Monday, June 6, 2011

Comparision between light weight concrete and conventional concrete


Light weight concrete
One of the main advantages of conventional concrete is the self weight of concrete. Density of normal concrete is of the order of 2200 to 2600. This self weight will make it to some extend an uneconomical structural material.
  1. Self weight of light weight concrete varies from 300 to 1850 kg/m3.
  2. It helps reduce the dead load, increase the progress of building and lowers the hauling and handling cost.
  3. The weight of building on foundation is an important factor in the design , particularly in case of weak soil and tall structures. In framed structure , the beam and column have to carry load of wall and floor. If these wall and floor are made of light weight concrete it will result in considerable economy.
  4. Light weight concrete have low thermal conductivity.( In extreme climatic condition where air condition is to installed the use of light weight concrete with low thermal conductivity is advantageous from the point of thermal comfort and low power consumption.
  5. Only method for making concrete light by inclusion of air. This is achieved by a) replacing original mineral aggregate by light weight aggregate, b) By introducing gas or air bubble in mortar c) By omitting sand fraction from concrete. This is called no – fine concrete.
  6. Light weight aggregate include pumice, saw dust rice husk, thermocole beads, formed slag. Etc
  7. Light weight concrete aggregate exhibit high fire resistance.
  8. Structural lightweight aggregate’s cellular structure provides internal curing through water entrainment which is especially beneficial for high-performance concrete
9.  lightweight aggregate has better thermal properties, better fire ratings, reduced shrinkage, excellent freezing and thawing durability, improved contact between aggregate and cement matrix, less micro-cracking as a result of better elastic compatibility, more blast resistant, and has better shock and sound absorption, High-Performance lightweight aggregate concrete also has less cracking, improved skid resistance and is readily placed by the concrete pumping method
  1. Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement.
  2. No fine concrete is made up of only coarse aggregate , cement and water.These type of concrete is used for load bearing cast in situ external walls for building. They are also used for temporary structures because of low initial cost and can be reused as aggregate.
High density concrete
  1. The density of high density concrete varies from 3360 kg/m3 to 3840 kg/m3.They can however be produced with density upto 5820 kg/m3 using iron as both fine and coarse aggregate.
2. Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or manufactured aggregates such as iron or lead shot. The density achieved will depend on the type of aggregate used. Typically using barites the density will be in the region of 3,500kg/m3, which is 45% greater than that of normal concrete, while with magnetite the density will be 3,900kg/m3, or 60% greater than normal concrete. Very heavy concretes can be achieved with iron or lead shot as aggregate, is 5,900kg/m3 and 8,900kg/m3 respectively.
  1. They are mainly used in the construction of radiation shields (medical or nuclear). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures
  2. The ideal property of normal and high density concrete are high modulus of elasticity , low thermal expansion , and creep deformation
  3. Because of high density of concrete there will be tendency for segregation. To avoid this pre placed aggregate method of concreting is adopted.
  4. High Modulus of Elasticity, Low thermal Expansion ,Low elasticity and creep deformation are ideal properties.
  5. The high density. Concrete is used in construction of radiation shields. They are effective and economic construction material for permanent shielding purpose.
  6. Most of the aggregate specific gravity is more than 3.5

Types of admixtures to use in construction


Admixtures  are  sometimes  used  in  concrete  mixtures to  improve  certain  qualities,  such  as  workability, strength,   
durability,   water tightness,   and   wear resistance. 
They   may   also   be   added   to   reduce segregation, reduce
  the  heat  of  hydration,  entrain  air, and accelerate or retard  
setting and hardening. We should note that the same results 
can often be obtained  by  changing  the  mix  proportions 
 or  by selecting  other  suitable  materials  without  
resorting  to the  use  of  admixtures 
 (except  air-entraining  admixtures when  necessary).
Whenever  possible,  comparison should be made 
between these alternatives to determine which  is 
 more  economical  or  convenient. Any admixture
 should be  added  according  to  current specifications 
and under the direction of the crew leader.


Workability Agents
           Materials, such as hydrated lime and bentonite,  are  used  to  improve  workability.  These  materials 
increase   the   fines   in   a   concrete   mix   when  
 an aggregate  is  tested  deficient  in  fines 
 (that  is,  lacks sufficient  fine  material).


Air-Entraining Agents

            The   deliberate   adding   of   millions   of  
 minute disconnected air bubbles to cement paste, 
if evenly diffused,  changes  the  basic  concrete 
 mix  and increases durability, workability, and strength. 
The acceptable  amount  of  entrained  air  in  
a  concrete  mix, by volume, is 3 to 7 percent. 
Air-entraining agents, used with types I, II,
 or III cement, are derivatives of natural  
wood  resins,  animal  or  vegetable  fats, 
 oils, alkali   salts   of   sulfated   organic   
compounds,   and water-soluble soaps. 
Most air-entraining agents are in liquid 
form for use in the mixing water. 

Accelerator 

                 The  only  accepted  accelerator 
 for  general concrete work is calcium chloride 
with not more than2 percent by weight of 
the cement being used. This accelerator is 
added as a solution to the mix water and is
 used to speed up the strength gain. 
Although the final strength is not affected,
 the strength gain for the first 7 days is greatly
 affected. The strength gain for the first 7 days
 can be as high as 1,000 pounds per square 
 inch  (psi)  over  that  of  normal  concrete 
 mixes.


Retarders


             The accepted use for retarders is to reduce
 the rate hydration.This   permits   the   placement 
  and consolidation  of  concrete  before  initial  set.
  Agentsnormally used are fatty acids, sugar, and 
starches