This article will discuss the melting point of concrete and will cover topics such as the definition of melting point and the innate resistance of concrete against fire. Topics such as the physical and chemical changes happening on a concrete under fire, explosive spalling, and thermal cracking will also be elaborated in this blog post.
Melting Point of Concrete
As a composite building material, concrete is simply not like pure substances with a distinct melting point. In fact, there is a high chance that concrete undergoes thermal decomposition and that its components transform to other compounds before it hits its melting point. Nevertheless, by rough approximation, the melting point of concrete is about 1,500 degrees Celsius, owing to the individual melting point of its components.
The melting point of quartz sand is about 1,650 degrees Celsius while the melting point of cement is approximately 1,550 degrees Celsius. At this range of temperatures, water molecules are severely agitated and transition to steam, causing damages through explosive spalling and thermal cracking.
Melting Point
Melting point is the temperature at which the material transitions from solid to liquid state. This normally happens when the material absorbs heat naturally from the surroundings or artificially by putting external heat. Microscopically, the molecules of a solid substance are well-compact and highly structured. The introduction of thermal energy excites each solid particle, disrupting its structure and pushing away each other. The distance between particles becomes wide and the arrangement becomes extremely random, finally achieving the successful transition to a liquid state.
Clear visualization of melting is illustrated by phase change of water from a solid ice to just water. The melting point of each building material solely depends on their microscopic identities. For instance, tungsten has an extremely high melting point while gallium metal may melt in one’s hands in an instant.
Fire Resistance of Concrete
Fire resistance is defined as the ability of the material to resist and protect itself against the effects of fire as well as its capacity to perform its function well even under extreme temperature conditions. Concrete is widely considered as fire-resistant which is further enhanced by the addition of admixtures or chemicals that synergistically contribute to this purpose. In theory, the performance of concrete when exposed to high temperatures depend on several factors some of which are the quality of aggregates used in the mixture, the available moisture in the matrix, and the extent and concrete area that is exposed to the extreme conditions.
Physical and Chemical Changes
As mentioned, concrete is a composite building material and its reaction with fire and extremely high temperatures is complex compared to pure substances. In terms of reaction, concrete may undergo either reversible changes when the temperature subsides or irreversible damages that could affect the performance of the structure in the long run.
For example, concrete has water internally which normally transitions to steam at a temperature of a hundred degrees Celsius. This boiling point may increase to about 140 degree Celsius due to the initial pressure in the concrete matrix. When more and more water molecules change their phase into gas, excessive pressure build-up happens which may exceed the strength of concrete, resulting in cracking damages.
In addition, calcium hydroxide crystals present in the structure naturally come in a hydrated form. With the increase in temperature of up to 400 degrees Celsius, the hydrated form of calcium hydroxide becomes dehydrated, creating additional pressure build-up in the matrix.
Furthermore, the increased temperature affects the aggregates in the mixture. For quartz-based aggregates, the minerals transform at a temperature of about 575 degrees Celsius which expands the volume occupied by the material. On the other hand, at 800 degrees Celsius, limestone-based aggregates decompose irreversibly.
Ultimately, the exposure to high temperatures could result in concrete structure’s collapse which may happen in various ways. For reinforced slabs, the introduction of too much thermal energy may lose the tensile strength of steel reinforcements, thereby causing the structure to lose its integrity. The bond between the cement paste and the reinforcement may weaken due to high temperature, creating loose connections between matrix components.
Spalling
Spalling is defined as the dissevering of a concrete matrix due to its exposure to an extremely abrupt rising of temperature which is normally encountered during fire. The different forms of spalling are aggregate spalling, corner spalling, surface spalling, and explosive spalling. At the first half an hour of exposure to a very high temperature, concrete may experience all forms of spalling except corner spalling which only happens when the entire structure has weakened from the initial damages caused by the aggregate, surface, and explosive forms. Typically, concrete turns to corner spalling after approximately one and a half hour of heating exposures.
The aggregate form of spalling normally produces minor popping sounds while both surface and explosive spalling exudes intense and vicious explosive sounds. It is important to note that among all spalling forms, explosive spalling is considered to be the most damaging and threatening not only to the structure but also to the properties on its vicinity.
In theory, the cause of spalling is the critical build-up of pressure within the matrix. As the temperature of the structure is increased from fire exposure, the water present within the matrix rapidly transforms into a highly-energized steam which continuously finds its escape away from the structure. Consequently, the pressure build up results in exceeding the maximum strength that the structure could withstand, leaving no other choice but to explode in several bulks and pieces.
Thermal Cracking
Thermal cracking is normally associated with the reasons as to why spalling happens. When the moisture present in a concrete matrix expands by phase transition due to high temperature the structure may also experience opening cracks other than the violent spalling. These openings allow fire to be in direct contact with the reinforcements of concrete, causing the metal to expand and produce stress that ultimately lead to irreversible damages.
Conclusion
This blog post discussed the melting point concrete. It was clearly explained in this article that because concrete is a composite material, its melting point is quite difficult to determine since thermal decomposition of individual components may commence before the melting process begins. From the values of the melting points of its raw materials, concrete may roughly melt at approximately 1,500 degrees Celsius.
Furthermore, the technical term “melting point” is defined in this post. Melting point was described as the temperature where the particles of the material start to transition from the solid phase to the liquid phase. Parallel to water, it is when ice becomes liquid once again.
Concrete’s remarkable resistance to fire as well as the physical and chemical changes that happen within its matrix was also elucidated in this blog post. Damages such as thermal cracking and explosive spalling were taught in this article.
For any questions and suggestions about this article, please feel free to submit your thoughts in the comment section below.
Frequently Asked Questions (FAQs): Melting Point of Concrete
What is the maximum temperature concrete can withstand?
Significant structural damages may happen to concrete at temperatures of around 65 to 93 degrees Celsius. Standards adapted this range by specifically specifying that the temperature limit of reinforced concrete is at a maximum of 93 degrees Celsius.
How hot can concrete get before it explodes?
A research made by the Swiss Federal Laboratories for Materials Science and Technology observed that concrete explodes at a heating temperature of 600 degrees Celsius.
What material has the highest melting point?
Among the available materials on earth, tungsten has the highest melting point of 3,414 degrees Celsius which makes it a suitable filament material in light bulbs. In addition, tungsten is also utilized as electrode, heating element, and heavy metal alloys.
How much heat does concrete produce?
At a basis of a hundred pounds of cement, the Portland Cement Association approximates that concrete can produce heat and increase its temperature by 15 degrees Fahrenheit on a maximum. The generated heat is from the chemical reaction between cement and water which is known as hydration reaction that happens during the curing process.
How do you make concrete heat resistant?
Heat-resistant concrete may be produced by the addition of a high-calcium fly ash to the main raw materials of concrete. The added material could increase the fire resistance of concrete to about 400 degrees Celsius, and can still undergo normal curing process at room temperature.
Will concrete crack with heat?
Yes, concrete may crack with the application of excessive amounts of thermal energy. The materials that compose concrete expand at high temperatures, and the expansion builds up the internal stress felt by the matrix. When the maximum resistance is reached, thermal cracking occurs.
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