Properties of concrete, properties of reinforced concrete, properties of concrete in civil engineering, properties of fresh concrete, long term and short term properties of concrete

Workability 

The amount of mechanical work ,or energy , required to produce full compaction of concrete without segregation.

Workability is not a fundamental property of concrete; to be meaningful it must be related to the type of construction and methods of placing,compacting,and finishing. Concrete that can be readily placed in a massive foundation without segregation would be entirely unworkable in a thin structural member. Concrete that is judged to be workable when high frequency vibrators are available, for construction would be unworkable if hand tamping are used.  

Segregation of concrete

The separation of different size of aggregate in concrete is called segregation. Thus , segregation can be defined as concentration of individual constituents of a heterogeneous mixture so that their distribution is no longer uniform. 

Segregation in concrete is a case of particle segregation in concrete applications, in which particulate solids tend to segregate by virtue of differences in the size, density, shape and other properties of particles of which they are composed 

Bleeding of concrete  

Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic.

Bleeding is a form of segregation in which water present in the concrete mix is pushed upward due to the settlement of cement and aggregate. Bleeding is a form of local concentration of water in some special positions in concrete, usually  the bottom of coarse aggregates the bottom of the reinforcement, and the top surface of the concrete member ,during placing and compaction, some of the water in the mix tends to rise to the surface of the freshly placed concrete. 

Factor affecting the workability of the concrete .

The factor that affect the workability of concrete are following.

1- Water content.

Water content is the most important factor influencing the workability of concrete, After adding water to a concrete mix, the water is absorbed by the surface of the particles of the cement and aggregates.Additional water fills the space among the particles and lubricates the particles by a water film 

Incresing the amount of water will increase the amount of water for lubrication and hence improve the fluidity and make it easy to be compacted.

2-Cement content 

Cement content influence the workability of concrete of two ways. First ,for given  water cement ratio, the larger the cement content ,the higher is the total water amount inthe concrete; hence the consistency of concrete will be enhance .

3- Aggregate .

The particle size of coarse aggregate influences the paste requirements for coating through the surface area .The larger aggregate have smaller surface area than smaller aggregates with the same volume. Subsequently the amount of the paste available for lubrication is increased for concrete with large aggregates,and consistency is improved. 

4- Use of Admixture 

Mineral admixture as well as chemical admixture can influence the workability of concrete. An air entraining agent increases the paste volume and improves the consistency of concrete.

5- Temprature 

High temprature decrease the workability . because hydration rate is higher and loss of water is faster at a higher temperature.