The Properties of Steel and Concrete at High Temperatures - Coursework Example

Name : xxxxxx Tutor : xxxxxxx Title : HAMAN, BMF Institution : xxxxxxx @2016 Abstract This paper details the properties of steel and concrete at high temperatures. It entails all experimental findings of both standard concrete and structural concrete under high temperature conditions such as those in fires. The paper also details the role of fire in inducing structural collapse. Several examples to validate this concept is also given in the report. Introduction Concrete is a construction component that is mostly used in a standard combination with steel reinforcements. Such a concrete is referred to as reinforced concrete. When a building is subjected to high temperatures such as in case of fires, the concrete and steel undergo some changes in terms of their mechanical properties. Such mechanical changes are the cause of structural failure of the building under construction. Several tests have been done experimentally by scholars and experts to determine the exact mechanical property changes of steel and concrete at elevated temperatures. Such experiments are performed at controlled conditions and the results is the exact determination of mode of failure of concrete and structural steel on exposure to fires3. Concrete used in structural members should satisfy given fire safety standards specified in the various building codes in use. Therefore, fire safety of either steel or concrete structures in a building are determined in terms time of resistance to fires in regards to stability, structural integrity as well as temperature transmission. Various buildings have been documented to have collapsed as a result of fire and the exact description of the failure have been determined. Thus, fire response to concrete structures as well as steel structure or reinforced concrete structures are dependent of the mechanical or thermal properties of concrete or steel2. These properties are dynamic and varies with temperature as well as the composition of the concrete mix. Properties of concrete at high temperetures The properties of concrete on grounds of its mechanical changes at high temperatures can be viewed expressed by determine properties such as; Compressive strength. Tensile strength. Modulus of elasticity. Stress-strain response The magnitude of such properties mentioned above in concrete at high temperatures determines the mechanical properties of concrete at such conditions. Compressive strength Design of fire resistance for concrete highly depends on the compressive strength of concrete at elevated temperatures. Such increased temperatures are used to represent situations or conditions of fire. At room temperature, the magnitude of compressive strength for concrete highly depends on water-cement-water ratio, size of the aggregates, types of stress subjected to the concrete as well as the curing conditions. The effect of compressive strength on concrete in elevated temperature conditions is an area that is well researched and experimented. It is however consistently documented that during elevated temperatures, there is a significant loss in compressive strength3. Figure 1: Variation of compressive strength with change in tempereture Tensile strength Concrete is typically weak in tension as compared to its compressive strength. Therefore, concrete has increased ease of propagating cracks while under tensile loads. Concrete is always very weak in tension and its tensile strength is found to be only 10% of its compressive strength when used as a unitary structural member. Thus in calculation of strengths of concrete both at room and elevated temperatures, tensile strength is normally neglected. However, it is an important property because tensile strength in concrete is known to propagate cracks and results in ultimate damage of the structural member under consideration2. Figure 2: Variation of tensile strength with change in tempereture Modulus of elasticity Modulus of elasticity of concrete has been found to undergo a drastic reduction with increase in temperature. This is attributed to the fact that, at high temperatures, the cement products in concrete that are hydrated undergo bond breakage within the structure of cement reduce elasticity. However, reduction in modulus of elasticity as a result of rise in temperature depends on aggregate of concrete mix5. Figure 3: Variation in elastic modulus of concrete with change in tempereture. Stress – strain response The stress-strains common in concrete at increased temperatures such as during fires results in deformations in concrete exposed to fire. Therefore, increased temperatures results in increased stress and strain effect resulting in collapse as a result of deformation4. Typically, due to reduction in compressive strength as well as rise in ductility of concrete during fires, the slope of the stress strain diagram for concrete will assume the one shown below. Figure 4: Stress-strain response of normal strength concrete at elevated temperatures. Properties of structural steel at increased temperatures Compressive strength The elevated temperatures in steel results in drastic reduction in compressive strength. This is attributed to the increased molecular deformation in the metal as a result of high temperatures. Tensile strengths The tensile strength of steel at elevated temperatures increases up to a certain limit before it undergoes plastic deformation. The behavior of steel at elevated temperature can be graphically represented and its phases of transformation represented in a standard graph2. Modulus of elasticity The modulus of elasticity of steel drastically increased with an increase in temperature. Thus, at elevated temperatures, there is increased tensile strength that consequently result in increased modulus of elasticity1. Ductility Ductility of steel is known to increase when it is subjected to elevated temperatures. Ductility is a property associated with the molecular state of the metal and is highly influenced by temperature. Role of fires in inducing structural collapse The exact mechanism of structural collapse as a result of fire is under a lot of contention. Structural collapse due to elevated temperatures in normally referred to as spalling. However, the most agreed cause of collapse in concrete member structures is attributed to the low permeability of concrete as well as the act of moisture migration of moisture in concrete as a result of high temperatures. Two theories have been formulated to explain the role of fire in inducing collapse of structures3. i) Pressure build up theory During heating as a result of elevated temperatures, there is a pore pressure build up. There is generation of very high water vapour pressure generated at such high temperatures and since such vapour cannot escape, it is likely to generate an effective pore pressure that surpasses the tensile strength of concrete. Thus, this results in junks of concrete falling from the concrete member of the structure2. The pore pressure is known to progressively implement this effect until total failure or collapse of the building results. Depending on the fire as well as concrete characteristics, the falling off of concrete junks may be explosive. ii) Restrained thermal dilation This theory considers spalling as a result of restrained thermal dilation on the surfaces close to the hot surface. This normally results in development of compressive stresses in a manner that is parallel to the heated surface. The compressive stresses ultimately lead to brittle fracture of concrete. Thus, it eventually results in structure failure or collapse.2 Spalling has been found to be a major occurrence in both high strength as well as low strength concrete. However, high strength concrete has been found to be at a risk of collapsing than low strength concrete. An example of fire induced collapse shown in the figure below is believed to be of the explosive nature. Figure 5: Relative spalling in NSC and HSC columns under fire conditions. Conclusion Concrete at elevated temperatures undergo various changes that makes it unfit for structural use. It destroys the integrity as well as stability of the structure under consideration. Elevated temperature is known to be one major reasons for collapse of buildings and should be put into consideration during design. Design for high temperature resistance would enable a building to survive a given level of elated temperatures. References 1. CREMASCO, M. (2012). Analysis of the effects of anti-icing agents on the mechanical properties of concrete. Waterloo, Ont, University of Waterloo. 2. INTERNATIONAL CONFERENCE ON MECHANICS AND PHYSICS OF CREEP, SHRINKAGE, AND DURABILITY OF CONCRETE AND CONCRETE STRUCTURES, HELLMICH, C., PICHLER, B., & KOLLEGGER, J. (2015).CONCREEP 10: Mechanics and Physics of Creep, Shrinkage, and Durability of Concrete and Concrete Structures : Proceedings of the 10th International Conference on Creep, Shrinkage, and Durability of Concrete and Concrete Structures, September 21-23, 2015 Vienna, Austria. http://ascelibrary.org/doi/pdf/10.1061/9780784479346. 3. LI, G., & WANG, P. (2013). Advanced analysis and design for fire safety of steel structures. Berlin, Springer. http://dx.doi.org/10.1007/978-3-642-34393-3. 4. HARVEY, P. D. (1982). Engineering properties of steel. Metals Park, Ohio, American Society for Metals. 5. CANADIAN INSTITUTE OF STEEL CONSTRUCTION. (1973). Properties of structural steel sections and selected data. [Willowdale, Ont.], Canadian Institute of Steel Construction. 6. AMERICAN INSTITUTE OF STEEL CONSTRUCTION. (2001). Manual of steel construction. [Chicago, Ill.?], American Institute of Steel Construction. Read More