The increased use of high strength concrete in buildings has resulted in concern regarding the behavior of such concrete in fire. In this research, the factors that influence the thermal and structural behavior of high strength concrete columns under fire conditions are discussed. This research paper has three purposes. First, understanding the complex process high strength concrete columns undergo when a definite high temperature is applied. Second, identifying the different effects extremely high temperatures have on the structural material, how that affects the building, and how choosing a different concrete can alter the outcome. Finally, the varied consequences when different ranges of temperatures are applied, and what aggregate to include in the mixture to make the concrete even stronger.
To bring such knowledge and consciousness I decided to organize this paper into three different sections. In the first section, I start by defining what is concrete, the different types of it and their properties, how the use of High Strength Concrete has impacted over the years, and how the addition of admixtures like super plasticizers can help reduce the risk of spalling. In the second section, I discuss the behavior concrete has under different high elevated temperatures, the paper concludes with a third section discussing the importance use of high strength concrete and a list of recommendations
In recent years, the construction industry has shown significant interest in the use of high strength concrete. This is due to the improvements in structural performance, such as high strength and durability, compared to traditional, normal strength concrete. Not until the early 1900’s engineers and technologists became involved in optimizing the strength of concrete though it has been a material that has been used throughout history. “The use of High-Strength Concrete (HSC) has become more commonplace in the building and transportation industry in the United States (US) because of its beneficial economical and material properties” (Myers 1). One of the major uses of HSC is for columns.
Concrete structural members “exhibit good performance under fire situations” (Kodur et al 1). However, studies, which will be explained, show that the performance of those structural members made with high strength concrete is different from those of normal strength concrete, which may not present a good performance during a fire. Measurements need to be taken previous the construction of the columns because of the high risk those structural members have to spall.
“Spalling, which refers to a sudden and violent breaking away of a surface layer concrete” (Phan 2) in other words spalling can create a condition that can weaken the condition of a column creating the possibility of a building to collapse. Compared to normal strength concrete, high strength concrete is believed to be more susceptible to this pressure build-up because of its low permeability; however super plasticizers are commonly added to solve this problem. The use of HSC in high-rise buildings helps achieve more efficient floor plans through smaller vertical members, and has also proven to be the most economical alternative by reducing both the total volume of concrete and the amount of steel required for a load-bearing member. Also, the framework is a large portion of the cost of constructing a column, smaller column sizes reduce the amount of form work needed and result in further cost savings. These advantages balance the fact that high strength concrete does not work well under a high temperature due to the low water-cement ration.
The behavior of concrete when subjected to high temperatures can be a complex phenomenon to understand, that’s why it is necessary to have a clear idea of what is concrete and how is it made. Contrary to popular belief, concrete and cement are not the same thing; cement is usually just a component of concrete. “In its simplest form, concrete is a mixture of three basic components water, aggregate (rock, sand or gravel), and cement, which usually in powder form acts as a binding agent when mixed with the other two ingredients. This mixture will be poured and harden into the durable material with which we are all familiar. Typically, a batch of concrete can be made by using 1-part cement, 2 parts dry sand, 3 parts dry stone, and ½ part water” (Portland Cement Association).
“Depending on the finishes and performance characteristics of the concrete, it divides into four main types: normal strength concrete, high strength concrete, very-high strength concrete, and ultra-high-performance concrete” (Portland Cement Association 2). The previous concretes differ from one another because of their ability to withstand the greatest pressure. “normal strength concrete can withstand a pressure from about 10 MPa to 40 MPa, high strength concrete has a compressive strength until 100 MPa; however, very high strength concrete has a compressive force that ranges from 100-150 Mpa” (Portland Cement Association). “Ultra-high-performance concrete is a rather new type of concrete that is constantly being developed. It is characterized by being steel fiber-reinforced cement material with a compressive strength in excess of 150 MPa, up to and possibly exceeding 250 MPa” (LYONS).
For purposes of this research, my focus would be on high-strength concrete. One of the major uses of high strength concrete in buildings is for columns. “The columns form the main load-bearing components in a building” (Kodur et al 1). “Columns are structural elements that transmit, through compression, the weight of the structure above to other structural elements above” (Cartwright). Since columns are the elements, in a structure, keeping it from collapsing, it is necessary to understand the behavior of those important elements during the occurrence of a fire.
Materials expand or contract when subjected to temperatures. When free to deform, concrete will expand due to fluctuations in temperature. The size of the concrete structure whether it is a bridge, a highway, or a building does not make it immune to the effects of temperature. In a fire, concrete starts experimenting slight changes in its structure as soon as it comes in contact with the high temperatures of the flames.
High strength concrete is produced primarily through use of relatively low water/cement ratio and incorporate silica. “Because this leads to a reduced permeability relative to normal weight concretes, there has been a concern that the HSC may be more susceptible to explosive spalling under fire conditions due to the buildup of pore pressure in the cement paste” (Naus 87). A study conducted by Oak Ridge National Laboratory, states the structural effects of concrete in different temperatures. The material properties of HSC vary differently with temperature than those of NSC. The differences are more pronounced in the temperature range from 25°C to about 400°C, where higher strength concretes have higher rates of strength loss than lower strength concretes. These differences become less significant at temperatures above 400°C. “Compressive strengths of HSC at 800°C decrease to about 30% of the original room temperature strengths” (87).
The variations in structural composition, before spalling, of concrete depending on the temperature can be divided into three main stages— “(1) an initial stage of strength loss (25°C to approximately 100°C), (2) a stage of stabilized strength and recovery (100°C to approximately 400°C), and (3) a stage above 400°C characterized by a monotonic decrease in strength with increase in temperature” (87). HSC has a higher rate of compressive strength loss in the temperature range between 100°C and 400°C compared to NSC. Explosive spalling failure occurs more in HSC than in NSC, because of the silica flume, at temperatures ranging from 300◦C to 650◦C, but this can be solved by adding super plasticizers. As soon as the concrete reaches a temperature of 300◦C it marks the beginning of a high rate of decrease in the strength and structural properties of all concretes (88). In comparison to other type of concrete, HSC is the better choice because as engineers we seek to find the most efficient and economic alternative, which will still fulfill our need and high strength concrete, is the best fit.
After the extensive research done in the topic I have reach to the conclusion that high strength concrete, in comparison to normal strength concrete, is a better option at the time of raising a building, because it has the characteristic to endure up to 600◦C without cracking. Also it proved that if silica is added into a concrete mix, the risk of spalling increases and can cause the collapse of any edification, however the effects that high temperatures have on this specific type of concrete are manageable, since constructing with high strength concrete may reduce the risk of spalling, adding an aggregate other than silica makes the structure more bearable, and at the same time the most viable option, and it is the most use nowadays in constructions. In addition to the conclusion I decided to write a brief list of recommendations for the use of high strength concrete.
The risks and hazards found on a construction can have a potential impact that can later become in a natural disaster. It is recommended to have knowledge of the different types of risk a construction is subjected.
It has been proved that the risk of spalling reduces depending on the type of concrete its used for the structure. It is highly recommended to use high strength concrete due to its characteristic of having a higher allowable thermal stress.
In case of a fire the first step that is recommended is to pull the nearest fire alarm as you exit the building. When evacuating the building, be sure to feel the doors for heat before opening them to make sure there is no fire danger on the other side, if there is smoke in the air lay on the ground to reduce inhalation exposure.