21-07-2012, 11:33 AM
natural environmental action
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Introduction
In the glorious past all the structures were built out of stone. As the time passed by, cement mortar was extensively used. A revolution took place in 1864 and the evolution of concrete took place. Concrete occupies a leading position in modern construction.
All buildings are subjected to show natural environmental action like wind, rain, snow and also biological growths. On the other hand, there are comparatively short duration actions like cyclone, flood, earth quake, fire etc. Such actions lead to damage, delay in appropriate remedial action with respect to deterioration would also lead to damage.
Fire and Concrete
The behavior of concrete in fire depends on its mix proportion and constituents and is determined by complex physiochemical transformations during heating.
In addressing fire the objective may be two fold. One is to consider structural scheme and material that with stand fire in different degrees. The intent is to delay catastrophe, by protective measures and/or introducing redundancy in structural system. The other objective is to worn on appropriate way to under taken restoration (or) retrofit of fire demand structure.
In the world of construction, fire is definitely a danger that has to be prevented and fought all possible means. Although the probability is low, fire may occur anywhere in any season and in any phase in the life time of the building.
Some of the circumstances in which the concrete experiences high temperatures are:
1. Furnaces used in the factories - 1700oC,
2. Concrete structure accidentally set on fire - 1000oC
3. Underground structures used for the disposal of nuclear wastes – 1350oC
4. Rocket launching pads – 2500oC
So both NSC and HSC are subjected to elevated temperatures, hence their strength and
deformation characteristics are to be clearly understood in order to assess the structural safety of structure.
Factors affecting concrete at elevated temperatures:
a) Effect of exposure time:
There is a variation of % residual compressive strength with temp for the different exposure time i.e., 60 and 120 minutes. It may be seen that the effect of exposure time was more pronounced. It can also be seen that the effect of exposure time was more noticeable at high temperatures (4000C-5000C).
b) Effect of cooling method:
There is a variation of residual compressive strength with temp and the method of cooling. It may be set that, in general, the method of cooling did not have a significant influence on residual compressive strength, particularly below 2000C. However the water cooled specimens exhibit slightly lower strength values than the air cooled once especially at temp above 2000C.
c) Effect of water cement ratio:
This reported that effect of temperature on compressive strength of concrete is independent of water cement ratio within the range normally used. Concrete with strengths more than 55N/mm2 are more susceptible to spalling and many result in low fire performance. In case of unstressed tests is no considerable loss in the compressive strength due to variations in w/c ratio. In case of stressed specimens the loss in the compressive strength is above 10-20% for w/c ratio 0.33 and the loss is about 30% for w/c ratio 0.57.
d) Effect of type aggregate:
Different type of aggregate influences the strength –temp characteristics. One of the important factors influencing compressive strength of concrete when heated is the type of aggregate used.
Many commonly used aggregate breakdown physically and/or chemically when heated to certain temp. It has been observed that if aggregate cement paste bond fail chemically on heating (or) as a result of thermal in compatibility between the aggregate and cement paste. The concrete will exhibit a significant reduction in strength.
Behavior of concrete at elevated temperatures:
Many parameters are responsible for the complex behavior of the concrete. The parameters that are responsible for the complex behavior of the concrete are:
a) Spalling of concrete:
Strength loss and spalling are more under loading conditions during fire. Carbonate aggregate (lime stone) provides higher fire resistance and better spalling resistance than siliceous aggregates during fire. Carbonate aggregate (lime stone) provides higher fire resistance and better spalling resistance than siliceous aggregates (quartz). By employing closer tie spacing and close ties, the fire resistance can be increased.