foundation system of burj al arab

Deterioration of Timber Foundation

All natural and man made materials in the world are susceptible to decay or deterioration. According to national science foundation, deterioration is a time dependent phenomenon and every infrastructure may deteriorate due to

• Aging of constituent or constructed materials
• Climate exposure
• Excessive use
• Lack of adequate maintenance
• Difficulties faced in appropriate inspection methods.

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


Fungal decay:


Such decay of timber foundation may be due to

Wet rot
• Dry rot
• Molds

Wet 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.

Dry rot:


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:


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.

The deposit present on the surface of timber is often taken as an indication of having excessive moisture.


Insect decay:


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.

Other factors:

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

 Moisture
 Fire
 Prolonged heating.

Timber piles:

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.

Timber foundations:


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)

Why is Curing of Concrete Required?

To produce a good quality of concrete, both in respect of strength and durability, a properly placed well-proportioned mix must receive curing during initial stage of hydration under suitable environment. The term environment is important as temperature, relative humidity and wind velocity is the vital factor in this process.

What is curing?


The procedure taken for promotion of hydration of cementitious materials is called curing. The tasks consist of control on movement of moisture and temperature from concrete to environment and as well from environment to concrete. In brief curing can be defined as maintaining warm and moist environment around or into concrete just after placement to continue hydration of cement particles until the expected properties of concrete are developed up to a sufficient degree to ensure service requirement of finished member.

Objective of curing:


• To keep specimen saturated or retain saturation as nearly as possible until spaces filled with thin fresh cement paste are filled by hydration products of cement particles to a desired extent. In actual, concrete placed at site, effective curing are stopped always nearly long before maximum expected hydration has been taken place.

• To prevent loss of moisture from concrete; this is essential not only because of adverse effect on strength development but also because of formation of plastic shrinkage, reduction of abrasive resistance, increase in permeability. Thus curing is important for better durability and improved strength.

Durability aspect:


Plastic shrinkage cracking:

When water comes out from any porous body that is not gained rigidity yet, it gets contracted. Such movement of moisture in the early state of concrete is also observed. Some volume change is associated with formation of hydration products and some are associated with moisture loss due to evaporation from exterior surface of concrete when mix is in plastic state. When proper protection is not taken such loss may occur under the suction of underlying soil or dry concrete. Such contraction is termed as plastic shrinkage as concrete remains in plastic state at the time of contraction.

Again plastic shrinkage depends on environmental condition like temperature wind velocity and relative humidity as they influence amount of moisture loss from surface of plastic concrete. However, rate of moisture loss cannot be used alone to determine plastic shrinkage as it is more dependent on rigidity of concrete mix.

Cracking appeared on surface when amount of loss of water from unit area is more than amount of water that come out on surface through bleeding process and becomes significant. Such cracking is termed as plastic shrinkage cracking. Thus evaporation just after placing of concrete must be prevented completely to eliminate such cracking.

Permeability:


Excessive drying of cement paste will increase permeability of it; this is probably due to shrinkage which may rupture some portion of gel formed in capillaries and open new passage along which fluid can travel. Prolong curing period of concrete (wet-curing) having very high water-cement ratio to 7 days instead of 1 day was reported to decrease permeability of concrete by a factor of five. This permeability was tested for permeability of concrete against water, not other fluid.

A change in surrounding relative humidity from 100% to 94% was reported to increase capacity to absorption water of concrete which is an indication of formation of continuous system of larger pores in concrete. Curing at ambient relative humidity less than about 80% was reported to yield increase in volume of pores larger than 37 nm that are an important issue in durability of concrete.

Abrasion resistance:


In case of concrete road or pavement (may be of industrial floor), if top surface is dried out rapidly under the exposure of sun with drying wind, shrinkage stress in plastic concrete is exerted. The resulting dried concrete is weak and cannot withstand large magnitude of stresses due to numerous cracks are developed on the surface. The defective gel structure of concrete will yield weak surface having poor wearing quality.This result less abrasion resistance and under the action of traffic will create dust in dry season and mud in rainy season.

Laitance appeared on the surface also results identical situation, but it is a form of segregation. Good curing is essential to achieve good wearing surface; European Standard (ENV 206) suggested to maintain curing period of two times that of normal curing in this regard.

Strength development:


Concrete gains strength thorough hydration of cement. All particles of cement are not hydrated at a time. This process takes time, though rate is fast at the beginning but hydration continues over a long time; with the increase in duration, rate decreases. The extent of development of hydration product and consequent formation of gel depends on hydration.

Water loss may take place by two ways one is losing water to environment, the other is self-desiccation occur due to chemical reaction during hydration of cement. It is established that hydration of cement only can take place in capillary spaces spaces filled with water. Such self-desiccated water must be replaced with water from external source; this means we have to ensure ingression of water into concrete.

It should keep in mind that only half of water exist in cement paste is effective to take part in chemical combination. This statement is valid even total water present in the mix is not adequate for required chemical reaction.

Strength and permeability relation:


It should be extended that to develop satisfactory strength of concrete, all cement particles must not be hydrated and actually practically it is very rare to achieve this. Concrete quality depends mainly on gel/space ratio of paste. However, if space filled with water in fresh concrete is more than volume to be filled with hydration products, hydration will be greater which will result higher strength and lower permeability.

Management of heat of hydration:

During hydration of cementitious materials considerably high heat is released; such heat of hydration generated within concrete is harmful considering volume stability.If generated heat can be removed by any means, the ill effect of heat can be decreased; in this regard, we can suggest water curing, above figure shows strength development under curing by wet covering and ponding.

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