Materials - Effects of temperature changes on construction materials

• Construction Principles
• Further Education (Key Stage 5)

Effects of temperature changes on construction materials

Unit 1: Construction Principles

Learning Outcomes

By the end of this lesson, you should be able to:

• Understand and describe the difference between sensible and latent heat
• Understand the effect of temperature change on construction materials
• Calculate thermal expansion for a material
• Describe the process of evaporation

Introduction

All construction materials respond in some way to temperature changes in their environment.

Heat energy that causes a change in the temperature of a material is called sensible heat i.e. you can SENSE it! 

The amount of sensible heat required to produce a given rise in temperature varies from material to material and even between the solid, liquid and gas phases of the same material.

Energy that does not cause a temperature rise, but is absorbed by the material as it undergoes a phase change, is called latent heat ('hidden heat'). 

For a given material, the latent heat of fusion (phase change from solid to liquid) will be different from the latent heat of vaporisation (phase change from liquid to gas).

Effect of temperature change on material properties

Effects of temperature changes on construction materials

Changes of state

If a solid material is subjected to a continuous input of heat energy, its temperature will begin to rise until it reaches the material’s melting point.

During the change of state, continued heating will lead to no further increase in temperature until the solid has completed its change into a liquid.

After this, continued heat will produce a rise in temperature, but at a different rate, until the boiling point is reached. 

Again, during the phase change, continued heat will lead to no further increase in temperature until all the liquid has changed into gas. 

Once the change of state is complete, continued heating will produce a rise in temperature at another rate.

Linear expansivity

A change in the temperature of a material is associated with an expansion or contraction of its size in all directions.

The amount by which the size changes for a given change in temperature differs from material to material and is defined by the material’s coefficient of linear expansion.

Worked Example

A 15m long steel beam is exposed to a annual temperature changes of between -5 to 25 degrees Celsius.

Calculate the change in length caused by thermal expansion?

A 15m long steel beam is exposed to a annual temperature changes of between -5 to 25 degrees Celsius.
Calculate the change in length caused by thermal expansion?

1. 0.54 mm
2. 5.4 mm
3. 54 mm
4. 540 mm

Evaporation

Evaporation is the process where a liquid changes to a gas, even at temperatures below its boiling point.

Evaporation happens because, although the average kinetic energy of all the molecules in a liquid is low, individual molecules can have sufficient energy to escape from the surface as a gas.

As these high-energy molecules leave the liquid they take their energy with them. The average energy left in the liquid reduces as does its temperature. This is why humans sweat to aid cooling, as the evaporation of water from our skin reduces its temperature.

Evaporation

Evaporation is a vital process in a range of applications associated with construction, from drying out timber, bricks or other porous materials that might otherwise retain moisture to the evaporation of solvents from paints,

allowing them to dry.

The rate of evaporation can be increased by:

raising the temperature

increasing the surface area

reducing the relative humidity of the air

Evaporation

The rate of evaporation can be increased by:

 

• raising the temperature
  
• increasing the surface area of a liquid from which molecules can escape
  
• reducing the relative humidity of the air immediately above the surface of the water (an effective method is by providing an air current over the surface of the liquid to carry away the gas molecules as they emerge).

1. Which of these best describes 'sensible heat'?

1. Heat energy that is passed from one solid material to another
2. Heat energy that does not cause a change in temperature of a material, but is absorbed during phase change
3. Heat energy that causes a change in temperature of a material
4. The heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).

2. Which of these best describes 'latent heat'?

1. Heat energy that is passed from one solid material to another
2. Heat energy that does not cause a change in temperature of a material, but is absorbed during phase change
3. Heat energy that causes a change in temperature of a material
4. The heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).

3. If you continue to heat water once it has reached boiling point, what will occur?

1. The temperature of the water will continue to rise relative to the heat energy being applied.
2. Separation of the hydrogen/oxygen molecules occurs to form hydrogen gas and pure oxygen gas.
3. The temp of the water will remain at 100'C, and the additional heat will be absorbed to change the state.
4. The triple point of water will be reached, in which all three phases of water will exist; solid, liquid, gas.

5. I am process where a liquid changes to a gas, even at temperatures below its boiling point, what am I?

4. What does the term 'Coefficient of Linear Expansion' mean?

1. the amount by which a material expands or contracts upon heating with each degree rise in temperature.
2. the amount by which a material expands or contracts upon being subjected to an external stress
3. the linear relationship between elastic deformation and stress
4. the expandable elements within a material that give rise to a change in state.

Final Q. A 50m run of copper pipe, with a coefficient of linear expansion of 0.000015, is exposed to temperatures ranging from -10 to 35'C.

Calculate the predicted thermal expansion of the pipe in millimetres.

Learning Outcomes

By the end of this lesson, you should be able to:

• Understand and describe the difference between sensible and latent heat
• Understand the effect of temperature change on construction materials
• Calculate thermal expansion for a material
• Describe the process of evaporation

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