4 major of building deterioration agents

• Weather agent 
• Chemical agents 
• BIological agents 
• Mechanical agents 

Weathering agents of building dete4rioration 

- Solar radiation 

- Moisture 

- Wind 

- Driving rain 

- Atmospheric gases 

Solar radiation 

- Solar raditaiton is received at the surface of the earth both directl & diffused long-wave radiation . 

- The intensity of solar radiation reaching the earth is reduced because of reflection , scattering ,& adsorption of the radiation by water vapour , ozone , air & dust particles . 

The intensity of solar radiation rteaching the earth is reduced because of reflection , scattering & adsorption of the radiation by water vapour , ozone , air & dust particles . 

Ultra-violet radiation is of shorter wavelength belonging to the region beyojnd the volet end of the visible spectrum , while infra -red radiation is of longer wavelength beyond the red end of the visible spectrum . 

Solar radiation affects building materials in 2 ways i.e. phtotochemical reaction & thermal movement . 

Solar reactionm : photochemical Effects 

- The only chemical effect of visible & infra-red  radiation s to speed up the rate of feterioration caused by other agents .
- During the attack by ultra violet radiation , the main chain of the olymers may be broken in isolated locations or reactive areas in large molecules may react with other chains . The resultants cross-linking of the moleculres makes the material harder & more brittle .

- Exposure to ultra-violet radiation can also cause changes or loss of colour of the oranic materials , particularly the blues & greens . A good example of this is the yellowing & surface delamination of glass fibre reinforced polyesters sheet .

- Most building materials are opaque & are therefore susceptible to attack by radiation . But the radiation absorption coefficeient s vary considerably from materials to material , depending on the colour & texture .
- Solar radiation is abrorbed when it strikes a materials . As a materials warm up , it expand & as it cools off , it contract .

- A further effect of temperature is the change in viscosity that occurs in liquids & in some organic materials  , such as bitumens & sealants . As the materials is heated , it beconmes thinner & flows more easily . As it cools it thickens & at a sufficiently low temperature it can be quite brittle .

- Most chemical reactions increase in rate with increasing temperature  . For example , the degradation reactions responsible for the breakdown of plastic sheets are initiated by ultra violet readiation , but the rate of deterioration i s largely dependent on temperature .

Moisture
- Moisture is the principle agent of deterioration & is probably also the agent witht the greatest influence on the properties of materials .
- In many cases , moisture is a prerequisite for physical , chemical or biological reactions to take place .

Example include :
- Changes in relative humidity can lead to dimensional changes in materials , with dformation , crazing or cracking .
- Rain , especially when driven by strong winds , can erode & dissolve certain soft materials .


Moisture include
- When water freezes in the pores of materials such as bricks , stones & concrete , stresses are produced which may cause spalling of the surface .
- Presence of moisture can promorete corrosion of metals , efflorescence & other chemical reactions .

- Moisture also creates an environments for fungal growth as well as atack by insects in organic materials .
- Giants hailstones can cause damage to glass surfgace & roofing tiles .

Wind
- mWind can cause dampnsss by driving moistur einto or thouigh a building fabric & excessive heat uncontrolled air changes .

- The consequent compression & suction forces may result in the loading or lifting of the surface . Suction forces tend to remove roofs or covering along the vulnerable edges of a building .

Driving Rain
- Driving rain is rain carried along at an angle to the vertical by wind so that it impinges on the vertical surfaces of the building .

- The effect of driving rain is that the vertical surfaces dacing the wind now receive rainwater although at an angle . The run-off from the vertical surfaces will increase the loadss on any abutting horizontal or sloping surface unless the water is drained away separately .

- When a droplet of rain driven by wind stikes the building surface , it fall apart &  its kinetic energy may force part of the droplet into the pores of the material used in the construction .

- The accumulation of moisture within the material may cause expansion of the materials while slower surface drying cause contraction . The resultant stresses set up may lead to disintegration of the surface layer .

Atmospheric Gases

- Atmospheric gases include sulphur dioxide , carbon dioxide , oxygen & ozone .
- In the presence of moisture they contribute to the formation of acids that attack certain materials such as metals , concrete m other cementitious products & stones .
- Sulphur dioxide gases with other hydrocarbon emissions from power stations & other industrial pollutants , form what is generallyu known as acid rain .
- Its inujurious attack on building materials inclues erosion of brickwork & corrosion of metals .
- The presence of carbon diozide in the atmosphere is generally benefiticial is reacting with lime products to form a protective carbonated layers .
- Hoewver if carbionation is allowed to proceed as far as reinforcement in concrete , it can cause the corrosion of the steel bar .
- Oxygen can cause oxidation of organic material such as paints , plasters & sealants & is particularly severe on unprotected steelwork .
- Ozone , which is an unstable modification of oxygen , is very much more reactive towards organic materials . It plays a major role in the degradation of rubber , mastics , bituminous compounds , paints & plastics , which can result in their embrittlement & eventual failure .