These factors result a structural system to become obsolescent as a whole which results a weak system that is required to be repaired, rehabilitated and retrofitted and sometimes even replaced to ensure adequate safety for occupants or users. Here our concern is deterioration of timber foundation.Causes of decay of timber foundation:
In case of timber foundation, decay is the appropriate term instead of deterioration. The decay of timber foundation may be caused by different factors; the main factors are as follows:
• Fungal decay
• Insect decay
• Other factors including chemical and chemical causes
Such decay of timber foundation may be due to
• Wet rot
• Dry rot
Wet rot decay of timber is very common in timber building which is responsible for about 90% of timber decay, used as building materials. Such fungus develops in timber when it remains persistently in wet condition having moisture content of about 50~60 percent.
A study on birch and pine showed that in most cases, loss of weight of wood mass decreased significantly after 3 month (more precisely 12 weeks) by fungus. Depending on developed fungus, the weight loss is altered; it is tried to quantify based in initial moisture content. To determine the pattern of decay and associated weight loss, the factor like moisture content and availability of oxygen is important.
Identification of wet rot:
As foundation is placed under ground level, to visualize component of foundation, it must be made exposed removing earth. The pile/its timber cap or timber foundation subjected to wet rot, can be identified by following symptoms
• The surface of timber seems to be black due to formation of fungus
• Timbers is often felt spongy and soft when touched with finger at the point of discoloration
• If portion of decayed timber is dried out, this is found to have crack and seems fragmented
• The painted surface of timber often found to have damaged finishing, but sometimes leaving painted surface unchanged rotting may progress from back.
Fungus like toadstools and mushrooms are usually responsible for dry rot. Both belong to same group, the reproduction of which is as spores. Such spores can be formed in large numbers under favorable condition. The required moisture content is (20~30)% within timber.
Mechanism of decay:
These fungi digest cellulose of wood and an enzyme known as cellulase is produced. This enzyme is not capable to attack polymer exist in wall of cell which render rigidity (known as lignin).The brittle matrix of lignin is cracked to take form of cubical pieces. The area where fungus attack initially strands are formed which have ability to reach surrounding areas, thus strands spreads to new areas which have ability to supply water and necessary nutrients. These strands are called rhizomorphs. The diameter of strands may be as large as 6 mm (1/4 inch). When moisture content of timber decrease below about 20%; the dormant period of fungus is started and in this condition it can still alive up to 9 month ~1 year).
Appearance of dry rotted timber:
The word completely rotted by dry-rotted fungus means
• Dry-rot fungus often produces deep cracks across timber grain that can be identified with white sheets.
• A dull brown appearance can be noticed with naked eye. Exposed surface of foundation under light makes fungus to take color of lemon-yellow
• The surface crumbled due to feeding of timber
• Sometimes large mushroom-like flat bodies can be grown through paint or finishing like plaster.
• Loss of weight, though in case timber foundation weight-loss cannot be identified except some fractured portions are inspected.
• Initial resinous smell is lost
Such type of decay is also called brown rot; in this process, fungus destroys cellulose of wood but left lignin almost unaffected and wood takes a unique brown color. As a consequence of this, entire structural strength of timber is almost lost.
How is dry rot different from wet rot?
Dry rot is reported to be the most severe type of fungal decay for timber components both in super-structure and sub-structure and spreads out and cause much destruction of timber. Wet rot fungus are very common but they are less serious as decay is often confined within wet portion of timber. The moisture supply must be constant i.e. timber always remains wet.
Porous surface of timber is the ideal condition to grow and develop of wet rot fungus as example high moisture content in porous body of about 50% whereas a moisture content of about 20% is required to grow dry rot fungus. If attention is not paid to moisture accumulation and left untreated, severe structural problems may be led by wet rot as timber member loss strength significantly.For suitable moisture content to grow of wet-rot, it is required to have consistent moisture source; let’s find out the causes-this could be due to lack in underground plumbing fitting, improper drainage around the apron of building.
Molds usually formed over the surface of timber. The porosity of timber element is increased upon formation of molds which permits moisture to penetrate through the surface and helps to keep wet; thus fungus involved wet-rotting are developed and wet-rot decay started.
|Molds formed over the surface of timber of crawling space|
The deposit present on the surface of timber is often taken as an indication of having excessive moisture.
There are different types and species of insects that attack timber. Some of them, like beetles, are formed to bore wood and consume wood directly. Other insects are interested about wood which is already subjected to fungal decay or about damp wood. Some insects, like termites, make complex colonies of adult, winged, soldiers and workers. Termites often make holes into interior portion of timber and live there; but make no mistake to leave outer shell unaffected for their own protection.
There have other factors which can result decay in timber. Some of them are
• Mechanical wear
• Chemical decay
• Decomposition of mass by physical agents like
|Deterioration of timber pile by marine bores|
Timber piles usually are made of trunks of tree, of which, branches are trimmed off carefully and usually small end is chosen to drive as a point. Generally there have limitations about dimension of butt and tip end with tolerance about misalignment.
Timber pile especially can be susceptible to decay because timber exposed in this condition can be perished by various types of organisms.As an example, the portion of timber piles that are immersed in river or marine can be subjected to severe attack by organism developed or lived in this environment like marine bores. Similarly portion of timber piles above groundwater level can also be subjected to deterioration due to insect attack and growth of fungus.
To reduce insect attack and fungal decay, timber piles can be treated by suitable preserving chemical. In this treatment process, the timber piles are placed in pressurized vessel/tank which is filled with creosote or any other chemical that can reduce deterioration. The pressure applied, forced the chemical to penetrated into pores exist in wood which will leave a thick coating over the surface of pile perimeter. Creosote treated timber piles usually can serve as long as the expected design life of structure.
Portion of piles that exist below mud line was neither reported to have deterioration nor marine bores are attacked them. Below level of high tide zone or above mud line, the maximum deteriorations are found as marine bore, various marine borers are active in this zone. Creosote treated piles may lose their strength when they are used to support blast furnaces or any other structures that may subjected to high temperature for long time; this time dependent strength loss was reported by Coduto, 1994.
From the above discussion it can be concluded that if timber piles remain permanently below water table, it will last apparently indefinitely. The problem arises, when they are wetted for some period and then dried to ambient environment and this alternate cycle continues through its life time.
The lifetime we are talking about, in this case, may be as low as one year except timber piles are treated with preservative. Though not relevant, the end of pile that receive driving load can be damaged due to crushing of fiber from hammer energy which is called brooming.
Unlike timber piles, most foundations are constructed now are of concrete. However, some older foundations (mainly of historic buildings) may have timber foundations which are susceptible to deterioration. According to international building code timber can be used as foundation only for type v constructions and they should be treated according to AWPA U1.
Such treatment are not essential where foundation are placed completely below water level (below lowest level seasonal fluctuation)or when timber is used to cap wood piles, installed to be projected over water level of marsh or submerged lands. Compressive stress normal to grain of timber footing that is not treated and supported by treated piles should not more than 70% of allowable stress defined in AF & PA NDS for selected grade and species of timber.