# Effect of Recycled Concrete on the Flexural Behavior of Concrete-Filled FRP Tubes

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## Abstract

This paper presents the results of short span concrete-filled fiber reinforced polymer (FRP) tubes filled with different recycled coarse aggregate (RCA) replacement ratios. The mechanical response of recycled concrete-filled FRP tubes (RCFFTs) under flexural load was examined to investigate the influence of RCA as an in-filled material. The experimental results were compared with the simplified finite element model to estimate the failure behavior of the test units. RCFFTs with a 100% RCA replacement ratio showed a similar bending behavior compared to natural aggregate concrete-filled FRP tubes. The flexural behavior of 100% recycled aggregate concrete could be complemented and utilized because of the high strength and high stiffness of the FRPs.

## Keywords

concrete-filled FRP tube recycled coarse aggregate FEA## 1 Introduction

Construction waste material has gradually increased due to the reconstruction of old deficient structures. In such a situation, it is preferable to use recycled concrete as a structural member to obtain new results and to minimize environmental pollution caused by industrial activities. It has been reported that the compressive strength of concrete constructed with 100% recycled coarse aggregate (RCA) is lower than the compressive strength of normal concrete owing to the high water absorption of RCA (Xiao et al. 2012; Etxeberria et al. 2007). Several experimental studies on the structural behavior and mechanical strength of recycled concrete beams have been performed (1973; Ignjatović et al. 2017; Fathifazl et al. 2011; Silva et al. 2015).

The tension of recycled aggregate concrete must be reinforced by tension carrying material. Fiber reinforced polymers (FRPs) have been extensively considered as an alternative material for tension carrying reinforcement (Lee et al. 2017a, b, c, d). Especially, concrete-filled FRP tubes have been widely investigated as next generation structural members that combine the advantages of FRP and concrete material. The important role of FRP tubes in such a system is to replace steel, while the concrete has a similar role as in general reinforced concrete structures. Additionally, the FRP tubes create shear and flexural reinforced covers, whereas concrete offers good compressive strength and stability for structures against lateral buckling (Mirmiran and Shahawy 1996).

Several researchers have studied circular tubes using FRP (Barbero and Tomblin 1994; Lin et al. 1996; Bambach et al. 2009; Haedir et al. 2009; Lee and Lee 2004; Roberts and Al-Ubaidi 2001; Fam and Son 2008; Shao and Mirmiran 2005, 2007; Fam and Rizkalla 2001; Burgueno and Bhide 2004; Ahmad et al. 2005; Mirmiran et al. 2000; Zhu et al. 2005; Ahmad et al. 2008; Fam and Cole 2007). Significant research efforts have been made to understand concrete-filled FRP tube (CFFT) systems under axial loading or axial-flexural loading (Barbero and Tomblin 1994; Lin et al. 1996; Bambach et al. 2009; Haedir et al. 2009; Lee and Lee 2004; Roberts and Al-Ubaidi 2001). However, a comprehensive review of the literature shows that the behaviors of short span beams have received little attention (Roberts and Al-Ubaidi 2001; Fam and Son 2008; Shao and Mirmiran 2005, 2007; Fam and Rizkalla 2001; Burgueno and Bhide 2004; Ahmad et al. 2005). Short span beams should be considered simultaneously for complex behavior including bending and shear. In particular, FRP tubes may be at risk of buckling in the case of short spans. Therefore, the structural examination of bending and shear of short span beam FRP tube needs to be studied sufficiently. Mirmiran et al. (2000) observed that beams with a/D ratios (ratio of shear span to outside diameter) of about 2.0 failed in the flexural mode. Zhu et al. (2005) and Ahmad et al. (2008) carried out load tests on ten specimens with a/D ratios between 0.9 and 6.25 and D/t ratios between 16 and 63. It was confirmed that none of the test beams failed in the shear mode. All tested beams failed due to rupture of the FRP tube under longitudinal tension. In addition, short-span concrete-filled FRP beams exhibited higher bending capacity due to the diagonal compression developed in in-filled concrete through arching effects. Fam and Cole (2007) examined the shear behavior of 10 short-span CFFTs reinforced with either steel or FRP longitudinal rebar. All test units failed in terms of the flexure at a/D ratios of about 1. However, all previous research is based on natural aggregate CFFTs. Further experimental and numerical study is necessary to ensure the mechanical behavior of recycled CFFTs under flexural loading.

Several researchers have studied thin-walled circular tubes under bending conditions (Mirmiran et al. 2000; Zhu et al. 2005; Ahmad et al. 2008; Fam and Cole 2007). Some researchers investigated the mechanical behavior of recycled concrete-filled steel tubes (RCFST). Yang and Han (2006) investigated the typical failure modes of RCFST columns with various RCA replacement ratios and found RCFST columns failed due to local buckling, which is the same mechanism as in natural aggregate concrete-filled steel (NCFST) tube columns. The ultimate strength of the 100% RCFST columns is about 10% lower than that for conventional columns. RCFST columns showed better ductility than NCFST columns. Further, RCFST columns have a slightly poorer but generally similar ultimate capacity compared to normal concrete-filled steel tube columns. Wu et al. (2012) and Yang and Ma (2013) investigated the cyclic performances of RCFST columns. Several parameters, including concrete type, axial load ratio, and different sources of demolished concrete, were considered. Huang et al. (2012) developed a theoretical model to predict the mechanical behavior of RCFST under axial compression and obtained that the RCA replacement ratio has a reasonable result on the mechanical behavior of the confined concrete. Most studies on recycled concrete-filled structures are related to steel tube columns. It should be noted that the short span recycled concrete filled FRP tubes (RCFFT) beam has a different load transfer mechanism compared to the relatively long RCFST columns.

Xiao et al. (2012) investigated axial loading tests on recycled concrete columns confined by GFRP material and steel material under axial compressive loading. The key parameters were the recycled coarse aggregate replacement and the type of tube. It was obtained that the maximum stress of RCA confined with GFRP tubes decreases while the corresponding strain increases. Dong et al. (2013) further investigated the structural behavior of RCFST columns wrapped with CFRP. External CFRP wrapping increased the stiffness of RCFST columns significantly due to the restraint on hoop deformations during compressive loading.

No research was found on how the mechanical behavior of RCFFT differs from natural aggregate concrete-filled FRP tubes under bending loads. As the construction industry develops recently, new structures are constructed. A large number of construction and demolition waste aggregates are generated as exist structure is broken down. The interest with recycled materials derived from construction and demolition waste is growing all over the world. Therefore, if recycled aggregate concrete obtained from construction and demolition waste aggregate is used as a structural material, effective construction material circulation can be achieved. Recycled aggregate may be lacking in mechanical performance as a structural material. However, the recycled aggregate can be combined with other materials and exhibit sufficient structural performance. Therefore, this study investigates the effect of structural performance by mixing recycled aggregate in FRP tube. This study investigates the possibility of utilizing recycled concrete as in-filled material for FRP tubes in order to increase the use of recycled materials. The significance of this study lies in extending the application of concrete-filled FRP tubes to the practical uses of RCA. The main objective of this research is to study the mechanical behavior of short span RCFFT beams and to evaluate the structural performance of the proposed system with varying RCA replacement ratios. Several experimental test units are designed and fabricated with varying RCA. The considered RCA replacement ratios are 0%, 25%, 50%, and 100%. The hollow FRP tube alone is also tested to investigate the in-filled effect of concrete as a reference test unit. Five test units were tested under flexural loadings. In addition, simplified finite element analysis (FEA) models of the RCFFT beam are presented and compared with the experimental results.

## 2 Experimental Program

_{2}/± 45

_{3}/90

_{2}], as shown in Fig. 2a. The orientation of the filaments can also be cautiously organized so that successive layers are supplied or oriented inversely from the prior layer. Figure 1b and c show the fabrication process for lamina orientation at 90° and ± 45°, respectively. The FRP tube was then hardened in a dry oven for 10 h, as shown in Fig. 1d. For in-filled material, ordinary Portland cement (OPC) with a compressive strength of 27.0 MPa was used to fabricate test specimens. Figure 1e shows the prepared aggregate. As discussed earlier, the coarse aggregates are natural coarse aggregates and recycled coarse aggregates (RCA). The ratio of recycled aggregate concrete is shown in Table 1. RCFFT-0 is a mixture of 0% recycled aggregate and RCFFT-25 is 25% recycled aggregate. RCFFT-50 is a specimen mixed with natural aggregate and recycled aggregate by 50%, and RCFFT-100 is a specimen mixed with recycled aggregate only. Samples of 100 × 200 mm concrete cylinders were prepared according to the same RCA replacement ratio as their respective test units (ASTM C 39 2001). During the concrete casting, the compact vibration was performed for the consolidation three times according to the height. Compressive strength tests were carried out using a hydraulic actuator at a constant displacement rate of 1 mm/min. Table 1 shows the compressive strengths of filled concrete. The FRP tube was filled with recycled aggregate concrete, as shown in Fig. 1f.

Mix proportions and properties of concrete.

Specimen | RCFFT-0 | RCFFT-25 | RCFFT-50 | RCFFT-100 |
---|---|---|---|---|

RCA (kg/m | 0 | 214 | 427 | 854 |

NCA (kg/m | 854 | 640 | 427 | 0 |

Cement (kg/m | 354 | 354 | 354 | 354 |

Sand (kg/m | 848 | 848 | 848 | 848 |

Water (kg/m | 202 | 202 | 202 | 202 |

W/C | 57 | 57 | 57 | 57 |

Compressive strength (MPa) | 30.98 | 29.64 | 29.60 | 26.74 |

Figure 2 shows an overview of the instrumentation and test setup. During a static load test, displacement transducers set up at mid-span and quarter-span to measure deflection. The strain gauges were installed at the bottom surface of the FRP tube in the longitudinal direction to analyze the strain change at each load level. Strain gauges were installed in the hoop direction at the mid-span to test the confining effect at each load level.

The experimental work investigated the load carrying capacity of each specimen as well as failure patterns beyond the ultimate load. The specimens were examined by four-point bending. A vertical servo-controlled hydraulic actuator was located 90 mm apart and at equal distance from the center of the specimen, as shown in Fig. 2a. Because the cross section of the specimen is circular, a half-circular shaped steel frame was attached to the hydraulic actuator, thus supporting the prevention of lateral instability of the specimens, as shown in Fig. 2. The experiment was controlled using the displacement-control method to ensure experimental safety. The load rate was 0.6 mm/min up to failure of the specimens. A data acquisition system was used with a sample rate of 1 Hz.

## 3 Finite Element Analysis

_{t}) correctly, and it is usually expected to be approximately 8% of compressive strength (σ

_{c}). A descending line in this study is used to model this tension stiffening. A smeared model is used to represent the discontinuous crack behavior when cracking of concrete occur. It is recognized that the cracked concrete element can transport the tensile stress in the normal direction to the crack, which is chosen tension stiffening (ASCE Task Committee on Concrete and Masonry Structure 1982). The concrete material is supposed to be initially isotropic, before tensile cracking or compressive yielding. Each element has five fibers per one integration point at which cracking and yielding are checked and the state determination based on material behavior is performed. Cracking occurs when the element’s principal stress surpasses the tensile strength of concrete. The element stiffness matrix was informed based on the uniaxial stress–strain relationship defined earlier. An incremental Newton–Raphson iterative procedure is applied to find the equilibrium position at each integration point.

It is noted that the RCA replacement ratio was not considered in the FEA model and the compressive strength of the FEA model was assumed to be 26.74 MPa because FE analysis was conducted to provide a reference for the experimental results and to obtain the structural response of in-filled concrete inside the FRP tube, which is difficult to obtain during testing.

## 4 Experimental and Numerical Results

_{c}) was calculated as Eq. (3) based on the compressive strength (

*f*

_{ck}) according to the ratio of recycled aggregate. The deflection of the specimen was compared with the theoretical deflection (δ) formula. The equation of deflection is given by Eq. (4).

Compared load–deflection.

Specimen | Experimental deflection (mm) | Theoretical deflection (mm) | Error rate (%) |
---|---|---|---|

HFT | 10.70 (at 136.88 kN) | 10.42 (at 136.88 kN) | 2.6 |

RCFFT-0 | 6.32 | 6.21 | 1.7 |

RCFFT-25 | 6.57 | 6.33 | 3.6 |

RCFFT-50 | 6.58 | 6.45 | 2.0 |

RCFFT-100 | 6.86 | 6.58 | 4.1 |

Load–deflection results.

Load (kN) | HFT (mm) | RCFFT-0 (mm) | RCFFT-25 (mm) | RCFFT-50 (mm) | RCFFT-100 (mm) |
---|---|---|---|---|---|

150 | 10.70 (at 136.88 kN) | 6.32 | 6.57 | 6.58 | 6.86 |

250 | – | 11.01 | 11.24 | 11.26 | 12.00 |

350 | – | 15.90 | 16.66 | 17.01 | 17.59 |

450 | – | 21.60 | 23.07 | 23.38 | 23.85 |

According to the compressive strength of the recycled concrete cylinder given in Table 1, the compressive strength of the natural concrete cylinder was 31.0 MPa while that of all recycled concrete was 26.7 MPa, a 16% difference. However, the strength of the CFFTs with all natural aggregate concrete is more than 6.7% higher compared to the CFFTs with all RCA. This indicates that the structural weakness of recycled concrete was reinforced by FRP material. Thus, recycled concrete can be applied as in-filled material for future construction.

## 5 Conclusions

- 1.
The flexural strength of RCFFTs is significantly higher than that of a hollow FRP beams. RCFFTs filled using 100% recycle concrete improved the strength more than 3 times compared with HFT. It is confirmed that the FRP specimen filled with the recycled aggregate concrete significantly improved the flexural strength by restraining the recycled aggregate concrete filled with the FRP tube.

- 2.
The flexural strength of the CFFTs with all RCA is 6.7% lower compared to CFFTs with all natural aggregate concrete. It was confirmed that the structural performance of recycled aggregate concrete could be complemented and utilized because of the high strength and high stiffness of the FRPs.

- 3.
As the load increased, the FRPs filled with the recycled aggregate concrete proceeded to fracture as the initial cracks formed in the filled concrete. As the cracks in the inner concrete progressed, the strain rapidly increased in the lower part of the mid-span of the FRP. The final failure mode was brittle fracture as the fibers of the FRP tube were locally fractured.

- 4.
The proposed finite element model in this study can predict the load level and circumferential deformation. The proposed FEA model predicted behavior similar to the experimental failure mode.

## Notes

### Authors’ contributions

HL studied the experimental results on this page and proposed a finite element analysis model. HJ performed compressive strength tests and flexural tests and analyzed the experimental results. WC managed this paper collectively. All authors read and approved the final manuscript.

### Acknowledgements

This research was supported by a grant (2017R1A2B4010467 and 2017R1C1B1006732) from the National Research Foundation of Korea through Ministry of Science, ICT and Future Planning, South Korea.

### Competing interests

The authors declare that they have no competing interests.

### Availability of data and materials

Not applicable.

### Funding

This study is a basic research project conducted with support from the National Research Foundation of Korea through government funds (Ministry of Science, ICT and Future Planning, South Korea). The Project Numbers are 2017R1A2B4010467 and 2017R1C1B1006732.

### Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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