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INTERNAL ASSESSMENT EXAMINATION – I, FEB 2020
Fourth Semester
B.E – Civil Engineering
CE8404 – Concrete Technology
Regulations 2017
Answer Key
Answer ALL questions
PART A
1.
Mention the test adopted to test the properties of cement in field.
a. Open the bag and take a good look at the cement, there should not be any
visible lumps
b. Thrust your hand into the cement bag should feel cool feeling
c. Take a pinch of cement and feel between the fingers. It should give a smooth
feeling not a gritty feeling
d. Take a hand full of cement and throw it on a bucket full of water, the particle
should float for sometime before they sink.
2.
Distinguish 43 grade and 53 grade of cement.
Concrete cube of size 150mm×150mm×150mm subjected to compressive strength
test after 28 days of curing the characteristic compressive strength value should
not be less than 43N/mm2 in case of 43 grade cement and in case of 53 grade
cement it should not be less than 53 N/mm2
3.
The impact strength of the given coarse aggregate is 50%. Justify whether the
given aggregate can be used for the construction purpose.
No it cannot be used for construction purpose because the value ot npact strength
should be less than 30%.
4.
Classify fine aggregate based on its size.
Coarse sand,Medium sand and Fine sand
5.
Define admixture. Explain the classification of admixture.
Admixtures are ingredients other than cement, fine aggregate and coarse aggregate
to improve the quality of concrete. The addition of an admixture may improve the
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concrete with respect to its strength, hardness, workability, water resisting power
etc.
Chemical and mineral admixtures are the types of admixtures.
6.
What is the purpose of retarders?
Retarders increases the setting time of concrete mix and reduce the water cement
ratio. Up to 10% water reduction is achieved
7.
Differentiate plasticizers and superplasticizers.
Plasticizers are defined as chemical admixtures added to wet concrete mix to
impart adequate workability properties
Superplasticizers produce extreme workability and achieve reduction of water
content without loss of water cement ratio i.e workability.
PART B
8. (a) Explain the methods to find out the abrasion value of coarse aggregate.
Abrasion of aggregates measures the resistance of aggregates against
wearing.
•It is an important property of concrete in roads and in floor surfaces
subjected to heavy traffic. The most frequently used test method is the Los
Angeles Abrasion Test.
•Los Angeles Abrasion Test: The aggregate of specified grading is placed in
a cylindrical drum, mounted horizontally. A charge of steel balls is added and
the drum is rotated a specified number of revolutions. The tumbling and
dropping of the aggregate and the balls result in abrasion and attrition of the
aggregate. The resulting grading should be compared with the standard
limitations.
(b) Discuss the role of various major compounds of cement and its hydrated
products in the properties of cement.
FUNCTIONS OF CEMENT INGREDIENTS
• Lime (CaO) – Correct proportion
• Silica (SiO2) – Imparts strength
• Alumina (Al2O3) – Quick setting property
• Calcium sulphate (CaSO4) – Increase the “initial setting time”
• Iron Oxide (Fe2O3) – Colour, Hardness
• Magnesia (MgO) - Colour, Hardness
• Sulphur (S) – Sound cement
• Alkalis - Efflorescence
SETTING ACTION OF CEMENT
• Sticky cement paste Rock like state
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• 28 days – 70% strength
• 1 year – 90%
Compounds formed
• Tricalcium aluminate (3CaO, Al2O3)
• Tetra-Calcium alumino ferrite (4CaO, Al2O3,Fe2O3)
• Tricalcium Silicate (3CaO, SiO2)
• Dicalcium Silicate (2CaO, SiO2)
• C3A – 4 to 14% (Celite)
• C4AF – 10 to 18% (Felite)
• C3S – 45 to 65% (alite)
• C2S – 15 to 35% (Belite)
• Bogue’s compounds
Hydration of cement
• The chemical reactions between cement and water
• The liberation of heat is called heat of hydration
9. (a)
Briefly describe the following test on aggregate: specific gravity test,
crushing test and impact test.
Specific Gravity
Indian Standard Specification IS : 2386 (Part III) of 1963 gives various procedures to find out
the specific gravity of different sizes of aggregates
Specifc Gravity = C/A - B
Apparent Specifc Gravity = C/C – B
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Water Absorption = 100(B – C)/C
A = Weight of saturated aggregate in water = (A1 – A2)
B = Weight of saturated surface dry aggregate in Air
C = Weight of oven dried aggregate in Air
A1 = Weight of aggregate and basket in water
A2 = Weight of empty basket in water
Aggregate Crushing Value test
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Aggregate Impact Value test
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(b) Explain the setting time of cement test and standard consistency test in detail
VICAT APPARATUS
• Consistency – plunger
• Initial setting time – square needle
• Final setting time – needle with annular collar
SETTING TIME TEST
To detect the deterioration of cement due to storage
INITIAL SETTING TIME
FINAL SETTING TIME
INITIAL SETTING TIME
Cement 300g + water
• Cement paste filled in vicat mould
• Square needle – 1mmx1mm
• Beginning – complete penetration
• Repeated upto penatration of 5mm to 7mm
• Initial setting time found
FINAL SETTING TIME
• Needle with collar
• Sharp point & annular collar
• Time at which the needle makes an impression & collar fails to do so
CONSISTENCY TEST
• To determine percentage of water required
• Cement 300g + 30% by weight of water
• Mixed thoroughly
• Fill the mould
• “Time of gauging”
• Plunger attached
• 5mm to 7mm from bottom
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10.
(a)
What are super plasticizers? How are these helpful in modifying the
properties of concrete.
Superplasticizer
• Developed in the 1960’s
• First used in Japan and Germany
• Introduced in the U.S. in the 1970’s
• Neutralizes the surface charges
• Causes dispersion
• Releases water in cement particles
• Reduces viscosity
Typical superplasticizers include
• Sulfonated melamine formaldehyde condensates.
• Sulfonated naphthalene formaldehyde condensate.
• Lignosulfonates.
• Polycarboxylates.
Superplasticizers (High-Range Water Reducers)
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Main Purpose Of Superplasticizer
• Produce Flowing Concrete (7”-9” Slump Increase)
• Easy Placement (Picture: Concrete Will Fill All Gaps Between Rebar)
• High-strength (Reaches Up To 14,000 Psi In 7 Day Break)
Effect on Fresh Concrete
Effect on Hardened Concrete
Effect on Durability
(b) Explain retarders and accelerators in detail.
Retarders
A retarder is an admixture that slows down the chemical process of hydration so that concrete
remains plastic and workable for a longer time than concrete without the retarder.
1. Retarders are used to overcome the accelerating effect of high temperature on setting properties of
concrete in hot weather concreting.
2. Very useful when concrete has to be place in very difficult conditions and delay may occur in
transporting and placing.
3. Gypsum and Calcium Sulphate are well known retarders.
4. Other examples are: starches, cellulose products, sugars, acids or salts of acids, lignosulphonic
acids, hydroxylated carboxyilic acid.
• Amount. Excess Retarders should be used in proper amount will cause indefinite setting time.
• At normal temperatures addition of sugar 0.05 to 0.10 percent have little effect on the rate of
hydration, but if the quantity is increased to 0.2 percent, hydration can be retarded to such an extent
that final set may not take place for 72 hours or more.
• Casting and consolidating large number of pours without the formation of cold joints.
• Grouting oil wells, where temperature is about 200 °C, at a depth of 6000 meters.
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Accelerators
Accelerating admixtures are added to concrete to increase the rate of early strength development
Why accelerators?
1. Permit earlier removal of formwork
2. Reduce the required period of curing
3. Advance the time that a structure can be placed in service
4. Partially compensate for the retarding effect of low temperature during cold weather concreting
5. In the emergency repair work.
Commonly used materials as an accelerator:
Calcium chloride (Not used now)
Some of the soluble carbonates
Silicates fluosilicates (Expensive)
Some of the organic compounds such as triethenolamine (Expensive)
Benefits of Accelerators
• Accelerators are so powerful that it is possible to make the cement set into stone hard in a matter of
five minutes are less.
• With the availability of such powerful accelerator, the under water concreting has become easy.
• Similarly, the repair work that would be carried out to the waterfront structures in the region of
tidal variations has become easy.
• The use of such powerful accelerators have facilitated, the basement waterproofing operations.
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