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HSC normally suffers from low stiffness and poor strain capacity after exposure to high temperature. High strength confined fibrous concrete (HSCFC) is being used in industrial structures and other high rise buildings that may be subjected to high temperature during operation or in case of an accidental fire. The proper understanding of the effect of elevated temperature on the stress-strain relationship of HSCFC is necessary for the assessment of structural safety. Further stress-strain model of HSCFC after exposure to high temperature is scarce in literature. Experimental results are used to generate the complete stress-strain curves of HSCFC after exposure to high temperature in compression. The variation in concrete mixes was achieved by varying the types of fibre, volume fraction of fibres, and temperature of exposure from ambient to 800°C. The degree of confinement was kept constant in all the specimens. A comparative assessment of different models on the high strength confined concrete was also conducted at different temperature for the accuracy of proposed model. The proposed empirical stress-strain equations are suitable for both high strength confined concrete and HSCFC after exposure to high temperature in compression. The predictions were found to be in good agreement and well fit with experimental results.

It is well known that the higher load carrying capacity of high strength concrete is normally accompanied by more brittle behaviour; however, this property can be compensated in a rational manner through incorporating the confinements by lateral reinforcement with and without fibres [

Experimental evidence of a total of 105 compressed high strength reinforced concrete circular columns confined by lateral steel with and without fibres after exposure to high temperature was selected to study the residual stress-strain relationship of confined concrete. A summary of this previous experimental work with details of specimens and variables considered is given in Table

Properties of specimens.

Specimens | Exposure | | Longitudinal steel | Transverse steel | Fibre volume fraction | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|

Number & diameter | | Dia. | Spacing | | | SF | PP | | |||

CBH, CB2H, CB3H, CB4H, CB5H, CB6H, CB8H | Ambient, 200, 300, | 71.36 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | | | |

CCH, CC2H, CC3H, CC4H, CC5H, CC6H, CC8H | Ambient, 200, 300, | 71.92 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | 1 | | |

CDH, CD2H, CD3H, CD4H, CD5H, CD6H, CD8H | Ambient, 200, 300, | 71.83 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | | 0.1 | |

CEH, CE2H, CE3H, CE4H, CE5H, CE6H, CE8H | Ambient, 200, 300, | 71.68 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | | | |

CFH, CF2H, CF3H, CF4H, CF5H, CF6H, CF8H | Ambient, 200, 300, | 71.11 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | 1.5 | | |

CGH, CG2H, CG3H, CG4H, CG5H, CG6H, CG8H | Ambient, 200, 300, | 71.18 | 6Nos 8 | 650 | 6 | 42 | 2.26 | 510 | | 0.2 | |

The experimental results of CBH series of high strength confined concrete after exposure to high temperature [

(a) Comparison of experimental and predicted residual stress-strain curves of CBH series specimens. (b) Comparison of experimental and predicted residual stress-strain curves of CCH series specimens. (c) Comparison of experimental and predicted residual stress-strain curves of CDH series specimens. (d) Comparison of experimental and predicted residual stress-strain curves of CEH series specimens. (e) Comparison of experimental and predicted residual stress-strain curves of CFH series specimens. (f) Comparison of experimental and predicted residual stress-strain curves of CGH series specimens.

The existing analytical model of [

The test results of [

The stress-strain model being proposed here has been defined in terms of confined strength,

Expression for peak confined stress:

Expression for peak confined strain:

Expression for strain at 50% of peak confined stress in descending side:

(a) Prediction of peak stress of confined concrete. (b) Prediction of strain in confined concrete at peak stress. (c) Prediction of postpeak strain of confined concrete at 50% of peak stress.

By redefining the different parameters in (

Analytical model for concrete confinement by rectilinear reinforcement at different exposure temperatures proposed by various researchers was studied. All three models were applied to high strength confined concrete specimens of CBH series. However no model provided the most accurate prediction for the stress-strain curve of high strength confined circular column at different temperature. Therefore, a model for representative postfire behaviour of high strength confined concrete and high strength fibre reinforced confined concrete was proposed. The following conclusions were drawn from this work. The models proposed by [

Cross-sectional area of concrete in the column specimen section

Cross-sectional area of core concrete in the column specimen section

Area under the stress-strain curve of confined concrete

Area under the stress-strain curve of unconfined concrete

Gross area of column cross section

Gross sectional area of longitudinal steel

Cross-sectional area of a tie bar

Core diameter centre to centre of parameter tie

Temperature difference between surface and centre of specimen

Young’s modulus of steel bars at ambient temperature

Elastic modulus of steel bars exposed to elevated temperature

Initial tangent modulus of elasticity of confined concrete

Initial tangent modulus of elasticity of heated confined concrete

Any stress value on the stress-strain curve

Standard cylinder compressive strength of plain concrete on the day of testing

Peak stress of confined concrete

Compressive strength of unconfined concrete

Stress at the inflection point of confined concrete stress-strain curve

Nominal average lateral confining pressure acting on the core concrete

Effective lateral confining pressure at peak confined stress

Effective lateral confining pressure acting on the core concrete after being exposed to high temperature

Nominal lateral confining pressure in heated concrete

Yield strength of lateral confining steel

Yield strength of longitudinal reinforcement

Residual yield strength of reinforcing bar after being exposed to high temperature

Confinement effectiveness coefficient

Peak concrete load in the load-strain curve after deducting the contribution of steel

Peak confined concrete load

Maximum total applied load on the confined concrete column specimen

Theoretical concentric capacity of specimen

Gross concrete area force

Core concrete area force

Centre to centre spacing of confining ties

Clear spacing of ties

Exposed temperature (°C)

Any strain value on the stress-strain curve

Axial strain corresponding to

Axial strain corresponding to the stress,

Axial strain at which the load drops to 50% of the peak confined concrete load

Axial strain at which the load drops to 85% of the peak confined concrete load

Axial strain at peak confined load

Strain at peak load of unconfined concrete specimen at ambient temperature

Strain at peak load of unconfined concrete specimen exposed to elevated temperature

Longitudinal reinforcement ratio in the core section

Volumetric ratio of longitudinal steel

Volumetric ratio of transverse reinforcement.

The authors declare that there is no conflict of interests regarding the publication of this paper.