Last edited 17 Feb 2021

Reversible and irreversible expansion in construction


[edit] Introduction

Most building materials suffer movements which can be caused by changes in temperature, changes in moisture content, subsidence and so on. Porous building materials, for example, can suffer from sudden changes to their original moisture content. In some materials, this sudden change can occur immediately after the process of manufacture and can continue during storage and distribution; it will depend on the properties of the material.

Changes can include expansion, contraction, deformation and so on and can sometimes lead to problems such as cracking and water penetration, or even failure.

Very broadly, expansion can be either reversible or irreversible.

[edit] Irreversible expansion

As an example of irreversible expansion, due to the intense heat involved, newly-fired clay bricks will be very dry as they emerge from the kiln. Their very low moisture content combined with sudden exposure to the atmosphere will mean they can absorb moisture from the air until they have reached a point of normal or atmospheric moisture level. This increase in moisture content causes expansion of the brick which is irreversible: the brick is marginally larger than when it was originally formed and will not shrink back to its original size.

Irreversible expansion can also be seen in calcium silicate bricks (made from mainly sand, lime and quartz). Formed in an autoclave under high heat, moisture and pressure, they are more saturated than clay bricks and after the process will immediately shrink until they have reached a moisture content that is in equilibrium with the of the prevailing air.

In both cases, before newly manufactured bricks are used on site, time is usually allowed for this irreversible expansion to be completed, otherwise cracking may occur if they are used immediately. In many cases, this is achieved by the time taken to store and distribute the bricks to the end user.

[edit] Reversible expansion

Reversible expansion usually occurs as a result of moisture absorption when materials are in use, e.g on a building in an exposed location. The material may expand when wet and contract as it dries out. This cycle may be repeated for the life of the material and can be accommodated with the correct provision of movement joints.

Thermal expansion may also be reversible. For example, metals will tend to expand when they become hot and contract when they cool.

For more information see: Thermal expansion.

Moisture present in buildings may freeze during cold weather, expanding as the water turns to ice, then thawing as temperatures increase. This repeated freeze-thaw cycle can result in significant damage, such as cracking brickwork, bursting pipes and so on.

For more information see: Frost attack.

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