2 Density assessment Workshop session: Timber properties KDA 503-335: Timber, its origin and characteristics Student Aim This exercise aims to determine the base and air-dried densities of a timber sample To this, you will determine the mass of the test piece, its volume and its moisture content Using these results, you can then calculate the base density and the air-dry density of the samples Background The density of wood is a measure of the amount of wood substance present in the timber Density varies between species of tree, between trees of the same species and within a tree Density is highly correlated with many of wood’s strength properties It can also be used to select timber or individual piece of timber for specific end uses The mass of wood in a given volume changes with the shrinkage and swelling caused by changes in moisture content and so, the term density is meaningless unless the conditions under which it is determined are specified The most common density values used for timber are basic density and air-dry density Basic density is determined when the moisture content of the sample is at or above the fibre saturation point and is based on the volume of the test piece when tested and the mass when oven-dried Air-dry density is determined when the moisture content of the test piece is below fibre saturation point and in Australia is normally quoted for a standard moisture content of 12 percent using shrinkage values published* for the species Methodology This procedure has four steps: The mass of the test piece is determined Its volume is established by linear measurement and subsequent calculation The moisture content is then determined using AS 1080.1 The base density and the air-dry density are then calculated Mass of the test piece (M) • Cut a test piece from the board no less than 500 mm from the board’s end The piece should be 25 mm wide and as square as possible • Scrape or sand off all loose splinters and sawdust and weigh the piece This should be immediately after it is cut from the board Determine its mass to an accuracy of 0.01g Volume of the test piece (V) • Using vernier caliper, carefully measure the width, breadth and depth of the piece • Calculate the pieces volume Moisture content of the test piece (W) The procedure for oven drying samples is: Cut two samples at least 500 mm from the end of the board The timber should be sound and relatively free of gum veins, resin pockets and knots The first sample should be at least 40 mm long and weigh not less than 50 grams when dry Scrape or sand off all loose splinters and sawdust and weigh the piece Number the pieces for identification Place the samples in an oven controlled at 103° ± 2°C Remove the samples from the ovens after the expected drying time Weigh the samples immediately Record the weight and the identifying number Return the samples to the oven After to hours, reweigh the samples A sample is considered to be dry if the second weight is within 0.2 % of the first Calculate the moisture content (Initial weight of the sample [1] - Oven dry weight of the sample) [2] / Oven dry weight of the sample [2] x 100 ([3]/[2] x100) Through all the steps, ensure that dust, debris, or sawdust does not contaminate the scales or samples MC % = Weight of water in timber × 100 Oven dry weight of timber KDA 503 Timber, its origins and characteristics – Block Worksheet Calculating density Basic density is calculated using the formula below: D= M 100 × V (100 + W ) Where: D= Basic density in kilograms per cubic metre M= Mass of test piece at time of test in kilograms V= Volume of test piece in cubic meters W= MC% of the test piece based on oven dry-mass in percentage units Air-dry density is calculated using the following formula: AD = AD = M (100 + 12 ) 100 × × V (100 + W ) (100 − (r + t )(W − 12 )) Air-dry density in kilograms per cubic metre M= Mass of test piece at time of test in kilograms V= Volume of test piece in cubic meters W= MC% of the test piece based on oven dry-mass in percentage units r&t= Unit shrinkage percent in radial (r) and tangential (t) directions respectively For the species being tested, r= t= Definitions Basic density the density of timber calculated from the green (or fully saturated) volume of the test piece and the mass of the test piece when oven-dry Air-dry density an expression of the mass of timber at equilibrium moisture content, standardized at 12 percent, per unit volume of timber at the equilibrium moisture content Unit shrinkage shrinkage per percent change in moisture content below fibre saturation point KDA 503 Timber, its origins and characteristics – Block Worksheet Results Sheet Basic density calculation Item Result Comment M kg V m3 W % Then apply the calculation below D= M 100 × V (100 + W ) Item Result Density Comment kg/m Air dry density calculation Item Result Comment M kg V m3 W % r&t Then apply the calculation below AD = M (100 + 12 ) 100 × × V (100 + W ) (100 − (r + t )(W − 12 )) Item Result AD Comment kg/m Moisture content calculation Sample No Initial weight [1] g g g g Oven dry weight [2] g g g g Weight of water lost by drying [3] g g g g % % % % Interim weights Initial weight - Oven dry weight [1] - [2] Moisture content Then apply the calculation below MC % = Weight of water in timber × 100 Oven dry weight of timber KDA 503 Timber, its origins and characteristics – Block Worksheet ... is within 0 .2 % of the first Calculate the moisture content (Initial weight of the sample [1] - Oven dry weight of the sample) [2] / Oven dry weight of the sample [2] x 100 ([3]/ [2] x100) Through... timber KDA 503 Timber, its origins and characteristics – Block Worksheet Calculating density Basic density is calculated using the formula below: D= M 100 × V (100 + W ) Where: D= Basic density. .. dry-mass in percentage units Air-dry density is calculated using the following formula: AD = AD = M (100 + 12 ) 100 × × V (100 + W ) (100 − (r + t )(W − 12 )) Air-dry density in kilograms per cubic