Study on Exposure of High Temperature on Performance of Concrete

Abstract

Concrete is a heterogeneous material with relatively inert aggregate that is held together by hydrated Portland cement paste. When concrete is exposed to high temperatures, changes in mechanical & physical properties occur. Concrete at elevated temperature is sensitive to the temperature level, heating rate, thermal cycling and temperature dura- tion. Changes in mechanical & physical properties may result in undesirable structural failure. Therefore the properties of concrete retained after a fire are of still importance for determining the load carrying capacity and for reinstating fire-damaged constructions procedure. Experimental investigation is conducted on four different types of concrete mixes namely M25(PCC), M25(FRC), M60(PCC) & M60(FRC) exposed to different el- evated temperatures namely 300C, 500C, 700C & 900C, respectively. Focus of this study is to evaluate mechanical properties & physical properties of concrete mixes ex- posed to different elevated temperatures. Mechanical properties taken into considerations are compressive strength, split tensile strength, exural strength, Modulus of Elasticity & bond strength etc. Physical properties incorporated in this study are spalling effect, Cracking, weight loss etc. Hooked steel fibres have been incorporated in fibre reinforced concrete mixes namely M25(FRC) & M60(FRC). Plain concrete specimens cast from dif- ferent mixes have been water cured for duration of 28 days. After curing, specimens have been exposed to different elevated temperatures at 300C, 500C, 700C & 900C, respectively. Specimens have been exposed to different temperature ranges for duration of 1 hour at target temperature as per the experimental time-temperature curve. Ex- perimental time-temperature curve has been derived from IS:3809(1979) standard codal time-temperature curve. After temperature exposure, specimens have been allowed to cool for 24 hours duration at room temperature till steady state condition is achieved. After completion of cooling period, destructive testing has been carried out. Experimental investigation demonstrates that there is a minor damaging effect in terms of spalling for M25(PCC) & M25(FRC) at 300 C & 500C, respectively. Damaging effect in terms of spalling is found to be higher in M25(FRC) as compared to M25(PCC) ex- posed at 700C & 900C, respectively. This might be due to the expansion of steel fibres which causes debonding of concrete from steel fibres. From investigation, it is found that M60(PCC) and M60(PCC) are prone to damage in terms of spalling even at 300C & 500C, respectively. Weight loss & Crack initiation in M60(PCC) mix is found to be higher as compared to M60(FRC) mix for each temperature range, respectively. Visual observation shows that M60(PCC) & M60(FRC) mixes are more vulnerable to damage as compared to M25(PCC) & M25(FRC) exposed to each temperature range. Experimental inspection shows that incorporation of steel fibres in M25(FRC) & M60(FRC) in unheated condition are found to be enhancing mechanical properties. Percentage loss in mechanical properties for M25(PCC) & M60(PCC) are higher for each temperature range, respectively. Percentage loss in mechanical properties for M25(FRC) & M60(FRC) are lower as compared to M25(PCC) & M60(PCC) mixes for each temperature range, re- spectively. Steel fibre incorporation is found to be mitigating strength loss in mechanical properties, such as compressive strength, split tensile strength, exural strength & modu- lus of elasticity, for M25(FRC) & M60(FRC) for each temperature range, respectively. It has been found from test result that there is no any beneficial effect of steel fibre on resid- ual bond strength for M25(FRC) & M60(FRC) for each temperature range, respectively.Destructive failure pattern of M25(PCC) & M60(PCC) mixes are found to be sudden while for M25(FRC) & M60(FRC) it is gradual, which is due to steel fibre incorporation. Additionally, behaviour of RC columns have been studied exposed to 900 C. RC columns from 4 mixes have been cast having dimension of 150 150 1000 mm with equal amount of reinforcement. In each mix of columns, average result of two columns have been con- sidered as final result. Mechanical properties of RC columns such as ultimate failure load, defection, stress-strain, failure modes & cracking patterns have been included in this study. Test results demonstrate that in unheated condition, steel fibre incorporation in RC columns[M25(FRC) & M60(FRC)] enhances ultimate load carrying capacity in mi- nor amount. Incorporation of steel fibre enhances displacement ductility for M25(FRC) & M60(FRC) columns in unheated and heated conditions, respectively. Plain concrete columns[M25(PCC) & M60(PCC)] showed sudden failure while that of fibre reinforced columns[M25(FRC) & M60(FRC)] showed gradual failure after exposed to 900C. Fibre reinforced columns having grade M25(FRC) & M60(FRC) undergo large vertical and ax- ial deformation before failure. Incorporation of steel fibre in M25(FRC) & M60(FRC) is found to reducing crack development and arresting the crack propagation.