Properties of Concrete

Properties of Concrete

What Are The Properties of Concrete?

Most people commonly use the words cement and concrete interchangeably. It's important to note that cement is only one component of three concrete properties and the term "concrete" is a mixture of the three.

Concrete properties consist of three basic components: water, aggregate (rocks and sand), and portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates.

This combination or mixture, when poured, hardens into this durable material called concrete that we are all familiar with.

The Three Basic Components of Concrete

1. Portland Cement - The cement and water form a paste that coats the aggregates and sand in the mix. The paste hardens and binds the aggregates and sand together.

2. Water - Water is needed to form a chemical reaction with the cement (hydration) to form heat. This starts the drying and hardening process. Water is also used to provide workability with the concrete.

The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio the stronger the concrete.

3. Aggregates - Sand is the fine aggregate and crushed stone and gravel are the coarse aggregate. These two materials help strengthen the concrete.

Some Desired Properties of Good Concrete

1. A workable mix that is easy to place and consolidate.

2. A high quality concrete using the lowest water-cement ratio possible, using the proper size coarse aggregate practical for the job you're doing, using the optimum ratio of fine to coarse aggregate.

3. For hardened concrete resistance to freezing and thawing, deicing chemicals, wear resistance, strength, and low permeability (water tightness).

These are the basics of concrete properties to produce a concrete mix. There are other admixtures that are added to improve workability, durability, and set times.

Curing of Concrete

Curing of Concrete

Curing may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete. Curing in the early ages of concrete is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more. If curing is not done properly, the strength of concrete reduces. Cracks develop due shrinkage. The durability of concrete structure reduces.

The following curing methods are employed:

(a)  Spraying of water

(b)  Covering the surface with wet gunny bags, straw etc.

(c)  Ponding

(d)  Steam curing and

(e)  Application of curing compounds.

(a) Spraying of water: Walls, columns, plastered surfaces are cured by sprinkling water.

(b) Wet covering the surface: Columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw.

(c) Ponding: The horizontal surfaces like slab and floors are cured by stagnating the water to aheight of 25 to 50 mm by providing temporary small hunds with mortar.

(d) Steam curing: In the manufacture of pre-fabricated concrete units steam is passed over the units kept in closed chambers. It accelerates curing process, resulting into the reduction of curing period.

(e) Application of curing compounds: Compounds like calcium chloride may be applied on the curing surface. The compound shows affinity to the moisture and retains it on the surface. It keeps the concrete surface wet for a long time.

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.

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.

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.

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.

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.

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.

Characteristics of stones

Characteristics of stones

In order to ensure suitable selection of stone of particular work, one must be conversant with its composition, characteristics, uses and place of availability.

Granite

1. Igneous rock

2. Composed of quart, felspar and mica and minerals

3. Available in grey, green, brown and pink and red

4. Hard and durable

5. High resistance to weathering

6. The texture varies with its quality

7. Specify gravity 2.7 and compressive strength 700 to 1300kg/cm2

8. Used for ornamental, road metal, railway ballast, aggregate for concrete; for construction of bridges, piers and marine works etc.

Balast

1. Igneous rock

2. It is compact, hard and heavy

3.Available in red, yellow grey, blue and greenish black color

4. Specific gravity is 3 and compressive strength varies 1530 to 1890 kg/cm2.

5. Used for ornamental, rail road ballast, aggregates for concrete etc.

Sand Stone

1.Sedimentary rock

2.It is available in variety of formations fine grained, coarse grained compact or porous

3.Available in white, green, blue, black, red and yellow.

4.Specific gravity 2.65 to 2.95

5.Compressive strength is 650kgs / cm

6.Used for ashlars works

Lime Stone

1. Sedimentary rock: It is available in a variety of forms which differ from one another in color Compaction, texture, hardness and durable

a. Compact lime stone

b. Granular lime stone

c. Magnesia lime stone

d. Kanker lime stone

f. Used for paving, road metal, etc

Artificial stones: These are also known as cast stones or reconstructed stones. Artificial stones may take up various forms such as

a. Cement concrete: This is the mixture of cement, fine aggregates, coarse aggregates and water. It may be cast in site or pre-cast if steel is used with cement concrete, it is known as reinforced cement concrete.

b. Mosaic tiles: Pre-Cast concrete tiles with marble chips at top surface are known as tiles. They are available in different shades and widely adopted at present.

c. Terrazo :This is a mixture of marble chips and cement. It is used for bathrooms residential buildings, temples 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.

Shrink Mixed Concrete

Shrink Mixed Concrete

Concrete that is partially mixed in a plant mixer and then discharged into the drum of the truck mixer for completion of the mixing is called shrink mixed concrete. Central mixing plants that include a stationary, plant-mounted mixer are often actually used to shrink mix, or partially mix the concrete. The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete.

Transit Mixed Concrete

Transit Mixed Concrete

While ready mixed concrete can be delivered to the point of placement in a variety of ways, the overwhelming majority of it is brought to the construction site in truck-mounted, rotating drum mixers. Truck mixers have a revolving drum with the axis inclined to the horizontal. Inside the shell of the mixer drum are a pair of blades or fins that wrap in a helical (spiral) configuration from the head to the opening of the drum. This configuration enables the concrete to mix when the drum spins in one direction and causes it to discharge when the direction is reversed.

To load, or charge, raw materials from a transit mix plant or centrally mixed concrete into the truck, the drum must be turned very fast in the charging direction. After the concrete is loaded and mixed, it is normally hauled to the job site with the drum turning at a speed of less than 2 rpm.

The traditional truck-mixer has discharged concrete at the rear of the truck. Front discharge units, however, are rapidly becoming more popular with contractors. The driver of the front discharge truck can drive directly onto the site and can mechanically control the positioning of the discharge chute without the help of contractor personnel.

Currently, because of weight laws, the typical truck mixer is a 7 to 8.5 m3. The drums are designed with a rated maximum capacity of 63%  of the gross drum volume as a mixer and 80% of the drum volume as an agitator. Generally, ready mixed concrete producers, load their trucks with a quantity at or near the rated mixer capacity. Fresh concrete is a perishable product that may undergo slump loss depending on temperature, time to the delivery point on the job site, and other factors.

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.

Checks Needed at Site During Concreting

Checks Needed at Site During Concreting

Proper co-ordination between the Ready Mix Concrete supply and placing and compacting gangs.

·     Proper signaling or communication at site is necessary.

·     Workability of concrete within accepted limits.

·     Adequacy of cohesiveness of concrete for pump ability.

·    Ensure that water addition or chemical admixtures are not added during transportation by RMC unauthorized persons and without the knowledge of the site in charge of the consumer.

·     Temperature of concrete at the time of receipt at site (if specified).

·     Continuous and steady supply at site and speedy unloading of the Monitor speed and progress of placing to avoid formation of cold joints transit mixers.

·     Monitor proper  placement without segregation.

·     Monitor placement of concrete at the closest possible point to its final location.

·     Arrange for curing as soon as finishing is completed. This is specially required in case of slabs, pathways and roads in hot/warm weather.

·     Retempering should be prohibited as experiments shows the addition of water to RMC truck at the construction site may result in substantial reduction in strength. The reduction in strength was found to be proportional to the increase in slump. Large increase in slump means higher reduction in strength. When the amount of water added is not controlled, reduction of strength may be as high as 35%. In cases where controlled amount of water is added to restore the slump within the specification’s limits (100 ±25 mm), the reduction of strength may be below 10%.

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.

Use of Fly Ash Based in Concrete

Use of Fly Ash Based in Concrete

To have a better performance characteristics in terms of durability of concrete, fly ash is been used successfully in concrete. The improvement in gel structure caused by pozzolanic action of fly ash leads to a very impervious concrete. These factors improve the resistance of concrete against external aggression. Since Fly ash is an environmental hazard, therefore by effectively using it in concrete it can be consumed constructively and thus contribute to the ecological balance. Many prestigious and critical structures have been built using either PPC or  by blending fly ash directly in concrete. The names of the famous Petronus towers & Eurotunnel can be definitely quoted in this regard. However , the  construction industry has offered some resistance in using PPC or fly ash in structural concrete. With advent of RMC the doubts regarding controls in using this material are slowly dispelled and engineers have become more open to the idea of using fly ash as pozzolan for partial replacement of cement in concrete.

“Limestone is the raw material for the manufacture cement & limestone is limited resource.” It cannot last long it ever. We need to realize the importance of this fact. We need to use our cement rationally. Whereas we talk about huge figure describing development and building needs of our country, concrete still remains the most widely used and environmental friendly construction material to achieve this.