Special Type of Concrete - Light Weight Concrete
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 manufacturing 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/cum for fine aggregates and 960 kg/cum 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 minimizes expansion and contraction.
d) Protect concrete slab cracking.
There are two types of artificial light weight materials available to produce Light Weight concrete.
1. Hemalite
2. Vermiculite.
Method of making, mixing proportions of light weight concrete made with artificial aggregates:
Insulating concrete can be prepared either by machine mixing or hand mixing. Where area less than 500 sq.m. then hand mixing is preferred and for more than 500 sq.m area machine mixing preferred.
Hand mixing:
1. Place the required quantity of light weight material in mixing place and spread the material.(say 3 , 4 , 6 part depend on the material used)
2. Add one bag of cement to it and mix it thoroughly with out adding water.
3. Add about 60 lit of water to the mix and mix it thoroughly to decide any addition water is required.
4. To check the water requirement press a handful of mix and when firmly gripped it would just release little water and no more. Care to be taken to avoid bailing and if it occurs it should be punctured.
Machine mixing:
1. Add about 60 lit of water to the mixer machine.
2. Add one bag of cement to it and mix well to form a slurry.
3. Add the required quantity of light weight material to the mixer drum .
4. Mix well to get uniform consistency.
5. If bailing occurs then add a little water and mix well.
Laying , Finishing and Curing:
Surface Preparation :
The surface to receive light weight concrete should be cleaned free from dust and loose materials.
Laying and finishing:
1. Lay the light weight concrete in place without any delay.
2. The concrete laid to be raked or spread so as to eliminate air voids to the required thickness.
3. The concrete can be laid in bays and a screed topping of 20 to 25 mm thick in the ratio of 1:2:3 (Cement :Sand : Stone chips). The thickness and proportion depends on the load of the topping screed.
4. The screed to be cured for a minimum period of 7 days.
5. When used ass water proofing : Light weight concrete should not be exposed to rain or direct weather where it is likely to contact water. Due to its porous nature water readily seeps in to it and will lessen the insulating value.
6. To have best insulating value water proofing to be done over the screed to avoid water penetrating inside light weight concrete.
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 manufacturing 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/cum for fine aggregates and 960 kg/cum 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 minimizes expansion and contraction.
d) Protect concrete slab cracking.
There are two types of artificial light weight materials available to produce Light Weight concrete.
1. Hemalite
2. Vermiculite.
Method of making, mixing proportions of light weight concrete made with artificial aggregates:
| Mix Proportion | |||
|---|---|---|---|
Description | Vermiculite | Hemalite For Roof | Hemalite For Floor |
Cement | 1 | 1 | 1 |
Light weight material | 6 | 4 | 3 |
Water | 50 to 60 liters | 70 to 80 liters | 50 to 60 liters |
Insulating concrete can be prepared either by machine mixing or hand mixing. Where area less than 500 sq.m. then hand mixing is preferred and for more than 500 sq.m area machine mixing preferred.
Hand mixing:
1. Place the required quantity of light weight material in mixing place and spread the material.(say 3 , 4 , 6 part depend on the material used)
2. Add one bag of cement to it and mix it thoroughly with out adding water.
3. Add about 60 lit of water to the mix and mix it thoroughly to decide any addition water is required.
4. To check the water requirement press a handful of mix and when firmly gripped it would just release little water and no more. Care to be taken to avoid bailing and if it occurs it should be punctured.
Machine mixing:
1. Add about 60 lit of water to the mixer machine.
2. Add one bag of cement to it and mix well to form a slurry.
3. Add the required quantity of light weight material to the mixer drum .
4. Mix well to get uniform consistency.
5. If bailing occurs then add a little water and mix well.
Laying , Finishing and Curing:
Surface Preparation :
The surface to receive light weight concrete should be cleaned free from dust and loose materials.
Laying and finishing:
1. Lay the light weight concrete in place without any delay.
2. The concrete laid to be raked or spread so as to eliminate air voids to the required thickness.
3. The concrete can be laid in bays and a screed topping of 20 to 25 mm thick in the ratio of 1:2:3 (Cement :Sand : Stone chips). The thickness and proportion depends on the load of the topping screed.
4. The screed to be cured for a minimum period of 7 days.
5. When used ass water proofing : Light weight concrete should not be exposed to rain or direct weather where it is likely to contact water. Due to its porous nature water readily seeps in to it and will lessen the insulating value.
6. To have best insulating value water proofing to be done over the screed to avoid water penetrating inside light weight concrete.
