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Physical properties and mechanical characteristics

 
 
 
Physical properties and mechanical characteristics

Dimensions, thickness and number of plies

The dimensions of plywood panels are usually expressed in mm and presented in a pre-established order:

  1. grain direction of the face veneer
  2. perpendicular direction
  3. thickness

In France, the most common formats are 2,500 × 1,220 mm and 3,100 × 1,530 mm for hardwood plywood, and 2,500 x 1,250 mm for softwood. The 3,100 x 1,830 mm format also exists for certain applications.

Plywood thickness commonly ranges from 3 mm to 50 mm depending on its construction and purpose.

In some cases, the number of plies is indicated, because two plywood panels of the same thickness can display different characteristics depending on the number of plies.

Density

The average densities of plywood are:

  • 450 kg/m3 for 100 % poplar plywood
  • 500 kg/m3 for 100 % okoume plywood
  • 600 kg/m3 for 100 % maritime pine plywood

Dimensional stability

Wood is a naturally hygroscopic material: depending on the hygrothermal conditions (temperature and relative humidity of the air (water vapour)), the wood will reach an equilibrium by absorbing or releasing humidity. These variations in humidity can lead to dimensional variations within the material. However, this phenomenon is reversible and does not affect the mechanical characteristics of the products.

Plywood’s hygroscopic behaviour is similar to that of solid wood. Nevertheless, because of the presence of glue, plywood’s humidity content varies more slowly.  Plywood’s hygroscopic equilibrium is 1 to 2 % lower than that of solid wood, and ranges from 7 to 13 %.

Dimensional variations in plywood panels are insignificant and much smaller than in solid wood, meaning that they perform excellently in terms of dimensional stability.

This stability is related to two factors:

  • the presence of glue, which acts as a barrier against the absorption of humidity
  • the plywood’s cross-grain construction, which counters the natural tangential shrinkage of the wood (thus countering the resulting deformation).

Strength characteristics and elastic modulus

The elastic modulus indicates a plywood’s stiffness and serves as a basis on which to calculate the deflection of panels under stress. Depending on the plywood’s structure, its flexural strength and elastic modulus can differ significantly depending on whether they are measured lengthwise or widthwise. Very high modulus values can be obtained in a longitudinal direction by adjusting the structure.

However, the average value measured is a constant and is characteristic of the variety or varieties of wood used.

 

The table below indicates the average flexural strength and elastic modulus of plywood panels with five or more plies:

Essences
CaractéristiquesOkoumé et feuillus tendressapelli, sipo et tropicaux dursPin maritime
(MV comprise entre 450 et 550 kg/m3)(MV comprise entre 550 et 650 kg/m3)LongitudinalTransversal
Résistance en flexion moyenne courante (MPa)35-405050-7525-40
Module d’élasticité moyen courant (MPa)400060006000-80002000-4500
Puncture resistance: Monnin hardness
Monnin hardness (N) is determined by the depth to which a 30 mm diameter steel cylinder penetrates a plywood panel when placed under a given load.

Contreplaqués NF Extérieur CTB-X
EssenceOkouméPeuplierPin maritime
Dureté2 à 31,5 à 2,52 à 3
Bois massif (par comparaison)
EssenceSapinPin sylvestreChêne
Dureté1,5 33,5

Fire rating

The regulations applicable to accommodation units (31 January 1986) rest on five basic principles: the reaction to fire of construction components, the fire resistance of structures and components, evacuation, smoke extraction and the subdivision of buildings.

      a) Reaction to fire describes the behaviour of a material in the presence of a flame or a temperature rise, and whether or not it serves to fuel and spread a fire.

Depending on the application, the requirements set forth may take into account several criteria relating to this reaction to fire: flammability, contribution to fire, release of smoke and creation of burning debris.

In the case of Construction Products subject to CE marking requirements, all these criteria are taken into account for the classification of products.

A material’s reaction-to-fire class (Euroclass) is determined based on the results of standard tests. According to standard NF EN 13986 relating to wood-based panels for construction, raw plywood panels with a thickness of 9 mm or more and a density of 400 kg/m3 or more are conventionally assigned to the following class:

D, s2, d0
(According to the equivalence tables available, Euroclass D, s2, d0 equates to French regulatory standard M3)

Masse volumique (Kg/m3)Epaisseur (mm)Conditions de montageClassement murs et plafondsClassement sols
9Posé directement sur la paroi support
40015Posé sur une lame d’air à cavité fermée sur une paroi A2-s3,d0D-s2, d0D -d1
18Posé sur une lame d’air à cavité ouverte sur une paroi A2-s3,d0

It is common for plywood to be fireproofed, without this altering the performance of the panels.  The levels of performance attained place this plywood in class

B –s2, d0
(According to the equivalence tables available, Euroclass B, s2, d0 equates to French regulatory standard M1)

      b) Fire resistance is defined as the length of time for which a construction product exposed to fire retains the characteristics that enable it to perform the function for which it is intended.

The average combustion rate of plywood to be considered for structural calculations (see « Eurocode 5″) is:

1 mm /min
for plywoods more than 20 mm thick and with a density of 450 kg/m3

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Water vapour permeability

Water vapour permeability is expressed by the water vapour resistance factor, which is determined according to standard EN ISO 12572/2001.

This factor is expressed by two values obtained using the « wet-cup » and « dry-cup » test methods, respectively.

 

The average water vapour resistance factors commonly used today, as presented in EN 13986, range from 50 to 110 (« wet-cup » method) and from 150 to 250 (« dry-cup » method) for plywood panels with a density of between 300 and 1,000 kg/m3.

Thermal conductivity

The thermal conductivity coefficient (l) of plywood varies according to species, density and the adhesives used.

According to EN 13986, the average thermal conductivity coefficient of plywood ranges from 0.09 to 0.24 for plywood panels with a density of between 300 and 1,000 kg/m3.

Acoustic absorption coefficient

In accordance with EN 13986, the average acceptable acoustic absorption coefficients for plywood panels are

  • 0.10 (250 to 500 Hz frequency range)
  • 0.30 (1,000 to 2,000 Hz frequency range)

Use class

Standard NF EN 314-2 defines the tests to be performed to check a plywood panel’s bonding class.

These classes determine the conditions in which the plywood can be used:

  • Class 1: dry interior environments (often expressed in terms of compliance with 636-1)
  • Class 2: humid interior environments (often expressed in terms of compliance with 636-2)
  • Class 3: exterior applications (often expressed in terms of compliance with 636-3)

EpreuvesImmersion 24 h dans de l’eau à 20°CImmersion 6 h dans de l’eau à 100°CImmersion 4 h à 100 °C – Séchage 16 h à 60 °C – Immersion 4 h à 100 °CImmersion 72 h à 100 °C
Classe 1 (milieu sec)
Classe 2 (milieu humide)
Classe 3 (milieu extérieur) NF Extérieur CTB-X

Biological durability

Wood decay fungi need favourable humidity and temperature conditions in order to develop: At least 20-22 % humidity (when wood’s rate of humidity falls below 20-22 %, fungus growth slows rapidly) and an optimal temperature of between 20 and 30°C (below 20°C and above 30°C, fungus growth is disrupted.

Thus, when plywood is used in dry interior environments (e.g., interior fittings and joinery), where humidity levels rarely exceed 13 %, there is no risk of attack by wood decay fungi.

For other uses, the risk is directly proportional to the length of time for which the panels are re-humidified (the natural hygrothermal variations of ambient air cannot generate sufficient levels or periods of humidity for there to be risk).

When exposed to particularly extreme conditions, plywood needs to be protected or specially treated at the installation stage, so as to minimise the risk of the material getting wet and/or limit the duration of exposure to moisture (by optimising ventilation or the natural drying process).

 

NF Extérieur CTB-X plywood panels are manufactured using species that stand up well to wood decay fungi, which makes them more naturally resistant.

In addition, the risk of attack by wood-boring insect larvae (death watch beetle, lyctus beetle, longhorn beetle, etc.) on wood-based panels is very low.

Environmental and sanitary performance