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

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 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.

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

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.

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.

The Many Uses of Ready-Mix Concrete

The Many Uses of Ready-Mix Concrete

Concrete is a popular building material. You can see it in homebuilding sites, commercial building sites and government projects such as bridges and highways. It is easier to complete big projects like these by using ready-mix concrete. Completing projects on a tight deadline is possible and achieved in less than half the time it would when not using this product.

Ready-mix concrete comes from manufacturers and plants ready for delivery and mixing at work or project sites. With the use of a concrete mixer, one can get a precise mixture of concrete for the project. Using this type of cement material eliminates too much confusion at the work site. It also saves considerable amount of time because it requires less time to prepare than concrete prepared from scratch.

Cements, sands, aggregates (gravels) and water are the main contents of the mixture. It has the same ingredients as the ones that are not except that instead of carrying and mixing the raw materials at the site, the ready-mix arrives at the site on mixer trucks already pre-mixed and ready to use. Adding different additives and aggregates offsite and then delivering onsite for a different project based upon the specification of the customer is another feature of the ready-mix concrete. Different textures, finishes and colors are also available in ready-mix form.

Using ready-mix concrete eliminates having to carry and mixing the materials on site, which is a painstaking process. It eliminates errors that come with wrong measurement of water and the concrete materials. Using ready-mix saves time and effort. Big projects take less time to complete after pouring in the mixture using the transit mixer. The quality of the product is also much better than those that come in a non-ready mixed form.

Ready-mix concrete has a big potential in a lot of building projects. Aside from using it on big infrastructure projects such as building bridges, highways and huge buildings, we see this type of concrete as the choice of homeowners when building driveways, walkways made of concrete. Some homeowners choose concrete for their kitchen and bathroom countertops and as floorings. Stained concrete floors and counters provide a rustic look and patina adding more character to the interior of the home.

When using ready-mix concrete one must work with caution. One must ensure that there is enough space for the transit mixer. In addition, the location should be strong enough to handle the weight of the transit mixer and the concrete. The workers practice caution when working with the concrete mixer, by avoiding standing near the way of the mixer, especially when operating and pouring concrete. Accidents can and do happen which can endanger the well-being of the workers.

Characteristics of sand

Characteristics of sand

·     It should be chemically inert

·     It should be clean and coarse. It should be free from organic matter.

·     It should contain sharp, angular and durable grains.

·     It should not contain salts, which attract the moisture from atmosphere.

·  It should be well graded (i.e.) should contain particles of various sizes in suitable proportions.

Manufacture of bricks

Manufacture of bricks

To minimize breakages in cold weather, increase the cement content of the mix or the curing period before moving blocks.

Ordering and stockpiling materials

Aggregates and cement should be ordered in good time. Stocks should be sufficient to prevent stoppages due to lack of material.

As a rough guide, using an aggregate: cement ratio of 8:1 by loose volumes, three and a half bags of cement and a cubic meter of aggregate will be enough to make about 400 bricks. The number of blocks produced from the same quantity of material will depend on block size and whether they are solid or hollow.

Aggregates must be stockpiled in such a way that contamination is prevented and mixing of different types is prevented.

Cement must be stored in such a manner that it is kept dry. Cement in bags should be used within one month of being delivered.

Batching

Cement, if supplied in bags, should preferably be batched by the full bag. Cement supplied in bulk may be weighed (preferable) or batched by loose volume (not recommended).

It is important to batch all materials accurately. Batching containers, e.g. wheelbarrows, buckets, drums and wooden boxes, should be loosely filled to the brim and struck off flush with it. To avoid errors, there should be enough containers for a full batch to be made without using any container more than once. Dented or broken containers must not be used. The amount of water to be added to the mix is judged by eye and by doing some simple tests

Water content

Water content is critical. The mixture must be wet enough to bind together when compacted, but it should not be so wet that the blocks slump (sag) when the mould is removed. A common mistake is the use of mixes that are too dry, resulting in incomplete compaction. The moisture content should be as high as possible as this allows better compaction and thus gives the best strength.

Mixing

Hand mixing with the use of shovels should be done on a level concrete slab or steel plate.

First spread the sand out 50 to 100 mm thick. Then distribute the cement, and stone if any, evenly over the sand. Mix aggregate and cement until the color is uniform. Spread the mixture out,

sprinkle water over the surface and mix. Continue with this process until the right amount of water has been mixed in.

Molding

Hand operated machines should be used as instructed by the manufacturer.

The mould of a powered machine should be filled until approximately six to eight cycles of compaction are required to bring the compacting head to its stops. Too little or poor compaction should be avoided as it results in greatly reduced strengths.

Curing

Curing is the process of maintaining a satisfactory moisture content and a favorable temperature in the blocks to ensure hydration of the cement and development of optimum strength.

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.

Aggregates as Building & Construction Materials

Aggregates as Building & Construction Materials

Aggregates - Grading: Aggregates is derived from igneous, sedimentary and metamorphic rocks or is manufacture from clays, slag etc. The properties of concrete are directly related to those of its constituents and should be hard, strong, durable, and free from clay, loam, vegetables and other such foreign matters. The presence of clay or dirt coating prevents the adhesion of cement on the surface of aggregates and ultimately retards the setting and hardening of cement and reduces the strength, durability and soundness of concrete. Depending upon their size, the aggregates are classified as (i)Fine Aggregative (ii) coarse aggregates.

Fine Aggregates: The material, most of when passes through 4.75mm I.S. sieve size, is termed as fine aggregates. It should not contain more than 1 to 8% of fine particles, which may be obtained from sea, river, lake or pit may be used as fine aggregates but care should be taken all its impurities must be removed.

Coarse Aggregates: The material whose particles are of such size as are retained on 4.75mm, I.S sieve are called coarse aggregates. The size of the coarse aggregates used depends upon the nature of work. The maximum size may be 23mm for mass concrete such as dams etc. and 63mm for plain concrete. Crushed hard stone and gravel is the common materials used as coarse aggregates for structural concretes. Coarse aggregates usually obtained by crashing granite, gneiss, crystalline lime stone and good variety of sandstone etc.

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

Stone

Crushed Stone

All the building structures are composed of different types of materials. These materials are either called building materials or materials of construction. It is very essential for a builder, may be an architecture or engineer or contractor, to become conversant thoroughly with these building materials. The knowledge of different types of material, their properties and uses for different purposes provides an important tool in the hands of the builders in achieving economy in material cost. The material cost in a building ranges 30 to 50 percent cost of total cost construction. In addition to material economy, the correct use of material results in better structural strength, functional efficiency and esthetic appearance.

Classification of Rocks:

Building stones are obtained from rocks occurring in nature and classified in three ways.

1. Geological classification

2. Physical classification

3. Chemical classification

Geological Classification:

According to this classification, the rocks are of the following types.

Igneous rocks: Rocks that are formed by cooling of Magana (molten or pasty rocky material) are known as igneous rocks. Eg: Granite, Basalt and Dolerite etc.

Sedimentary rocks: these rocks are formed by the deposition of production of weathering on the pre-existing rocks. Examples: gravel, sandstone, limestone, gypsum, lignite etc.

Metamorphic rocks. These rocks are formed by the change in character of the pre-existing rocks. Igneous as well as sedimentary rocks are changed in character when they are subject to great heat and pressure. Known as metamorphism. Examples: Quartzite, Schist, Slate, Marble and Gneisses.

Equipment Required in Ready Mix Concrete

Equipment Required in Ready Mix Concrete

Following are the equipments required in Ready Mix Concrete

1.Batching plant

2.Transit mixer

BATCHING

Batching plants are classified as

1.Manual

2.Semiautomatic

3.Fully automatic

STORAGE

Storage of the raw materials is done by following methods: -

INLINE BINS Inert raw materials like fine & coarse aggregates are stored in bins called as

“Inline Bins” where the trucks carrying fine & coarse aggregate can dump the material easily.

The aggregates required are fed by the means of aggregate belt conveyer. On the aggregate belt conveyer the aggregates are weighed automatically by means of computer form the computer room presents on the plant.

SILOS

Cement & Flash are stored in airtight container called as “Silos”. The required quantity of cement & fly ash is extracted by the silos. There are two cement silos and one silo of fly ash.

Material Required for Ready Mix Concrete

Material Required for Ready Mix Concrete

Admixture: A substance added to the basic concrete mixture to alter one or more properties of the concrete; ie fibrous materials for reinforcing, water repellent treatments, and coloring compounds.

·     Air-entraining admixtures (mainly used in concrete exposed to freezing and thawing cycles)

·     Water-reducing admixtures, plasticizers (reduce the dosage of water while maintaining the workability)

·     Retarding admixtures (mainly used in hot weather to retard the reaction of hydration)

·     Accelerating admixtures (mainly used in cold weather to accelerate the reaction of hydration)

·     Super plasticizer or high range water-reducer (significantly reduce the dosage of water while maintaining the workability)

·     Miscellaneous admixtures such as corrosion inhibiting, shrinkage reducing, coloring, pumping etc.

Aggregate: Inert particles (i.e. gravel, sand, and stone) added to cement and water to form concrete.

Cement: Dry powder that reacts chemically with water to bind the particles of aggregate, forming concrete. Portland cement is typically used in concrete production.

Fly ash: Fly ash is a by-product from coal-fired electricity generating power plants. The coal used in these power plants is mainly composed of combustible elements such as carbon, hydrogen and oxygen (nitrogen and sulfur being minor elements), and non-combustible impurities (10 to 40%) usually present in the form of clay, shale, quartz, feldspar and limestone.

Scope of Ready Mix Concrete

Scope of Ready Mix Concrete

Long, Long years ago, their where simple houses but in 21st century we can see houses constructed in R.C.C. Therefore concrete got more importance than any other construction material. So the use of concrete is increasing day by day.

For construction most of the contractors and builders have to collect the raw materials required for the construction before starting actual works. These materials should be stored at the site properly. This technique can be possible when there will be more empty space at the construction site which is not possible in congested areas. At this time there is one solution to overcome all these problems that is nothing “READYMIX CONCRETE”.

By using R.M.C we can save the time and money required for the labors. In following places ready mix concrete can be used:-

·     Major concerting projects like dams, roads, bridges, tunnels, canals etc.

·     For concreting in congested areas where storage of materials is not possible.

·     Sites where intensity of traffic makes problems.

·     When supervisor and labor staff is less.

·     To reduce the time required for construction etc.

·     Huge industrial and residential projects.

Check by Consumer Before Ordering the Ready Mix Concrete

Check by Consumer Before Ordering the Ready Mix Concrete

The following need to be looked into by the consumer:

·     Reliability of the plant and transit mixers for consistent and continuous concrete supply as per requirement.

·     Calibrations of all measuring devices and their accuracy.

·     Mode of operation of plant should preferably be fully automatic and not manual.

·     Quality of materials proposed to be used.

·     Adequacy of quantity of materials proposed to be used.

·     Compliance of concrete specifications based on the mix parameters specified.

·     Adequacy of testing facilities

·     Time likely to be taken by transit mixers from plant to site and back.