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    FRESH CONCRETE: Characteristics of Fresh Concrete


    FRESH CONCRETE: CHARACTERISTICS OF FRESS CONCRETE



    Basic characteristics of fresh concrete

    The following, are some basic characteristics of fresh concrete, that a civil engineer must know. Fresh concrete, is concrete that is newly mixed, prepared and/or placed, which has not undergone hardening.

    ·        Workability and Stability

    Workability:


          Workability is the ease with which concrete can be transported or moved from the mixer or mixing point, to the final placement point, the final compacted shape, formwork or mould.

    Properties of workability

    Properties of concrete workability include:
    •      Consistency
    •   Mobility
    •   Compactibility

    Consistency is the degree of wetness or fluidity of the concrete mix.
    Mobility is how transferrable the concrete mix is.
    Compactibility is the degree to which the concrete could be fully contacted. That is, the ease to which air voids could be expelled out of the fresh concrete.

    Workability Tests

    Workability tests include:
    •     Slump Test
    •        Compacting Factor Test
    •        V.B. Consistometer

    Factors Affecting Workability

    i.                   Cement and water
    ii.                 Admixtures
    iii.              Aggregates
    iv.               Time
    v.                 Ambient Conditions e.g. humidity, temperature, wind speed, etc.




    Slump Types
    Slump Types: These are the results you can get from your slump tests

    Collapse slump occurs where the mix is too rich, or there is too much water in the mix.

    Shear slump, occurs where the mix is devoid of much cement, in other words, where the cementitious element is insufficient.

    True slump occurs where all elements of the mix are in the right proportion. In other words, cementitious, as well as moisture are in right quantities.

    Admixtures are substances or chemicals added to concrete to alter or change some properties of the concrete. Examples of admixtures include:
    •         Water reducing agents
    •         Air-entraining agents
    •             Retarders, etc.

    Water reducing agents and air-entraining agents are good admixtures for improving concrete workability.

    Stability: 


    Stability as a property of fresh concrete, signifies the uniformity of the concrete mix. If concrete is not stable, it will affect the concrete in two major ways viz:

    Segregation: 

    Segregation, is the tendency for coarse aggregates to separate from the fine aggregates. There is high tendency for segregation to occur in a wet mix, as well as in a hard mix.

    Bleeding: 

    Bleeding in fresh concrete, is a situation where the hard or solid part of the mix, settle below, while the liquid portion, settle on top of the mix due to different specific gravities. The wet portion of the mix, later flows out, leaving behind, a solid mix, devoid of adequate water and cement.


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