Effect of fly ash and corrosion on bond behavior in reinforced concrete

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T E C H N I C A L P A P E R

Effect of fly ash and corrosion on bond behavior in reinforced concrete

Qingsong Zhou

¹

| Caifeng Lu

^1,3

| Wei Wang

²

| Shenghuai Wei

¹

| Bangfa Xi

¹

1Jiangsu Key Laboratory of

Environmental Impact and Structural Safety in Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou, China

2Department of Architecture, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan

3JiangSu Collaborative Innovation Center for Building Energy Saving and

Construction Technology, Xuzhou, China

Correspondence

Caifeng Lu, Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou, 221116, China.

Email: [email protected]

Wei Wang, Department of Architecture, Graduate School of Engineering, The University of Tokyo, Tokyo, 113-8654, Japan.

Email: [email protected]

Funding information

JiangSu Collaborative Innovation Center for Building Energy Saving and Construction Technology., Grant/Award Number: SJXTBS1715

Abstract

Reinforcing steel bar can exert its strength and ductility by bonding to concrete. However, corrosion of steel bars reduces the bonding effect. This study mainly investigated the coupling effects of fly ash (FA) and corrosion on bond behavior. The impressed current method was utilized to achieve the target corrosion degrees. Three crucial factors that have impact on bond behavior was considered, which were FA content, corrosion degree and stirrups. The bond behavior was discussed using the bond-slip curves. The results showed that the slight corrosion (less than 2.1%) can improve the bond behavior, the supple- ment of 15% FA can result in a higher ultimate bond strength and to a substantial extent offset the reduction in bond strength owing to the severe corrosion of rebar. The effects of stirrups on bond strength mainly focused on fly ash concrete (FAC) and had an almost negligible impact on ordinary concrete. The results and discussions can be of great significance to establish the bond-slip constitutive model under multi-factor coupling conditions and also can give some important advices to the design of engineering structures.

K E Y W O R D S

bond property, confinement, fly ash concrete, impressed current method, pull-out test, reinforcement corrosion

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I N T R O D U C T I O N

Bond performance is a crucial index for evaluating the cooperative work of reinforced concrete (RC) structures.

However, the corrosion of steel bars remarkably reduces

the bond performance of RC structures.¹ The products generated from steel corrosion will form the corrosion- induced expansion force around reinforcement, leading to cracking and spalling of concrete cover,^2,3which pro- vides a more convenient channel for harmful substance (such as chloride ions, CO2molecules, moisture and oxy- gen) to get into concrete and attack the steel bars. This adverse effect can accelerate the corrosion propagation rate,^4,5 and then destroy the bond behavior. Once the bond performance is destroyed, the service life of RC

Discussion on this paper must be submitted within two months of the print publication. The discussion will then be published in print, along with the authors’closure, if any, approximately nine months after the print publication.

DOI: 10.1002/suco.201900264

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structures will be reduced and a serious of chain reac- tions will occur. It was reported that billions of pounds had already been spent on repair and maintenance because of the steel corrosion in concrete.⁶ Meanwhile, due to the extensive practice of concrete for the construc- tions in marine environment, the corrosion of reinforcing bars induced by chloride erosion has become much more serious.⁷In severe cases, it can prompt buildings to col- lapse and pose a threat to people's lives. For example, the Berlin Congress Hall was destructed before the end of their life cycle as the result of the corrosion of reinforcement.⁸ Therefore, a method is urgently needed to improve the decreased bond properties caused by steel corrosion, which can effectively guarantee people's safety and also can provide enormous economic benefits.

Many researchers have cast their spotlight on bond performance of RC structures and this problem has been studied extensively and intensively.^6,8^–¹⁴ However, the majority of these studies mainly focused on the bond property of ordinary Portland cement concrete (OPC)^6,8–13 and only a few researchers systematically examined the comprehensive factors that have impacts on the performance. Al-Sulaimani et al. conducted an experiment on the pull-out specimens and the beam with different degrees of corrosion. They showed that the bond strength increased at the initial stage of reinforcement corrosion, but with the corrosion loss increasing, the bond strength decreased significantly.¹⁰ Zhao et al.

showed that when the steel bar was mildly corroded, the ultimate bond strength of the plain steel bar was 2–3 times greater than that of the bar without corrosion. Even when the corrosion was more than 4%, the strength was still greater by more than 40%. They also point out that the effect of corrosion on plain bars was more significant than that of deformed bars. Meanwhile, the bond-slip constitutive relation considering the corrosion of steel bars was given.¹³In order to compare the effects of corrosion on ordinary aggregate concrete and recycled aggregate concrete (RAC), Zhao et al. conducted a pull-out test. The results showed that the effects did not deviate greatly with the variety of concrete type and the existence of stirrups could reduce the difference in the bond perfor- mances of ordinary aggregate concrete and the RAC.¹⁴ By taking the comprehensive characteristics of the bond failure into account, Berto et al. applied the numerical simulation method to investigate the effects of corrosion factor on the failure procedure. They found that the damage type law was more suitable for failure analysis of bond property in the case of corrosion.¹⁵ However, the model they built ignored the effects of stirrups and mineral admixtures on bond performance. Ma et al. proposed an empirical model to predict the bond strength, they showed that the effects of corrosion could be neglected

when the corrosion degree was less than 2.4%,¹² which was significantly different from the conclusions drew by the previous scholars.

A successful alternative way, the addition of fly ash (FA), has been developed to cover the shortages brought out by the OPC. Investigations have proved that the concrete blended with FA has a better functional performance.¹⁶^–¹⁸ However, as a kind of environmental- friendly and high-performance material compared to the OPC, yet there has still been a shortage of extensive and detailed research on the bond behavior of fly ash concrete (FAC).¹⁹^–²¹Arezoumandi et al. reported that the addition of high-volume fly ash (70%) had no noteworthy effect on bond performance as the bond strength was comparable to that of ordinary concrete, or merely marginally developed.¹⁹ Arezoumandi et al. concluded that the bond strength increased with an increase in the replacement level of FA in concrete,²²this differs from their previous research. And, Lu indicated that when the FA content increased to a roughly certain value (30%), the total porosity and harmful pore content increased significantly,²¹ which can seriously degrade the bond behavior. Li et al.

investigated the bond behavior of early aged FAC after fire, they demonstrated that the residual bond strength of FAC was related to curing age, ambient temperature, cooling method and specimen storage period after cooling.¹¹Due to these contradictory results and relatively less research, it is urgent to do experiments to inspect the bond property of the FAC with corroded bars, and comprehensive factors should be taken into consideration on the deterioration of the bond performance.

This paper studied the differences of bond performance between the OPC and the FAC based on the precondition of corroded reinforcements through experiments. The impressed current method was utilized to get the target corrosion degrees and three FA replacement levels (i.e., 0, 15%, and 30%) were employed using the super-substituted method. The effects of stirrups were also put into contras- tive analysis in various types of concrete in order to give the fully consideration to the actual situation. By the experimental results, the factors covered in this research were discussed particularly. Finally, conclusions were obtained and suggestions for engineering design were also presented on account of this study work.

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E X P E R I M E N T A L D E T A I L S 2.1

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Materials and mixture proportions of concrete

The cementitious materials were class F fly ash and ordinary Portland cement 42.5 (P. O42.5). The chemical

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composition and physical properties of the materials are shown in Tables 1 and 2, respectively. Crushed stone with a maximum particle size of 20 mm was accepted as coarse aggregate. Its specific gravity and water absorption were 2.70 g/cm³ and 0.21%, respectively. The natural river sand with a specific gravity of 2.65 g/cm³, water absorption of 1.22%, and fineness modulus of 2.42 was adopted. All materials conformed to the Chinese specifi- cations for the FAC (SAC²³ GB175; SAC²⁴ GB/T1596;

MOHURD²⁵GB/T50146).

The HRB335 ribbed steel bars (16 mm in diameter) were used for corrosion. Meanwhile, the HRB335 stirrups (8 mm in diameter) were chosen for constraint. Table 3 shows the mechanical properties of the HRB335 steel bar.

Table 4 presents the details of the concrete mixture proportions. The utilization of polycarboxylic super- plasticizer was to ensure the workability of the concrete mixtures. Meanwhile, according to the specification, the super-substitute technique was used to add the FA and the excess coefficient (K) was 1.3 (MOHURD²⁵ GB/T50146).

2.2

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Test specimens

The dimension of the concrete cube specimens was 150 mm×150 mm×150 mm. Figures 1 and 2, respectively, present the different components that make up the specimen and their dimensions and a physical map of the cast-in-situ specimen. Two similar circular holes whose diameters are slightly greater than the deformed HRB335 bar were arranged in the center of two opposite edges of the molds. A 70 mm PVC tube was embedded into the loading end to eliminate the measurement errors caused by the extrusion damage

of the bearing plate. The steel bar implanted in the test piece can be divided into four sections according to their functions, as shown in Figure 1, the lengths of the steel bar external to the concrete at the loading end and the free end were 220 mm and 30 mm, respectively. The bonded areas have a length of 80 mm. This assembly method enabled easy installation of displacement meters which were employed to determinate the bond slip values at the both ends of the steel bar.

Besides, for the test pieces considering confinements, the 8 mm diameter stirrup was installed around the deformed bar. The distance between concrete surface and stirrup outer surface is 23 mm (i.e., the thickness of concrete cover).

The original quality of the steel bars was determined using the following steps. Firstly, the lengths of test sections were measured by a steel rule and the steel bars were cut to test sections by a bar cutter.

Then, the original weight of test sections was measured using an electronic scale. After that, the test sections of steel bars were used to manufacture the pull- out test specimens.

The concrete mixtures were poured into the premade wooden molds and then they were compacted using a vibrator. After that, they were placed in a natural environment and kept in the molds for 24 hr. Then, the molds were stripped and the specimens were put in water and cured at 20 ± 2C for 28 days. Finally, the specimens were moved into a curing chamber for 90 days, in which the relative humidity and temperature were maintained at 80 ± 5% and 20 ± 2C, respectively.²⁶For each concrete mixture proportion, 15 cubic specimens of dimensions 100 mm×100 mm×100 mm were constructed in common plastic molds to measure the compressive strength of the concrete at 7, 14, 28, 60 and 90 days, respectively.

T A B L E 1 Chemical composition of cement and fly ash (%)

Oxide Na2O MgO Al2O3 SiO2 K2O CaO Fe2O3 MnO TiO2 P2O5 SO3

Cement 0.17 2.5 7.0 22.5 0.78 59 3.3 0.03 0.31 0.1 1.8

Fly ash 0.51 0.75 32.8 54.5 1.4 2.7 4.1 0.02 1.3 0.15 0.4

T A B L E 2 Physical properties of cement and fly ash

Properties Cement Fly ash

Specific gravity 3.15 2.4

Fineness (% retained in 45μm) — 7%

Specific surface area (m²/kg) 328 308

Water demanded (%) — 84

Loss on ignition (%) 4.14 6.2

T A B L E 3 Tensile mechanical properties of HRB335 steel bar

Mechanical property HRB335 steel bar

Yield strength (MPa) 369

Tensile strength (MPa) 471

Elongation percentage (%) 27.3

Elastic modulus (GPa) 2.087

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2.3

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Corrosion and test procedure

The impressed current method was used to accelerate corrosion in order to obtain the target corrosion degree.

The corrosion loss can be calculated using Equation (1).

T=ð^2Δw⁼^MÞ Ne

I ð1Þ

where T: conduction time; Δw: corrosion mass of steel bar; M: molar mass of iron; N: Avogadro's constant; e:

electron charge; I: current intensity.

The amount of corrosion of steel bar can be controlled by the current intensity and the period when the steel bar was connected to the power supply. To be specific, during the corrosion procedure, according to the target

amount of corrosion products and the preset current intensity (in mA), the length of time can be determined.

Based on the surface area A (in cm²) of the reinforcing steel bars, the current intensity can be preset according to (0.01–0.02 mA·cm^–²)·A. Meanwhile, the insulation mea- sures were taken so as to prevent any loss in the current intensity.

Figures 3 and 4, respectively, show the schematic diagram and the actual diagram of the corrosion procedure.

The steel bar and the copper electrode were respectively connected to the wires extending from the anode and the cathode of the power supply to realize an effective close T A B L E 4 Mixture proportions of concrete

Concrete material dosage (kg/m³) Concrete

type

The measured compressive strength (MPa)

Water/binder

ratio Cement

Fly

ash Water Sand Gravel Admixture

AF0 34.43 0.5 390 0 195 586 1,189 4.68

AF15 37.92 0.45 368 84 195 563 1,189 4.68

AF30 35.54 0.42 325 181 195 536 1,189 4.68

Note:F0, zero fly ash; F15 and F30, 15% and 30% fly ash; A, the same designed concrete grade.

F I G U R E 1 The specimen components and their dimensions

F I G U R E 2 Undemoulded specimen

F I G U R E 3 Impressed current method

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loop. Before the power supply was switched on, the specimens were fully immersed in the 5% NaCl solution for 5 days. Besides, the exposed parts of the steel bars were coated with epoxy resin in order to prevent the parts being corroded by the chloride ions.

The bond strength can be determined by pull-out test according to the standard (SAC²⁷ GB50152). The loading device can adopt the electro hydraulic servo system, which is shown in Figure 5. Two YHD displacement meters were installed on the test specimen.

The meter at the free end was required to touch the end of the steel bar, while the other meter was placed on the surface of the test specimen. As they were in full contact, the two displacement meters both had initial readings. Therefore, with reference to these initial readings, the relative displacements of the steel bar and concrete were measured. In addition, in view of the corrosion in reinforcement, the loading rate was car- ried out according to the standard (SAC²⁷ GB50152).

The static strain measurement and analysis system TST3826E acquisition instrument was used to record the experimental data. Figure 6 presents the actual laboratory loading device.

Previous research indicated that there are non- negligible effects on the steel corrosion by utilizing the FA additions.²⁸ Namely, during the whole accelerated corrosion procedure, the corrosion loss of OPC was greater than that of FAC. It can be speculated that the theoretical degrees of corrosion calculated by Equation (1) and the experimental corrosion degrees of steel bars were always different. Therefore, after the pull-out test procedure, it is also necessary to determinate the actual degree of corrosion. The corroded reinforcement which was bonded to the concrete was taken out from the broken specimen and the remained concrete attached to the surface of reinforcement was scraped off. Then, it was F I G U R E 4 Actual diagram of

accelerated corrosion of reinforcing steel bars

F I G U R E 5 Schematic diagram of loading device

F I G U R E 6 Actual laboratory loading device

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cleaned with 12% HCl solution. After being neutralized in the saturated solution of Ca(OH)2, the test sections were washed with water and put into an oven to dry for 4 hr. Finally, the test sections were weighted with an electronic scale. The percentage of corrosion can be calculated using Equation (2).

ηA=m_b−m_a

mb ×100% ð2Þ

whereηA: corrosion percentage;mb: original weight; ma: the weight after removing the corrosion products.

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E X P E R I M E N T A L R E S U L T S A N D D I S C U S S I O N

The bond strength includes three aspects, which are chemical adhesion, friction and mechanical interaction.

Due to the steel bars used in this experiment were all ribbed bars, the bond strength is mainly produced by mechanical interaction. In this section, the results of pull-out test are listed and the failure modes of specimens are concluded. Different influence factors on bond behavior of OPC and FAC are discussed.

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Experimental results

Ultimate bond strength is a crucial parameter for reflecting the bond performance of RC structures.

According to the code (SAC²⁷ GB50152), Equation (3) can be applied to calculate this parameter.

τ= F

πdl ð3Þ

whereτ: ultimate bond strength; F: external applied load;

d: the diameter of rebar;l: the anchorage length.

Table 5 shows the details of test results.

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Failure modes

For the pull-out specimens, it was reported that there were three main failure modes^12,29: (a) the failure caused by pulling out of the reinforcement, (b) the splitting failure of concrete, and (c) the combination of the first two failure modes. In this experiment, the failure modes of test specimens mainly focused on the first two failure modes. Figure 7 shows some of the representative failed specimens. For the failed specimens, some had just one T A B L E 5 Results of the pull-out test

Bond slip/mm Specimen

number

Corrosion degree (%)

Applied load (F)/(kN)

Ultimate bond

strength/MPa SL SF

Residual stress (MPa)

AF0-W1 0.00 51.74 9.98 1.61 0.57 2.05

AF0-W2 0.90 54.26 11.17 1.71 0.69 2.49

AF0-W3 1.80 55.82 11.28 1.58 0.26 2.53

AF0-W4 5.50 34.30 7.38 0.73 0.29 3.18

AF0-W5 12.14 21.35 4.30 0.43 0.18 1.30

AF15-W1 0.00 55.77 11.27 2.02 1.33 2.38

AF15-W2 1.30 56.08 11.46 2.11 1.25 2.27

AF15-W3 2.10 57.22 11.69 1.63 0.92 2.41

AF15-W4 5.20 35.88 7.78 0.78 0.35 3.37

AF15-W5 11.17 21.80 5.09 0.52 0.21 1.80

AF30-W1 0.00 53.78 10.64 1.68 0.87 2.18

AF30-W2 1.83 55.68 11.64 0.86 0.38 2.62

AF30-W3 5.40 35.02 7.36 0.78 0.31 2.34

AF30-W4 11.34 21.25 4.89 0.49 0.17 2.24

AF0-Y1 0.00 52.83 10.33 1.78 1.37 2.49

AF15-Y4 5.30 36.71 15.58 0.92 0.41 6.42

AF30-Y3 6.10 30.21 6.41 0.74 0.39 2.83

Note:W, without stirrup; Y, with stirrup; A, C30 strength grade; SL, slip value at the loading end; SF, slip value at the free end.

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crack (Figure 7a–c), and some had more than one crack (Figure 7d–f).

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Effect of fly ash content on bond performance

The experimental results indicate that the effects of FA on bond performance mainly present as following three aspects: (a) the ultimate bond strength (τ^u); (b) the slip value corresponding to the ultimate bond strength; and (c) the residual stress.

Figure 8a shows the bond-slip curves for Specimens AF0-W1, AF15-W1 and AF30-W1, which had different FA content. At the initial stage of loading, the relative position of reinforcing bar and concrete only changed a little with the increase of the applied load. This variation continued to develop until the external load rise to the value that was large enough to overcome the ultimate bond strength. The bond stress and slip distance roughly exhibited a linear relationship before the strength peak.

When bond stress struck the peak, and then continued to apply the load, the bond strength fell sharply, and subse- quently it tended to flatten out. The curve changing laws appeared no significant deviation within the tested fly ash contents.

Figure 8b illustrates the ultimate bond strengths of specimens with various FA content. The ultimate bond strengths of Specimens AF0-W1, AF15-W1 and AF30-W1 peaked at 9.98 MPa, 11.27 MPa and 10.64 MPa, respectively. Therefore, the ultimate bond strength of the test specimen with 15% FA increased by 12.9% as compared with that of the OPC specimen. But when FA replacement level increased to 30%, compared to 15% FA replacement, the ultimate bond strength declined 5.59%.

Among these three levels of substitution, 15% FA replacement displayed a better bond performance.

To make a better comparison, the rust phases included in this experiment can be divided into four effective corrosion stages according to the various corrosion degrees, the more backward the corrosion process was, the greater the amount of corrosion would be. Figure 8c shows the relationship between the slip value when the bond stress reaches its maximum (defined as the initial slip value) and the different corrosion degrees of rebar in concrete. The slip value at the moment indicated a downward trend with an increase of rust amount, which showed no obvious difference under the three substitution quantities. It is worth pointing that the major differences mainly concentrate on the value that was affected by the FA content. Specifically, FA replacement at the 15% level displayed a relatively large F I G U R E 7 (a–f) Failure modes of typical pull-out specimens

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slip value, specimens with 30% and 0% FA have roughly equivalent slip values.

The reasons why fly ash changed the bond behavior might be mainly due to the micro-structures of the OPC

influenced by the kind of small particle size material. In fact, the concrete would become a more homogeneous material by blending cement paste with some FA particles. Therefore, the FAC has a larger contact area with internal reinforcement as compared to that of the OPC, which increase the chemical adhesion and mechanical interaction of these two materials. The FAC shows abso- lute superiority at this time in view of that the chemical adhesion plays a major role in the early stage of loading.

Further, as the particle size distribution of the FAC is optimized, the matrix of cementitious materials produced by FAC results in a better mechanical property,²²its abil- ity to avoid splitting is greater, which brings an advan- tage that it should take more force to produce a relative replacement. Besides, the cement paste with FA particles can form a denser concrete/steel interface, this may increase the compressive force of concrete on the surface of rebar,²¹thus giving rise to the increase of friction. The factors are probably able to explain why the FAC has a different bonding behavior. From the specific level of FA content in the experimental study, the concrete with 15%

FA content has a more optimized micro-structure which is embodied in the reduced porosity and the narrowed pore diameter. Hence, a better bond strength appeared in this test block. Accordingly, more external loads were required, the slip value corresponding to the ultimate bond strength was greater. When FA replacement level increased to 30%, from the pore structure point of view, although the pore size becomes smaller, the total pore content and harmful pore content also increase,²¹which makes the region of concrete/steel interface become loose. This effect adversely decreased the bond strength.

Therefore, in the actual structure, the FA content is pref- erably controlled at about 15%.

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Effect of corrosion on bond performance

Figures 9a, 10a and 11a, respectively, show the bond-slip curves of Specimens AF0, AF15 and AF30 for the reinforcing steel bars with different degrees of corrosion. For the specimens with small percentage of corrosion, the bond-slip curves are divided into five stages: initial slip stage, slip stage, split stage, falling stage and residual stage. In general, the initial slip stage witnessed a significantly increasing process of bond stress, the second stage pointed out that the bond stress gradually increased with the increasing slip distance, and then the bond strength can be well reflected by the peak value in the split stage, the bond stress would come across a continuous decreas- ing process (falling stage) and the residual stage would become visible at the end of the test procedure. The F I G U R E 8 (a) Bond-slip curves for specimens with different

FA replacement. (b) Ultimate bond strength for specimens with different FA replacement. (c) The relationship between the initial slip value and corrosion degree

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evolution process seemed to be well in accordance with that of the pure concrete.³⁰For the specimens with large percentage of corrosion, however, the split stage was

practically non-existent, and the falling stage was relatively slow. Taking Specimen AF0-W4 as an example, when the bond strength was 7.38 MPa, the average slip F I G U R E 9 (a) Bond-slip curves for F0. (b) Ultimate bond strength for F0

F I G U R E 1 0 (a) Bond-slip curves for F15. (b) Ultimate bond strength for F15

F I G U R E 1 1 (a) Bond-slip curves for F30. (b) Ultimate bond strength for F30

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value was 0.51 mm, but when the slip value was 1.19 mm, the bond stress only decreased by 1.12 MPa.

Figures 9b, 10b and 11b show the ultimate bond strength(τ^u) of concrete with different FA replacement in the case of different corrosion degrees. For Specimens AF0, the τ^u of the reinforcement with 1.80% corrosion loss was greater than that with zero corrosion. Theτu of Specimen AF0-W1 (η = 0%) and Specimen AF0-W3 (η = 1.8%) were 9.98 MPa and 11.28 MPa, respectively.

Therefore, the τ^u of Specimen AF0-W3 (η = 1.8%) was 1.13 times of that of Specimen AF0-W1 (η = 0%). How- ever, the higher degree of corrosion, the lowerτ^uof concrete and steel bar. For example, the τu of Specimen AF0-W5 with a corrosion percentage of 12.14% was only 0.431 times of that of Specimen AF0-W1 (η = 0%), as shown in Figure 9b. In other words, a relatively high corrosion loss decreased theτu a lot. For the concrete with

15% FA replacement, theτufor 0%, 1.3%, 2.1%, 5.2% and 11.17% corrosion loss were 11.27 MPa, 11.46 MPa, 11.69 MPa, 7.78 MPa, and 5.09 MPa, respectively. That is to say, in the case ofη=2.1%, the bond performance was improved a lot. For the concrete with 30% FA replacement, under the condition of 1.83% corrosion degree, the τuwas also the highest of the four values. Both AF15 and AF30 test specimens had the similar phenomenon that the mild corrosion loss had a relatively better bond performance.

Figure 12 shows a comparative study of theτuof the concrete with different FA replacement. In general, the bond behavior of concrete with 30% and 0% FA were worse than that of concrete with 15% FA replacement at the corresponding degree of corrosion. For example, the τ^u of AF15-W3 was 1.036 and 1.004 times than that of AF0-W3 and AF30-W2, respectively. However, these F I G U R E 1 2 Contrast diagram of ultimate bond strength for different fly ash contents

F I G U R E 1 3 Concrete/steel interface of slight corrosion of reinforcement bars

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three sections had a similar amount of steel corrosion.

This also explains from another angle that a better bond strength can be obtained by adding an appropriate amount of FA in the case of a small amount of steel corrosion. Another phenomenon that is worth paying close

attention to, however, is that the ultimate bond strength of AF0-W4, AF15-W4 and AF30-W3, respectively, were 7.38, 7.78 and 7.36 MPa, the effect of FA on bond behavior in the case of rebar corrosion was not as good as that without corrosion, which was also the pattern displayed F I G U R E 1 4 (a) Bond-slip curves for specimens with/without the stirrup. (b) Ultimate bond strength for specimens with/without the stirrup. (c) The variation of initial slip value for specimens with/without the stirrup. (d) Schematic cracking diagram of the specimens with/

without stirrups

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by the test pieces of AF0-W5, AF15-W5 and AF30-W4.

This might be ascribed to the reason that the test specimens of AF0-W4, AF15-W4 and AF30-W3 actually all contained the corrosion rebar, the bond behavior under that circumstance may be attributed to the coupled effect of FA and the corrosion of rebar and, more importantly, the latter maybe contribute more of the loss or the improvement of bond behavior than the FA.

As illustrated in Figure 13, when the rebar is slightly rusted, the original porous and loose concrete and steel interface area filled with many corrosion products, which further increased the compressive stress of the concrete on the rebar. Meanwhile, the surface of rebar becomes uneven due to the corrosion, this can result in the modifi- cation of the roughness. These two elements may contribute to the development of the friction. Consequently, low corrosion loss had a higher bond strength. This filling effect coupled with the advantages of adding an appropriate amount of FA discussed in the Section 3.3 together give rise to the better bonding performance of F15.

As the amount of rust production increases, more rust product particles fill into the interface area. When the strain induced by the expansion force of rust exceeds the ultimate tensile strain of concrete, cracks in concrete tension will first appear at the intersection of concrete and steel threads. This is equivalent to the pregeneration of tensile stress inside the concrete before the external load is applied, which seriously influences the bond behavior. Meanwhile, corrosion can lead to flattening of the cross ribs and reduce of the cross-sectional area of rebar.³⁰Therefore, the friction and mechanical interaction all declined in varying degrees, this also result in the stress concentration of steel bar during the pull-out test. In view of the above reasons, the bond strength fell off dramati- cally. The differences between the FAC and the OPC in the case of severe corrosion may be the FAC have a more optimized cement matrix, as mentioned in the Section 3.3. This makes it more invulnerable to generate splitting damage, thus showing better bond properties.

3.5

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Effect of stirrups on bond performance

The effect of stirrups on bonding properties is demonstrated concretely in Figure 14a–c. The slip occurred earlier in the specimens without stirrups as compared to those with stirrups. Taking AF0-W1 and AF0-Y1 as an example, when the bond stress was 0.98Mpa, the slip distance of the AF0-W1 was 0.11 mm. However, no slip value appeared in the specimen with stirrup. The ultimate bond strength (τ^u) of the AF0-W1 was 9.98 MPa, the specimen with stirrup exceeded 9.98 MPa, theτ^uof the AF0-Y1 was slightly greater than the AF0-W1. However, this variation was not obvious and even

could be ignored and the same situation also appeared in the comparative test pieces of AF30. Only specimens with 15%

FA content showed significant differences, the τu of the AF15-Y4 block was 15.58 MPa, which was nearly twice that of the AF15-W4. This might be attributed to the fact that the stirrups constrain the concrete around the rebar, thus giving rise to an increase in the radial force along the circumference of the reinforcement, the positive effect results in an enhance- ment of the friction. For another, the concrete cover cracking and spalling sometimes starts from the micro cracks in concrete. In the initial stage of loading, micro cracks appear in the concrete in contact with the transverse ribs of reinforcement. Then, the cracks extended to the surface of concrete under continuous loading. However, the extended cracks were restricted by the stirrups, which resulted in increasing in the length of the time of energy release. In other words, it should take more load to achieve the ultimate bond strength of the test specimens with stirrups. Therefore, a slight increase in bond strength was shown in the specimens without FA and corrosion. However, the severe corrosion of rebar par- tially offsets the increase in the bond strength due to the stirrups, hence, the AF30 specimens including in this part exhibit the same characteristic as the specimens of AF0.

Meanwhile, the slight rust and the beneficial effects of the stirrups coupled with the addition of appropriate FA result in a substantial increase in bond strength at 15%.

The residual stress in Specimens AF0-W1 and AF0-Y1 was 2.05 MPa, 2.49 MPa, respectively. The AF0-W1 specimen showed a 17.7% residual stress reduction as compared with the specimen AF0-Y1. This might be precisely due to the presence of the stirrups in concrete. To be specific, when the concrete cover cracked, the stirrups still played a part by restricting the un-cracked part of concrete. Besides, the tensile force induced by the stirrup prevented the cracks from con- tinuing to develop, as shown in Figure 14d. Consequently, in the specimens with or without fly ash, the stirrup specimens showed relatively large residual stress.

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C O N C L U S I O N S

This paper mainly investigated the effect of fly ash and steel corrosion on bond behavior between deformed bar and concrete, the conclusions are:

1 The differences of bond behavior between the specimens after adding fly ash and the original concrete specimens are mainly reflected in two aspects, namely the ultimate bond strength and the initial slip value.

The bond strength of concrete with 15% fly ash is the highest, but it decreases when the content of fly ash increases to 30%, and the bond strength without fly ash is the lowest. The concrete with 15% fly ash also has a

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relative higher initial slip value, however, the concrete with 0% and 30% fly ash show the similar regularity.

2 The slight corrosion can increase the bond strength instead, which combines with the advantages of an appropriate fly ash, giving rise to the greatly enhanced bond strength. Severe corrosion will markedly weaken the bond strength, but owing to the presence of fly ash, the bond strength still exhibits certain advantages compared with that of ordinary concrete with the same degree of steel deterioration.

3 Stirrups can enhance the bond strength due to the existence of the radial force along the circumference of reinforcement. But there are limits to this improvement, especially in the aspect of severe corrosion, it is because the severe corrosion can have the opposite effect so as to offset the increase by stirrups. After cracking, the residual stress in the specimens with stirrups is also slightly greater than that in the specimens without stirrups.

This study mainly investigated the effect of rebar corrosion and fly ash on the bond behavior in reinforced concrete, without taking the slight corrosion of stirrups generated by the pure chloride contamination process into consideration. However, it should be noted that when the corrosion degree of stirrups increases to a certain level by which some negative effects can be induced.^3,31 Hence further investigations after considering the effects of stirrups corrosion as well as some other factors, such as the diameter of rebar, different concrete mix proportions and loading conditions and so on, are urgently needed to make this issue clearer.

A C K N O W L E D G M E N T S

The funding for this work was provided through a grant (SJXTBS1715) from the JiangSu Collaborative Innovation Center for Building Energy Saving and Construction Technology.

N O T A T I O N S T conduction time

Δw corrosion mass of steel bar M molar mass of iron N Avogadro's constant e electron charge I current intensity η^A corrosion percentage mb original weight

ma the weight after removing the corrosion products

τ ultimate bond strength F external applied load d the diameter of rebar l the anchorage length

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A U T H O R B I O G R A P H I E S

Qingsong Zhou, Jiangsu Key Labo- ratory of Environmental Impact and Structural Safety in Engineer- ing, School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou 221116, China.

Caifeng Lu, Jiangsu Key Labora- tory of Environmental Impact and Structural Safety in Engineering, School of Mechanics and Civil Engi- neering, China University of Min- ing & Technology, Xuzhou 221116, China.

Wei Wang, Department of Archi- tecture, Graduate School of Engi- neering, The University of Tokyo, Tokyo 113-8654, Japan.

Shenghuai Wei, Jiangsu Key Lab- oratory of Environmental Impact and Structural Safety in Engineer- ing, School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou 221116, China.

Bangfa Xi, Jiangsu Key Laboratory of Environmental Impact and Struc- tural Safety in Engineering, School of Mechanics and Civil Engineer- ing, China University of Mining &

Technology, Xuzhou 221116, China.

How to cite this article:Zhou Q, Lu C, Wang W, Wei S, Xi B. Effect of fly ash and corrosion on bond behavior in reinforced concrete.Structural

Concrete. 2020;1–14.https://doi.org/10.1002/suco.

201900264