Bond performance of thermal insulation concrete under freeze–thaw cycles

Construction and Building Materials 104 (2016) 116–125

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Bond performance of thermal insulation concrete under freeze–thaw cycles Yuanzhen Liu a, Y. Frank Chen a,b,⇑, Wenjing Wang a, Zhu Li a a b

Department of Civil Engineering, Taiyuan University of Technology, Taiyuan, China Departement of Civil Engineering, The Pennsylvania State University, Middletown, USA

h i g h l i g h t s
TIC is utilizable for structures in cold areas due to adequate frost resistance.
An expression is developed to describe the bond strength of damaged TIC.
The porous structure in GHB is beneficial to the frost-proof durability of TIC.
The rebar diameter leads to apparent deterioration and shift of bond property.

a r t i c l e

i n f o

Article history: Received 18 June 2015 Received in revised form 30 October 2015 Accepted 6 December 2015 Available online 14 December 2015 Keywords: Thermal insulation concrete Bond property Freeze–thaw Bond strength Splitting tensile strength

a b s t r a c t Thermal insulation concrete (TIC) mixed with a sufficient volume of glazed hollow beads (GHBs) is an innovative material and has been proven to achieve an excellent balance between the mechanical and thermal insulation performances due to its self-insulation property. However, the bond between the reinforcing bars (rebars) and the TIC must be good enough in long-term sense for wide practical applications using the TIC. This paper focuses on the experimental study on the deterioration of the bond property between the TIC and rebars in freeze–thaw environment. A total of 132 pullout specimens and 216 cubic specimens made of TIC were prepared, which cover three concrete strength grades, three rebar diameters, and six anchorage lengths. Parts of the specimens were exposed to various number of rapid freeze–thaw cycles prior to their failure. The combination of pull-out and mechanical performance test was carried out to assess the frost damage on bond performance. The frost damage on bond property was quantified based on the bond strength, slip, splitting tensile strength, and relative dynamic modulus of elasticity. The usage of GHB against the harmful effects of freeze–thaw cycles was also analyzed by comparing the failure characteristics of normal concrete to that of the TIC. Interestingly, the bond performance of the TIC is found to be affected by the rebar diameter and anchorage length, rather than the concrete strength. The rebar diameter results in the apparent decrease of bond strength and the variation of failure modes. Lastly, an analytical expression was developed to relate the bond strength with the splitting strength of damaged TIC, which considers the effects of rebar diameter and anchorage length. The proposed equation correlates well with the experimental results. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The enormous building energy consumption and the greenhouse gas emission are becoming as inevitable issues to the global climate change. More than 0.69 billon of hydrocarbon energy consumption comes from the public and residential buildings in China ⇑ Corresponding author at: Department of Civil Engineering, The Pennsylvania State University, Middletown, PA 17057, USA. E-mail address: [email protected] (Y.F. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2015.12.040 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

[1]. Due to the thicker insulation layer and the more complex construction technology required in cold areas, reducing the thermal conductivity of structural material is deemed as an effective solution to achieving energy efficiency by altering the thermal physical properties of building envelopes [2]. Thermal insulation concrete (TIC) mixed with a sufficient volume of glazed hollow beads (GHBs) is an innovative material and has been proven to achieve an excellent balance between the mechanical and thermal insulation performances due to its selfinsulation property. The compressive strength of TIC ranges from

Y. Liu et al. / Construction and Building Materials 104 (2016) 116–125

20 to 50 MPa, which is similar to the strength of the normal concrete (NC). The thermal conductivity of TIC ranges between 0.30 and 0.50 W/(m K), which is obviously lower than that of NC having the thermal conductivity up to 1.51 W/(m K) according to the China’s code [3]. The insulation property of TIC is apparently superior to the conventional structural concrete. Several researchers have investigated the behavior of TIC including mix design, thermal insulation properties, microstructure [4], and mechanical properties [5]. Relevant research, such as mechanical properties and seismic behavior of TIC mixed with construction waste, was carried out [6–8], where the stress–strain constitutive relationship and failure behavior of TIC were also discussed. The available research results showed that the behavior of NC and TIC are very similar under uniaxial loading conditions and that TIC is more ductile than NC for the same strength grades. Previous research on TIC also looked at the bond performance between reinforcement and TIC [9], in which a relation between the bond strength and the tensile strength was proposed based on the experimental results. In China, a TIC with C35 strength grade had been used in a 12-floor residential building with frame-shear wall [10], where an 82.6% reduction for coal consumption compared with normal concrete buildings was obtained. In cold environment, reinforced concrete structures are often connected with temperature cycles that reduce the expected durability of the system. To evaluate the durability of the concrete, the frost resistance of TIC had been tested previously [11], which shows that the dynamic elastic modulus of TIC ranges from 60.5% to 85.2% under 300 cycles being considered satisfactory to the structures exposed to cold environment. However, not only the material, but also the bond between the reinforcing bars (rebars) and the TIC must be good enough in long-term sense for wide practical applications using the TIC. It can be concluded from the previous research [12–14] that the residual load-carrying capacity of frost-damaged concrete needs to be quantified in terms of bond properties. Fagerlund showed that the bond capacity and slip at the maximum bond strength changed significantly by frost damage [12]. Experimental results from Petersen showed that the internal damage’s location affected notably the bond decreases and the slip increase [13]. Hanjari showed that the relations for undamaged concrete could not be used directly for frostdamaged concrete [14]. There are several kinds of mechanisms explaining the frost damage in concrete including ‘‘Hydraulic pressure” [15,16] and ‘‘Microscopic ice lens growth” [17]. However, much less attention has been given to the effect of frost damage on the bond performance of concrete. Based on the limited available publications, two major viewpoints can be summarized as follows. One is that the frost damage on bond performance was quantified by the number of freeze–thaw cycles [18]. Shih regarded cyclic temperature as the decisive factor affecting the maximum bond resistance of concrete. Similar results can be obtained from the studies of Ji et al. [19] and He et al. [20]. Another viewpoint from the previous research is that the damage degree depends more on the material properties and internal structure of concrete than on the exterior environment such as the number of freeze–thaw cycles [13,21,22]. Meanwhile, the internal structure of concrete may be strongly influenced by the frost damage caused due to the increase of inner pressure and the associated micro cracks. For the reasons mentioned above, a series of experiments were carried out in this study to investigate the deterioration of bond property between TIC and reinforced bars. Based on the conclusions obtained from our previous research, which agrees with the second viewpoint, the study in this paper focuses on the severity of the damage. That is, the changing in bond property, rather than the correlation between the damage level and the number of freeze–thaw cycles, was used to quantify the frost damage.

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The combination of pull-off and strength test was carried out to assess the frost damage on bond performance. In this study, the change of bond property after freeze–thaw cycles was quantified based on the bond strength, slip, compressive strength, splitting tensile strength, and relative dynamic modulus of elasticity. The usage of GHB against the harmful effects of freeze–thaw cycles was also analyzed by comparing the failure characteristics of normal concrete to that of the TIC. 2. The experimental program In this study, several tests were carried out to evaluate the frost-damaged concrete and the undamaged concrete specimens. The first kind of specimens were exposed to 100 rapid freeze–thaw cycles prior to their failure. Besides, parts of the specimens with C35 strength grade were exposed to 30, 60, and 100 rapid freeze–thaw cycles to investigate the mechanical and bond responses against the various freeze–thaw cycles. The frost damage on the bond property between reinforcement and concrete was experimentally determined by the pull-out test recommended in RILEM [23]. The deterioration on mechanical properties, including compressive strength and splitting tensile strength, was determined by the strength test according to Chinese Standard [24]. The dynamic modulus of elasticity, intended to assess the frost damage on TIC and explain the frost-resistance mechanism, was experimentally determined by the test recommended in Chinese Standard [25]. 2.1. Materials and specimens 2.1.1. Concrete In this study, the mechanical properties of TIC using three different strength grades (C30, C35, C40) were evaluated. The mixture proportions for the three different strength grades were designed and tested six times for each strength grade. The average result from the six specimens was used in each experiment. The concrete mixture proportions for the test specimens are shown in Table 1, in which the GHB particles were used as thermal insulation aggregates to reduce the thermal conductivity [6]. Polycarboxylate high-efficiency water-reducer was used in the mixtures for this study as the chemical additive with water reducing rate of 35– 40%. 2.1.2. Reinforcement In this research, hot-rolled ribbed bar (HRB) was chosen, covering three different nominal diameters of reinforcement. All ribbed bars had similar rib pattern, as shown in Table 2. The geometrical dimensions such as the spacing between ribs (sR), the rib height (hr), and the tensile elongation (d5) are presented in Table 2 along with the mechanical properties of reinforcement. 2.1.3. Specimens Two kinds of specimens were fabricated for distinct research purposes: cubic concrete specimens for frost effect characterization tests; pull-out specimens for bond performance tests. The average result from six specimens was used for each experiment. The specimens for characterizing the frost effects were prepared according to the Chinese Standard [24,25]. In total, 216 TIC and 72 NC specimens were cast in steel forms, as summarized in Table 3. The specimens were cured in a standard curing room at a temperature of 20 ± 2 °C and 95% humidity for a period of 28 days. For the pull-out tests as recommended in RILEM, concrete cube specimens with the unit dimension of 150 mm were used and a bar was inserted along any principal axis of the cubic specimen. For comparison purposes, a total of 132 pull-out specimens were prepared, which cover three concrete strength grades (C30, C35, and C40), three rebar diameters (£12, £18, and £25), and six anchorage lengths. The anchorage length of the reinforcement is equal to 5 times the bar diameter. While the remaining part of the reinforcement was insulated from the concrete by means of a plastic sleeve. Besides, the embedded length, ranging from 1.5 to 12.5 times the bar diameter, was used to analyze its influence on the degradation of bond property, as summarized in Table 4. The steel molds adopted in the pull-out testing were designed according to the requirement of Chinese Standard for Test Method of Concrete Structures [26], where the steel bars were embedded horizontally in the steel molds. As such, the casting direction was perpendicular to the longitudinal direction of bars. The tests were carried out at the age of 28 days, using the equipment shown in Fig. 1. 2.2. The experimental method In this study, several tests were made on the reference specimens and frostdamaged concrete. The compressive and splitting tensile strengths were tested according to the GB/T 50081 Standard [24]. YAW-1000 compression-testing machine with microcomputer controlled electro-hydraulic servo system was used

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Table 1 Concrete mixture proportions. Strength grade

Water/Cement

Gravel (kg/m3)

Sand (kg/m3)

Cement (kg/m3)

Silica fume (kg/m3)

GHB (kg/m3)

Superplasticizer (%)

C30 C35 C40

0.53 0.50 0.48

970 986 1010

490 468 440

329 384 417

25 29 31

156 156 156

3 3 3

Table 2 Characteristics of reinforcement. Type

£ (mm)

fy (MPa)

fu (MPa)

hr (mm)

sR (mm)

d5 (%)

HRB HRB HRB

12 18 25

384.2 415.1 397.8

554.8 579.4 561.8

0.99 1.35 1.70

7.9 11.1 15.2

31 30 28

Table 3 Specimens for frost effect characterization. Frost effect characterization

Concrete type

Specimen dimension

Strength grades

Freeze–thaw cycles

Quantity

Compressive strength

TIC NC

Cubic 150  150  150

C30, C35, C40 C35

0, 30, 60, 100 0, 30, 60, 100

72 24

Splitting Tensile strength

TIC NC

Cubic 150  150  150

C30, C35, C40 C35

0, 30, 60, 100 0, 30, 60, 100

72 24

Dynamic modulus of elasticity

TIC NC

Cuboid 100  100  400

C30, C35, C40 C35

0–100 0–100

72 24

Table 4 Specimens for pull-out test. Concrete type

Strength grade

Rebar diameter

Anchorage length

Freeze–thaw cycles

Quantity

TIC

C30 C35

£12 £12

5d 1.5d 3d 5d 6.5d 8d 12.5d 5d 5d 5d 5d

0, 100 0, 100 0, 100 0, 30, 60, 100 0, 100 0, 100 0, 100 0, 100 0, 100 0, 100 0, 30, 60, 100

12 12 12 24 12 12 12 12 12 12 24

C40 C35

NC

£18 £25 £12 £12

3. Results and discussion 3.1. Dynamic modulus of elasticity

to estimate the mechanical properties. A loading pattern was used to control the specimens during the strength tests at a constant loading rate between 0.3 and 0.8 MPa/s. WAW-2000 universal testing machine with microcomputer controlled electrohydraulic servo system was used to estimate the bond performance. The test was stopped when either pull-out failure or splitting of the surrounding concrete was observed. The loading rate can be calculated by Eq. (1) proposed by Chinese Standard for Test Method of Concrete Structures [26]. 2

V F ¼ 0:03d

specified in GB/T 50082-2009. Twenty-four hour freeze–thaw cycles at temperatures from +20 °C to 20 °C were applied. The DT-16100 dynamic modulus tester was used to estimate the dynamic modulus of elasticity and the mass loss percentage of the specimens under various freeze–thaw cycles.

ð1Þ

where VF is the loading rate and d is the bar diameter. The loading rates were fixed to the constant speeds of 4.32, 9.72, and 18.75 kN/min. The applied forces and displacements were measured using a data acquisition system. The slips at the unloaded end Sf and loaded end Sl of the bar were recorded by the displacement gauge up to a precision of 0.001 mm. As shown in Fig. 1, one gauge was positioned at the unloaded end of the bar and another two at the loaded end. These three displacement gauges were connected to the static resistance strain indicators (Type CM-1L-10), which were in turn accessed by the data acquisition system. After 28 days of curing, the specimens were placed in a rectangular cubic water container and then exposed to freeze–thaw cycles in a temperature controlled chamber of CDR-3 automatic quick concrete freeze–thaw tester, as shown in Fig. 2. The specimens were covered with 5 mm layer of demineralized water on all surfaces and subjected to repeated freezing and thawing. During the test, the temperature in the freezing chamber for all specimens followed the regime

The dynamic modulus of elasticity being a nondestructive measurement is usually used to determine the frost resistance of concrete. The change in the dynamic modulus of elasticity after different freeze–thaw cycles, the so-called ‘‘relative dynamic modulus of elasticity”, can be used to evaluate the freeze-proof durability of concrete. The relative dynamic moduli of elasticity for TIC with three strength grades and NC, versus different freeze–thaw cycles, are shown in Fig. 3. The increase in freeze–thaw cycle number led to the gradual expansion of the specimen and the internal pressure, thus reducing the dynamic modulus of elasticity of TIC. The damage in TIC during the freeze–thaw process started from the destruction of its pore structure. It can be seen from Fig. 3 that after 100 freeze–thaw cycles, the relative dynamic moduli of elasticity of TIC with C30, C35, and C40 grades are retained at 80.34%, and 92.87% of its initial value, respectively. For TIC, it is observed that the higher strength grade leads to a less loss in dynamic modulus of elasticity and hence the better freeze-proof durability. Comparison of the relative dynamic moduli of elasticity between TIC and NC with the same strength grade can be made based on Fig. 3. Slighter differences between them in the initial phase of 10–20 freeze–thaw cycles are noted. The relative dynamic moduli of elasticity for TIC-35 are 97.78% and 93.48% after 10 and 20 cycles, respectively, while the corresponding values for NC-35 are 94.51% and 90.79%. This shows that the destruction on the pore structure of TIC and NC is similar in the initial phase of freeze– thaw. However, the relative dynamic modulus of elasticity of NC

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· Displacement gauge 150mm 150mm

Plastic sleeve Reinforced fixture

Spherical joint

310mm

Loading reinforcement

Static resistance strain indicator

Fig. 1. Details of pull-out specimens and testing equipment.

Fig. 2. Concrete pull-out specimens in freeze–thaw condition.

3.2. Compressive and splitting tensile strength

Fig. 3. Relative dynamic modulus of elasticity versus freeze–thaw cycles.

drops to 71.03% after 100 freeze–thaw cycles, which is 15.6% lower than TIC. It can be stated that TIC exhibits better freeze-proof durability than ordinary concrete with an increasing freeze–thaw cycles. This improvement in freeze–thaw resistance of TIC is probably due to the addition of GHB particles which serve as a kind of solid air-entraining agent and thus improve the freeze-proof durability of concrete (more details described later in this paper).

The compressive and splitting tensile strengths of TIC and NC under the various freeze–thaw cycles are listed in Table 5. The failure patterns of the specimens from the compressive and splitting tensile tests under freezing–thawing cycles are shown in Figs. 4 and 5, respectively. In Table 5, the ‘mean’, ‘range’ and ‘variance’ values represent the average strength, minimum and maximum strengths, and variance of testing strengths for the six tested specimens. As shown in Fig. 4, it is obvious that the freeze–thaw cycles change the failure modes of TIC. As to TIC-0, the first vertical crack occurs around the middle portion of specimens, which appears thin and short. Then, this crack extends quickly with the increased loading, which in turn aggravates the strength damage to the corner regions of specimen. However, unlike the failure behavior of TIC-0, more serious stress concentration occurs along the edges of specimen experiencing freeze–thaw cycles. More visible cracks were therefore initiated and propagated in the frost-damaged TIC. The more freeze–thaw cycles it experiences, the more vertical cracks develop along the edges of specimen. For TIC under 100 freeze–thaw cycles, when the compression loading ascended to the maximum one, the specimens would eventually fail in the form of short columns.

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Table 5 Test results for compressive and splitting tensile strengths. Strength

Freeze–thaw cycles

fcu (MPa)

0 cycle

30 cycles

60 cycles

100 cycles

f t;s (MPa)

0 cycle

30 cycles

60 cycles

100 cycles

Strength grade TIC30

TIC35

TIC40

NC35

Mean Range Variance Mean Range Variance Mean Range Variance Mean Range Variance

34.40 32.78–37.02 2.17 29.76 27.89–32.72 3.11 26.04 24.34–28.33 2.01 24.36 22.55–28.04 3.10

38.85 36.19–41.58 3.53 35.31 32.95–38.04 2.86 31.70 29.13–34.75 3.50 29.29 27.16–33.12 3.69

43.83 41.44–45.88 2.01 40.28 37.80–43.18 3.58 37.82 35.44–40.76 3.67 35.94 33.55–39.88 4.22

39.28 36.79–42.06 3.62 34.25 32.60–36.91 1.99 30.24 27.89–33.00 2.88 25.69 23.84–28.90 2.67

Mean Range Variance Mean Range Variance Mean Range Variance Mean Range Variance

2.12 2.00–2.23 0.01 1.95 1.76–2.16 0.02 1.82 1.69–1.93 0.01 1.75 1.49–1.83 0.02

2.27 2.12–2.40 0.01 2.04 1.85–2.21 0.02 1.91 1.72–2.09 0.03 1.82 1.68–2.01 0.02

2.42 2.26–2.59 0.01 2.26 2.02–2.43 0.02 2.08 1.93–2.27 0.01 1.93 1.78–2.14 0.02

2.32 2.14–2.51 0.02 2.09 1.93–2.25 0.01 1.89 1.76–2.05 0.01 1.52 1.37–1.65 0.01

Fig. 4. Failure pattern of TIC under compressive tests.

Fig. 5. Failure pattern of TIC under splitting tensile tests.

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The splitting tensile cracks of thermal insulation concrete were apparently formed at the weakest part of section. With the number of freezing–thawing cycles increases, it can be seen that the failure surface of concrete becomes irregular and that more mortar and stones are relaxed and dropped from the failure surface. The results show that the tensile capacity between cement mortar and coarse aggregate is affected by freezing–thawing cycles, as shown in Fig. 5. Based on the results, it appears that 18.0% and 29.2% reductions in compressive strength, and 17.4%–20.2% reduction of TIC in splitting tensile strength, due to frost damage. Compared to the results of TIC, a larger reduction appears in the NC specimens-up to 34.5% in splitting tensile strength and 34.6% in compressive strength. The reduction in splitting tensile strength was slightly smaller than that of compressive strength, which is different from what has been reported on normal concrete [22,27,28]. For NC, extensive cracks will likely be formed due to the freeze–thaw cycles [22]. These cracks are perpendicular to the direction of tensile testing, resulting in a more rapid deterioration rate in the splitting tensile strength than the compressive strength. For TIC, the hydration products in the inner structure of concrete become loose due to the freeze–thaw cycles, leading to a reduction of compressive strength. While, with the special porous structure formed by the large volume of GHBs in TIC, there would have less extensive cracks appearing under freeze–thaw situations in the TIC than in the NC. Consequently, the deterioration of splitting tensile strength is alleviated due to the positive effect from the large quantity of GHBs. 3.3. Deterioration of bond performance and affecting factors 3.3.1. Ultimate bond strength and corresponding slip In this study, the pull-out test was carried out on both undamaged and damaged specimens. The average bond strength s is calculated using Eq. (2).

s¼

P

ð2Þ

pdla

where P, d, and la are the applied road, bar diameter, and anchorage length, respectively. The ultimate bond strengths and corresponding slips of the specimens are summarized in Table 6, in which the values in the parentheses represent the minimum and maximum results from the six tests. For the purpose mentioned above, a variable cNs is proposed in this paper to quantify the deterioration percentage of bond strength after N freeze–thaw cycles, as expressed by

cNs ¼

su  sNu su

ð3Þ

where sNu represents the ultimate bond strength after N freeze–thaw cycles and su is the initial ultimate bond strength of undamaged specimens. Similarly, cNs can be used to quantify the increase percentage of slips after N freeze–thaw cycles, as described by

cNs ¼

su  sNu su

ð4Þ

where sNu represents the slip related to the ultimate bond strength after N freeze–thaw cycles and su is the initial value of undamaged specimens. The deterioration percentage of bond strengths and the increase percentage of slips after 100 freeze–thaw cycles, versus different strength grades, reinforcement diameters, and anchorage lengths, are shown in Fig. 6. As indicated in Fig. 6, the bond performance of the TIC is found to be influenced by each of the three primary factors to various extents. It appears that the reduction in the ultimate bond strength of TIC is 12.7–47.6% for frost-damaged concrete resulting in a significant increase in ultimate slip of 30.3–143.7%. 3.3.2. Effects of strength grade The deterioration percentage of bond strengths and the increase percentage of slips remain at about 20% and 30% respectively when the strength grade of TIC increases from C30 to C40. It should be noted that the deterioration of bond property of TIC cannot be alleviated by the higher strength, which disagrees with what has been concluded from the material test [11]. During the material testing, the durability of concrete material is quantified by the deterioration of compressive strength, while the duration of bond property of TIC concerns on the interaction force between rebar and concrete. The bond property, including cohesive, friction, and interlock force, is linearly correlated with the tensile capacity of concrete instead of compressive strength. It is generally understood that the increasing rate of tensile strength is much lower than compressive strength. Therefore, the higher strength grade contributes less on the tensile strength in undamaged concrete. Moreover, the decrease of tensile strength under freeze–thaw conditions depends more on the inner pressure and the associated micro cracks than on the stiffness of hydration products. So, it can be concluded that the strength grade has no apparent effect on the failure mode of reinforced TIC after freeze–thaw cycles. Consequently, the higher strength grade has little influence on the deterioration of ultimate bond strength. 3.3.3. Effects of rebar diameter The bond performance of the TIC is found to be significantly affected by the rebar diameter. The failure modes of undamaged TIC with £12 and £18 were characterized by the shear sliding

Table 6 Ultimate bond strengths and corresponding slips of the specimens from the pull-out tests. Specimens

TIC30-HRB£12-5d TIC35-HPB£12-5d TIC35-HRB£12-1.5d TIC35-HRB£12-3d TIC35-HRB£12-5d TIC35-HRB£12-6.5d TIC35-HRB£12-8d TIC35-HRB£12-12.5d TIC35-HRB£18-5d TIC35-HRB£25-5d TIC40-HRB£12-5d NC35-HRB£12-5d

Undamaged

Damaged

su (MPa)

Su (mm)

Failure mode

s100 (MPa) u

(mm) S100 u

Failure mode

22.35 (20.38–24.16) 15.72 (14.62–16.67) 38.93 (34.89–42.04) 26.85 (24.04–30.01) 23.61 (20.76–26.53) 22.7 (20.45–24.06) 19.45 (17.25–20.81) 15.29 (14.02–17.13) 17.4 (14.80–19.74) 12.29 (10.45–13.89) 26.43 (23.33–29.07) 24.8 (22.34–27.78)

1.61 0.36 0.79 1.18 1.52 1.61 1.70 1.90 0.80 0.42 1.14 1.44

Shearing Shearing Shearing Shearing Shearing Shearing Shearing Yielding of bar Shearing Splitting Shearing Shearing

18.11 (16.35–20.16) 10.39 (9.38–11.43) 29.88 (26.18–32.27) 20.96 (18.84–22.83) 18.98 (17.15–21.07) 18.27 (15.73–19.84) 16.29 (14.68–18.05) 13.35 (11.73–14.83) 9.11 (7.81–10.43) 6.46 (5.55–7.36) 20.73 (18.20–22.80) 15.38 (13.37–16.83)

2.11 2.79 1.25 1.65 2.00 2.11 2.23 2.48 1.40 1.03 1.54 2.14

Shearing Shearing Shearing Shearing Shearing Shearing Shearing Shearing Splitting Splitting Shearing Shearing–splitting

(1.48–1.71) (0.33–0.38) (0.73–0.90) (1.09–1.29) (1.09–1.29) (1.43–1.82) (1.52–1.89) (1.73–2.11) (0.68–0.91) (0.36–0.47) (0.99–1.27) (1.26–1.56)

(1.85–2.36) (2.52–3.12) (1.14–1.39) (1.49–1.82) (1.75–2.17) (1.89–2.36) (2.07–2.42) (1.95–2.75) (1.21–1.60) (0.89–1.17) (1.38–) (1.89–2.36)

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1.600

Strength grade

1.400

1.437

Rebar diameter

Anchorage length

1.200 1.000 0.752

0.800

0.578

0.600

0.476

0.400 0.200 0.000

0.315 0.190

0.312 0.196

C30

C35

0.396

0.341 0.216

0.474

0.312

0.312 0.232

0.196

C40

12

18

25 γτ

100

0.219

1.5d

0.196

3d

5d

0.309 0.195

6.5d

0.309 0.162

8d

0.303 0.127

12.5d

γs

100

Fig. 6. The changing percentages of bond performance and slips vs. the three key affecting factors.

along the gross perimeter of the reinforcement. Different failure modes, however, were observed from the pull-out specimens after 100 freeze–thaw cycles. The damaged specimen with £12 retains the shearing failure mode and has a less deterioration on bond performance. The larger diameters result in the apparent decrease of bond strength by 47.6% and a significant change on the ultimate slip, up to 143.7%. Moreover, the shifting mode of splitting failure was found in the damaged specimens with £18 rebar, as shown in Fig. 7. The bondslip curves for this kind of damaged specimens were affected significantly by the freeze–thaw condition, as shown in Fig. 8, which is very similar to the behavior of normal concrete observed by Ji and Song [29]. The effects of rebar diameter are generally attributed to the cracks and their development. According to the experimental result reported by Tepfers [30], there are internal ring cracks formed between reinforcing bars and concrete. The intersecting

undamaged

frost-damaged

of the internal ring cracks and the vertical cracks caused by loading could result in a significant increase of slip, and thus the decrease of bond strength. Either sufficient concrete cover or transverse reinforcement is beneficial to restrict the extension of cracks along the longitudinal direction of bars. For the harmful effect from freeze–thaw cycles in this experiment, the vertical cracks in the specimens with larger diameter rebars (£18 and £25) could extend rapidly across the concrete cover to the surface. As a result, the shifting mode of splitting failure mode was found in the reinforced TIC with thinner concrete cover. The bond strength corresponding to the splitting failure mode was much lower than the shearing mode. Correspondingly, because the intersecting of cracks separates the concrete into isolated parts, the deterioration rate of slips jumps in the tested specimens from 31.2% to 143.7%. 3.3.4. Effects of strength anchorage length It is also noticed that the deterioration percentage of the bond strengths of specimens with HRB of £12 reduces from 23.3% to 12.7% with the anchorage length increased from 1.5d to 8d. Correspondingly, the increase percentage of slips drops from 57.8% to 30.3%. It can be stated that the longer anchorage length is beneficial to the frost-proof durability on bond performance. Although the bond performance of the TIC was affected by anchorage length, no distinct difference in terms of the increase of slips was found when the anchorage length ranged from 5d to 12.5d. This indicates that the extra anchorage beyond 5d contributes little to the alleviation of slip increase. 3.4. Correlation between bond strength and splitting strength

Fig. 7. Failure mode of TIC with HRB of £18.

Some relevant material characteristics, such as compressive strength, tensile strength, and splitting tensile strength were used to estimate the bond strength between reinforcement and concrete [31,32]. Eqs. (5) and (6) were proposed by Orangun et al. [33] and Xu [34], respectively.

20

0cycle 16

qffiffiffiffi

100cycles /MPa

12 8

su ¼ 0:083045 f 0c ð1:2 þ 3c=d þ 50d=la Þ

ð5Þ

su ¼ ða1 þ a2 d=la Þða3 þ a4 c=dÞf t;s

ð6Þ

0

4 0 0

1

2

3

4

5

6

7

s/mm Fig. 8. Bond-slip response of TIC with HRB of £18.

8

In Eq. (5), f c is the compressive strength of concrete and c is the concrete cover. In Eq. (6), the parameter a1, a2, a3, and a4, may be determined by analyzing and processing the experimental data with the least-square method. Because of the differences in the test methods, this study adopts Eq. (6) with modification to describe the relationship between the bond strength and material characteristics of TIC. The analytical relationship for TIC modified based on the experimental results is described by

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Y. Liu et al. / Construction and Building Materials 104 (2016) 116–125

sNu ¼ ð0:67 þ 1:44d=la Þð2:31 þ 1:43c=dÞf Nt;s

ð7Þ

The pull-out test results and the calculated bond strengths based on Eqs. (5) and (6) were summarized and compared in Table 7, where scu represents the calculated bond strength. It is found that the modified equation (Eq. (7)) correlates well with the experimental results for the specimens with shearing failure mode. Eq. (5) would underestimate the ultimate bond strength of the material after the freeze–thaw cycles. It should also be noted that Eq. (7) is suitable to describe the bar-concrete interaction and agrees with what has been reported by others [35]. For the specimens with larger diameters (TIC35-HRB£18-5d and TIC35HRB£25-5d), splitting failure is likely and the bond strength could be overestimated by Eq. (7). Hence, the splitting tensile strength is considered as a more appropriate parameter to estimate the bond strength of reinforced TIC with shearing failure mode. 3.5. Differences in bond performance deterioration between TIC and NC As an observation, the bond behavior of undamaged TIC is very similar to that of NC. However, due to the distinction on the inner structure between TIC and NC, the deterioration of bond behavior for damaged TIC is relieved somewhat as compared to NC. The bond-slip curves of TIC and NC with HRB£12 under various freeze–thaw cycles are shown in Fig. 9. For TIC, it can be seen from Fig. 9 that the slope of the ascending branch of the bond-slip response decreases with frost damage slightly and that the descending branches of damaged concrete are smoother than the undamaged one. The reductions of ultimate bond strengths are 14.2%, 18.4%, and 19.6% and the corresponding increases of slips are 11.1%, 20.2%, and 31.2% respectively under 30, 60, and 100 freeze–thaw cycles. The residual bond strength

remains above 0:4su when the number of the freeze–thaw cycles ranges from 0 to 100. The shearing failure mode was observed in both the undamaged specimens and frost-damaged ones with HRB£12. As for the damaged normal concrete, a higher degree of deterioration was observed. The slope of the ascending branch of NC decreases quicker than that of TIC, resulting in 38.0% reduction in ultimate bond strengths and 48.6% increase in slips after 100 freeze–thaw cycles. Besides, the shearing–splitting failure mode was observed even in the specimens with smaller diameters of 12 mm. The bond-slip curves of the specimens with such failure pattern descends quickly, as shown in Fig. 9. The residual bond strength remains at about 0:4su under 30 and 60 cycles, and disappears under 100 cycles. The splitting cracks corresponding to the pullout of reinforcement were clearly observed, as shown in Fig. 10. The different failure modes demonstrate that the freeze–thaw resistance property of TIC is better than that of NC. This improvement in freeze–thaw resistance of TIC can be physically explained by the influence of GHB particles in that the GHB acts as a kind of solid air-entrained agent against the harmful effects of freeze– thaw cycles, as shown in Fig. 11. According to the mechanisms proposed previously by others [15,16], the inner structure of concrete subjected to freeze–thaw cycles could be damaged by hydraulic and frost-heaving pressures, generated by moisture migration and its volume expanding. The pressure exceeding the tensile strength of concrete may damage the concrete by introducing the micro and macro cracks. As shown in Fig. 11, GHB having internal honeycomb pores and vitrified closed surface has good water-keeping ability and stable structure. Hence, the special pore structure can intercept the moisture migration as well as block the crack extending of concrete, which is beneficial to alleviate the damage caused by hydraulic pressure.

Table 7 Calculated bond strengths of frost-damaged TIC specimens.

s100 (MPa) u

Specimens

TIC30-HRB£12-5d TIC35-HRB£12-1.5d TIC35-HRB£12-3d TIC35-HRB£12-5d TIC35-HRB£12-6.5d TIC35-HRB£12-8d TIC35-HRB£12-12.5d TIC35-HRB£18-5d TIC35-HRB£25-5d TIC40-HRB£12-5d

d/la

18.11 29.88 20.96 18.98 18.27 16.29 13.35 9.11 6.46 20.73

c/d

0.2 0.667 0.33 0.2 0.154 0.125 0.08 0.2 0.2 0.2

28

5.75 5.75 5.75 5.75 5.75 5.75 5.75 3.67 2.5 5.75

Orangun

scu (MPa)

scu =s100 u

scu (MPa)

scu =s100 u

17.66 31.25 21.95 18.36 17.09 16.29 15.05 13.18 10.26 19.47

0.975 1.046 1.047 0.968 0.936 1.000 1.127 1.447 1.588 0.939

11.66 23.28 15.71 12.79 11.75 11.10 10.09 9.98 8.40 14.16

0.644 0.779 0.749 0.674 0.643 0.681 0.756 1.096 1.301 0.683

28

TIC

0cycle 30cycles 60cycles 100cycles

24 20

NC

0cycle 30cycles 60cycles 100cycles

24 20

16

/MPa

/MPa

Xu

12

16 12

8

8

4

4 0

0

0

1

2

3

4

5

s/mm

6

7

8

9

10

0

1

2

3

4

5

s/mm

Fig. 9. Different bond-slip responses of frost-damaged TIC and NC.

6

7

8

9

10

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Y. Liu et al. / Construction and Building Materials 104 (2016) 116–125

(a) Shearing failure mode of TIC with HRBØ12

(b) Shearing-splitting failure mode of NC with HRBØ12 Fig. 10. Different failure modes between TIC and NC after freeze–thaw.

Fig. 11. The inner structure and function of GHBs.

Additionally, the frost heaving would be harmless when this process is properly confined within a pore diameter less than 200 lm, as also reported by Wu and Lian [36]. As shown in Fig. 11, a large proportion of expanded diameters of GHBs range from 20 to 80 lm, which provide a good extra space for the water expanding. The GHBs, therefore, function as solid air entraining agent to replace the air voids in concrete. This kind of porosity in concrete could reduce the frost-heaving pressure by allowing the water to expand more freely and thus improves the frost-proof durability to freeze–thaw cycles. 4. Conclusions The combination of pull-out and mechanical performance test was carried out on the reinforcing bars embedded in TIC to assess the frost damage on bond performance. The experimental results obtained from this study lead to the following conclusions:

strengths, the results indicate that TIC is satisfactory on frost resistance and may be utilized in engineered structures in cold areas. (2) The deterioration trend on bond strength under freeze–thaw cycles is similar to that on splitting tensile strength. Therefore, the splitting tensile strength, rather than the compressive strength, should be used to estimate the bond strength of reinforced TIC with shearing failure mode. (3) The major factor affecting the deterioration of bond performance of TIC is the rebar diameter, which leads to the apparent drop of bond strength and the shifting of failure modes. (4) The special porous structure of TIC caused by GHBs appears to be beneficial as it could reduce the frost damage and improve the frost-proof durability of concrete. This issue deserves a further study.

Acknowledgements (1) Based on the relative dynamic modulus of elasticity, no significant freeze–thaw deterioration in thermal insulation concrete appears after 100 freeze–thaw cycles. Considering the slight decrease in compressive and splitting tensile

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant No. 51308371), the Natural Science Foundation of Shanxi Province,

Y. Liu et al. / Construction and Building Materials 104 (2016) 116–125

China (Grant No. 2014011033-1) and China Scholarship Council. However, the opinions expressed in this paper are solely of the authors. References [1] Building Energy Conservation Research Center, Tsinghua University, 2014 Annual Report on China Building Energy Efficiency, Architecture and Building Press, Beijing, China, 2014 (Only available in Chinese). [2] M.S. Al-Homoud, Performance characteristics and practical applications of common building thermal insulation materials, Build. Environ. 40 (3) (2005) 353–366. [3] GB 50176, Thermal Design Code for Civil Building, Ministry of Housing and Urban–Rural Development of the People’s Republic of China, Beijing, China, 1993 (Only available in Chinese). [4] Lin Zhao, Wenjing Wang, Zhu Li, Y. Frank Chen, Microstructure and pore fractal dimensions of recycled thermal insulation concrete, Mater. Test. 57 (4) (2015) 349–359. [5] Yuanzhen Liu, Wenjing Wang, Yu Zhang, Zhu Li, Mechanical properties of thermal insulation concrete with a high volume of glazed hollow beads, Mag. Concr. Res. 67 (13) (2015) 693–706. [6] Wenjing Wang, Lin Zhao, Yuanzhen Liu, Zhu Li, Mechanical properties and stress–strain relationship in axial compression for thermal insulation concrete using construction waste, Constr. Build. Mater. 71 (11) (2014) 425–434. [7] Wenjing Wang, Lin Zhao, Yuanzhen Liu, Zhu Li, Mix design for recycled aggregate thermal insulation concrete with mineral admixtures, Mag. Concr. Res. 66 (10) (2014) 492–504. [8] Gang Ma, Yu Zhang, Yuanzhen Liu, Zhu Li, Seismic behavior of recycled aggregate thermal insulation concrete (Ratic) shear walls, Mag. Concr. Res. 67 (3) (2015) 145–162. [9] Xiaohong Zheng, Bonding Performance Experimental Study of Bearing and Thermal Insulation Glazed Hollow Bead Concrete, Master Thesis, Taiyuan University of Technology, Taiyuan, China (Only available in Chinese). [10] S. Luo, Research of Mix Ratio Test and Durability of Thermal Insulation Glazed Hollow Bead Concrete for Jincheng Phoenix Town 19# Project, Master Thesis, Taiyuan University of Technology, Taiyuan, China, 2012 (Only available in Chinese). [11] Liu Yuanzhen, Zheng Xiaohong, Li Zhu, Xu Li, Qin Shangsong, Experimental study on frost resistance of vitrified microsphere bearing and insulation concrete, Construct. Technol. 42 (9) (2013) 83–85+105 (Only available in Chinese). [12] G. Fagerlund, G. Somerville, J. Jeppson, Manual for assessing concrete structures affected by frost, Division of Building Materials, Lund Institute of Technology, Lund, Sweden, 2001. [13] L. Petersen, L. Lohaus, M.A. Polak, Influence of freezing-and-thawing damage on behavior of reinforced concrete elements, ACI Mater. J. 104 (4) (2007) 369– 378. [14] K. Zandi Hanjari, Load-Carrying Capacity of Damaged Concrete Structures, Licentiate Thesis, Chalmers University of Technology, Gothenburg, Sweden, 2008. [15] T.C. Powers, A working hypothesis for further studies of frost resistance of concrete, ACI 41 (1945) 245–272. [16] G. Fagerlund, The significance of critical degrees of saturation at freezing of porous and brittle materials, in: C.F. Scholer (Ed.), Durability of Concrete, American Concrete Institute, Detroit, 1975, pp. 13–65.

125

[17] J.S. Max, Micro-ice-lens formation in porous solid, J. Colloid Interface Sci. 243 (2001) 193–201. [18] T.S. Shih, G.C. Lee, K.C. Chang, Effect of freezing cycles on bond strength of concrete, J. Struct. Eng. ASCE 114 (3) (1998) 717–726. 1988. [19] X. Ji, Y. Song, Y. Liu, Effect of freeze–thaw cycles on bond strength between steel bars and concrete, J. Wuhan Univ. Technol. – Mater. Sci. Ed. 23 (4) (2008) 584–588. [20] Shiqin He, Jinxin Gong, Haichao Wang, Bond mechanism and degradation model between reinforcement and concrete subjected to deicer-frosting cycles, Ind. Construct. 35 (12) (2005) 19–22 (Only available in Chinese). [21] W. Sun, Y.M. Zhang, H.D. Yan, R. Mu, Damage and damage resistance of highstrength concrete under the action of load and freeze–thaw cycles, Cem. Concr. Res. 29 (9) (1999) 1519–1523. [22] K. Zandi Hanjari, Peter Utgenannt, Karin Lundgren, Experimental study of the material and bond properties of frost-damaged concrete, Cem. Concr. Res. 41 (3) (2011) 244–254. [23] RILEM, Technical Recommendations for the Testing and Use of Construction Materials, RILEM, 1992. [24] GB/T 50081, Standard for Test Method of Mechanical Properties on Ordinary Concrete, Ministry of Housing and Urban–Rural Development of the People’s Republic of China, Beijing, China, 2002 (Only available in Chinese). [25] GB/T 50082, Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete, Ministry of Housing and Urban–Rural Development of the People’s Republic of China, Beijing, China, 2009 (Only available in Chinese). [26] GB/T 50152, Standard for Test Method of Concrete Structures, Ministry of Housing and Urban Rural Development of the Peoples Republic of China, Beijing, China, 2012 (Only available in Chinese). [27] G. Fagerlund, A service life model for internal frost damage in concrete, Division of Building Materials, Lund Institute of Technology, Lund, Sweden, 2004. [28] M. Hassanzadeh, G. Fagerlund, Residual strength of the frost-damaged reinforced concrete beams, in: III European Conference on Computational Mechanics Solids, Structures and Coupled Problems in Engineering, Lisbon, Portugal, 2006. [29] Xiaodong Ji, Yupu Song, Experimental research on bond behaviors between steel bars and concrete after freezing and thawing cycles, J. Dalian Univ. Technol. 48 (02) (2008) 240–245 (Only available in Chinese). [30] Ralejs Tepfers, Cracking of concrete cover along anchored deformed reinforcing bars, Mag. Concr. Res. 31 (106) (1979) 3–12. [31] CEB-FIP, Model Code 1990: Design Code, Thomas Telford Service, London, United Kingdom, 1990. [32] Sun-Woo Kim, Hyun-Do Yun, Evaluation of the bond behavior of steel reinforcing bars in recycled fine aggregate concrete, Cem. Concr. Compos. 46 (2014) 8–18. [33] C.O. Orangun, I.O. Jirsa, J.E. Breen, A reevaluation of test data on development length and splices, ACI J. 74 (3) (1977) 114–122 (1977). [34] You-lin Xu, Experimental Study on Bond Anchorage Behaviors between Deformed Reinforcing Bars and Concrete, Doctoral Dissertation, Tsinghua University, 1990 (Only available in Chinese). [35] CEB-FIP Task Group Bond Models, Bond of Reinforcement in Concrete, Bulletin No. 10, International Federation for Structural Concrete, 2000. [36] Zhongwei Wu, Huizhen Lian, High Performance Concretes, China Railway Publication House, Beijing, China, 1999 (Only available in Chinese).