C T: Mixing and Transportation of Concrete Unit 4 Part 2

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.

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