Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

Materials & Design 83 (2015) 257–269

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Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete Sun-Woo Kim a, Hyun-Do Yun b,⇑, Wan-Shin Park a, Young-Il Jang a a b

Department of Construction Engineering Education, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Architectural Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea

a r t i c l e

i n f o

Article history: Received 13 February 2015 Revised 21 May 2015 Accepted 4 June 2015

Keywords: Recycled coarse aggregate (RCA) concrete Pull-out test Bond strength Density Compressive strength Nonlinear regression analysis

a b s t r a c t This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Great quantities of construction and demolition waste materials are produced annually all over the world. Due to the depletion of high quality aggregate in many parts of the world and the desire for sustainability, recycled aggregate derived from construction and demolition waste is an alternative as partial replacement for natural aggregate in concrete mixtures. In Korea, 187 thousand tons per day of construction and demolition waste is generated, which corresponds to 48.9% of the total waste, and 84.4% of this amount is reused or recycled [1]. However, even though there was an experimental study for flexural behavior of reinforced concrete beams with recycled aggregate [2], most of the reused or recycled aggregate is applied to the construction of roads, soil cover, and concrete secondary products, such as concrete block, due to the difficulties and cost increases associated with the recycling process. Many studies of the reuse of waste concrete have shown that concrete made with recycled coarse aggregate (RCA) has lower mechanical properties (compressive, tensile, and flexural strength) than normal concrete because of its lower density values and higher levels of water absorption and porosity, mainly due to

⇑ Corresponding author. E-mail address: [email protected] (H.-D. Yun). http://dx.doi.org/10.1016/j.matdes.2015.06.008 0264-1275/Ó 2015 Elsevier Ltd. All rights reserved.

the heterogeneous nature of the recycled aggregate [3]. Many studies show common results that normal aggregate can be replaced up to 30% by recycled aggregate in new structural concrete [4–6]. As a result, current standards and specifications [7–10] impose limitations on the use of replacement recycled aggregate in new concrete, particularly in structural concrete. In addition to these limitations, in Korea, the minimum quality of density above 2.5 and water absorption below 3.0 of RCA for structural concrete are specified in the Recycled Aggregate Quality Standard [11]. As stated above, the quality of the recycled aggregate affects the mechanical properties of recycled aggregate concrete (RAC) and the structural performance of reinforced concrete members that contain RAC. In recent, in order to evaluate the effect of RA on the shrinkage behavior, compressive and tensile strength of concrete, statistical analyses [12–14] were performed on the collated data from several studies. The steel-to-concrete bond that allows longitudinal forces to be transferred from the reinforcement to the surrounding concrete is one of the most important factors in designing reinforced concrete or precast concrete, especially for the development and splices of the reinforcement. The bond performance between conventional concrete and rebar is well known, and development and splice lengths are calculated based on the attainable bond stress over the length of the embedment of the reinforcement. In order to use recycled aggregate for structural concrete, the factors that

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Table 1 Bond strength for deformed bar [33]. Pull-out

Splitting

Good

All other

bond cond. pffiffiffiffiffiffi 2:5 f ck

bond cond. pffiffiffiffiffiffi 1 :25 f ck

Good bond cond.

All other bond cond.

unconfined

stirrups

pffiffiffiffiffi0:25 f 7:0 20ck

(a) NCA

8:0

pffiffiffiffiffi0:25 f ck 20

(b) RCA

unconfined

stirrups

pffiffiffiffiffi0:25 f 5:0 20ck

5:5

pffiffiffiffiffi0:25 f ck 20

(c) NFA

Fig. 1. Aggregate types used in this study.

Aggregate

Maximum grain size (mm)

Specific gravity (%)

Water absorption (ton/m3)

Bulk density

Fineness modulus

NCA RCAa NFA RCA1 RCA2

25 25 5 25 25

2.69 2.48 2.59 2.59 2.26

0.57 3.01 1.43 1.59 6.28

1.553 1.511 1.580 1.616 1.298

6.66 6.38 2.85 6.45 6.34

Blending ratio of RCA = 2.5 (RCA1):1.0 (RCA2).

NCA RCA1

80

RCA2 NFA

60 KS Limits

40

20

0 0.1

1

10

100

Sieve (mm) Fig. 2. Grain size distribution of natural and recycled aggregates.

2. Steel-to-concrete bond strength The nominal bond strength generally is determined by a wedging action that is enhanced by the crushed concrete stuck to the front of lugs and hoop stresses in the surrounding concrete [36]. The peak bond strength and stiffness values are due mostly to the interlocking among the reinforcement, the concrete struts radiating from the bar, and the undamaged outer ring [37]. Assuming a uniform bond, the nominal bond stress (s) is given as:

s¼

Table 2 Physical characteristics of aggregate used in this study.

a

100

Cumulative percentage passing (%)

affect the steel-to-concrete bond performance should be evaluated through testing and analysis because the mechanical properties of RAC are different from or inferior to those of normal concrete. Most study results concerning the bond strength of RAC [15–25] indicate that the bond strength of RAC decreased as more recycled aggregate was replaced. Even though some researchers [26–29] have reported that the bond strength of RAC is similar or superior to that of normal concrete because the improved integrity and mechanical properties of the concrete conglomerate can enhance the bond strength, it is commonly found that the bond strength of RAC is dependent on the recycled aggregate replacement ratio (RAr). However, most study have drawn the experimental results only or proposed the predictive equations using a little amount of experimental datum, although steel-to-bond behavior is one of the most important factor for reinforced concrete structures. In order to evaluate the effect of RCA on the steel-to-concrete bond strength, this study presents the results of an experimental investigation into the bond behavior between RAC and deformed steel bars, with the main variables being the RCA replacement ratio (RCAr) and water-to-cement ratio (w/c) of the concrete mixture. In order to simulate actual concrete mixing conditions for construction in practice, all materials were added at the plant and mixed using ready-mix concrete trucks. The results of twenty four pull-out tests are presented, and the bond strength levels of the RAC are compared to current codes [30–33] and equations proposed by other researchers [34,35]. Test results from other studies also have been compiled to suggest a predictive model for the bond strength of deformed steel bars in RCA concrete.

Pu

pdb ld

ð1Þ

where Pu is the maximum pull-out load, db is the diameter of the bar, and ld is the embedded bar length. 2.1. Equations in code provisions The American Concrete Institute (ACI) 318 [30] and the Canadian Standards Association (CSA) CAN3-A23.3 [31] codes provide equations that account for the developmental length of

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S.-W. Kim et al. / Materials & Design 83 (2015) 257–269 Table 3 Mix proportions, slump, and air contents of the various concrete mixtures. Unit weight (kg/m3)

w/c

RCAr (%)

S/a

W

C

S

NG

RCA1

RCA2

0.51

0 30 60 100

0.48

177

347

827

937 656 375 0

0 189 378 630

0 76 151 252

0.46

0 30 60 100

0.46

176

385

782

954 668 382 0

0 192 385 641

0 77 154 257

0.33

0 30 60 100

0.42

163

494

695

987 691 395 0

0 199 398 664

0 80 159 265

Slump

Air content

AD

(mm)

(%)

2.43

125 125 150 160

5.1 4.5 4.3 5.0

2.7

110 125 140 160

5.0 4.7 4.8 4.7

4.94

145 140 160 140

3.5 3.6 3.4 2.8

Note: w/c is water-to-cement ratio; S/a is sand-to-aggregate ratio; W is water; C is cement; and AD is admixture (water-reducing agents).

deformed rebar. The minimum required bond strength can then be calculated based on the calculated development length as:

s¼

f y Ab

ð2Þ

pdb ld

where fy is the specified yield strength of the tested rebar, and Ab is the area of the tested rebar. According to the ACI code provision [30], the development length (ld) of No. 19 and smaller deformed bars in tension can be calculated as:

0

1

Bf y wt we kC ld ¼ @ qffiffiffiffi Adb 0 2:1 f c

ð3Þ

where Wt is the location factor that can be taken as 1.3 if the horizontal reinforcement is placed such that more than 300 mm of fresh concrete is cast below the development length of the bar, and 1.0 in all other cases. Therefore, for this investigation, Wt was taken as 1.0 to calculate the development length of the rebar because the bar was placed vertically in less than 300 mm of fresh concrete. We was taken as 1.0 because uncoated reinforcement was used in this study. k is the lightweight aggregate concrete factor, taken as qffiffiffiffi 0 f c =1:8f ct [30]. The CSA code [31] presents a similar equation to calculate the development length of rebar. The main difference between the CSA and ACI codes is the bar diameter factor. In the ACI code, this factor is reduced from 1.0 to 0.8 if the bars are equal to or less than 6 mm in diameter; in the CSA code, the factor is reduced from 1.0 to 0.8 if the bars are equal to or less than 20 mm in diameter. Therefore, the predicted bond strength values that were calculated with the ACI code are about 23% lower than those predicted by the CSA code. Australian Standard 3600 [32] recommends the following equation:

Table 4 Description of test specimens. Specimen

C (mm)

db (mm)

ld (mm)

w/c

RCAr (%)

PLA PMA PHA

75

16

64

0.51 0.46 0.33

0, 30, 60, 100

Note: P = pull-out specimen, L/M/H = low/medium/high compressive strength of concrete, A0/30/60/100 = RCAr.

qffiffiffiffi

s ¼ 0:265 f 0c

 c þ 0:5 db

ð4Þ

where c is the radius of a cylindrical concrete prism. In the CEB-FIP model code [33] for bond of embedded steel reinforcement, the bond strength s, between the deformed bars and the surrounding concrete under monotonic loading is defined as shown in Table 1. 2.2. Empirical equations Equations that represent the bond between the reinforcing bars and the concrete have been suggested by several researchers, as follows. Orangun et al. [34] proposed the following formula:

qffiffiffiffi

s ¼ 0:083045 f 0c 1:2 þ 3:0

c db þ 50 db ld

 ð5Þ 0

where c is the minimum concrete cover (mm) and f c is the concrete’s compressive strength (MPa). Darwin et al. [35] proposed a modified expression for the bond strength as:

s ¼ 0:083045 

    qffiffiffiffi c C max db 0 þ 75 f c 2:12 þ 0:5 0:92 þ 0:08 db C min ld

ð6Þ

where Cmin = min (Cx, Cy, Cs/2), Cmax = max [min (Cx, Cs/2), Cy], Cx is the side cover, Cy is the bottom cover, and Cs is the spacing between the bars. 3. Experimental program 3.1. Materials The RCA used in this study was obtained by processing waste concrete from the demolition of apartment buildings in Korea. The compressive strength of the original waste concrete was 21 MPa. For the processing of the RCA, RCA1 (high quality) and RCA2 (low quality) were blended in a ratio of 2.5:1.0 to meet the quality requirements of the Korean Standard (KS). Crushed stone was used for the natural coarse aggregate (NCA) and river sand was used for the natural fine aggregate (NFA). The maximum grain size of all the coarse aggregate was 25 mm in accordance with KS F 2527 [38]. Fig. 1 shows the three types of aggregate used in this study, and Table 2 lists their physical characteristics. A comparison of the grain size distribution for the natural and recycled aggregates is shown in Fig. 2. It can be seen that the

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Casting direction

150

Self leveling mortar 810

PVC pipe A

150

A

PlanView

SectionA-A Fig. 3. Typical pull-out specimen.

1 LVDT at the unloaded end

Concrete block Steel base plate with a hole at center

Top of test machine 2 LVDTs at the loaded end Deformed steel rebar

Loading direction

Fig. 4. Setup of pull-out testing.

NCA and NFA grading respectively conforms to the requirement of KS F 2526 [38] of 2.5–40.0 mm and 0.15–5.0 mm, also the RCA1 and RCA2 lie between the previous two, corresponding to the requirement of 2.5–40.0 mm. In general, increasing the maximum size of coarse aggregate leads to an increase of the compressive strength of normal strength concrete. As can be seen in Fig. 2, all aggregates show continuous grain size distributions for enough compaction of mixture, and NCA has more amount of coarse aggregates that has a grain size above 10 mm. Common Portland cement Type 1 conforming to KS L 5201 [38] was used as the binder in this study. The cement had a specific gravity of 3.15, plain surface area of 3.06 m2/g, and initial and final setting times of 3 h 35 min and 5 h 35 min, respectively. The designed mix proportions of the concrete specimens are listed in Table 3. The replacement ratios of the RCA are termed as the ratio of the RCA to the total coarse aggregate (by weight). In Korea, the maximum replacement ratio of RCA is restricted to 30% for structural use [11], so 30%, 60%, and 100% of RCAr were chosen for concrete mix. In order to simulate actual concrete mixing conditions for construction in practice, all materials were added at the plant and mixed using ready-mix concrete trucks. Due to their high water absorption characteristics, the RCAs were

presoaked prior to mixing in the ready-mix trucks. The amount of water used to presoak the recycled aggregate was calculated based on the saturated surface dry condition. Ferreira et al. [39] reported that the pre-saturation method negatively affects the concrete behavior, especially the durability performance. However they also concluded that behavior differences are not significant for commercial large-scale concrete production by batcher plant and ready-mix concrete truck. The main variables used in this study are the water-to-cement ratio (w/c) and the RCAr. The concrete mixtures were divided into three groups, with the water-to-cement ratios (w/c = 0.51, 0.46, and 0.33), and four different RCAr values (0%, 30%, 60%, and 100%) were considered for each group. The fresh concrete properties including slump, air content, and unit weight were determined for each mixture. As can be seen in Table 3, the slump of concrete increases as the RAr increases. It is common that the slump decreases when natural aggregates are substituted with recycled aggregates due to the higher water absorption value of RCAs, as well as the porous surface texture of the aggregates [40]. However, because of pre-saturating method used in this study, it can be inferred that the water released from RA increases the effective w/c ratio in concrete mix. For air content, in case of w/c = 0.51 and 0.46, the amount of entrapped air varied

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40

Elastic modulus (GPa)

Compressive strength (MPa)

50 40 30 20 w/c = 0.51

10

w/c = 0.46

0

25

50

75

30

Ec = 261

20 15 10

Proposed NC RC

0 20

100

30

RCAr (%)

40

50

60

Compressive strength (MPa)

(a) compressive strength

(b) elastic modulus vs. compressive strength

3.0

1.0

2.5

0.8

2.0 0.6 1.5

λ

Splitting tensile strength (MPa)

fcu + 11643

25

5

w/c = 0.33

0

35

0.4 1.0

w/c = 0.51

w/c = 0.51

0.2

w/c = 0.46

0.5

w/c = 0.46

w/c = 0.33

0.0

0

25

50

75

w/c = 0.33

100

0.0

0

25

RCAr (%)

50

75

100

RCAr (%)

(d) splitting tensile strength Fig. 5. Compressive test results.

Table 5 Mechanical properties of concrete. RAC

f c (MPa)

ec (106)

Ec (GPa)

fsp (MPa)

k

RAC0L RAC30L RAC60L RAC100L RAC0M RAC30M RAC60M RAC100M RAC0H RAC30H RAC60H RAC100H

29.26 26.52 28.53 27.08 33.42 31.46 30.66 29.49 44.13 39.50 43.80 42.44

2265 2192 2355 2084 2050 1839 2237 2096 2251 2380 2242 2538

20.15 17.60 18.70 19.39 20.17 20.56 18.97 19.59 24.46 21.57 22.00 22.67

1.81 1.94 2.11 1.89 2.49 2.67 2.11 1.81 2.45 2.38 2.55 2.32

0.62 0.73 0.73 0.68 0.86 0.88 0.72 0.63 0.68 0.70 0.74 0.66

0

to ASTM C234 [41], and all the specimens had the same concrete cover-to-bar diameter ratio (C/db = 4.7). Fig. 3 shows a typical pull-out test specimen with vertical rebar. Cubic specimens measuring 150 mm  150 mm  150 mm were cast to embed a deformed bar; then, concrete was placed in the cubic cast. The anchored length of the rebar was planned to 4db (64 mm) at the nonloading face in order to prevent restraint from compression. The other embedded portion was covered with PVC pipe to keep the bar from bonding with the concrete. The loaded end was long enough (660 mm = 810 mm  150 mm) to allow for pull-out testing. The opposite end protruded to allow the attachment of a linear variable differential transducer (LVDT). 3.3. Test setup and instrumentation

between 4.0% and 5.1% of the total volume of fresh concrete, while the air contents for w/c = 0.33 varied between 0.28% and 3.5%. However, the amount of entrapped air does not showed significant differences between normal concrete and RAC. Deformed steel rebar with a diameter of 16 mm (D16) was used for this investigation. Tensile tests of the reinforcement were conducted for five tensile samples, and the D16 rebar showed a yield strength value of 428 MPa at 0.24% strain, an ultimate strength value of 612 MPa, and elastic modulus value of 162 GPa.

3.2. Pull-out test specimens The bond behavior of deformed rebar with a diameter of 16 mm in concrete was studied by conducting direct pull-out tests of deformed rebar embedded in normal concrete and in RAC specimens. As listed in Table 4, four types of RCAr served as the main variables for this study. All specimens were fabricated according

The test setup is presented in Fig. 4. Pull-out tests, designed according to CSA S806-02 [42], were performed for the deformed steel rebar centrally embedded in the concrete cube. The pull-out tests were conducted in a rigid electro-hydraulic test frame using a specially fabricated mild steel rig that was connected tightly to the test machine. During loading, which was applied monotonically at the rate of 1.2 mm per minute, the pull-out specimen was pressed against a rigid 50-mm thick mild steel restrainer plate into the shape of a partial concave. A convex rigid steel plate was placed between the restrainer and the pull-out specimen to ensure uniform contact and to prevent eccentric loading. The test was performed by pulling the embedded rebar downward from the specimen. The applied load was measured by a pressure sensor whose signal was fed to an automatic data acquisition system. The average loaded end slip was measured using two LVDTs, and the net slip at the unloaded end was measured using a single LVDT. The output of the LVDTs was recorded using the data acquisition system. After reaching peak loads, the specimens were

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35

35

25 20 15 10 5 0

25 20 Splitting

15 10 5 0

0

1

2

3

4

5

6

1

2

Slip (mm)

3

4

15 10 5 4

5

20 15 10 Splitting

5 1

2

4

5

15 10 5 4

5

20 15 10 5 2

3

4

5

10 5 3

10 5 1

2

4

5

6

4

5

6

35

25 20 15 10 5

RCAr = 100%

Splitting

30 25

Splitting

20 15 10 5 0

0

1

2

3

4

5

6

Slip (mm)

Slip (mm)

(a) PLA series

3

Slip (mm)

0

0

RCAr = 60%

15

0

Bond stress (MPa)

Bond stress (MPa)

15

6

20

RCAr = 100%

20

5

Splitting

25

6

30

25

4

0 1

35 RCAr = 100%

3

Splitting

Slip (mm)

30

2

2

30 Splitting

0

35

1

1

35 Splitting

Slip (mm)

0

5

Slip (mm)

25

6

RCAr = 30%

10

0

0

0

6

15

6

Bond stress (MPa)

20

5

Splitting

20

RCAr = 60%

30

Bond stress (MPa)

Bond stress (MPa)

3

35

25

4

0 0

RCAr = 60%

3

25

Slip (mm)

30

3

2

Splitting

30

25

6

35

2

1

35

Slip (mm)

1

5

Slip (mm)

0

0

10

0

Bond stress (MPa)

Bond stress (MPa)

Bond stress (MPa)

20

3

15

RCAr = 30%

25

2

20

6

30

0

Bond stress (MPa)

5

35 RCAr = 30%

1

25

Slip (mm)

30

0

Splitting

0 0

35

RCAr = 0%

Splitting

30

Bond stress (MPa)

30

Bond stress (MPa)

Bond stress (MPa)

35 RCAr = 0%

RCAr = 0%

30

(b) PMA series

0

1

2

3

4

5

6

Slip (mm)

(c) PHA series

Fig. 6. Bond stress-slip curves of pull-out specimens.

unloaded to capture the descending branch of the load-slip relationship, and the mode of failure was noted. The pull-out tests were terminated when any of the following conditions occurred: (i) pull-through or rupture of the rebar, (ii) splitting of the surrounding concrete, or (iii) unloaded end slip at 6.0 mm. 4. Experimental results and discussion 4.1. Mechanical properties of RAC For each mix, 25 cylinders (D = 150 mm) were cast in steel molds and kept in a mist room at 23 °C and 95% relative humidity for 24 h until demolding. The cylinders were then placed in water at 23 °C for a total curing period of 28 days. Twenty cylinders were

used to determine the compressive strength (fcu), and five cylinders were used to determine the tensile splitting strength (fsp). The cross-sectional area of each cylinder was calculated using an average of three diameter measurements taken in two intersecting directions at the mid-height of the specimen. For compressive testing, the cylinder was placed at the center of a pressure plate, and loading was applied at a rate of 0.6 ± 0.4 MPa per second until failure. The compressive strength, splitting tensile strength, and elastic modulus were determined in accordance with ASTM C39 [43], ASTM C496 [44], and ASTM C469 [45], respectively. Compressive strength, splitting tensile strength, and the lightweight aggregate concrete factor k are plotted in Fig. 5, and the results of the compressive tests are presented in Table 5; each test result is the average of the twenty specimens. The compressive

S.-W. Kim et al. / Materials & Design 83 (2015) 257–269

(a) PMA0

(b) PMA60

(c) PHA0

(d) PHA60

263

Fig. 7. Splitting crack patterns of PMA and PHA specimens.

(a) PMA0

(b) PMA60 Fig. 8. Cross-sections of PMA series specimens.

strength and elastic modulus levels of the RAC were about 10% lower than those of the normal concrete specimen, respectively, as expected. In addition to the effect of density and water absorption of RAC, the grain size distribution of RAC affects the compressive strength because RAC has less amount of coarse aggregate that has a grain size above 10 mm than normal concrete. Compressive strain at peak load shows no particularly significant difference between conventional and recycled aggregated concrete, although many studies [46–48] reported that RAC shows an increase in the peak strain and higher strain characteristics in both ascending and descending branches in a compressive stress–strain curve due to the presence of mortar adhered to the recycled aggregate. However, the water absorption of recycled aggregate in the studies [46–48] ranged between 3.3% and 9.5%, which is much larger than

that (3.01%) of the recycled aggregated used in this study. It may be regarded as the most important characteristic that could influences overall compressive behavior of RAC. For the elastic modulus of concrete, Somna et al. [49] reported that the elastic modulus of RAC was approximately 19% lower than that of the conventional concrete, and even 25–26% lower elastic modulus of RAC were observed by other researcher [50–52]. Corinaldesi [53] also reported that 15% lower elastic modulus is achieved by using 30% finer coarse recycled aggregates, and the elastic modulus is more dependent on the compressive strength of concrete. In this study, as seen in Table 5, the elastic modulus of RAC were approximately 10% lower than that of normal concrete. Some equations for describing the relationship between the elastic modulus and the compressive strength suggested by

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S.-W. Kim et al. / Materials & Design 83 (2015) 257–269

35 RCAr = 0%

Bond strength (MPa)

30

RCAr = 30% RCAr = 60%

25

RCAr = 100%

20 15 10

CEB-FIP-S AS3600

5 0

CSA ACI

0

10

20

30

40

50

Compressive strength (MPa) Fig. 9. Variation of bond strength with compressive strength of concrete.

other researcher [53–59], and an equation is proposed and is graphed in Fig. 5(b). For RAC specimens with w/c values of 0.51 and 0.33, no noticeable effects of the recycled aggregate on the splitting tensile strength values were evident as reported in Tuyan et al. [60], whereas the strength levels of the RAC60M and RAC100M specimens were 15–27% lower than that of the RAC0M specimen with the w/c of 0.46. This was due to the absorption capacity of the adhered mortar on the surface of the recycled aggregate, especially the effectiveness of the interfacial transition zone (ITZ) of the RAC [61]. Due to the good bond characteristics between aggregate and the mortar matrix, RAC could produce higher splitting tensile strengths than normal concrete [62–64]. However, it was also reported by Duan and Poon [65] that excessive amount of adhered mortar lead to a decrease in tensile strength. 4.2. Bond behavior characteristics of RAC For each RCAr, the bond stress s was calculated as the stress between the rebar and the surrounding concrete along the embedded portion of the rebar, using Eq. (1). Fig. 6 shows the relationship between the bond strength and the free end slip for all the pull-out specimens. As shown in the figure, the bond strength levels of all the specimens had a tendency to decrease as the RCAr increased. For the PLA series (w/c = 0.51), splitting did not occur and bond failure was caused by pull-out. No noticeable difference in bond behavior characteristics between the normal and RAC pull-out specimens was evident in the ascending and descending branches,

and similar adhesive forces of about 10 MPa at the micro-slip stage were measured. All PLA specimens with RCA showed bond stress levels below 20 MPa, whereas the bond stress level of the PLA0 specimen was 21.69 MPa. For some specimens in the PMA series (w/c = 0.46), except the PMA100 specimen, longitudinal splitting cracks broke out throughout the whole cover concrete, and the bond failed abruptly. In the case of the PMA30 and PMA60 specimens, splitting failure occurred at about 2 mm of slip after peak stress. The PMA100 specimen showed 20.88 MPa of peak stress, which was relatively lower than for the other specimens, and bond failure occurred without splitting cracks. For the PHA series (w/c = 0.33), the concrete split parallel to the bar, and the resulting crack propagated to the surface of the concrete cube. All specimens showed about 29 MPa of peak bond stress, and there was no noticeable effect of the RCAr on the bond stress of the concrete. Splitting crack patterns and sections of specimens that failed by splitting are shown in Figs. 7 and 8, respectively. As shown in Fig. 7, no difference in the crack propagation between the normal concrete and RAC specimens was observed. Inclined cracks appeared toward the corners of the PHA specimens whereas only cracks perpendicular to the extreme face were propagated in the case of the PMA specimens, because higher strength concrete generally has greater fracture energy resulting in the abrupt propagation of cracks. No shearing-off in the concrete below the ribs was evident for the normal and RCA concrete specimens, as shown in Fig. 8. A noticeable point is that the PMA60 specimen failed by splitting even though its bond strength was 21.55 MPa, which is slightly lower than that of the PLA0 specimen that failed by pull-out. Based on the pull-out test results for the PMA0 specimen, the splitting crack occurred when the bond strength reached 25 MPa. However, the PMA30 and PMA60 specimens showed splitting failure at about 20 MPa of bond stress and around 2 mm of slip, which is evident by the descending curve after the peak stress as seen in Fig. 6(b). It is well known that the tensile spitting strength of concrete is related to the bearing stress at the rib face and the bond splitting stress of the cover concrete during pull-out loading. The degradation of the bond splitting tensile stress of the cover concrete is affected not only by the roundness of the coarse aggregate particles, as shown in Fig. 1, but also by the weaker ITZ between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete [66]. As shown in Fig. 9, the bond strength levels for all the pull-out specimens are 27–89% higher than those predicted by the various equations. In particular, the bond strength predicted using ACI, CSA and AS3600 equations show values significantly lower than those experimentally obtained because the equations of code provisions

Table 6 Bond stress levels of pull-out specimens. Specimen

0

f c (MPa)

s (MPa)

PLA0 29.26 21.69 PLA30 26.52 17.65 PLA60 28.53 19.17 PLA100 27.08 18.73 PMA0 33.42 25.32 PMA30 31.46 21.94 PMA60 30.66 21.55 PMA100 29.49 20.88 PHA0 44.13 29.42 PHA30 39.50 29.92 PHA60 43.80 28.77 PHA100 42.44 28.84 pffiffiffiffiffiffi a s ¼ 2:5 f ck for pull-out failure (CEB-FIP-P). pffiffiffiffiffi0:25 b s ¼ 7:0 20f ck for splitting failure (CEB-FIP-S).

qffiffiffiffiffi

s= f 0c 4.01 3.43 3.59 3.60 4.38 3.91 3.89 3.84 4.43 4.76 4.35 4.43

Calculated bond strength (MPa) Orangun

Darwin

CEB-FIP

ACI

CSA

AS3600

11.80 11.23 11.65 11.35 12.61 12.23 12.08 11.84 14.49 13.71 14.43 14.21

12.89 12.27 12.72 12.40 13.77 13.36 13.19 12.94 15.83 14.97 15.77 15.52

13.52a 12.87a 13.35a 13.01a 7.96b 7.84b 7.79b 13.58a 8.53b 8.30b 8.52b 8.45b

2.84 2.70 2.80 2.73 3.04 2.94 2.91 2.85 3.49 3.30 3.47 3.42

3.55 3.38 3.51 3.42 3.79 3.68 3.63 3.56 4.36 4.12 4.34 4.28

6.72 6.40 6.63 6.46 7.18 6.97 6.88 6.75 8.25 7.81 8.22 8.09

265

35

35

30

30

30

25

25

25

20 15 w/c = 0.51

10 5

20 15 w/c = 0.51

10

w/c = 0.46

0 20

40

60

80

100

0

RCA r (%)

1

2

15 w/c = 0.46 w/c = 0.33

0 2.45

3

Average water absorption of aggregate (%)

(a) RCAr

w/c = 0.51

5

w/c = 0.33

0 0

20

10

w/c = 0.46

5

w/c = 0.33

τ (MPa)

35

τ (MPa)

τ (MPa)

S.-W. Kim et al. / Materials & Design 83 (2015) 257–269

2.55

2.65

2.75

Average density of aggregate (%)

(b) Average water absorption

(c) Average density

1.2 τ ratio f'c ratio

1.1 1.0 0.9 0.8

0

25

50

75

100

1.2

Strength reduction ratio

1.2

Strength reduction ratio

Strength reduction ratio

Fig. 10. Effect of RCAr and physical properties of RCA on bond strength.

τ ratio f'c ratio

1.1 1.0 0.9 0.8

0

RCAr (%)

(a) w/c = 0.51

25

50

RCAr (%)

(b) w/c = 0.46

75

100

τ ratio f'c ratio

1.1 1.0 0.9 0.8

0

25

50

75

100

RCAr (%)

(c) w/c = 0.33

Fig. 11. Effect of water-to-cement ratio on the relationship between compressive strength and bond strength.

are focused on the development and splice lengths of reinforcement in concrete structure and consider the splitting tensile failure due to low value of C/db. In particular, bond strengths predicted by CEB-FIP for splitting failure are lower than those for pull-out failure as presented in Table 1, and are similar with those predicted by AS3600. In addition to the underestimation of the predictive equations, no factor for the RAC was considered in the code provisions and empirical equations. The measured bond strength levels and the bond strength levels calculated using the different equations discussed in Section 2 are listed in Table 6. As listed in the table, the contribution of the compressive strength of concrete on the bond strength shows a tendency of decrease as compressive strength increases. 4.3. Effect of RCA on bond strength It is well known that recycled aggregate has lower density and higher water absorption values than normal aggregate so that the mechanical properties of RAC is inferior to normal concrete. Hence, in order to evaluate the effect of RCA on the bond strength as well as on the mechanical properties of RAC, an investigation into variables such as RCAr, density, and water absorption of RCA was undertaken. This study deals with RCA only, so the average water absorption of coarse aggregate was calculated as: P ½each coarse aggregate weight  relev ant water absorption ð%Þ WAav e ¼ Total weight of coarse aggregate

ð7Þ The average density of coarse aggregate was calculated as:

qav e ¼

P ½each coarse aggregate weight  relev ant density Total weight of coarse aggregate

ð8Þ

Fig. 10 describes the effect of RCA on bond strength in terms of (a) RCAr, (b) water absorption, and (c) density. As shown in

Fig. 10(a), the bond strength decreased as the RCAr increased for all the specimens with different water-to-cement ratios. As more RCA was replaced, the average water absorption value increased and the average density value decreased, as shown in Figs. 10(b) and (c). The average water absorption and density values of the RCA show a strong correlation with the bond strength of the RAC. As stated in other studies and code provisions, the water absorption and density of RCA are important and critical factors for investigating the effect of RCA on the mechanical properties and bond strength. However, in the case of the w/c of 0.33, very slight differences can be seen for the main variables, i.e., RCAr, water absorption, and density. Fig. 11 shows the relationship between the strength reduction ratio (strength of RAC divided by that of normal concrete) and the RCAr in order to investigate the effect of the compressive strength of RAC on the bond strength. A strong relationship evidenced by a relatively good correlation appears to exist between the compressive strength and the bond strength of the RAC. As the RCAr increased, the trend of the strength reduction ratio became similar to that of the bond strength. As mentioned above, the effect of the RCAr on the compressive strength and bond strength is not noticeable provided that the w/c is 0.33. Hence, it is clear that the compressive strength also affects the contribution of the physical properties of RCA on the bond strength of RAC. 4.4. Analysis of parameters for bond strength of RAC Based on the test results for this study, compressive strength, average density (q), and water absorption (WA) were adopted as the parameters to depict the degradation characteristic of bond strength with an increase in the RCAr. Fig. 12 shows the effects of these parameters on the bond strength. As shown in Fig. 12(a), a strong relationship is evident between the compressive strength of the RAC and its bond strength as indicated by a good correlation.

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S.-W. Kim et al. / Materials & Design 83 (2015) 257–269

Bond strength ratio (τRAC / τNC)

1.1 w/c = 0.33

1.0

0.9 w/c = 0.46

0.8

w/c = 0.51 w/c = 0.51

0.7 0.86

0.88

0.90

0.92

0.94

w/c = 0.46

0.96

Bond strength ratio (τRAC / τNC)

0.9

w/c = 0.51

w/c = 0.46

0.8

w/c = 0.51

0.98

1.00

0.7 0.86

1.02

0.88

0.90

0.92

0.94

w/c = 0.46

0.96

w/c = 0.33

0.98

1.00

Compressive strength ratio ( f'RAC / f'NC)

Average aggregate density ratio ( ρ RAC / ρNC )

(a) compressive strength

(b) average density (ρ) of coarse aggregate

1.1

w/c = 0.33

1.0

0.9 w/c = 0.51

0.8

w/c = 0.46 w/c = 0.51

w/c = 0.46

w/c = 0.33

0.7 0

w/c = 0.33

1.0

w/c = 0.33

Average water absorption ratio (Ǝ RAC / Ǝ NC)

Bond strength ratio (τRAC / τNC)

1.1

1

2

3

4

5

6

1.02

1.1

1.0

0.9

0.8

0.7 0

Average water absorption ratio (WA RAC / WA NC)

1

2

3

4

5

6

Average water absorption ratio (WA RAC / WA NC )

(d) relationship between ρ and WA

(c) average water absorption (WA) of coarse aggregate

35

35

30

30

30

25 20 15 10 τ_test τ_cal

5 0

0

20

40

60

80

100

25 20 15 10 τ_test τ_cal

5 0

0

20

40

RCAr (%)

(a) w/c = 0.51

Bond strength (MPa)

35

Bond strength (MPa)

Bond strength (MPa)

Fig. 12. Effect of parameters on bond strength.

60

80

100

RCAr (%)

(b) w/c = 0.46

25 20 15 10 τ_test τ_cal

5 0

0

20

40

60

80

100

RCAr (%)

(c) w/c = 0.33

Fig. 13. Effect of compressive strength on bond strength.

The figure shows that when the compressive strength of the concrete increases by 10%, its bond strength increases by 20%. However, in the case that the w/c is 0.33, the effect of the compressive strength of the concrete on the bond strength is much slighter than with a lower w/c. Like the effect of compressive strength, the average density of the RCA has a clear relationship with the bond strength of the RAC, as shown in Fig. 12(b). In the case of average water absorption of the RCA, a relationship that is similar to that for average density can be seen in Fig. 12(c). However, the bond strength increases in an inverse relationship to the average water absorption of the RCA. As expected, a linear correlation is evident between the average density and water absorption of the RAC, as seen in Fig. 12(d). For an average aggregate density decrease of 8%, the average water absorption increases by about 5 times. It is

inferred that the attached mortar and the porosity of the RCA caused degradation in terms of bond strength related to compressive strength. In sum, the bond strength of the RAC was affected by the parameters, i.e., the compressive strength of the RAC, and the average density and water absorption of the RCA. Clearly, a linear relationship exists between the bond strength and the parameters, but higher compressive strength values led to a decrease in the effect of the parameters. 4.5. Bond strength predictive equation Based on the evaluation of the effects of the parameters on the bond strength of the RAC, it was found that the bond strength

267

S.-W. Kim et al. / Materials & Design 83 (2015) 257–269 Table 7 Comparison of predicted values with pull-out test results in this study. 0

w/c

RCAr (%)

f c (MPa)

qnc

qave

stest (MPa)

rtest

scal (MPa)

rcal

stest /scal (rtest/rcal)

0.51

0 30 60 100 0 30

29.26 26.52 28.53 27.08 33.42 31.46

2.690

2.690 2.634 2.576 2.496 2.690 2.634

21.69 17.65 19.17 18.73 25.32 21.94

– 0.814 0.884 0.864 – 0.867

– 17.41 18.26 16.16 – 23.40

– 0.803 0.842 0.745 – 0.924

– 1.014 1.049 1.159 – 0.938

60 100

30.66 29.49

2.576 2.496

21.55 20.88

0.851 0.825

22.64 21.22

0.894 0.838

0.952 0.984

0 30 60 100

44.13 39.50 43.80 42.44

2.690 2.634 2.576 2.496

29.42 29.92 28.77 28.84

– 1.017 0.978 0.980

– 28.83 29.04 28.84

– 0.980 0.987 0.980

– 1.038 0.991 1.000

0.46

Predicted normalized bond strength (rcal )

0.33

Nonlinear regression analysis was used to develop a multivariable model to predict the bond strength of RAC. For this study, the dependent variables are the compressive strength of the RAC and the average density of the RCA. Based on the pull-out test results in this study, the following model was developed:

1.8

R2 = 0.92

1.6 1.4 1.2 1.0



r ¼ 0:92 

0.8

qav e qnc





 3

6 ð0:04f 0c Þ

ð10Þ

0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Measured normalized bond strength (rtest ) Fig. 14. Predictive model for bond strength of RAC.

decreases as a ratio of the average density of the coarse aggregate to the average density of the normal coarse aggregate. However, the degradation of the bond strength with the density of the coarse aggregate slows down as the water-to-cement ratio becomes lower. Therefore, it can be stated that the ratio of the bond strength of RAC to the bond strength of normal concrete is affected by parameters such as the density of the RCA and the compressive strength of the RAC. Thus, a degradation proportion of the bond strength of RAC to that of normal concrete (r) can be expressed as:

Fig. 13 examine the accuracy of the developed model. The parameters used in the model and the predicted bond strength levels are listed in Table 7. Fig. 14 shows the predicted normalized bond strength (rcal) versus measured normalized bond strength (rtest). It can be seen that the developed model can predict the bond strength of RAC reasonably well. In order to verity the compatibility of the predictive model, the effect of the normalized density (qave/qnc) on the model curve was examined. As shown in Fig. 15, the model curves developed based on the test results from this study tend to converge near 1.0 as the compressive strength increases. However, the model underestimates the bond strength ratio when the compressive strength of the concrete is below 30 MPa. To check the validity of the developed model, other experimental results found in previous studies [20–26], as listed in Table 8, were collected and examined. To consider the variables of the collected data, Eq. (10) has been modified as:

h r ¼ 1:039  ½0:925  qqAVE 

4 ð0:04F c Þ3

NC

sRAC r¼ sNC

i



 0 F c ¼ 30; if f c 6 30

ð11Þ

0

ðF c ¼ f c ; other caseÞ

ð9Þ

To improve the compatibility of the model (Eq. (10)), the range of compressive strength levels below 30 MPa has been modified as Eq. (11). The modified bond strength ratio curves can be seen in

1.2

Bond strength ratio

1.0 Table 8 Physical properties of coarse aggregate and compressive strength of concrete in other studies.

0.8 0.6

ρave /ρnc = 0.70

Researcher

ρave /ρnc = 0.80

0.4

ρave /ρnc = 0.90

0.2

w/c = 0.51

ρave /ρnc = 0.99

w/c = 0.46 w/c = 0.33

0.0

0

10

20

30

40

50

60

70

Compressive strength (MPa) Fig. 15. Bond strength ratio curve with normalized density.

80

Breccolotti and Materazzi [20] Guerra et al. [21] Hassanean et al. [22] Prince and Singh [23] Kim et al. [24] Kim and Yun [25] Singla [26]

Density of coarse aggregate NCA

RCA

2.70 2.69 2.56 2.67 2.64 2.61 2.34

2.50 2.23 2.44 2.50 2.61 2.48 2.60

0

f c (MPa)

37–56 46–53 28–35 24–29 43–45 29–33 25–52

268

S.-W. Kim et al. / Materials & Design 83 (2015) 257–269

1.2

Bond strength ratio

1.0

ρave /ρnc = 0.99

(4)

0.8

ρave /ρnc = 0.90

0.6

ρave /ρnc = 0.80 ρave /ρnc = 0.70

0.4 0.2 0.0

(5) 0

10

20

30

40

50

60

70

80

Compressive strength (MPa) Fig. 16. Modified bond strength ratio curves with normalized density.

(6)

Predicted normalized bond strength

1.8 Breccolotti [20]

1.6

R2 = 0.70

Guerra [21]

1.4

Hassanean [22] Prince [23]

1.2

(7)

Kim [24]

1.0

Yun [25]

0.8

Singla [26] This study

0.6 0.4 0.2 0.0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Measured normalized bond strength Fig. 17. Validity of predictive models for bond strength of RAC.

Fig. 16, and the results are shown in Fig. 17. It can be seen that the degradation characteristic of RAC bond strength can be fitted by Eq. (11). 5. Conclusions The following observations and conclusions can be made and drawn on the basis of the pull-out test results and regression analysis in this study. (1) The slump of concrete increases as the RAr increases because the water released from RA increases the effective w/c ratio in concrete mix. However, the amount of entrapped air does not significant differences between normal concrete and RAC. (2) The grain size distribution of RAC affects the compressive strength because RAC has less amount of coarse aggregate that has a grain size above 10 mm than normal concrete, and the RCA has no particularly significant effect on the compressive strain at peak load due to relatively lower absorption of recycled aggregated compared with that used in other studies. The elastic modulus of RAC were approximately 10% lower than that of normal concrete, and an equation to best fit this test results are proposed. (3) For the splitting tensile strength, no noticeable effects of the RCA were evident. This was due to the absorption capacity of the adhered mortar on the surface of the recycled aggregate,

(8)

especially the effectiveness of the ITZ of the RAC. Due to the good bond characteristics between aggregate and the mortar matrix, RAC could produce higher splitting tensile strengths than normal concrete. The PMA100 (w/c = 0.46) specimens showed 20.88 MPa of peak stress, which is relatively lower than that shown by the other specimens, and bond failure occurred without splitting cracks. For the PHA series (w/c = 0.33), the concrete split parallel to the bar, and the resulting crack propagated to the surface of the concrete cube due to greater fracture energy. The bond strength predicted using ACI, CSA and AS3600 equations show values significantly lower than those experimentally obtained because the equations of code provisions are focused on the development and splice lengths of reinforcement in concrete structure and consider the splitting tensile failure due to low value of C/db. The average water absorption and density values of the RCA exhibited strong correlations with the bond strength of the RAC. However, when the w/c was 0.33, a very slight degradation of bond strength was observed for the variables, i.e., RCAr, water absorption, and density. Hence, it is clear that compressive strength also affects the contribution of the physical properties of RCA on the bond strength of RAC. The bond strength of the RAC was affected by the parameters: the compressive strength of the RAC, and the average density and water absorption of the RCA. The contribution of the compressive strength of concrete on the bond strength shows a tendency of decrease as compressive strength increases. A linear relationship was observed between the bond strength and the parameters, but higher compressive strength levels diminished the effects of the parameters. Using nonlinear regression analysis, a bond strength predictive model is suggested that takes into account important parameters (the density of RCA and compressive strength of RAC) that affect the bond strength of RAC. In the process of validating the developed model based on other studies’ test results, the proposed model has been modified and improved; hence, it can be inferred that the degradation characteristic of the bond strength of RAC can be predicted reasonably well. With the predictive model proposed, more simple calculation for the development and splice length of steel rebar in structural concrete with RCA could be available.

Acknowledgement This study was financially supported by research fund of Chungnam National University, Daejeon, South Korea. References [1] Ministry of Environment, Statistics on waste occurrence and disposal, , 2012. [2] W.C. Choi, H.D. Yun, Long-term deflection and flexural behavior of reinforced concrete beams with recycled aggregate, Mater. Des. 51 (2013) 742–750. [3] K.H. Younis, K. Pilakoutas, Strength prediction model and methods for improving recycled aggregate concrete, Constr. Build. Mater. 49 (2013) 688– 701. [4] WRAP, Performance Related Approach to Use Recycled Aggregates, Waste and Resources Action Programme, Oxon, 2007. [5] W.W.V.C.M. Tam, Recycled aggregate from concrete waste for higher grades of concrete construction, PhD thesis, Department of Building and Construction. Hung Kong, City University of Hung Kong, 2005. [6] T.C. Hansen, Recycled of Demolished Concrete and Masonry, E & FNSPON, London, 1992. [7] BS 8500-2, Concrete Complementary British Standard to BS EN 206 Part 1–Part 2: Specification for Constituent Materials and Concrete, British Standard Institution, London, 2006.

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