Degradation of construction materials

[edit] Introduction

Construction materials can be very durable but there are many cases of building materials failing prematurely. All building materials can suffer from deterioration and decay if they are not protected and maintained correctly. Common reasons for the degradation of construction materials include:

Natural agents.
Moisture.
Shrinkage.
Exposure conditions.
Corrosion in metals.
Loading.
Chemical.

[edit] Natural agents

Some materials can decay due to natural processes including:

Ageing- natural deterioration over time from wear and tear.
Weather – exposure to the elements over time caused for instance by wind, frost and rain.
Timber decay- including dry rot, insects, wet rot and fungal attacks.
Ultraviolet light – radiation from sunlight exposure.

[edit] Moisture

Moisture penetration can cause damage to wood fibres and materials such as plaster, brickwork, concrete and stonework. Capillary action causes water to move through materials and this can result in deterioration.

[edit] Shrinkage

Shrinkage generally occurs in timber. Freshly felled timber will contain significant amounts of moisture, but when used in construction it is likely to dry and shrink, causing cracks and gaps in the timber itself, especially in very dry areas.

Conversely, kiln-dried timber can expand when used in areas with high humidity. The best method to avoid the size changes in timber is to allow it to acclimatise to the conditions in which it will be used, before installation.

[edit] Exposure conditions

Exposure conditions might include:

Weathering - the combined action of the elements, such as rain, wind, frost and sunlight.
Thermal ageing - in high temperatures the chemical processes involved in material degradation can be accelerated.
Humidity – extreme levels of humidity can have an expediting effect on the degradation of materials.

[edit] Corrosion in metals

Corrosion in metals is the result of chemical processes, typically when the metal is exposed to liquid. The most common occurrence of corrosion in metals is oxidation, which causes rust in iron.

[edit] Loading

Loading conditions causing degradation can be classified as:

Shock – dynamic loads causing impact in low toughness materials.
Static – can cause creep deformation in materials over time.
Cyclic – constant varying dynamic loading can cause fatigue in materials.

[edit] Chemical

All materials are generally susceptible to chemical degradation. This can be caused by:

Alkalis – especially when alkaline cement paste and silica-based aggregates react and can cause cracking.
Sulphates – sulphates can be present in water and can react with concrete and cause levels of stress.
Leaching – some porous materials can be susceptible when minerals are dissolved in liquid solutions which will, in turn, weaken materials.
Acid rain – pollution in the atmosphere combines with water and forms acid rain. This rain can dissolve calcite in marble or limestone.