Flexural behavior of large scale semi-precast reinforced concrete T-beams made of natural and recycled aggregate concrete

Engineering Structures 198 (2019) 109525

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Flexural behavior of large scale semi-precast reinforced concrete T-beams made of natural and recycled aggregate concrete

T

Mohamed F.M. Fahmya, , Lamiaa K. Idrissb ⁎

a b

Dept. of Civil Eng., Faculty of Eng., Assiut Univ., Assiut, Egypt Higher Institute of Engineering and Technology, El-Menia, Egypt

ARTICLE INFO

ABSTRACT

Keywords: T-beams RAC blocks Recycled aggregate Filler Semi-Precast Flexural

This study aimed to investigate the flexural behavior of T-section semi-precast reinforced concrete (RC) beams composed of recycled aggregate concrete (RAC) and natural aggregate concrete (NAC). The experimental program consisted of four large-scale T beams, including three semi-precast T-beams and a reference RC T-beam made of traditionally cast high strength natural aggregate concrete (HSNAC). The web of all the semi-precast beams composed of a precast U-shaped part, which contained the longitudinal and transverse reinforcement and was cast of HSNAC, and an inner RAC core. Normal strength RAC (NSRAC) or high strength RAC (HSRAC) was used as web core filler, and HSNAC or HSRAC was used for the beam flange. Two techniques were adopted to fill the web core: the traditional concrete casting and the placement of well-sized precast concrete blocks. The results indicated the applicability of RAC and NAC in one section to achieve comparable strength, deformability, and failure mode to those of the monolithically cast beam with NAC. In addition, using precast RAC blocks as filler in the web core of RC beams is a promising construction technique, which can be adopted as a critical design parameter to control development, propagation, and width of flexural and shear cracks to a large extent.

1. Introduction Since the recognition of concrete, the concrete industry has contributed to the construction of many traditional and important structures. With the aging of existing reinforced concrete (RC) buildings/ bridges, many of them need to be replaced due to various reasons, and there is an urgent need to increase the number of buildings to the high number of the earth's population. Therefore, it is necessary to recycle a large amount of construction waste, which is difficult and unfavorable to disposal costs [24]. For future generations, the protection of natural resources of aggregates has become an inevitable necessity to protect the environment and achieve sustainable development. Therefore, many researchers have conducted a variety of studies to determine the possibility of using construction waste in modern buildings. Compared with the behavior of natural aggregate (NA) concrete, several experimental studies considered the behavior of medium and high strength recycled aggregate concrete (RAC) under different loading conditions [7,10,24,23,30,9]. Others have studied the effect of the replacement ratio of coarse and fine aggregate on the behavior of RAC [25,18,13,31]. On the other hand, the study of the flexural behavior of RAC structural members has triggered the interest of researchers [27,19,17,34,4,38,2]. Similarly, the shear behavior of RAC structural ⁎

members has been studied by others [19,3,26,8]. A number of studies have shown that the durability of recycled concrete is not as good as that of natural aggregate concrete (NAC) [14,21,32,11,12,16]. In addition, the shrinkage rate of recycled concrete is larger than that of NAC, and its growth rate is directly related to the concentration of recycled aggregate (RA) [37,28,15,6]. From the perspective of durable structure, RA should come from high quality concrete, such as the demolition parts of prefabricated or pre-stressed concrete structure [31,13,5]. Due to the diversity of RAC sources, this suggestion/limitation may be an obstacle to the wide application of RAC in the production of structural components. Other studies have also discussed the possibility of using mineral admixtures to improve the overall durability of RAC [16], however, this may require additional construction costs. On the other side, in the design of RAC beams, the most important constraint factor is the potential mid-span deformation caused by its low effective modulus of elasticity. With the increase of cross section height, this can be easily compensated; some experimental studies have considered this behavior [29]. However, this solution may oppose the international community's call for sustainable societies by reducing carbon emissions, or may adversely affect the seismic response of modern structures due to structural weight gain. The RAC application in the precast structural systems is a promising

Corresponding author. E-mail addresses: [email protected], [email protected] (M.F.M. Fahmy).

https://doi.org/10.1016/j.engstruct.2019.109525 Received 23 February 2019; Received in revised form 7 August 2019; Accepted 7 August 2019 Available online 20 August 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

opportunity and can provide designers with the ability to properly deploy RAC without affecting the long lifecycle of modern structures. In addition, the combination of RAC and precast technology can achieve the purpose of reducing construction cost by using appropriate RAC grade. This was put forward by Xiao et al. [35] by dividing the cross section of the column into an external precast hollow section and an internal core cast on-site using RAC. This has been suggested as a reasonable solution to reduce the chance of shrinkage of the RAC and accelerate the construction process. Obviously, it is possible to choose the appropriate concrete grade as the inner core of RC columns under combined axial and lateral loads is a low stress area, so Xiao et al. [35] filled the column core with low strength RAC. Recently, Yang et al. [39] proposed a new type of T-shaped semi-prefabricated beams consisting of two parts: the outer part contains a high-performance U-shaped concrete, steel stirrups, and a longitudinal steel shape, and the inner part was poured in-situ with traditional concrete strength. Wu et al. [33] proposed the use of a green technique, in which external steel tubes were filled with precast concrete blocks made of coarsely crushed demolished concrete lumps. The test results strongly support the future adoption of such construction technology because it behaves similarly to that of traditional concrete-filled steel tube columns. The aim of this study was two-fold: the first was to examine the flexural behavior of semi-prefabricated T-beams, in which NA was used to produce U-shaped web as the precast part of the beam to be filled onsite with RAC, and the second was to find out the change in the flexural behavior of semi-precast beams with precast RAC blocks (segments) as filler. Precast RAC blocks were used as filler to determine pre-cracked sections, so as to control the development of cracks and the beam deformability. In addition, the construction of 100% prefabricated system can generally be accelerated, and the on-site labor work can be reduced. Furthermore, the use of precast RAC blocks as filler can make the structure easier to dismantle and faster to restore the constituent materials in the future. The experimental program included three largescale semi-precast beams with RAC filler of different grades compared with that of a conventionally cast high strength NAC (HSNAC) beam. All the semi-precast beams were designed according to the traditional design method of RC beams. The results included the failure mode, the load-deflection response, and the strain distribution along the longitudinal beam reinforcement.

(a) Recycled aggregate obtained from crushed demolished concrete cubes and cylinders Coarse recycled aggregate

Fine recycled aggregate

(b) Separation of the recycled aggregate to coarse and fine aggregate using sieve of 5mm size Fig. 1. Recycled coarse aggregate used in the production of NSRAC and HSRAC. Table 1 Basic property of the aggregate used. Aggregate type

Crush index (%)

Unit weight (t/m3)

Specific gravity

Water absorption rate (%)

Coarse

22.1 14.8 –

1.34 1.62 1.68

2.50 2.55 2.60

2.17 0.80 1.00

Fine

Recycled Natural Natural

due to the irregular texture of the RCA. It is worth noting that although the water absorption ratio of the coarse recycled aggregate was low, it was necessary to use additional water in concrete mix design to consider its effect. On the other side, in order to ensure high strength RAC, sikament was used as water-reducer, and its content (2.5% of the cement content) was taken into account when calculating the total water content. In addition, silica-fume was used in the design of HSRAC: 5% of the cement content was replaced by silica-fume. HSNAC was prepared with natural coarse aggregate and fine aggregate, and the design cubic compressive strength of 28-day-old concrete was 50 MPa. Properties of the natural aggregate are shown in Table 1. Similar to HSRAC mixing design, sikament was 2.5% of the cement content, and 5% of the cement weight was replaced by silica fume. The HSNAC was used for the reference beam, the U-shaped parts of the webs of all the semi-precast beams, and the flange of a semiprecast beam. Details of the concrete mix are shown in Table 2. The compressive strength was determined by the average test results of three standard cubes with 150 mm side length, and the tensile strength was determined by the splitting test of three standard cylinders with 150 mm diameter and 300 mm length. The compressive strength and tensile strength of concrete were measured using a testing machine with a capacity of 1500 kN. Table 3 includes the 28-day test results of cubic and cylindrical specimens for each test beam. It is worth mentioning that the curing conditions of cubes and cylinders were similar to those of the corresponding test beams. This can explain the observed lower compressive strengths than the designed compressive strengths of HSNAC and HSRAC. The test results of the concrete cubes and the concrete cylinders, which were cured in water for 28-days, are given in Table 3.

2. Materials This section describes the materials used to produce all the test beams and lists their mechanical properties. 2.1. Concrete In this study, recycled coarse aggregate was produced from a large number of pretested concrete cubes and cylinders, which are usually provided by construction companies to the Concrete Structure Laboratory of Assiut University, Egypt, to assess the mechanical properties of concrete used in real construction projects; see Fig. 1. RCA was obtained by crushing the pre-tested cubes and cylinders with an electric crusher. The regenerated aggregate consisted of fine and coarse components, which were separated by a sieve of 5 mm size: the coarse aggregate was retained on a sieve of 5 mm size. The design compressive strength of common RC structures in Egypt is usually between 25 and 40 MPa, and the aggregate used is natural and obtained from areas approved by the government. In this study, two grades of RAC concrete were designed to fill the web core of three semi-precast beams: normal strength RAC (NSRAC) and high strength RAC (HSRAC) with design concrete cube compressive strengths of 25 MPa and 50 MPa, respectively. Table 1 shows the basic properties of the rough part of RAC for both NSRAC and HSRAC. The mixing characteristics of NSRAC and HSRAC are shown in Table 2. Coarse aggregate in both mixes was 100% RCA; and both grades required more sand than that can be used in NAC 2

Engineering Structures 198 (2019) 109525

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Table 2 Mixing components of the different concrete types. Concrete type

RCA replacement percentage (%)*

Cement (kg/m3)

Sand (kg/m3)

NCA (kg/m3)

RCA (kg/m3)

Water (kg/m3)

Silica fume (kg/m3)

Sikament (kg/m3)

HSC HSRAC NSRAC

0 100 100

475 507 396

500 1020 804

1200 0 0

0 673 846

166 155 217

25.0 27.0 0.0

12.5 13.4 0.0

Note: RCA is recycled coarse aggregate, NCA is natural coarse aggregate, HSC is high strength concrete, HSRAC is high strength recycled aggregate concrete, and NSRCA is normal strength recycled aggregate concrete.

ACI 318-14 code (equal to 167 kN). That is, the shear reinforcement ensured a flexural dominant failure mode. The ratio of shear span to depth ratio (a/d) is equal to 3, where a is the shear span of the test beam. The first beam was designed as a reference RC T-beam constructed with high strength concrete (HSC). Designation of this beam is T.N50, in which T indicates traditionally cast beam and N50 refers to natural aggregate concrete of a cubic compressive strength equal to 50 MPa. The other three beams were semi-precast T-beams, in which different types and grades of concrete were considered for examination, as shown in Fig. 2(c–e). Precisely, the beam web was divided into two parts, i.e. the outer precast U-shaped part and the web core. The Ushaped part included both the flexural and shear reinforcement and was cast with HSNAC. The web core was cast using normal strength RAC (NSRAC) or high strength RAC (HSRAC) for two beams, and precast NSRAC blocks of 300mmx200mmx140mm were used to fill the web core of the third beam. For the beam flange, HSNAC was used for one semi-precast, and HSRAC was used for the other two beams. Therefore, the designation of the three semi-precast beams consists of two sets to illustrate the types and grades of concrete used to cast the beam web core (C) and the beam flange (F). The first set is related to the web core, consisting of the letter C (core) and R50 or R25, in which R represents RAC with a designed compressive strength of 50 MPa or 25 MPa, respectively. The letters RB are used to specify that the web core was filled with precast recycled concrete blocks. The second set introduces the types and grades of the concrete flanges, consisting of the letter F (flange) and N50 or R50, in which N and R are NAC and RAC, respectively, and 50 is the design compressive strength in MPa. For example, the web core and the flange of the C.R50-F.N50 beam were RAC and NAC, respectively, with designed compressive strengths of 50 MPa. For the C.RB25-F.R50 beam, the web core was precast RAC blocks of 25 MPa designed compressive strength, and the beam flange was RAC with 50 MPa designed compressive strength.

Table 3 Compressive strength and tensile strength of concrete after 28 days curing. Specimen

T.N50 C.R50-F.N50 C.R25-F.R50 C.RB25-F.R50

Average concrete compressive strength (MPa)

Average concrete tensile strength (MPa)

U

C

F

U

C

38.8 40.4 46.8 40.9

38.8 49.6 26.5 18.1

38.8 40.4 36.1 40.9

2.8 3.0 3.4 2.9

2.8 3.5 2.3 1.9

Note: U denotes the outer precast part, C refers to the web core, F points to the beam flange, T is traditionally cast, N is natural, R is recycled, B is block, and 25 and 50 are the design compressive strength.

2.2. Steel Reinforcement details of the test beams were made of steel bars with diameters of 8 mm, 10 mm, and 12 mm. Uniaxial tension tests were carried out on a testing machine with a capacity of 300 kN to determine mechanical properties of all the steel types. Table 4 lists the yielding/ proof strength and ultimate strength of all the steel bars. 3. Experimental program Four large-scale T-beams were fabricated and tested under fourpoint loading test. All the beams had a T-shaped section with the same concrete dimensions: the beam thickness was 630 mm, the width and the thickness of the flange were 500 mm and 100 mm, respectively, and the thickness and the width of the web were 530 mm and 250 mm, respectively. Such type of beams can be used in the construction of precast RC T-beam bridges across short-width obstacles such as water channels, and the beam flange shall be provided with shear connectors to achieve the required composite action to the concrete deck. All test beams were designed in accordance with ACI 318-14 [1]. The flexural reinforcement bars of all the beams were seven steel bars of a diameter of 12 mm, i.e. total cross-section area was 792 mm2, which was higher than the minimum flexural reinforcement (419 mm2). The compression reinforcement consisted of 2 steel bars with a diameter of 12 mm and four steel bars with a diameter of 10 mm, see Fig. 2. The steel reinforcement was designed to ensure a tension-controlled response (when the compressive strain reaches 0.003, the maximum tensile strain shall be equal to or greater than 0.005). Transverse reinforcement with 8 mm diameter and spacing of d/4 (d is the beam effective depth = 570 mm) was adopted. Hence, the flexural capacity (equal to 268 kN) calculated from the shear strength of the test beams exceeded that determined by the rectangular stress block method according to

3.1. Casting of the test beams One of the test beams was cast in accordance with the well-known steps of the traditional casting technique, in which steel cage was firstly produced according to the details of a predesigned tension, compression, and shear steel reinforcements. The steel cage was placed in a steel mold, while hardened cement-based pieces were used to ensure the required thickness of the concrete cover layer around the steel cage. Finally, the concrete mix prepared in the laboratory was poured into the steel mold; see Fig. 3(a–c). For the semi-precast beams, the details of the cage were the same to those of the T.N50 beam, but the casting steps were different. Additional internal steel mold was necessary to create the beam web core, so it was carefully manufactured to meet the required dimensions (150 mm width and 400 mm depth). After placing the steel cage and satisfying the required concrete cover, HS concrete containing natural aggregate was poured on the bottom tension side of the beam and the pouring depth was 130 mm. Then, the inner steel mold was placed to create the web core. In order to avoid uplifting of the internal mold during the compaction process of the cast concrete, lateral supports (steel angles) with vertical legs were affixed to the steel mold, as shown

Table 4 Mechanical properties of the reinforcement bars. Rebar diameter (mm)

Yield strength (MPa)

Ultimate strength (MPa)

Elastic modulus (GPa)

8 10 12

305 578 509

451 678 645

200 200 200

3

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

P/2

1750

200

P/2

500

1750

8 / 141 mm

8 / 141 mm

630

8 / 141 mm

200

96

104 104 200

415

141 141 141 141 141 141 141 141 141 141 141 141

141 141 141 141 141 141 141 141 141 141 141 141

4000

96 104104 200

250

100

2 d 10

2 d 10

100

630

NS-RAC

7 d 12 250

(c) C.R50-F.N50 beam

(d) C.R25-F.R50 beam Additional part joining the stirrup’s legs

100

2 d 10

HS-RAC

NS-RAC

2 d 10 130 200

630

500 2 d 12

7 d 12 250

(b) T.N50 beam

500 2 d 12

130

130

7 d 12

2 d 10

HS-RAC

HSC

630

630

2 d 12

2 d 10

100

2 d 10

2 d 12

HSC

2 d 10

(a) Reinforcement details of all beams

NS-RAC prism (300*140*200) NS-RAC prism (300*140*200)

Open stirrup

7 d 12 250

d 8 / 141 mm

(e) C.RB25-F.R50 beam

(f) Stirrups details

Fig. 2. Schematic drawings for the reinforcement details and concrete dimensions of all beams.

in Fig. 4(a); this technique helped also to ensure the accurate thickness of the concrete cover around the internal steel mold. Fig. 4(b) shows the casting of the U-shaped part. After the final setting time of the precast concrete part passed, the internal steel mold was removed (Fig. 4(c)). Therefore, the concrete casting process can be completed according to the type of concrete required for the web core and the flange of the test beam. It is noteworthy that open stirrups were used to facilitate the placement and removal of the internal steel mold, see Fig. 4(d). Therefore, before casting the beam, each stirrup was joined to an

(a) Prepared steel cages for the test beams

additional part that completed the closed shape of the stirrup, as shown in Fig. 2(f) and Fig. 4(d). Another casting process is proposed, which provides a reasonable solution to accelerate the construction steps of semi-precast structures and also 100% precast structures. After the U-shaped part was cast, precast plain concrete blocks can be used to fill the web core. These concrete blocks were prepared according to the size of the web core. In this study, concrete blocks were prepared one day before the web core of the semi-precast beam was cast. According to the required

(b) Placement of the steel cage in a casting mold

(c) Traditional cast beam

Fig. 3. Traditional casting process of the T.N50 beam. 4

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

Steel angles with vertical legs

The web core after removing the mold

(a) Placement of inner steel mold

(b) Casting of the outer U shape part of the beam

(c) Removal of the inner steel mold

(d) Transverse reinforcement of the beam flange, and RAC casting of the web core

Fig. 4. Casting process of the semi-precast C.R50-F.N50 and C.R25-F.R50 beams (cast-in-situ RAC for the web cores, and the flanges were cast-in-situ with NAC and RAC, respectively).

(a) Placement of the first row of the internal precast concrete blocks

(c) Grouting the first row of the RAC blocks

(b) A whole row of the internal precast concrete blocks

(d) After filling the web core with the RAC blocks, the beam flange was ready to be cast

Fig. 5. Casting process photos of the semi-precast C.RB25-F.R50 beam (precast RAC blocks at the web core and cast-in-place RAC at the flange of the beam).

readings of all the instruments used in the loading process. The loading rate was 50 kN/min, which was the suitable loading speed for observing the cracking process. The loading rate of all the test beams was the same.

dimensions, the wood mold was 145 mm in width, 300 mm in length, and 200 mm in height (internal dimensions). After removing the internal steel mold in the C.RB25-F.R50 beam, concrete blocks were placed to fill the bottom half of the beam web core, as shown in Fig. 5(a, b). High fluidity cement-based grouting was used to fill any gap between the concrete blocks or between all the precast parts of the web, Fig. 5(c). Then, the upper half of the web core was filled with concrete blocks, which were also grouted, Fig. 5(d). Ultimately, the beam flange was cast with HSRAC.

4. Experimental results and discussion The compressive strengths of the U-shaped parts of the C.R50-F.N50 and C.RB25-F.R50 beams were comparable to that of the reference beam, but the C.R25-F.R50 beam had a compressive strength 12% higher than the average of the test results, as shown in Table 3. For the flange of the test beams, the variation of the concrete compressive strength with respect to the reference beam was ( ± 5%). Hence, the main influential parameters were the characteristics of the concrete that filled the web core (high or normal strength RAC) and the technique to fill the web core (cast-in-situ or precast concrete units). In terms of the performance of the reference beam, the test results of the semi-precast beams were evaluated considering the observed crack patterns, the recorded tensile axial strains in the main steel reinforcement, the measured compressive strains of concrete, the final failure modes, the load-deflection relationships, and the distribution of the axial strains along the beam cross-section at the beam mid span. Table 5 summarizes the characteristic parameters of the load-deflection response including the initial flexural cracking load, the steel yield load, and the maximum flexural strength and the corresponding beam deflections of all the test beams.

3.2. Test setup and instrumentation All the beams were tested on a 5000 kN testing machine. Each beam was supported on hinge and roller supports that were spaced at 4000 mm. The machine load was divided into two point loadings using a rigid spreading beam, as shown in Fig. 6(a). The distance between the central lines of the applied loads was 500 mm. A load cell of 5000 kN capacity was used to measure the applied load, and seven linear variable displacement transducers (LVDTs) were installed on the beam to measure the beam deflection at five points along the beam span including the mid-span point, and to measure the crack width at two sections (i.e. the mid-span section and the section at the applied load). In addition, a strain gauge was attached to the outermost compression face of the beam at the middle cross section, and another strain gauge was affixed at 160 mm distance from the compression face of the beam at the same cross-section. Furthermore, as shown in Fig. 6(b), 11 strain gauges were connected to the middle steel bar of the outer layer of the tensile reinforcement. Data acquisition system was used to collect the 5

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

Testing machine

Load cell

Load cell Spreading beam Specimen LVDTs Data acquisition system

(a) Photos for test setup and Applied Load Load cell

Spreading beam

SG12 SG 13 Vl. LVDT SG11 250 Roller mm support

Vl. LVDT

Vl. LVDTs SG9

SG10 500 mm

500 mm

Data acquisition system

SG8

250 mm

Crack width measure SG3 SG6 SG5 SG4

SG7 250 mm

250 mm

250 mm

250 mm

250 mm

SG2 500 mm

SG1 500 mm

250 mm Hinge support

(b) Schematic drawing

Fig. 6. Test set-up and instrumentation of the test specimens.

4.1. Crack pattern

between the concrete blocks can be regarded as a T-section with hollow web core. The initial cracking load of this beam was calculated twice according to the section transformation method, i.e. one considering the whole T-section and the other considering the T-section with hollow web core. The area of different types of RAC and steel bars were replaced with equivalent areas of NAC having the same axial stiffness. Mander et al. [22] model and the Xiao et al. [36] model were adopted to define the elastic modulus of NAC and RAC, respectively. The calculated initial cracking loads were 60.2 kN and 55 kN for the whole section and the hollow web core section of this beam, respectively. That is, neglecting the existence of pre-cracked sections at the interfaces of the concrete blocks caused an overestimation of 12% in the first cracking load. The initial cracking load of the T.N50, C.R50-F.N50, and

The first cracking of the reference beam (T.N50) was at an applied load of 80 kN, and it was observed at the pure flexural zone. Compared with the reference beam, when the web core of the semi-precast C.R50F.N50 beam was HSRAC, the first cracking load increased by 23%. On the other hand, the semi-precast beams with NSRAC at the web core cracked initially under lower loads, i.e. 58.8 kN and 53.8 kN for the C.R25-F.R50 and C.RB25-F.R50 beams, respectively. This can obviously be attributed to the mechanical characteristics of the RAC used in the web core. For the C.RB25-F.R50 beam, the existence of pre-cracked sections at the interfaces between adjacent RAC blocks can be another reason for the observed early cracking: the beam section at the interface Table 5 Characteristic points on the load-deflection relationship of the test specimens. Specimen

Cracking Pcr (kN)

T.N50 C.R50-F.N50 C.R25-F.R50 C.RB25-F.R50

80.0 98.5 58.8 53.8

Yielding δcr (mm)

1.65 1.67 1.68 1.55

Ki (kN/mm)

48.4 59.1 35.0 34.7

Maximum

Initial local yielding

The end of the elastic stage

Py,i (kN)

δy,i (mm)

Py (kN)

δy (mm)

Ky(kN/mm)

181 223.8 161.3 212.5

6.67 8.47 6.76 8.99

250.0 283.8 250.0 300.0

9.78 11.1 11.7 13.8

25.6 25.6 21.4 21.7

6

Pmax (kN)

δmax (mm)

317.5 361.3 317.5 348.8

126.4 129.0 67.4 110.0

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

(a) T.N50 248 197

227 223

231

302 180 155 180

152

226 120 200 280 170 270 190

225

113 175

278 178 181 178 80 91

173 155

205 178

165

160

240 180 160 120 120 210 220243 170 222 199 250 210 260 208 173 225 245 249

165

(b) C.R50-F.N50

190 220

150 115 168 131 141 173 155

185 140

260

160 173 115 151 128 124 126

140 135 100

208 153

220

197 172 229 239 206

178

290

157

279 239

120

(c) C.R25-F.R50 247 208

225

195

185

148

110 235 60 177 200 100 162 236 210 178

60

267 216

224

105210 135 75 236 196

150 264

266 100 136 85 96

180

206 123 190

166

247 222

203

162

(d) C.RB25-F.R50

203

222 130 130

160

210

211 220 180 134 117 198 103 215 132

228

80

68 217 125

190

121

86 80

123 83

220 135

133

78

213 133

97 86

208 211

220

85

77

236 236

Fig. 7. Crack patterns of the test beams up to the yielding load of the reference beam (black cracks correspond to the initial local yield load, and blue cracks represent the new and propagated cracks after the initial yield load). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

C.R25-F.R50 beams were also determined, which are equal to 69 kN, 73.5 kN, and 66.0 kN, respectively. The experimental results and the theoretical calculations reveal that the serviceability limit of semiprecast RC beams was greatly influenced by the grade of RAC in the web core and the casting technique. Since different types and grades of concrete were used in the semiprecast beams, it was necessary to carefully track the observed changes in crack development and propagation. Fig. 7 shows the crack patterns of all the test beams under two different load levels. Black and blue represent the cracks developed up to 181 and 250 kN, respectively, which were the initial local yield load and the yield load of the reference beam. These values were defined based on the recorded readings of the strain gauges attached along the longitudinal beam reinforcement, as it will be discussed in the following section. The crack pattern of the T.N50 beam up to 181 kN was characterized by multiple flexural cracks in the pure bending zone, some of which extended upward to nearly 450 mm of the beam height. In addition, most of the flexural-shear cracks between the applied loads and the beam supports extended to more or less 400 mm in height. At an applied load of 250 kN, additional flexural and flexural-shear cracks occurred between the applied loads and the beam supports, i.e. out of the pure flexural zone, and some flexural-shear cracks were observed to reach the bottom

of the beam flange. However, when HSRAC was in the web core of the semi-precast C.R50-F.N50 beam, there were fewer flexural and flexuralshear cracks. In addition, spacing between cracks that came close to the flange of the beam seemed to be wider. Considering the influence of the grade of RAC, the overall evaluation of NSRAC in the web core of the C.R25-F.R50 beam revealed that its crack morphology was similar to that of the reference beam under 250 kN load. Nevertheless, the number of the flexural cracks in the bending zone was still lower than that of the reference beam. On the other hand, the C.RB25-F.R50 beam with prefabricated NSRAC blocks had several closely spaced flexural cracks in the pure bending zone and few flexural-shear cracks in the other parts of the beam. Furthermore, compared with the crack patterns of the C.R25-F.R50 and the T.N50 beams, the use of precast NSRAC blocks ensured that cracks distributed within a shorter length of the beam span, and the cracks out of the pure flexural zone were mostly arrested from extending to the flange of the beam. Fig. 8 shows the crack pattern of the test beams after reaching the yield load and was plotted in red. Additional flexural cracks appeared in the pure bending zone of the T.N50, C.R50-F.N50 and C.RB25-F.R50 beams, some of which extended upward to the flange of the beams. In addition, some flexural-shear cracks developed near the beam supports and propagated along the direction of the applied loads. On the other 7

Engineering Structures 198 (2019) 109525

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(a) T.N50 289 302 180 155

248 197

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Fig. 8. Crack patterns of the test beams after yielding of the reference beam.

hand, the C.R50-F.N50 and C.R25-F.R50 beams experienced the development of flexural cracks that reached 300 mm in depth, but could not grow further, and similarly the observed new flexural-shear cracks. It is noteworthy that the cracks encountered after yielding were observed at different deflection levels of the test beams.

strains (SG12) measured at the outermost fiber of the beam compression side and the applied load; see Fig. 6(b). This relationship of all the test beams was almost bilinear. The slope of the first linear part varies depending on the characteristic strength of the concrete web, and its end corresponded to the yield load; Table 5 lists the yield load of all the test beams. The higher the compressive strength of the web core, the higher the slope of this part. After steel yield, the test beams had the same slop of the second branch of the load-strain relationship, which can be attributed to a small difference in concrete compressive strength at the flange of the beams. In addition, the measured compressive strain of all the test beams increased significantly. Fortunately, the SG12 of the C.R25-F.R50 beam worked to the end of the test, and its maximum compressive strain was about 4800με. This can be attributed to the concentration of critical flexural cracks at some sections due to the low strength of the concrete used in the web core, which led to a severe increase in the steel tensile strains, which was associated with a considerable increase of the compression strain compared with the other beams. The concrete resistance on the compression side of the beam deteriorated at a beam deflection of 68 mm, and then the beam showed a decrease in the flexural strength. For the other beams, although the strain gauges broken at the deflection of 50–60 mm, which was less than the maximum deformability of the beams, the corresponding strain readings were 2000–3500με. That is, with further loading,

4.2. Axial strains Fig. 9 shows the relationship between the induced axial strain in the main reinforcement and the applied load of all the test beams. Obviously, the strain gauges SG3 to SG9 along the longitudinal reinforcement of the beam recorded a significant increase in the tensile strains of the beam after the steel bar yielded. Generally, the tensile strain of the steel bars in the pure bending zone was about 20–25 times the yield strain (2000με). When the web core of the semi-precast beams was HSRAC (Beam C.R50-F.N50), strain gauges SG3 and SG4 recorded more than 25 times the yield strain; see Fig. 9(b). On the other hand, the tensile strains of the precast beams with cast-in-situ NSRAC or precast NSRAC blocks at the web core were equal to 12–16 times the steel yield strain. General conclusion, the longitudinal reinforcement had a comprehensive contribution to the deflection of all the test beams. Fig. 10 shows the relationship between the concrete compression 8

Engineering Structures 198 (2019) 109525

Applied load (kN)

Applied load (kN)

M.F.M. Fahmy and L.K. Idriss

400 350 300 250 200 150 100 50 0 400 350 300 250 200 150 100 50 0

(a)

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Fig. 9. Applied load versus the tensile axial strain of all the test beams.

4.5. Pre-cracking stiffness

Applied load (kN)

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Fig. 10. Compressive strain at the beam mid span versus the beam maximum deflection.

As mentioned in the previous sections, the use of NSRAC in the web core of two test beams was associated with a decrease in the cracking load. The cracking load (Pcr) and the corresponding displacement (δcr) of all the test beams are listed in Table 5. The table also includes the pre-cracking stiffness (Ki) of the test beams, which was determined as Ki = Pcr/δcr. It is obvious that Ki is sensitive to the concrete grade used in the beam web core: when the web core was HSRAC (Beam C.R50F.N50), the pre-cracking stiffness was 22% higher than that of the reference T.N50 beam. On the other hand, the pre-cracking stiffnesses of the C.R25-F.R50 and C.RB25-F.R50 beams were about 28% lower than that of the reference beam, which was due to the low concrete properties of the web core.

concrete strains can exceed these values.

4.6. Yielding load

(T.N50) SG12 (RH) (C.R50-F.N50) SG12 (UH-CRH-FH) (C.R25-F.R50) SG12 (UH-CRN-FRH) (C.RB25-F.R50) SG12 (UH-CRNB-FRH)

-6000

-5000

-4000 -3000 -2000 Axial strain ( )

-1000

0

Table 5 summarizes the initial local yield load (Py,i), the yield load (Py) of the test beams and the corresponding mid-span deflection. As shown in Fig. 13, it was helpful to define both loads by plotting the axial strain measured along the longitudinal steel bar of the beam under different load levels and superimposing the horizontal line representing the yield strain (2000με) on the same graph. The initial local yield load refers to the load that caused the yield strain at a section or simultaneously at several sections along the beam. The yield load, typically, defines the end of the elastic stage of the beam load-deflection relationship. Under this load, most of the length of the outermost tensioned steel bar of the test beams reached the yield strain value. In addition, the strain distribution curve may defeat losing its regularity due to a sudden increase in the strain readings at one or more sections along the beam span. From Table 5, the initial yield load of the C.R25F.R50 beam was the lowest, but its yield load was almost equal to that of the reference beam. The initial and yield loads of the semi-precast C.R50-F.N50 and C.RB25-F.R50 beams were higher than those of reference beam: the increases in the initial yield load were 24% and 17%, respectively, and the increases in the yield load were 14% and 20%, respectively. Obviously, the increase in the yield load was due to the increase in the initial yield load, which was mainly dependent on the crack distribution within the flexural zone. Fig. 7 shows that there were a limited number of flexural cracks in the pure bending zone of the C.R50-F.N50 and C.RB25-F.R50 beams compared with the other two beams under 181 kN load. Namely, most of the flexural zone was intact, so the main steel bars and the surrounding concrete resisted the applied

4.3. Failure mode The test time after achieving the yield load of the beams was shorter than that of the previous loading stage due to the rapid propagation and widening of cracks and the development of additional cracks. The concrete crushing of all the test beams was observed. That is, all the beams experienced a perfect flexural failure mode, starting from the yield of the main reinforcement and after reaching a reasonable deformation level, concrete finally crushed. Fig. 11 shows photographs of the ultimate failure mode of the test beams. Note that the longitudinal main reinforcement of any tested beam did not fracture until the end of the test. 4.4. Load-Deflection relationship Fig. 12(a, b) present the relationship between the beam mid-span deflection and the applied load for the semi-precast C.R50-F.N50 beam and the C.R25-F.R50 and C.RB25-F.R50 beams, respectively. The loaddeflection response of the reference beam was plotted on both figures for comparison. The effects of the studied parameters on the precracking stiffness, the beam elastic stiffness, the yield load, the maximum flexural strength, and the maximum beam deformability are thoroughly discussed as follows: 9

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M.F.M. Fahmy and L.K. Idriss

(a) T.N50

(b) C.R50-F.N50

(c) C.R25-F.R50

(d) C.RB25-F.R50

Applied load (kN)

Applied load (kN)

Fig. 11. Final failure mode of the test beams.

400 350 300 250 200 150 100 50 0 400 350 300 250 200 150 100 50 0

4.7. Elastic stiffness

(a)

Obviously, as can be seen from Fig. 12, the elastic responses of all beams were roughly the same. Table 5 lists the calculated elastic stiffness (Ky) of all the test beams; Ky = Py/δy, where Py and δy are the beam yield load and the corresponding deflection. The ratio of the elastic stiffness of the C.R50-F.N50 beam to that of the reference beam showed that they shared exact elastic stiffnesses. However, when the web core was filled with NSRAC, this ratio was lower than unity. For example, a reduction of approximately 15% was found between the elastic stiffness of the C.R25-F.R50 and C.RB25-F.R50 beams and that of the T.N50 beam.

RH T.N50 C.R50-F.N50 UH-CRH-FH

(b)

4.8. Maximum flexural strength

T.N50 RH C.R25-F.R50 UH-CRN-FRH C.RB25-F.R50 UH-CRNB-FRH

The load–deflection relationship, shown in Fig. 12, shows that the flexural strengths of all the semi-precast beams were higher than that of the reference beam, except the C.R25-F.R50 beam almost reached the same flexural strength of the reference beam. The flexural strength of the C.R50-F.N50 and C.RB25-F.R50 beams increased by 11.4% and 9.8%, respectively, compared with the reference beam, as seen in Table 5. The increase in the yield load seems to be the reason for the increase in the flexural strength of these beams. The yield loads of the two beams increased by 13.5% and 20%, respectively. In other words, according to the increase of the yield load, the post-yield stage moved upward. The C.R25-F.R50 beam had a typical yield load and maximum flexural load of the reference beam. For the two semi-precast beams with NSRAC web core, although the material characteristics of all parts of the cross-section were comparable, they displayed different flexural strengths. In fact, the C.RB25-

0 10 20 30 40 50 60 70 80 90 100110120130140 Mid span deflection (mm)

Fig. 12. Load-deflection curve of the reference beam compared with those of the semi precast beams made of (a) HSRAC and HSC and (b) NSRAC, HSRAC, and HSC.

load and thus a delay in the yield of the steel bars was observed. Under the applied loads of 100 kN and 150 kN, Fig. 13 also shows that the strain values of the C.R50-F.N50 and C.RB25-F.R50 beams along the beam span were the lowest compared to the other beams.

10

Engineering Structures 198 (2019) 109525

(a)

0

P = 50 kN P = 100 kN P = 150 kN P = 200 kN P = 250 kN P = 275 kN

Axial strain ( )

RH T.N50

500 1000 1500 2000 2500 3000 3500 4000 Distance from the roller support (mm)

5000 P= 50 kN C.R50-F.N50 4500 UH-CRH-FH P= 100 kN 4000 P= 150 kN (b) P= 200 kN 3500 P= 250 kN 3000 P= 275 kN 2500 P= 300 kN 2000 1500 1000 500 0 0 500 1000 1500 2000 2500 3000 3500 4000 Distance from the roller support (mm)

)

5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

Axial strain (

Axial strain (µ )

Axial strain (µ )

M.F.M. Fahmy and L.K. Idriss

5000 P=50 kN UH-CRN-FRH 4500 C.R25-F.R50 P=100 kN 4000 P=150 kN (c) 3500 P=200 kN 3000 P=250 kN P=275 kN 2500 2000 1500 1000 500 0 0 500 1000 1500 2000 2500 3000 3500 4000 Distance from the roller support (mm) 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

UH-CRNB-FRH C.RB25-F.R50

(d)

0

Fig. 13. Strain distribution along the beam tensile reinforcement at different loading levels for all specimens (Note: the unequal distribution of the strain records at the applied point loads of the C.R50-F.N50 beam was attributed to that strain gauge 7 (as shown in Fig. 6(b)) was not directly intersected by a critical flexural crack, as shown in Fig. 7(b), Fig. 8(b), and Fig. 11(b)).

P= 50 kN P=100 kN P=150 kN P=200 kN P=250 kN P=275 kN P=300 kN

500 1000 1500 2000 2500 3000 3500 4000 Location of strain gauges (mm)

F.R50 beam contained pre-determined cracked sections, which were hidden in the web core according to the size of the concrete blocks used. This was helpful to produce a good distribution of the flexural cracks, which in turn ensured lower induced strains in the longitudinal reinforcement, as shown in Fig. 13(c and d). Therefore, the elastic stage of the C.RB25-F.R50 beam ended at 300 kN, which was the highest yield load reached by all the test beams. Ultimately, further research is necessary to examine the possibility of using concrete blocks of different sizes and concrete grades to enhance the performance of semi-precast beams.

increase in the axial strains. This response indicated that the pre-introduced cracked sections in the web core triggered several sections along the span of the beam to contribute to the beam deformability at the same time. Fig. 15(a, b) displays the cracks in the web core and the cracks observed on the side surfaces of the U-shaped part. Obviously, the web core had wide cracks at the interfaces of the concrete blocks, which corresponded to the cracks early developed on the side surface of the U-shaped part. Consequently, limited damage level was observed on the beam tension side. The second type of response is shown in Fig. 14(c), and it was observed for the semi-precast C.R25-F.R50 beam, in which the web core was filled with cast-in-situ NSRAC. The strain readings of two gauges suddenly increased at the same time. With further loading, other gauges contributed one by one to the deformability of the beam at different deflection values through the rapid increase in strain readings. That is, there was a stress concentration at the beam sections that early contributed to the beam deformability. Therefore, this beam exhibited an early degradation of the flexural strength. Although the steel strain-deflection relationship of the reference beam was similar to that of the C.RB25-F.R50 beam to some extent; however, the rate of increase in the axial strain of all beam sections was lower, as shown in Fig. 14(a). In other words, the longitudinal beam reinforcement at the examined sections gradually contributed to the beam deformability. The reinforcement of the C.R50-F.N50 beam, in which the web core was HSRAC, exhibited to somewhat a mixed behavior between the first response and the second response [Fig. 14(b)], which was attributed to the mechanical characteristics of the concrete core compared with those of the C.R25-F.R50 beam.

4.9. Ultimate deformability It was planned to remove the applied load on the test beams when the beam flexural strength decreases to 80% of the maximum achieved strength. However, three test beams (T.N50, C.R50-F.N50, and C.RB25F.R50) were loaded up to the maximum available clearance between the bottom beam surface and the lower head of the test machine. The load on the C.R25-F.R50 beam was stopped to avoid any instability problems because of the obvious enlargement of the flexural cracks in the pure bending zone. For a fair comparison between the test beams, Table 5 includes the deflection of all the test beams under the maximum flexural strength. Obviously, both the C.RB25-F.R50 and C.R50-F.N50 beams showed a quite similar deformability to that of the reference beam; however, when cast-in-situ NSRAC material was used in the web core of the semi-precast beam, Beam C.R25-F.R50 failed to do the same. Since the cross-section of any of the test semi-precast beams was composed of two types of concrete of equal or different grades, it would be interesting to explore all possible causes/effects that led to the unequal maximum deformation capacity. Fig. 14 presents the relationship between the axial tensile strain of the longitudinal beam reinforcement and the beam deflection measured at different beam sections. The starting point of the inelastic stage (the post-yield stage) was defined on each part of the figure via a vertical line corresponding to the yield deflection (δy) of the beam. From this figure, the longitudinal beam reinforcement had two different responses to the applied loads in the inelastic stage of loading. The first type of response was for the C.RB25-F.R50 beam (Fig. 14(d)), in which the gauges SG4 to SG8 at the middle zone of the beam showed a simultaneous steep rise. Then, when the beam deflected about 20 mm and further, the rate of increase became lower. When the beam deflection was about 65 mm, the gauges SG3 and SG9 showed a sudden

4.10. Ductility ratio The displacement ductility ratio of the test beams is an important design parameter to measure the deformation capacity of inelastic members. It can be defined as the ratio of the maximum deflection to the deflection of the beam when the steel is yielded. Based on the test results shown in Table 5, the ductility ratios of the T.N50, C.R50-F.N50, C.R25-F.R50, and C.RB25-F.R50 beams were calculated to be equal to 12.9, 11.6, 5.8, and 8.0, respectively. This result refers to the RAC grade used in the web core as a critical design parameter, where the use of HSRAC was associated with a reduction of 10% in the beam ductility ratio with respect to the result of the reference beam. However, the use of low strength RAC resulted in a 55% reduction of the beam ductility. 11

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

(a)

20000 10000

Axial strain (

)

0 50000 0

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25 (b) 50 75 100 UH-CRH-FH C.R50-F.N50 Mid span deflection (mm)

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)

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SG3 SG4 SG5 SG6 SG7 SG8 SG9 UH-CRNB-FRH

C.RB25-F.R50 (d (d) Simultaneous and deflection Mid span (mm) 40000 steep increase

Axial strain (

Axial strain ( )

50000

30000 SG2 SG3 SG4 20000 SG5 SG6 10000 SG7 SG8 0 125 150 0

SG3 SG4 SG5 SG6 SG7 SG8 SG9 25

50 75 100 125 Mid span deflection (mm)

150

Fig. 14. Strain distribution along the beam tensile reinforcement at different loading levels for all specimens.

(a)

(b) Precast concrete blocks

Fig. 15. The outer cracks (a) and the inner cracks (b) of the C.RB25-F.R50 beam.

This reduction can be controlled using precast concrete blocks in the web core, which successfully increased the ductility ratio of the beam by 38% compared to the semi-precast beam filled with cast-in-place NSRAC. When the maximum compressive strain was 0.003, the

corresponding maximum axial strain of the most tensioned longitudinal reinforcement was above 0.03 for all beams, and the ratios of the measured deflections to the corresponding maximum deflections of the test beams were 0.35, 0.52, 0.66 and 0.56 for the T.N50, C.R50-F.N50, C.R25-F.R50, and C.RB25-F.R50 beams, respectively. Obviously, the 12

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

400

Applied load (kN)

5. Evaluation of the beam flexural strength

300

Fig. 18 shows the strain distribution through the beam depth at the mid-span section under different load levels. All parts of the figure revealed the linear strain distribution along the depth of the beam. That is, the use of different types/grades of concrete and different casting techniques did not significantly affect the well-known behavior of monolithically cast RC beams. So, the flexural strength of the semiprecast beams can be determined by the existing methods such as the fiber section method and design codes approaches. This figure also confirms the slight difference in the depth of the neutral axis of the test beams measured from the upper side of the beam because they had comparable compressive strengths at the beam flange. In addition, the semi-precast beam design satisfied the conditions of tension-controlled section. The flexural capacities of the test beams were determined in accordance with both the fiber section method and ACI 318-14 code.

250 200 150 100

C.R50-F.N50 UH-CRH-FH C.RB25-F.R50 UH-CRNB-FRH

50 0

0

5

10

15

Crack width at the beam mid span (mm)

400

20

(b)

350

Applied load (kN)

than those of the precast U-shaped part, see Fig. 17(a & c).

(a)

350

300 250

5.1. Fiber section method

200 150 50 0

The following assumptions were considered: (1) the strain distribution along the beam depth was linear; (2) perfect bond condition was assumed between the steel reinforcement and surrounding concrete; (3) a full interaction (perfect bond condition) was assumed between the different parts of any of the tested semi-precast beams (4) compressive stresses on concrete fibers were determined using Mander et al. [22] model and the Xiao et al. [36] model for NAC and RAC, respectively [Fig. 19(a)]; (5) concrete compressive strength was defined using the experimental results given in Table 3; however, a modification according to Kotsovos [20] model was adopted to consider the development of triaxial stress condition rather than the uniaxial stress state; (6) the tensile contribution of uncracked concrete was neglected; and (7) The stress-strain behavior of steel was assumed bilinear as shown in Fig. 19(b), and characteristics of all steel bars are summarized in Table 4. The cross-section flexural strength was calculated at an ultimate compressive strain of 0.0035 με for all the test specimens. To increase the accuracy of calculating the flexural strength, the beam section was divided into several strips every 10 mm, as shown in Fig. 19(c). The proposed fiber section can safely predict the flexural strength (Pfib) of the test beams, where the ratios between the predicted values to the experimental results were ranged between 0.87 and 0.99, as shown in Table 6. A higher accuracy in calculating the flexural response of such semi-precast beams can be guaranteed when other design influential parameters such as the interaction between the precast parts and the cast-in-situ concrete or concrete blocks are considered; see Fig. 15(a, b), which show that the crack map of the precast U-shaped part was different from that of the web core.

T.N50 RH C.R50-F.N50 UH-CRH-FH C.RB25-F.R50 UH-CRNB-FRH

100

0

5

10

15

20

Crack width at the loading point (mm) Fig. 16. Applied load versus crack width at the beam mid span for the test beams.

semi- precast beams can achieve acceptable ductility after meeting the design requirements of the design codes. This is a good indication for using axial tensile strain of 0.005 as a reasonable limit to determine the reinforcement ratio to realize the required tension control performance. However, further research should be conducted to ensure that the ductility ratio is comparable to that of conventionally cast beams. 4.11. Crack width Crack width is one of the important design indices to measure the serviceability of RC structural elements. Fig. 16 shows the relationship between the crack width of two sections (i.e. the mid-span section and the loading point section) and the applied load. According to the crack patterns of the test beams, it was observed that the C.R50-F.N50 and C.RB25-F.R50 beams cracked approximately at the mid-span section. Along with the reference beam, the C.R50-F.N50 and C.RB25-F.R50 beams experienced the development of a major flexural crack at the loading point section. The clear difference was in the post-yielding stage. The two parts of Fig. 16 show that the width of the developed cracks of the C.RB25-F.R50 beam was the lowest. The strain readings are directly related to the width of the developed cracks, so Fig. 17(a & c) present the maximum measured axial strains and the average of the induced strains along the entire length of the steel bar of the test beams, respectively. In order to compare among the test beams before the steel bars yield, the maximum limit of the horizontal axis in Fig. 17(b & d) is 3000με. Fig. 17(c) shows that the measured stains of the reference beam and the C.R25-F.R50 beam under any load were comparable and higher than those of the other beams. This result highlighted that the use of NSRAC in the web core could reach a comparable serviceability limit to that of the reference beam; Fig. 17(d) confirmed this conclusion. Furthermore, Fig. 17(b & d) declare that the use of NSRAC blocks as filler was associated with lower axial strains with respect to the reference beam. Ultimately, the width of the developed cracks can be highly controlled by filling the web core with precast concrete blocks or concrete with higher characteristics

5.2. ACI 318-14 code For ACI 318-14 code, the above assumptions were also followed except that the ACI rectangular block was assumed for the stress distribution at the beam compression side (the maximum mean compression stress equal to 0.85 of the measured concrete compressive strength), the ultimate concrete strain is 0.003 at the compression side, and the steel reinforcement has the elastic-perfectly plastic behavior. To consider the hardening effect of the longitudinal reinforcement, the probable flexural strength was determined according to steel stress equal to 1.25 times the steel yield strength (ACI 318-14). Table 6 summarizes the calculated nominal flexural strength (Pn.), and the probable flexural strength (Pprob.). Using the ACI 318-14 code, unsurprisingly, Pn. of any of the test beams was lower than the maximum achieved flexural strength as steel hardening was not considered. On the other hand, the calculated 13

Engineering Structures 198 (2019) 109525

T.N50 RH C.R50-F.N50 UH-CRH-FH C.R25-F.R50 UH-CRN-FRH C.RB25-F.R50 UH-CRNB-FRH

(a) 0

400 350 0 300 250 0 200 0 150 0 100 0 500 0

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(c) 0

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Applied load (kN)

Applied load (kN)

400 350 300 2500 2000 1500 100 50 0

Applied load (kN)

M.F.M. Fahmy and L.K. Idriss

T.N50 RH C.R50-F.N50 UH-CRH-FH C.R25-F.R50 UH-CRN-FRH C.RB25-F.R50 UH-CRNB-FRH

10000 20000 30000 Average axial strain (

40000 )

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400 350 300 250 200 150 100 50 0

400 350 300 250 200 150 100 50 0

RH T.N50 UH-CRH-FH C.R50-F.N50 UH-CRN-FRH C.R25-F.R50 UH-CRNB-FRH C.RB25-F.R50

(b) 0

500

1000 1500 2000 2500 Maximum axial strain (µ )

3000

RH T.N50 UH-CRH-FH C.R50-F.N50 C.R25-F.R50 UH-CRN-FRH C.RB25-F.R50 UH-CRNB-FRH

(d) 0

500

1000 1500 2000 2500 Average axial strain ( )

3000

Fig. 17. (a) Maximum measured axial tensile strain, (b) Magnified part of Fig. 17(a), (c) average of the induced tensile strains, and (d) Magnified part of Fig. 17(c) along the longitudinal beam reinforcement.

Cross section depth (mm)

600

Cross section depth (mm)

700

600

RH

T.N50

700

P= 100 kN P= 150 kN

500 400

P= 200 kN

300

P= 250 kN

P= 100 kN P= 150 kN P= 200 kN

400

P= 250 kN

300 200

100

100

(a)

0 700

UH-CRH-FH

C.R50-F.N50 Axial strain (µ )

500 400 300

(c)

0 700

P= 100 kN P= 150 kN P= 200 kN P= 250 kN P= 275 kN P= 300 kN

UH-CRNB-FRH

C.RB25-F.R50 Axial strain (µ )

600 500 400 300

P= 100 kN P= 150 kN P= 200 kN P= 250 kN P= 275 kN P= 300 kN

200

200 100

-2000

C.R25-F.R50

500

200

0

UH-CRN-FRH

600

100

(b) 0

2000 Axial strain (µ )

4000

6000-2000

0

(d) 0

2000

4000

6000

Axial strain (µ )

Fig. 18. Strain distribution through the beam depth at the beam mid span section at different load levels.

probable strength to the experimental measured flexural strength (Pmax,exp) ratio (Pprob/Pmax,exp.) was slightly conservative for the C.R50F.N50 and C.RB25-F.R50 beams; as shown in Table 6.

sustainability and durability. In order to reduce the construction cost, NSRAC can be used for the web core. Filling the core with RAC blocks instead of cast-in-situ RAC can ensure high deformation and controllable damage level. In addition, it can accelerate the construction of 100% precast systems. However, further examination is still required to define the shear behavior of such type of beams and set design-based details of the concrete blocks (segments) inside the web core.

6. Consideration for the construction of semi precast RC beams with RAC Design engineers are recommended to use RAC in the core web of semi precast beams, while the other parts of the beam should be based on NAC. This may be an alternative to replace a part of NA with RA in the concrete mix to meet the durability requirements. That is, the web core could be 100% RAC, which should be enclosed by NAC that includes the reinforcement cage of the beam to ensure the required

7. Conclusions The results showed that the integrity of the semi-precast beams was verified whether cast-in-situ RAC or precast RAC blocks were used as filler. In addition, RAC blocks were very effective in controlling the 14

Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

E= 0.01 Es

Mander et al. (1988) model for NAC Xiao et al. (2005) model for RAC

Es co

c

s

c

y

(a) Concrete stress-strain relationship

(b) Steel stress-strain relationship

bf

c

Fs4

tf z

Fs3 N

N

,

t Fs2

s2 s1

Fs1

bw

(c) Fiber section and strain and stress distribution Fig. 19. Fiber section method to determine the flexural strength of the test beams.

crack pattern of the semi-precast C.RB25-F.R50 beam, which achieved a comparable flexural response to that of the reference beam. Several conclusions were drawn from the test results and summarized as follows:

2. The initial cracking load and the elastic stiffness of the test semiprecast beams were dependent not only on the concrete mechanical characteristics of the external U-shaped part, but also on the grade of the RAC filler (NSRAC or HSRAC) in the web core. The higher the tensile strength of the concrete filler, the higher the initial cracking load and the elastic stiffness of the semi-precast beam. 3. The use of concrete blocks inside the web core of the C.RB25-F.R50 beam introduced internal pre-cracked sections, which further reduced the initial flexural cracking load besides the use of the low grade concrete blocks. 4. The grade of the concrete filler had no negative impact on the beam yielding load. 5. Deformability of the semi-precast beam filled with cast-in-place lowgrade RAC was lower than that of the semi-precast beam filled with

1. Following the design guidelines for traditionally cast RC beams; flexural was the main failure mode of all the semi-precast beams tested. Compared with the reference beam, the following observations were realized: The number of flexural and flexural-shear cracks, crack spacing, and crack propagation depend on the RAC grade used in the web core. Using precast NSRAC blocks as filler for the web core positively changed the crack map. The number of flexural-shear cracks extending to the flange of the C.RB25-F.R50 beam was the least.

• •

Table 6 Evaluation of the flexural capacity of the test beams using ACI 318–14 code and the fiber section approach. Specimens

T.N50 C.R50-F.N50 C.R25-F.R50 C.RB25-F.R50

Fiber section

ACI318-14

Pfib. (kN)

Pfib./Pmax,exp

Pn (kN)

Pn/Pmax,exp.

Pprob. (kN)

Pprob./Pmax,exp.

315 315 306 309

0.99 0.87 0.96 0.89

263 263 262 263

0.83 0.73 0.83 0.75

325 325 324 326

1.02 0.90 1.02 0.93

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Engineering Structures 198 (2019) 109525

M.F.M. Fahmy and L.K. Idriss

high-grade RAC. On the other hand, the proposed filling technology using low grad precast RAC blocks successfully achieved the same deformation capacity as the reference beam. However, the interaction between the concrete blocks and the inner surface of prefabricated parts, the size of the concrete blocks, and the configuration along the length and depth of the beam are design influential parameters that should be studied. 6. The semi-precast beams with different types of concrete flanges (NAC and RAC) made of comparable compressive strengths to that of the traditionally cast RC beam achieved approximately the same flexural strength. 7. The linear strain distribution of the mid-span section of all the test beams was observed before the steel bars yield, indicating the applicability of the available theoretical approaches to calculate the flexural strength of semi-precast beams made of NAC and RAC with reasonable accuracy.

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In general, the performance of the semi-precast beam filled with precast reinforced concrete blocks indicates that further tests and numerical evaluation are needed. This technology can greatly improve the deformability of RC beams, reduce the contribution of shear or shearflexural cracks to the beam deformability, reduce the construction cost of modern structures, and promote the application of RAC in 100% precast structures. Declaration of Competing interest The authors have no conflict of interest whatsoever. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.engstruct.2019.109525. References [1] ACI. Building Code Requirements for Structural Concrete (ACI 318–14) and Commentary, American Concrete Institute, 38800 Country Club Drive. Michigan: Farmington Hills; 2014. p. 48331. [2] Alnahhal W, Aljidda O. Flexural behavior of basalt fiber reinforced concrete beams with recycled concrete coarse aggregates. Constr Build Mater 2018;169:165–78. [3] Aly S, Ibrahim M, Khattab M. Shear behavior of reinforced concrete beams casted with recycled coarse aggregate. Eur J Adv Eng Technol 2015;2(9):59–71. [4] Arezoumandi M, Smith A, Volz JS, Khayat KH. An experimental study on flexural strength of reinforced concrete beams with 100% recycled concrete aggregate. Eng Struct 2015;88:154–62. [5] Brandes MR, Kurama YC. Use of recycled concrete aggregates in precast/prestressed concrete. Proc Eng 2016;145:1338–45. [6] Bravo M, de Brito J, Evangelista L, Pacheco J. Durability and shrinkage of concrete with CDW as recycled aggregates: benefits from superplasticizer’s incorporation and influence of CDW composition. Constr Build Mater 2018;168:818–30. [7] Breccolotti M, Materazzi AL. Structural reliability of eccentrically-loaded sections in RC columns made of recycled aggregate concrete. Eng Struct 2010;32(11):3704–12. [8] Chaboki HR, Ghalehnovi M, Karimipour A, de Brito J, Khatibinia M. Shear behaviour of concrete beams with recycled aggregate and steel fibres. Constr Build Mater 2019;204:809–27.

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