Preparing and Placing of Concrete

Preparing and Placing of Concrete

The following steps are involved in the concreting:

1.Batching

2.Mixing

3.Transporting and placing and

4.Compacting.

1. Batching: The measurement of materials for making concrete is known as batching. The following two methods of batching is practiced:

(a) Volume batching

(b) Weight batching

(a) Volume Batching: In this method cement, sand and concrete are batched by volume. A gauge box is made with wooden plates, its volume being equal to that of one bag of cement. One bag of cement has volume of 35 litres. The required amount of sand and coarse aggregate is added by measuring on to the gauge box. The quantity of water required for making concrete is found after deciding water cement ratio. For example, if water cement ratio is 0.5, for one bag of cement (50 kg), water required is 0.5 × 50= 25 kg, which is equal to 25 litres. Suitable measure is used to select required quantity of water.

(b) Weight Batching: This is the recommended method of batching. A weighing platform is used in the field to pick up correct proportion of sand and coarse aggregates. Large weigh batching plants have automatic weighing equipments.

2.Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand and coarse aggregate, first in dry condition and then in wet condition after adding water. The following methods are practiced:

(a) Hand Mixing

(b) Machine Mixing.

(a)  Hand Mixing: Required amount of coarse aggregate for a batch is weighed and is spread on an impervious platform. Then the sand required for the batch is spread over coarse aggregate. They are mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed by shovels. After uniform texture is observed water is added gradually and mixing is continued.

(b) Machine Mixing: In large and important works machine mixing is preferred. A typical concrete mixer. Required quantities if sand and coarse aggregates are placed in the drum of the mixer. 4 to 5 rotations are made for dry mixing and then required quantity of cement is added and dry mixing is made with another 4 to 5 rotations. Water is gradually added and drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this stage uniform and homogeneous mix is obtained.

3. Transporting and Placing of Concrete. After mixing concrete should be transported to the final position. In small works it is transported in iron pans from hand to hand of a set of workers. Wheel barrow and hand carts also may be employed. In large scale concreting chutes and belt conveyors or pipes with pumps are employed. In transporting care should be taken to see that seggregation of aggregate from matrix of cement do not take place.

Concrete is placed on form works. The form works should be cleaned and properly oiled. If concrete is to be placed for foundation, the soil bed should be compacted well and is made free fromloose soil.

Functions of Various Ingredients

Functions of Various Ingredients

Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.

Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents shrinkage of cement. When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.

Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete. The variation of strength of concrete with water cement ratio. To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixingand water cement ratio of 0.5 to 0.6 is used for hand mixing.

Concrete

Concrete

 Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. This can be easily moulded to desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete and Ferro cement.

Plain Concrete

Major ingredients of concrete are:

1.Binding material (like cement, lime, polymer)

2.Fine aggregate (sand)

3.Coarse aggregates (crushed stone, jelly)

4.Water.

A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture.

Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the

absence of mix design the ingredients are proportioned as 1:1:2, 1:11/2 :3, 1:2:4, 1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.

In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works

How to test for a good brick?

 

How to test for a good brick?

Drop a brick vertically from a height of 1 m. A good quality brick will not break.

Strike two bricks against each other. Good quality bricks will produce a clear ringing sound on contact.

Perfect Mix

For a 4"thick wall (partition walls in the middle of the house), keep mortar proportion as =1:4 • (cement: sand)

For a 9"wall (outer wall), keep mortar proportion as = 1:6 (cement: sand)

Good Practice

Begin work at the corners, first to a height of 3 or 4 layers with base extending in steps.

Place all bricks on their bed. The depression on top provides space for the mortar to bond well. Use line-string, plumb bob, and spirit level for checking alignment, and to keep vertical and horizontal lines straight.

Soak your bricks in water for 8 hours at least before use, else it will absorb moisture from mortar.

What is Ready Mix Concrete Composed of?

What is Ready Mix Concrete Composed of? 

Ready mix concrete is composed of standard concrete ingredients and additives, according to the intended use of the concrete. "Ready mix" is not a special type of concrete. Instead, the term describes the way the concrete is delivered to a job site--already mixed. Ready mix concrete is the solution to the problem of having to mix concrete components in large quantities while maintaining consistently precise properties.

Features

By volume, ready mix concrete is compose of 60 to 75 percent aggregate including sand, gravel and stone. Any clean, hard, non-reactive, non-porous rock or sand may be a suitable aggregate. Size and shape of aggregate are selected according to the desired strength and texture of the concrete, how the ready mix concrete will be placed, and also cost.

Function

Cement is the binder in ready mix concrete, and it represents 10 to 15 percent of ready mix concrete volume. The most commonly used cement, Portland cement may contain limestone, marl, shale, blast furnace slag, silica sand and iron ore. Ingredients may vary; it's the chemical constituents of these--calcium, silicon, aluminum, iron and gypsum--that are important in ready mix concrete.

Effects

The components of ready mix concrete cannot become solid without water. Water molecules, when combined with calcium silicate, react in a chemical process called hydration. The hydrated compounds of water and calcium silicate form a dense crystaline structure that binds the aggregate into a single solid form. So technically concrete does not dry. Instead, it cures chemically.

Significance

The ratio of water to cement in ready mix concrete is the critical determining factor in concrete strength. Water not used up by the hydration reaction remains a part of the concrete, causing it to be less strong. Ideally, only enough water would be added to the mix to react with the cement, and no more. In practice though, this makes for concrete that is difficult to work with, so more water is added than necessary for hydration. Depending on the intended use, ready mix concrete is composed of 15 to 20 percent water by volume.

Potential

Ready mix concrete may also contain fly ash, silica fume, blast furnace slag or metakaolin as a substitute for a percentage of the Portland cement. These components, like Portland cement, react with water to form a monolithic solid. The advantage of their use is the improvement of concrete strength and durability. Fly ash, silica fume and furnace slag are byproducts of power generation or industrial processes, so their use benefits the environment by reducing waste.

Masonry

Masonry

Masonry is defined as the art of construction in which building units, such as clay bricks, sand-lime, bricks, stones, Pre-cast hallow concrete blocks, concrete slabs, glass bricks, combination of some of these building units etc are arranged systematically and bonded together to form a homogeneous mass in such a manner that they can with stand point to other loads and transmit then through the mass without fail or disintegration.

Masonry can be classified into the following categories.

1. Stone masonry

2. Brick masonry

3. Hallow block concrete masonry

4. Reinforced masonry

5. Composite masonry

These can be further sub-divided into varies types depending upon workmanship and type of materials used.

Objects of foundations

Objects of foundations

Every structure consists of two parts. (1) Foundation and (2) Super structure. The lowest artificially prepared parts of the structure which are in direct contact with the ground and which transmit the loads of the structure to the ground are known as Foundation or Substructure. The solid ground on which the foundation rest is called the “foundation bed” or foundation soil and it ultimately bears the load and interacts with the foundations of buildings.

Objects of foundations

Foundations are provided for the following purposes.

1.                        To distribute the total load coming on the structure on large area.

2.                        To support the structure.

3.                        To give enough stability to the structures against various distributing forces such as wind, rain etc.

4.                        To prepare a level surface for concreting and masonry work. The general inspection of site of work serves as a good for determine the type of foundation, to be adopted for the proposed work and in addition, it helps in getting the data w.r.to the following items.

    i.            Behavior of ground due to variations in depth of water table.

ii.            Disposal of storm water at site.

iii.            Nature of soil by visual examination.

iv.            Movement of ground due to any reason etc.

Building Plinth

Building Plinth

This is the portion of structure between the surface of the surrounding ground and surface of the floor, immediately above the ground. As per Byelaws, the plinth should not be less than 45cm. The basic requirements of plinth area

1.                       To transmit the load of the super-structure to the foundation.

2.                       To act as a retaining wall so as to keep the filling portion below the raised floor or the building.

3.                       To protect the building from damp or moisture penetration into it.

4.                       It enhances the architectural appearance of the building.

Building Foundation

Building Foundation

The foundation is the most critical part of any structure and most of the failure is probably due to faulty foundations rather than any other cause. The purpose of foundation is to transmit the anticipated loads safety to the soil.

Basic requirements:

·     To distribute the total load coming on the structure over a large bearing area so as to prevent it from any movement.

·     To load the bearing surface or area at a uniform rate so as to prevent any unequal or relative settlement.

·     To prevent the lateral movement of the structure.

·     To secure a level or firm natural bed, upon which to lay the courses of masonry and also support the structure.

·     To increase the suitability of the structure as a whole, so as to prevent it from overturning or sliding against such as wind, rain, frost etc.

Component parts of building

Component parts of building

The building basically consists of three parts namely,

1) Foundation

2) Plinth and

3) Super structure

Foundation : It is the lowest artificially prepared part below the surface of the surrounding ground which is indirect contact with sub-strata and transmits all the loads to the ground (or sub-soil).

Plinth : It is the middle part of the structure, above the surface of the surrounding ground up to the surface of the floor immediately above the ground. Its function in the building is same as of sub-structure in the case of the bridge.

Super structure : It is that part of the structure which is constructed above the plinth level (i.e., ) ground level A building in general made of the following structural components.

1. Foundation

2. Plinth

3. Walls and piers in super structure

4. Ground, basement and upper floors

5. Doors and windows

6. Sills, Lintels and weather shades

7. Roofs8. Steps and stairs

9. Finishes for walls

10.Utility fixtures

Each of these components is an essential part of a building and requires due consideration in design and construction for their functional performance.

Type of Buildings

Type of Buildings

Residential buildings: All those buildings in which sleeping accommodation is provided for residing permanently or temporary with or without looking or dinning or both facilities are termed as residential building. Ex: Apartments, Flats, Bungalows, Dormitories, private houses, Hotels, Hostels, Cottages, Hole day camps, clubs, hotels, Inns etc.

Educational buildings: All those buildings which are meant for education from nursery to university are included in this group Ex: schools, colleges, universities, training institutes etc.

Institutional Buildings: This group includes any building or part thereof, which is used for the purposes such as medical, health, recovering health after illness, physical or mental diseases, care of infants or aged persons, panel detention etc. These buildings normally provide sleeping accommodation for the occupants.

Assembly Buildings: This group includes any building or part or a building where groups of people assemble or gather for amusement; recreation, social, religious, patriotic or similar purpose for example museums, gymnasiums, restaurants, places of worship, passenger stations, public transportation services, open air theatres, swimming pools etc.

Business Buildings: This group includes any building or part or a building which is used for purposes such as transaction of business, keeping of accounts and records etc; dispensaries and clinics, banks, city halls, court halls, libraries etc.

Mercantile Buildings: This group includes any building or part of a building which is used for shops, stores, market, for safe and display of products or waves either whole sale or retail.

Industrial Buildings: This group includes any building or part of a building or structure in which product of different kinds and properties are fabricated, assembled or processed. For example, laboratories, assembly plants, laundries, gas plants, power plants, refineries, diaries etc.

Storage Building: This group includes those building structures which are primarily used for the storage structures which are primarily used for the storage or sheltering of goods, waves or merchandise vehicles or animals, for example warehouses, cold storages, freight depots, store houses, transit sheds, truck terminals, garages etc.

Hazardous Building: This group includes those building structures which are used for the storage, handling, manufacture or processing of materials which are liable to burn with extreme rapidity and prove hazards to health; building or building contents. Hazards may be due to fire; poisonous fumes or gases, explosions, ignitions etc from materials subjected to various operations. Buildings used for storage of gases under high pressure or for storage and handling of highly flammable liquids or explosives, fireworks etc are included in this group.

Curing of concrete

Curing of concrete

Curing of concrete is one of the essential requirement of process of concreting. Curing is process of keep the set concrete damp for some days in order to enable the concrete gain more strength.

Purposes:

·     Curing protects concrete surfaces from sun and wind

·     Presence of water is essential to cause the chemical action which a companies the setting of concrete

Concrete Compaction - Methods

Concrete Compaction - Methods

Concrete should be placed and compacted immediately after mixing. The concrete should be placed within 30 to 40 minutes to prevent the danger of concrete getting its initial set, before laying the concrete, the shuttering should be cleaned of all of dust or debris. Crude oil or grease etc is usually applied to the shuttering before concreting to prevent the shuttering absorbing the water from the concrete or getting stuck to it. In placing the concrete, care should be taken to see that it should not be thrown from heights. Concrete should be laid in layers 15 to 30 cm (6” to 12”) in thickness and each layer should be properly compacted before laying the next one.

Compaction of concrete should be proceed immediately after placing. The function of compaction of concrete is to expel the air bubbles in the mass and make it impermeable in addition to its securing the desired strength. The concrete mass should be consolidated or compacted till the cream of the cement starts appearing on the surface. Over compaction may lead to segregation of concrete while-under-compaction may leave air voids in concrete and results in honey combing. Compaction may be done by hand or mechanical device.

Hand compaction : The hand compaction may be done by rodding, tamping or hammering. Tamping is usually adopted for compacting concrete for slabs or other such surfaces. Rodding is done for thin vertical members. Hammering is done for massive plain concrete works and for compacting an almost dry concrete the surface is beaten with heavy flat bottom rammers till the thin film of mortar start appearing on the surface.

Mechanical compaction: Mechanical compaction is done by the use of vibrators. Vibrators are of three types 1. Internal 2. External 3. Surface. Internal vibrators are commonly used in large works for flat surface compaction. In this the vibrator is immersed in the full depth of concrete layer. The vibrator should be kept in one position for about 3 minutes and then removed and placed another position. External vibrators are placed against the form work and are only adopted for thin section of members or in places where internal vibrators cannot be used with ease. Surface vibrators are generally employed in concrete road construction. Compaction of concrete by use of vibrators permits the use of stiff concrete mix of high strength and ensure better compaction than that obtained by the method of hand compaction

Uses of Concrete

Uses of Concrete

·     For heavy loaded R.C.C columns and R.C.C arches of long spans

·     For small pre cast members of concrete like fencing poles, telegraph poles etc. watertight construction.

·     For water tanks, bridges, sewers etc.

·     For foot path, concrete roads.

·     For general work of RCC such as stairs, beams, columns, slabs, etc.

·     For mass concrete for heavy walls, foundation footings etc.

Sand

Sand

Sand is an important building material used in the preparation of mortar, concrete, etc.

·     Sources of Sand: Sand particles consist of small grains of silica (Si02). It is formed by the decomposition of sand stones due to various effects of weather. The following are the natural sources of sand.

·     Pit Sand: This sand is found as deposits in soil and it is obtained by forming pits to a depth of about 1m to 2m from ground level. Pit sand consists of sharp angular grains, which are free from salts for making mortar, clean pit sand free from organic and clay should only be used.

·     Rive Sand: This sand is obtained from beds of rivers. River sand consists of fine rounded grains. Color of river sand is almost white. As the river sand is usually available in clean condition, it is widely used for all purposes.

·     Sea Sand: This sand is obtained from sea shores. Sea sand consists of rounded grains in light brown color. Sea sand consists of salts which attract the moisture from the atmosphere and causes dampness, efflorescence and disintegration of work. Due to all such reasons, sea sand is not recommendable for engineering works. However be used as a local material after being thoroughly washed to remove the salts.

Uses of Cement

Uses of Cement

1. Cement mortar for masonry work, plaster, pointing etc.

2. Concreter for laying floors, roofs and constructing lintels, beams, weather sheds, stairs, pillars etc.

3. Construction of important engineering structure such as bridges, culverts, dams, tunnels storage reservoirs, light houses, deckles etc.

4. Construction of water tanks, wells, tennis courts, septic tanks, lampposts, roads, telephone cabins etc.

5. Making joints for drains, pipes etc.

6. Manufacture of pre cast pipes, piles, garden seats, artificially designed urns, flowerpots, etc dustbins, fencing posts etc.
7. Preparation of foundations, watertight floors, footpaths etc.

Types of Cement

Types of Cement

In addition to ordinary cement, the following are the other varieties of cement.

·     Acid Resistance Cement : This is consists of acid resistance aggregates such as quartz, quartzite’s, etc, additive such as sodium fluro silicate (Na2SiO6) and aqueous solution of sodium silicate. This is used for acid-resistant and heat resistant coating of installations of chemical Industry. By adding 0.5 percent of unseed oil or2 percent of ceresil, its resistance to water is increased and known as acid water resistant cement.

·     Blast Furnace Cement: For this cement slag as obtained from blast furnace in the manufacture of pig iron and it contains basic elements of cement, namely alumina, lime and silica. The properties of this cement are more or less the same as those of ordinary cement and prove to be economical as the slag, which is waste product, is used in its manufacture.

·     Colored Cement: Cement of desired color may be obtained by intimately mixing mineral pigments with ordinary cement. The amount of coloring may vary from 5 to 10 percent and strength of cement if it is exceeds 10 percent. Chromium oxide gives brown, red or yellow for different proportions. Colored cements are used for finishing of floors, external surfaces, artificial marble, windows.

·     Expanding Cement : This type of cement is produced by adding an expanding medium like sulpho – aluminate and a stabilizing agent to ordinary cement. Hence this cement expands where as other cement shrinks. Expanding cement is used for the construction of water retaining structures and also for repairing the damaged concrete surfaces.

·     High alumina Cement: This cement is produced by grinding clinkers formed by calcining bauxite and lime. The total content should not be less than 32 percent and the ratio by weight of alumina to lime should be between 0.85 and 1.30.

Functions of Ingredients in Cement

Functions of Ingredients in Cement

1.Lime : Lime is the important ingredient of cement and its proportion is to be maintained carefully. Lime in excess makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is in deficiency the strength of the cement is decreased and it causes cement to set quickly.

2. Silica : This also an important ingredient of cement and it gives or imparts quick setting property to imparts strength to cement.

3. Alumina : This ingredient imparts quick setting properly to cement. Express alumina weakens the cement.

4. Calcium Sulphate : This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.

5. Magnesia : The small amount of this ingredient imparts hardness and color to cement.

6. Sulphur : A very small amount of sulphur is useful in making sound cement. If it is in excess, it causes the cement to become unsound.

7. Alkalies : Most of the alkalies present in raw material are carried away by the flue gases during heating and only small quantity will be left. If they are in excess in cement, efflorescence is caused.

Grading of Aggregates

Grading of Aggregates

Grading of aggregates consists of proportioning the fine and coarse aggregates in such a ratio, so as to get strongest and densest mix with the least amount of cement. Grading the aggregates is so graded as to have minimum voids when mixed with all ingredients, and water should render a concrete mass of easy workability.

The grading of aggregates are done by the following methods

·     By trail – In this method, proportioning of aggregates as to give heaviest weight for same volume, yield the densest concrete

·     By finesse modules method (sieve analysis method): in this method, the samples of both coarse and fine aggregates are passed through a set of nine standard sieve and the percentage of sample retained on each of the said sieves is determined. The total of these percentages divided by 100 gives the finesses modulus of sample

·     By minimum voids method: This method is based on the fact, that so obtain dense concrete the quantity of cement should also be slightly in excess of voids more that the fine aggregates. In this method the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. The percentage of voids I aggregate, “X” given by the equation.

X = (V1 – V2) x 100

                                                                                       V2

Where v1, volume of water filled

Where v2, volume of aggregates.

·     By arbitrary standards: It is a commonly adopted method of propitiating the aggregates in a concrete mix for small works of moderate importance. This method is not recommended for large works or important works in this method, the volume of cement, sand and coarse aggregates are taken in the proportion of 1:n:2n respectively. The quantity of water to be used a varied suit the workability descried.

Ex:                           1:1:2 M250 rich mix for columns, beams

                                   1:1:3 – M200 Water retaining structures etc

                                  1:3:6 – M150 slab’s columns roads etc

                                  1:3:6 – M100 – foundations,

                                   1:4:8 - For mass concrete.

Qualities of a good building stone

The following are the qualities or requirements of a good building stone.

1. Crushing strength: For a good building stone, the crushing strength should be greater than l000kg per cm2.

2. Appearance: Good building stone should be a uniform color, and free from clay holes, spots of other color bands etc capable of preserving the color for longtime.

3. Durability: A good building stone should be durable. The factors like heat and cold alternative wet and dry, dissolved gases in rain, high wind velocity etc affect the durability.

4. Fracture: For good building stone its fracture should be sharp, even and clear.

5. Hardness: The hardness greater than 17, treated as hard used in road works. It is between 14 to 17, medium hardness, less 14 said be poor hardness.

6. Percentage wear: For a good building stone, the percentage wear should be equal to or less than 3 percent.

7. Resistance to fire: A good building stone be fire proof. Sandstone, Argillaceous stone resists fire quite well.

8. Specific gravity: For a good building stone the specific gravity should be greater than 8.7 or so.

9. Texture: A good building stone should have compact fine crystalline structure should be free from cavities, cracks or patches of stuff or loose material.

10. Water absorption: For a good building stone, the percentage absorption by weight after 24 hours should not exceed 0.60.

11. Seasoning: Stones should be well seasoned before putting into use. A period of about 6 to 12 months is considered to be sufficient for proper seasoning.

12. Toughness Index: Impact test, the value of toughness less than 13 – Not tough, between 13 and 19 – Moderate, greater than 19- high