Advantages of concrete

·        Variety of Specification

Concrete can be manufactured to an inexhaustible range of specifications to suit all applications. This is possible by using different proportions of the natural ingredients or by the use of different materials.

·        Variety of Surface Finish

Building in concrete provides an extraordinary range of surface finishes that can be applied either when the concrete is still wet or once it has hardened, providing the opportunity for architectural expression to go hand in hand with structural integrity.

·        Flexibility of Shape and Form

Concrete can be moulded into any shape by using appropriate formwork. This capability can be used to provide bespoke design solutions to specific problems and also aesthetically pleasing finishes which often eliminate the need for further fixings, e.g. false ceilings.

·        Durability

Well designed and well placed concrete offers exceptional durability and long life in any structure. Concrete structures built over 100 years ago, indeed as long ago as the Romans, are still in active service today.

·        Environmentally Friendly

Concrete consists of naturally occurring materials, produces no emissions and needs no toxic preservatives. This is becoming an increasingly important issue.

·        Fire Resistance

Concrete is naturally and inherently fire resistant and needs no additional application of fire protection.

Solar drying

Solar drying

Solar seasoning offers a compromise between the low energy requirement of air drying and the speed of kiln drying. A number of different configurations have been tried, including a double skinned inflatable kiln. Because the energy input is variable, the kilns often have effective insulation to hold the heat inside at night time. Some kilns developed overseas can generate temperatures of over 40C inside even though the temperature outside is – 20C.

Solar-drying, which can take nearly twice the time required for kiln-drying, is well suited to high-grade applications such as furniture. Solar-drying is really a slower and gentler form of kiln drying. Some seasoned hardwood producers use air-drying down to fibre saturation point (FSP) prior to stacking the timber in the solar kiln.

Air drying

Air drying

The traditional method of seasoning timber was to stack it in air and let the heat of the atmosphere and the natural air movement around the stacked timber removes the moisture. The process has undergone a number of refinements over the years that have made it more efficient and reduced the quantity of wood that was damaged by drying too quickly near the ends in air seasoning.

The basic principle is to stack the timber so that plenty of air can circulate around each piece. The timber is stacked with wide spaces between each piece horizontally, and with strips of wood between each layer ensuring that there is a vertical separation too. Air can then circulate around and through the stack, to slowly remove moisture. In some cases, weights can be placed on top of the stacks to prevent warping of the timber as it dries.

Air-drying is necessarily a slow process, particularly for hardwoods, typically taking 6 to 9 months to reach a moisture content in the range 20% to 25%.

Kiln seasoning

Kiln seasoning

A number of commercial processes for seasoning of timber are available, the most common of which is kiln-drying. Kiln seasoning accelerates the process of seasoning by using external energy to drive the  moisture out. The timber is stacked in much the same way as it is for air drying, and is placed inside a chamber in which the conditions can be varied to give best seasoning results. Air is circulated around the charge (stacked timber) and the temperature and humidity can be varied to give optimum drying. Each species has different cell characteristics and therefore requires different drying schedules. Typically the timber may be in the kiln for a period of between two days to one week. All untreated structural pine and some commercial hardwoods are seasoned, mostly using kilns that are often heated by sawmill by-products or gas.

After kiln seasoning, there is often some damage to cells near the surface of the wood.  (All of the moisture passes through those cells.)  They have in fact collapsed, but can easily be ‘pumped back up’ in a reconditioning chamber.  This chamber introduces steam for a period and puts some moisture back into the outer cells and removes the effect of seasoning collapse. Kiln-seasoning of softwoods such as pine species is generally a fairly quick process, seasoning of hardwoods tends to be a much longer process. This is mainly due to the different (closed) cell structure of hardwoods.

Once the sawn hardwood material reaches fibre saturation point or slightly below (at a moisture content of about 20% to 25%), it is then placed in kilns usually for up to 10 ‑ 14 days (depending upon the thickness of the sawn timber) in order to bring the moisture content down to between 10% and 15%. This drying process must be strictly controlled and monitored in order to avoid drying degrade.

Moisture content of wood

Moisture content of wood

 Water is stored in wood in two main forms:

·                     As free water in the vessels and/or cells, used to move nutrients within the tree.

·                     As cell (or bound) water, which is an integral part of the cell walls.

The process of seasoning removes all of the free water and most of the bound water. In the removal of the bound water, the wood cells change in size and shape, so this part of the process must be carried out with careful control over drying rate.

When the timber is first cut, the initial reduction in moisture content is a result of free water loss. This usually occurs without any significant dimensional changes to the timber as the loss of moisture represents the drainage of pores in the timber. If the environmental conditions are favourable, the moisture loss continues until all the free water is released to the atmosphere. This point is known as the fibre saturation point (fsp). The fibre saturation point varies a little with each piece of timber, but it is generally taken to be at a moisture content of between 25% and 30%. The loss of free water will occur relatively quickly in small cross-sections of timber, even if the timber is exposed to rain.

After all of the free water has been lost, the timber is still classed as unseasoned as its moisture content is above 25%. Further drying is required to bring the moisture content below 15% so that it can be classed as seasoned. This additional reduction in moisture content, below fibre saturation point, requires a reduction in the amount of bound water in the wood's cell wall, which in turn requires more energy and occurs more slowly than the loss of free water. It also results in a reduction in the size of the cell walls, which causes the timber to shrink in size.

Seasoning of timber

Seasoning of timber

Seasoning is the process of drying timber to remove the bound moisture contained in walls of the wood cells to produce seasoned timber. Seasoning can be achieved in a number of ways, but the aim is to remove water at a uniform rate through the piece to prevent damage to the wood during drying (seasoning degrade).

Seasoned timber has an average moisture content of around 15% or less.  (It is permissible to have occasional moisture readings as high as 18%, but the bulk of any package of seasoned timber will have a moisture content of less than 15 %.)

Seasoned timber tends to have superior dimensional stability than unseasoned timber and is much less prone to warping and splitting in service. In higher grades of timber, particularly hardwoods, the process of seasoning can enhance the basic characteristic properties of timber, increasing stiffness, bending strength and compression strength.

strength and stiffness of timber

Wood itself is fibrous. Cells are long and slender and are aligned with the long axis of the trunk. It is these fibres that give the grain in the wood, not the growth rings. They also make the properties of wood quite anisotropic with much higher stiffness and strength parallel to the grain than across the grain.

The structure of wood can be likened to a bunch of parallel straws (representing the fibres or grain of the wood), which are bonded together using a weak glue. When load is applied parallel to the axis of the straws (a), they are very strong in tension and have reasonably good compressive strength until they start to buckle. However, if the load is applied perpendicular to the axis of the straws (b), they will tend to crush under compression and are weakest in tension, where the “glue” bond fails and the straws literally tear apart.

Creep

Creep is the term used to describe the changes in microscopic structure of wood that causes deflection of timber over a given time under a given load. (It is not the instantaneous deflection that occurs due to the changes in the level of load.

Natural Defects

Planed lumber can have defects that have occurred during the tree's growth. These include:

Decay, Rot or Unsound Wood Decay results from fungal activity. The wood loses its strength and may become soft or “punky”. Decay can be seen in a variety of forms. These forms are known as “white speck”, “honeycomb”, and “peck”.

Knots Knots are created where branches grew from the stem of the tree. Size and number of knots will affect the grade of the lumber. Additionally, the wood of the knot may fall out and leave a hole. This happens when the branch forming the knot dies and the wood is not bonded to the trunkwood.

Burl Burl is a defect caused by an injury in the living tree. This causes distortion of the grain and affects the strength of the lumber.

Shake A shake is a lengthwise separation of the wood. Shakes are most likely caused by wind damage. When the tree tissues are not elastic enough to withstand the stress as the tree is bent by wind, the tissues separate. If the separation is between or along the annual rings, it is known as a cup shake. If the separations radiate from the heartwood, they are known as heart or star shake. Shakes may also be caused in the drying process.

Timber Breaks Timber breaks are tiny cracks that zigzag across the grain of a board or plank. Their cause is unknown but may be due to wind stresses or hard contact with the ground during felling.

Sap Stain Sap stain is a variation in the natural colour of the wood. This is unsuitable for certain grades of lumber because of appearance requirements. The usefulness of stained wood is reduced to uses where natural finishes will be applied. Stained wood is divided into light, medium, and heavy stained categories. Sap stain will not progress when the wood has been kiln dried and planed.

Heart Stain The heartwood is naturally a darker colour than the sapwood. However, if the colour is in irregular patches, it is called heart stain. This is often a dark or reddish colour, but may range from pink to brown. Heart stain is unsuitable for certain grades of lumber because the strength can be affected.

Bark Pockets Pockets of bark may be present in lumber as a result of the stem of the tree growing outwards around a branch that has broken off. Bark pockets can also occur when scar tissue is formed after an injury.

Insect Damage Wood is prone to attack from a fairly wide array of insects.

Drying Defects Planed lumber may have defects that have occurred during the Drying Process:

 use ful links........

timberfor construction     www.timberinconstruction.co.uk

great timber  www.greatbuildings.com/types/construction/timber.html