Air pollutants can also be broadly classified into two general groups- Primary air pollutants and secondary air pollutants.
A. Primary Air Pollutants
Primary air pollutants are those emitted directly from identifiable sources.
Examples of primary air pollutants: 1) Fine particles(less than 100 μ in diameter) 2) Coarse particles (greater than 100 μ in diameter) 3) Sulphur compounds 4) Oxides of nitrogen 5) Carbon monoxide 6) Halogen compounds 7) Organic compounds 8) Radioactive compounds
B. Secondary Air Pollutants
Secondary air pollutants are those which are produced in the air by the interaction among two or more primary pollutants,or by reaction with normal atmospheric constituent, with or without photo-activation.
Examples of secondary air pollutants: 1) Ozone 2) Formaldehyde 3) PAN (Peroxy acetyl nitrate) 4) Photochemical smog 5) Formation of acid mists (H2SO4) due to the reaction of sulphur dioxide and dissolved oxygen. when water droplets are present in the atmosphere
The following 15 precautions should be taken during the preparation of good-quality concrete:
1. The cement and aggregates should be adequate in quality and it should ensure that they meet the required specifications to achieve the desired strength and durability of concrete.
2. The aggregate should be hard and durable. And, it shall be properly graded in size.
3. The cement should be of sufficient quantity and quality to produce the required strength and water-tightness.
4. The water shall be free from organic material or any deleterious minerals. The quantity of water should be such that it produces the needed consistency.
5. To make a good quality of concrete you should follow the proper water-cement ratio specified in the mix design. Avoid adding excessive water, which can weaken the concrete strength and affect its durability.
6. Mixing should be done thoroughly so as to produce homogeneity.
7. During the transportation of concrete, there should be no segregation of or separation of materials in concrete take place.
8. Concrete delivered at the point of placing should be uniform & have the proper consistency.
9. The concrete should not be thrown from a height to avoid segregation.
10. It should be deposited in an even horizontal layer of uniform thickness. Concrete should fill every part in these forms.
11. Until the concrete becomes hard, it is necessary to ensure that the temperature of the concrete is maintained above the freezing point.
12. When the placing operation is suspended for some time, groves must be made in the finished work joining of future work.
13. In easy of R.C.C slab, placing of concrete should be started from the end and done width wise.
14. The old concrete surface must be made rough, cleaned and cement grouted before placing fresh concrete over it.
15. The finished work should be cured properly for a specified time.
Conclusion: By following these precautions, you can significantly increase the chances of preparing good quality concrete. For any specific guidance, you must consult a professional engineer or local concrete expert.
The process of keeping the concrete surface moist for a certain period after its finishing is called curing.
Necessity of curing
Curing prompts continuous hydration of cement and thereby helps in acquiring full strength. Curing also helps in improving:
The durability,
Impermeability,
Weather-resisting qualities,
And reduces shrinkage.
If curing is not done properly complete hydration of cement will not take place. Due to this concrete will not acquire its full strength and shrinkage cracks will be developed.
The concrete is cured by any of the following methods, depending upon the type of construction work, Following are the 9 methods for curing of concrete.
1. Covering
In this method of concrete curing, the newly laid concrete surface is covered by wet gunny bags or hessian, which are wetted periodically. This curing method is suitable for horizontal as well as vertical and sloping surfaces.
9 Methods Of Curing Of Concrete
2. Ponding
Ponding is one of the best methods for curing of concrete, mostly practised in India. In this curing method, the whole surface is divided into rectangular or square enclosures by constructing small clay or mud bunds and these enclosures are filled with water periodically forming small ponds.
This method is suitable for curing horizontal concrete surfaces such as the floor, pavements, etc. Though this curing method is very good, the water requirement for this method is very large, and sometimes mud bunds may be washed out by water or water may be leaked through mud bunds.
3. Immersing In Water
Pre-cast concrete members are often cured by immersing them underwater. This curing method is not acceptable everywhere due to the lack of water.
4. Steam Curing
In this curing method, steam under pressure is sprayed over the concrete surface. This curing method is typically adopted in the case of pre-cast members like concrete sleepers.
5. Sprinkling
In this concrete curing method, the concrete surface is kept wet by sprinkling water frequently over the surface. In this case, the water requirement is very large, therefore, it is acceptable for that place where water is sufficiently available for this method.
6. Curing with Chemical
In this chemical curing method, water is sprinkled over the surface after adding a certain amount of hygroscopic salt such as NaCl, CaCl, etc. which absorbs moisture from the atmosphere.
7. Membrane Curing
In the membrane curing method, the concrete surface is kept covered by a waterproof membrane such as wax emulsion, bitumen emulsion, etc.
The bituminous compounds are black in colour. The heat absorbed by such substances is therefore much higher. They end up raising the concrete surface temperature which is inappropriate. For this reason, certain non-black modified compounds are used. These types of compounds are known as ”Clear Compounds”, which help to reduce heat absorption.
The membrane prevents the evaporation of water from the concrete surface. In order to obtain the best results, the membrane is applied after one or two days of actual water curing.
8. Electrical Curing
The electrical curing method is applicable for cold climate regions. This method is not preferable in ordinary climate regions.
In this method, concrete can be cured by passing an electric current( must be an alternative current) through the concrete. Prevention must be taken during curing by the electrical method.
As they have many disadvantages, this method is not much more acceptable for concrete curing.
9. Curing By Infrared Radiation
Infrared radiation is another method for curing of concrete, which is applicable in very cold climate regions. The infrared radiation method is mostly used in Russia. It is claimed that this method helps to get more rapid strength than steam curing.
Best Concrete Curing Methods For Different Types of Structures
Types of Structure
Best Methods for Different Types of Concrete Structure
For horizontal members, ponding is the best-suited method, other methods like sprinkling, steam curing, chemical, and membrane Curing are also preferable.
vertical members like columns, walls, etc.
For curing vertical members, covering and sprinkling are the best-suited methods.
Pre-cast members
For pre-cast members, steam curing and immersing in water are the best methods.
Concrete structure in very cold weather
For very cold regions, electrical curing and curing By infrared radiation method may be used.
FAQs
How many methods are used for concrete curing?
There are different methods used for curing of concrete, Following 9 methods are used for concrete curing:
Covering
Ponding
Immersing In Water
Steam Curing
Sprinkling
Curing With Chemical
Membrane Curing
Electrical Curing
Curing By Infrared Radiation
What is the best method of curing concrete?
When the best methods for curing are asked, it will always be covering, water spray, and ponding. However, these curing methods require more water than others. These methods are not applicable where water deficiency is the main headache.
What is the minimum curing period?
The minimum period for curing of concrete to gain maximum strength(approximately 90%) is 28 Days
What happens if curing is not done?
Curing ensures the hydration of cement. If the curing is not done, there will be a lack of water which will cause insufficient hydration and this can lead to cracks and poor strength development in the concrete.
Can curing be done without water?
Yes, curing can be done without water, where water deficiency is more some other methods are used for concrete curing, such as chemical curing, electrical curing, membrane curing, etc.
How much strength gain after 7-day curing period?
Concrete gains 55 to 65 % strength after 7 day curing period.
When a binding material (cement or lime), fine aggregate (sand or surkhi), coarse aggregate such as crushed stone, broken bricks, etc. and water are mixed in suitable proportion, they form an easily workable mix, known as plastic or green concrete. When this plastic concrete becomes hard like a stone, this is termed as hardened concrete or simply as concrete.
Classification of Concrete
The concrete is classified as given below :
1) According to specification
a) Nominal mix concrete
The concrete which is prepared according to prescriptive specification i.e. the proportion of constituents and their characteristics is termed as nominal mix concrete.
b) Designed mix concrete
The concrete which is prepared according to performance-oriented specifications i.e. strength, workability, etc. is termed as designed mix concrete.
2) According to the level of control
a) Controlled concrete
Controlled concrete is that concrete, in which the preliminary test is conducted for designing the mix. In addition, to mix proportioning, level of control is also exercised in the selection of materials batching, mixing, transportation, compaction and curing along with necessary checks and tests for quality acceptance.
b) Ordinary concrete
Ordinary concrete is one where no preliminary tests are performed for designing the mix.
3)According to binding materials
a) Cement concrete
The concrete consisting of cement, sand and coarse aggregate mixed in suitable proportion in addition to water is called cement concrete.
b) Lime concrete
The concrete consisting of lime, fine aggregate and coarse aggregate mixed in suitable proportion in addition to water is called lime concrete. The strength of this concrete is less but it is cheaper than cement concrete.
4) According to design
a) Plan cement concrete. b) Reinforced cement concrete. c)Pre-stressed concrete.
5) According to purpose
a) Vacuum concrete. b) Air entrained concrete. c) Lightweight concrete. d) Sawdust concrete. e) High early strength concrete. f) White and coloured concrete.
Concrete is generally graded according to its compressive strength as per IS-156,2000, there are fifteen grades (M10 to M80) of concrete in the designation of concrete mix, the letter M refers to the concrete mix and the number refers to the specified characteristic strength of concrete at 28 days expressed in MPa.
The grade M10, M15, M20 are term as ordinary concrete.
The grade M25, M30, M35, M40, M45, M50, and M55 are termed as standard concrete.
The grades M60, M65, M70, M75, and M80 are termed as high strength concrete.
The Concrete of Grade M10
The concrete of grades M10 or lower is suitable for the lean concrete base, simple foundation, foundation of masonry walls and other simple works.
The Concrete of Grade M20
The concrete of grade M20 or higher is suitable for reinforced concrete works.
The Concrete of Grade M30
The concrete of grade M30 or higher is suitable for pre-stressed concrete works.
For Concrete in Seawater
For concrete in seawater or exposed directly along sea coast shall be at less M20 grade in the case of PCC concrete and M30 in the case of RCC works.
Permeability is defined as the property of a porous material that permits the passage or seepage of water (or other fluids)through its interconnecting voids.
A material having continuous voids is called permeable. Gravels are highly permeable, while stiff clay is the least permeable.
4 Importance Of Study Of Seepage Analysis
The study of seepage of water through soils is important for the following engineering problems:
Determination of rate of settlement of a saturated compressible soil layer
Calculation of seepage through the body of earth dams, and stability of slopes.
Calculation of uplift pressure under the hydraulic structure and their safety against piping
Groundwater flow towards wells and drainage of soils.
DARCY’S LAW
The law of flow of water through the soil was first studied by Darcy. This law states that the rate of flow or discharge per unit time (q) is proportional to the hydraulic gradient (i).
This law only for laminar flow conditions in a saturated soil mass. If q is the discharge per unit time through a soil mass of cross-sectional area ‘A’ under hydraulic gradient ‘i’.
Then, q ∞ i.A or, q= K.i.A
Where, K is a constant, known as the coefficient of permeability. If a soil sample of length ‘L’ and cross-sectional area ‘A’ (measured perpendicular to the direction of flow) is subjected to a differential head of water (h1-h2), then
Limitation of Darcy’s Law
Darcy’s law is valid only for laminar flow conditions in the soil mass.
Coefficient Of Permeability
The coefficient of permeability is defined as the average velocity of flow that will occur through the total cross-sectional area of the soil mass under a unit hydraulic gradient. It is denoted by ‘K’ Its unit is the same as the unit of velocities such as Cm/sec or m/sec or m/hr, etc.
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Following are the 7 Factors Affecting The Permeability of Soil:
1) Size of particles
The permeability varies approximately as the square of the grain size. The relationship is [latex] K = CD^{2} \frac{e^{3}}{1+e}\frac{\gamma_{w} }{\mu } [/latex], where [latex] D [/latex] is the effective diameter of the soil particles in cm. From the above equation, we can say, the permeability of soil is directly proportional to the square root of the particle diameter. If the soil particle is large then permeability will be high, and if the soil particle is small then permeability will be less.
This happens because the large particles contain large volumes of voids, and other hand small particles contain small volumes of voids. Hence, the flow of water through soil mass will be more in the case of large particles.
2. Specific Surface and Shape
The permeability of coarse-grained soil is inversely proportional to the specific surface of particles at a given porosity. Relationship is [latex] K = \frac{1}{K_k\eta (S_s)^2} \times \frac{n^3}{1-n^2} [/latex] , where [latex] S_s [/latex] is the specific surface of particles.
3) Properties of the pore fluid
The permeability of soil is directly proportional to the unit weight of water and inversely proportional to its viscosity. Again the viscosity changes with the change in temperature. Normally, with the increase in temperature viscosity decreases and hence permeability increases.
4) Void ratio of soil
The permeability (k) is directly proportional to c.e3/ (1+e), where ‘c’ is a factor and ‘e’ is the void ratio. Hence permeability decreases with a decrease in void ratio & vice-versa.
5) The structural arrangement of soil particles
The permeability may vary with different geometric arrangements and shapes of voids.
If the particles are arranged in a flocculated structure, then the permeability will be low. On the other hand, if the particles are arranged in a dispersed structure, then permeability will be high.
The permeability of soil also depends on structural defects like – cracks.
6) Entrapped air and organic impurities
The permeability of soil is greatly reduced if the air is entrapped in the voids. The presence of organic foreign matter in soil mass also decreases the permeability.
7) Adsorbed water
The adsorbed water surrounding the fine soil particles reduces the effective voids available for the passage of water and thereby reduces the permeability.
The uniformity coefficient is a measure of particle size range and given by the ratio of D60 and D10 size of particles. It is denoted by Cu.
Thus, Cu = D60/D10
Where,
D60 = Size of the particle corresponding to 60% finer and,
D10 = Size of the particle corresponding to 10% finer.
Coefficient of curvature(Cc)
The coefficient of curvature is a measure of gradation of particles and given by the following expression:
Cc = (D30)2/D60.D10
Where,
D60 = Size of the particle corresponding to 60% finer.
D30 = Size of particle corresponding to 30% finer.
D10 = Size of the particle corresponding to 10% finer.
The size D10 is some times called effective size. The value of Cu is nearly equal to unity for a uniformly of poorly graded soil. The value of Cc is more than 1.0 but less than 3.0 for well-graded soil. Cu is greater than 6 for sands.
