Timber
Timber has long been used by man especially in building construction. It is the most useful material available for wall, floor, roof and other structural framing. All commercial timbers can be classified into softwoods (such as Pine, Fir and Damar Minyak) and hardwoods (such as Chengal, Meranti and Kapur), depending on the characteristics of their grains, weight and moisture content.
In general, timbers either of softwoods or hardwoods have a moisture content of between 12 to 15 per cent. Normally, a well dried timber has a moisture content of 12%. If the moisture content of the wood exceeds 20%, fungal rots, insect infestation and termite attack will eventually take place. This will further lead to structural failure.
Before timber is being used for building construction, it is important for the material to be seasoned and preserved. The primary aim in seasoning timber (either of air or kiln seasoning) is to render timber as stable as possible, for the timber increases its strength properties as it dries. On the other hand, the preservation of timber, usually by chemical processes either before manufacture or after, concentrates on fungicidal preservation, flame-proof protection and water-repellence application.
Stone
Stone has been used in building construction for thousands of years. Stone comes in different types and properties ranging from the hard impervious such as granite, slate, marble to the softer and pervious sandstone and limestone. Although stones will last for many hundred years, its tendency of decay in any kind of weather is possible.
Weathering occurs in three situations. First, the attacks from soluble salts especially when it comes up from the ground where there is no damp course, in locations near seas or from a heavily polluted atmosphere. Second, trouble arising from the slow build-up of soot deposits and dust, leading to possible onset of decay due to small vegetation organisms. Third, the straight forward erosion by wind and rain. Stone will become saturated when it is exposed excessively to driving rain. As a result, its surfaces becomes marked and rough. Besides weathering, stone may also decay through faulty materials and workmanship.
stone can be deteriorate by the wetting or drying cycles either causing frost damage or introducing soluble salts that crystalise, expand and "blow" the stone. These salts may come from a polluted atmosphere, from the mortars, may be inherent within the stone or from the ground through rising damp or road salt splashes.
Sulfation is an especially common form of decay in urban areas. The calcium carbonate of limestone converts to calcium sulfate by the action of acid rains. Initially a hard protective skin of sulfate is formed but this is the beginnings of a blister that expands and blows off, exposing soft friable decayed stone behind. Cleaning is excellent maintenance, as calcium sulfate is soluble in water and the skin can be washed away, before it causes damage. The skin is always unsightly because it binds black tarry and sooty pollution by products.
Organic growth with its aerial roots and rapid growth is a menace as it quickly penetrate the joints and can threaten the whole stability of a building. Organic acids secreted by suckers and tendrils can damage the surface and the mortar. A heavy growth keeps the wall moist whilst inhibiting healthy rain washing, and can accelerate frost damage. If climbing plants are desirable, they need to be maintained and their growth controlled, perhaps by covering the building with stainless steel wire to keep the plant from clinging to the building itself.
Limestone is also affected by salt attack. road salt can caused crystallisation on the stone face - not destructive but causing unsightly efflorescence. Crystallisation behind the face can damage the stone.
Below is some of the picture of stone deterioration
Brickwork or burnt clay block
Brickwork or burnt clay block has been used in many historic buildings in Malaysia, particularly the ones built during the British occupation. Some of the colonial buildings have exposed brick walls and others are plastered and painted. Old bricks are slightly different than modern bricks. The texture of modern bricks looks closer and smoother, and the edges are straighter and sharper compared to the old materials. Colour and size are also different. Brick may decay through weathering process including sulphurous smoke caused by polluted atmosphere, water penetration through small holes and openings of the brick as well as mortar joints; and dampness in wall caused by no damp course in locations near sea or river. Brick may deteriorate due to harmful vegetation and also mould or fungal growth that accumulate in the brick surface. Brick can also decay due to cracks caused by structural movements. Such structural movements may come from building foundations when subsoil is compressed through the decades or centuries followed by wall deflections due to the foundation weakness or an uneven loading distribution from above wall structure.
Plaster
Plaster tend to deteriorate over a period of time. Plaster normally contains lime, sand and water; and sometimes chopped animal hairs to give tensile strength. Plaster are used widely in decorative panels, ceiling renderings, cornices and internal walls. Causes of deterioration include direct exposure to driving rain, condensation, evaporation, air pollution, aerosols, capillaries, thermal stresses, vegetal causes, insect attacks, animals and human activities. Plaster may become cracked due to either shrinkage or movement in the substrate. Shrinkage usually occurs early in the life of the building but substrata movement is often the reason for failure in historic situations.
Deterioration can be caused by direct or induced mechanical stresses. Much of these depend on changes linked to the humidity present in the masonry, both due to external causes (rainwater) and internal (diffusion of vapour from inside to outside). These phenomena are linked with consequent micro-variations in volume (freezing-thawing), chemical or electrochemical phenomena (efflorescence, oxidation, incompatibility of a chemical type) and biophysical pathologies (moulds, algae).
The presence of a considerable amount of damp behind the facing can be caused by particular geometry of the facade (stringcourses, drip moulding, balconies, etc.) that facilitate water penetration and lead to water stagnation, capillary rising and saline migration towards the rendered surface.
In many historic buildings, defective plastered rendering occurs mostly on external walls, columns and ceiling. In a humid tropical climate, the defects of rendering are normally caused by biological attacks arising out of penetrating rain, evaporation, condensation, air pollution, dehydration and thermal stress. Other causes may come from mould or harmful growth, insects, animals and traffic vibration. Prior to being decomposed and broken apart, plastered rendering may become cracked due to either shrinkage or movement in the substrate itself.
Steel
Deterioration of steel at the material level stems primarily from incompatibility
factors, such as chemical attack, that lead to corrosion. There are a large
number of ways that steel can corrode. A partial list of corrosion types includes
chloride-accelerated, concentration cell, crevice, deposit, electrochemical, electrolytic,
galvanic, pinpoint, and thermogalvanic. Section loss may occur due to corrosion,
reducing the strength or stability of a steel member or system. Proper surface treatment
of steel, such as corrosion resistant paint, often can reduce or prevent the occurrence
of corrosion.
No comments:
Post a Comment