are observed higher than respective unconfined NAC

252

specimens. Effect of confinement on the ultimate strain is observed more prominent with the rise in

253

RA replacement ratio. For example, R0-S20-B specimens show 5 times higher ultimate strain than

254

R0-B specimens, whereas R100-S20-B specimens show 6 times higher ultimate strain than R100-B

255

specimens. This may be attributed to the confinement pressure, which improves the ultimate strain

256

performance of RAC. Similar behaviour is observed in previous studies [33, 66] for FRP confined

257

RAC specimens.

258

4.3.4. Modulus of elasticity

259

Figure 10 presents the effect of confinement pressure and replacement ratio of RA on the average

260

elastic modulus of concrete specimens, which is calculated through the initial slope of the axial

12

261

stress strain curve following [74]. For all the specimens, the reduction in elastic modulus is noticed

262

with the rise in RA replacement ratio. For example, R100-A, R100-B and R100-C specimens show

263

49%, 40% and 41% decrease in average elastic modulus than R0-A, R0-B and R0-C specimens,

264

respectively. The reduction in elastic modulus of concrete specimens may be related to the lower

265

elastic modulus of RA (owing to porous adhered mortar), whereas, the difference in the decrease of

266

elastic modulus for each series is attributed to the difference in the concrete mix design. Similar

267

behaviour was reported in the previous studies as well [72, 75].

268

The rise in confinement pressure results in a reduction in average elastic modulus. For example,

269

R20-A specimens show an average elastic modulus of 19.9 GPa, which reduces to 19.8 GPa, 18.7

270

GPa and 18.1 GPa for R20-S40-A, R20-S30-A and R20-S20-A specimens, respectively. Similarly,

271

R100-B specimens show an average elastic modulus of 20.3 GPa, which decreases to 18.7 GPa,

272

18.5 GPa and 16.6 GPa for R100-S40-B, R100-S30-B and R100-S20-B specimens, respectively. In

273

previous studies, researchers [36, 76] also reported the decrease in elastic modulus of NAC with the

274

rise in pressure through confinement steel. However, there should be no effect of confinement on

275

the elastic modulus of concrete specimens. Therefore, a decrease in elastic modulus was reported as

276

a random error in the previous study [76]. The reduction of elastic modulus was attributed to the

277

presence of steel spirals that produced gaps between steel spirals and surrounding concrete due to

278

concrete shrinkage [77]. Concrete cross-sectional area may be reduced at the location of spirals

279

owing to the shrinkage of concrete and may result in reduced rigidity and elastic modulus of

280

confined specimens through a reduction in the effective concrete area. More spirals result in a large

281

decrease in effective concrete and cause a higher decrease in elastic modulus of confined

282

specimens.

13

283

4.3.5. Toughness

284

Energy absorption capacity i.e., toughness was calculated as the area under the stress strain curve

285

up to the ultimate strain of concrete specimens [78, 79]. Figure 11 presents the effect of

286

confinement pressure and replacement ratio of RA on the average toughness values of concrete

287

specimens. For unconfined specimens of series A, the increase in toughness is observed with the

288

rise in replacement ratio of RA. This may be related to the reduction in the slope of descending

289

branches of stress strain curves of unconfined specimens in series A with the rise in RA

290

replacement ratio, owing to the production of low strength concrete. However, all the other

291

confined and unconfined specimens show a decrease in toughness with the rise in the replacement

292

ratio of RA. For instance, R0-S40-B specimens show an average toughness of 0.6 MPa, which

293

decreases to 0.4 MPa for R100-S40-B specimens. Similarly, R0-S40-C specimens depict an average

294

toughness of 0.38, which decreases to 0.32 for R100-S40-C specimens. The decrease in toughness

295

may be related to the lower performance of RAC as compared to NAC [78]. Similar behaviour was

296

observed in a previous study [77].

297

The rise in confinement pressure results in an increase in toughness. Values of toughness for all the

298

confined specimens in each series are observed higher than respective unconfined NAC specimens.

299

4.3.6. Specific toughness

300

Specific toughness calculated as the ratio of the toughness of each concrete mix to its corresponding

301

compressive strength is reported as a better measure of toughness in accordance with past studies

302

[80, 81]. Figure 12 shows the effect of confinement pressure and replacement ratio of RA on the

303

specific toughness of concrete specimens. For unconfined specimens of series A, the increase in

304

specific toughness is noticed with the rise in RA replacement ratio. This may be related to the

305

production of low strength concrete in series A with the rise in replacement ratio of RA, which is

14

306

less brittle in nature than NAC. However, decrease in specific toughness is noticed with the rise in

307

RA replacement ratio for unconfined specimens in series B and C. For instance, R100-B and R100-

308

C specimens show 2% and 1% decrease in average specific toughness than R0-B and R0-C

309

specimens, respectively. The reduction in toughness may be related to the brittle behaviour of RAC

310

than NAC as reported in past studies [77, 82].

311

For the confined specimens, rise in confinement pressure and RA replacement ratio result in an

312

increase in specific toughness. As obvious, specific toughness values of all the confined specimens

313

in each series are observed higher than respective unconfined NAC specimens. Effect of

314

confinement on specific toughness is noticed more prominent with the rise in RA replacement ratio.

315

For example, R0-S20-B specimens show 6 times higher specific toughness than R0-B specimens,

316

whereas R100-S20-B specimens show 7 times higher specific toughness than R100-B specimens.

317

This may be attributed to the confinement pressure, which improves the specific toughness of RAC

318

[77].

319

4.4.

320

4.4.1. Stress strain performance

321

In this work, Mander model [24] for steel spiral confined NAC is considered to estimate the stress

322

strain behaviour of steel spiral confined RAC, which is expressed by Eqs. (1) - (5).

Analytical modeling

= 323

. . −1+

Ɛ ≤ Ɛ

(1)

where =

Ɛ Ɛ

15

(2)

=

− =

= 5000 324

Ɛ (unit in MPa)

(3)

(4)

(5)

where fcc and Ɛ are the peak stress and peak strain (i.e., strain at peak stress) of the steel spiral

325

confined concrete specimens, respectively; Ɛ

326

steel spirals); Esec and Ec are the secant modulus at peak stress and modulus of elasticity of steel

327

spiral confined concrete specimens, respectively; fco is the peak stress of unconfined specimens.

328

Based on Eqs. (1) – (5), fcc, Ɛ

329

Mander et al. [24]. In this study, these parameters are predicted using different existing models for

330

steel spiral confined NAC. For this purpose, an up-to-date and large test database is used

331

considering the test results available in the literature as well as in this study regarding the stress

332

strain behaviour of steel spiral confined NAC. Based on the prediction results, a suitable and best-

333

performing model is selected and these parameters are further regressed considering the

334

experimental data for steel spiral confined RAC. Owing to the availability of scant work in the

335

literature about steel spiral confined RAC, test results only from this study are used to model the

336

role of replacement ratio of RA. Considering the difference in modulus of elasticity of RAC than

337

NAC, Eq. (5) in Mander model [24] also needs to be modified to consider the effect of replacement

338

ratio of RA. The new model can be used for both steel spiral confined NAC and RAC.

339

4.4.2. Test database and existing models

340

Although detailed analytical and experimental studies are present in the literature about the stress

341

strain behaviour of steel spiral confined NAC, most of the available test data involves the effect/role

and Ɛ

is the ultimate strain (i.e., strain at first fracture of

are the important parameters of the model proposed by

16

342

of longitudinal steel reinforcement. Therefore, limited test data is present in the current literature

343

about the pure role of confinement on concrete strength. Table 5 depicts the detail of the

344

experimental database used in this study regarding the axial stress strain behaviour of steel spiral

345

confined NAC specimens with no effect of longitudinal steel reinforcement. Experimental database

346

includes unconfined compressive strength (fco) ranging from 15 – 85 MPa, confining pressure (fl)

347

ranging from 1.0 – 28.3 MPa, confinement ratio (fl/fco) ranging from 0.03 – 0.83 and peak stress

348

ratio (fcc/fco) ranging from 0.68 – 4.0.

349

Table 6 presents the details of existing stress strain models to estimate the peak stress and peak

350

strain of steel spiral confined NAC. All the presented models are empirical models and were

351

regressed through experimental results. Some models [24, 37, 38, 83, 84] are applicable to both

352

circular and rectangular sections. However, some models [35, 73, 85, 86] are only developed for

353

circular sections. In Table 6, s′ and s are clear and center to center spacing of steel spiral,

354

respectively; fs is the strength of spiral reinforcement; fy and dsp are the yield strength and the

355

diameter of steel spiral, respectively; Al and As are the cross-sectional area of longitudinal and spiral

356

reinforcement, respectively; D is the diameter of confined specimen; and Ɛ is the peak strain of the

357

unconfined concrete specimens. The performance of the models is evaluated through the following

358

error-index [73]: =

|

|

. −

.|

.|

(6)

359

where n, Ana. and Exp. are a total number, analytical results and experimental results of confined

360

specimens, respectively. On the basis of error-index, the parameters of top-performing stress strain

361

models for steel spiral confined NAC are further improved and a novel stress strain model for steel

362

spiral confined NAC and RAC is developed to consider the effect of replacement ratio of RA. 17

363

4.4.3. Modulus of elasticity

364

In order to calculate Young’s modulus of concrete specimens (Ec), Eq. (5) presented by Mander et

365

al. [24] is modified to consider the effect of replacement ratio of RA through regression analysis of

366

the experimental data of RAC from this study, which gives: = (5000 − 423! " + 2341!)

(unit in MPa)

(7)

367

where, R defined as the RA replacement ratio ranges from 0 to 1 (i.e., 0 for 0% replacement ratio of

368

RA and 1 for 100% replacement ratio of RA). It is worth mentioning that Eq. (7) degenerates into

369

Eq. (5) for R equal to zero (0).

370

4.4.4. Peak stress

371

The evaluation of existing models to estimate the peak stress of steel spiral confined NAC is

372

presented in Fig. 13(a). The comparison shows that the peak stress of steel spiral confined NAC can

373

be well predicted by existing models. However, Hoshikuma et al. [38] model overestimates the

374

peak stress of steel spiral confined NAC, particularly at high confinement pressure, with an error

375

index of 35% (Table 7). All the other models show the error-index less than 20%. Wei and Wu [73]

376

model depicts the least error index of 10%. Therefore, Wei and Wu [73] model (Table 6) is used as

377

the base model for further modeling in this study for steel spiral confined RAC.

378

Based on Richart et al. [35], Wei and Wu [73] presented Eq. (8) to determine the peak stress of steel

379

spiral confined NAC. Coefficient k1 is proposed by Wei and Wu [73] considering an up-to-date and

380

large test database. This model is extended in this study through regression analyses of the

381

experimental database to consider the role of the replacement ratio of RA and is presented in Eq.

382

(9):

18

= 1 + $% 383

&

(8)

where $% = 5.35

&

'(.%)

− 0.546! (.+, &

=

2

&

'%

(unit in MPa)

-.

(9)

(10)

384

where, fl is the confinement pressure, determined through Eq. (10). The above model degenerates

385

into the original model developed by Wei and Wu [73] for R = 0.

386

The evaluation of the proposed model to estimate the peak stress of steel spiral confined NAC and

387

RAC is presented in Fig. 13(b). Peak stress values predicted by the proposed model are noticed

388

close to the test results with an error index of 10% as presented in Table 8.

389

4.4.5. Peak strain

390

Different models are present in the literature to estimate the peak strain of steel spiral confined

391

NAC as presented in Table 6. Figure 14(a) depicts the evaluation of existing models to estimate

392

the peak strain of steel spiral confined NAC. The peak strain of steel spiral confined NAC is

393

underestimated and overestimated by Li et al. [83] model and El-Dash and Ahmad [85] model,

394

respectively. The error indexes for the models by Li et al. [83] and El-Dash and Ahmad [85]are

395

higher than 45% (Table 7). All the other models depict an error-index ranging from 25-40%.

396

Models such as Mander et al. [24], Razvi and Saatcioglu [37] and Wei and Wu [73] show an error

397

index of 25.25%, 26% and 27.27%. Mander et al. [24] model to estimate the peak strain of steel

398

spiral confined NAC involves predicted peak stress value of steel spiral confined NAC. Similarly,

399

Razvi and Saatcioglu [37] model involves many coefficients to predict the peak strain of steel spiral

19

400

confined NAC. Therefore, models such as Mander et al. [24] and Razvi and Saatcioglu [37] are not

401

effective to estimate the performance of steel spiral confined RAC. In this study, Wei and Wu [73]

402

model (Table 6) is used as the base model. Regression analyses of the experimental database for

403

steel spiral confined RAC gives Eq. (11): /00

/01

4

"

4

5

= 1 + 220.6 + 3661 35 6 + 734 5 8 5 9 (unit in MPa) 01

01

01

(11)

404

The evaluation of the proposed model to predict the peak strain of steel spiral confined NAC and

405

RAC is presented in Fig. 14(b). Peak strain values predicted by the proposed model are noticed

406

close to the test results, with an error index of 24% (Table 8).

407

4.4.6. Ultimate strain

408

Few models are present in the literature to estimate the ultimate strain of steel spiral confined NAC

409

at first fracture of steel spirals. Scott et al. [87] proposed Eq. (12) to estimate the ultimate strain of

410

steel spiral confined NAC at first hoop fracture: :

= 0.004 + 0.9< =

5>

@ (unit in MPa)

?((

(12)

411

where ρs is the volumetric ratio of steel spirals. Mander et al. [24] used an energy balance approach

412

to predict the ultimate strain of steel spiral confined NAC. However, the application of the method

413

was observed inconvenient [73]. Furthermore, it has recently been found that the widely adopted

414

energy balance approach is invalid and should be modified [88]. Priestley et al. [89] proposed a

415

model as presented in Eq. (13): :

= 0.004 + 2.8:

20

B

&

C

(13)

416

where Ɛ is the fracture strain of the steel spirals. On the basis of these models, Wei and Wu [73]

417

developed the following model for steel spiral confined NAC at first fracture of steel spirals: : :

= 1.75 + 900:

B

&

C

(14)

418

Figure 15(a) depicts the performance of these existing models to estimate the ultimate strain of

419

steel spiral confined NAC. The comparison shows that the ultimate strain predicted by Scott et al.

420

[87] model generally overestimates the test results of steel spiral confined NAC with an error index

421

of 185% (Table 9). However, Priestley et al. [89] model and Wei and Wu [73] model depict the

422

error indexes of 28% and 26%, respectively. Therefore, Wei and Wu [73] model (Eq. (14)) is used

423

as the base model, which is modified through regression analyses of the experimental database for

424

steel spiral confined RAC to give: : :

= 1.75 − 1.73! " + 2.98! + 900:

B

&

C

(15)

425

Figure 15(b) shows the evaluation of the proposed model to estimate the ultimate strain of steel

426

spiral confined NAC and RAC, with an error index of 17% (Table 10).

427

4.4.7. Model Performance

428

Figure 16 depicts the performance of the proposed model as well as other existing models to

429

estimate the stress strain behaviour of steel spiral confined NAC with the test results of present and

430

past studies. The comparison indicates that the stress strain behaviour of steel spiral confined NAC

431

can be well estimated by all the models. However, models such as El-Dash and Ahmad [85], Assa

432

et al. [86] and Shah et al. [84] underestimates the stress strain curves of steel spiral confined NAC.

433

Similarly, Hoshikuma et al. [38] model overestimates the stress strain curves. Furthermore, Li et al.

434

[83] model and Hoshikuma et al. [38] model are ineffective to estimate the descending part of the 21

435

curves of steel spiral confined NAC. The stress strain curves estimated by the models such as

436

Mander et al. [24], Razvi and Saatcioglu [37], Wei and Wu [73] and proposed model are noticed

437

closer to the test results.

438

Figure 17 depicts the performance of the proposed model to estimate the stress strain behaviour of

439

steel spiral confined RAC with the test results of this work. The proposed model generally performs

440

well. Based on this study, the proposed model can be effectively used to predict the stress strain

441

behaviour of both steel spiral confined NAC and RAC.

442

4.4.8 Allowable replacement ratio of RA

443

To gain the peak strength of steel spiral confined RAC similar to NAC, the allowable amount of RA

444

can be calculated by putting peak stress of unconfined NAC specimens (fco) in Eq. 8 equal to peak

445

stress of steel spiral confined RAC specimens (fcc). In this case, k1 in Eq. 9 is equal to zero, giving: R=

%,⋅FG59H.H H⋅IJI 501

(unit in MPa)

(16)

446

The proposed relationship is developed based on fl ranging from 1.77 – 3.53 MPa, fco ranging from

447

25 – 62 MPa, and RA replacement ratio ranging from 0 – 100%. Using the relationship, typical

448

results are presented in Fig. 18. For instance, 33%, 55% and 100% RA replacement ratio can be

449

used with a confinement pressure of 3 MPa to manufacture concrete with a strength of 60 MPa, 40

450

MPa and 25 MPa, respectively. In this way, the allowable amount of RA can be determined for any

451

confinement steel without compromising concrete strength.

452

4.4.9 Comparison with existing stress strain model

453

Previously, Munir et al. [77] developed a stress strain model based on another smaller database (fco

454

= 50 MPa, fl from 1.77 – 3.53 MPa, and RA from 0 – 100%) for steel spiral confined RAC. That

455

database consists of test results using a different source of RA. Table 11 shows the comparison 22

456

between the model by Munir et al. [77] and the proposed model. The general form of both models is

457

similar. However, coefficients are different in the two models. Clearly, these coefficients are related

458

to the material properties of RA as the sources of RA are different in the two studies.

459

Due to scant work present in the literature about steel spiral confined RAC, it is impossible to

460

develop a general model at the moment that relates the values of these coefficients to the material

461

characteristics of RA. However, such a model should exist and can be developed when the material

462

characteristics of RA can be quantified by an index (or indexes) in the future. Extensive research

463

works are needed before such indexes can be developed for RA.

464

5. Summary & Conclusions

465

Non-eco-friendly and costly techniques are required to enhance the performance of RAC. On the

466

other hand, the improvement of concrete strength owing to confinement by lateral reinforcement is

467

disregarded in the current concrete design practice. This study aims to use pre-existing lateral

468

reinforcement to enhance the behaviour of RAC. As steel spiral reinforcement is required in

469

construction, improving the performance of RAC by considering this un-utilized lateral steel can be

470

a sensible and cost-effective solution. This study aims to analyze the behaviour of steel spiral

471

confined RAC. For this reason, axial stress strain behaviour of steel spiral confined concrete

472

specimens with different confinement pressure, replacement ratio of RA and target strength is

473

considered. Following conclusions are made on the basis of this work:

474

•

Steel spirals can offset the negative effect of the replacement ratio of RA on concrete strength.

475

All the confined specimens of series A except R100-S40-A show average peak stresses higher

476

in comparison with unconfined R0-A specimens. Similarly, all the confined specimens of series

477

B except R50-S40-B, R100-S40-B and R100-S30-B show average peak stresses higher in

478

comparison with unconfined R0-B specimens. In case of series C, only the peak stresses of R023

479

S40-C, R0-S30-C, R0-S20-C and R20-S20-C specimens are observed higher than unconfined

480

R0-C specimens, whereas, all the other specimens show peak stresses still lower in comparison

481

with unconfined R0-C specimens.

482

•

Increase in peak and ultimate strain of concrete specimens is noticed with the rise in

483

confinement. Peak and ultimate strains of all the confined specimens in each series are observed

484

higher than respective unconfined NAC specimens.

485

•

For all the specimens, the reduction in elastic modulus is noticed with the rise in the

486

replacement ratio of RA. Further decrease in elastic modulus of specimens is noticed with the

487

rise in spiral quantity. However, this is attributed to the decrease in the effective concrete area

488

due to the shrinkage of concrete at the location of the spiral. For large concrete members in

489

practice, use of steel spirals should not reduce the elastic modulus of concrete.

490

•

Due to scant research work related to the steel spiral confined RAC, existing models cannot

491

well estimate the stress strain behaviour of steel spiral confined RAC. A new model is

492

developed based on a large experimental program, which can be effectively used to predict the

493

stress strain performance of both steel spiral confined NAC and RAC. The new model provides

494

guidelines for the design of RAC members. Furthermore, a relationship is also developed in this

495

study to determine the allowable content of RA for steel spirals without compromising the

496

concrete strength, which would be useful in constructions with RA.

497

•

Source of RA significantly affects the material properties of RAC. As a result, RA from

498

different sources would lead to different models. To overcome the problem, the model

499

developed from this work takes a general form in which the parameters of the model are related

500

to the source of RA. Two sets of model parameters have been obtained so far for two different

501

sources of RA. More extensive research works are planned in the future to develop a method 24

502

that identifies/quantifies characteristics of RA from different sources, from which the model

503

parameters can be evaluated using the identified RA characteristics.

504

6. Acknowledgments

505

Authors would also like to thank the students of Shenzhen University (Bin Xi, Qianli Zhong,

506

Yanxiang Ta, Yuhao Ren, Yanchao Yue and others) for their assistance during the experimental

507

work. Scholarships provided by RMIT to the student authors are also highly acknowledged.

508

7. References

509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538

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27

631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677

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678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724

[66] J.G. Teng, J.L. Zhao, T. Yu, L.J. Li, Y.C. Guo, Behavior of FRP-confined compound concrete containing recycled concrete lumps, Journal of Composites for Construction 20(1) (2016) 10.1061/(ASCE)CC.1943-5614.0000602. [67] L. Ferreira, J.d. Brito, M. Barra, Influence of the pre-saturation of recycled coarse concrete aggregates on concrete properties, Magazine of Concrete Research 63(8) (2011) 617-627. [68] J.-J. Xu, Z.-P. Chen, T. Ozbakkaloglu, X.-Y. Zhao, C. Demartino, A critical assessment of the compressive behavior of reinforced recycled aggregate concrete columns, Engineering Structures 161 (2018) 161-175. [69] R.V. Silva, J. de Brito, R.K. Dhir, The influence of the use of recycled aggregates on the compressive strength of concrete: a review, European Journal of Environmental and Civil Engineering 19(7) (2015) 825-849. [70] J. Pacheco, J. de Brito, C. Chastre, L. Evangelista, Experimental investigation on the variability of the main mechanical properties of concrete produced with coarse recycled concrete aggregates, Construction and Building Materials 201 (2019) 110-120. [71] J. Xu, X. Zhao, Y. Yu, T. Xie, G. Yang, J. Xue, Parametric sensitivity analysis and modelling of mechanical properties of normal- and high-strength recycled aggregate concrete using grey theory, multiple nonlinear regression and artificial neural networks, Construction and Building Materials 211 (2019) 479-491. [72] J. Xiao, J. Li, C. Zhang, Mechanical properties of recycled aggregate concrete under uniaxial loading, Cement and Concrete Research 35(6) (2005) 1187-1194. [73] Y. Wei, Y.-F. Wu, Compression behavior of concrete columns confined by high strength steel wire, Construction and Building Materials 54 (2014) 443-453. [74] J.A. Carneiro, P.R.L. Lima, M.B. Leite, R.D. Toledo Filho, Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete, Cement and Concrete Composites 46 (2014) 65-72. [75] U.J. Counto, The effect of the elastic modulus of the aggregate on the elastic modulus, creep and creep recovery of concrete, Magazine of Concrete Research 16(48) (1964) 129-138. [76] X. Ren, K. Liu, J. Li, X. Gao, Compressive behavior of stirrup-confined concrete under dynamic loading, Construction and Building Materials 154 (2017) 10-22. [77] M.J. Munir, Y.-F. Wu, S.M.S. Kazmi, I. Patnaikuni, Y. Zhou, F. Xing, Stress-strain behavior of spirally confined recycled aggregate concrete: An approach towards sustainable design, Resources, Conservation and Recycling 146 (2019) 127-139. [78] M. Nematzadeh, A. Baradaran-Nasiri, Residual properties of concrete containing recycled refractory brick aggregate at elevated temperatures, Journal of Materials in Civil Engineering 30(1) (2018) 10.1061/(ASCE)MT.1943-5533.0002125. [79] J.K. Yail, J. Yongcheng, Axial load-bearing concrete confined with carbon fiber- reinforced polymer sheets in acidic environment, ACI Structural Journal 114(03) (2017) 775-786. [80] L.G. Li, C.J. Lin, G.M. Chen, A.K.H. Kwan, T. Jiang, Effects of packing on compressive behaviour of recycled aggregate concrete, Construction and Building Materials 157 (2017) 757-777. [81] C.S. Poon, Z.H. Shui, L. Lam, Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures, Cement and Concrete Research 34(12) (2004) 22152222. [82] Y.-c. Guo, J.-h. Zhang, G.-m. Chen, Z.-h. Xie, Compressive behaviour of concrete structures incorporating recycled concrete aggregates, rubber crumb and reinforced with steel fibre, subjected to elevated temperatures, Journal of Cleaner Production 72 (2014) 193-203. [83] B. Li, R. Park, H. Tanaka, Stress-strain behavior of high-strength concrete confined by ultrahigh- and normal-strength transverse reinforcements, ACI Structural Journal 98(3) (2001) 395-406. 29

725 726 727 728 729 730 731 732 733 734 735 736 737 738

[84] S.P. Shah, A. Fafitis, R. Arnold, Cyclic loading of spirally reinforced concrete, Journal of Structural Engineering 109(7) (1983) 1695-1710. [85] K.M. El-Dash, S.H. Ahmad, A model for stress-strain relationship of spirally confined normal and high-strength concrete columns, Magazine of Concrete Research 47(171) (1995) 177-184. [86] B. Assa, M. Nishiyama, F. Watanabe, New approach for modeling confined concrete. I: Circular columns, Journal of Structural Engineering 127(7) (2001) 743-750. [87] B.D. Scott, R. Park, M.J.N. Priestley, Stress-strain behavior of concrete confined by overlapping hoops at low and high strain rates, ACI Journal 79(1) (1982) 13-27. [88] W. Yu-Fei, C. Yugui, Energy balance method for modeling ultimate strain of confined concrete, ACI Structural Journal 114(2) (2017) 373-381. [89] M.N. Priestley, F. Seible, G.M. Calvi, Seismic design and retrofit of bridges, John Wiley & Sons, New York, 1996. [90] A.S. Shamim, T.T. Murat, Reinforced concrete columns confined by circular spirals and hoops, ACI Structural Journal 90(5) (1993) 542-553.

739

30

740

TABLES

741

Table 1 – Physical characteristics of NA and RA Aggregate category NA RA

Water absorption (%) 1.30 6.85

Apparent specific gravity 2.66 2.55

Density (kg/m3) 1513 1414

Crushing value (%) 27.0 31.0

742 743

Table 2 – Details of concrete mix design RA Series replacement Cement ratio (%) 0 201.63 20 201.63 A 50 201.63 100 201.63 0 288.06 20 288.06 B 50 288.06 100 288.06 0 313.67 20 313.67 C 50 313.67 100 313.67

Constituents (kg/m3) Sand

NA

RA

Water

Extra water

462.53 462.53 462.53 462.53 528.82 528.82 528.82 528.82 539.83 539.83 539.83 539.83

865.11 691.58 432.21 --710.18 568.15 355.09 --672.44 537.96 336.22 ---

--161.63 404.03 807.98 --132.74 331.86 663.71 --125.69 314.22 628.44

100.82 100.82 100.82 100.82 135.39 135.39 135.39 135.39 78.42 78.42 78.42 78.42

11.25 19.98 33.09 55.35 9.23 16.48 27.35 45.46 8.74 15.60 25.90 43.05

Admixture (ml/m3) ----------------967.86 966.70 964.95 962.05

744 745

Table 3 – Mechanical characteristics of steel wire Nominal area (mm2) 7.55

Yield Ultimate Elastic Strain at ultimate Diameter Peak load strength strength modulus strength (mm) (N) (MPa) (MPa) (GPa) (%) 3.1 6504.25 730 861.76 200.3 2.78

746

31

Ultimate strain (%) 7.72

747 748

Table 4 – Parameters of concrete specimens Specimen R0-A R20-A R50-A R100-A R0-S40-A R20-S40-A R50-S40-A R100-S40-A R0-S30-A R20-S30-A R50-S30-A R100-S30-A R0-S20-A R20-S20-A R50-S20-A R100-S20-A R0-B R20-B R50-B R100-B R0-S40-B R20-S40-B R50-S40-B R100-S40-B R0-S30-B R20-S30-B R50-S30-B R100-S30-B R0-S20-B R20-S20-B R50-S20-B R100-S20-B R0-C R20-C R50-C R100-C R0-S40-C R20-S40-C R50-S40-C R100-S40-C R0-S30-C R20-S30-C R50-S30-C R100-S30-C R0-S20-C R20-S20-C R50-S20-C R100-S20-C

Series

A (25 MPa)

B (40 MPa)

C (60 MPa)

Replacement ratio of RA (%) 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100 0 20 50 100

32

Pitch of steel spirals (mm) ---

40

30

20

---

40

30

20

---

40

30

20

749

Table 5 – Detail of test database Researchers Wei and Wu [73] Richart et al. [36] Assa et al. [86] Li et al. [83] Mander et al. [25] Sheikh and Toklucu [90] Present study

12 18 24 14 15

Concrete strength fco (MPa) 36.4 14.7 25.0-85.0 52.0-82.5 24.0-32.0

Confining pressure fl (MPa) 3.2-12.9 1.0-10.4 2.5-28.3 1.8-20.2 1.0-4.3

203-356

27

29.7-30.5

150

108

26.3-61.8

Diameter (mm)

Number

150 254 145 240 500

33

0.09-0.36 0.07-0.71 0.03-0.83 0.03-0.39 0.03-0.15

Peak stress ratio fcc/fco 1.48-2.93 1.20-3.97 1.00-3.82 1.01-2.50 1.24-1.80

1.7-6.8

0.06-0.22

1.21-1.70

NAC

1.8-3.5

0.03-0.13

0.68-2.03

NAC + RAC

Confinement ratio fl/fco

Type of concrete NAC NAC NAC NAC NAC

750

Table 6 – Details of stress strain models for steel spiral confined NAC Model Richart et al. [35]

Peak stress = 1 + 4.1

= 1 +

%$Assa et al. [86] Wei and Wu [73]

= 1 + 3.36

Razvi and Saatcioglu [37]

Li et al. [83]

: :

&

El-Dash and Ahmad [85]

Mander et al. [24]

Peak strain

&

L

= 1 + $% L

−2

&

= 1 +

%$&

&

= 2.254Y1 + 7.94\

&

− 2\

Hoshikuma et al. [38]

= 1 + 3.83 = 1 + 0.73

: :

= 1 + 21.5

: :

− 1.254

= 1 + 21.15 +

= 1 + $"

&

= 2.254Y1 + 7.94

&

21 < <

8

K K

&

: :

= 1+ 52

:

=:

: :

= 1 + 384 2

% (1

Q &

− 18

+ 5$? [)

8

=

2

K

-.

:

L

=

2

K

M1 − N

= 0.0028 − 0.0008$? 4 & = 0.5$ < K ; < = -. .L 1− 2- ; . L = . − P $ = Q 1−< 4 & < = Z-"

"

+ 0.00195

= 0.00218 + 0.0332 = 0.00245 + 0.0122

34

<

<

& K K

Circular

Circular Circular Circular & rectangular

Circular & rectangular

%

\ = 3.1

+ 0.0296

Circular

. O 1.25(.G (."G PQ $% = 5.1 M O 2 8 < K 66 4 $" = ; < = -. . %.+ PQ 16 K 2K .T X Q =S F , -. (.?) V -. U 2 ; $% L = 5.35 & '(.%) & = -. & = 0.5$ < K .L 1− 4 2< = ; $ = -. 1−< 4 & L . = . − P Q ; < = Z-" 2 ; $% = 6.7 & '(.%+ & = -. $% & 40 [= ; $? = S . 2 , 1.08 &

L

&

<

&

]

&

> 52 _`

\ = (21.2 − 0.35

= 1.491(10'G )

:

:

&

= 1 + 20.6

:

:

− 18

: : Q

− 1.254

Shah et al. [84]

= 1+ 52

Applicable section

Parameters

&

=

2

< =

) K

-.

4 -.

&

Circular & rectangular

]

≤ 52 _`

Circular & rectangular Circular & rectangular

Table 7 – Performance of models to estimate the peak stress and peak strain of steel spiral confined NAC Prediction of fcc/fco Error Index (%) Richart et al. [35] 11.22 El-Dash and Ahmad [85] 12.46 Assa et al. [86] 14.47 Wei and Wu [73] 9.77 Mander et al. [24] 11.33 Razvi and Saatcioglu [37] 12.51 Li et al. [83] 18.87 Shah et al. [84] 18.71 Hoshikuma et al. [38] 34.85 Proposed 9.77 COV = Coefficient of variation SD = Standard deviation Model

fcc, Ana. / fcc, Exp.

COV (%)

SD

1.00 0.88 0.85 1.01 1.05 1.08 1.01 0.80 1.30 1.01

12.69 18.70 9.72 11.27 12.25 11.35 36.77 13.55 18.05 11.27

0.13 0.21 0.11 0.11 0.12 0.11 0.37 0.17 0.14 0.11

Prediction of Ɛ /Ɛ Error Ɛ , Ana. / COV Index Ɛ , Exp. (%) (%) 27.31 0.96 31.46 108.93 0.49 40.54 39.65 0.75 31.75 27.27 0.96 31.48 25.25 1.07 30.78 26.00 0.90 28.10 46.25 1.47 64.11 35.28 0.83 33.40 29.76 1.33 32.05 27.27 0.96 31.48

SD 0.33 0.83 0.42 0.33 0.29 0.31 0.44 0.40 0.24 0.33

Table 8 – Evaluation of the proposed model to predict the peak stress and peak strain of steel spiral confined NAC and RAC Prediction of fcc/fco Model Proposed

Error Index (%) 9.85

fcc, Ana. / fcc, Exp.

COV (%)

SD

0.97

11.39

0.12

Error Index (%) 24.04

Prediction of Ɛ /Ɛ Ɛ , Ana. / Ɛ , Exp.

0.97

COV (%)

SD

27.88

0.29

Table 9 – Performance of models to estimate the ultimate strain of steel spiral confined NAC Model Scott et al. [87] Priestley et al. [89] Wei and Wu [73] Proposed

Error index (%) 185.30 26.13 28.07 26.13

Prediction of Ɛcu

Ɛcu, Ana. / Ɛcu, Exp.

2.65 1.09 1.03 1.09

COV (%) 98.45 39.50 40.76 39.50

SD 2.61 0.43 0.42 0.43

Table 10 – Evaluation of the proposed model to predict the ultimate strain of steel spiral confined NAC and RAC Model Proposed

Error index (%) 17.16

Prediction of Ɛcu Ɛcu, Ana. / Ɛcu, Exp. COV (%) 1.05 25.36

35

SD 0.27

Table 11 – Comparison between the parameters of proposed and Munir et al. [77] model Coefficients A General model '% $% = 5.35 & '(.%) − ( !a + b!) & a : ! ! & = 1 + M20.6 + 2 8 + b O : : & = 1.75 + !a + b! + 900: B C : = (5000 − ! a + b!) a & +b & R= c

B

0.546

Munir et al. [77] -0.41

3661

a

0

Munir et al. [77] 0.7

-921

734

-1.73

0.83

423 18.65

Present study

b

0.78

Munir et al. [77] 2

0

2

2.98

0

2739

2341

3.5

0

36

Present study

-

Munir et al. [77] -

1.06

-

-

2

1

-

-

4416

2

2

-

-

2

1.1

2

1.282

1

Present study

Present study

FIGURES

Fig. 1 – Particle size analysis of normal and recycled aggregates

Fig. 2 – Stress strain behaviour of steel wire

37

Fig. 3 – Test set-up

38

Fig. 4 – Failure patterns of concrete specimens: (a) R0-A; (b) R100-A; (c) R0-B; (d) R100B; (e) R0-C; (f) R100-C; (g) R50-S40-B; (h) R50-S20-B

39

(a)

(d)

(b)

(e)

(c)

(f)

40

(g)

(j)

(h)

(k)

(i)

(l)

Fig. 5 – Effect of replacement ratio of RA on the stress strain behaviour of (a) unconfined specimens - Series A, (b) unconfined specimens - Series B, (c) unconfined specimens - Series C, (d) confined S40 specimens - Series A, (e) confined S40 specimens - Series B, (f) confined S40 specimens - Series C, (g) confined S30 specimens - Series A, (h) confined S30 specimens Series B, (i) confined S30 specimens - Series C, (j) confined S20 specimens - Series A, (k) confined S20 specimens - Series B, and (l) confined S20 specimens - Series C 41

(a)

(d)

(b)

(e)

(c)

(f)

42

(g)

(j)

(h)

(k)

(i)

(l)

Fig. 6 – Effect of confinement pressure on the stress strain performance of (a) R0-A specimens, (b) R0-B specimens, (c) R0-C specimens, (d) R20-A specimens, (e) R20-B specimens, (f) R20-C specimens, (g) R50-A specimens, (h) R50-B specimens, (i) R50-C specimens, (j) R100-A specimens, (k) R100-B specimens, and (l) R100-C specimens

43

(b) (a)

(c) Fig. 7 – Effect of confinement pressure and RA replacement ratio on the peak stress of specimens: (a) series A, (b) series B and (c) series C

44

(b) (a)

(c) Fig. 8 – Effect of confinement pressure and RA replacement ratio on the peak strain of specimens: (a) series A, (b) series B and (c) series C

45

(b) (a)

(c) Fig. 9 – Effect of confinement pressure and RA replacement ratio on the ultimate strain of specimens: (a) series A, (b) series B and (c) series C

46

(b) (a)

(c) Fig. 10 – Effect of confinement pressure and RA replacement ratio on the elastic modulus of specimens: (a) series A, (b) series B and (c) series C

47

(b) (a)

(c) Fig. 11 – Effect of confinement pressure and RA replacement ratio on the toughness of specimens: (a) series A, (b) series B and (c) series C

48

(b) (a)

(c) Fig. 12 – Effect of confinement pressure and RA replacement ratio on the specific toughness of specimens: (a) series A, (b) series B and (c) series C

49

(a)

(b) Fig. 13 – Evaluation of the models for peak stress: (a) existing and proposed models for steel spiral confined NAC and b) proposed model for steel spiral confined NAC and RAC

50

(a)

(b) Fig. 14 – Evaluation of the models for peak strain: (a) existing and proposed models for steel spiral confined NAC and b) proposed model for steel spiral confined NAC and RAC

51

(a)

(b) Fig. 15 – Evaluation of the models for ultimate strain: (a) existing and proposed models for steel spiral confined NAC and b) proposed model for steel spiral confined NAC and RAC

52

(a) R0-S40-A

(d) R0-S30-A

(b) R0-S40-B

(e) R0-S30-B

(f) R0-S30-C (c) R0-S40-C

53

(g) R0-S20-A

(j) Wei and Wu [73]

(h) R0-S20-B

(k) Assa et al. [86]

(i) R0-S20-C

(l) Li et al. [83]

54

(m) Mander et al. [25] Fig. 16 – Performance of existing and proposed stress strain models for steel spiral confined NAC

55

(i) R20-S40-A

(v) R20-S30-B

(ix) R20-S20-C

(ii) R20-S40-B

(vi) R20-S30-C

(x) R50-S40-A

(iii) R20-S40-C

(vii) R20-S20-A

(xi) R50-S40-B

(iv) R20-S30-A

(viii) R20-S20-B

(xii) R50-S40-C

56

(xiii) R50-S30-A

(xvii) R50-S20-B

(xxi) R100-S40-C

(xiv) R50-S30-B

(xviii) R50-S20-C

(xxii) R100-S30-A

(xv) R50-S30-C

(xix) R100-S40-A

(xxiii) R100-S30-B

(xvi) R50-S20-A

(xx) R100-S40-B

(xxiv) R100-S30-C

57

(xxv) R100-S20-A

(xxvi) R100-S20-B

(xxvii) R100-S20-C

Fig. 17 – Performance of proposed stress strain model for steel spiral confined RAC

Fig. 18 – Relationship between allowable amount of RA and confinement pressure

58