Light Weight Concrete

One of the biggest disadvantage of conventional concrete is its heavy weight. This heavy weight make it, to some extent an uneconomical structure material. This problem is addressed by the use of light weight concrete which offers the following advantages:

Aerated Concrete

• It is obtained by introducing air or gas into a slurry consisting of portland cement or lime and finely crushed siliceous filler, so that a uniform cellular structure is formed when the mix hardens.
• It is also referred as gas concrete, foam concrete or cellular concrete.
• Aerated concrete is manufactured by the following methods:
  (i) By the formation of gas by some chemical reaction when the mix is in liquid or plastic stage.
  (ii) By mixing preformed stable foam with slurry.
  (iii) By using finely divided metal with slurry and reacting it with calcium hydroxide to give out large
  quantity of hydrogen gas that leads to the formation of a cellular structure.
• The density of aerated concrete varies from 400 kg/m3 to about 800 kg/m3. Lower density grades are employed for insulation purpose, medium density grades for manufacture of building blocks or load bearing walls and high density grades are used in conjunction with steel reinforcement.

No-Fines Concrete

• It is made up of only coarse-aggregate, cement and water.
• Normally aggregates of size passing through 20 mm and retained on 10 mm are used and aggregate cement ratio varies from 6 : 1 to 10 : 1.
• Unlike the conventional concrete in which water/cement ratio is the primary factor determining strength, the strength of no-fines concrete is dependent on the water/cement ratio, aggregate/cement ratio and unit weight of concrete.
• When conventional aggregates are used, no-fines concrete show a density of about 1600 to 1900 kg/m3 but when light weight aggregates are used, the density may come down to upto 360 kg/m3.
• The bond-strength of no-fines concrete is very low and hence reinforcement is not used along with no-fines concrete. However if reinforcement is required it is advisable to coat reinforcement with cement paste to improve the bond and also to protect it from rusting.
• It lacks cohesiveness due to absence of fine-aggregates, thus it requires long time for formwork removal.
• There is no standard method (slump/compaction factor) to measure the consistency/workability of no-fines concrete.

GENERAL PRECAUTIONS IN CEMENT CONCRETE CONSTRUCTION

Following precautions should be observed:

• Cement should be fresh and free from set cement particles.
• Aggregates should be well graded and free from dirt, clay, silt and other deleterious materials.
• Mixing water should be free from harmful chemicals and foreign materials.
• The quantity of concrete prepared each time should be used and finished within 30 minutes or initial setting time of cement.
• Concrete should be prepared on a rigid, impervious and water tight platform without loosing any cement or water.
• The quantity of cement, sand, coarse aggregate should be correctly measured either by volume or mass as desired in specifications of the mix proportion. Measurement by mass is more accurate and results in better quality construction.
• Complete mixing of ingredients either by hand or by mixer should be ensured.
• Care should be taken to avoid bleeding and segregation during transporting or placing concrete.
• Form work or surface on which concrete is to be laid, should be cleaned and moistened with water before laying concrete.
• The form work or other base on which concrete is to be laid should be checked for its rigidity, shape, size and other details and should be prepared well before concreting.
• Shuttering and centering should be checked and ensured thoroughly before concreting operation.
• Reinforcement and other components to be embedded in concrete should be thoroughly checked for their position, size, concrete cover and correct quantity before laying the concrete.
• Laid concrete should be compacted thoroughly by manual tamping or mechanical vibration in layers of 150 to 450 mm. Care should be taken to avoid segregation and bleeding due to improper
  compaction.
• During intermittent or concreting in different spells suitable construction joints should be formed to obtain a proper bond between the previously laid concrete and freshly laid concrete by using
  special techniques.
• In long concrete structures, provide suitable expansion and contraction joints.
• During extreme weather conditions, precautions for concreting should be taken by using special admixtures in suitable proportion and applying protective techniques of construction.
• Concrete construction should be cured for sufficient period (14 to 28 days) as specified.
• Shuttering and centering should be removed after a predetermined period and in a predetermined sequence.
• Patch work or surface finish if any on the concrete surface should be done immediately on removal of shuttering and such a work should be properly cured for sufficient period (14 to 28 days).

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