Concrete
It is obtained by mixing cementing material,water ,and aggregate,and sometimes admixture in required proportions.
Concrete=cementing material+aggregates+ water,
Advantages of concrete
1)- concrete is economical in the long run as compared to other engineering materials
2)- concrete possesses high compressive strength ,and the corrosive and weathering effects are minimum. When properly prepared its strength is equal to that of a hard natural state.
3)-The formwork can be reused a number of times for similar jobs resulting in economy.
4)- concrete can be pumped and hence it can be laid in sufficient position .
5)- concrete is durable ,fire resistant and require very little maintenance.
Disadvantages of concrete
1)- concrete has low tensile strength and hence cracks easily.
2)- fresh concrete shrinks on drying and hardened concrete expands on wetting.
3)- concrete under sustained loading undergoes creep, resulting in the reduction of prestress in the prestressed concrete construction.
4)- concrete is liable to disintegrate by alkali and sulphate attack.
5)- concrete is not entirely impervious to moisture and contains soluble salts which may cause efflorescence.
Uses of concrete in comparison to other building materials
1)- concrete can be used for construction in very wide range of environments with the use of special admixture and special cements
2)- cement concrete can also be used in combination with other materials for structure subjected to different types of forces.
3)- concrete construction are used for covering very large spans
4)- cement concrete can also be advantageously used for pre cast construction to replace may conventional scare and costly material such as timber.
CONSTITUENTS OF CONCRETE
Cement - Cement is adhesive and cohesive material that acts as a bonding material .
Ordinary portland cement
Portland cement consist of the following constituents:
1)- lime. (CaO) ------60%-67%
2)- silica (SiO2)----------17%-25%
3)- Alumina(Al2O3) ------------3%-8%
4)- iron oxide(fe2O3) ------------0.5%-6%
5)- Magnesium oxide(MgO)---------0.1%- 4%
6)- soda and potash ---------0.2%-1%
7)- Sulphur trioxide( SO3) ---------1%-3%
8)- Calcium sulphate ( CaSO4)----3%-5%
Lime
Lime imparts strength and soundness to the cement. If it is present in excess quantity, it makes the cement unsound (causes it to expand and disintegrate).
It imparts strength to the cement due to the formation of dicalcium and tricalcium silicates.
If silica is present in excess quantity, the strength of cement increases but at the same time, its setting time is prolonged.
Alumina
It imparts quick setting property to the cement.
Excess excess alumina weakens the cement.
Iron oxide
Iron oxide imparts color to cement. At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite. Tricalcium alumino-ferrite imparts hardness and strength to cement.
Magnesium oxide
It imparts colour to cement.and helps in chemical reaction.
Calcium Sulphate
It increases the initial settings time of cement .
Bogue compunds
There are four bogue compounds that originate at the time of the hydration of cement.
Tri calcium silicate (3CaO · SiO2),
dicalcium silicate (2CaO · SiO2),
tricalcium aluminate (3CaO · Al2O3), and
tetra-calcium aluminoferrite (4CaO · Al2O3Fe2O3).
In an abbreviated notation differing from the normal atomic symbols, these compounds are designated as C3S, C2S ,C3A and C4AF
Types of cement
1)-Ordinary portland cement
It is obtained by pulverising clinker consting essentially of proportioned mixture of argillaceous materials until it approach fusion along with 2% to 5% of gypsum .
Properties
1)- Minimum initial setting time is 30 minutes .
2)- Maximum final setting time is 600 minutes.
3)- the residue of ordinary portland cement when sieved through IS no.9 should not exceed 10%
Uses
1) this is all purpose cement which may be used in all important structure except in the situation where soil or ground water contains sulphate .It possesses very low sulphate resistance.
2) Low heat cement
This cement is less reacts than ordinary portland cement and is obtained by increasing the properties of C2S and reducing C3S and C3A .the rate of development of strength is slow but the ultimate strength is same as that OPC.
Properties
1)- Less heat is evolved during setting low heat cement.
2)- When the tested by Le Chatelier method and autoclave test the expansion should not be more than 10 mm and 0.8% respectively.
3)- The minimum initial setting time should not be less than 60 minutes. and final setting time should not be more than 600 minutes.
Uses
Low heat cement is used for large mass concrete works such as dam ,large raft foundation etc.
(3) Rapid hardening cement
This type of cement has High content of tri calcium silicate C3S and finer grinding due to which it attains greater strength at early stage .The strength developed in rapid hardening cement after 3 days is approximately the same as developed in ordinary portland cement after 7 days
Fineness. The residue of this cement does not exceed 5% when sieved through IS sieve No.9
Soundness Expansion when tested by Lechatlier apparatus should not be more than 10 mm.
Setting times. Initial setting time should not be less than 30 minutes and final setting time more than 600 minutes.
Uses it is used where high early strength is required.
1)- Rapid hardening cement is used for repairs of roads and bridges.
2)- When load is applied for a short period of time then rapid hardening cement is more suitable.
3)- This cement is used where a rapid strength development is required.
(4) High alumina cement
High Alumina cement is manufactured by grinding clinkers of calcining bauxite and ordinary lime .in which the total amount of alumina content should not be less than 32 % and should maintain the ratio by weight of alumina to the lime between 0.85 to 0.30.
Uses
(1)in construction concrete ,where rapid strength development is required, even at low temperatures
(2)- it is used as ultra rapid strength development and controlled expansion are required.
(5)-Quick setting cement
This type of cement is manufactured by reducing the gypsum and adding small amount of aluminium sulphate to accelerate setting time of cement .the initial setting time starts in 5 minutes and final setting take place within 30 minutes.
Uses Quick setting cement is used where works is to be completed very short period and for concreting in static or running water .
(6)-Sulphate Resistance cement
Sulphate resisting cement is used to reduce the risk of sulphate attack on concrete and thus is used in construction of foundations where soil has high sulphate content .This cement has reduced content of C3A and C4AF.
Uses it is used in construction exposed to severe sulphate action by water and soil in places like canals linings ,culverts, retaining walls, siphons etc.
(7)-Portland pozzolana cement
Portland pozolana cement is prepared by grinding pozzolanic clinker with portland cement .it is also produced by adding pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending portland cement and fine pozzolana.it is cheaper than the ordinary portland cement
Uses it is used in marine structures, sewage works , sewage work and for laying concrete under water such as bridges ,piers,dams and mass concrete works.
Tests of cement
(1)- consistency of cement the standard consistency of cement paste is defined as the consistency which will allow a vicat plunger having 10 mm diameter and 50 mm length to penetrate to 33-35 mm deep from the top surface of the mould .This test is performing to find out the water required to make a cement paste of standard consistency.To find out the initial & final setting time and soundness of cement ,the consistency of cement should be known.
Mould size 40 mm deep and 80 mm in diameter.
Apparatus use
(1)- Vicat's apparatus
(2)-Weighing Balance
(3)- Gauging Trowel
(4)- Stop Watch
Procedure
(1)-The standard consistency of a cement paste is the consistency which will permit the vicat plunger to penetrate to a point 5 to 7 mm from the bottom of the vicat mould.
(2)- Initially a cement sample of about 300 gm is taken in a tray and is mixed with with a known percentage of water by weight of cement ,say starting from 26% and then it is increased by every 2% until the normal consistency is achieved.
(3)- Prepare a paste of 300 gm of cement with a weighed quantity of portable or distilled water .
(4)- Fill the vicat mould with this paste ,the mould resisting upon a non- porous plate . After completely filling the mould smothen the top surface of the paste , making it level with top of the mould.
(5)- Place the test block in the mould , together with the non- porous resisting plate ,under the rod bearing the plunger ; lower the plunger gently to touch the surface of the test block and quickly release,allowing it to sink into the paste .this operation shall be carried out immediately after filling the mould .
(6)- Prepare trial paste with varying percentage of water and test as described above until the amount of water necessary for making up the standard consistency as defined in step 1 is found.
2 Intial setting time
In this time cement can be molded in any desired shape without losing its strength. This is the time at which the hardening of the cement starts and the only time available for mixing the paste and placing it in position. if this time less then 30 min then can't molded in any shape in many situations
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