# Bond Behavior between Steel Rebar and RCA Concrete after Exposure to Elevated Temperatures

Authors: Wanjie Zou; Jiongfeng Liang; Dawei Liu; Guangwu Zhang
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

To explore the bond behavior between steel rebar and recycled coarse aggregate (RCA) concrete after exposure to elevated temperatures, an experimental study was carried out. The results demonstrated that the bond strength of RCA concrete pullout specimens decreased greatly with increasing temperature. As the exposure temperature elevated, the slope of the ascending portion of the bond-slip curves gradually declined, and the descending portion of the curves tended to flatten. A modified model was developed to predict the bond strength between RCA concrete and steel rebar after exposure to elevated temperature, and the predicted results showed a very good fit in the experimental test results. Besides, the proposed bond-slip relations for steel rebar in RCA concrete after elevated temperatures showed satisfactory agreement with test results.


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## Body

## 1. Introduction

In recent years, recycling and utilization of construction waste as aggregates in new concrete have been studied by many researchers [1–8]. Their works were carried out to examine mechanical properties, durability of recycled aggregate concrete (RAC), and structural behavior of RAC members [9, 10]. For example, Letelier Gonzalez et al. [11] investigated the behavior of three beam-column joints under cyclic loading. It found that the behavior under cyclic loading of the concrete made with 30% replacement of natural by recycled aggregates was quite similar to ordinary concrete. Peng et al. [12] tested 6 rectangular squat recycled concrete wall specimens to understand the behavior of squat reinforced concrete shear walls and promote the application of recycled concrete in structures and developed a simplified analytical method to predict the peak loads of squat walls failed in flexure or a mixed flexural-diagonal compression mode. Bond behavior was a crucial factor affecting the composite action between the two components. Therefore, a number of studies have been conducted to inspect research studies [13–19]. They showed the influence of different parameters (such as RCA replacement ratio, water/cement ratio, type of steel rebar, diameter of steel rebar, and steel rebar location) on the bond of steel rebar in RAC. For instance, Xiao and Falkner [20] investigated the bond behavior between recycled aggregate concrete (RAC) and steel rebars. It was found that the bond strength between the RAC and the plain rebar decreases with an increase of the RCA replacement percentage, whereas the bond strength between the RAC and the deformed rebar has no obvious relation with the RCA replacement percentage. Pandurangan et al. [21] compared the effect of treatment methods such as acid treatment, thermal treatment, and mechanical treatment on the bond strength of reinforcement with recycled aggregate concrete.It is well known that the fire resistance of concrete structures is very important. Therefore, many researchers were making efforts toward residual mechanical behavior, such as stress-strain relationship, splitting tensile strength, and compressive strength, of RAC after elevated temperatures or fire [22–27]. For example, Liang et al. [28] investigated the influences of high temperatures on the mechanical properties of concrete containing recycled fine aggregate. The results showed that the mass loss, compressive strength, elastic modulus, and splitting tensile strength of concrete specimens containing recycled fine aggregate declined significantly as the temperature rose. Liang et al. [29] presented the effects of elevated temperatures on the strength and compressive stress-strain curve of recycled coarse aggregate concrete with different replacement percentages and proposed stress-strain relations for recycled aggregate concrete after exposure to elevated temperatures. However, few studies have been conducted on bond behavior between steel rebar and RAC after high temperatures. Yang et al. [30] studied the bond behavior of steel rebar in RAC after exposure to elevated temperatures of 300°C, 400°C, and 500°C; 500 of 300 bond behavior of steestrength decreased with increasing temperature, but the peak slip increased with increasing temperature. Some previous works were conducted on the bond between steel bars and concrete, the bond between fibres and concrete, and the bond in other fibre-reinforced composites [31–34].The objectives of this study are establishing the constitutive equation of bond-slip relationship between RCA concrete and steel rebar after high temperatures, predicting bond strength between RCA concrete and steel rebar after high temperatures, and analyzing the influences of high temperatures, recycled coarse aggregate content, and heat exposure time on the bond behavior.


## 2. Experimental Program

### 2.1. Materials

Common Portland cement (C) of grade 32.5R, river sand (S) with a fineness modulus of 2.7 and a moisture content of 0.6%, coarse aggregates, and tap water (W) were used in the concrete mixes. The used coarse aggregates included natural coarse aggregate (NCA) and recycled coarse aggregate (RCA) acquired from waste concrete brought in the reclamation depot, which is in the range of 5–20 mm. The physical properties of RCA are shown in Tables1 and 2 which provide the concrete mix proportions, which varied the replacement ratio of RCA in the concrete (i.e., the RCA replacement ratio was 0%, 50%, and 100%, respectively.) The deformed 10 mm steel bars (HRB335) with a yield strength of 375 MPa were used in this pullout tests.Table 1
Physical properties of RCA.
Grading (mm)
Bulk density (kg/m3)
Apparent density (kg/m3)
Water absorption (%)
Silt content (%)
Aggregate crushing value (ACV) (%)
5–20
1385
2490
8.47
5.5
13.2
ACV
=
M
2
/
M
1
×
100, where M1 is the mass of the test specimen (g) and M2 is the mass of the material passing the 2.36 mm test sieve (g).Table 2
Mix proportions of the recycled coarse aggregate concrete.
Tag
Replacement (%)
RCA (kg/m3)
C ( kg/m3)
S (kg/m3)
NCA (kg/m3)
W (kg/m3)
RAC0
0
0
513
474
1218
195
RAC50
50
609
513
474
609
195
RAC100
100
1218
513
474
0
195


### 2.2. Specimen Preparation

A total of 108 pullout specimens were designed and manufactured according to Chinese code (GB 5015292-1992) [35], as shown in Figure 1. Table 3 lists the variation parameter and the number of tested pullout specimens for each temperature. To obtain the compressive strength of concrete, 54 cubic specimens were also prepared, as shown in Table 3. After being cured for 2 days, the specimens were demoulded, stored at ambient temperature of 20°C for 28 days, and then placed in a high temperature furnace for heat treatment. In order to attain uniform distribution of heat across the specimens, the target temperature of 200, 400, and 600°C was maintained constant according to the exposure time. And, the heating rate in this test was 10°C/min [36].Figure 1
Dimensions of test specimens.Table 3
The variation parameter and the number of tested specimens.
No.
r (%)
T (°C)
t (h)
n
1
n
2
NC
0
20
1
6
3
0
200
1
6
3
0
400
1
6
3
0
600
1
6
3
0
600
0.5
6
3
0
600
2
6
3
RAC50
50
20
1
6
3
50
200
1
6
3
50
400
1
6
3
50
600
1
6
3
50
600
0.5
6
3
50
600
2
6
3
RAC100
100
20
1
6
3
100
200
1
6
3
100
400
1
6
3
100
600
1
6
3
100
600
0.5
6
3
100
600
2
6
3
r denotes the RCA replacement ratio; T is exposure temperature; t is exposure time; n1 is the number of tested pullout specimens; n2 is the number of tested cubic specimens.


### 2.3. Testing

The loading setup for the pullout test used a UTM-300 microcomputer-controlled electrohydraulic servo tester, as shown in Figure2. The applied pull load and free-end slip of tested specimens were measured with the electrohydraulic servo tester and a linear variable differential transformer (LVDT), respectively. The following equation was used to calculate the bond strength between rebar and concrete:(1)τu=Puπdla,where τu is the bond strength, Pu is the peak tensile load (N), d is the diameter of the steel rebar (mm), and la is the embedded length of the steel rebar (mm).Figure 2
Schematic of pullout test setup.


## 2.1. Materials

Common Portland cement (C) of grade 32.5R, river sand (S) with a fineness modulus of 2.7 and a moisture content of 0.6%, coarse aggregates, and tap water (W) were used in the concrete mixes. The used coarse aggregates included natural coarse aggregate (NCA) and recycled coarse aggregate (RCA) acquired from waste concrete brought in the reclamation depot, which is in the range of 5–20 mm. The physical properties of RCA are shown in Tables1 and 2 which provide the concrete mix proportions, which varied the replacement ratio of RCA in the concrete (i.e., the RCA replacement ratio was 0%, 50%, and 100%, respectively.) The deformed 10 mm steel bars (HRB335) with a yield strength of 375 MPa were used in this pullout tests.Table 1
Physical properties of RCA.
Grading (mm)
Bulk density (kg/m3)
Apparent density (kg/m3)
Water absorption (%)
Silt content (%)
Aggregate crushing value (ACV) (%)
5–20
1385
2490
8.47
5.5
13.2
ACV
=
M
2
/
M
1
×
100, where M1 is the mass of the test specimen (g) and M2 is the mass of the material passing the 2.36 mm test sieve (g).Table 2
Mix proportions of the recycled coarse aggregate concrete.
Tag
Replacement (%)
RCA (kg/m3)
C ( kg/m3)
S (kg/m3)
NCA (kg/m3)
W (kg/m3)
RAC0
0
0
513
474
1218
195
RAC50
50
609
513
474
609
195
RAC100
100
1218
513
474
0
195


## 2.2. Specimen Preparation

A total of 108 pullout specimens were designed and manufactured according to Chinese code (GB 5015292-1992) [35], as shown in Figure 1. Table 3 lists the variation parameter and the number of tested pullout specimens for each temperature. To obtain the compressive strength of concrete, 54 cubic specimens were also prepared, as shown in Table 3. After being cured for 2 days, the specimens were demoulded, stored at ambient temperature of 20°C for 28 days, and then placed in a high temperature furnace for heat treatment. In order to attain uniform distribution of heat across the specimens, the target temperature of 200, 400, and 600°C was maintained constant according to the exposure time. And, the heating rate in this test was 10°C/min [36].Figure 1
Dimensions of test specimens.Table 3
The variation parameter and the number of tested specimens.
No.
r (%)
T (°C)
t (h)
n
1
n
2
NC
0
20
1
6
3
0
200
1
6
3
0
400
1
6
3
0
600
1
6
3
0
600
0.5
6
3
0
600
2
6
3
RAC50
50
20
1
6
3
50
200
1
6
3
50
400
1
6
3
50
600
1
6
3
50
600
0.5
6
3
50
600
2
6
3
RAC100
100
20
1
6
3
100
200
1
6
3
100
400
1
6
3
100
600
1
6
3
100
600
0.5
6
3
100
600
2
6
3
r denotes the RCA replacement ratio; T is exposure temperature; t is exposure time; n1 is the number of tested pullout specimens; n2 is the number of tested cubic specimens.


## 2.3. Testing

The loading setup for the pullout test used a UTM-300 microcomputer-controlled electrohydraulic servo tester, as shown in Figure2. The applied pull load and free-end slip of tested specimens were measured with the electrohydraulic servo tester and a linear variable differential transformer (LVDT), respectively. The following equation was used to calculate the bond strength between rebar and concrete:(1)τu=Puπdla,where τu is the bond strength, Pu is the peak tensile load (N), d is the diameter of the steel rebar (mm), and la is the embedded length of the steel rebar (mm).Figure 2
Schematic of pullout test setup.


## 3. Results and Discussion

### 3.1. Bond Strength

The influence of high temperature (T) on bond strength of steel rebar in RCA is shown in Figure 3. It was found that the bond strength of tested specimens decreased with the increase in temperature regardless of recycled coarse aggregate content (r), which was similar to the previous study [30]. After the temperature of 200°C, the reduction in bond strength of NC, RAC50, and RAC100 was about 16%, 15%, and 16%, respectively. As the temperature continued to increase, the bond strength reduced continuously. After exposure to 400°C, it was reduced by 33%, 39%, and 38% for NC, RAC50, and RAC100, respectively. There was a significant loss in the bond strength for all concrete mixes after 600°C. The bond strength loss was 60%, 65%, and 66% for NC, RAC50, and RAC100, respectively. The reason may be that the resistance of a steel bar against pulling out is provided mainly by adhesion and friction between reinforcing bar and concrete. Adhesion is lost at early stages of loading when slip is initiated. Therefore, resistance to pulling out is mainly provided by friction. As the specimens exposed to high temperature is deteriorated, the friction is weakened, and the resistance to pulling out is lessened.Figure 3
Influence of temperature on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)The influence of RCA content on bond strength of steel rebar in RCA is displayed in Figure4. It was concluded that the bond strength had been less affected due to recycled coarse aggregate content increased. For NC specimens, the bond strength decreased by 2.41 MPa–8.83 MPa from 200°C to 600°C; the bond strength of RAC50 specimens decreased by 2.01 MPa–8.54 MPa from 200°C to 600°C; the bond strength of RAC100 specimens decreased by 1.98 MPa–8.22 MPa from 200°C to 600°C. That is, bond strength loss of all pullout specimens in 200°C–600°C range was comparable regardless of recycled coarse aggregate content.Figure 4
Influence of RCA content on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)The influence of heat exposure time on the bond strength of steel rebar in RAC is shown in Figure5. It was found that the bond strength declined with increasing exposure time (t). The bond strength loss for NC, RAC50, and RAC100 was 6.17 MPa, 6.21 MPa, and 6.56 MPa after exposure to 600°C for 0.5 h, respectively. For an exposure time of 1 h at 600°C, the bond strength loss was about 63% for different recycled coarse aggregate contents. And, the bond strength loss significantly increased to about 88% after two hours of exposure for all pullout specimens of different concrete mixes.Figure 5
Influence of exposure time on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


### 3.2. Bond-Slip Relation

The influence of high temperature on bond-slip relation of all pullout specimens is presented in Figure6. It concluded that the shape of bond-slip curves of heated pullout specimens was not the same as those of unheated pullout specimens. But both bond-slip curves had four stages, namely, microslip stage, internal cracking stage, pullout stage, and descending stage. And, the slope of the ascending part of bond-slip curves gradually declined with increasing temperature; meanwhile, the peak bond stress was continually reduced. That is to say, as the temperature increased, the initial stiffness of the specimens decreased. And, as the temperature rose, the descending portion of the curves tended to flatten. The influence of RCA content on bond-slip relation of all pullout specimens is shown in Figure 7. The shape of bond-slip curves of pullout specimens with different recycled coarse aggregate content at the same temperature was similar. It was also similar to linear ascending and descending trend of the bond-slip curves. Figure 8 presents the effect of exposure time on bond-slip relation of all pullout specimens. The bond-slip curves of pullout specimens after exposure to different times had a similar trend. However, the bond stress was lowest for pullout specimens after an exposure time of 2 h followed, in sequence, after an exposure time of 1 h and 0.5 h, respectively. And, the effect of exposure time on the slip corresponding to the peak bond stress was not obvious.Figure 6
Influence of high temperature on the bond-slip relationship: (a)r = 0%, (b) r = 50%, and (c) r = 100%.
(a)
(b)
(c)Figure 7
Influence of recycled coarse aggregate content on the bond-slip relationship: (a) room temperature (RT) and (b)T = 600°C.
(a)
(b)Figure 8
Influence of exposure time on the bond-slip relationship: (a)T = 600°C, r = 0%; (b) T = 600°C, r = 100%.
(a)
(b)


### 3.3. Bond Strength Provisions

At present, many different modes to predict the bond strength between concrete and steel bars have been proposed [37–46]. For example, the CEB-FIP code [47] recommended an equation as follows:(2)τu=2.5fc′,where fc′ is the compressive strength of concrete.Here, considering the effect of elevated temperature and exposure time on bond strength, a simple modified mode for predicting the bond strength of steel rebar in RAC after exposure to elevated temperature was given as(3)τu=2.5fc′−0.0113ΔTt,where fc′ is the compressive strength of concrete and ΔT is the difference between exposure temperature in °C and room temperature in °C. It ranges between 20 and 600°C (20°C≤ΔT≤600°C); t is exposure time in hours.The results calculatedτ0cal using equation (3) compared to the test results τ0exp are summarized in Table 4. And, the comparison of tested and predicted bond stress values is shown in Figure 9. It found that the predicted equation revealed a very good fit in the test results.Table 4
Tested and predicted bond strength.
No.
T (°C)
t (h)
f
c (MPa)
s
0 (mm)
τ
0exp (MPa)
τ
0cal (MPa)
Error (%)
NC
20
1
37.92
0.37
14.71
15.39
4.62
200
1
35.34
0.88
12.32
12.83
4.14
400
1
32.12
1.44
9.81
9.87
0.61
600
1
25.21
1.98
5.85
5.99
2.39
600
0.5
27.80
2.24
8.53
8.55
0.23
600
2
21.21
1.79
1.99
2.24
12.56
RAC50
20
1
30.53
0.43
13.11
13.81
5.34
200
1
27.62
0.92
11.12
11.10
0.18
400
1
24.15
1.51
7.93
7.99
0.76
600
1
20.26
2.05
4.56
4.69
2.85
600
0.5
22.41
2.32
6.89
7.20
4.49
600
2
19.82
1.86
1.85
1.86
0.54
RAC100
20
1
28.73
0.54
12.54
13.40
6.86
200
1
25.72
0.97
10.52
10.64
1.14
400
1
23.11
1.64
7.72
7.72
0.00
600
1
19.16
2.13
4.26
4.39
3.05
600
0.5
18.22
2.48
5.94
6.04
1.68
600
2
17.61
1.91
1.27
1.22
3.94Figure 9
Comparison of the tested and predicted on bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


### 3.4. Model for Bond-Slip Relationship after High Temperature

Some models had been proposed to predict the bond-slip relation between RAC and steel rebar at room temperature [13–21]. Here, the analytical expression advised by Xiao and Falkner [20] was modified for describing the bond-slip relationship between RCA concrete and steel rebar after high temperatures by redefining some parameters. The normalized bond-slip relationship between RCA concrete and steel rebar after high temperature was approximated by the following equation:(4)y=xa,0≤x<1,xbx−12+x,x≥1.In equation (4), x=s/s0, y=τ/τ0,τ is the bond stress, τ0 is the peak bond stress, s is the slip, and s0 is the slip corresponding to the τ0. a and b are constants to be determined. Based on data regression analysis, the value of the parameter a is 0.3, and b is given as follows:(5)b=1+0.01T1+0.04tb0,where T is temperature in °C, t is exposure time in hours, and b0 is 0.15, 0.13, and 0.12 for NC, RAC50, and RAC100 at room temperature, respectively.To check the efficiency of the suggested bond-slip relationship equation, the bond stress and slip were calculated by equations (4) and (5) and compared with the experimental test data, as shown in Figure 10. It was found that the suggested bond-slip relationship equation was in good agreement with those obtained experimentally.Figure 10
Comparison of the tested and predicted on bond-slip relationship. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


## 3.1. Bond Strength

The influence of high temperature (T) on bond strength of steel rebar in RCA is shown in Figure 3. It was found that the bond strength of tested specimens decreased with the increase in temperature regardless of recycled coarse aggregate content (r), which was similar to the previous study [30]. After the temperature of 200°C, the reduction in bond strength of NC, RAC50, and RAC100 was about 16%, 15%, and 16%, respectively. As the temperature continued to increase, the bond strength reduced continuously. After exposure to 400°C, it was reduced by 33%, 39%, and 38% for NC, RAC50, and RAC100, respectively. There was a significant loss in the bond strength for all concrete mixes after 600°C. The bond strength loss was 60%, 65%, and 66% for NC, RAC50, and RAC100, respectively. The reason may be that the resistance of a steel bar against pulling out is provided mainly by adhesion and friction between reinforcing bar and concrete. Adhesion is lost at early stages of loading when slip is initiated. Therefore, resistance to pulling out is mainly provided by friction. As the specimens exposed to high temperature is deteriorated, the friction is weakened, and the resistance to pulling out is lessened.Figure 3
Influence of temperature on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)The influence of RCA content on bond strength of steel rebar in RCA is displayed in Figure4. It was concluded that the bond strength had been less affected due to recycled coarse aggregate content increased. For NC specimens, the bond strength decreased by 2.41 MPa–8.83 MPa from 200°C to 600°C; the bond strength of RAC50 specimens decreased by 2.01 MPa–8.54 MPa from 200°C to 600°C; the bond strength of RAC100 specimens decreased by 1.98 MPa–8.22 MPa from 200°C to 600°C. That is, bond strength loss of all pullout specimens in 200°C–600°C range was comparable regardless of recycled coarse aggregate content.Figure 4
Influence of RCA content on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)The influence of heat exposure time on the bond strength of steel rebar in RAC is shown in Figure5. It was found that the bond strength declined with increasing exposure time (t). The bond strength loss for NC, RAC50, and RAC100 was 6.17 MPa, 6.21 MPa, and 6.56 MPa after exposure to 600°C for 0.5 h, respectively. For an exposure time of 1 h at 600°C, the bond strength loss was about 63% for different recycled coarse aggregate contents. And, the bond strength loss significantly increased to about 88% after two hours of exposure for all pullout specimens of different concrete mixes.Figure 5
Influence of exposure time on the bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


## 3.2. Bond-Slip Relation

The influence of high temperature on bond-slip relation of all pullout specimens is presented in Figure6. It concluded that the shape of bond-slip curves of heated pullout specimens was not the same as those of unheated pullout specimens. But both bond-slip curves had four stages, namely, microslip stage, internal cracking stage, pullout stage, and descending stage. And, the slope of the ascending part of bond-slip curves gradually declined with increasing temperature; meanwhile, the peak bond stress was continually reduced. That is to say, as the temperature increased, the initial stiffness of the specimens decreased. And, as the temperature rose, the descending portion of the curves tended to flatten. The influence of RCA content on bond-slip relation of all pullout specimens is shown in Figure 7. The shape of bond-slip curves of pullout specimens with different recycled coarse aggregate content at the same temperature was similar. It was also similar to linear ascending and descending trend of the bond-slip curves. Figure 8 presents the effect of exposure time on bond-slip relation of all pullout specimens. The bond-slip curves of pullout specimens after exposure to different times had a similar trend. However, the bond stress was lowest for pullout specimens after an exposure time of 2 h followed, in sequence, after an exposure time of 1 h and 0.5 h, respectively. And, the effect of exposure time on the slip corresponding to the peak bond stress was not obvious.Figure 6
Influence of high temperature on the bond-slip relationship: (a)r = 0%, (b) r = 50%, and (c) r = 100%.
(a)
(b)
(c)Figure 7
Influence of recycled coarse aggregate content on the bond-slip relationship: (a) room temperature (RT) and (b)T = 600°C.
(a)
(b)Figure 8
Influence of exposure time on the bond-slip relationship: (a)T = 600°C, r = 0%; (b) T = 600°C, r = 100%.
(a)
(b)


## 3.3. Bond Strength Provisions

At present, many different modes to predict the bond strength between concrete and steel bars have been proposed [37–46]. For example, the CEB-FIP code [47] recommended an equation as follows:(2)τu=2.5fc′,where fc′ is the compressive strength of concrete.Here, considering the effect of elevated temperature and exposure time on bond strength, a simple modified mode for predicting the bond strength of steel rebar in RAC after exposure to elevated temperature was given as(3)τu=2.5fc′−0.0113ΔTt,where fc′ is the compressive strength of concrete and ΔT is the difference between exposure temperature in °C and room temperature in °C. It ranges between 20 and 600°C (20°C≤ΔT≤600°C); t is exposure time in hours.The results calculatedτ0cal using equation (3) compared to the test results τ0exp are summarized in Table 4. And, the comparison of tested and predicted bond stress values is shown in Figure 9. It found that the predicted equation revealed a very good fit in the test results.Table 4
Tested and predicted bond strength.
No.
T (°C)
t (h)
f
c (MPa)
s
0 (mm)
τ
0exp (MPa)
τ
0cal (MPa)
Error (%)
NC
20
1
37.92
0.37
14.71
15.39
4.62
200
1
35.34
0.88
12.32
12.83
4.14
400
1
32.12
1.44
9.81
9.87
0.61
600
1
25.21
1.98
5.85
5.99
2.39
600
0.5
27.80
2.24
8.53
8.55
0.23
600
2
21.21
1.79
1.99
2.24
12.56
RAC50
20
1
30.53
0.43
13.11
13.81
5.34
200
1
27.62
0.92
11.12
11.10
0.18
400
1
24.15
1.51
7.93
7.99
0.76
600
1
20.26
2.05
4.56
4.69
2.85
600
0.5
22.41
2.32
6.89
7.20
4.49
600
2
19.82
1.86
1.85
1.86
0.54
RAC100
20
1
28.73
0.54
12.54
13.40
6.86
200
1
25.72
0.97
10.52
10.64
1.14
400
1
23.11
1.64
7.72
7.72
0.00
600
1
19.16
2.13
4.26
4.39
3.05
600
0.5
18.22
2.48
5.94
6.04
1.68
600
2
17.61
1.91
1.27
1.22
3.94Figure 9
Comparison of the tested and predicted on bond strength. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


## 3.4. Model for Bond-Slip Relationship after High Temperature

Some models had been proposed to predict the bond-slip relation between RAC and steel rebar at room temperature [13–21]. Here, the analytical expression advised by Xiao and Falkner [20] was modified for describing the bond-slip relationship between RCA concrete and steel rebar after high temperatures by redefining some parameters. The normalized bond-slip relationship between RCA concrete and steel rebar after high temperature was approximated by the following equation:(4)y=xa,0≤x<1,xbx−12+x,x≥1.In equation (4), x=s/s0, y=τ/τ0,τ is the bond stress, τ0 is the peak bond stress, s is the slip, and s0 is the slip corresponding to the τ0. a and b are constants to be determined. Based on data regression analysis, the value of the parameter a is 0.3, and b is given as follows:(5)b=1+0.01T1+0.04tb0,where T is temperature in °C, t is exposure time in hours, and b0 is 0.15, 0.13, and 0.12 for NC, RAC50, and RAC100 at room temperature, respectively.To check the efficiency of the suggested bond-slip relationship equation, the bond stress and slip were calculated by equations (4) and (5) and compared with the experimental test data, as shown in Figure 10. It was found that the suggested bond-slip relationship equation was in good agreement with those obtained experimentally.Figure 10
Comparison of the tested and predicted on bond-slip relationship. (a) Bond strength. (b) Bond strength loss.
(a)
(b)


## 4. Conclusions

(1)
With the increase in temperature, the bond strength of pullout specimens reduced. At temperatures of 200°C to 600°C for 1 hour, the bond strength of pullout specimens decreased by 15%–66% relative to the bond strength at room temperature. And, recycled coarse aggregate content had less effect on the bond strength. At temperatures of 200°C for 1 hour, the bond strength of the RAC50 and RAC100 specimens declined by approximately 2%. At temperatures of 400°C for 1 hour, the bond strength of the RAC50 and RAC100 specimens declined by 5.17% and 4.75%. At temperatures of 600°C for 1 hour, the bond strength of the RAC50 and RAC100 specimens declined by 8.54% and 8.22%. Increasing exposure time declined the bond strength.
(2)
The slope of the ascending portion of the bond-slip curves gradually declined with increasing temperature, the descending portion of the curves tended to flatten. The recycled coarse aggregate content or exposure time did not have an obvious effect on the shape of bond-slip curves of pullout specimens after high temperature.
(3)
The modified model was developed to predict the bond strength for steel rebar in RCA concrete after exposure to elevated temperature. It agreed well with the experimental test results.
(4)
Good agreements between the bond-slip relationship model proposed and those obtained experimentally were reached.



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*Source: 5230295-2020-02-10.xml*