Concrete

Concrete definition

Concrete is construction material. It is a mixture of cement, sand, fine aggregate and coarse aggregate and water.

Portland cement is commonly is used as a type of portland cement. Concrete is the major building material in the construction industry. It can be easily molded into a durable structure member.

In building construction material, concrete is used in beams, columns, slab, foundation, and another place is used concrete.

In concrete, sand is used as a fine aggregate, gravels and crushed stones are used as coarse aggregate. When the cement acts as a building material which forms a paste with water and on hardening their cement paste holds coarse and fine aggregates together to form a solid mass.

Components Of concrete

Cement, aggregate, water are the ingredients of concrete Aggregate may be fine aggregates sand or coarse aggregates like gravel and crushed stone.

Cement

It uses as the binding material and acts as a binder for coarse and fine aggregates. Generally, ordinary portland cement uses to form concrete. Initial setting time of good cement should no be less than 30 minutes and the final setting time should not exceed 10 hours.

Aggregate

The granular material generally inert chemicals such as natural sand, gravels, crushed stones, and air-cooled iron blast furnace slag, are call aggregate. If only cement uses for the construction, the avoid these cracks, aggregates uses. These aggregates bounds together by means of a cement. The aggregates are classified into two categories such as fine and coarse aggregates. The material which passes from sieve number 480 calls fine aggregate and which retains on sieve number 480 calls coarse aggregates. These should clean, sharp, angular, well-graded, hard, durable and free from foreign matters.

 

Density of sand

The Density of sand material returns sand density based on sand conditions (wet/ dry in bulk / packaged).

The density of the sand is vary if the sand is compacted (bulged) or loose and if it is wet or dry. When packed, the grains of sand are forced to form a narrower formation, and more matter is in the volume

Density of sand (fine aggregate) is ranging between 1450 – 2082 kg/m3 depending on different condition like wet, dry, loose, dry packed and wet packed.

Locally available river sand have density 1710 kg/m3 is used and specific gravity is 2.65 and finance of modulus of river sand is 5.24.

m /crush sand used as partial replacement of fine sand in construction line and the density of M /crush sand is 1750 kg/m3, specific gravity and fineness of modulus is found to be 2.73 and 4.66 respectively.

The average density of the different sand conditions is as follows:

● Loose sand: density of loose sand in kg/m3 is 1442 . It is dry sand that has been moved or agitated to loosen the natural packaging process.

● Dry sand: density of dry sand in kg/m3 is 1602. It is sand in its undisturbed natural form, where it has been partially compacted by rain and gravity over time, but is now dry

● Packed sand: density of packed sand in kg/m3 is 1682. Sand that has been packed manually or mechanically (compacted)

● Wet sand: density of wet sand in Kg/m3 is 1922. This is the sand that has been in a natural and naturally compressed environment that is now wet.

● Wet packed sand: 2082 kg / m3. This is compacted sand that is also almost saturated with water.

Density of coarse aggregate

● density of coarse aggregate: there are two types of aggregate used in construction fine aggregate and coarse aggregate. the size of fine aggregate is 10mm & 12mm and the size of coarse aggregate is 20 mm and 40mm used in construction

as we know density of fine aggregate is more compare to density of coarse aggregate because fine aggregate have less presence of air voids than coarse aggregate. So density of aggregate 10mm is higher of other aggregate size.

Density of coarse aggregate 20mm and 40mm ranging between 1200 – 1450 kg/m3 and density of fine aggregate 10mm and 12mm ranging between 1600 – 1750 kg/m3

Chemical admixtures

These are the ingredients in concrete other than materials, also calls mineral additives, contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. When a typical example nature pozzolans, fly-ash, ground granulates blast-furnace slag and silica fume.

When the concrete is placed, these components must be cured at satisfactory moisture content and temperature must be carefully maintained for a sufficiently long time adequate development of strength of the concrete.

Water

The water uses in marking concrete should conform to the drinking water standard. It is the least expensive but most important ingredient of concrete. The water, which uses for making concrete should be free from acids, Alkalies, salts, oil, grease, and also decayed vegetable matters. The amount of water should be of that, which will produce concrete that requires quality.

Classification of concrete

They can be classified into two different categories depending on their density and strength-

·         The classification depends upon the density of concrete

·         The classification depends upon the strength of concrete

Density of concrete

Depend upon density, concrete is classified as normal weight, lightweight and heavyweight concretes. Normal weight concrete produces by using natural sand and crushed stone. Lightweight concretes produces by using weight aggregates such as pumice, or processed and bloated aggregate. This concretes uses for application in which a load of gravity is to reduce, e.g. for reducing the load on founding. Heavyweight concretes are produces by using high-density aggregates. They also use for increasing the weight of a structure for stability purposes.

Strength of concrete

Depended upon the strength concretes is classified as ordinary, standard, and high-strength concretes. Which is having maximum strength less than 20 MPa is called standard concretes and these having strength between 40-80MPa is called high-strength concretes.

When the proportions of a constituent of materials give in the for a different type of concretes-like ordinary, standard and high-strength concretes.

Properties

·         It has high compressive strength.

·         Free from corrosion and there is no appreciable effect of atmospheric agents on it.

·         It hardens with age and the process of hardening continues for a long time after the has attained suffered sufficient strength. When it is the property of cement which gives it a distinct place among the building materials.

·         They find rapidly with steel and as it is weak in tension, the steel reinforcement is placed in cement at suitable places to take up the also tensile stresses. This is termed as the RCC.

·         It is proved to be more economical than steel. This is due to the fact that sand and aggregates forming the bulk of cement, to the extent of about 80-90%, are usually available at a moderate cost. The formwork, which is of steel or timber, can be used over and again or for another purpose after it is removed.

·         It has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing. There should be proper grading ad consolidating of the aggregates. The minimum water-cement ratio should be adopted.

·         They form a hard surface, capable of resisting abrasion.

·         It should be remembered that apart from other materials, Comes to the site in the form of raw materials only. Its final strength and quality depend entirely on the local conditions on also handling it. However, the materials of which concretes compose may be subject to rigid specifications.

Advantages

·         It is much economical than steel.

·         They have much compressive strength.

·         It may also use as a soundproofing form.

·         They can be used for the purpose of the lining of the tunnel.

·         It acts as a waterproofing material.

·         It can be easily handled and molded into any desired shape.

·         The ingredients are meet locally available.

·         Its surface is capable of resisting abrasion.

Disadvantages

·         It requires to provide expansion joints in case of long structures.

·         It is required to provide contraction joints to avoid drying shrinkage.

·         These structures are heavy in weight and hence subject to heavy creep. When It requires more time for curing to develop strength.

·         These have low tensile strength.

 

CONCRETE CREEP

Creep is defined as the long term deformation under a sustained load. Water within the hardened cement paste is forced to more as a result of the applied load. This movement of moisture is the primary cause of creep deformation. Some movement also occurs due to the propagation of microcracks.

The movement due to creep can be greater than the elastic strain on loading. Creep can continue over a long period of time (more than 30 years in some cases) after the application of the load.

Many of the factors that affect shrinkage and modulus of elasticity also affect the creep is a similar way.

Following are the cases, which affect creep in concrete:

• When the aggregate volume is increased, creep will be less as the aggregate is more rigid than the cement paste.
• When the water/cement ratio is increased, the higher water/cement ratio will result in a more porous and weaker cement paste which will deform more under a given load.
• When the natural aggregate is replaced by a lightweight artificial aggregate, the lightweight aggregate will be more porous and less rigid than the natural aggregate.
• If the applied load is increased, the creep also increases.