28-12-2012, 05:05 PM
The Effects of Fire onConcrete and Steel
The Effects of Fire on Concrete and Steel.pptx (Size: 1.63 MB / Downloads: 108)
Introduction
Concrete can be exposed to elevated temperatures during fire or when it is close to furnaces and reactors.
The mechanical properties of concrete,such as strength, elastic modulus and volume deformation, decrease remarkably upon heating resulting in a decrease in the structural quality of concrete.
High temperature is one of the most important physical deterioration processes that influence the durability of concrete structures and may result in undesirable structural failures.
Therefore, preventative measures such as choosing the right materials should be taken to minimize the harmful effects of high temperature on concrete.
The high temperature behavior of concrete is greatly affected by material properties, such as the properties of the aggregate, the cement paste and the aggregate- cement paste bond, as well as the thermal compatibility between the aggregate and cement paste.
BENEFITS OF USING CONCRETE
Concrete is non-combustible (i.e. it does not burn).
Concrete is inherently fire resistant (i.e. it does not support the spread of fire).
Concrete has a slow rate of heat transfer (making it an effective fire shield).
Concrete does not produce any smoke, toxic gases or emissions in a fire situation.
Concrete does not contribute to the fire load of a building.
Under typical fire conditions, concrete retains most of its strength..
Skillful mix design further refines concrete’s inherent performance.
For the vast majority of applications, concrete does not require any additional,
costly fireproofing measures.
Fire damage to concrete is typically minimal, requiring only a minor clean up.
Concrete has been given the highest possible material classification for its fire resistance.
Bespoke concrete mixes can be designed to cater for extreme fire loads.
Discoloration
Permanent colour changes may accompany dehydration of cement paste.
As the temperture increases the colour of siliceious or limestone aggregate concrete changes through pink or red to gray at about 600C to buff at about 900 C
Proper evaluation of steels subjected to fire typically requires estimating the temperature and duration of the fire.
For clean unpainted steel, a yellowish brown color indicates a temperature of 460-480ºF while a blue color indicates a temperature of 600-640ºF.
Weightloss
the weight loss of the concrete specimens resulting from elevated temperature.
As seen in Fig.the unit weights decrease with increasing temperature. Weight reduction takes place in the specimens due to the release of water. Because of the release of bound water from the cement paste, air voids are formed in the concrete. The structural integrity of the specimens deteriorates as confirmed by the increase in weight reduction with increased temperature.
The reduction in weight confirms the loss of mass by the concrete material and the increase in the proportion of air voids .
The average weight loss of the nine samples was 4 % at 400 1C, 6% at 600 1C and 8% at 800 1C.
Thermal Expansion of concrete
Thermal expansion is not a linear function of temperature, but increases with increasing temperature.
The expanded shale lightweight concrete with natural sand replacement of some of the fine aggregate has the most nearly linear and lowest expansion-versus-temperature relationship over the temperature rangeof 70 to 1600 F (21 to 871 C).
In structures where expansion of the heated concrete is resisted by surrounding elements, thermal expansion information is indispensable in designing for fire.