Fibre Saturation Point of Timber

The fibre saturation point is an important moisture content parameter for timber. It refers to the point at which the cell walls of the timber are saturated with water, while the cell cavities contain no water. This is a critical moisture content level that affects the physical properties and behavior of timber.

Explanation of the Correct Answer (Option B)

When the cell walls are saturated with water and the cell cavities contain no water, the timber is said to be at its fibre saturation point. This is the correct answer because it accurately describes the condition at which the fibre saturation point occurs. Let's break down the different options to understand why the other options are not correct:

- Option A: When the moisture content is 12%, it does not necessarily mean that the cell walls are saturated with water. The fibre saturation point occurs when the cell walls are fully saturated, which may require a higher moisture content than 12%.

- Option C: When the cell walls are dried and the cell cavities contain water, the timber is in a state of over-drying. This is not the fibre saturation point, as the cell walls should be saturated with water at this point.

- Option D: When the cell walls are dried and the cell cavities contain no water, the timber is in a completely dry state. This is below the fibre saturation point, as the cell walls should be saturated with water at this point.

Importance of Fibre Saturation Point

The fibre saturation point is an important parameter to consider in timber engineering and construction because it affects the physical properties and behavior of timber. Some key points to note about the fibre saturation point are:

1. Dimensional Stability: The fibre saturation point is closely related to the dimensional stability of timber. Below the fibre saturation point, timber tends to shrink or swell depending on the change in moisture content. Above the fibre saturation point, the timber is relatively stable in terms of its dimensions.

2. Strength and Stiffness: The moisture content at the fibre saturation point is also important for determining the strength and stiffness properties of timber. Timber is typically stronger and stiffer in the dry condition compared to the fully saturated condition.

3. Decay Resistance: The fibre saturation point is also relevant to the decay resistance of timber. If the moisture content remains below the fibre saturation point, the timber is less susceptible to decay and fungal growth.

In conclusion, the fibre saturation point of timber occurs when the cell walls are saturated with water and the cell cavities contain no water. This moisture content level is critical for understanding the behavior and properties of timber.