Sunday, February 9, 2014
Light weight aggregates can be either natural like diatomite, pumice, scoria, volcanic cinders, etc, or manufactured like bloated clay, sintered fly ash or foamed blast furnace slag. Light weight aggregates are used in structural concrete and masonry blocks for reduction of the self weight of the structure. The other usages of light weight aggregate are for better thermal insulation and improved fire resistance.
Unlike for normal weight concrete aggregates from natural source there is no IS specification for light weight aggregates. However IS 9142 covers specification for artificial light weight aggregate for concrete masonry units, and IS 2686 covers cinder aggregate for use in lime concrete for the manufacture of precast blocks. However IS 456 envisages the use of bloated clay and sintered fly ash aggregate with proven performance in structural concrete.
The main requirement of light weight aggregates is their low density, some specification limit the bulk density to 1200 kg/[censored] for fine aggregates and 960 kg/[censored] for coarse aggregates for use in concrete. Both fine and coarse aggregates may be lightweight. Alternatively light weight coarse aggregates can be used with natural sands.
The following are the characteristic of light weight aggregates which require consideration for use in structural concrete are as follows :
a) Some light weight aggregates may contain closed pores or voids in the material, apart from high water absorption of the order of 8 to 12 percent. The closed pore system inside the aggregate mass will not be accessible to mixing water but they will displace an equal amount of mixing water or paste( equal to the volume of the pore). The water absorption of the aggregate will absorb part of mixing water when the aggregate comes in contact with water and required water will not be available for the hydration of cement . So the relation between relative density, water absorption, moisture content of the aggregates on the one hand and workability, density and compressive strength on the other may have to be established.
b) Being artificially produced by sintering or pelletizing , more of the synthetic aggregates may have a smooth surface and rather regular shapes which may reduce the bond characteristic with the mortar and thereby result in lower compressive strength.
c) If, during mixing the light weight aggregates get crushed , the void structure is broken down resulting in a coarse surface texture which may lower the workability.
d) The modulus of elasticity of concretes made with light weight aggregates is lower than the normal weight concrete may be ½ to ¾ . Creep and shrinkage of concrete are also greater(will vary from equal to about double) compared to normal weight concrete, having the same compressive strength.
Lightweight concrete will have a range in weight from about 1280 to 1920 kg/m3 (80 to 120 lb/ft3) depending on the lightweight material used. Some lightweight aggregate will produce concrete of 1760 kg/m3 (110 lb/ft3) and have compressive strengths comparable to conventional concrete
Advantages:
a) It reduces the weight as filling material where structural problem arise.
b) It prevents ingress of heat
c) Reduces heat loss in winter minimises expansion and contracation.
d) Protect concrete slab cracking.
Unlike for normal weight concrete aggregates from natural source there is no IS specification for light weight aggregates. However IS 9142 covers specification for artificial light weight aggregate for concrete masonry units, and IS 2686 covers cinder aggregate for use in lime concrete for the manufacture of precast blocks. However IS 456 envisages the use of bloated clay and sintered fly ash aggregate with proven performance in structural concrete.
The main requirement of light weight aggregates is their low density, some specification limit the bulk density to 1200 kg/[censored] for fine aggregates and 960 kg/[censored] for coarse aggregates for use in concrete. Both fine and coarse aggregates may be lightweight. Alternatively light weight coarse aggregates can be used with natural sands.
The following are the characteristic of light weight aggregates which require consideration for use in structural concrete are as follows :
a) Some light weight aggregates may contain closed pores or voids in the material, apart from high water absorption of the order of 8 to 12 percent. The closed pore system inside the aggregate mass will not be accessible to mixing water but they will displace an equal amount of mixing water or paste( equal to the volume of the pore). The water absorption of the aggregate will absorb part of mixing water when the aggregate comes in contact with water and required water will not be available for the hydration of cement . So the relation between relative density, water absorption, moisture content of the aggregates on the one hand and workability, density and compressive strength on the other may have to be established.
b) Being artificially produced by sintering or pelletizing , more of the synthetic aggregates may have a smooth surface and rather regular shapes which may reduce the bond characteristic with the mortar and thereby result in lower compressive strength.
c) If, during mixing the light weight aggregates get crushed , the void structure is broken down resulting in a coarse surface texture which may lower the workability.
d) The modulus of elasticity of concretes made with light weight aggregates is lower than the normal weight concrete may be ½ to ¾ . Creep and shrinkage of concrete are also greater(will vary from equal to about double) compared to normal weight concrete, having the same compressive strength.
Lightweight concrete will have a range in weight from about 1280 to 1920 kg/m3 (80 to 120 lb/ft3) depending on the lightweight material used. Some lightweight aggregate will produce concrete of 1760 kg/m3 (110 lb/ft3) and have compressive strengths comparable to conventional concrete
Advantages:
a) It reduces the weight as filling material where structural problem arise.
b) It prevents ingress of heat
c) Reduces heat loss in winter minimises expansion and contracation.
d) Protect concrete slab cracking.
