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The large temperature variation has a harmful effect on concrete structures reinforced with fiber reinforced polymer (FRP) bars. This is due to the significant difference between transverse coefficient of thermal expansion of these bars and that of the hardened concrete. This difference generates a radial pressure at the

The use of non-metallic fiber reinforced polymer (FRP) reinforcement as an alternative to steel reinforcement in concrete structures, particularly in hostile and aggressive environments, is gaining acceptance mainly due to its high corrosion resistance, and its high mechanical performance [_{ct}). These thermal cracks may cause the degradation of the bond between GFRP bars and the surrounding concrete, and eventually, failure of the concrete cover if the confining action of concrete is not sufficient [

Although, temperature effects on mechanical properties of FRP are recognized by all documents reviewed, no guidance is given for the design of FRP reinforced concrete structures under temperatures variation effects [

This paper presents experimental and analytical studies of the effective thermo-structural behavior of one-way GFRP-reinforced concrete slabs having different concrete cover thicknesses. Slabs were subjected simultaneously to service loads namely, a sustained mechanical load of 20% of the flexural ultimate load of slabs and a temperature variation from −30 to 60 °C, which represents, generally, the global temperature variation in many countries (North American, Arabian Gulf countries,

A total of six series of concrete slab specimens reinforced with GFRP bars were fabricated for this experimental program, each series is constituted by three slabs: SA, SB, and SC. The slabs had dimensions of 500 mm wide, 195, 200, and 215 mm thick, 2500 mm total length, and 2000 mm span between supports (_{u}), as shown in

Mechanical properties of concrete used.

Slabs | Concrete cover, |
Bar diameter _{b } |
_{b} |
Longitudinal modulus of elasticity, _{C }(GPa) |
Compression strength (MPa) | Tensile strength (MPa) |
---|---|---|---|---|---|---|

SA195.25.16 | 25 | 15.9 | 1.57 | 26.17 ± 0.4 | 33.83 ± 1 | 1.94 ± 0.04 |

SA200.30.16 | 30 | 15.9 | 1.88 | 24.80 ± 0.8 | 30.39 ± 2 | 2.58 ± 0.04 |

SA215.45.16 | 45 | 15.9 | 2.83 | 26.17 ± 0.4 | 33.83 ± 1 | 1.94 ± 0.04 |

SA195.25.19 | 25 | 19.1 | 1.31 | 26.53 ± 0.3 | 34.77 ± 0.7 | 2.77 ± 0.15 |

SA200.30.19 | 30 | 19.1 | 1.58 | 27.34 ± 0.7 | 36.93 ± 2 | 2.92 ± 0.25 |

SA215.45.19 | 45 | 19.1 | 2.36 | 27.34 ± 0.7 | 36.93 ± 2 | 2.92 ± 0.25 |

Note: SA200.30.16 means: Slab SA, with thickness of 200 mm, reinforced with GFRP bars No. 16, concrete cover thickness

Environmental room including the four point flexural setup, slabs SA and SB.

Slabs were instrumented with strain gauges, thermocouples and LVDTs (linear variable differential transformers) in order to measure deformations, temperatures, and deflections, respectively. Each slab was instrumented by six strain gauges installed on three main bars in both transverse and longitudinal directions at mid-span of slabs. To measure the transverse and longitudinal strains of concrete, three strain gauges were placed on the lower tensioned surface of the slab during thermal tests. Then, three strain gauges were added on the upper compressed surface during the bending test. All strain gauges were installed on the mid-span. To measure the deflection and crack opening, two LVDTs and two sensor cracks were placed at mid-span and at 3/8 of the slab span. Each slab was instrumented by four thermocouples: two thermocouples are installed on two of the main bars, and tow thermocouples are installed on the upper and lower surface of slabs. Another one was attached inside the thermal room to measure the air temperature. All thermocouples were installed on the mid-span.

Concrete slab specimens SA and SB were tested under temperature variations. The temperatures was varied from 20 to −30 °C then from −30 to +60 °C using the thermal room shown in _{u}). The total loads are nearly 40 kN. When the mechanical strains were stabilized, all the strains were recorded and then zeroed in order to present the results in terms of thermal strains alone. The thermal strains are then corrected to take into account the thermal effect on the strain gauges according to the recommendations of the manufacturer. At the end of thermal cycles, all the strains were put to zero, and the three slabs SA, SB, and SC were subjected to four-points bending test up to failure to examine the effect of thermal and combined thermal and mechanical loads on the flexural behavior of slabs. The bending test results were published by Bellakehal

In the same conditions of slabs, six GFRP bars were tested (three bars N°16 and three N°19) under thermal and mechanical combined loads, each one was instrumented by two strain gages in the longitudinal and transverse directions of bar. The applied mechanical load is about 14% of the ultimate load of bars. This load representing the amount of axial load in GFRP bars of reinforcement of slabs SA (

Typical Thermal cycles measured at

Conditioned tensile test setup of GFRP bars.

For each slab, six standard concrete cylinders of 150 mm × 300 mm were cast and cured with water for 28 days at the room temperature under the same conditions as the slab specimens. Cylinders were tested to evaluate the compression and tensile strengths of concrete at 28 days and immediately before slab specimen tests. The tensile strength was determined by the splitting test. The modulus of elasticity is calculated as recommended by the code CAN/CSA-S806-12 [_{c} = 0.17 and α_{c} = 11.7 × 10^{−6}/°C, respectively, as the concrete used is an ordinary concrete. The mechanical properties of concrete are presented in

The mechanical and thermal properties of GFRP bars used in this study are presented in

Mechanical and thermal properties of GFRP reinforcing bars.

Bars diameter _{b} |
15.9 | 19.1 |
---|---|---|

Longitudinal Modulus of elasticity, _{fl} (GPa) |
47.0 ± 0.3 | 52.2 ± 1.2 |

Transverse modulus of elasticity, _{ft} (GPa) |
7.75 | 7.87 |

Poisson’s ratio in the longitudinal direction, ν_{lt} |
0.28 ± 0.005 | 0.28 ± 0.008 |

Poisson's ratio in the transverse direction, ν_{tt} |
0.38 | 0.38 |

Ultimate tensile strength (MPa) | 700 ± 24 | 691 ± 7 |

Guarantee tensile strength (MPa) | 683 | 656 |

Ultimate tensile strain (%) | 1.50 ± 0.06 | 1.33 ± 0.03 |

TCTE* (α_{ft}) [×10^{−6} ]/°C |
27.35 ± 0.35 | 22.45 ± 0.31 |

LCTE‡ (α_{fl}) [×10^{−6}]/°C |
6.81 ± 0.9 | 6.61 ± 0.1 |

Notes: * TCTE: Transverse Coefficient of Thermal Expansion; ‡ LCTE: Longitudinal Coefficient of Thermal Expansion.

_{u}), corresponding to FRP bar diameter N°16 and N°19, respectively.

From these figures, it can be seen that there is no big difference in longitudinal thermal strains of slabs SA and SB, particularly, for temperatures variation between −30 and +40 °C. However, for temperatures greater than 40 °C, the longitudinal strains of slabs SA, subjected to combined thermal and mechanical loads, are in general lower than that of slabs SB (subjected to thermal loads only) particularly for ratios of _{b}

Longitudinal strains of GFRP bars, _{b}

Longitudinal strains of GFRP bars, _{b}

Transverse strains of GFRP bars, _{b}

Transverse strains of GFRP bars, _{b}

Behavior of GFRP imbedded in concrete of the slab SA.200.30.16undercooling-heating cycle and sustained load.

Experimental transverse strains of GFRP bars for concrete slabs SA having different concrete cover and FRP bars diameter.

_{u}), corresponding to FRP bar diameter N°16 and N°19, respectively.

Transverse tensile concrete strains _{b}

Transverse tensile concrete strains _{b}

At the high temperature (60 °C), these Figures show that the thermal strains of slabs SA are generally lower than those of slabs SB. The difference is estimated by 5% to 15%. This is due to the decrease of the radial pressure and consequently the reduction of radial crack propagations through concrete cover from _{u}, on transverse tensile concrete thermal strains, is insignificant.

_{u}). Longitudinal thermal strains were measured only for the series of slabs having _{b}

Longitudinal tensile concrete strains _{b}

It should be noted that no thermal cracks have been observed at the outer surface of concrete cover at the end of experimental tests carried out on slabs under combined thermal and mechanical loads. Therefore, it can be concluded that ratios of concrete cover thickness to FRP bar diameter (_{b}

The analytical model is established to analyze the combined effect of thermal and mechanical loads on the behavior of a concrete cylinder concentrically reinforced with FRP bar. The model studied is based on the following assumptions:

A perfect bond between concrete and FRP bar.

The behavior of concrete and FRP bars is linear elastic.

The cross section of the concrete cylinder remains plane after deformation.

Absence of transverse reinforcing bars to evaluate only the contribution of the concrete cover to support the tensile stresses due to applied loads.

To determine thermal strains and stresses due to radial pressure

Where Δ_{a}_{c}: modulus of elasticity of concrete; _{ft}: modulus of elasticity of FRP bar in the transverse direction; ν_{c}: poisson’s ratio of concrete; ν_{tt}: poisson’s ratio of FRP bar in the transverse direction; _{b}_{b}_{b}

Masmoudi

Where α_{ft} is the transverse coefficient of thermal expansion of FRP bars, and α_{c} is the coefficient of thermal expansion of concrete.

This model has been modified to take into account the axial force in FRP bar due to applied mechanical loads. The circumferential strains in FRP bar (ε_{ft}) and in concrete (ε_{ct}) at the interface of FRP

Where ε_{fl} and ε_{cl} are, respectively, the longitudinal strains in the FRP bar and in the tensile concrete due only to the mechanical loading. The radial pressure _{ft} = ε_{ct}). Equations (3) and (4) give:

The radial stress (σ_{ρ}) and the circumferential stress (σ_{t}) in a concrete element situated at a radius (ρ) from the center of the concrete cylinder due to the radial pressure (

The maximum circumferential stress at the concrete/bar interface (ρ =

Concrete cylinder concentrically reinforced with FRP bar.

_{u}). It should be noted that the reference temperature 23 ± 1 °C.

It is observed that experimental results are widely higher than those obtained from the analytical model. This divergence is probably due to the development of circumferential cracks around FRP bars due to radial tensile stresses developed at low temperature added to those of the shrinkage effect. However, at high temperature, this divergence is due to radial cracks caused by the thermal expansion of FRP bars when the temperature increases. These cracks were not taken into account in the assumptions of the analytical model based on the theory of elasticity.

To determine transverse strains of FRP bars, the analytical model (Equation (3)) was modified to fit the experimental results obtained from FRP-reinforced concrete slabs tested under a temperature variation from −30 to + 60 °C applied simultaneously with a mechanical loading of 20% of the ultimate load of slabs. The proposed model is given by:

Transverse strain of FRP bars of slab SA.30.16—Experimental and analytical results comparison, _{b}

Transverse strain of FRP bars of slab SA.45.16—Experimental and analytical results comparison, _{b}

Eighteen large-scale bars FRP-reinforced concrete slabs were cast and tested. Slabs were divided in six series. Each one was constituted of three slabs. The first slab was subjected to both temperature variations and mechanical loads. The second slab was subjected only to temperature variation. The last one was the control slab and was stored under room temperature. The studied parameters used were concrete cover thickness, FRP bar diameter, and temperatures. The temperature was varied from −30 °C to +60 °C and the applied mechanical load was 20% of slab ultimate load (_{u}). The main objective of this study is to investigate the mechanical load effect on the behavior of GFRP bars reinforced concrete slabs subjected to large temperature ranges (−30 °C to +60 °C). Although, temperature effects have been a most difficult drawback of FRP-reinforced concrete members, there are no studies which investigate these members under large temperature variation simultaneously with mechanical loads, as the manner described in this study. It should be noted that these results are valid for slabs with materials used in this study. Based on the analysis of the analytical and experimental results in terms of thermal strains of concrete and GFRP bars, the following conclusions can be drawn:

The mechanical load effect of 20% of the ultimate load (_{u}) of reinforced concrete slabs has no significant effect on the transverse thermal strains of GFRP bars embedded in concrete under temperature variation from −30°C to +60°C.

At high temperature (>40°C), longitudinal thermal strains at the

The thermo-mechanical behavior of GFRP bars embedded in concrete of actual slabs is linear elastic.

The concrete cover thickness variation has no big effect on transverse thermal strains at

At high temperature, transverse tensile concrete strains at external surface of concrete cover were reduced under mechanical load. This reduction varied from 5% to 15%, for a temperature of +60 °C, due to the decrease of the radial pressure and consequently the reduction of radial crack propagations through concrete cover. While, for the low temperature (−30 °C), the mechanical load of 20% _{u}, has no remarkable effect on transverse tensile concrete strains.

Ratiosof concrete cover thickness to FRP bar diameter (_{b}

The transverse strains, at

The transverse strains, at

Concrete cover thickness

_{b}

FRP bar diameter

_{c}

Modulus of elasticity of concrete

_{ft}

Modulus of elasticity of FRP bar in the transverse direction

_{ct}

Tensile strength

_{u}

Ultimate load

_{c28}

Compressive concrete strength

Radial pressure

ratio of concrete cylinder radius “

_{ft}

Transverse coefficient of thermal expansion of FRP bars

_{c}

Coefficient of thermal expansion of concrete

Temperature variation

_{ft}

Circumferential strains in FRP bar at the interface of

_{ct}

Circumferential strains in concrete at the interface of

_{fl}

Longitudinal strain in the FRP bar due only to the mechanical loading

_{cl}

Longitudinal strain in the tensile concrete due only to the mechanical loading

_{c}

Poisson’s ratio of concrete

_{tt}

Poisson’s ratio of FRP bar in the transverse direction

_{lt}

Poisson’s ratio of FRP bar (force applied in longitudinal direction and strains measured in transverse direction)

Radius from the center of the concrete cylinder

_{ρ}

Radial stress in concrete cover

_{t}

Circumferential stress in concrete cover

_{t, max}

Maximum circumferential stress in concrete cover

Great thanks to the personnel of the Civil Engineering Department of the Sherbrooke University (Canada) for their helps. Also, Special thanks to “

The topic of this study was proposed by Ali Zaidi. The whole work was done by Hizia Bellakehal. The comments and revision were done by Ali Zaidi. Finally, it is noted that this work was supervised by Radhouane Masmoudi and Mohamed Bouhicha.

The authors declare no conflict of interest.