What if we have models over the period of 6 months showing different connections and different conditions – there is no way to make amendments so these need to be experiments
- Constant flow (water pump)
- Effect of water on timber
- Heated environment
- Timber in Brick & Mortar
- What is the material of the mortar
- What kind of brick
- What kind of configuration of brick
- Brick in Timber
- Concrete in Timber
- Steel in Timber
- This can be a whole system of water flow testing different materials and joints
The models should also be made in three parts
- Masterplan level (city)
- Design level (region)
- Material level (room)
From the book Timber Decay in Buildings: The Conservation Approach to Treatment, by Brian Ridout, 2013
- There are two camps: remedial timber treatment industry vs. environmental control (drying etc.)
- The epithet ‘preservative’, although doubtless a powerful marketing device, is an unfortunate generic name to apply to biocides for use on timber, because it implies that decay will inevitably occur unless the timber is given some form of treatment. Yet timber is easily preserved by a dry environment.
- Current legislation requires that precautionary treatments be justified
- Some organisations: BRE & British Wood Preserving and Damp Proofing Association
Chapter 1: Origins and Durability of Building Timber
- Wood is not the uniform material it appears to be
Between Softwood and Hardwood
- The terms ‘softwood’ and ‘hardwood’ are used extensively within the timber trade, and frequently lead to confusion. Softwood refers to the conifers, the needle-leafed or cone-bearing trees (for example pine and cedar), some of which provide quite hard timber. Hardwood is used to describe timber from the so-called broad-leafed trees (for example oak and mahogany) and includes species whose wood is in fact very soft. Nonetheless, the distinction between softwood from cone-bearing trees and hardwood from broad-leafed trees remains extremely useful.
- The softwood trees are botanically known as gymnosperms (from the Greek: naked-seeded) because the seeds develop exposed on the surface of a cone scale. The gymnosperms living today are the representatives of a group that extends back in time for more than 300 million years (Sporne, 1965). Modern softwood trees are mostly restricted to regions where the climate is harsh and the soil is poor in nutrients. Their ability to survive in these areas derives to a large extent from an ability to restrict water loss, by the possession of a water-conducting system controlled by valves, and by narrow waxy needles that restrict vapour loss.
- The hardwood trees are known botanically as angiosperms, or hidden-seeded plants, because the seeds develop enclosed in an ovary which eventually becomes a seed capsule. They are a more recent addition to the flora, and do not appear in the fossil record until about 100 million years ago (Sporne, 1974). The majority of the hardwoods have broad leaves to maximize light absorption, and an open water conducting system. They tend to favour environments where conditions are suitable for prolonged vigorous growth, although many are able to tolerate poor soils and harsh weather.
- These differences in efficiency of water transportation and habitat have a considerable effect on the structure of their timbers.
- At the molecular level, softwood trees and hardwood trees are similar, even though the exact make-up of the lignin and hemicellulose components may differ. At the structural level, however, there are differences that relate to environment and growth form. Softwood trees have leaves and wood structure that reduce water loss in harsh environments (Rundel and Yoder, 1998). But these environments, particularly the colder environments, impose restraints on form. Damage from high winds and snowfall is easier to avoid if the tree has a straight trunk and an open-branched structure (Figure 1.6): wind resistance is reduced and snow falls off as the branches bend down under its weight. The tree can maintain sufficient growth by maximizing conduction within the tracheids during the early vigorous growing season, when the tracheid walls are thin (earlywood cells), and maximizing strength by thickening the tracheid walls during the latter part of the year (latewood cells). The two types of tissue together produce a more or less distinctive annual ring. Storage requirements are low because the tree can maintain a longer growing period by minimizing water loss, and the soil is of poor quality.
- Hardwood trees have a rather different lifestyle. They favour milder climates where water and nutrients are more plentiful. These conditions mean that the tree can sustain a bulky growth form if competition will allow, producing a wide crown of leaves for photosynthesis. If growth is affected by direct competition from other trees, as in an oak woodland, then long straight trunks will be produced, pushing the leafy head towards the light (Figure 1.7). In contrast, parkland growth allows plenty of space, and produces oaks with a lower and more spreading form because light is readily available (Figure 1.8). In economic terms, the forest growth produces a greater volume of better quality timber. Modern forestry techniques aim to maximize growth by giving space, while providing enough competition to maintain quality. A thicker stem and wider crown require stronger wood, and the larger species tend to have denser timber and a high volume of structural fibres. Tree roots cannot function at soil temperatures below 4 ° C so water vapour lost from the leaves during cold periods cannot be replaced from the soil. Hardwood trees, with thin-cuticled broad leaves that maximize photosynthesis, must either drop their leaves when the ground is cold, and become dormant, or risk desiccation and the loss of leaves (and the nutrients they contain) as a result of frost damage. If leaf fall is part of the tree’s natural strategy then nutrients can be withdrawn, and the leaf sealed from the stem. The leaf then dies, changes colour, and falls off. Loss of leaves in winter also reduces wind resistance and snow deposition, thus reducing mechanical stress to the tree during adverse weather.
- Substantial storage facilities are required, and many hardwood trees have wide rays which add considerably to the decorative qualities of timber such as oak by producing a patterned grain. When the favourable season returns the stored material must be rapidly mobilized and photosynthesis maximized. This is aided by an open and direct water-conducting system.
- The start of heartwood production varies. In European redwood in southern Norway it has been shown to commence around the pith after 20 years (Uusvaara, 1974) but the starting point apparently depends upon latitude. Bruun and Wilberg (1964) demonstrated heartwood production after 30– 40 years in Finnish redwood, whereas Hägglund (1935) showed that it commenced after about 25 years in southern Sweden and 70 years in northern Sweden.
- (In the living tree) The woody tissue of the tree is protected from attack by the living sapwood, which is mostly too wet to be susceptible to colonization by pathogens (Hudson, 1986), and is also able to respond in a variety of ways to infection. Sap rots do sometimes occur in living trees if the sapwood is damaged, but the decay is usually localized.
- If a wound occurs in the sapwood zone of a living tree then the tissue can respond by compartmentalizing the damage and any consequent infection (Shigo, 1983). The wound is healed by the stimulation of growth hormone production, which in turn stimulates the production of callus tissue over the surface. Damaged and diseased sapwood is isolated by the production and deposition of tyloses, gums, resins and other toxic materials in wall-like zones which box in the injury (Shain, 1979). If the wound penetrates to the inert heartwood, then the barrier will be incomplete and decay may occur.
- It is the dead heartwood in the living tree which is vulnerable to a greater or lesser degree, depending on its natural durability. This difference in durability between the sapwood and heartwood is, as described presently, reversed when the tree dies. Most fungi appear to enter the heartwood zone via small dead branches, sapwood wounds, broken tops and roots. Decay may be categorized as a butt rot if it is at the base of the tree, or a heart rot if it is further up the stem. The molecular and structural organization of wood is not the only influence on durability. Age and extractives also have a bearing on durability and resistance to decay.
- The period of juvenile growth varies in length and the density/ strength of the timber increases away from the centre. Eventually, the cells reach a maximum length with nearly vertical microfibrils in the S2 layer, and the tree may be considered to be mature. At about this time (usually about 10– 20 growth rings from the centre) the cycle of earlywood and latewood is fully established in those species where this occurs (Krahmer, 1986).
- A tree may live for a very long time, but eventually vigorous growth slows and the tree becomes senile. Increase in trunk height ceases first; increase in diameter may continue for many years. Growth rings become narrow, and latewood production declines as lignin production decreases, so that the timber produced is brittle. Accompanying these changes are changes within the heartwood, which progress outwards from the pith. These changes, which frequently include the breakdown of extractives and the formation of minute compression fractures, increase the timber’s susceptibility to decay. The centre of the tree is usually invaded by fungi which slowly destroy the core, leaving a hollowed trunk.
Sorption of Water by Timber
- Distortion caused by shrinkage and swelling is generally recoverable to a large extent, provided that no plastic deformation of the cell walls has occurred. Permanent distortion may, however, arise if there is lateral restraint, perhaps caused by panel surrounds or floorboard fixings.
- The shrinkage of timber from green to air dry was important in traditional timber-framed construction. Joint pegs provide an interesting example. Oak was usually worked more or less green and would therefore shrink. If the joint pegs behaved in a similar fashion then the joints would loosen, but if the pegs were made from dry timber, which would expand as it absorbed moisture, the joints would be tightened.
- Dimensional changes which are a response to diurnal or seasonal humidity fluctuations are generally called ‘movement’, and are of importance in good quality joinery and carpentry. If timber with a high movement value is used, or timbers with different movement values are mixed, or timber is worked at a substantially higher moisture content than it will achieve in its intended environment, then loose joints and undesirable gaps are likely to occur. Tables that group commercially available timbers by their movement values are published by the Building Research Establishment and other similar organizations. Values are usually quoted for dimensional change from fibre saturation down to 12% moisture content.
- Sometimes changes in the building environment make undesirable movement inevitable. This type of damage frequently occurs when central heating is installed in rooms with window linings and dado panelling. Heat from radiators placed in front of the windows may cause localized lowering of humidities to 25– 35% and timber moisture contents to 6– 8%. The resulting movement of the timber will be readily visible, and is frequently mistaken for decay. If some concealed decay is present, perhaps as a result of water penetration 200 years earlier, substantial buckling and distortion may occur. This is a major reason why active dry rot is sometimes reported from dry buildings after restoration works.
- Responses to moisture vary according to the age of the timber.
- Decay commences in softwood timber by the breakdown of pit membranes, thus increasing porosity. This process is sometimes encouraged, particularly for species which are difficult to treat with preservatives, by storage in water. Increased porosity can, however, lead to excessive and uneven uptake of preservative, which may cause surface bleeding, and to difficulties with the application of glues and coatings. Decay by brown and white rot fungi will substantially alter moisture uptake. It will be remembered that water molecules are held by the cellulose/ hemicellulose within the timber and it is these structural polymers that are destroyed by brown rots. Brown rots therefore cause a sharp drop in equilibrium moisture contents, particularly during the early stages of decay. White rots remove lignin and the proportion of cellulose exposed increases. Timber decayed by white rots therefore tends to have a higher equilibrium moisture content once about 60% weight loss has been exceeded. Moisture meter readings should not normally be taken from decayed sections of timber.
- The moisture level in Guangzhou is high – there is high level of air moisture and also seasonal monsoons. How does this affect timber structures there?
Chapter 3: Post-Harvest Changes and Decay
Effects of Moisture Contents
- It has been shown that it is difficult for most decay organisms to exploit the living tree (see Section 1.5). The nutrient-rich sapwood, the most vulnerable part of the actively growing tree, is too wet for colonization by most organisms, and the living tissue can produce growths that check the spread of pathogens. Decay fungi therefore mostly enter via dead or damaged tissue, particularly branch wounds and roots. The fungus still has to overcome the natural resistance of the tree, and also any gums, resins or phenolic compounds produced at the site of wounds, which inhibit spore germination and compartmentalize fungal growth. It is easier for fungi to attack the juvenile wood at the centre, where the concentration of antiseptic extractives and the moisture content are lower. The most common forms of decay in standing trees are therefore butt or heart rots.
- Because of its moisture content, sapwood may weigh twice as much when green as it does after oven drying. Heartwood is usually drier in softwoods (and in some hardwoods); in some pines, for example, moisture levels are about 120% of the dry weight in the sapwood and 35% in the heartwood. This situation changes when the tree is felled. The moisture content is slowly reduced as the timber dries, to about 17– 20% in the wood yard and perhaps about 15% in a cool building. In a heated building the moisture content may drop to less than 9%. As the water content drops and the air content increases, a variety of changes, including decay, may occur. Decay remains a possibility until the timber moisture content is too low for the relevant organisms to survive.
- The progressive moisture loss, from living tree through felled log, conversion and finally incorporation into a building, presents a wide range of conditions which different decay organisms may exploit, although the extractives may remain an insurmountable obstacle in some timber species. Sapwood, which was a resistant material in the living tree, becomes highly susceptible to decay as it dries, because of its elevated nutrient content.
- Wood decay fungi and insects vary widely in the moisture contents they can tolerate. Most require relatively high timber moisture levels in order to thrive. Some groups can cope with drier conditions, but as moisture content decreases, so does the variety of decay organisms. Moisture levels below about 18% provide a harsh environment for all except a few specialized insects, which probably derive a proportion of their water requirements from the breakdown of cellulose. In temperate climates, most of these specialists are beetles; in warmer regions termites have a major economic impact. In general, fungi in buildings will not damage timber with a moisture content below about 22%, whereas colonies of most wood-boring beetles will not thrive at moisture levels below about 12%. A few may continue their attack at moisture levels as low as 8%. These differences enable the classification of biological hazards (see Section 3.5).
- It is important to remember that damp timber provides an environment which will ultimately be colonized and decayed by a succession of organisms. The types of organisms that take part in this progression and the speed of attack depend on parameters which include moisture content, temperature and the different chemicals present in the wood. Preservative treatments may inhibit or destroy some organisms, but preservatives are really only changing the environment, and sooner or later a suitably tolerant sequence of colonizers will reach the damp timber and commence its destruction. These colonizers need not themselves be decay organisms. Benign microfungi, for example, may modify the toxins so that decay fungi may develop. Wood is only immune from decay if it is kept dry.
- Decay organisms may be classified by their ability to degrade the components of the timber, and they will range from cell content feeders in sapwood, to those capable of destroying the entire cell wall of sapwood and heartwood.
- So basically how moisture works in timber is that the green timber comes with lots of water, then during the conversion process, the timber is dried out to a certain percentage of water content. If the percentage is not low enough, organisms might start to attack it. Preservative treatments might help but really still depends on the drying. These organisms eat away at the wood mass and increase its porosity (decreases density). Timber that is always submerged in water does not have the same issue because due to the absence of air, not many organisms can attack at the timber. (is that true?) So it is really a combination of heat, moisture and air that creates decay in timber.
Nutrient availability after conversion: the potential for decay
- Wood is not an easy food source for decay organisms to exploit. Timber conversion may increase the problem for decay organisms because soluble nutrients tend to be carried to the surface as the timber dries and large amounts of this surface zone are lost when the timber is planed and worked.
Damage to timber can be caused by
- Fungus (moulds and stains or soft rot)
- Mould and stain fungi (fungi imperfecti) are mostly cell-content feeders that exploit the nutrients in sapwood. They usually do little significant damage (Viitanen and Ritschkoff, 1991b), although the bulk of pigmented hyphae produced by stain fungi may cause the discoloration of damp timber. They do, however, have a primary part to play in the natural cycle of decay by increasing porosity and detoxifying some natural fungicides. Colonization by moulds is facilitated by some types of bacteria that break down the pit membranes within the sapwood (Blanchette et al., 1990). Stain fungi are more independent than other moulds, and travel from cell to cell by boring fine holes in the walls. Viitanen and Ritschkoff (1991b) grew a range of common moulds on redwood and spruce. They concluded that the lowest air relative humidity for growth on sapwood was 80%, and that growth was very slow at this humidity level.
- Soft rot damage progresses slowly as a surface decay in wet timber, and the wood has a fine surface checking, similar in appearance to brown rot damage, when it dries. The fungi can, however, tolerate a wide range of environmental conditions, and dry timber which is intermittently wetted may eventually be destroyed (Savory, 1955). Savory also showed that the brash surface frequently found on timbers where there were no other indications of fungal decay, could usually be attributed to soft rot fungi. Significant rot damage is more common in timber exposed in soil or aquatic environments than it is in buildings. This restriction has been ascribed to a lack of additional
- The primary division of decay into white rots and brown rots, according to the colour of the visible damage, was proposed by Hartig in 1874. The fungi that cause white rot (Basidiomycete; Figure 3.3) grow within cell cavities and attack all constituents of the cell wall from the lumen inwards. Some white rot fungi commence by attacking hemicelluloses and lignin, whereas others utilize all cell-wall components at the same rate. Decay commences with the depolymerization of the hemicellulose. The resulting decay appears as a mass of white fibres, and weight loss may eventually exceed 95% (Zabel and Morrell, 1992). White rot decay is predominantly associated with hardwoods (Hudson, 1986). The reason seems to be that most of the white rot fungi have difficulty in attacking the lignin unit which predominates in softwood lignin (see Figure 1.2). White rot fungi in buildings tend to thrive in wetter sites than brown rot fungi; thus they are frequently found in external window sills and below substantial roof leaks. This view is perhaps supported by laboratory decay tests, which showed that white rots required more water than brown rots to achieve an optimal wood-weight loss (Highley and Scheffer, 1970). Examples are oak rot (see Section 8.5.2) and some others of the ‘pore’ fungi (Polyporaceae).
- Surface degradation caused by mechanical damage
- The combined effects of light, wind and water movement produce stresses which result in small surface checks and cracks. Surface material loosened in this fashion is eventually lost and erosion takes place. This process is mostly very slow, published estimates ranging from 1 to 7 mm per century (Kühne et al., 1972; Browne, 1960). Erosion varies with the type of tissue: the softer earlywood is likely to be preferentially removed, leaving behind ridges of latewood (Feist and Mraz, 1978). A severe artificial form of this type of damage will occur when softwood is cleaned by grit blasting (see Section 12.3).
- Both alkaline and acid attack may occur in damp timber if there are partially embedded iron fastenings (Figure 3.9). The resulting electrochemical reaction is driven by differential oxygen availability (Baker, 1974). In essence, the exposed portion of the fastening acts as a cathode while the concealed section acts as an anode. Electron flow from the anode to the cathode produces corrosion products at the anode which may be hydrolyzed to free acid. A progressive and concomitant increase in alkalinity at the cathode produces an increasingly alkaline environment so that surface degradation also occurs. Blue/ black staining frequently spreads from the cathode, particularly in oak, as soluble iron salts react with tannins to form iron tannate.
Acidity and Corrosion of metals by timber
- Fresh oak tends to contain particularly high levels of acetic acid. This can often cause metal corrosion, so fixings in oak must be corrosion resistant.
- It is worth noting that kiln drying tends to raise acidity
Drying and wetting: A historical perspective on timber decay within buildings changes in moisture content after felling
- Timber is not a homogeneous solid like a metal which contracts and expands equally in all directions. It has different internal arrangements along the grain and across it, so that it shrinks and swells unevenly as it loses or gains moisture. Green mature timber shrinks little along its length, more across the radial face and much more across the tangential face. Tangential shrinkage is usually 1.5– 2 times as great as radial shrinkage. Unequal shrinkage causes stresses within the log that lead to splits along the weakest tissue, which is the rays. Sawn timber subjected to uneven drying will warp, the severity and type of distortion being dependent on the manner in which the log was cut.
- Sawn building timbers therefore have to be dried to a level that does not suit the majority of decay organisms, but in a controlled fashion so that distortions are minimized. This process is known as seasoning, a term that probably derives from the practice of leaving oak logs and ships’ frames for several seasons in order to rid them of saps liable to ‘ferment’ (Bowden, 1815, pp. 85– 93).
- Nowadays seasoning is usually undertaken in a more controlled fashion and its purpose is to make the timber as stable as possible with regard to both distortion and decay. Seasoned timber is also lighter (therefore less costly to transport), stronger, holds nails better and is easier to machine, paint and glue. It also usually accepts chemical preservatives readily because the wood cells are empty of liquid. Ideally the less durable timbers should be converted as soon after felling as practicable because they are susceptible to a wide variety of decay organisms.
- If the bark is completely removed from the log before or after felling, then uneven drying and the release of stresses may cause severe permanent shakes to develop from the outside inwards. This reduces the amount of usable timber that can be cut from the log. However, if the bark is left on the log to retard drying, a different set of problems occurs; these include end splitting and staining as well as attack by wood-boring beetles and fungi.
- Different timbers vary considerably in their anatomical structure and in their physical properties, and there is a wide variation also within any one species. It follows, therefore, that although the drying of all wood, regardless of its species and size, is governed by the same physical laws, there are great differences in the drying conditions that can safely be imposed. Water seasoning is worth a mention here even though it is not a method of drying, because it is an old-established practice. If logs remain waterlogged then starch is removed and with it the risk of powder post beetle damage when the timber dries. Fungi are also inhibited but bacterial attack may increase the porosity of the sapwood. This is not necessarily a disadvantage because it may aid the penetration of preservatives.
- Air drying (Figure 11.1) is probably the oldest technique available. The timbers must be stacked in a way that allows adequate, even air flow and prevents distortion. Correct stacking is of considerable importance, as the diameter of the felled trees decreases and the risk of distortion, particularly cupping, in the resulting smaller section timber increases (Wengert and Denig, 1995). Planks and other timbers are normally stacked horizontally with thin spacing timbers of uniform size, known as stickers, between them. Stickers are placed so as to form vertical lines throughout the stack, usually at about 300 mm centres, in order to avoid unequal stresses. Stickers have to be kept thoroughly dry in order to avoid staining in susceptible timbers (Wengert, 1990). The rate of drying may be controlled to some extent by the thickness of the stickers and their distance apart. Heavy weights may be placed evenly on top of the stack to minimize distortion. If stacks are too large for adequate ventilation then the air becomes saturated and drying ceases. Maximum dimensions have been given as 4 m wide and 5 m high with a 1 m minimum wide passageway between stacks. A suitable cover is constructed over the top to protect the stack from rain. The edges of the covering should extend beyond the sides of the pile to prevent rainwater from dripping onto the timber (Desch and Dinwoodie, 1981; Rietz and Page, 1975). The cover may be a shed, with or without fan-assisted ventilation (Figure 11.2).
- If carefully piled, the timber may dry to a moisture content of about 17– 20%. The time taken depends on numerous factors, including type and thickness of timber, season of the year (see Section 12.7), weather, and dry conditions within the yard. The latter is important because if the area around the stack is cluttered and damp then drying will be impaired. Good foundations (i.e. a flat surface that is sufficiently firm to take the weight of the timber) are also required to prevent deflection (Food and Agriculture Organisation, 1986). If conditions are satisfactory, 25 mm-thick redwood should air dry in about 3 months. Hardwoods such as beech and oak of similar dimension may take 7 or 8 months, and it is usual to add one year for each extra 25 mm of thickness (Desch and Dinwoodie, 1981). End shakes are the most common form of defect during drying; they occur because of fast evaporation from end grain. They may be avoided by using a sealant, or by using some form of end covering.
- Natural seasoning does not produce dry enough timbers for many purposes and requires a large amount of yard space for a long time. The timber industry in many parts of the world is having to become more efficient and competitive. As the cost of logs and the demand for dry timber rise, storage time and drying degrade have to be kept to a minimum if profits are to be maintained. Kiln drying (Figure 11.3) is therefore frequently more economical, but a range of defects may still occur. There are two conventional types of ventilated kiln. In the first the conditions are uniform and the timber remains stationary within the kiln for a set period of time while being dried by a current of hot, moist air. The second type of kiln is long and conditions become progressively hotter and less humid along its length. The timber is slowly moved from one end to the other as drying proceeds. The second type of kiln is more difficult to control, and is normally only used for softwoods, which can safely withstand faster drying than hardwoods. These kilns may assist in avoiding a high proportion of kiln-induced defects which have been shown to be caused by variations in moisture content in both the timber and the kiln. Defects are mostly produced by localized variations in conditions within the kiln. Figure 11.2 Air drying under cover in a drying shed. Timber is stacked in the kiln in the same manner as for air drying, but great care has to be taken because the risk of defects occurring is far higher under the more severe kiln-drying regime. It is essential to ensure that loads are composed of similar species and sizes. Different species of timber are dried according to schedules that have been developed by trial and error in order to hasten drying and reduce defects. However, the satisfactory kiln drying of timber depends ultimately upon the degree to which the individual pieces of wood are uniform in structure, upon careful monitoring and upon the skill of the kiln operator. It has not been unusual for up to 25% of pieces passing through a kiln to be subsequently downgraded due to kiln-induced defects (Hart, 1990). The cost implications of this may be substantial (data from Southern Pine Lumber Mills). Figure 11.3 Drying kilns at Henry Venables timber yard. Moisture movement from the core of the timber to the surface is a relatively slow process. If drying proceeds too rapidly because the air is too dry, then the outside of the timber may dry and set while the inside remains damp. The result is known as ‘case hardening’ (Figure 11.4), and differential drying when the plank is resawn will cause severe cupping (Stumbo, 1990). If case hardening continues too far then shrinkage of the tissue at the centre will be restrained by the hardened shell. The result is a mass of small shakes and defects at the centre which are invisible from the surface of the timbers. This defect is known as honeycombing (Figure 11.5).
- If the timber is very green and evaporation of moisture occurs too rapidly, moisture is forced from the timbers faster than air can be drawn in to replace it. In this case the diminishing volume of water causes the cell walls to cave in, especially in the spring wood, and the whole timber shrinks and warps. This is known appropriately as collapse (Figure 11.6). Most of these defects can be corrected by changing the drying schedule, provided that they are detected early by careful monitoring, and that they do not derive from abnormalities in the wood (Lamb, 1990). Collapse may be avoided by predrying. Most modern kilns today dry a charge of softwood every 24 h although some operate on a 12 h drying cycle (McConnell, 1990). Hardwoods must be dried at a much slower rate if defects are to be minimized, and 25 mm thick oak dried from green will take about 5– 6 weeks. This period may be halved if the timber is air dried first. One problem with conventional kilns is that the water collected from the timber may contain up to 3% of organic extractives, some of which may be hazardous (Singer et al., 1995). Waste disposal must therefore be taken into consideration. These condensates may also cause significant corrosion damage to the kiln (Little and Moschler, 1994).
(Above) Honeycomb shakes are separations of the fibres caused by drying stresses. These occur after case hardening when the interior of the timber dries.
- Several other methods of drying have been tried with varying success (an excellent overview is provided by Milota and Wengert, 1995). Drying by dehumidification seems likely to become of increasing importance, particularly for drying pretreated timber, and drying with vacuum kilns is becoming an accepted practice in many parts of Europe. Commercial vacuum kilns can produce a vacuum which halves the boiling point of the water within the timber; the moisture can therefore be driven out at a lower temperature than in a conventional kiln. Timber is frequently dried to a moisture content specified for a particular end use, and its moisture content when supplied should preferably be within about 2% of that specified. Dimensional change for every percentage variation in moisture content may be small, but may nevertheless affect gluing because a glue layer may only be 0.23 mm thick (Taylor, 1994). Care must be taken after drying to ensure that the moisture content remains stable, and that it is not altered by inappropriate transportation, on-site storage, or other building works. The average moisture contents that modern softwood timbers attain in dry buildings are generally about 12– 15% in domestic roofs, and perhaps 12– 14% in floor timbers (above ground floor) and joinery. Heating systems may reduce moisture levels to about 6– 9%, when cracking or joint separation may occur, whereas suspended ground floor timbers may have moisture contents in excess of about 18% – high enough to allow insect attack if sapwood is present.
- Recent amendments to BS4978 and BS5268 (Part 2) 1977 concerning dry stress-graded structural timbers, under 100 mm thick and for interior use, require that they be graded at a maximum average moisture content of 20%. If these timbers are subsequently pretreated with a water-based preservative then they must be redried to the same level. Timbers must then be marked as ‘Dry’ or ‘KD’ (and ‘Wet’ if undried, and therefore for outdoor use). Kiln drying will make a substantial difference to the cost of timber for repairs. Kiln-dried oak costs about three times as much as green (Venebles, 1993)
Figure 11.6 Collapse is a kilning defect that occurs in some timbers if a rapid loss of moisture causes cells to deflate.
- Large-dimension softwoods also present a problem, and sizes over about 100 × 300 × 6000 mm are likely to be green and prone to shrinkage.
Material considerations of project
- For the project
- Skin that wraps around urban spaces to denote a new urban territory
- Opportunities for vertical attachment of timber (flexibility)
- The skin becomes occupyable at certain spaces – for attachment
- How does the skin meet historical materials (bricks & timber & oyster)
- How does the skin meet new materials (concrete & steel)
- Properties of skin (which is a line)
- Linear (directional)
- How it meets other walls
- How it meets ground
- How does it open
- How does it extend vertically and horizontally
- How does a line/skin embody volume
- Properties of timer
- How it meets the ground
- How it meets the roof
- How it interacts with water
- Duration of its exposure to water (constant, periodic)
- Type of exposure to water (dripping, soaking, splashing)
- Exposure to type of water (humidity, rain, snow)
- How it interacts with time
- How could it be not only in terms of visual difference, but there is a deeper meaning to the material conferred by time – its demise.
- How it interacts with activity/play
- What type of timber is it
- Quality of the timber sourced
- Where timber is sourced
- How timber is treated
- Replacement for the same material that has similar properties to timber
- Natural material that reacts with the elements
- Rendered/montage elevation of part of project (100m2 @ 1:20/1:50)
- Hand-size mini model of project (or part of it) that distils the material/formal character of project
- 3x precedents:
- they should be videos – that show the passing of time
- one architectural
- one natural/found (a forest, an airplane, a potato)
- one found image/photo of a phenomena, a situation, a texture… a painting… a print.
- Photos from fieldwork
The workshop began with a mock up tests of little elements – wall, roof and brick. This was an exercise to get familiar with the local materials, crafts and masons. It then moved on to combination of the different designs into a single pavilion, with input from Gao. When design was confirmed, the construction began with the collection of materials and then the actual building of the structure.
Design was not so important in this workshop, especially when there was not a function allocated to the pavilion, other than to look onto the Guest House. It was meant as a temporary structure, for two years and will be demolished when the community centre is built on the same location. I learnt more about the design of joints and elements than the design of the whole structure. However, it was interesting that the aim of the pavilion was to be a catalogue of different material connections and possibilities in the village. If the design actually was that, it would have been very interesting.
People’s opinions about materials – mud bricks and not able to reuse it; grey bricks and its beauty; mortise and tenon and needless to use nails; social stigma
Chinese architecture is inward-looking. There are joints that you don’t see and the magic happens on the inside.
Bricks are divided into several types in this village.
- Grey bricks (Qing Zhuan) are traditional materials used in the Guest House. They are soft bricks, fired slowly with charcoal in special urns, over water. After they are fired, they are then clamped and sanded against each other to create a smooth surface on the facing side. This process takes a long time. The masons are very proud of the historical technique and the beauty that it creates. However, they no longer do it because of the complexity and effort. The mortar used for this is then usually gypsum mixed with organic glues found in crops.
- Red bricks (Hong Zhuan) are common materials found now in the village and area. They used to be made in the village by the villagers but now can be bought simply on the market. They are fired in urns quickly and so they are harder and cannot be sanded. The result is a rough finish. This kind of bricks come in two sizes, 10, 20, 30 and a smaller size. The mortar used is normally gypsum or cement with gypsum for decorative lining.
- Mud bricks (Tu Zhuan) are traditional materials used in common houses in the village and area. They are made with sand and water and air dried. If left out to dry in the sun, the surface might dry too quickly and crack. The way to test the mud bricks is to drop it on the ground from head height and see if it withstands the impact. There is also mud mortar made from the same material and could be used as temporary or permanent joints. Mud bricks cannot be reused.
Brick construction techniques
We wanted to test the wall and the points of insertion for timber – a typical condition found in the timber-brick buildings. Together with the mason, we first laid out the typical wall types used in construction in the village (Appendix a) and then tried one typical way of inserting a timber beam into the brick wall (Appendix b). The current way of insertion either leaves a hole in the wall while constructing it, and hence disrupts the pattern, or knock out a hole in the wall post-construction which damages the structural stability of the wall. We wanted to test if there is a way to allow for flexibility and stability at the same time. The different ways of new walls were made to create holes at regular intervals along the length of the wall. Timber elements such as beams, purlins, staircases or even furniture can then be inserted into the wall. This idea of flexibility also comes from the buildings in the village which are constructed at different times for different purposes. Flexibility allows for practical expansion of the buildings. The title of the mock up was engineered holes.
However, during the testing of these mock ups, there needed to have a tie-brick in between the two walls to ensure structural stability. Working the tie brick into the patterns then became a challenge and in the end we resorted to overlaying the layers of bricks in cantilevers to eliminate the need for tie bricks (Appendix c). Another important point to consider is the position of the beams. The location of holes in the wall has been arbitrary but the angle of the roof is standardised to be 1 in 2 (40cm length, 20cm height) in order to make sure that the roof tiles can sit properly and angle is sufficient for water drainage, but also because of the use of bricks and half bricks. Therefore, adjustments also needed to be considered to the wall to allow for the beam position.
There are also holes made in the upper parts of the walls of typical brick building so as to insert beams into it to build the second storey. The holes are then later filled with mortar or half bricks. In the construction of a typical brick room, bricklaying begins from the corners and moves inward. The bricks in the middles could be half-bricks.
Mortar placement was an interesting exercise. The typical way of placing it was to scrape on the sides and none in the middle or the side of the bricks. During actual construction, this was deemed to be unsafe as there is insufficient amount of cement/mortar between the bricks. Also, the masons use mortar to level the bricks because of the uneven sizes of the recycled bricks. This was also deemed to be structurally unsound.
In terms of foundation, the retaining walls and the columns should be tied in together to withstand horizontal forces. However, during the construction of the pavilion, some areas were not tied in together and the columns have to be rebuilt.
During construction of the pavilion, we wanted to test this technique and insert a staircase into the wall. However, one of the major problems with this is that the wall must be tall enough and have a sufficient amount of mass above the staircase in order to hold the timber elements down. Having a half wall does not allow for such a technique and this brick wall became more decorative than functional.
Material and gathering
Typical timber in the area is Chinese fir, Kampur and pine. Kampur is the cheapest and most common one. There are some historic forests in the area but to fall trees in there would need the trees to have died first. In India apparently they nail steel bars in a circle around the tree trunk and the tree will die. Kampur and fir are planted and felled with permit. The best kind of timber is regrown timber (chopped once and regrown from the same stump) and they are not easy to be corrupted. Typical ways to waterproof the trees is to apply layers of Tong Oil, which prevents it from termites and also rain. It is also advisable to lift the timber 20cm away from the ground to prevent water from entering into it.
On the hills behind the Big House Group of Fairy Water Village, there is a forest of Eucalypts which is forested to make paper. It is grown by the government as it grows very rapidly. However, because of the its need for a massive amount of water, it damages the soil.
Historic trees could also apply for licensing and protection. Once the tree is licensed, anyone trying to harm it is punishable by law.
Termites were present on site and a termite expert was invited to examine the site and the Guest House. Termites are scared of sunlight and dig tunnels to protect themselves from the sun. They leave traces which are visible as elevated tunnels of soil. Once disturbed, they escape and are difficult to find again. The best way to deal with termites is to first leave it and then leave possibilities to trace it. To get rid of termites, either find the queen termite, extinguish her and then the colony will die, or poison the termite/soil. The second way is the most typical but is harmful for health. When there are young children, it is advisable to leave the building for a week before opening it to this particularly vulnerable group. There might also be a type of poison that affects the termites as they communicate with each other.
Recycled timber is available cheaply around the village and there are houses with new, unused timber or old timber from demolished houses. When MM brought us around the place, it was interesting that she would point out the old buildings and how we can demolish it to get this piece of timber or that wall of bricks. Obviously these buildings do not matter in her eyes or even the villagers’ eyes. The contrast between the attitude towards this building and towards the Guest House is quite different. The quality of recycled timber is not very clear. It was not a good idea to do material viewing at night. We could have insisted more on buying new timber because that would have saved us time and labour costs in processing the old timber pieces.
Using recycled timber was difficult in terms of communication with the carpenter. However, I do feel that this carpenter is a special case. The rest of the people were much easier to communicate with. We (me and Jeff) began with showing him the 3D Rhino model of the design. It is understandable that this is not very clear. We then moved on to giving him plans and sections of the structure. However, spending the first two days doing mock ups we realised that the plans and sections do not communicate our ideas to him and he is building things that are not out intentions. Following that we decided that the easiest way is to make a physical model so that he could see the structure – also very importantly the different levels of the ground. The model was made of chopsticks and scrap wood pieces and was labelled heavily to simplify the process for the carpenter. This was successful as the discussions were able to focus on the model and we could pin-point which joint and beam we are discussing etc. The carpenter also referred to the model often when he is working to ensure that he is making the joints right.
The model also served very interestingly as a conversation piece between us and the villagers/curious onlookers. I had placed a pile of paper near the model to invite people to write their comments, although this was not very successful. However, having that physical and visual representation of what we are trying to build is an important communication tool to reach the villagers. It might also be that any structure that we build in the future, there is a communication through physical models. Maybe the design involvement and workshop could use models instead of drawings.
Timber is fixed onto each other with mortise and tenon joints (Appendix e). We observed the carpenters making these joints and recorded the process. When a new set of equipment arrived we sanded the chisels and axe and participated in the making process, starting with making a work bench (Appendix f). Beams and columns are locked into each other with mortise and tenon joints and end pegs. The beams could be stacked on top of each other and connected together with pegs to make thicker beams. The beams have to be vertically staggered to minimise compromise to the timber and not creating a four-way hole at any one point of the timber. The direction of the beams (vertically or horizontally) is controlled not by the position of the mortise but by the shape of the tenon (angled or straight). The pieces are made off-site and then transported on-site for assembly.
Connection between timber and brick became problematic. This is because the brick columns were standing alone and unstable when finished and there is a need to lock in the bricks column with the timber column. THe more continuously connected the structure is the more stable it is. The current practice is to place the timber on top of the brick. The team was unsure that this is sufficient joining between the two different materials. There were two proposed solutions, one to add a metal plate on the bottom of the brick, subject to the availability of steel plates. The other is to make timber parts on all columns which then act as clamps to prevent the timber from sliding off (Appendix d).
The other issues on structural stability included the viability of timber columns that have cracks in them. There were doubts whether the cracking is a cause for concern. The carpenter was not worried as apparently only cracked wood is able to dry properly. However, some of the timber has huge cracks across the length of the wood and we ultimately decided to use a thicker piece of timber to replace these cracked ones. The other option is to use steel ties to tie around the timber, however, these were not readily available (Appendix d).
The structural stability of the two standing columns were an issue. The proposed solutions included knocking them down and making a new wall tied into the columns so that the columns could at least withstand one direction of the horizontal forces. Negotiations with the workers resulted in a compromise using a different solution as they have already rebuilt columns twice and are very happy about redoing work again. This solution is to make a timber clamp in between the two columns and hold them together. This could also be applied to the other side and thus unsure that on both directions there is resistance (Appendix d).
Construction on Site/Brickwork
Clearing the site – work on site started with clearing the vegetation and grass. This included burning the site. However, this could only be done when the vegetation has been cut and left out to dry for at least a night. This way we can ensure that it will actually burn.
Fengshui – there is not much belief in Fengshui in the area. MM and Gao seem to have more respect for the Fengshui than the actual villagers do. This is something interesting – Fengshui is now viewed as a part of the culture of Chinese architecture and something to be respected and imposed from the outsider, as a method to reconnect to the past almost.
One of the final steps to completing the pavilion is to incorporate the landscape designs for the pavement, walls and benches.