Behavior Of Concrete Exposed To Fire Literature Review Examples

Background:

Concrete slabs used in the many construction sites are evaluated for their ability to resist fire. The fast paced advances in the structural engineering and design with regards to fire safety induced the need for a more precise and accurate information and theoretical background for concrete behaviors during fires [1]. The many cases of fire on concretes reported was found out to be due to the thermal instability of concrete structures when subjected to accidental fire called “spalling”. Spalling is the phenomenon where concrete is ejected during fire thereby resulting to the complete reduction of the cross-section resistance and the bearing capacity of the concrete structures. The cost for repairs and immobilization for concretes that has undergone spalling is very high. Concrete Spalling risk assessment is done through experiments since it is difficult to point out theoretical knowledge on the physical structural origins of concrete spalling [2]. Spalling covers various damage phenomena that occur in various layers of the concrete structures when exposed to fire. Mostly, it is due to combination of various factors such as the pore pressure and the thermal stresses. It is also influenced by factors such as the cracking in the concrete and around the reinforcements, the internal micro-cracking, temperature gradients, and the chemical transitions affecting loss of strength [3]. Concrete spalling reduces the fire resistance of concrete when subjected to high temperatures. The most dangerous type of spalling is the explosive spalling which happens under conditions of high explosive energy. When this happens, the concrete shrapnel fly at high speeds causing grave damages to the environment and allowing more oxygen to increase the fire severity. The heating of one side of the concrete creates two fronts: the moisture front and the heat front. These fronts move away from the heated concrete face turning towards the unheated side. The movement of the fronts at certain speeds cause the water clogged as concrete moisture turn into vapor. This process of transformation builds up the increase of vapour pressure that may reach a point where the concrete explodes. Moreover, the steel reinforcements found in the concrete structures impede moisture movement that many allow development of pore pressure [4]. In fire exposure of concrete slabs, there is movement in the axial expansion of the slab that pushes the external wall panels load at the junctions of walls and floors. This phenomenon causes conditions of eccentric loading which gradually results to its collapse [5].

Objectives:

Researches for concrete spalling risks and the various experiments thereof are important in understanding the behavior of concretes when subjected to fire. Results of various experiments contribute to the understanding of concrete behaviors when exposed to fire. Specifically, this critical analysis of literatures for concrete slabs behavior in fire aims to:

Determine the process of spalling and explosive spalling phenomenon happening to concrete slabs.

Identify the different parameters involved and changed when concrete slabs are subject to fire.
Compare various experimental results in the determination of concrete slabs behavior
Determine the influencing factors in the concrete structures failures when exposed to heat and fire
Methodology:
The understanding of the behavior of concrete slabs when exposed to fire is done through the analysis of the various experiments. The results of the various experiments are collected for the assessment of the concrete slabs behavior under the conditions of extreme heat such as fire. Mindeguia [2] performed an experiments where the structural models are subjected to various mechanisms such as thermomechanical process and the hygro-thermal process. Results of the experiments are assessed to determine the major characteristics of concrete when subjected to high temperatures. The hygro-thermal behavior of the concrete when exposed to unidirectional heating is also determined. The observed results from the experiment contribute to the explanation of the behaviors of concrete when exposed to fire [3]. An experiment was also done by Ali [4] where concrete slabs were placed on top of combustion chambers with loads applied to them attaché with measuring devices to produce results that would help in the assessment of the behavior of concrete in fire. Another experiment was done where of a series of furnace tests with concrete slabs is used to evaluate the deflection of concrete slabs at the mid-span and axial aspects when they are exposed to fire[5]. A big fire test examined by Huang [2] was also conducted where a seven story concrete building was subject to fire. Along the experiment, the concrete slabs with various were analyzed. A model that takes into account the behavior of the concrete under fire conditions was formulated by the author.

Findings/ Result:

The experimental results contribute to the analysis of concrete structural mechanisms and their behaviors when exposed to fire. Assessment can be done through the concrete properties that are dependent on the temperature such as the thermal expansion and the as permeability. There are also assessments done based on the types of with variables included such as the pore gas pressure fields and the temperature. Expectedly, there is a higher amount of spalling to more compact concretes. The gas pore pressures for compact concretes are also higher when exposed to relatively slow heating. The gas pore pressure is observed to be greatly reduced when concretes consists of pp fibres. Furthermore, there is a decrease in the concrete gas pore pressure as the severity of the thermal loads is increased. This observation explains the strong link between the material damage due to fire and the hygrothermal transfers due to gas pore pressure. Low gas pressure leads to relatively low concrete spalling [2]. In an experimentation were concrete slabs were placed on top of combustion chambers with loads applied to them produced results that helps in the assessment of the behavior of concrete in fire. Based on the results, it is observed that the increase in fire severity normally results to explosive spalling. Reinforced concrete slabs with normal strength exhibits larger thermal gradients between their surface, slab core, and the steel reinforcements. Increasing the severity of fire causes rapid deflection rate to the concrete slabs. Also, the increase in the hydrocarbon fire exposure of slabs results to higher degree of explosive spalling [4].

Discussion:

Structural end point – This is the point when the concrete slab specimen failed to support the design load
Flame Passage end point – This is the point when the specimen incur cracks and fissures
Heat Transmission end point – This is the point when the unexposed surface temperatures of the specimen rise above the ambient average temperature of 250oF or 325 oF at any point in the specimen.
The heat transmission that occurs in the concrete reveals the behavior of the concrete slab. This transmission is affected by the following factors: the type of aggregate, the concrete thickness, and the concrete’s condition of moisture. The figures 1 and 2 display the relationship of the concrete thickness and fire endurance for both the lightweight and the normal weight concretes, and the effect of the type of aggregates on the concrete slabs, respectively.
Figure 1 Figure 2
The behaviors of the concrete slabs when subjected to fire and high temperature are dependent on the other elements of the concrete structure. The concretes that are prestressed and reinforced are affected by the tensile stress resistance of steel and the compressive strength of the entire concrete [1]. It follows then that the behavior of the concrete slabs and beams when exposed to fire depends on the steel’s tensile strength and the concrete’s compressive strength. This is true with the experimentation done by Ali [4] where the reinforced concrete slabs with normal strength exhibits larger thermal gradients between the steel reinforcements, the slab core, and the slab surfaces. Mindeguia [3] also reveals that in compact concretes, the amount of spalling and the gas pore pressures are higher when exposed to the heat. Moreover, the presence of the aggregates in the concrete slab structure also affects the behavior of the concrete exposed to fire. Concrete slabs that are made of lightweight or carbonate aggregates retain around 75% of their strength at high temperatures while concrete of siliceous aggregates retain only half of their strength at high temperatures of around 1000oF [1]. An experiment of a series of furnace tests with concrete slabs is used to evaluate the deflection of concrete slabs at the mid-span and axial aspects when they are exposed to fire. It was found out that there is a higher rate of mid-span deflection on the slabs when exposed to hydrocarbon fire. However, the deflection can be reduced by the employment of coarse aggregate on lightweight concretes. The lowering of about 30% on the deflection rate is due to the lower thermal conductivity and lower thermal expansion coefficients. Furthermore, there is a recommended addition of soffit protection for the reduction of mid-span deflection on the concrete slabs. Based on the experiments, the slabs with the soffit protection experienced smaller deflection rate. There was about 25% lesser rates of deflection compared to the unprotected slabs [5].
Figure 3: Effect of Soffit Protection
The behavior of concrete when subjected to fire is dependent on the elements and support of the concrete structure. It should also be noted that concrete beams must be properly supported where structural design must emphasize on the load intensity capacity, the cover thickness, the reinforcement design, and the concrete type. In the presence of fire, these factors contribute to the failure and endurance of the concrete [1]. In the study done by Huang [2], it is concluded that there is a very significant effect of the spalling of concrete on the concrete’s thermal behavior. The spalling of the concrete are dependent on the concrete structures and the degree of thermal restraint on the concrete slabs. It is seen that force of the compressive membrane educes concrete spalling impact on restrained concrete slabs such as the floors [2]. Furthermore, the heat transmission throughout the concrete governs the restrained concretes. However, the factors affecting their strength and failure under high temperature includes concrete thickness, aggregate type in the concrete, and the moisture condition of the concrete [1].

References:

Gustaffero, A. “Factors Influencing the Fire Resistance of concrete”. Fire Technology. (1996):
187-195
Mindeguia, JC., Pimineta, P. Carre, H., and La Borderie, C. “Experimental Analysis of
Concrete Spalling Due to Fire Exposure”. European Journal of Environmental and civil
Engineering, 17[6] (2013): 453-466.
Huang, A. “The behavior of reinforced concrete slabs in fire”. Fire Safety Journal. 45(2010):
271-282
Ali, F., Nadjai, A., Abu-Tair, A. “Explosive spalling of normal strength concrete slabs subjected
Cooke , G. “Behavior of precast concrete floor slabs exposed to standardized fires”. Fire
Safety Journal 36 (2001): 459-475