Exploring the Melting Point of Concrete

Concrete is one of the most widely used construction materials due to its strength, durability, and versatility. However, when exposed to high temperatures, concrete can undergo significant changes that affect its structural integrity. To understand how concrete behaves under heat, it is crucial to delve into its melting point and the factors that influence it.

1. Understanding Concrete:

Concrete is a composite material made up of cement, aggregates (such as sand and gravel), water, and often additional additives. The chemical reaction between cement and water, known as hydration, creates a solid mass that binds the aggregates together. This hardened material provides the strength and stability characteristic of concrete.

2. Melting Point of Concrete:

Unlike metals that have distinct melting points, concrete does not have a precise melting point. Instead, it undergoes a gradual process known as thermal decomposition when exposed to high temperatures. The behavior of concrete under heat can be better understood by examining its constituent materials.

While concrete does not have a specific melting point like metals, it undergoes thermal decomposition and experiences changes in its properties when exposed to high temperatures. Understanding the behavior of concrete under heat is vital for designing structures that can withstand fire and extreme conditions. By considering the composition of concrete, aggregate selection, mix proportions, moisture content, and incorporating fire protection measures, engineers can create more resilient structures that can withstand the effects of heat and maintain their structural integrity.

When Does Concrete Melt?

Concrete is an artificial building material. It consists of a mixture of cement, water and aggregate (sand and gravel). Hence, there is no precise temperature at which concrete melts. Depending on its composition, concrete melts at temperatures between 1150°C and 1200°C. By the way carbon steel melts at temperatures between 1425°C and 1540°C.

The exact melting process is hardly dealt with in the literature. According to Schneider (in German; page 18) it is probably the case that the cement stone matrix melts first and then the aggregate stones. The photo below shows a small test body made of concrete that I examined at temperatures of up to 1200°C. It can be seen that the concrete has partially melted.

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.

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.

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.

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.

Spalling

For example, before a concrete column melts, parts of the column can spall off. There is always some unbound water in concrete. Depending on the type of construction, the proportion is between approximately 2% to 5% (with a normal reinforced concrete column) up to 10% (with a steel composite column). In the event of a fire, the free water heats up and evaporates at 100°C.

If the water cannot escape through the capillary pores of the concrete, a lot of pressure builds up. Because concrete can only absorb low tensile stresses, explosive spalling can occur. These can be prevented or reduced with suitable reinforcement. You can find further information, for example, in the dissertation of Dr.-Ing. Meyer-Ottens.

By the way, high-strength concrete is particularly at risk of spalling in the event of a fire. It has a denser structure than normal-strength concrete and therefore fewer capillary pores. This makes it difficult for steam to escape. For this reason, there are special reinforcement rules for high-strength concrete in Eurocode EN 1992-1-2.