Concrete Slump Test for Workability

WORKABILITY OF CONCRETE BY SLUMP CONE TEST

Ref : IS: 7320-1974, IS: 1199-1959, SP: 23-1982.

https://archive.org/details/gov.in.is.7320.1974

http://www.iitk.ac.in/ce/test/IS-codes/is.1199.1959.pdf

https://law.resource.org/pub/in/bis/S03/is.sp.23.1982.pdf

SLUMP CONE TEST

A concrete is said to be workable if it can be easily mixed, placed, compacted and finished. A workable concrete should not show any segregation or bleeding. Segregation is said to occur when coarse aggregate tries to separate out from the finer material and a concentration of coarse aggregate at one place occurs. This results in large voids, less durability and strength. Bleeding of concrete is said to occur when excess water comes up at the surface of concrete. This causes small pores through the mass of concrete and is undesirable.

Unsupported fresh concrete flows to the sides and a sinking in height takes place. This vertical settlement is known as slump. The slump is a measure indicating the consistency or workability of cement concrete. It gives an idea of water content needed for concrete to be used for different works. To measure the slump value, the test fresh concrete is filled into a mould of specified shape and dimensions, and the settlement or slump is measured when supporting mould is removed. The slump increases as water-content is increased. For different works different slump values have been recommended. The following table indicates the relationship between degree of workability and slump value.

Slump test is adopted in the laboratory or during the progress of the work in the field for determining consistency of concrete where nominal max., size of aggregates does not exceed 40 mm. The pattern of slump indicates the characteristics of concrete in addition to the slump value. If the concrete slumps evenly it is called true slump. If one half of the cone slides down, it is called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence. Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation. Any slump specimen, which collapses or shears off laterally gives incorrect results and at this juncture the test is repeated only true slump should be measured.

Although, slump test is popular due to the simplicity of apparatus used and simple procedure, unfortunately, the simplicity is also often allows a wide variability and many time it could not provide true guide to workability. For example, a harsh mix cannot be said to have same workability as one with a large proportion of sand even though they may have the same slump.

Apparatus:

The slump cone experiment is conducted in an apparatus called slump cone. This apparatus essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as under: Bottom diameter: 20 cm, Top diameter: 10 cm, Height: 30 cm and the thickness of the metallic sheet for the mould should not be thinner than 1.6 mm.

Slump cone, tamping rod, metallic sheet, weighing machine and scale.

Material: Cement, sand, aggregate and water

Procedure:

1. Clean the internal surface of the mould thoroughly and place it on a smooth horizontal, rigid and non-absorbent surf ace, such as of a metal plate.

2. Consider a W/C ratio of 0.5 to 0.6 and design mix of proportion about 1:2:4 (it is presumed that a mix is designed already for the test). Weigh the quantity of cement, sand, aggregate and water correctly. Mix thoroughly. Use this freshly prepared concrete for the test.

3. Fill the mould to about one fourth of its height with concrete. While filling, hold the mould firmly in position

4. Tamp the layer with the round end of the tamping rod with 25 strokes disturbing the strokes uniformly over the cross section.

5. Fill the mould further in 3 layers each time by 1/4th height and tamping evenly each layer as above. After completion of rodding of the topmost layer strike of the concrete with a trowel or tamping bar, level with the top of mould.

6. Lift the mould vertically slowly and remove it.

7. The concrete will subside. Measure the height of the specimen of concrete after subsidence.

Record the water content ratio.

Record the slump values in mm.

The slump value indicates that the concrete has Very low/ Low/ Medium/ High degree of workability.

Dos and Donts in an Interview

Interview Do's…

• Do a practice run or mock interview before the real employer interview
• Do research the company: know who they are, what they do and how they do it
• Do dress for the job
• Do arrive 10-15 minutes prior to the interview time – get specific directions
• Do know the name of the person who is interviewing you
• Do know what job you are being interviewed for
• Do take copies of your resume, your letter and your reference page
• Do accentuate the positive – and BE POSITIVE!
• Do shake the employers' hand
• Do provide specific work examples and facts

Interview Don'ts…

• Don't be late – shows lack of pre-planning
• Don't chew gum, smoke, or wear strong perfumes or colognes
• Don't avoid eye contact with the interviewer
• Don't answer your questions with merely a “yes” or “no”
• Don't talk with your hands
• Don't argue with the interviewer
• Don't begin your answer with “I already told you that”…or “As I said earlier”
• Don't be quick to answer
• Don't take your cell phone in the interview – leave it in the car or at home
• Don't rely on your cover letter and resume to do the selling – you need to sell yourself!

 

Fineness of Cement

DETERMINATION OF FINENESS OF CEMENT BY SIEVING

Ref : IS 4031 (Part-1):1996 - Methods of physical tests for hydraulic

cement, Part 1: Determination of fineness by dry sieving http://www.iitk.ac.in/ce/test/IS-codes/is.4031.1.1996.pdf

FINENESS OF CEMENT

Cement is obtained by grinding various raw materials after calcination. The degree to which cement is ground to smaller and smaller particles is called fineness of cement.

Apparatus:

IS-90 micron sieve conforming to IS: 460 (Part 1-3)-1985; Weighing balance; Gauging trowel; Brush.

Material: Ordinary Portland Cement

Procedure:

1. Weigh accurately 100 g of cement to the nearest 0.01 g and place it on a standard 90 micron IS sieve. Note the weight as W1

2. Break down any air-set lumps in the cement sample with fingers.

3. Agitate the sieve by giving swirling, planetary and linear movements for a period of 10 minutes or until no more fine material passes through it.

4. Collect the residue left on the sieve, using brush if necessary, and weigh the residue. Note the weight as W2

5. Express the residue as a percentage of the quantity first placed on the sieve to the nearest 0.1 percent.

6. Repeat the whole procedures two more times each using fresh 100 g sample.

Fineness of cement = (W2/W1)/100

Some Important Points

The fineness of cement has an important role on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat.

Finer cement offers a greater surface area for hydration and hence the faster development of strength although the ultimate strength is not affected.

Fineness also provides more cohesiveness to concrete and avoid separation of water at the top of concrete (called bleeding). However, increase in fineness of cement increases the drying shrinkage and cracking of the concrete.

Fineness of cement is tested either by sieving or by determination of specific surface using air-permeability apparatus.

The specific surface is defined as the total surface area of all the particles in cm² per one gram of cement. Although determination of specific surface is more accurate to judge fineness of cement, it is rarely used except for specific purpose. In contrast sieving is most commonly used method to determine fineness of cement and is quite good for field works.

Bleeding in Concrete

BLEEDING IN CONCRETE

What is Bleeding:

Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic.

The bleeding in concrete is not harmful if the rate of evaporation of water is equal to the rate of bleeding.

Causes of Bleeding:

The Prime factor for bleeding in concrete is the high dosage of Water cement ratio. Higher water-cement ratio weakens concrete and leads to excessive bleeding.

The cement type and fine aggregates can play a role in determining the bleed rate. The fewer fines you have in your mix, the more bleeding will occur.

Types of Bleeding:

• Normal bleeding refers to a uniform seepage of water over the entire surface of the structure.

• Channel bleeding refers to water rising through particular paths.

Effects of Bleeding:

• Concrete loses its homogeneity

• Due to bleeding, when the top surface is worked with trowel, the aggregate goes down and cement paste forms at the top surface forming Laitance which decreases the wearing capacity and decreases its life.

• This Laitance decreases the bond between successive concrete lifts

• Concrete becomes permeable

• Delays the surface finishing

• Decreases the pumping ability

Measures to reduce Bleeding:

• Reduce water content. Use lower slump mix

• Use finer cements

• Increase amount of fines in the sand

• Use supplementary cementitious materials

• Use air entraining admixtures

Segregation in Concrete

SEGREGATION OF CONCRETE

What is Segregation:

Segregation in concrete is a case of particle segregation in concrete applications. Common forms are

• Separation of Coarse aggregate from the concrete mixture,

• Separation of Cement pastes from the concrete during its plastic stage.

• Separation of water from the concrete mix (Bleeding in concrete)

Segregation in concrete is commonly thought of as separation of some size groups of aggregates from cement mortar in isolated locations with corresponding deficiencies of these materials in other locations.

Segregation also occurs due to over-vibration or compaction of concrete, in which cement paste comes to the top and aggregates settles at the bottom.

Internal Factors for Segregation:

Segregation could result from internal factors such as concrete that is not proportioned properly and not mixed adequately, or too workable a mix.

External Factors for Segregation:

Segregation from external factors include too much vibration, improper transportation, placement, or adverse weather conditions.

Segregation is also caused by dropping concrete from more than 1 m.

Measures to reduce Segregation:

• Concrete should not be dropped from more heights

• Concrete should be placed through temporary inclined chutes for heights more than 1 m.

• The angle of inclination may be kept between 1:3 and 1:2 for smooth travel of concrete.

• The delivery end of chute should be as close as possible to the point of deposit.

• Adding air entraining agents, admixtures and pozzolanic materials increase the viscosity, thereby reduce segregation.

• Segregation can be controlled by maintaining proper proportioning the mix.

• Water content should not be more than the desired amount.

• Formwork should not have any leakages.

Tests for Segregation:

• Random ultrasonic testing should be conducted for any presence of segregation.

• Segregation can be rectified by pressure grounding with special chemical compounds.

• After rectifying the defects by pressure grouting core test has to be performed to ensure that the strength of concrete has reached to the desired level.

Bulking of sand

BULKING OF SAND

Ref : IS 2386-3: 1963 - Methods of test for aggregates for concrete, Part 3 http://www.iitk.ac.in/ce/test/IS-codes/is.2386.3.1963.pdf

BULKING OF SAND

The increase in the volume of sand due to increase in moisture content is known as bulking of sand. A film of water is created around the sand particles which forces the particles to get a side from each other and thus the volume is increased.

Due to the bulking, fine aggregate shows completely unrealistic volume. Therefore, it is absolutely necessary that consideration must be given to the effect of bulking in proportioning the concrete by volume. If cognisance is not given to the effect of bulking, in case of volume batching, the resulting concrete is likely to be under-sanded and harsh.

It will also affect the yield of concrete for given cement content. To compensate the bulking effect extra sand is added in the concrete so that the ratio of coarse to fine aggregates will not change from the specified value. Maximum increase in volume may be 20 % to 40 % when moisture content is 5 % to 10 % by weight.

Fine sands show greater percentage of bulking than coarse sands with equal percentage of moisture.

Apparatus: Beaker, 1000ml measuring jar, brush, scale, mixing tray.

Material: Fine aggregate, water.

Procedure:

1. Put sufficient quantity of dry sand into the beaker until it is about one-thirds full.

2. Level off the top of the sand and measure the height (H₁) by pushing a steel rule vertically down through the sand at the middle to the bottom. Measure weight of the sand.

3. Add 2% of water; mix it thoroughly in the container. Smooth and level the top surface measure the height (H₂) of soil. Find the height percentage increment.

4. Repeat the same procedure with increasing amount of water by 2% until percentage increment of sand height is reduced and attends original level.

5. Plot a graph of percentage increment of sand height against percentage of water.

Observations:

Initial Height of sand in the Jar (H1) : _______ mm

Weight of fine aggregate : ________ g

                                                        Typical Curve Showing Bulking of Sand

Table 1: Observation Sheet

Sl. No.	% of Water	Volume of water (in ml)	Height of sand (H2)	Bulking = {(H2-H1)/H1}x100
1
2
3
4
5

Setting Time of Cement

SETTING TIME OF CEMENT

IS 8142 (1976): Method of test for determining setting time of concrete by penetration resistance [CED 2: Cement and Concrete]

Ref: http://www.iitk.ac.in/ce/test/IS-codes/is.8142.1976.pdf

Initial Setting Time - The elapsed time, after initial contact of cement and water, required for the mortar (sieved from the concrete) to reach a penetration resistance of 3.43 N/mm² ( 35 kgf/cm² )

Or

The time at which cement starts hardens and completely loses its plasticity is called Initial setting time of cement.

Or

The time available for mixing the cement and placing it in position is an Initial setting time of cement. If delayed further, cement loses its strength.

The initial setting time for OPC is 30 min

Final Setting Time - The elapsed time, after initial contact of cement and water, required for the mortar (sieved from the concrete) to reach a penetration resistance of 26.97 N/mm² ( 275 kgf/cm²)

Or

The time at which cement completely loses its plasticity and became hard is a final setting time of cement.

Or

The time taken by cement to gain its entire strength is a Final setting time of cement.

The final setting time for OPC is 600 min

Test for finding Initial and Final Setting time of Concrete

APPARATUS

Vicat apparatus conforming to IS: 5513-1976.

Balance of capacity 1kg and sensitivity 1 gram.

Gauging trowel conforming to IS: 10086-1982.

PROCEDURE

• Unless otherwise specified this test shall be conducted at a temperature of 27 +20 C and 65 + 5% of relative humidity of the Laboratory.

• Prepare a paste of 300 grams of cement with 0.85 times the water required to a give a paste of standard consistency IS: 4031 (Part 4) 1988.

• The time of gauging in any case shall not be less than 3 minutes not more than 5 minutes and the gauging shall be completed before any sign of setting occurs.

• Count the time of gauging from the time of adding water to the dry cement until commencing to fill the mould

• Fill the vicat mould with this paste making it level with the top of the mould.

• Slightly shake the mould to expel the air.

• In filling the mould the operator hands and the blade the gauging trowel shall only be used.

Initial Setting Time

• Immediately place the test block with the non-porous resting plate, under the rod bearing the initial setting needle.

• Lower the needle and quickly release allowing it to penetrate in to the mould.

• In the beginning the needle will completely pierce the mould

• Repeat this procedure until the needle fails to pierce the mould for 5 + 0.5mm.

• Record the period elapsed between the time of adding water to the cement to the time when needle fails to pierce the mould by 5 + 0.5mm as the initial setting time.

Final Setting Time

• Replace the needle of the vicat apparatus by the needle with an annular ring

• Lower the needle and quickly release.

• Repeat the process until the annular ring makes an impression on the mould.

• Record the period elapsed between the time of adding water to the cement to the time when the annular ring fails to make the impression on the mould as the final setting time.

REPORT

Report the initial setting time and final setting time in minutes.

PRECAUTION

The time of gauging in any case shall not be less than 3 minutes not more than

5minutes

The range of three results of properly conducted tests by the same operator with the same machine using similar materials on different days shall not exceed 4 minutes, and the average setting times for two sets

of tests each consisting of three similar batches shall not depart more than

20 minutes from the average of the two.