CONCRETE TECHNOLOGY (WORKABILITY OF CONCRETE)

CONCRETE TECHNOLOGY

WORKABILITY OF CONCRETE

Workability is a measure of the ease with which a fresh mix of concrete or mortar can be handled and placed. Workabilty is defined as the amount of useful internal work necessary to produce full compaction. The useful internal work is a physical property of  concrete alone and is the work or energy required to overcome the internal friction between the individual particles in the concrete.

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.

Workability can be measured by the concrete slump test, a simplistic measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod in order to consolidate the layer. When the cone is carefully lifted off, the enclosed material will slump a certain amount due to gravity. A relatively dry sample will slump very little, having a slump value of one or two inches (25 or 50 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by using the flow table test.

Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio. Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.

High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.

After mixing, concrete is a fluid and can be pumped to the location where needed.

Factors affecting workability of concrete:

1.Water content

2  Mix Proportions

3. Shape of Aggregates

4.  Surface Texture

5.Grading of Aggregates

6.Use of Admixtures

 1.Water content

Water content in given volume of concrete will have significant influence on the workability. The higher the water content for cubic meter of concrete, the higher the fluidity of concrete, which is the one of the important factor affecting the workability of concrete. Increase of water content is the last resource to be taken for improving the workability even in case of uncontrolled concrete. For controlled concrete one cannot arbitrarily increase the water content. More water can be added, provided correspondingly higher quantity of cement is also added to keep water cement ratio constant. So that strength remains the same.

2  Mix Proportions

Aggregate cement ratio is an important factor influencing the workability. The higher the aggregate cement ratio, the leaner is the concrete, in which less quantity of paste is available for providing lubrication per unit surface area of aggregate and hence the mobility of aggregate is restrained. In case of rich concrete with lower aggregate cement ratio, more paste is available to make the mix cohesive and fatty to give better workability.

3. Shape of Aggregates

The shape of the aggregates will influence the workability in good measure. Angular, elongated and flaky aggregate makes the concrete very harsh when compared to rounded aggregates. Rounded aggregates will have less surface area and fewer voids than angular (or) flaky aggregates and also rounded aggregates will reduce greatly the frictional resistance also. River sand and gravel provide greater workability to concrete than crushed sand and aggregates. The importance of shape of aggregates will be of great significance of present day high strength and high performance concrete, when we use very low water cement ratio in order of about .25.

 4.  Surface Texture

The total surface area of rough textured aggregate is more than the surface area of smooth rounded aggregate of same volume. Rough textured aggregate will show poor workability and smooth or glassy textured aggregate will give better workability. A reduction of inter particle frictional resistance offered by a smooth aggregates also contributes to higher workability.

5.Grading of Aggregates

A well graded aggregate is the one which has least amount of voids in a given volume. Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect. With excess amount of paste, the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide past each other with the least amount of compacting efforts. The better the grading, the less is the void content and higher the workability for the given amount of paste volume.

6.Use of Admixtures

Most important factor which affects the workability is the use of Admixtures, Plasticizers and Super plasticizers greatly improve the workability. Use of air-entraining agent being surface active, reduces the internal friction between the particles thus increasing I workability.

TEST FOR MEASURING THE WORKABILITY OF CONCRETE

1.Slump test

2.Compaction factor test

3.Vee-Bee test

4.Vibro-workabilty test

1.Slump Test

The concrete slump test is used for the measurement of a property of fresh concrete.The test is an emprical test that measures the workability of fresh concrete. More specifically, it measures consistency between batches. The test is popular due to the simplicity of apparatus used and simple procedure. The slump test is used to ensure uniformity for different batches of similar concrete under field conditions and to ascertain the effects of plasticizers on their introduction.

The slump test result is a measure of the behaviour of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete.

 The apparatus used for the slump test is a mould and a tamping rod.

The mould  for the slump test is a frustum of a cone.

The dimensions for the mould is given below

    

For tamping the concrete , a steel tamping rod of 16 mm dia and 0.6 m long with bullet end is used.

The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as true slump, shear slump or collapse slump. If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated. A collapse slump is an indication of too wet a mix. Only a true slump is of any use in the test. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate Very dry mixes; having slump 0 - 25 mm are used in road making, low workability mixes; having slump 10 - 40 mm are used for foundations with light reinforcement, medium workability mixes; 50 - 90 for normal reinforced concrete placed with vibration, high workability concrete; > 100 mm.

Procedure for slump test

1.                 Dampen the slump test mold and place it on a flat, moist, nonabsorbent, rigid surface, like a steel plate.

2.                 fill the cone 1/3 full and uniformly rod the layer 25 times to its full depth

3.                 Fill the cone with a second layer until 2/3 full by volume and rod 25 times uniformly, ensuring that the rod just penetrates into the first layer.

4.                 Overfill the cone with the third layer and rod uniformly, 25 times, with the rod just penetrating into the second layer.

5.                 Strike off the excess concrete level with the top of the cone by a screening and rolling motion of the tamping rod.

6.                 Remove any spilled concrete from around the bottom of the cone.

7.                 Immediately remove the mould from the concrete by raising it carefully in a vertical direction without lateral or torsional motion.

8.                 Measure the difference between the height of the mould and the height of the specimen at its highest point to the nearest 6.3mm.The distance will be the slump of the concrete.

The height of the concrete mix varies from one sample to another. Concrete samples with lower heights are predominantly used in construction and the samples having high slumps are commonly used to construct roadway pavements. After the slum cone test procedure gets completed, discard the concrete used for it. This concrete should not be used for other tests.

COMPACTION FACTOR TEST

Compacting factor of fresh concrete is done to determine the workability of fresh concrete by compacting factor test as per IS: 1199 – 1959. The apparatus used is Compacting factor apparatus.

 

Procedure to determine workability of fresh concrete by compacting factor test.

i)                    The sample of concrete is placed in the upper hopper up to the brim.

ii) The trap-door is opened so that the concrete falls into the lower hopper.

iii) The trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.

iv) The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades.

v) The concrete in the cylinder is weighed. This is known as weight of partially compacted concrete.
vi) The cylinder is filled with a fresh sample of concrete and vibrated to obtain full compaction. The concrete in the cylinder is weighed again. This weight is known as the weight of fully compacted concrete.

Compacting factor = Weight of partially compacted concrete / weight  of fully  compacted concrete