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.

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.

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