What is Workability of Concrete? | Requirements and Factors Affecting Workability of Concrete
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What is Workability of Concrete?
The workability of concrete can be defined as the property of fresh concrete mix that will meet the diverse requirements of mixability, stability, transportability, placeability, mobility, compactability and Finishability of the fresh concrete. This property of the concrete is composite in nature. All the aspects that are included in workability of concrete cannot be explained in a single definition.
As per Indian Standard Code, IS: 6461 (Part V11) -1973,
Workability can be defined as that property of freshly mixed concrete or mortar which determines the ease and the homogeneity with which it can be properly mixed, placed, compacted, and finished.
The optimum amount of workability will change from one situation to another. The workability required for a mix that is required to be pumped into large sections with reinforcement in minimum amount will not be equal to the workability of a mix that is needed to be poured into a thin section that is reinforced heavily.
The terms Consistency and Plasticity might be used to express the workability of a concrete mix. But consistency refers to how wet the mix is and a mix that is merely consistent may not have all the requirements mentioned in the initial statement. A very wet mix will result in segregation and bleeding when they are placed in the formwork. Plasticity is the property of cohesiveness to hold the individual grains of the mix together in the whole cement matrix.
Factors Affecting The Workability
There are certain factors that will affect the workability of concrete. The concrete workability is primarily dependent on the materials use in concrete mix, their mix proportions and the environmental conditions. Each factors and their influence on concrete workability are mentioned briefly.
Influence of Mix Proportions on the Workability of Concrete
In the case of a concrete mix, that consist of cement - aggregate - water system, the aggregates will occupy mostly 70 to 75 % of the total volume of the concrete. And it is always recommended to have larger quantities of aggregates to achieve economy.
The total specific area of the aggregates used has to be reduced. This is done by the Proper choice of the shape, size and the proportion of both the fine and the coarse aggregates. The void content in the mix can be reduced by using different size fractions. A highly densely packed particles will require more water content to overcome the difficulty in mobility. The figure - 2 below shows the effect of aggregate cement ratio on the workability.
The intrinsic properties of the cement paste and the workability requirement is determined by the water - cement ratio of the mix.That is is there enough cement paste in order to surround the aggregates in the mix and also for filling up the voids.
The water content is the primary factor that will govern the workability of the fresh concrete. The workability will increase with the increase in the water content.
Influence of Properties of Aggregate on the Workability of Concrete
The effect of aggregates on the properties of aggregates can be briefly enlisted as below:
- The use of coarser aggregates of large size or rounded aggregates for the same volume of the aggregates will help in increasing the workability. These aggregate type reduces the specific area and the particle interference. The workability is decreased by the use of elongated aggregates as they have larger particle interference.
- The specific surface are will increase by the use of finer particles. This will demand of higher water content.
- As specific area have an important role over workability, the grading of aggregates tend to have critical importance compared to the grading of the coarse aggregates. Or the ratio of C.A : F.A must be chosen so that there is no increase in the specific area.
- In general condition those mixes with higher water cement ratio supposed to have fine gradinf and those with lower water-cement ratio have coarser grading. The effect of water content and the aggregate size on the workability is shown in the figure-3 below.
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| Fig.3: Effect of Water Content and Size of Aggregates on the Workability of Concrete |
Influence of Environmental Conditions on the Workability of Concrete
The water content of the concrete mix have to be increased on a hot day so that the desired workability is maintained. The amount of mixing water that is used to bring a change in the mix also will affect the temperature.
Influence of Time on the Workability of Concrete
With time there is loss of water content from the mix. This is because of the evaporation process going on with time. A part of the water is absorbed by the aggregates, some is lost for evaporation and some is taken for the hydration process. The loss of water will depend on the Type of Cement,Amount of Cement, Concrete Mix Proportion, Workability and Temperature.On a average, a 120mm slump concrete mix might lose 50mm slump in the first one of the casting. In terms of compacting factor, the workability will decrease by 0.10 during 1 hour. This is more pronounced for the concrete with plasticizers.
Requirements of Workability
The Requirements of Workability are:
- The workability of the concrete must be such a way that it can be properly mixed, placed in the formwork and properly compacted with less effort without the chances for segregation and bleeding.
- The workability is chosen based on the compacting equipment, section size and reinforcement concentration
- Workability is high for sections that are heavily reinforced or when they have inaccessible parts
- The workability requirement should be dealt with situation. Minimum workability with satisfactory placement and compaction must be followed.
Read Article :Segregation in Fresh Concrete and Bleeding in Fresh Concrete
Table.1: Requirements of workability for different conditions of concrete placement.
Degree of Workability
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Slump, mm
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Compaction Factor
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Suitability
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Very Low
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0-25
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0.78
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Roads that are vibrated by power operated machines
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Low
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25-50
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0.85
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Roads that are vibrated with hand operated machines, light reinforced sections with vibrations, mass foundation without vibration
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Medium
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50-100
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0.92
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Flat slabs, less workable, manually compacted areas
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High
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100-180
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0.95
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Sections of Congested Reinforcement. Not for Vibration
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Workability is a parameter that a mix designer is required to specify in the mix design process. A concrete which has high consistency and which is more mobile need not be of right workability for a particular job.
Every construction work of concrete requires a particular workability. A concrete that is considered workable when used in the thick section is not workable when considered its use in thin section. Workability of concrete depends on the:
- Ø Thickness of the section
- Ø Extent of reinforcement
- Ø Mode of compaction
Different concrete mixes with same consistency can have different workability characteristics. The presence of excess mixing water, undermixing and overmixing are common causes of many problems that are related to the physical properties of the concrete in its fresh state and the hardened state.
According to ASTM, workability can be defined as the “Property determining the effort that is required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity”. Workability is necessary to compact concrete to the maximum possible density.
Need For Sufficient Workability
The strength of Partially compacted concrete compared to that of fully compacted concrete in terms of strength ratio.
Rstrength = fck of partially compacted concrete / fck of fully compacted concrete
In terms of Density Ratio
Rdensity = Density of partially compacted concrete/density of Fully compacted concrete
With the reduction in the density, the voids will increase. The figure below shows the relation between the density ratio and the strength ratio. From the figure, it is clear that the strength ratio decreases with the decrease in the density ratio.This indirectly shows that the strength reduction is due to the increase in the voids (lower density). The presence of voids about 5% may reduce the strength by as much as 30%.
Fig.. The relationship between the Strength Ratio and the density Ratio
