Flexural Behaviour of Full-scale Basalt FRP RC Beams – Experimental and Numerical Studies

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Procedia Engineering 108 ( 2015 ) 518 – 525

Peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering doi: 10.1016/j.proeng.2015.06.114

ScienceDirect

7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD’2015)

Flexural behaviour of full-scale basalt FRP RC beams –

experimental and numerical studies

Dawid Pawłowski

a,*

, Maciej Szumigała

a

a_{Faculty of Civil and Environmental Engineering, Poznan University of Technology, Piotrowo 5, 60-965 Poznań, Poland}

Abstract

Basalt fiber-reinforced polymer (BFRP) bars are a relatively new material. Due to lack of experience in its use, behaviour of BFRP reinforced concrete (RC) members should be fully investigated. Furthermore, existing design codes for fiber-reinforced polymer (FRP) RC structures do not consider this type of reinforcement. This paper presents the results of an experimental and numerical study of the flexural behaviour of a series of simply supported BFRP RC beams under short-term static loads. The beams were varied in terms of the reinforcement ratio and the influence of this parameter was analysed. It had a significant effect on the stiffness and flexural strength of the beams. The members then were analyzed by the Finte Element Method. Good agreement between the results of the experimental and numerical studies were observed.

Selection and peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering.

Keywords: composite materials; BFRP bars; BFRP RC beams; flexural behaviour; FEM

1. Introduction

Durability of building structures is one of the most important features of present design [1]. Standard steel bars do not have corrosion resistance, hence traditional reinforced concrete (RC) structures are very sensitive to damage in aggressive environment [2]. This problem does not affect fiber-reinforced polymer (FRP) bars, which exhibit such properties as corrosion resistance, electromagnetic neutrality and high cuttability [3,4]. As a result it can have

* Corresponding author.

E-mail address: [email protected]

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many applications – in structures used in marine environments, in chemical plants, when electromagnetic neutrality is needed, or in temporary structures.

FRP bars (especially in the case of glass FRP) have low modulus of elasticity as well as high tensile and low shear strength [5]. Moreover, they do not exhibit any yielding before failure and they behave almost linearly up to tensile rupture. Due to their mechanical properties, deflections and cracking in FRP RC beams are larger than these found in traditional RC members. Consequently, the design of FRP RC beams is often governed by the serviceability limit states [6, 7].

Basalt fiber-reinforced polymer (BFRP) bars are the newest type of FRP reinforcement used in civil engineering. The mechanical properties of basalt bars are similar to those of glass [8-10], so it can be supposed that BFRP and GFRP RC members can be designed according to the same design rules [5]. Nevertheless, BFRP reinforcement is a relatively new material, so behaviour of BFRP RC elements should still be thoroughly examined.

The main aim of this study was to evaluate the failure mechanisms, deflections and ductility of simply supported BFRP RC beams depending on the reinforcement ratio. This paper presents some chosen results of a larger research programme in which 12 beams have been tested under four-point bending. The results of experiments were compared with the results of the Finite Element Method (FEM) analysis.

2. Experimental programme

Tests of 3 simply supported BFRP RC beams subjected to four-point bending were carried out in the laboratory of the Institute of Structural Engineering at Poznan University of Technology. The beams were designed to fail by concrete crushing or by reinforcement rupture. Three different amounts of BFRP reinforcement were used. The reinforcement ratios, balanced reinforcement ratios and designations of the beams are presented in table 1.

Table 1. Characteristics of specimens.

Beam designation Main bar (mm) Reinforcement ratio, ρ (%) Balanced reinforcement ratio, ρfb (%) * BFRP 3#7 3#7 0.19 0.27 BFRP 3#9 3#9 0.32 0.18 BFRP 5#9 5#9 0.54 0.18 *according to ACI 440.1R-06 [4] 2.1. Test specimens

Fig. 1 illustrates the geometry and the reinforcement of test specimens. All the beams had a cross-section of

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0.20 × 0.30 m2_,_{a total length of 3.05 m and a span of 2.70 m. The shear reinforcement consisted of 8 mm round} steel stirrups placed at intervals of 100 mm. In the pure bending zone no stirrups were provided. Two 8 mm steel bars were used as top reinforcement to hold the stirrups. Reinforcing steel grade B500SP was used.

2.2. Material properties – concrete

All the beams were made of C30/37 concrete. The mechanical properties of this material were evaluated from cubic specimens (compressive strength fck,cube) and calculated according to formulas included in Eurocode 2 [11]. They are presented in table 2.

Table 2. Mechanical properties of concrete. Specified compressive strength class* C30/37 Cube compressive strength, fck,cube (MPa) 52.3

Modulus of elasticity, Ecm (GPa)** 33.8

Tensile strength, fctm (MPa)** 3.6

*according to EN-206:2001 [12] **according to PN-EN 1992-1-1 [11] 2.3. Material properties – BFRP bars

BFRP ribbed bars were used as the flexural reinforcement. The experimentally determined mechanical properties of reinforcement [13] are shown in table 3.

Table 3. Mechanical properties of BFRP reinforcement.

Equivalent diameter (mm) 6.74 (7*) 8.65 (9*) Tensile strength, fu (MPa) 1185 1485

Modulus of elasticity, Ef (GPa) 52.8 56.3

Ultimate Strain, εfu (Ĩ) 22.5 26.2

*Nominal diameter 2.4. Experimental setup

The beams were tested under static four-point bending. The load was applied in displacement control mode at a displacement rate of 1.0 mm/min. During the test deflections, compression and tension strains in the middle span of the beams were measured. Data was constantly being collected by a data acquisition system. Crack widths were measured after every 10 kN loading. Test stand of the beams is shown in Fig. 2.

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3. Experimental results

3.1. Failure mode and ultimate load

The beams were designed to fail in different manners. According to code [5] BFRP 3#7 should fail by reinforcement rupture, whereas concrete crushing was expected in BFRP 3#9 and BFRP 5#9.

As can be observed in table 4, beam BFRP 3#7 failed suddenly due to reinforcement rupture. Beam BFRP 3#9 failed in the same manner, even though its reinforcement ratio was larger than the balanced reinforcement ratio (table 1). However, in the case of member BFRP 3#9 it was not a typical reinforcement rupture – two external BFRP bars ruptured 70 cm from the center of the beam, in the place where they were connected to a stirrup. They were probably damaged during assembly. Beam BFRP-5#9 failed by concrete crushing. Its failure was not sudden – behavior of the beam exhibited some ductility.

Table 4. Failure mode and ultimate load of the beams. Beam Ultimate

load Pu (kN)

Failure mode Ultimate compr. concrete strain εcu (Ĩ)

Photo of the beam

BFRP 3#7 63.9 Reinforcement rupture -1.2 BFRP 3#9 85.9 Reinforcement rupture/concrete crushing -4.1 BFRP 5#9 132.3 Concrete crushing -3.8

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On the basis of the results, it can be said that the reinforcement ratio has a significant influence on the flexural strength and failure mode of BFRP RC beams. Furthermore, there is good agreement between the results of these experiments and the results obtained for FRP RC flexural members using other types of non-metallic reinforcement [14,15,16].

3.2. Load deflection

Fig. 3 shows load-deflection (midspan and at Point A – the point of application of force) curves for all the beams. In the case of element BFRP 3#9, measurement of midspan displacements was interrupted before failure of the beam because of damage to the transducer holder. Its final deflection was evaluated on the basis of displacement of Point A.

Fig. 3. Midspan (a) and point A (b) deflections.

All the beams behaved almost linearly until failure. This is the result of the mechanical properties of BFRP bars, which exhibit a linear elastic behavior under tensile loading. Because of the low modulus of elasticity of BFRP reinforcement, ultimate deflections of the beams were about six times greater than these permissible (SLS graph in Fig. 3a – deflection limit=L/250).

It is clear from Fig. 3 that the reinforcement ratio had a considerable effect on the stiffness of the beams. As expected, deflections of these elements increased with the decrease in the reinforcement ratio.

Table 5. Deflections and service loads.

Beam Ultimate load Pu Deflection d=L/250

Pu (kN) dmax (mm) L/ dmax PL/250 (kN) PL/250/Pu

BFRP 3#7 63.9 63 43 25.0 0.39

BFRP 3#9 85.9 >58 <47 30.5 0.36

BFRP 5#9 132.3 78 35 37.0 0.28

Table 5 presents deflections and span-to-deflection ratios for ultimate loads as well as loads for permissible deflections (equal to about L/250) of the beams. For the ultimate loads, the value of the span-to-deflection ratio varied between 35 and 43. In comparison to the service deflection limit of L/250 this value was relatively low, thus the design of all the beams was governed by the serviceability limit state.

As can be observed in table 5, service loads for the beams were about 28 %, 36 % and 39 % of the limit loads for BFRP 5#9, BFRP 3#9 and BFRP 3#7, respectively. These values corresponds well with the values obtained for RC elements with other types of FRP reinforcement [6,17,18].

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4. Numerical simulation

4.1. FE model of the beams

The finite element (FE) model of considered beams was implemented in ABAQUS environment [19]. The analysis was performed on 2D model and the following assumptions were adopted:

x concrete damage plasticity (CDP) model of concrete [20] was assumed, x tension stiffening effect was taken into account,

x BFRP reinforcement was assumed as a linear elastic isotropic material (Fig. 4a),

x steel reinforcement (grade B500SP) was assumed as a linear elastic-plastic material with isotropic hardening (Fig. 4b),

Fig. 4. The stress-strain relationship of BFRP (a) and steel (b) bars.

x the reinforcement was modelled as 2-node truss elements embedded in 4-node elements of plane stress (Fig. 5).

Fig. 5. Scheme of the 2D FE model.

The numerical model of the beams consisted of two different types of finite element: x T2D2 – 2-node 2D truss elements,

x CPS4R – 4-node plane stress elements with reduced integration.

The concrete was modelled as concrete damage plasticity material, which is based on the brittle-plastic degradation model [21]. For concrete under uniaxial compression, the stress-strain curve shown in Fig. 6a was adopted.

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Fig. 6. Response of concrete to uniaxial loading in compression (a) and in tension (b).

The tension stiffening effect was taken into account by applying a modified Wang&Hsu [22] formula to describe the behaviour of concrete under tension (Eq. 1, Fig. 6b):

°

¿

°

¾

½

!

¸¸

¹

· ¨¨

©

§

d

cr t n t cr ctm t cr t t c t

f

E

H

V

H

V

,

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where Ec is the modulus of elasticity of concrete, εt is the tensile strain of concrete, εcr is the tensile strain at

concrete cracking, fctm is the average tensile strength of concrete and n is the rate of weakening (n=0.50).

4.2. FE results

Fig. 7 shows the numerical and experimental load-deflection curves for beam BFRP 3#7 and BFRP 5#9. The results of the numerical analysis correspond well with the results obtained in the experiments.

Fig. 7. Experimental (EXP) and numerical (FEM) load-midspan deflection curves.

5. Conclusions

This paper presents the results of a numerical and experimental study of the flexural behaviour of BFRP RC beams. Based on these results, the following conclusions may be drawn:

x The reinforcement ratio has a significant effect on the flexural behaviour of BFRP RC beams. An increase in the reinforcement ratio results in an increase in the ultimate loads and in the stiffness of the beams.

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x There are two failure modes of BFRP RC members. When the reinforcement ratio ρf is greater than the balanced

reinforcement ratio ρfb (according to ACI 440.1R-06), the beams fail by concrete crushing. This type of failure

is not sudden – behavior of the beam exhibits some ductility. However, when the reinforcement ratio ρf is lower

than the balanced reinforcement ratio ρfb (according to ACI 440.1R-06), the beams fail suddenly due to

reinforcement rupture.

x Due to the mechanical properties of BFRP bars, the beams behave almost linearly until failure, which takes place at relatively large deflections.

x Design of the beams is governed by the serviceability limit states.

x FRP bars are very sensitive to damage during assembly. Special care should be taken to avoid unexpected damage to this type of reinforcement.

x There is good agreement between the experimental and numerical results. In the future numerical calculation may become a good alternative to laboratory tests.

Acknowledgements

The laboratory tests were partially supported by the Ministry of Science and Higher Education under doctoral grant 01/11/DSMK/0291. The tests specimens were donated by Depenbrock Polska Sp. z o.o. Sp. k.

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