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

Preparation of concrete mix

Preparation of concrete mix

There are two types of concrete mixing

 (i)Hand mixing

(ii)Machine mixing

Hand Mixing: This method of mixing concrete is resorted to when the quantity if concrete to be used in a work is insufficient to warrant the necessity of machine. This is used with advantage in places where machinery cannot be used on account of their non-availability or in works near a hospital where the noise of machine is not desirable. Hand mixing is done on a clean, hard and impermeable surface. Cement and sand are first mixed dry with the help of shovels until the mixture attains uniform color. Aggregative are then added to this mixture and the whole mixture is then turned by shovels until the stone pieces uniformly spread throughout. After this, desired are quantity of water is poured into the heap from a can fitted with a rose. The mass is then turned until a workable mixture is obtained. It is advised to add 10% extra cement to guard against the possibility of inadequate mixing by this method.

Machine Mixing: The machine used for mixing concrete is termed as concrete mixer. Two types of concrete mixers are in common are

1. Continuous mixers

2.Batch mixers

Continuous mixers are employed in massive construction where large and continuous flow of concrete is desired. The process of feeding the mixing is more or less automatic. The machine requires careful supervision so as to obtain the concrete mix of desired consistency.

In batch type of concrete mixer. The desired proportion of materials are fed into the hopper of a drum in which the materials get mixed by the series of blades or baffles inside the mixer. Batch mixers are further two types 1. Tilting drum type 2. closed drump type. In the first type, components are fed in the revolving drum in a tilted position and after sometime the concrete mix is discharged by tilting the drums in the opposite direction. In the latter type the drum remains rotating in one direction and emptied by means of hopper which tilts to receive the discharge.

While using the mixer, coarse aggregates should be fed first, sand and cement should be put afterwards. In this revolving state, the components get mixed while water is poured with the help of can. The concrete should be for atleast 2 minutes, the time being measured after all the ingredients including water have been fed into the drum.

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.

Constituents - Requirements

Constituents - Requirements

The main constituents of concrete are:

·     Cement / Lime: Before introduction of ordinary Portland cement, lime was used as cementing material. At present most of the cement concrete works in the building construction is done with ordinary Portland cement. But other special varieties of cement such as rapid hardening cement, high alumina cement are used under certain circumstances. The cement should comply with all standard specifications.

·     Fine Aggregates: The material, which is passed through 4.7625mm B.S. test sieve, is termed as fine aggregates. Usually natural river sand is used as fine aggregates. But places where natural sand is not available economically, finely crushed stone may be used as fine aggregates.

·     Coarse Aggregates: The material retained on 4.7625mm size B.S. test sieve is termed as coarse aggregates. Broken stone is generally used as coarse aggregates. For thin slabs, and walls, the maximum size of coarse aggregates should be limited to one third the thickness of the concrete section.

·     Water : Water to be used in the concrete work should have the following properties.

1.                        It should be free from oils

2.                        It should be free from acids or alkalies

3.                        It should be free from Iron, Vegetables matter or other substance, which is likely to have adverse effect on concrete.

4.                        It should be fit for drinking purpose

Concrete

concrete

Cement concrete is a mixture of cement, sand, pebbles or crushed rock and water. When placed in the skeleton of forms and allowed to cure, becomes hard like a stone. Cement concrete is important building material because of the following reasons.

·     It can be moulded into any size and shape of durable structural member.

·     It is possible to control the properties of cement concrete.

·     It is possible to mechanize completely its preparation and placing processes.

·     It possesses adequate plasticity for mechanical working.

The cement concrete has the following properties

·     It has high compressive strength

·     It is free from corrosion

·     It hardens with age and continues for a long time after concrete has attained sufficient strength

·     It is proved to be economical than steel

·     It binds rapidly with steel and it is weak in tension, steel reinforcement is placed in cement concrete at suitable places to take up tensile concrete or simply R.C.C.

·     It forms a hard surface, capable of resisting abrasion stresses. This is called reinforced cement.

·     It has tendency to be porous to avoid this proper grading & consolidation of the aggregates, minimum water-cement ratio should be adopted.

Bulking of Sand

Bulking of Sand

The presence of moisture in sand increases the volume of sand. This is due to fact that moisture causes film of water around the sand particles which result in the increase of volume of sand. For a moisture content of 5 to 8 percent, the increase in volume may be about 5 to 8 percent, depending upon the grading of sand. The finer the material, the more will be the increase in volume for a given moisture content. This phenomenon is known as bulking of sand.

When moisture content is increased by adding more water, sand particles pack near each other and the amount of bulking of sand is decreased. Thus the dry sand and the sand completely flooded with water have practically the same volume.

For finding the bulking of sand, a test is carried out with following procedure.

·     A container is taken and it is filled two third with the sample of sand to be tested.

·     The height is measured, say 20cm.

·     Sand is taken out of container.

·     The container is filled with water.

·     Sand is then slowly dropped in the container and it is thoroughly stirred by means of a rod.

·     The height of sand is measured say 16cm, then bulking of sand =

                         20-16                4

              =   ----------------  =  --------     or 25%

                       16                       16

Grading of Sand

Grading of Sand

According to the site of grains, sand is classified as fine, coarse and gravelly

·     Sand passing through a screen with clear opening of 1.5875mm is known as fine sand. It is generally used for masonry works.

·     Sand passing through a screen with clear openings of 7.62mm is known as gravely sand. It is generally used for plastering.

·     Sand passing through a screen with clear opening of 3.175mm is known as coarse sand. It is generally used for masonry work.