What is Fresh Concrete ? Factors Affecting Workability of Fresh Cement ?

Fresh Concrete

Fresh concrete or plastic concrete is a freshly mixed material which can be moulded into any shape. The relative quantities of cement, aggregates and water mixed together, control the properties of concrete in the wet state as well as in the hardened state. 

There are two sets of criteria that we must consider when making concrete; 

1) Long-term requirements of hardened concrete, such as, strength, durability, and volume stability, 

2) Short-term requirements, like workability. However, these two requirements are not necessarily       complementary.

For fresh concrete to be acceptable, it should: 

1. Be easily mixed and transported. 

2. Be uniform throughout a given batch and between batches. 

3. Be of a consistency so that it can fill completely the forms for which it was designed. 

4. Have the ability to be compacted without excessive loss of energy. 

5. Not segregate during placing and consolidation. 

6. Have good finishing characteristics.  

Factors Affecting Workability of Fresh Cement-

• Water Content of the Mix- This is the single most important 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 move past one another more easily. Therefore, finer particles, necessary for plastic behaviour, require more water. Some side-effects of increased water are loss of strength and possible segregation.

• 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 "over sanded" mix will be permeable and less economical. A concrete deficient of fines will be difficult to finish and prone to segregation.

• 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. It is important to distinguish between total water content, which includes absorbed water, and free water which is available for improving workability.

• 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.

• Loss of Workability -- Workability will decrease with time due to several factors; continued slow hydration of C3S and C3A 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 rapid-hardening cements will result in rapid hydration and increased water requirements, 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 cements and aggregates and may result in reduced workability.