Dimensional change – calculate expansion of bamboo

Water and bamboo – what happens when the two are combined?

Bamboo is hygroscopic, as is all wood. This means that wherever there is moisture – even in the air – the bamboo will absorb it. If the air dries out the moisture will be “drawn” out of the bamboo. This results in dimensional change of the bamboo.

When this happens, the bamboo will either expand or contract. The force at which this movement occurs can be astounding – we have heard of wood pulling rawl bolts out of concrete. It is for this reason that we need keep this dimensional change in mind in our designs.

So, let’s look at how to calculate this dimensional change of bamboo so that you can compensate for it in your projects.

Some basics first

The bamboo fibers, that make up the bamboo culm, run along the length of the culm. These fibers are typically longer than the fibers found in wood (thus the incredible tensile strength of bamboo).

These fibers allow the “sap” that feeds the tree to move freely up to the leaves. The fibers are loaded with “sap”. When the bamboo culm is cut down, these fibers begin to dry out and the bamboo starts to shrink. This shrinking happens mostly in the thickness of the fibers. A very small amount of shrinkage occurs along the length.

This shrinkage is characteristic of all wood.

Fiber saturation point

These fibers have a moisture content of between 0% (completely dry) and a maximum 25-30%. This is commonly referred to as the moisture content (MC). In the wood drying process, a state is reached which is referred to as the fibre saturation point. This is the point where only water bound in the cell walls remains – all other water, called free water, has been removed from the cell cavities.

Bamboo is normally dried to a point where it is in equilibrium with the atmospheric moisture content or relative humidity. Typically, this moisture content (MC) is around 8%. Most timbers are kiln dried to between 6% and 15% before milling.

Equilibrium moisture content (EMC)

Hailwood-Horrobin EMC graph

Hailwood-Horrobin EMC graph

The equilibrium moisture content (EMC) is the moisture content at which the wood is neither gaining or losing moisture; this however, is a dynamic equilibrium and changes with relative humidity and temperature.

This table details to common EMC values for typical atmospheric ranges to which bamboo will be exposed.

[table id=18 /]

Relative humidity (RH)

Relative humidity (RH) is the amount of water vapour in the surrounding air.

A rule of thumb is that a relative humidity (RH) of 25% results in an EMC of 5%. A relative humidity (RH) of 75% results in an EMC of 14%. Thus a 50 % swing in relative humidity (RH) produces an EMC change of 10%.

Bamboo flooring will perform best when the environment is controlled to stay within a relative humidity (RH) range between 30% and 50% and a temperature range between 16 and 28 degrees Celsius, about the same comfort range most humans enjoy.

Now for some technical details

Dimensional change coefficient

Bamboo has a tangential dimensional change coefficient (the amount the fibers will expand or contract) of 0.00144.

Moisture content (MC)

Moisture content for flooring installations varies between 4% and 18%. The moisture content required depends on the wood species, the local geographic climatic conditions and the current season.

Most Oak flooring, for example, is milled at 6% to 9%. Solid bamboo is typically milled between 8% and 10% Strand woven bamboo is typically milled between 9% and 11%.

Calculating the expected movement in bamboo

The dimensional change coefficient of bamboo is used to calculate the expected shrinkage or swelling. This is done by multiplying the change in moisture content (MC) by the dimensional change coefficient of bamboo. The result is then multiplied by the width of the board or flooring plank.

Example:

A bamboo flooring plank ( dimensional change coefficient = 0.00144), 144 mm wide is subjected to a moisture content (MC) change from 8% to 12%. The MC change is thus 4%.

Calculation:

(MC change  x  dimensional change coefficient)  x width of plank  = total movement across width of plank.

Therefore:

4 x .0.00144 = 0.0058

0.0058 x 144 mm = 0.8294 mm movement across the width of the plank can be expected.

Thus a bamboo floor, 4 m wide, can move up to 23 mm across the width of the floor when a change of 4% MC is experienced. It is therefore easy to see why expansion gaps along the edges of the floor are so critical.

In practice, this movement will diminish across a complete floor, as the planks’ proximity to each other tends to restrain this movement. So one can view this calculation as a maximum movement expected for the anticipated change in moisture content (MC).

Bamboo plywood

Bamboo plywood is a lot more stable than a solid plank; it does not expand and contract as much as solid wood.

Here are a few tips to keep in mind when working with bamboo plywood:

  • Avoid cross-grain assemblies – this will reduce the stress on the joints
  • Allow the tops to move freely – the bigger the top the more allowance you need. Use;
    • blocks that allow movement
    • elongated screw holes (parallel to the grain)
    • figure 8 or Z clips
  • Use a sliding dovetail if the joint has to fit across the grain
  • Use an elasticized glue
  • Use a panel and frame construction – with room for the panel to move
  • Apply a finish. All finishes are permeable to moisture, but they do slow down the rate of absorption or egress.