Que4.4. Define plasticity. What are the factors affecting the plasticity of concrete?
Answer Plasticity of Concrete
i. A concrete is said to be workable if it’s fluently transported, placed, compacted and finished without any isolation.
ii. Plasticity is a property of lately mixed concrete, and a concrete is a admixture of cement, total, water and amalgamation. Factors Affecting of Plasticity of Concrete Following are the factors affecting of plasticity of concrete
1. Water Content Workability of concrete increases with increase in water content.
2. Aggregate/ Cement rate The advanced the total/ cement rate, the slender is the concrete, performing in lower plasticity.
3. Size of Aggregate For a given volume of water and paste, bigger size of summations will give advanced plasticity.
4. Shape of Aggregate Better plasticity is assured to rounded aggregate than angular, elongated or short total.
5. Grading of Aggregate This is one of the factors which will have maximum influence on plasticity. A well canted total can lead to good plasticity.
6. face Texture of Aggregate Rough textured total will show poor plasticity and smooth or glassy textured total will give better plasticity.
7. Use of Admixture The right way of perfecting plasticity is to use chemical cocktails similar as plasticizers, super plasticizers, air entraining agents,etc.
Que4.5. Mention the different tests which are generally acclimated to measure plasticity and explain any one test in detail.
Answer Test for Measure Plasticity Following are the test used for measure plasticity Depression test.
ii. Compacting factor test.
iii. Vee- Bee test.
Concrete Depression Test Procedure
1. originally, the internal face of the mould is gutted precisely. oil painting can be applied on the face.
2. The mould is also placed on a base plate.
3. The mould is filled with fresh concrete in three layers. Each subcaste is tamped 25 times with a sword rod.
4. After filling the mould, redundant concrete should be removed and the face should be leveled.
5. also the mould is lifted gently in the perpendicular direction and also unsubstantiated concrete will slump. The drop in height at the centre point is measured to nearest 5 mm or0.25 inch and it’s known as ‘ depression ’.
Que4.6. How do you conduct compacting factor test in laboratory.
Answer contraction Factor Test
1. The contraction factor test gives the geste of fresh concrete under the action of external force.
2. In this test, the contraction achieved through a free fall of concrete determines its plasticity.
Contraction Factor Test Procedure
1. The concrete sample is placed in the upper hopper.
2. also the door of hopper is opened. The sample drops into lower hopper filling it to overflowing.
3. The trap door of the lower hopper is also opened and the sample falls into the cylinder which is also filled to overflowing.
4. The fat concrete is removed from the top of the cylinder with the help of a trowel.
5. The outside face of cylinder is wiped and gutted.
6. The cylinder is also counted and it’s recorded as weight of incompletely compacted concrete.
7. The cylinder is again filled with concrete in layers not exceeding 50 mm in consistence. Each subcaste is completely compacted with tamping rod.
8. The cylinder is again counted after wiping and drawing the outside face of cylinder. This weight is recorded as the weight of completely compacted concrete.
9. The compacting factor is also calculated from the formula Compacting factor = Weight of incompletely compacted concrete/ Weight of completely compacted concrete.
Que4.7. Explain the Vee- Bee test of determining plasticity with neat sketch.
Answer 1. The test is suitable for stiff concrete mixes having low or veritably low plasticity.
2. Compared to the depression and compacting factor tests, the Vee- Bee test has the advantage that the concrete in the test receives a treatment analogous to what it would in factual practice.
3. The test consists of moulding a fresh concrete cone in a spherical vessel mounted on a wobbling table(Fig.4.7.1).
4. When the concrete cone is subordinated to vibration using a standard vibrator, it starts to enthrall the spherical vessel by way of getting remoulded.
5. The remoulding is considered complete when the concrete face becomes vertical.
6. The time( in seconds) needed for the complete remoulding is considered as a measure of plasticity and is expressed as the number of Vee- Bee seconds.
7. The end point of the test, when the concrete face becomes vertical, has to be caught on visually.
Que4.8. What’s the effect of time and temperature on plasticity?
Answer Effect of Time on Workability
1. lately mixed concrete stiffens with the passage of time. This is different from the hardening of the blend.
2. As time passes, water is lost due to immersion by summations if they are not formerly impregnated. Some water is lost due to evaporation, especially if the concrete is exposed to hot rainfall and wind also plasticity drop.
Effect of Temperature on Plasticity
1. When temperature increases, also in the same proportion plasticity of fresh concrete diminishments.
2. The reason that stands before is “ when temperature increases also evaporation rate also increases due to that hydration rate diminishments and hence, concrete will gain strength before ”.
3. Due to fast hydration of concrete, a hardening comes in concrete and that decreases the plasticity of fresh concrete.
Que4.9. Write about isolation and its causes. How reduce isolation of concrete?
Answer isolation It’s defined as the separation of the ingredients of a homogeneous admixture of concrete. It’s caused by the differences in sizes and weights of the constituent patches. Causes of isolation in Concrete
1. Transporting concrete composites for long distances.
2. inadequately proportioned blend, where sufficient matrix isn’t there to bind the summations.
3. Dropping concrete from further than 1m.
4. wobbling concrete for a long time.
Remedial Measures To reduce isolation, well canted summations are used and concrete is placed with enough contraction.
ii. The concrete shouldn’t be dropped from a height of further than1.5m.