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Concrete
The various concrete grades are denoted by M 10, M 15, M 20, etc. In which M denotes mix of concrete followed by the compressive strength number. A mix is the respective proportion of constituent materials Cement: Fine aggregates: Coarse aggregate. Compressive strength number indicates the compressive strength of 150mm size cube at 28 days, expressed in N/mm2.
The Concept of quality control (reference: Shivkumar M. Goti)
For the structure to be durable, of adequate strength and for aesthetic, accomplishing a quality concrete is of supreme importance. Low strength and low durable concrete structures have sustained several damages causing a loss in terms of repair, downtime and life loss in past decades. So, in order to achieve a quality and a durable structure, maintaining the quality and standard of concrete is a prime need.
Reinforced Concrete:
Lightweight concretes are used, depending upon their composition, for thermal insulation., for protecting steel structures, they are also used in long span bridge decks, and even as building blocks.Aerated Concrete is a variety of extremely lightweight concrete (density 480-800 Kg/m3). This is obtained by using a cement, sand and powdered fuel ash as constituents.
High-Density Concrete:
Cement
Aggregates
Water
Admixtures
Concrete Mix Design
Special Concrete
Concrete is a composite material composed of gravel or crushed stone (coarse aggregates), sand (fine aggregates), and hydrated cement (blinder). [Sengupta and Menon, NPTEL]
Figure 1, shows a petrographic section of concrete. The matrix consists of aggregates, hydrated cement, and tiny voids
a. Aggregates
The aggregates are granular materials obtained from rocks and crushed stones. They may also be obtained from synthetic materials like slag, shale, fly ash and bloated clay. The sand obtained from river beds or quarries is used as fine aggregates. The fine aggregate along with hydrated cement paste to fill the space between the coarse aggregate, called as cement-sand mortar. The aggregate in normal concrete forms this skeletal matrix, it's about 60-65 percent by volume.
b. Cement
Cement is a blended mixture of argillaceous and calcareous material which develops the binding ability in a matrix of constituent element. The cement and water form a paste that coats aggregates in the mix. The paste hardens and binds the aggregates together.
c. Water
Water is needed to chemically react with the hydrated cement and to provide workability with the concrete. The amount of water in the mix pounds compared with the amount of cement is called water/cement ratio (W/C). The lower W/C ratio, the stronger will be the concrete (high strength and reduce permeability).
d. Admixtures
Modern engineering concrete has additional ingredients other than the above materials. These are chemical admixtures like accelerator, set retarders, water reducers, etc and Mineral admixtures such as fly ash, silica fume, rice husk and pozzolana or similar other materials. This admixture is added to concrete during production, in order to improve the performance of concrete in fresh and the hardened state.
(Reference http://nptel.ac.in/courses/105102012/1)
Desirable properties of concrete
Following are the desirable properties of a good concrete;
1. Strong
The concrete should be strong in compression as it will be weak in tension. The concrete used in a construction of load carrying structures such as dams, construction of bridges, pier and abutment of bridges, etc. In these structure reinforcements are used with concrete to extend the strength of concrete, then it will be able to resist the loads.
2. Durability
The concrete should be able to resist weathering action, such a wind, rain, storm, and variation of temperature. So when we used the concrete in the construction of dams, sewer line or in seawater then it should be able to withstand the action of chemical salts. Then the concrete will be safe from environmental problems.
3. Watertight
When used in the construction of water-retaining structures like the culvert, a retaining wall of dams or in water canal the concrete must be watertight. If the concrete is not watertight reinforcing steel bars which we used in the concrete can be rust (corrosion).
4. Workability
The concrete must be easily workable when used in construction. For that, we have to use the water in a limit. Then concrete will be workable and strong. But if the water ratio is maximum in the concrete it leads to segregation of coarse aggregate and fine aggregate and also bleeding of concrete. When the concrete bleeds then it results in strength loss.
5. Density
The concrete must be compacted and there should be no voids or hollow left in the concrete. The weight of concrete must be 2400 kg/cubic meter. So that is the good quality of concrete.
6. Resist to Wear and Tear
When we used concrete in the construction of road and floor, so the concrete must be able to resist abrasive action.
Advantages and disadvantages of concrete
Following are the advantages of concrete structures;
- Ingredients of concrete are locally available, which make it economical
- Unlike natural stones, concrete is free from defects and flaws
- Concrete can be manufactured to desired strength with an economy
- The durability of concrete is high
- It can be cast to any desired shape
- The casting of concrete can be done on the working site
- Maintenance of concrete is almost negligible
- The deterioration of concrete is not appreciably with age
- Concrete makes building a fire-safe due to its incombustible nature
- Concrete can withstand high temperature
- Concrete is resistant to wind and water. Therefore, it is very useful in storm shelters
Following are the disadvantages of concrete structures;
- Compare to other binding material, the tensile strength of concrete is relatively low
- Concrete is less ductile
- The weight of the concrete is high compared to its strength
- Concrete may contain soluble salts, which causes efflorescence
Types of concrete
The classification of concrete is based on following factors;
- Type of binding material
- According to density
- According to purpose
- According to the additives used
- Based on strength
- Cement concrete (cement used as a binder)
- Lime concrete (lime used as a binder)
- Cement-lime concrete (both cement and lime used as a binder)
- Ultra-light concrete (density < 1200 kg/m3)
- Light-weight concrete (density 1200-1800 kg/m3)
- Normal weight concrete (density 2400 kg/m3)
- Heavy-weight concrete (density > 3200 kg/m3)
- Vacuum concrete
- Pumpable concrete
- Self-compacting concrete
- Air entrained concrete
- Normal concrete
- Fiber reinforced concrete
- Shrinkage-compensating concrete
- Polymer concrete
Based on strength (compressive strength),
- Low-strength concrete < 20 MPa
- Moderate strength concrete 20-50 MPa
- High strength concrete 50-200 MPa
- Ultra-high strength concrete > 200 MPa
The various concrete grades are denoted by M 10, M 15, M 20, etc. In which M denotes mix of concrete followed by the compressive strength number. A mix is the respective proportion of constituent materials Cement: Fine aggregates: Coarse aggregate. Compressive strength number indicates the compressive strength of 150mm size cube at 28 days, expressed in N/mm2.
Table 1, provides details of various grade of concrete in use (IS 456:2000)
Group
|
Grade designation
|
Mix Ratio
|
Specified characteristic strength of 150 mm cube at 28 days in N/mm2
|
Ordinary concrete
|
M10
|
1:5:10
|
10
|
M15
|
1:4:8
|
15
|
|
M20
|
1:3:6
|
20
|
|
Standard concrete
|
M25
|
1:2:4
|
25
|
M30
|
Design mix
|
30
|
|
M35
|
Design mix
|
35
|
|
M40
|
Design mix
|
40
|
|
M45
|
Design mix
|
45
|
|
M50
|
Design mix
|
50
|
|
M55
|
Design mix
|
55
|
|
High strength concrete
|
M60
|
Design mix
|
60
|
M65
|
Design mix
|
65
|
|
M70
|
Design mix
|
70
|
|
M75
|
Design mix
|
75
|
|
M80
|
Design mix
|
80
|
Note:
- Minimum grade of concrete for Plain Cement Concrete is M 15
- Minimum grade of concrete for Reinforced Cement Concrete is M 20
What is nominal mix and design mix state their difference
Nominal mix
|
Design mix
|
It is produced at site level on the volumetric basis. Are based on observation and experience without reference to material properties. Adopted for small quantities
|
It is produced on the weight basis. Are based on material properties where a check is applied to each every property of ingredients. Adopted for large volumes
|
The Concept of quality control (reference: Shivkumar M. Goti)
For the structure to be durable, of adequate strength and for aesthetic, accomplishing a quality concrete is of supreme importance. Low strength and low durable concrete structures have sustained several damages causing a loss in terms of repair, downtime and life loss in past decades. So, in order to achieve a quality and a durable structure, maintaining the quality and standard of concrete is a prime need.
In manufacturing, the customer as a user recognizes the quality as a measure of excellence or a state of being free from defects, deficiencies and significant variations. The quality control application in concrete construction is;
- Properties of the cement used in the concrete.
- Properties of the concrete mix designed for use in the structure.
- Control of the coarse aggregates and fine aggregates going into the concrete.
- Mixing of the concrete.
- Transport of the concrete to the construction site.
- A slump of the concrete.
- Curing of the concrete.
- Control of water addition.
- Vibration/Compaction of the concrete.
- Preparation of areas where different concrete pours is done.
- Control of compression test samples
- Control of formwork removal.
Where does quality control begin?
It begins with the production of material used in concrete (Sampling and testing):
- Portland cement
- Pozzolana
- Coarse and Fine Aggregate
How does quality control continue?
Handling and stockpiling
Batching and Mixing
Sampling and testing fresh concrete
1. Slump
2. Air Content
3. Unit weight
4. Temperature
Transporting and placing the freshly mixed concrete.
To know the quality of concrete, We can do the several tests
Tests on Fresh Concrete:
The Slump test
The Compacting Factor Test
Tests on Hardened Concrete
Compression Test
Tensile Strength Test (Split Cylinder Test)
Flexural Strength Test
Cement
It is defined as, "a blended mixture of argillaceous and calcareous material which develops the binding ability in a matrix of the constituent element". It is used for construction that sets, hardens and adheres to other materials, binding them together.
Cement is seldom used on its own, but rather to bind the sand and aggregate together. Cement is used with fine aggregate to produce Mortar for masonry, or with sand and aggregate to produce Concrete.
It is one of the most commonly used concrete types. In this type of concrete, the essential constituents are cement, sand and coarse aggregates designed and mixed with a specified quantity of water. The ratio of essential constituents may be varied within wide limits. A very commonly used design mix is, Nominal Design Mix is 1:2:4.
Plain concrete is mostly used in the construction of pavements and in buildings, where very high tensile strength is not required. It is also used in the construction of Dams.
Among the most important properties of ordinary concrete, the following may be mentioned.
· Density: 2200 – 2500 Kg/m3
· Compressive Strength: 200 – 500 Kg/m2
· Tensile Strength: 50 – 100 Kg/m2
· Durability: Very Satisfactory
Reinforced Concrete:
It is also called RCC (Reinforced Cement Concrete). In this concrete type steel in various forms is used as reinforcement to give very high tensile strength. In fact, it is because of the combined action of plain concrete (having high compressive strength) and steel (having high tensile strength).
Lightweight Concrete: (Reference https://civilguides.com/different-types-of-concrete/)
Any types of concrete having a density less than 1920 Kg/m3 is known as lightweight concrete. Various types of aggregates that are used in the manufacturing of lightweight concrete include natural materials like pumice and scoria, artificial materials like expanded shales and clays and processed materials like perlite and vermiculite. The single important property of lightweight concrete is its very low thermal conductivity.
For example, Thermal conductivity – the k value, for plain concrete may be as high as 10-12. But the thermal conductivity of Lightweight concrete is about 0.3.
High-Density Concrete:
This type of concrete is also called heavyweight concrete. In this concrete type, the density varies between 3000-4000 Kg/m3. These types of concrete are prepared by using high density crushed rocks as coarse aggregates. Among such materials, Barytes is the most commonly used material, which has a specific gravity of 4.5. They are mostly used in atomic power plants and other similar structures. Because it provides good protection from all types of radiations.
Pre-stressed Concrete:
It is a special type of reinforced concrete in which the reinforcement bars are tensioned before been embedded in the concrete. Such tensioned wires are held firmly at each end while the concrete mix is placed. The result is that when concrete sets and hardens, the whole concrete members so the cast is put into compression.
This sort of arrangement makes the lower section of the reinforced concrete also stronger against tension, which is the principal cause of the development of tension cracks in un-tensioned reinforced concrete. Since pre-stressing involves the use of jacks and tensioning equipment, the pre-stressed concrete is also cast in the factories. Some of its advantages are following.
· The potential compressive strength of concrete gets considerably increased.
· The risk of development of tension cracks in the lower sections of beams is considerably reduced.
· The resistance to shear is greatly reduced. This eliminates the necessity of stirrups to a great extent.
· Lighter members can be used than the un-tensioned (normal) reinforced-concrete.
The pre-stressed concrete is greatly favored in the construction of;
· Bridges.
· Long span Roofs.
· Most structures with the heavy dead load.
Precast Concrete:
This term refers to numerous types of concrete shapes that are cast into molds either in a factory or at the site. However, they are not used in construction until they completely set and hardened in a controlled condition.
Some of the examples of Precast Concrete are; precast poles, fence posts, concrete lintels, staircase units, concrete blocks and cast stones, etc. These structural and decorative members are prepared in a well-equipped place where all arrangements are made for;
· Perfect proportioning of the ingredients of concrete.
· Thorough mixing of the cement, aggregates, and water to obtain the mix of the desired design and consistency.
· Careful handling during transport and placement in the perfect design molds.
· Perfect curing, under the controlled conditions of temperature and humidity. Even steam curing is used to obtain precast products having high strength in much less time.
The latest trend in the construction industry is to shift more and more to prefabricated concrete units in building construction.
Air Entrained Concrete: (Reference https://civilguides.com/different-types-of-concrete/)
It is a specially prepared plain concrete in which air is entrained in the form of thousands of uniformly distributed particles. The volume of air, thus, entrained may range between 3-6 percent of the concrete. The air entrainment is achieved by adding a small quantity of foaming or gas-forming agents at the mixing stage. Fatty acids, fatty alcohols, and resins are some common air entraining agents.
Air entrained concrete is more resistant to;
· Scalling.
· Deterioration due to freezing and thawing.
· Abrasion.
Cement
It is defined as, "a blended mixture of argillaceous and calcareous material which develops the binding ability in a matrix of the constituent element". It is used for construction that sets, hardens and adheres to other materials, binding them together.
Cement is seldom used on its own, but rather to bind the sand and aggregate together. Cement is used with fine aggregate to produce Mortar for masonry, or with sand and aggregate to produce Concrete.
Water
Admixtures
Concrete Mix Design
Special Concrete
Aggregates is a granular material such as river sand, gravel, crushed stones, crushed hydraulic-cement concrete or blast furnace (iron) slag used with hydraulic cementing medium to produce either concrete or mortar.
The aggregates are primarily used for providing bulk to the concrete, nearly 70 to 80 percent of the volume of concrete is occupied by aggregates. The reduces shrinkage and effect economy. The study of aggregates involves;
1. Classification
2. Source
3. Size
4. Shape
5. Texture
6. Strength
7. Specific gravity and bulk density
8. Moisture content
9. Bulking factor
10. Cleanliness
11. Soundness
12. Chemical properties
13. Thermal properties
14. Durability
15. Sieve analysis
16. Grading
1. Classification
The most common classification of aggregates on the basis of bulk specific gravity is;
(a) Lightweight aggregates;
(b) Normal weight aggregates; and
(c) Heavy-weight aggregates.
Lightweight aggregates weigh less than 1100 kg/m3. The light weight is due to cellular or high porous microstructure. They have high absorption values. Heavy-weight aggregates weigh more than 2080 kg/m3 and can range up to 4484 kg/m3. Heavy-weight aggregates are commonly used for radiation shielding, counter-weights and other applications where high mass to volume ratio is desired. Normal weight aggregates weight ranges from 1520-1680 kg/m3 and used to produce a normal weight concrete. 
Normal aggregates can further be classified as;
(i) Natural aggregates:
These aggregates are obtained from the natural deposit of sand and gravel or from quarries by cutting rocks. Cheapest among them will be the natural sand and gravel which have been reduced to their present size by natural agents such as water, the wind, and snow etc. River deposits are the most common and have good quality.
The second most commonly used source of aggregates is quarried bedrock material. Crushed aggregates are made by breaking down natural bedrocks into requisite graded particles through a series of blasting, crushing and screening, etc.
(i) Artificial aggregates:
Amongst the artificial aggregates brick ballast and air-cooled blast furnace slag are most common. Broken brick may be used for mass concrete but is not used for reinforced concrete work unless the crushing strength is high. Blast furnace slag is not commonly used on account of the possible corrosion of steel due to the sulphur content of slag. Concrete made with blast furnace slag aggregate has good fire resisting qualities. Other artificial aggregates such as foamed slag, expanded clay, shale, and slate are also used for producing light weight concrete.
1. Source
All the natural aggregates are obtained from the bed rocks. In the earth-crust three exists three types of rocks namely; Igneous, sedimentary, and metamorphic. This classification is made on the basis of the rock formation. The cooling of molten magma or lava on the surface of the crust or deep beneath the crust results in the Igneous rock formation. These rocks are hard, tough, and dense.
Igneous rock or metamorphic rocks are subjected to weathering agencies such as sun, rain, and wind. These weathering agencies decompose, fragmentise, transport and deposit the particle of rock, below the oceanic bed where they are cemented by cementing materials. The cementing material may be carbonaceous, siliceous, or argillaceous in nature.These sedimentary rock formation subsequently get lifted up and becomes continent. The sedimentary rocks are quarried and concrete aggregates are obtained. The sedimentary aggregates vary from soft to hard, porous to dense, and light to heavy.
Both igneous rock and sedimentary rocks may be subjected to high temperature and pressure which causes metamorphism which changes the structure and texture of rocks. Metamorphic rocks possess a foliated structure. The properties of aggregate depend on chemical and mineral composition, petrographic description, specific gravity, hardness, strength, physical and chemical stability, pore structure, etc.
Igneous rocks: Granite, Dolerite, Basalt etc.
Sedimentary rocks: Gravel, Sandstone, limestone, Gypsum, Lignite etc.
Metamorphic rocks: Marble, etc
Is the most elusive property of concrete, which is difficult to define and measure. Many attempts have been made to define workability as a function of the requirement of fresh concrete. The requirements of fresh concrete can be summarized as follows;
a. Stability: The mix should be stable against segregation and bleeding
b. Mobility: The mix should be sufficiently cohesive and mobile so that it can be cast into required shape and placed properly around the reinforcement.
c. Compactability: The mix should be amenable to proper and through compaction.
d. Finishability: It should be possible to achieve the desired surface finish.
These diverse requirements of stability, mobility, compactability and finishability of fresh concrete are collectively referred as a workability.
IS 6461 (Part VII) -1973 defines workability as “ that property of freshly mixed concrete or mortar which determines the ease and homogenzity with which it can mix, placed, compacted and finished………..”
The quality of concrete, satisfying the above requirement is termed as workable concrete. Workable concrete is one which exhibits very little internal friction between particle and particle or which overcomes the frictional resistance offered by the formwork surface or reinforced content in the concrete. The factors affecting the workability of concrete are;
a. Water content
b. Mix proportions
c. The size of aggregates
d. The shape of aggregates
e. Surface textures of aggregates
f. Grading of aggregate
g. Use of admixture
Date of Display: 20.02.2018 Date of Submission: 26.02.2018
Assignment No. 1
Date of Display: 10.02.2018 Date of Submission: 17.02.2018
1. Define Concrete? Explain in detail about its constituent materials
2. Enlist properties of good concrete
3. Write in detail about grades of concrete
4. State advantages and disadvantages of concrete
5. Write about importance of quality control of concrete
Assignment No. 2
Date of Display: 10.02.2018 Date of Submission: 19.02.2018
1) Define Cement? Review the early history of modern cement.
2) Describe in detail about the manufacturing of Ordinary Portland Cement (OPC).
3) Write a note on the chemical composition of OPC.
4) Write about R. H. Bogue’s, Le Chateler and Tornebohm identification of cement compounds and their equations.
5) Explain in detail about hydration of cement and heat of hydration
6) Discuss the role of the following compound in OPC;
a. Calcium silicate hydrates
b. Calcium hydroxide
c. Calcium aluminate hydrates
7) Discuss the structure of hydrated cement.
8) Classify cement as per ASTM Standards
9) Write about the types of cement? Explain any four in detail
10) Enlist physical test on cement? Explain any one in detail.
Assignment No. 3
Date of Display: 20.02.2018 Date of Submission: 26.02.2018
1) Write in brief about;
a. Classification of aggregates
b. Source of aggregates
c. Size of aggregates
d. Shape of aggregates
e. Texture of aggregates
f. Cleanliness and soundness of aggregates
g. Grading of aggregates
h. Specific surface and Surface index
i. Standard grading curve
j. Bulking of sand
2) A sample of coarse aggregate weighing 2 kg is sieved through the standard set of sieves as mentioned below
IS Sieve
|
80 mm
|
40 mm
|
20 mm
|
10 mm
|
4.75 mm
|
Weight retained (gm)
|
0
|
160
|
1380
|
270
|
140
|
3) A sample of fine aggregate weighing 2 kg is sieved through the standard set of sieves as mentioned below
IS Sieve
|
4.75 mm
|
2.36 mm
|
1.18 mm
|
600 mm
|
300 mm
|
150 mm
|
Weight retained (gm)
|
0
|
160
|
1180
|
270
|
240
|
150
|
4) Describe in detail about;
a. Compressive strength of aggregates
b. Impact strength of aggregates
c. Abrasive strength of aggregate
d. Elongation Index and Flakiness Index
Minor - I
1. Write a short note on quality control of fresh concrete.
2. Write a short note on quality assessment of hardened concrete.
3. What is hydration of cement? Discuss water requirement for hydration.
4. What are the Bogue’s compounds? Explain how each compound affects the properties of cement
5. Explain in detail wet manufacturing of cement
6. Explain in detail dry manufacturing of cement.
7. Enlist various types of cement; also specify each.
8. Describe laboratory test for (anyone);
a. Fineness of cement (dry sieving and Blain permeability apparatus)
b. Normal consistency of cement
c. Initial setting and Final setting time of cement
d. Soundness of cement
e. Compressive strength of cement
9. How will you determine absorption and moisture content of coarse aggregate? How does it affect concrete making?
10. Explain gap grading of aggregate. How does it affect the concrete strength?
11. What is the alkali-aggregate reaction? How does it affect the concrete strength?
12. A sample of coarse aggregate weighing 5 kg is sieved through the standard set of sieves as mentioned below
IS Sieve
|
80 mm
|
40 mm
|
20 mm
|
10 mm
|
4.75 mm
|
Weight retained (gm)
|
0
|
560
|
2380
|
1570
|
490
|
13. A sample of fine aggregate weighing 1 kg is sieved through the standard set of sieves as mentioned below
IS Sieve
|
4.75 mm
|
2.36 mm
|
1.18 mm
|
600 mm
|
300 mm
|
150 mm
|
Weight retained (gm)
|
0
|
90
|
385
|
290
|
175
|
60
|
14. Surface index of combined grading of aggregate samples given in question 12 and 13 is 0.55. Estimate the proportion of fine aggregate and coarse aggregate. Use data provided in question 12-13 for estimating surface index of FA and CA individually if, surface index of aggregate particles is;
IS Sieve
|
80-40
|
40-20
|
20-10
|
10-4.75
|
4.75-2.36
|
2.36-1.18
|
1.18-600µ
|
600-300µ
|
300-150µ
|
Less 150µ
|
Surface index
|
-2.5
|
-2
|
-1
|
+1
|
+4
|
+7
|
+9
|
+9
|
+7
|
+2
|
15. Write short note on bulk density of aggregate
16. What do you understand by bulking of sand? How it affects the concrete making.
Model Answer
Concrete Technology
Minor -I
1. Write a short note on quality control of fresh and hardened concrete. (05)
Answer:
For the structure to be durable, of adequate strength and for aesthetic, accomplishing a quality concrete is of supreme importance. Low strength and low durable concrete structures have sustained several damages causing a loss in terms of repair, downtime and life loss in past decades. So, in order to achieve a quality and a durable structure, maintaining the quality and standard of concrete is a prime need.
In manufacturing, the customer as a user recognizes the quality as a measure of excellence or a state of being free from defects, deficiencies and significant variations. The quality control application in concrete construction is;
1. Properties of the cement used in the concrete.
2. Properties of the concrete mix designed for use in the structure.
3. Control of the coarse aggregates and fine aggregates going into the concrete.
4. Mixing of the concrete.
5. Transport of the concrete to the construction site.
6. The slump of the concrete.
7. Pouring of the concrete.
8. Control of water addition.
9. Vibration/Compaction of the concrete.
10. Preparation of areas where different concrete pours is done.
11. Control of compression test samples
12. Control of formwork removal.
It begins with the production of material used in concrete (Sampling and testing):
Ø Portland cement
Ø Pozzolana
Ø Coarse and Fine Aggregate
Uniformity of concrete production will be no greater than the uniformity of materials used in the concrete.
One can start quality control from;
Handling and stockpiling
Ø Batching and Mixing
Ø Sampling and testing fresh concrete
1. Slump
2. Air Content
3. Unit weight
4. Temperature
Ø Transporting and placing the freshly mixed concrete.
To know the quality of concrete, We can do the several tests.
1. Tests on Fresh Concrete (for workability):
a. The Slump test
b. The Compacting Factor Test
2. Tests on Hardened Concrete:
a. Compression Test
b. Tensile Strength Test (Split Cylinder Test)
c. Flexural Strength Test
2. Explain in detail wet manufacturing process of cement. (05)
Answer:
In the wet process, the limestone brought from the quarries is first crushed to smaller fragments. Then it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to a fine consistency of slurry with the addition of water.
The slurry is a liquid of creamy consistency with a water content of about 35 to 50 percent, wherein particles, crushed to the fineness of Indian Standard Sieve number 9, are held in suspension.
The slurry is pumped to slurry tanks or basins where it is kept in an agitated condition by means of rotating arms with chains or blowing compressed air from the bottom to prevent settling of limestone and clay particles. Finally, the corrected slurry is stored in the final storage tanks and kept in a homogeneous condition by the agitation of the slurry.
The corrected slurry is sprayed on to the upper end of a rotary kiln against hot heavy hanging chains. The slurry on being sprayed against a hot surface of flexible chain losses moisture and becomes flakes.
These flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move from the upper end towards the lower end of the kiln subjecting itself to higher and higher temperature.
The fuel is either powered coal, oil or natural gas. By the time the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the temperature is in the order of 1500°C, about 20 to 30 percent of the materials get fused.Lime, silica, and alumina get recombined. The fused mass turns into the nodular form of size 3 mm to 20 mm known as clinker.
The clinker drops into a rotary cooler where it is cooled under controlled conditions The clinker is stored in silos or bins. The clinker weighs about 1100 to 1300 gms per liter. The liter weight of clinker indicates the quality of clinker.
The cooled clinker is then ground in a ball mill with the addition of 3 to 5 percent of gypsum in order to prevent flash-setting of the cement. The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement is bagged or filled into barrels for bulk supply to dams or other large work sites.
3. Describe laboratory test for (any one): (05)
a. Fineness of cement (dry sieving and Blain permeability apparatus)
b. Normal consistency of cement
c. Initial setting and Final setting time of cement
d. Soundness of cement
e. The compressive strength of cement
Answer:
To determine the fineness of cement sample by dry sieving
Apparatus used are IS 90-micron sieve, balance, gauging trowel and brush.
Control of particle size of cement is important in practice because it has an influence on the behavior of the cement. It has an important bearing 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 faster the development of strength, (as shown in Figure 1). The fineness of grinding has increased over the years. But now it has got nearly stabilized. Different type of cement is ground to different fineness. The particle size fraction below 3 microns has been found to have the predominant effect on the strength at one day while 3-25 micron fraction has a major influence on the 28 days strength. Increase in fineness of cement is also found to increase the drying shrinkage of concrete.
The fineness of cement is tested in two ways:
(a) By sieving.
(b) By determination of specific surface (total surface area of all the particles in one gram of cement) by air-permeability apparatus. Expressed as, cm2/gm, or m2/kg. Generally, Blaine Air permeability apparatus is used.
Figure 1: Effect of fineness on compressive strength
