WORKABILITY OF CONCRETE

WORKABILITY OF CONCRETE

   The term workability indicate the ease or difficulty with which the concrete is handled, transported and placed. The amount of water present in the concrete should be in proper ratio. If water is much more then required, it will result in non-uniform composition of concrete. If water is less than required it will result in difficulty to handle and place it in position. The concrete should enable to be fully compacted with gives efforts at site. Generally a higher water- cement ratio is required for good workability. In actual, function of water is to lubricate the lubricate the concrete. The concrete which is easy for handling and placing is an workable concrete. The lubricant should be in such a manner so it porduce concrete without segregation, without loss of homegeneity and to finish it sufficiently easily.

Factors of Workability 

•  Water Content Of Mix
• Influence Of aggregate Mix Proportions
• Aggregate Properties
• Time and Temperature
• Loss Of Workability
• Cement Of Characteristics
• Admixtures

Water Content Of Mix

 This is the single most importance fact or governing workability of concrete. A group of particles requires a certain amount of water. Water is absorbed on the particle surface, in the volumes between particles, and provides "lubrication" to help the particles past one another most easily. Influence Of aggregate Mix Proportions   Increasing the proportion of aggregates relative to the cement will decrease the workability of the concrete. Also, any additional fines will require more cement in the mix. An "oversanded" mix will be permeable and less economical. A concrete deficient of fines will be difficult to finish and prone to segregation. 

 Time And Temperature

  In general, increasing temperature will cause an increase in the rate of hydration and evaporation. Both of these effects lead to a loss of workability.

 Aggregate properties

  The ratio of coarse/fine aggregate is not the only factor affecting workability. The gradation and particle size of sands are important, Shape and texture of aggregate will also affect workability. Spherical shaped particles will not have the interaction problems associated with more angular particles. Also, spherical shapes have a low surface/volume ratio, therefore, less cement will be required to coat each particle and more will be available to contribute to the workability of the concrete. Aggregate which is porous will absorb more water leaving less to provide workability.

 Loss Of Workability 

  Workability will decrease with time due to several factors; continued slow hydration and during dormant period, loss of water through evaporation and absorption, increased particle interaction due to the formation of hydration products on the particle surface. Loss of workability is measured as "slump loss" with time.

 Cement Characteristics

 Cement characteristics are less important than aggregate properties in determining workability. However, the increased fineness of raid-hardening cements will result in rapid hydration and increased water requirement, both of which reduce workability. 

 Admixtures 

 In general, air entraining, water reducing, and set- retarding admixtures will all improve workability. However, some chemical admixtures will react differently with cement and aggregate and may result in reduced workability.