# Response of Glass Fiber Reinforced Polymer (GFRP)-Steel Hybrid Reinforcing Bar in Uniaxial Tension

- Minkwan Ju
^{1}, - Sangyun Lee
^{2}and - Cheolwoo Park
^{3}Email author

**11**:212

https://doi.org/10.1007/s40069-017-0212-9

© The Author(s) 2017

**Received: **4 July 2016

**Accepted: **9 July 2017

**Published: **7 December 2017

## Abstract

This study introduces a glass fiber reinforced polymer (GFRP)-steel hybrid bar with a core of a deformed steel bar (steel core). Six types of the hybrid cross section were considered, and a total of 48 tensile specimens were tested by the uniaxial tensile test to measure the tensile strength and modulus of elasticity of the GFRP hybrid bar. The results of the uniaxial tensile test revealed that the GFRP hybrid bar showed higher modulus of elasticity and lesser ultimate tensile strength than those shown by a normal GFRP bar. The stress–strain relationship showed a bi-linear behavior indicating good ductility against the brittle failure of a normal GFRP bar. Among all the steel core having a diameter of 19.1 mm, the bar with a core diameter of 9.53 mm exhibited the highest tangent modulus of elasticity. A tensile stress–strain model was suggested for the GFRP hybrid bar having an outer diameter of 19.1 mm and a core diameter of 9.53 mm. This was in good agreement with the experimental results. The suggested stress–strain model can be applied for structural design or analysis of concrete structures such as bridge deck slabs.

## Keywords

## 1 Introduction

Structural degradation of reinforced concretes (RCs) occurs mainly because of the corrosion of steel reinforcements due to the penetration of de-icing salts on RC bridge deck slabs or carbonation of concrete by environmental attacks during the service life. To prolong the service life of reinforced concrete, some research to prevent corrosion of reinforcing steel have been performed. Pritzl et al. (2014) found out that the acrylic coating was effective to prevent corrosion of steel bar. Choi et al. (2008, 2006) reported that epoxy-coated steel bar showed good performance under corrosive environment. Tae (2006) proved that the Cr-bearing rebar with over 7% of Cr content possessed excellent corrosion resistance.

Another innovative approach to prevent reinforcement from the corrosion is to use a glass fiber reinforced polymer (GFRP) bar as a reinforcement in reinforce concrete. Owing to their non-corrosiveness and high tensile strength, glass fiber reinforced polymer (GFRP) bars are considered as promising alternatives to RCs (Maranan et al. 2015; El-Gamal et al. 2009). Another advantage of GFRP bars is their low cost as compared to that of the fiber reinforced polymers (FRPs) made up of carbon or aramid (Zheng et al. 2012; Carvelli et al. 2010; El-Gamal et al. 2007). However, GFRP bars suffer from service-related issues which still need to be addressed such as large deflection or crack width in structural design because of the high deformability caused by the low modulus of elasticity (Mazaheripour et al. 2016). Several efforts have been made for enhancing the flexural stiffness of GFRP bar reinforced concretes by using the GFRP and steel reinforcements simultaneously (El-Refai et al. 2015; Lau and Pam 2010). It was found that these GFRP bar reinforced concretes showed an improved flexural capacity with less crack width and lower deflection in the serviceability. This approach may have a limitation due to a complicated arrangement of reinforcements during the construction and the lack of design specification because of the composite action between the GFRP and the steel bars. Several studies have been conducted with an aim to improve the flexural stiffness of FRP bars using hybrid rods. Nanni et al. (1994) developed a hybrid rod consisting of FRP braided skin made up of aramid or vinylon fiber and a steel core. It was found that the hybrid rod had a modulus of elasticity higher than that of the normal FRP bar and exhibited a bi-linear behavior in ductile manner. A hybridization approach was studied by You et al. (2007). They observed that the hybrid FRP bar with a core of carbon fiber showed an improvement in the modulus of elasticity. It was found that the modulus of elasticity of this hybrid bar was higher than that of glass fibers. On the basis of these studies, it can be stated that FRP-hybrid steel bars can be promising alternative reinforcements to normal GFRP bars. Seo et al. (2013) investigated the tensile characteristics of the GFRP hybrid bars with a GFRP outer surface having a diameter of 12.7 mm and a deformed steel core having a diameter of 9.0 mm. It was revealed that the hybridization could improve the modulus of elasticity and enabled the enhancement of the flexural stiffness of reinforced concretes. In this study, the GFRP hybrid bar with unique properties such as non-corrosiveness and high modulus of elasticity was produced. The GFRP hybrid bars can provide structural efficiency to the RCs having low crack width and deflection as compared to normal GFRP bars and exhibit better serviceability in flexure. The GFRP hybrid bars also contribute to the durability of concretes because of the non-corrosiveness of the GFRP surface. Moreover, GFRPs are relatively cost effective as compared to carbon or aramid fibers. With regard to the structural design, several guidelines have been proposed for designing the reinforced concretes using GFRP bars (ACI 440.1R-15 2015; AASHTO 2009; CSA S806-12 2012). The material and mechanical characteristics of the GFRP hybrid bars can be studied and reflected in those structural designs.

The current study aims to investigate the tensile property of the GFRP hybrid bars experimentally. A total of 48 tensile specimens were tested by the uniaxial tensile test in accordance with the ASTM test method (ASTM D 3916 2002). The test specimens had geometrical variables in a cross section consisting of an outer GFRP surface and a deformed steel bar (steel core). In order to compare the enhanced tensile strength of the GFRP hybrid bars with the steel core, the tensile test was conducted for the steel core of D10, D13, and D16, respectively. The important and fundamental mechanical properties including tensile strength, stress–strain relationship, and modulus of elasticity were investigated. In addition, a stress–strain model for the GFRP hybrid bar was suggested. The application of hybrid bar as a tensile reinforcement in structural designs was also discussed.

## 2 Description of GFRP and Deformed Steel Hybrid Bar

## 3 Experimental Program

### 3.1 Test Specimens

Details of the GFRP hybrid bar.

Specimen no. | Specimen ID | Nominal diameter (core diameter) (mm) | Pitch space of fiber ribs (mm) | Number of samples |
---|---|---|---|---|

1 | D13 | 12.7 | 12.8 | 6 |

2 | D13 (D10) | 12.7 (9.53) | 12.8 | 6 |

3 | D16 (D10) | 15.9 (9.53) | 15.9 | 6 |

4 | D16 (D13) | 15.9 (12.7) | 15.9 | 6 |

5 | D19 | 19.1 | 19.1 | 6 |

6 | D19 (D10) | 19.1 (9.53) | 19.1 | 6 |

7 | D19 (D13) | 19.1 (12.7) | 19.1 | 6 |

8 | D19 (D16) | 19.1 (16.1) | 19.1 | 6 |

### 3.2 Test Setup and Strain Measurement

*L*/2 and

*L*/4 of the FRP bar to investigate the strain variation in the longitudinal direction. Figure 4 shows the schematic of the test setup.

## 4 Results and Discussions

### 4.1 Load-Slip Relationship

*δ*

_{u}) and that at the fixed end (

*δ*

_{d}) was measured and compared before and after the tensile test. Figure 5 shows the measured average slip at each end. By averaging the change in the length and taking it as the absolute value, the maximum slip in the normal D13 and D19(D10) bars was found to be 4.0% (0.512 mm) and 2.6% (0.5 mm) for the pitch space of 12.8 and 19.1 mm, respectively. These values may be small enough not to affect the stress–strain relationship.

### 4.2 Tensile Stress–Strain Relationship

*L*/2 and

*L*/4, the even distribution of stress throughout the free length of the GFRP hybrid bar was confirmed. The values were almost identical so that it was known that the applied stress was evenly distributed along the longitudinal direction. For the D16 bar shown in Fig. 6b, the tensile stress–strain of the normal D13 GFRP bar was additionally imposed to indirect comparison with that of the D16 bar. Due to the hybridization of a steel core and a GFRP outer surface, the tensile stress–strain behavior was governed by a bi-linear manner. The modulus of elasticity for the GFRP hybrid bar and normal GFRP bar were compared. As a result, the hybrid bar showed an apparent increase in the modulus of elasticity by the yielding point of the steel core. This phenomenon was observed for all the hybrid bar specimens. It was found that the initial modulus of elasticity of the GFRP hybrid bar was governed by the core of the deformed steel bar. Hence, the present design of steel reinforced concrete was valid up to the initial modulus of elasticity. This state could determine the serviceability of the crack width and deflection. However, the tangent modulus of elasticity in the first linear branch exhibited much less stiffness as compared to that in the second linear branch. In order to compare the enhanced tensile strength of the GFRP hybrid bars with the steel core, the tensile test was conducted for the steel core of D10, D13, and D16, respectively. The yield strength was 400 MPa and the number of specimens was three for each diameter. Only one of the tensile stress data for each diameter was plotted in Fig. 6 because the tensile stress–strain behaviors of steel core waere identical. It was found that the increased ranges of tensile strength showed from 18.8 to 97.8% as compared to the yield strength of steel core. As the amount of GFRP outer surface increased, the enhanced effect of tensile strength was increased too. Among the GFRP hybrid bars, D19(D10) specimen showed the largest increase of tensile strength. It was judged that the hybridization of the GFRP and steel core can devote to the higher tensile strength than the steel core and the higher modulus of elasticity than the normal GFRP bar.

Tested tensile strength of the GFRP hybrid bar.

D13 | D13 (D10) | D16 (D10) | D16 (D13) | D19 | D19 (D10) | D19 (D13) | D19 (D16) | |
---|---|---|---|---|---|---|---|---|

1 | 1108.5 | 649.7 | 768.3 | 591.5 | 957.0 | 893.1 | 561.2 | 483.0 |

2 | 1125.1 | 704.0 | 818.8 | 611.0 | 872.6 | 910.2 | 576.0 | 498.4 |

3 | 1112.8 | 680.0 | 794.3 | 613.1 | 924.7 | 845.2 | 534.7 | 438.4 |

4 | 1174.9 | – | 805.6 | 611.6 | 858.2 | 854.6 | 535.3 | 490.9 |

5 | 1091.6 | – | 799.9 | 606.2 | 942.2 | 922.1 | 413.5 | 473.6 |

6 | 1108.5 | – | 805.8 | 634.1 | 865.7 | 970.0 | 536.7 | 472.7 |

Average tensile strength (MPa) | 1120.2 | 677.9 | 798.8 | 611.3 | 903.4 | 899.2 | 526.2 | 476.2 |

Standard deviation (MPa) | 24.4 | 22.2 | 15.5 | 12.5 | 39.3 | 42.0 | 52.7 | 19.2 |

*E*

_{T}branch. As the area of the GFRP increased, the modulus of elasticity at the first branch was decreased and the tangent modulus of elasticity was increased. The optimum relationship between the tensile strength and the modulus of elasticity for structural designs needs to be investigated and discussed.

### 4.3 Investigation of Tensile Strength, Modulus of Elasticity and Cross Sectional Ratio

*P*

_{1}and

*P*

_{2}corresponding to 50 and 25% of the ultimate load, respectively, and the corresponding strains

*ε*1 and

*ε*2. This equation showed an inverse relationship. Thus, if the area of GFRP outer surface was increased, an increase in the tensile strength was observed, while the magnitude of the modulus of elasticity decreased. This relationship is quite important to determine the design parameters of the GFRP hybrid bar as a longitudinal reinforcement. An optimum balance between the tensile strength and the modulus of elasticity should be established to obtain hybrid reinforcements with optimum tensile properties. Though the number of test variables used in this study was not sufficient, we aimed to solve the abovementioned issues using an approach illustrated in Fig. 8. The number of variables was limited by the compliance to the specified standard diameter of the reinforcement. A reasonable criteria for the sectional design was the intersection point between the positive and negative inclination curves for the tensile strength and modulus of elasticity for the D13, D16, and D19 specimens, respectively.

Keeping the difficulty in adjusting the sectional design according to the standard diameter in mind, the sectional design of the GFRP hybrid bar with a core of D10 steel core was considered to be the most appropriate design. The modulus of elasticity for this structural design was found to be 87.8 GPa, which was 154.9% of that of the normal D19 GFRP bar. Moreover, there were the least change of tensile strength within 0.5%.

It must be very important aspect for design using the GFRP hybrid bar as the experimental data in tensile can be converged well. In the result of the analysis, D19(D10) hybrid bar showed the most accurate relationship for yield stress and modulus of elasticity, while D16(D10) and D19(D16) showed good results except some of out of data. This result proved that D19(D10) hybrid bar has the most stable behavior in tensile test.

*E*) and tangent modulus of elasticity (

*E*

_{T}). The variation in

*E*

_{T}was found to be proportional to that in the area of the GFRP outer surface. Figure 9 shows the relationship between the tangent modulus of elasticity and the cross sectional ratio of the GFRP hybrid bar. After approaching the yield strain of the steel core, the tensile behavior of the second branch is governed by the GFRP outer surface.

Average modulus of elasticity and tangent modulus of elasticity.

D13(D10) | D16(D10) | D16(D13) | D19(D10) | D19(D13) | D19(D16) | |
---|---|---|---|---|---|---|

Average modulus of elasticity | ||||||

Average | 123.0 | 104.4 | 145.5 | 87.8 | 126.2 | 139.1 |

Standard deviation (GPa) | 12.7 | 3.8 | 12.7 | 4.4 | 10.2 | 26.5 |

Tangent modulus of elasticity | ||||||

1 | 25.0 | 52.3 | 18.5 | 40.1 | 22.5 | 10.9 |

2 | 22.2 | 34.0 | 57.1 | 42.8 | 16.9 | 10.6 |

3 | 23.0 | 65.0 | 14.1 | 41.9 | 37.1 | 5.2 |

4 | 15.7 | 57.1 | 19.9 | 40.7 | 46.8 | 8.5 |

5 | 26.3 | 34.6 | 17.6 | 44.8 | 25.3 | 5.0 |

6 | 22.0 | 31.1 | 17.8 | 42.2 | 49.7 | – |

Average | 23.3 | 45.7 | 30.2 | 42.1 | 33.0 | 8.1 |

Standard deviation (GPa) | 3.4 | 13.0 | 14.1 | 1.5 | 12.4 | 2.8 |

*E*is transferred to

*E*

_{T}because its behavior is governed by the low modulus of elasticity of the GFRP. The ultimate tensile properties such as the design tensile strength and the modulus of elasticity should be determined within the

*E*

_{T}branch. This is because the strain hardening property is still available for stress transfer to the concrete. Moreover, an interesting aspect was identified in Fig. 10. The

*E*

_{T}and cross sectional ratio showed a close correlation. Once the cross sectional ratio was determined, the

*E*

_{T}could be approximately estimated except for the D16 specimen series. This large discrepancy might be due to incomplete material configuration.

From the analysis of the relationship between the tensile strength and the modulus of elasticity, including the tangent branch, it was found that the design of the D19(D10) hybrid bar was appropriate and displayed high *E*
_{T} and low discrepancy in the cross sectional ratio. Therefore, it had an excellent tensile stiffness at the mechanical range of *E* and *E*
_{T} branch along with a sufficiently high tensile strength. A material model of *E*
_{T} with an appropriate safety factor may be analytically obtained according to the varying cross sectional ratios by attaining a quality control with an effective design process.

## 5 A Suggestion of Stress–Strain Relationship for the GFRP Hybrid Bar

*E*and

*E*

_{T}. The tangent modulus of elasticity is one of the important factors determining the tensile property of the GFRP hybrid bar. The

*E*

_{T}could be estimated by following its close correlation with the cross sectional ratio investigated above. The other change was using three kinds of fitting coefficients. These curve fitting factors were finally determined by an iteration process as compared to the experimental results. Therefore, Eq. (1) was introduced for the modified and suggested stress–strain model of the GFRP hybrid bar.

*f*

_{fu}= ultimate stress of the GFRP hybrid bar (MPa),

*ɛ*= characteristic strain,

*ɛ*

_{f}= strain according to ultimate stress,

*E*

_{T}= average tangent modulus of elasticity (MPa),

*E*= average modulus of elasticity (MPa)

*L*/2 was considered as representative data. The specimens under consideration exhibited the least discrepancy in stress–strain behavior. From the comparison, it was found that the stress–strain behavior of the suggested model was in good agreement with the experimental results. However, the discrepancy slightly increased upon reaching the ultimate stress. The average tensile strength was calculated to be 899.2 MPa, and the standard deviation was found to be 42.0 MPa. For the design value, the design tensile strength of the GFRP exposed to the surroundings with an environmental factor of 0.7 was determined to be 542.2 MPa. The suggested stress–strain model could accurately predict the tensile behavior including the branch of the tangent modulus of elasticity for the design tensile strength as well as the guaranteed strength.

## 6 Conclusions

- 1.
The tensile test revealed that the GFRP hybrid bar showed a low ultimate strength and large modulus of elasticity as compared to the normal GFRP bar. The bi-linear behavior of the GFRP hybrid bar indicated good ductility as compared to the brittle failure of the normal GFRP bar at the ultimate state without any sign of fracture. From the relationship between the tensile strength and modulus of elasticity, it was found that the reasonable criterion for the section design could be determined from the intersection point. D19(D10) specimen showed the largest increase of tensile strength up to 97.8% as compared to that of the yield strength of steel core. The hybridization of the GFRP and steel core could devote to the higher tensile strength than the steel core and the higher modulus of elasticity than the normal GFRP bar.

- 2.
The tangent modulus of elasticity is one of the important factors determining the strength limit state of the GFRP hybrid bar. From the relationship between the tensile strength and the tangent modulus of elasticity, it was found that the D19(D10) bar exhibited an enhanced modulus of elasticity of up to 54.9% as compared to that of the normal GFRP bar and the highest tangent modulus of elasticity among the D19 hybrid bars. Besides, it showed the most stable behavior in tensile test. Therefore, the cross sectional design of the D19(D10) hybrid bar was considered to be most appropriate. Bars with this cross sectional design can be used as reinforcements to design, analyze, and construct concrete bridge deck slabs.

- 3.
The conclusive hybrid bar was experimentally selected as D19(D10) hybrid bar in this study and the stress–strain model was proposed for the bar. The proposed model showed a good agreement with the experimental tests. The stress–strain model could accurately predict the tensile behavior including the branch of the tangent modulus of elasticity for the design tensile strength and strength. This stress–strain model can be applied to structural designs or finite element analysis for flexural RCs or concrete bridge deck slabs. Further investigation on various bar diameters and flexural performance needs to be carried out including the effect of the GFRP outer surface only to the tensile characteristics of the GFRP hybrid bars.

## Declarations

### Acknowledgements

This research was supported by a Construction Technology Research Project (17SCIP-B128706-01) funded by the Ministry of Land, Infrastructure and Transport and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03934809).

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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