a- What do you mean by the workability of concrete; why do we need workable concrete? b- How various properties of aggregates affect workability of concrete?

 

Q#  a-      What do you mean by the workability of concrete; why do we need workable concrete? 

b-      How various properties of aggregates affect workability of concrete?                                         

 

Solution#4 (to be typed here by the student)

Workability

Workability is a property of raw or fresh concrete mixture. In simple words, workability means the ease of placement and workable concrete means the concrete which can be placed and can be compacted easily without any segregation.

The property of concrete which determines the amount of useful internal work ,necessary to produce full compaction i.e workability is the amount of energy to overcome Friction while compacting. Also defined as the relative ease with which concrete can be mixed ,transported, moulded and compacted..

Compaction and workability are very close to each other. Workability can also be defined as the amount of useful internal work necessary to produce full compaction.

Workability is the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity

 

why do we need workable concrete?

    In every construction work we use different quantitative or qualitative terms to express workability. Before specifying workability for any work a concrete technologist must keep the following things in mind.

            1-Type of construction work

            2-Method of mixing

            3-Thickness of section

            4-Extent of reinforcement

            5-Mode of compaction

            6-Distance of transporting

            7-Method of placing

            8-Environmental condition

      Concrete that can be placed readily without segregation or separation in a mass dam could be entirely unworkable in a thin structural member.

     Workable concrete compacted by means of high frequency vibrators would be unworkable if vibrators could not be used and hand tamping and spading were required.

Concrete having suitable workability for a pavement might be unsuitable for use in a heavily reinforced section.

Answer B

water contents

Water/Cement Ratio or Water Content of Concrete

Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable. Higher water/cement ratio is generally used for manual concrete mixing to make the mixing process easier.

Mix Proportions

Rich concrete mix (cement content is high) is more workable because due to sufficient cement aggregates will have proper lubrication for easy movement which means more workability.

Size of Aggregates

Finer particles require more water for a larger surface, hence aggregate with finer particles need more water to make it workable. On the other hand, bigger particles have less surface area, demand less water for wetting surface and require less amount of paste for lubricating.

So bigger particles give higher workability for fixed water content. But maximum size of aggregate depends on some practical considerations like handling-mixing and placing equipment, concrete section, and spacing of reinforcement.

Grading of Aggregates

Well-graded aggregates tend to fill up voids and easily get workability. Less amount of water can make it workable. If grading is better, there will be fewer voids and excess paste will be available to give better lubricating effect. Due to excess paste, the mixture gets cohesive and prevent segregation. It also makes it get compacted easily i.e. increases the workability.

Use of Admixtures

There are some admixtures which can improve workability. Some admixtures are mixed intentionally to increase workability and some admixtures increase workability as a side effect of its main purpose.

The Surface Texture of Aggregates

The surface texture of aggregate can be either smooth or rough. A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength