What Is Workability of Concrete?
The diverse requirements of transportability, compatibility, mobility, stability, mixability, placeability, and finish ability of fresh concrete mentioned above are collectively referred.
It is difficult to define precisely all the aspects of workability in a single definition. IS 6461 (Part-VII)-1973 defines workability as the Property of freshly mixed mortar or concrete that determines the ease and homogeneity by which it could be mixed, placed, compacted, and completed.
Also, readd: IS Code for Civil Engineer [Q&a]
Ease is related to the theology of fresh concrete, which includes performance parameters of stability, mobility, and compatibility.
These parameters are redefined in theology in terms of forces involved in the transmission of mechanical stresses, resistance to segregation and bleeding, and resistance to flow by cohesive, viscous and frictional forces.
The equivalence of parameters defining workability and theology of fresh concrete is shown in as per below figure. These parameters are predominately dependent on the consistency of the mixture, as explained later.
On the other hand, parameters of mixability, placeability, compatibility, and finish ability are performance variables that are dependent on the consistency and homogeneity.
Thus, workability of fresh concrete is a complex system of two critical parameters, consistency and homogeneity.
A mixture could have a very fluid consistency and be very placeable, but if it segregates, it would not be considered to have good workability due to lack of homogeneity.
Thus, for optimal performance, consistency and homogeneity must be balanced.
The optimum workability of fresh concrete varies from situation to situation, e.g., the concrete which can be termed as workable for pouring into large sections with minimum reinforcement may not be equally workable for pouring into heavily reinforced thin sections.
Concrete might not be workable when compacted by hand; however, perhaps satisfactory when vibration is utilized.
Also, read: Core Cutter Method
Consistency Workability of Concrete:
Consistency is the relative mobility or ability of freshly mixed concrete to flow, and the usual measurements are slump for concrete, Flow for manor or grout. And penetration immunity for neat cement paste.
It’s thus a dimension of one dimension of workability. It doesn’t indicate whether or not a low viscosity combination gets the cohesion to be placed without segregation and bleeding.
Different concrete mixes with the same consistency can have different workability characteristics.
Important factors affecting consistency Include water content: cement content as Well as Its Particular Attributes; plasticity of the cement paste; aggregate content and its characteristics — air content: temperature: mixing conditions: chemical admixtures and mineral additives used.
Addition of superplasticizer improves consistency by dispersing the cement particles and reducing the viscosity of the cement paste.
Increasing the water content will also improve consistency and can be resorted to if the negative effects of extra water rep-resented by bleeding, segregation, and lower strength can be tolerated.
Also, read: Consistency Test of Cement
Homogeneity Workability of Concrete:
Homogeneity, which means uniform and stable distribution of cement, aggregate and water and resistance to segregation, is a critical physical property of plastic concrete.
This property does not have standardized test methods for its measurement.
The standardized tests for measurement of workability, namely flow, spread, and slump tests, measure consistency, not the homogeneity.
To measure the homogeneity properties, it is necessary to use a rheometer, an instrument that measures yield stress and plastic viscosity.
A reduction in viscosity increases flow and improves consistency. Low viscosity mortar with superplasticizer has relatively high stress and high plastic viscosity and therefore is more resistant to segregation and bleeding.
Low viscosity is essential for ease of placement with cohesion, and for resistance to segregation and bleeding that is necessary for homogeneity.
It should be noted that viscosity and plastic viscosity are different properties. The presence of excess mixing water, undermining and overtaxing are common causes of many problems related to the physical properties of concrete in plastic and hardened states.
The addition of superplasticizer improves the wetting out and mix-ability of the concrete mix. It reduces yield stress which means less mixing energy and time are required. It improves the homogeneity of the various mineral additives and admixture.
Also, read: Soundness of Cement Test
Factors Affecting Workability of Concrete:
The workability of fresh concrete Is Based Mostly on the properties of constituent materials, mix proportions, and environmental conditions.
Workable concrete exhibits very little internal friction between particles and Overcomes the frictional resistance provided by this formwork surface or reinforcement found in the concrete with an appropriate amount of compacting efforts.
Type of Factor Effecting of Workabily
- Influence of Mix Proportions
- Influence of Aggregate Properties
- Influence of Admixtures
- Effect of Time
Also, read: What Is Bulkage of Sand (Fine Aggregate )
1. Influence of Mix Proportions:
In the concrete comprising a cement—aggregate—water system, the aggregate occupy approximately 70 to 75 percent of Their Entire volume of concrete and Market demands that the quantity of aggregates should be as large as possible.
The total specific area of the aggregate is to be minimized to the extent possible by proper choice of size, shape, and proportions of fine and coarse aggregates.
In a well-graded aggregate different size fractions are so chosen as to minimize the void content, and such a mixture will need more water for lubricating effects to overcome the reduction in mobility due to dense packing of particles.
However, when the total voids are less, for the given amount of paste volume, an excess paste is available to give a better lubricating effect.
With an excess amount of paste, the mixture becomes cohesive and fatty, which prevents segregation of particles and lubricated aggregate particles slide past each other with the least amount of compacting effort.
It has been noticed that the change in the measured value of workability of concrete due to relative change in water content is independent of the composition of concrete within wide limits.
An increase of water content results in a monotonous increase in workability, but eventually, a state is reached where segregation and bleeding occur, and use of higher water content will result in the more serious problems of shrinkage and creep of hardened concrete.
However, the water content is limited to some maximum value given by the water-cement ratio, which is dependent on the target design strength of hardened concrete.
The water-cement ratio in itself deter-mines the intrinsic properties of cement paste.
The requirements of workability state that there is sufficient cement paste to surround or lubricate the aggregate particles as well as fill the voids in the aggregate.
In lean concrete, i.e., concrete with high aggregate cement ratio, less quantity of cement paste per unit surface area of aggregate is available for providing lubrication, and hence the mobility of aggregates is restrained.
On the other hand, in the event of rich concrete using reduced aggregate–cement ratio, even more, a paste is available to make the mix fatty and cohesive to provide far better workability.
Also, read: Lab Test on Aggregates at Site
2. Influence of Aggregate Properties:
The effect of aggregate properties on the workability of fresh concrete can be summarized as follows.
For the same volume of aggregate in concrete, the use of coarse aggregate of larger size and/or rounded aggregate gives higher workability because of reduction in total specific surface area and inter-particle frictional resistance
The lesser the surface area, the lesser will be the amount of water required for wetting the surface, and consequently, lesser cement paste will be required for lubricating the surface of aggregates to reduce internal friction.
The Effect of surface texture on workability is because of the fact that the total surface area of rough-textured aggregate is more than the surface area of a smooth rounded aggregate of the same volume.
Using angular, elongated or flaky aggregates results in low workability, primarily due to an increase in the void content and inter-particle interference.
This explains the reasons why river sand and gravel provide greater workability to concrete than crushed sand and aggregate.
The size and shape of the aggregate are of paramount importance in the case of present-day high strength, and high performance concretes where very low water-cement ratios of the order of about 0.25 are used.
The use of finer sand increases the specific surface area, thereby increasing the water demand for the same workability.
In other words, for the same water content, the use of finer sand decreases workability.
Due to the larger contribution to the total specific surface area, the grading of the fine aggregate is much more crucial than the grading of coarse aggregate.
Nonetheless, the Percentage of fine to coarse aggregates Must be so chosen as neither to Raise the total specific surface area (by an excess of fine aggregate) nor to increase the particle interference (due to deficiency in fine aggregate).
An unsuitable selection of score can produce honeycombing or segregation.
In a normal range of mixes though an increase in fines content decreases workability, in practice there is an optimum fine content for maximum workability, such that either an increase or decrease of fines reduces workability.
Normally, the mixes with greater water-cement ratio could demand a somewhat finer grading, and for mixes with low water-cement ratio (as in case of high-strength concrete) a coarser grading is preferable.
The effect of water content and aggregate size is shown in as per figure
The workability is also affected by the physical and chemical properties of cement, but to a much lesser extent than that by the aggregate properties.
The influence of cement properties may have to be taken into account, especially for richer mixes.
A rapid-hardening cement will have reduced workability as compared to ordinary Portland cement because of its higher specific surface and the fact that it hydrates more rapidly, and also the fineness of cement has an influence on bleeding.
Also, read: Procedure for Rcc Concrete
3. Influence of Admixtures:
The presence and nature of admixtures and mineral additives affect the workability considerably. , the plasticizers and superplasticizers im-prove the workability many folds.
It is to be noted that an initial slump of concrete mix, also called the slump of reference mix should be about 20-30 mm to enhance the slump manyfold at a minimum dosage.
Utilization of air-entraining agents that are normally surface-active reduces the inner friction between the particles.
The air bubbles may be considered as artificial fine aggregates of very smooth surface.
Addition they act as a type of ball bearing between the particles to slide past each other and provide easy mobility to the contaminants. In the same way, the fine glassy pozzolanic materials, in spite of increasing the surface area, provide better lubricating effects for giving better workability.
3. Effect of Environmental Conditions for Workability:
The workability of a concrete mix can also be influenced by the temperature of concrete and, thus, by the ambient temperatures.
On a hot afternoon, it will become essential to improve the water content of the concrete mix to be able to maintain the desired workability.
The sum of mixing water required to cause a certain shift in workability also increases with temperatures.
Effect of Time for Workability:
The fresh concrete loses workability with time, chiefly due to the reduction of moisture due to evaporation.
A part of mixing water can be absorbed by aggregate or lost by evaporation from the presence of sun and wind and a part of it is used in the chemical response of hydration of cement.
The reduction of workability varies with the Kind of cement, the concrete mix proportions, the first workability, and the temperature of the concrete.
On an average, a 125 mm slump concrete may lose about 50 mm slump in the first one hour. The workability in terms of compacting factor decreases by about 0.10 during the period of one hour from the time of mixing.
The decrease in workability with time after mixing may be more pronounced in concrete with admixtures like plasticizers.
For some particular total time after mixing, the loss in workability is small and initial level could be regained without loss in the strength of hardened concrete simply by adding extra water.
The effect of placing time on the workability is illustrated in the as per figure.
How to Measurement of Workability?
The quantitative assessment describing concrete as being of high or low workability or semi-dry Or plastic may mean unique things to different people.
The commonly used practice of defining this physical property by a numerical scale based on the empirical tests for its measurement has been found to be unsatisfactory in many situations, thus restricting its applications, in that many builders prefer to rely on subjective assessment rather than on empirical tests.
A number of different empirical tests are available for measuring the Of new concrete, but not one of them is fully satisfactory.
Each test measures just a particular element of it, and there’s actually no unique way which steps the workability of concrete in its totality.
But by assessing and controlling the uniformity of this workability, it’s easier to ensure a uniform quality of concrete and thus, uniform strength for a particular job.
Test of Workability of Concrete:
The empirical tests widely used are workability
- Slump Test.
- Competing Factor Test.
- Vee-Bee Consistency Test.
- Flow Test.
#1. Slump Test for Workability of Concrete–
The slump test is perhaps the most frequently used, primarily due to the ease of the apparatus required as well as the test procedure.
The slump test suggests the behavior of a compacted concrete cone below the action of gravitational forces.
The test is carried out with a mold known as the slump cone. The slump cone is placed on a horizontal and also non-absorbent surface and full of three equal layers of fresh concrete, and each layer being tamped 25 occasions with a typical tamping rod.mold is raised vertically without disturbing the concrete cone.
The subsidence of concrete in millimeters is known as the slump as shown in as per below figure — the concrete after the test when slumps evenly all around are called true slump.
In the case of very lean concrete, one-half of the cone may slide down the other, which is known as a shear slump; also, it can collapse in case of very moist concretes.
The slump test is fundamentally a measure of consistency or the wetness of this mix.
The test is suitable Just for concretes of medium to high workabilities (i.e., having slump values of 25 mm to 125mm). For very stiff mixes having zero slump, the slump test does not indicate any difference in concretes of different workabilities.
It must be appreciated that the different concrete of the same slump may, indeed, have different workabilities under the site conditions.
But, the slump test was found to be useful in ensuring that the uniformity among different batches of allegedly similar concrete under field conditions.
The slump test is limited to concretes with a maximum size of aggregate less than 38 mm.
#2. Competing Factor Test for Workability of Concrete-
The compacting factor test gives the behavior of fresh concrete under the action of external forces.
It measures the compatibility of concrete that is an important aspect of workability, by measuring the total amount of compaction attained for a given amount of work.
The compacting factor test has been held to be more accurate than slump test, especially for concrete mixes of medium and low workabilities, i.e., compacting factor of 0.9 to 0.8, because the test is more sensitive and gives more consistent results.
The test has been more popular in laboratory conditions.
For concrete of very low workabilities of the order of 0.70 or below, the test is not suitable, because this concrete cannot be fully compacted for comparison in the manner described in the test. The relationship between a slump and compacting factor is given in per figure.
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#3. Vee-Bee Consistency Test for Workability of Concrete-
The Vee-Bee test is suitable for stiff concrete mixes having low and very low workability Compared to the slump test and compacting factor test, the Vee-Bee test has the advantage that the concrete in the test receives similar treatment as it would in actual practice.
The test consists in molding a fresh concrete cone in a cylindrical container mounted on a vibrating table as shown in as per the figure.
The concrete cone, Even when Exposed to vibration by starting the vibrator, Begins to Devour the cylindrical container by way of Being remolded.
The remolding is deemed complete when the cement surface becomes horizontal.
The time necessary for the whole remolding in seconds is considered as a measure of workability and can be expressed as the number of Vee-Bee seconds.
Since the endpoint of the test—when the concrete surface be-comes horizontal—is To be discovered visually, it introduces a source of error that is more pronounced for concrete mixes of high workability and hence records low Vee-Bee time
For concrete of slump in excess of 125 mm, the remolding is so quick that time cannot be measured. The test is, therefore, not suitable for concrete of higher workability, i.e., a slump of 75 mm or above. An approximate relationship between a slump and Vee-Bee time is given in as per below figure.
#4. Flow Test for Workability of Concrete-
Flow test gives the satisfactory performance for concretes of the consistencies for which the slump test can be used.
The test consists of molding a fresh concrete cone on the top of the platform of the flow table, and in giving 15 jolts of 12.5 mm magnitudes.
The spread of this concrete, measured as the increase in diameter of the cone, is accepted as a measure of the movement or flow of the concrete.
The test suffers from the drawback that the concrete may scatter on the flow table with a tendency towards segregation.
#5. Kelly Ball Test for Workability of Concrete-
Kelly ball test, an ASTM method, is a simple field method for measuring the consistency of plastic concrete.
The apparatus is a simple portable metal ball that penetrates the surface of the concrete.
It can be performed on the in-place concrete much faster and accurately. Test results are accurate, reliable, and compare favorably with results obtained through the more commonly known and used slump test.
The Kelly ball test device consists of a cylindrical metal plunger with a hemispherically shaped metal ball of 150-mm diameter and 115-mm height at the bottom and a handle at the top as shown in as per figure.
The total assembly weighs 15 kg. A stirrup frame guides the plunger and acts as a reference for measuring the depth of penetration. The plunger is graduated for noting the penetration.
The semicircular bearing plates at each foot of the frame serve to prevent the frame from tilting. There is a movable pinch clamp which makes it easier to measure the depth of penetration.
This clamp is attached the top of the plunger where it remains until the ball has penetrated the concrete. Then the clamp is lowered until it comes in contact with the frame.
When the entire apparatus is removed from the concrete, the position of the clamp on the handle gives the depth of penetration of the ball.
The apparatus can be mounted on a simple wooden base with a tin can for holding rags for wiping the ball clean after each test. In addition to carrying the apparatus, it acts as a footboard for the person testing the wet concrete.
Estimation of Errors In Workability:
The workability tests are operator sensitive. Due to inherent errors in experimental measurements, the results of workability tests are generally stated to the following accuracies:
The experimental errors estimated by replicate measurements as reported in the literature arc given in Table as per below.
|Types of Workability||Range of Values||Standard Deviation|
|Vee-Bee Time||1-24 s||0.25σ|
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Workability of Concrete PPT