Solar radiation

• Solar radiation is received at the surface of the earth both directly & as diffused long-wave radiation .
• The intensity of solar radiation reaching the earth is reduced because of reflection , scattering & adsoprtion of the radiation by water vapour , ozone , air & dust particles .
• The intensity of solar radiation reaching the earth is reduced because of reflection , scattering & adsoprtion  of the radiation by water vapour , ozone air & dust particles .
• Ultra-violet radiation is of shorter wavelength belonging to the region beyond the violet 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 radiation : Phtotochemical Effects

1. The only chemical effect of visible & infra-red radiation is to speed up the rate of deterioration caused by other agents .
2. During the attack by ultra violet radiation , the main chain of the polymers may be broken in isolated locations or reactive areas in large molecules may react with other chains . The resultant cross-linking of the molecules make the materials harder & more brittle .
3. Exposure to ultra-violet radiation can also cause changes or loss of colour of the organic materials , particularly the blues & greens . A goods example of this is the yellowing & surface delamination of glass fibre reinforced polyester sheet .
4. Most building materials are opaque & are therefore susceptible to attack by radiation . But the radiation absorption coefficients vary considerably from material to material , depending on the colour & texture ( see Table 2.1) .

Aluminium   10-20
White -washed surface  10-15
White oil paint    20-30
Light coloured paint  35-45
White marble  40-50
Medium grey   60-70
Dark coloured paint   65-75
Bricks , concrete   70-75
Glossy black paint   80-85

• Solar radiation is absorbed when it strikes a material . As a material warms up , it expand & as it cools off , it will 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 material is heated , it becomes 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 radiation , but the rate of deterioration is largely dependent on temperature .