PSI - Issue 14

D. Anupama Krishna et al. / Procedia Structural Integrity 14 (2019) 384–394 A. Krishna et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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moisture and porosity. Exposure to elevated temperatures affects the mechanical and physical properties of concrete. Fire is one of the natural hazards that attack the building constructions. Subjecting concrete to a higher temperature (e. g., due to accidental fire etc.) leads to severe deterioration and it undergoes a number of transformations and reactions, thereby causing progressive breakdown of cement gel structure, reduced durability, increased tendency of drying shrinkage, structural cracking and associated aggregate color changes Yang(2018)].The most important effects of elevated temperature on concrete are: the dehydration of the cement paste, an increase in porosity, modifications in the moisture content, thermal expansion, alteration of pore pressure, loss of strength, thermal cracking due to incompatibility, thermal creep and thermal spalling due to excessive pore pressure. Water distribution and transport, whether in a gaseous or liquid form, play important roles in the local damage to concrete structures [Bazant et al. (1999)]. During heating, the endo-thermal nature of vaporization creates locally high thermal gradients and high vapour pressure, which can lead to tensile stresses exceeding the concrete’s strength [Noumowe and Debicki (2002)]. Since the early studies on heat exposed concrete in the past century reinforced concrete structures showed good performance in fire due to physical features of cement based material such as incombustibility and low thermal diffusivity. This latter property allows the external heat damaged layers to protect the inner core from attaining too high temperature, even in the case of long fire duration. On the other hand, concrete mechanical properties are significantly influenced by high temperature [Bamonte and Monte (2015)]. The behaviour of a concrete structural member exposed to fire is dependent, in part, on thermal, mechanical, and deformation properties of concrete of which the member is composed. Similar to other materials the thermo-physical, mechanical, and deformation properties of concrete change substantially within the temperature range associated with building fires. These properties vary as a function of temperature and depend on the composition and characteristics of concrete. The properties of High Strength Concrete (HSC) vary differently with temperature than those of Normal Strength Concrete (NSC). This variation is more pronounced for mechanical properties, which are affected by strength, moisture content, density, heating rate, amount of silica fume, and porosity [Kodur (2014)].The modes of concrete failure under fire exposure vary according to nature of fire, loading system, and types of structure. Moreover the failure could happen due to different reasons such as a reduction of bending or tensile strength, loss of shear or torsional strength, loss of compressive strength etc. [Bikhiet etal. (2014)] The present study aims to investigate the effect of elevated temperatures on the mechanical properties of normal strength concrete and high strength concrete. The effect of water cooling and air cooling on the mechanical behaviour of concrete is also a part of the work. The mechanical properties that are of primary interest in fire resistance design are compressive strength, tensile strength, elastic modulus, and stress-strain response of concrete under compression. To study these, cubes and cylinders of standard size have been casted and subjected to elevated temperatures and the properties evaluated for different regimes of temperature. The variation of the values with temperature thus obtained has been compared with the variation of values in the Eurocode (EN 1991-1-2-2004), ASCE (American Society of Civil Engineers) 1992 and Published literature. Nomenclature

Average temperature in the furnace ( o C) Time after the start of the fire (min)

T

t

Characteristic compressive strength of concrete at ambient temperature Characteristic compressive strength of concrete at a temperature of T C o

f c '

f cT '

Tensile strength of concrete at ambient temperature Tensile strength of concrete at a temperature of T C o

r c f rT c f

Reduction coefficient of characteristic tensile strength of concrete at a temperature of T C o

K T c t ,

Concrete Compressive stress at elevated temperature Ultimate Strain for concrete at ambient temperature Strain at maximum stress of concrete at elevated temperature Strain at maximum stress of confined concrete at elevated temperature

CT  cu 

max  oTc  c E crT E

Initial Modulus of Elasticity at ambient temperature Initial Modulus of Elasticity at a temperature of T C o