Chapter 2.pptx Agencies Causing detoration
- 1. Chapter 2 AGENCIES CAUSING DETERIORATION
- 2. INTRODUCTION In course of time, distress would appear in structures as a natural phenomenon. This natural metamorphosis is due to natural wear and tear. The old structures which were constructed in non-aggressive environment and without any special preventive measure are now very badly affected due to gradual deterioration of the environment around them. These structure need immediate attention for ascertaining the causes of their distress and for proper diagnosis and selection of methodology for rational solution and rehabilitation of the structures. DEFINITION OF DETERIORATION/DECAY In maintenance engineering, deterioration is defined as the gradual and continuous process of degeneration of a structure or its components which render it un usable. Deterioration or decay is the development of defect in a structure may be due to natural cause of ageing. If deterioration is not checked or is allowed to occur, decomposition of materials results and replacement becomes the only solution. The rate of deterioration depends on the resisting capability of materials.
- 3. FACTORS CAUSING DETERIORATION The various factors responsible for causing deterioration are classified as follows • Human factors • Chemical factors • Environmental factors • Miscellaneous factors 1. HUMAN FACTORS: Most of the deterioration of structures is due to human factors. It may be either due to lapses at the construction stage or due to misuse of the building by its occupants or due to improper and inadequate maintenance of the building. Following are the human factors causing deterioration of the building: (1) Use of poor construction materials. (2) Poor workmanship during construction. (3) Lack of supervision during construction. (4) Lack of knowledge about factors causing deterioration. (5) Failure to carry out routine maintenance. (6) Lack of awareness of maintenance needs among the users. (7) Failure to establish acceptable standard of maintenance. (8) Poor planning, budgeting and allocation of funds for maintenance work. (9) Having a casual approach towards repairs and a negative attitude of waiting until emergency measures are required. (10) Blatant abuse of buildings, its fittings, finishes etc. (11) Lack of awareness among occupants leading to damage of building
- 4. Continue 2. CHEMICAL FACTORS: Chemical reactions take place due to interaction of certain materials with surrounding environment. This leads to disintegration, softening or discoloration of materials or components. Corrosion is the result of chemical reactions of the materials with air and water of the atmosphere. 3. ENVIRONMENTAL FACTORS: Environmental factors refer to exposure of building components with atmospheric agencies such as sun, wind, rain, ground salts etc. The various environmental factors causing deterioration are: (i) Atmospheric moisture: Atmospheric moisture is regarded as the principal agent causing deterioration of the structures. Moisture is always present in the atmosphere and when surface temperature falls, condensation can occur. Water frozen in the pores of materials can cause spalling of the surface, cracking or disintegration. (ii) Temperature effects: Temperature changes cause temperature stresses in the materials of the structure. These stresses cause rupture and failure of the building materials. (iii) Gaseous pollutants of air: The ever-increasing pollution also is the major source causing deterioration of structures. SO2 is the most aggressive gaseous pollutant which causes corrosion of some metals and causes some stones to blister and spall. CO₂ also forms a weak acid, capable of slowly eroding limestone. The extent of carbonation in concrete has a marked influence on the corrosion rate of reinforcement. (iv) Solid contaminants: The dirt from the atmosphere also has adverse effect on the buildings. The dirt also contains some soluble salts. It absorbs water from the atmosphere and accelerates corrosion rate of metals and deterioration of some stone surface.
- 5. Continue 5. Groundwater and salts: Salts present in the groundwater rise in solution by capillary action. On evaporation of water, salts remain deposited on the surface and damage the building. Usually it causes efflorescence and defacing of building surfaces and finishes. More seriously if magnesium sulphate is present in groundwater, disintegration of rendering and masonry surface can occur. Acidic ground water can cause disintegration of concrete. 6. Biological agencies: Some construction materials get affected by biological agencies such as algae, moss, termites etc. The attack of termites on timber is very prominent. 4. MISCELLANEOUS FACTORS: The following are the miscellaneous factors causing deterioration of the structures: (i) Poor construction materials: Use of sub-standard construction materials, inadequate inspection of materials, poor storages facilities at site for construction materials and inconsistent mixing of materials at site are factors responsible for deterioration of buildings at a later stage. (ii) Poor design: Poor and faulty design leads to faster deterioration of structures. (iii) Poor workmanship: The following factors lead to deterioration of the structure • Failure to understand/follow exactly the specifications and drawing. • Lack of skilled labour. • Lack of supervision during construction. • Failure to replace the defective work noticed, if any. • Over-emphasis on quantity rather than quality of construction. (iv) Misuse of buildings: It includes the use of building for which it has not been designed. Blatant misuse of building, its fittings, furnishing etc, may lead to deterioration of the building.
- 6. EFFECTS OF VARIOUS AGENCIES OF DETERIORATION ON DIFFERENT BUILDING MATERIALS The choice of building material is governed by A. Ability to withstand the effect of climate B. Ability to fulfil the designed functions C. Reaction with surrounding materials D. Ease of maintenance and replacement E. Overall economic acceptability. The effects of various agencies of deterioration on different building materials is as follows: 1. BRICKS: Generally bricks have good durability. The most common effects of weathering on bricks are: 1. Efflorescence (deposition of white powdery materials causing disfigurement of bricks). 2. Spalling of the external surfaces. 3. Change in appearance. 2. TIMBER: The following are the effects on durability of timber: Timber decays as a result of destructive action of 'fungi' (called dry rot) growing on it. (ii) Dry rot requires a moisture content of about 20% and spreads very rapidly. (iii) Insect infestation (i.e. beetles, termites) destroys timber used in building. (iv) Exposure to natural weathering agents such as rain, wind and temperature contribute a lot to fast decay of timber. The following steps are taken to avoid deterioration of timber: (1) Proper seasoning of timber (reduction of moisture content to optimum level) is done to make it last longer. (2) Preservatives are used to preserve the timber from decaying. Such treatment is done to ensure a longer, trouble free life of timber.
- 7. Continue 3. CONCRETE: Concrete is a relatively durable material but its durability is affected due to the following factors: a. Freezing and thawing: Water entering in the pores (voids) of concrete freezes in cold climate. Due to increase in volume of water on freezing results into disintegration of concrete. b. Sub-soil salt attack: The water soluble sulphates in soil when comes in contact of concrete causes its expansion, spalling and disintegrations. The extent of damage of concrete will depend upon the amount and types of sulphate present in the ground water and quality of concrete. c. Alkali-aggregate reactions: Silica present in aggregates reacts with the alkalis of cement in the presence of water and causes expansion and subsequent damages to concrete. d. Corrosion of steel: Corrosion of steel bars in R.C.C. structures reduces the durability of concrete in contact with steel bars. Rusting of steel bars causes spalling and cracking of concrete. Deterioration is aggravated in case the concrete is permeable or concrete cover to steel reinforcement is inadequate. Effect of deterioration of concrete can be controlled by the following steps: (i) Using optimum water cement ratio. (ii) Using sound and fresh cement. (iii) Using durable, densely graded and non-reactive aggregates. (iv) Using proper batching and mixing equipments and methods. (v) Providing thorough and uniform compaction.(vi) Providing proper curing
- 8. Continue 4.METALS: Metals used in buildings have good durability. Durability of metals is affected by corrosion. Corrosion is a complex electro-chemical reaction. It is aggravated by the presence of dissolved atmospheric gaseous pollutants, dirt, admixtures. The risk of corrosion is increased when metals are in contact with other building materials such as bricks or plaster. Mild steel largely used in building construction is seldom exposed but has a protective coating of paint, bitumen or is enclosed by other materials, usually by concrete in R.C.C. Protective coatings reduce the rate of corrosion. In R.C.C. members, it is the concrete cover that cracks first. Therefore, the cover to steel in R.C.C. structure should be according to the exposure of the structure to the environment. When steel corrodes very much, then only we can see both broken concrete and corroded steel. The cast iron pipes buried in the ground also get rusted but do not deteriorate. The rusted part remains in its place and acts as a protection against further corrosion. Many cast iron pipes buried under ground last for very long periods in the field. Steel pipes do not last so long.
- 9. Continue 5. PAINTS: Paints are coating of coloured liquid materials applied on the surface of the finished parts of the building, which on drying, forms as an impervious coat and protects the surfaces from the effects of atmospheric agencies, decay of wood and corrosion of metal and also serves as a decorative surface. The following defects are observed in painting due to various agencies of deterioration: (i) Blistering: This is due to trapping of water vapour behind the painted surface. This forces the paint into little bubbles or blisters. (ii) Fading: When the painted surface is exposed to direct sunlight, gradual fading of colour due to loss of brightness of pigment occurs. (iii) Blooming: This is the development of dull patches due to presence of moisture or chilling of surface glossy cont. All other defects in paints are due to following reasons: (a) Use of poor material: All the paints should be selected of good quality in relation to their exposure condition and backing material. (b) Application on damp surface: Dampness breaks down the adhesion of the paint with the surface of the component causing flaking and cracking from the surface. (c) Poor workmanship: It is one of the main causes of paint deterioration and defects. Poor workmanship can be attributed to incorrect, inadequate or non-existent surface preparation. Over thinning of paint, in proper brush selection poor brushing techniques and failure to apply the specified number of coats may result in deterioration of paints
- 10. Continue 6. PLASTICS: A wide range of plastics are used in buildings. Polyvinyl chloride (PVC) has the widest application. Plasticised PVC is extensively used as floor covering, false ceiling under pitched roofs, membrane covering in flat roofs and plastic membranes for waterproofing. Rigid unplasticised PVC is used principally for domestic soil and vent-systems, rainwater disposal, drainage, wall cladding, dueting etc. Expanded PVC is also available for thermal insulation. Glass Reinforced Plastics (GRP) are also used for structural purposes. Special plastics are used for large drainage chamber, plumbing, drainage fittings and wall ties. Foamed plastic provide a cellular material used for thermal insulation. The following defects are observed in plastics: 1. Short wave solar radiations degrades plastics by causing embrittlement and change in surface appearance. 2. Moisture in general has little effect but can reduce bond strength between glass fibre and polyester resin. 3. Cracking of polyethylene in cold-water cisterns is caused by use of oil based jointing compounds. 4. PVC and polycarbons have high thermal expansion. Unless properly allowed for, the movement of PVC gutters and down pipes can cause joint failure and leakage. 5. Plastics creep under continued loads and special precautions are needed when stresses are high.
- 11. Continue 7. STONES: Natural stones are classified as belonging to one of the three main groups-igneous, sedimentary or metamorphic. The following deteriorations are observed in natural stones due to various weathering agencies: 1. The atmospheric pollution causes deterioration of limestones and sandstones. If wetting and drying are frequent due to rainfall, the surface of the stone gets slowly eroded. 2. Frost may also attack some limestones. 3. Marble is attacked by sulphuric gases. 4. Slates are used for roofing, cladding and D.P.C. Roofing slates are exposed to most severe conditions and can be affected by sulphuric gases. 5. One major cause of damage in all types of stones can be due to corrosion of embedded fixtures. Rusting of iron and steel ramps and dowels cause extensive damage to limestones and sandstones.
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