The Efect of Fly Ash on the Bond Strength of Steel Reinforcement and Concrete

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https://doi.org/10.1007/s40996-021-00617-8 RESEARCH PAPER

The Effect of Fly Ash on the Bond Strength of Steel Reinforcement and Concrete

Seyed Abbas Hosseini¹ · Mansour Bagheri²

Received: 21 November 2020 / Accepted: 2 March 2021

Abstract

Concrete is one of the essential construction materials that requires many natural resources for production and releases much pollution such as greenhouse gases. As a cement replacement material, fly ash is an essential tool to reduce the environmental impacts of cement production. The main objective of this study was to evaluate the bond strength between concrete and reinforcement for different fly ash levels. In the experiments, 5%, 15%, and 25% of Portland cement were replaced by fly ash, while the water-to-binder ratio (w/b) was kept constant at 0.42. Standard test specimens of 15 cm containing a 16-mm- diameter bar at the center were used in bond strength tests. Bond strength was measured at 7, 28, and 90 days of age through a direct pull-out test. The results show that adding fly ash with any percentage reduces the bond strength at an early age while replacing 15% cement with fly ash has more positive effects as it enhances bond strength up to 20% in the long term.

Evaluation of existing models for predicting bond strength also showed that these models’ average provided greater bond strength than experimental results in 28 days. Therefore, it is necessary to provide an appropriate model for estimating the bond strength of concrete containing fly ash.

Keywords Concrete · Bond strength · Admixtures · Compressive strength · Fly ash · Environment

1 Introduction

Cement production generates more than 5% of the world’s carbon dioxide alone, which causes irrecoverable damage to the environment (Huang et al. 2020). Due to the increasing demand for concrete, especially in developing countries, the CO₂ gas emission is also expected to increase in the next decade. The use of alternative materials instead of the primary concrete materials, mainly supplementary cementitious materials, is central to reducing fuel and resource consumption and lower CO₂ emission in recent years. In addition to replacing some cement, Pozzolans improve concrete properties including durability and mechanical properties and reduce environmental impacts (Atiş and Karahan

2009; Wang and Park 2015). Fly ash (FA) is a by-product of coal-fired power plants that improves concrete workabil- ity, lessens hydration, and reduces thermal cracking at an early age. The most crucial FA property is to enhance the long-term strength of concrete due to less concrete porosity and calcium silicate hydrate (CSH) (Chousidis et al. 2015;

Karalar 2020).

Many studies have focused on the effect of FA on mechanical properties, especially the compressive strength of concrete (Hadi 2008; Kurtoğlu et al. 2018; Sumer 2012).

Kumar and Prasad (2019) reported a substantial improve- ment in the strength of blended concrete mix with 15% FA (Kumar and Prasad 2019). Sumer (2012) reported a slight increase in compressive strength at 28 and 90 days by replacing 10 and 17% FA with cement (Sumer 2012). Cross et al.

(2005) found a significant decrease in the bond strength and compressive strength of concrete by completely replacing the cement with FA (Cross et al. 2005). Sadrmomtazi et al.

(2017) by replacing up to 25% of cement with FA concluded that by increasing amount of FA in mixtures. Compressive strength reduces at the ages of 7 and 28 days and increases at the age of 90 days (Sadrmomtazi et al. 2017).

* Seyed Abbas Hosseini [email protected] Mansour Bagheri

[email protected]

1 Faculty of Technology and Mining, Yasouj University, Choram, Iran

2 Department of Civil Engineering, Birjand University of Technology, Birjand, Iran

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The bond strength between the concrete and steel reinforcement ensures the proper behavior of the reinforced concrete structures in a composite element. The concrete compressive strength, reinforcement shape and diameter, cover thickness, and confinement because of transverse bars are the main influences on the bond strength (Al-Shannag and Charif 2017; Castel et al. 2006; Sancak et al. 2011). All factors affecting the mechanical properties of the concrete can also affect the bond strength between concrete and reinforcement (Diab et al. 2014; Hosseini and Etedali 2019).

Nili and Salehi (2011) concluded by replacing 15 and 25%

FA in high strength concrete that the replacement of FA in high-strength concrete reduces initial strength and, in the long term, compressive strength increases compared to the plain concrete (Nili and Salehi 2011). Gopalakrishnan et al.

(2005) reported the same bond strength as the control specimen for 15 cm cubic specimens made of concrete containing 50% of FA (Gopalakrishnan et al. 2005). Arezomandi et al. (2013) reported up to a 20% enhancement in the bond strength based on the direct pull-out test (Arezoumandi et al. 2013). Naike et al. (1989) reported an optimum value between 10 and 20% of FA when FA’s replacement improves the bond strength (Naik et al. 1989). Normally, FA’s replacement level for conventional concrete applications is around 15–20% (Bendapudi and Saha 2011).

Some construction codes and standards limit the maximum amount of cement substituted with FA to 35 percent.

The use of FA upper than these ranges has been the focus of many researchers (Cross et al. 2005). Despite broad research into the effect of replacing cement with FA, the bond strength of this kind of concrete has not been thor- oughly investigated, and there is just a limited number of studies on the bond strength of concrete containing FA and reinforcing steel bars. Due to considerable uncertainty in various stages of concrete construction and operation, there is a need for further study on the bond strength (Bagheri et al. 2020; Hosseini et al. 2020). In this paper, the effect of replacing 5, 15 and 25 percent of mix design cement with FA on the bond strength is evaluated. In addition to the bond strength, which is accomplished by the direct rebar pull-out test, the compressive strength is also measured to establish a relationship between compressive strength and the bond strength. The results are compared with the existing models.

2 Experimental Program

2.1 Materials

Materials used in this study include coarse and fine aggregates, cement, water, FA and superplasticizer. The coarse aggregate was crushed dolomitic limestone with a maximum size of 20 mm. The specific gravity of coarse aggregates, and

their water absorption were measured to be 2.72 and 1.24%, respectively. The fine aggregates were provided from the local river with a maximum of 5 mm and fineness modulus of 2.43. The particle size distribution of aggregates is shown in Fig. 1. These aggregates are modified according to ASTM C330/C33M (2018)(ASTM 2018a). ASTM type II Portland cement and FA class F with the chemical properties are used as shown in Table 1. The particle size distribution of binders is shown in Fig. 2 too.

2.2 Concrete Mixes

In order to perform concrete test samples, the water-to- binder ratio is taken as 0.42. To prepare one cubic meter of the control specimen, 1337 kg and 557 kg coarse and fine aggregates were used, respectively. The amount of cement and water for this control specimen is 350 kg and 147 kg, respectively. To investigate the effect of cement replacement with FA, 5%, 15%, and 25% of cement were replaced with FA. Table 2 shows the naming method and mixture properties of the test specimens. In this study, both the compressive

0 20 40 60 80 100 120

0.10 1.00 10.00 100.00

Passing (%)

Seive Size (mm)

Fine Agg, Lower limit Fine Agg ,Upper limit Coarse Agg, Lower limit Coarse Agg, Upper limit

Fine Aggregates Coarse Aggregates

Fig. 1 Particle size distribution of aggregates

Table 1 Chemical and physical properties of cement and FA

Composition Cement FA

CaO (%) 63.2 3.5

SiO₂(%) 22.6 54.5

Al₂O₃(%) 4.1 30.4

Fe₂O₃(%) 3.2 6.5

SO₃(%) 1.5 0.3

MgO(%) 2.6 1.8

Na₂O(%) 0.2 0.2

K₂O(%) 0.5 0.6

Density (kg/m³) 2900 2150

Blaine (m²/kg) 310 –

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strength and direct pull-out tests were performed at 7, 28, and 90 days after demolding the concrete samples (150-mm cubic specimens). A 16-mm-diameter ribbed steel bar is centered in the concrete specimen to perform the pull-out test. To prevent compressive stresses at the beginning of the rebar on the bond strength results, a PVC pipe was utilized at the top of the specimen to prevent rebar and concrete connection. The details of the concrete mold and bars are shown in Fig. 3.

The dimensions of the text box were selected so that the cubic sample placement is easy to perform. The test box was designed and constructed based on the maximum bearing capacity of the concrete. Based on the compressive strength of concrete considered in this experiment, the thickness of the box sheets was determined so that under the influence of the maximum possible force, the displacements made in the connecting rod and the box can be ignored. Considering the finite element analysis performed on the test box, the dis- placement was about 0.01 mm under the maximum tensile force effect. As shown in Fig. 4, the specimens were kept in the mold for 24 h after molding and protected from moisture loss. They were then cured in the water basin according to the ASTM C192/C192M–18 (2018a) until the desired age (ASTM 2018b). A total of 32 samples were made and tested.

The reported results are the average of three specimens.

For the pull-out test, a steel box with the dimensional properties shown in Fig. 5 is used.

3 Results and Discussion

3.1 Compressive Strength

The compressive strength test was performed according to BS EN 12390-2 (2009) standard, and the results are shown in Table 3 and Fig. 6 (BSI 2009). As it is evident from the results, at an early age (7 days), there was no significant difference in compressive strength in specimens with 5% and 15% FA, but for the specimen with 25% FA, a decrease in compressive strength is noticeable. As shown in Fig. 6, the ratio of the compressive strength to the control specimen for this case is 0.79. By increasing the samples’ age to 28 days, it is observed that there is no significance difference between the compressive strength of the control specimen and the

0 20 40 60 80 100 120

0.0001 0.001 0.01 0.1 1 10

Cumulative (%)

Particle size distribution(mm) Cement

FA

Fig. 2 Particle size distribution of binders

Table 2 Mixture properties of samples

Specimens w/c Cement (kg/m³) FA (kg/m³) FA (%)

Control 0.42 350 0 0

FA05 0.44 332.5 17.5 5

FA15 0.49 297.5 52.5 15

FA25 0.56 262.5 87.5 25

Fig. 3 Specimens made for rebar pull-out test

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samples with 5% and 15% FA. The difference between the control specimen and 25% FA was less than the specimen with 7 days’ age. The ratio of resistance at each age to the control specimen increased from 0.79 to 0.95. After 90 days, the resistance of all samples exceeded the compressive strength of the control sample, as shown in Fig. 6. In addition, the highest increase was related to the 15% replacement of cement with FA, which is 16%. Remarkably, 25% FA caused an increase in the specimen’s compressive strength.

Until the age of 28 days, it has less compressive strength than the control specimen, but at 90 days, the compressive strength ratio to the control was elevated to 6%.

3.2 Bond Strength

Several methods evaluate the bond strength between concrete and rebar (ACI 2003; Ramirez and Russell 2008).

The beam-end specimen based on ASTM A944-10 (2015) and direct pull-out test based on RILEM recommendations are the most common experimental methods to examine the bond strength (BSI 2015). In this research, the bond strength test is performed by pulling out the reinforcement, as shown in Fig. 5(b). In this experiment, since the length of the rebar involved in concrete is 10 cm, the ultimate bond strength value ( 𝜏_u ) is calculated using Eq. (1) based on the maximum tensile force applied to the reinforcement during the test:

where 𝜏_u is the ultimate bond strength, F is the maximum tensile force applied to the reinforcement before the mechanism occurs, db is rebar diameter, and Ld is the reinforcement length that is equal to 10 cm. The ultimate bond strength of the specimens is presented in Table 4.

(1) 𝜏_u =F∕(𝜋L_dd_b)

Fig. 4 Curing of concrete specimens

Fig. 5 Box made for pull-out test and test setup

Table 3 Compressive strength of samples at different ages Specimen Compressive strength (MPa)

7 days 28 days 90 days

Control 29.1 39.2 44.0

FA05 29.4 39.8 46.1

FA15 28.6 40.0 49.7

FA25 22.9 37.4 45.4

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The results show that after 7 days, for all specimens, the bond strength is lower than the control sample, and the bond strength ratio of the specimens containing FA to the control mix decreased with higher percentage of FA. By comparing the ratio obtained for the compressive strength in Fig. 6 and the bond ratio presented in Fig. 7, it is clear that adding FA on the bond strength was more than its effect on the compressive strength. The ratio of the compressive strength and

bond strength of FA15 to the control sample was obtained at 0.98 and 0.86, respectively. After 28 days, for FA05, no significant reduction was observed. For the FA15 specimen, the bond strength has increased from 11.25 to 12.7 MPa. Based on the preceding description, the replacement of cement by FA at this age, except for sample FA15, has reduced the bond strength. The FA25 specimen at this age has lower bond strength than the control sample. The age of the sample had a greater influence on the mechanism of failure of samples in the experiment. Figure 8 depicts the rebar pulling out at 7 days and the concrete crash at 28 and 90 days. The amount of FA did not affect the kind of specimen mechanism during the pull-out test.

By increasing the samples’ age to 90 days, all specimens had higher bond strength than the control sample and the highest increase was observed for the FA15 sample, which showed a 21% increase. At this age, the bond strength increased for samples with up to 15% FA, but for the 25%

replacement, this trend did not continue despite more bond strength than the control sample.

In the direct pull-out test, samples’ failure typically occurs in two ways; pulling out the reinforcement and crushing the concrete. The most critical parameter affecting the type of failure is the thickness of the concrete cover. The concrete thickness used in this research has been selected so that the failure mechanism is in the form of concrete crushing.

To offer a model based on the results of this study that can predict the bond strength with different FA percentages, the bond strength values are divided by their corresponding f_c^1/2 and plotted in Fig. 9. It is clear from this figure that the ratio of 𝜏_u / f_c^1/2 has increased over time for all percentages of FA.

After 7 days, with increasing FA content, the ratio of 𝜏_u / f_c^1/2 decreased with a mild slope, but for older ages, this trend has appeared in different forms; at 28 days, the ratio of 𝜏_u / f_c^1/2 has risen with increasing FA content to 15%, and after that, it decreased to 5% flay ash. The decrease in the bond strength at 7 days can be attributed to the lack of pozzolanic reaction initiation and dilution effects of FA at this age.

3.3 Comparison with Existing Models

Compared with the results of other researchers and existing models, some of the most applicable bond strength models are listed in Table 5. The main parameters in these models are compressive strength, concrete clear cover, and development length of reinforcement; hence, by placing the compressive strength results presented in Table 3, the models’ bond strengths were calculated and compared with the results obtained from the experiment.

In Figs. 10, 11, 12 and 13, these values were plotted along with the mean of model results. Based on the obtained results shown in these figures, it can be seen that at 7 days, the bond strength obtained from the experiment is lower

1.01 1.02 1.07

0.98 1.02 1.16

0.79 0.95 1.06

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

Normalized compressive strength

Specimen age

FA0 FA05 FA15 FA25

Fig. 6 Normalized compressive strength to the control mix

Table 4 Bond strength of samples at different ages Specimen Bond strength (MPa)

Control 7.25 11.25 12.35

FA05 6.85 11.10 12.85

FA15 6.25 12.70 14.80

FA25 5.15 10.55 13.15

Fig. 7 Normalized Bond strength to the control sample

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than the mean of the models, and with increasing FA, the bond strength reduces even below the minimum resistance predicted by the models. Overall, most models except the Hadi’s model (2008) do not provide a reasonable estimate of the bond strength at this age, and the values obtained even for the control sample are much higher than the laboratory results; therefore, the results obtained through these models are non-conservative for early age estimation of bond strength. At 28 days, for all specimens except FA15,

the bond strength is close to the models’ mean values. For 90 days of age, the bond strength of all specimens was higher than the mean of the models, but the difference in FA15 was higher than that in the others.

4 Conclusions

The effect of cement replacement with 5, 15 and 25% of FA on the bond strength between concrete and rebar was investigated, and the following results were obtained:

- In the long-term, the effect of 5 and 25% fly ash replacement on the compressive strength was similar, so that there was not much difference between the compressive strength of these samples with the sample without fly ash. For the sample containing 15% fly ash, the compressive strength has improved approximately 16% compared to the control sample.

- Replacing 15% of mix design cement with FA increased the bond strength by 13% and 21% for 28 days and 90 days, respectively.

- Replacing 5% and 25% cement with FA harmed the 28-day bond strength as a benchmark for measuring the bond strength although this effect was eliminated and the positive effects of FA were observed in the long term.

-The average of available models at an early age provides non-conservative results for the bond strength of concrete containing FA, but the model’s average is almost consistent with the experimental results of older ages.

Fig. 8 The effect of specimens age on failure mechanism

0.94 0.99 1.05

0.86 1.13 1.21

0.71 0.94 1.07

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Normalized compressive strength

Specimen age

FA0 FA05 FA15 FA25

Fig. 9 Bond changes relative to FA percentage

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Table 5 Bond strength models from the literature

c = minimum concrete clear cover, c_s = side cover, c_x = distance between reinforcement, c_y = bottom cover, d_b = reinforcement diameter, L_d = development length, f_c = cylinder compressive strength, f_ct = tensile strength of concrete, f_cu = cube compressive strength, h_r = rib height, s_r = rib spacing, C_min = min(c_x,c_s/2,c_y), C_max^* = max(min(c_x,c_s/2,c_y))

Model

number Bond Strength Model References

1 0.083045√

f^�

c

[(

1.06+2.12(_c

db

)) +(

0.92+.08(C∗ max C_min

) +75(_d

b Ld

))] Darwin et al. (1992)

2 0.083045

√ f^�

c

[ 1.2+3

(_c

d_b

) +50

(_d

b L_d

)] Orangun et al. (1977)

3 8.6

( ^c

db+.5 c db+5.5

)

f_ct Esfahani and Rangan (1998)

4 0.083045√

f^�

c

[

22.8−0.208(_c

d_b

)

−38.212(_d

b L_d

)] Hadi (2008)

5 √

f^�

c

[

0.1377+0.1539(_c

d_b

)

+2.673(_d

b L_d

)

+1.053(_h

r s_r

)] Diab et al. (2014)

02 46 8 1012 14 1618 20 2224

7 28 90

Bond Strength (MPa)

Time(Day)

Orangun et al. Darwin et al.

Hadi Esfahani & Rangan

Diab et al. Test results

Fig. 10 Bond strength based on available models for mix control

02 46 108 1214 1618 20 2224

7 28 90

Bond Strength (MPa)

Time(Day) Orangun et al.

Darwin et al.

HadiEsfahani & Rangan

Fig. 11 Bond strength based on available models for FA05

0 2 4 6 8 10 12 14 16 18 20 22 24

7 28 90

Bond Strength (MPa)

Time(Day)

Models average

0 2 4 6 8 10 12 14 16 18 20 22 24

7 28 90

Bond Strength (MPa)

Time(Day)

Models average

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Declarations

Conflict of interest The authors have no conflicts of interest to declare.

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