Materiali da costruzione avanzati - Parte 3

PRO:

- Easy to find

- Renewable

- Easy to work, used in past

- Low cost

- Low density, used for airplane

- Insulator, voids that increase the insulation capability Trunk transport food.

- Good mechanical charact Fibers are oriented in

- Good combustible, tree has sugar inside one direction

CON:

- Property related to species, some spices need very long time to grow

- Affected by moisture, great affinity of humidity

- Anisotropic

- Flammable

- Biological attack

- Viscoelastic behavior, structure of wood is polymeric

- Dimensional limits

- Presence of defects Growth rings, there are different colors because they depend on the season. In winter there is

the reduction in food transport and so the structure is closed, for this reason we have the

dark color. During summer there is a lot of food transport so the structure is open and the

color is light.

These different rings are visible only in environment like our because we have the seasonal

change.

Macrostructure Heartwood: the central part of the trunk, usually has dark color because is the oldest part.

Sapwood: is the external part and it is used to transport food, had light color.

Outer bark: protection, is the external skin.

Botanical differentiation of wood. We have two different kind of wood:

Softwoods (conifers: cedars, cypresses, firs, larches, pines ...)

Hardwoods (deciduous plants: Oak, Maple, Walnut, Beech, Birch, Mahogany ...)

Present of different constituent inside te structure.

In softwood there are capillarity that are called Tracheids, in winter the

capillaries are small but in spring and summer they are big to

transport food.

In hardwood there are small capillaries called Fibers and big capillaries

called Vessel, also here are visible the difference in dimensions caused

od winter and summer periods.

Rays, the structure of these is oriented in the direction of the trunk,

capillaries come from the bottom and reach the branches, bring the

water to the lives. There is the connection between the capillaries also

in transversal direction in order to move food in different direction.

Microstructure The voids of the capillaries are called lumen, the cellular hole give the resistance, there is also the

fundamental the connection between the lumens because if there is some problems that stop the transport

of food, through these connection the tree can found another solution to the food transport.

The pits have selective membrane that control the transport.

There are three different layers, S1, S2 and S3. The layer that give the maximum

performance is the S2 due to the high thickness. In this layer the fibers are

oriented in one direction, the angle is 10/30 degree respect the direction of the

trunk. The other layers the fibers are oriented in other direction, like in S1 the fibers

are not oriented.

Middle lamella connect the different cells. The composite of this oart is related to

the presence of cellulose fiber and matrix of hemicellulose and lignin (lignin is very

important and give the properties of wood).

We have also extractives present in hardwood and also water.

In lives there is the photosynthesis, this give the formation of sugar.

Cellulose has semi crystalline structure with high degree of

crystallinity, this give resistance and chemical stability, so is not so

easy to destroy the cellulose.

Polar unit help the absorption of water

In wood there are:

Cellulose (40-45%)

Hemicelluloses (20% softw, 15-35% hardw) —> low degree of polymerization, solubility in alkaline solution (NaOH)

generally in softwood is low than in hardwood

Lignin (25-35% softw, 17-25% hardw) —> produced only in wood and in the living cells, is not a carbohydrate (aromatic)

higher amount in softwood, this give the higher durability of softwood

Extractives (1-20%) —> gums, tannins, resins, oils, alkaloids, starches

present inside of the wood especially in the hardwood, contain different substances, and some of these are poison for insect and are

natural protection from the external attack.

Tannins go out from the wood when this is in contact with water, so this extractive change the behavior of the liquid.

Distribution in the cell wall (HW) M is middle lamella, that connect the different structural part part of food, is made made of lignin.

S2 is the part that give the resistance, so it is full of cellulose.

Effects of components on properties

Cellulose —> strength in axial tension, anisotropy, hygroscopicity, strong water absorption because there are OH ioni in the external oart of

the cells so there is polarity that help the absorption of water.

The water goes inside the cellulose fiber and there is a very good adhesion, but this create an increase in the volume because water change the

interaction between fibers, this is similar to what happens in polymers, there is the plasticization.

We can also say that if wood is full of water it is easier to broke because there is a decrease in mechanical performances.

Hemicelluloses and Lignin —> compressive strength, strength in wet condition, because fibers are oriented and there is the maximum resistance

in the fibers direction

Lignin —> dimensional stability, high presence of lignin give high stability

Extractives —> color, odor, taste, durability (natural defense), inflammability, gluing, water content, health problem (some of these substances

are toxic for us), corrosion of metals.

Moisture and dimensional changes

Moisture in the cells

- Free in the lumen

- Adsorption in the wall

There is water everywhere in a living tree, the high humidity is normally presence in tree. When we cut wood we reduce the moisture, wood go in

equilibrium with external humidity and lose water, this lose of water give some effect

In the living trees (green wood) —> 30-300% —> Moisture reduction after cutting

Water loss:

- in the lumen (free) —> no effect, because this water is not connected with the structure

- in the wall (adsorbed) —> Shrinkage (reduction in dimension) and swelling, water between the fibers, so the lose of water change the

characteristics. The lose of water in wall happens when all water in the lumen is lose. —> Variation in mechanical properties.

Fiber saturation point: under 30% of moisture in wood (cavities are empty) When we reduce humidity from 30% to a value closed to

zero we have increase in shrinkage, when increase the

internal humidity from value from zero to 30% there is

swelling.

In longitudinal direction the effect is very low but if we

consider the radial and tangential direction the swelling is

higher.

So is fundamental to control internal humidity before the used

of wood.

This controlled humidity is called seasoning (similar to the curing of concrete).

We can make natural or artificial seasoning. The goals is to bring wood in humidity conditions that is the same that we have in the film.

If we do not make the correct seasoning and use wood take from natural conditions, during time there is incredible shrinkage and so fracture, lose

of bonding and so on. Also if there is the opposite condition and the wood is too dried, in time there is absorption of water and so swelling.

The best one is the artificial seasoning because it is used high temperature and in this way we can kill the possible insects presents inside and the

spores of fungi. Worst effect in tangential direction because there is a strong shrinkage.

Shrinkage Variation of moisture in time, in years but also in night and day.

If we do not consider in the correct way the seasoning these are the effects

that we can have.

Mechanical properties

Increase the density we increase the mechanical performance, the resistance of air-dry wood is higher than the resistance of green wood.

For anisotropy if we apply load parallel to the fibers we have the maximum resistance, if we change the angle we have a decrease in the resistance.

For Douglas wood we have a resistance in tension that is around 140 MPa, but if the load is perpendicular to the direction of fiber the resistance

has around 3 MPa.

Presence of defects

These affect a lot the mechanical properties, these defects are normally present in wood. To avoid this problem we can use engineering wood

because this is only wood without defects.

Mechanical properties are affected by:

- Moisture

- Density

- Anisotropy

- Presence of defects

- Viscoelastic behavior, if we have a permanent load we can increase the thickness in order to reduce the stress because the formula is θ = F/A.

EN 338: Structural timber. Strength classes Number are related to the flexural stress that we have in

the wood. Like C20 means that the bending resistance is

20 MPa.

Letter C is for softwood and letter D is for hardwood.

Higher resistance in hardwood elements.

Design

EN 1995-1-1:2004 (Eurocode 5 - Design of timber structures - Part 1-1)

Basic requirements referred to EN 1990 (durability)

- Load-duration classes

- Service classes

- Service class 1: moisture content in the materials corresponding to a temperature of 20°C and the relative humidity of the surrounding air

only exceeding 65 % for a few weeks per year —> DRY CONDITION

- Service class 2: moisture content in the materials corresponding to a temperature of 20°C and the relative humidity of the surrounding air

only exceeding 85 % for a few weeks per year —> MOIST CONDITION

- Service class 3: climatic conditions leading to higher moisture contents —> HUMID CONDITION

Other properties:

Thermal properties

Low thermal conductivity (porous structure); , humidity and density (0.12-0.18 W/m°C at a moisture of 12% and 25°C)

High Specific heat (~ double vs steel and concrete; —> a lot of heat is necessary to increase wood temperature)

Coefficient of thermal expansion different in the different directions (lower in the axial direction and greater in the tangential direction)

Electrical properties

Electrical insulator (if it is dry)

Acoustical properties

Sound absorption properties (porosity, density and modulus of elasticity) (vibration —> musical instruments)

Durability:

Main cause of deterioration —> Biodeterioretion due to the presence of sugar

Wood —> source of food (especially sapwood)

In the living tree, the sapwood is protected by the bark and by active wound responses which can deliver natural phytotoxins to the site of a wound

After cutting —> attack is easier (we have to cut tree during winter period)

Lignin is more resistant to micro-organisms (only very few number can degrade it)

—> Hardwoods generally have less lignin than softwoods and the lignin they contain is also more readily degraded making them particularly

susceptible to attack

In most tree species, the conversion of sapwood to heartwood involves blocking pathways between cells, which dramatically reduces the

permeability of the heartwood, and the manufacture of natural toxic materials which provide some degree of protection against fungi and insects

(ext