CONCRETE IS…
No more three component system. Is a multi-component and multi-variant system. Concrete includes- cement, coarse and fine aggregates, water , mineral and chemical admixtures, additives, performance improvers etc. As some materials are natural and others are processed, site conditions –variation in quality is expected. Consistent quality with a minimum variation in the material is the target and to be maintained.
TYPES OF CEMENTS:
Ordinary Portland Cement(33 Grade) –IS 269
Ordinary Portland Cement(43 Grade) –IS 8112
Ordinary Portland Cement(53 Grade) –IS 12269
Rapid Hardening Cement- IS 8041
Sulphate Resisting Cement-IS 12230
Portland Pozzolana Cement (part 1)-IS 1489 (fly-ash based)
Portland Pozzolana Cement (part 11)-IS 1489 (calcined based)
Portland Slag Cement-IS 455
Oil Well Cement- IS 8299
High Alumina Cement-IS 6452 etc
Type of cement to be used for intended use.
APPLICATIONS OF OPC:
OPC 33 grade can be used in all general construction works where high strength is not required.
OPC 43 shall be used in pre-cast products, multistoried buildings, water tanks, canals etc.
OPC 53 shall be used in structures where high strength is required –elements of high rise buildings, railway sleepers etc .
SULPHATE RESISTING CEMENT:
If sulphates are more in soils/ water OPC is susceptible to attack. In such cases, a special type of cement SRC to be used. SRC has sulphate resisting property. SRC is recommended to use in – marine structures, foundations in aggressive soils, concrete pipes in marshy soils etc
IS:12230
PORTLAND POZZOLANA CEMENT (PPC):
Portland Pozzolana Cement (Fly ash based) IS 1489 Part I
Portland Pozzolana Cement ( Calcined clay based) IS 1489 Part II
PPC is manufactured by the intergrinding of OPC clinker and pozzolanic material. The common pozzolanic material are- fly-ash, clay powder etc.
WHAT IS POZZOLANA ?
A pozzolanic material is essentially a siliceous/ siliceous aluminous material which does not possess cementations property, but, when finely ground react with calcium hydroxide in the presence of water at ordinary temp. and forms compounds possessing cementations properties.
PORTLAND SLAG CEMENT (PSC):
PSC is obtained by mixing OPC clinker+ gypsum+ finely ground granulated blast furnace slag in suitable proportions or intergrinding OPC+ slag + gypsum
Granulated blast furnace slag is by-product from steel industry
IS 455: 1989 Specification of Portland Slag Cement
PSC and PPC are called as ‘Blended cements’ because they are manufactured by blending with the OPC and gypsum.
STRENGTH GAIN IN BLENDED CEMENTS:
OPC : CS+OH CSH +Ca(OH)2
PPC& PSC: CS+OH CSH+ Ca(OH)2
Ca(OH)2+ S CSH
more dense
Advantages of Blended Cements:
Low heat of hydration
Less permeability
Greater resistance to aggressive environments
Higher degree of durability & performance
Other advantages- Risk reduced due to ASR, continuous strength gain, easy workability etc.
APPLICATIONS OF BLENDED CEMENTS:
1. Can be used in all constructions where OPC 33,OPC43 are being used.
2. Most suitable in marine and aggressive environment constructions.
3. Mass concrete works and in hydraulic structures
QUALITY REQUIREMENT S OF CEMENT ..
1.Check for BIS mark and type of cement on cement bags
2.Check week no. and year of manufacturing
3.Check whether the company’s original packing is intact
4.Check the presence of ‘lumps’
5.Check randomly the weight of cement bag
6.Store cement in water tight sheds and on wooden planks or covered by water proof materials
7. Use cement as soon as possible and in order of arrival
8. Test certificate to be collected from the manufacturer
9. Send samples to recognized laboratories for testing for physical, chemical and mineralogical properties
10. Frequency and no. of samples to be sent for testing depends on the project.
AGGREGATES:
Aggregates play two important roles- to provide concrete with a rigid skeletal structure and reduce void space
Coarse aggregates: > 4.75 mm , Natural or artificial or recycled
Properties of coarse aggregates depend on nature of rock & texture,
Fine aggregates: -4.75 mm
Natural – river/ marine sand, Manufactured
Both shall comply with IS :383
Sources to be identified and aggregates to be evaluated for their suitability
PROPERTIES OF COARSE AGGREGATES:
Shall comply mechanical & mineralogical properties : Crushing value, impact value, abrasion value , soundness etc
Grading is a must & is described in terms of cumulative % by mass of aggregate passing particular IS sieves
Light weight aggregates:
That produce concrete having unit weight around 1100 kg/m3
Pumice, Scoria, Vermiculate etc material can be used as LWA
Can also be manufactured by sintering fly ash/ GGBS nodules
Successfully used in high rise structures, light weight panels etc.
QUALITY CHECKING OF COARSE AGGREGATES..:
Coarse aggregates must be hard,clean,durable,free from dust / organic matter
Use angular/ rounded aggregates. Particle shape influences workability
Elongated and flaky aggregates should be avoided as they reduce workability. They increase void & water demand
Grading to be carried out. Should neither be oversize ( not more than 5%) nor undersize (not more than 10%)
Higher size aggregates –40/ 80/100 mm can be used in road / foundations etc. preferable 20 mm for normal building construction
Water absorption should be less than 5%
Aggregates to be stored properly
FINE AGGREGATES:
Natural – River sand, marine sand, Fly ash
Artificial- manufactured sand
Fine aggregates shall comply with IS: 383-1970
Grading, silt content, moisture content, Bulk age, deleterious constituents, organic impurities are the controlling parameters
Zone II sand is preferable
QUALITY CHECKING OF FINE AGGREGATES..
Fine aggregates shall be free from clays, dirt, organic impurities, salts and dry
Shall comply with IS 383 and zone grade II are preferred.
Zone I sand is coarsest while Zone IV is finest. If zone II is not available correction in the quantity of sand can be made
Sand having fineness modules >3.2 is unsuitable for making concrete
TESTING OF WATER:
INCASE OF DOUBT REGARDING DEVELOPMENT OF STRENGTH, THE SUITABILITY OF WATER SHALL BE ASCERTAINED BY TESTS FOR COMPRESSIVE STRENGTH OF CONCRETE AND INITIAL SETING TIME OF CEMENT
28 DAYS STRENGTH ≥90 PERCENT OF CONTROLED MIX
INITIAL SETTING TIME NOT LESS THAN 30 MINUTES AND SHALL NOT DIFFER BY MORE THAN + 30 MINUTES FROM CONTROL
ADMIXTURES:
To improve specific property of concrete in fresh and/ or hardened states like
workability, self compaction, reduction in water demand etc
high strength, low permeability,early strength, corrosion protection etc.
ADMIXTURES TYPES:
Mineral admixtures
Fly-ash, Silica fume, Metakaolin, Rice husk ash etc
Chemical admixtures
many types depending upon what property to be modified
FLY-ASH:
Fly-ash is a by-product of thermal plant
Fly-ash can be used in cement , as a additive in concrete, special concretes and other building materials
It is collected from ESP's from thermal plants
Shall comply with IS :3812 (Pts 1&2)
Low lime fly-ash : Possess pozzolanic property, CaO content is <10% and produced from anthracite and bituminous coals (equal to **** –618 F class)
High lime fly-ash : Possess both pozzolanic and cementitious properties,CaO content >10% and produced from lignite and sub-bituminous coals (**** –618 C class
SILICA FUME:
By-product from Ferro-silicon industry
Essentially composed of amorphous silica with a very high surface area (20000 m2/kg)
Particles with an average dia.0.10 µm and a bulk density of 150-250 kg/m3
Acts both as pozzolona and filler
Requires high range water reducing/ super plasticizer
**** C-1240
RICE HUSK ASH:
Obtained by burning rice husks under controlled conditions
Can be used as additive or pozzolona
NCB has developed technology for manufacture of RHAM (Rice Husk Ash Masonry Cement)
Compressive strength 175-180 kg/cm2
CHEMICAL ADMIXTURES:
1.Accelerators
2.Retarders
3.Water reducers
4.High range water reducing
5.High range water reducing and retarding
QUALITY CHECKING OF ADMIXTURES:
Shall comply with IS: 9103
Compatibility test with cement
Optimum dosage
Sp.gravity for liquid admixtures, dry material content in liquid/solid admixtures,
Chloride content to be checked for every batch
For high workability flow in mm to be checked
Generally dosage of naphthalene-based super plasticizers is 0.5 – 1.5% by wt. Of cement
IS: 456 limits retarders (0.5%), plasticizers(1.0%) & super plasticizers(2.0%), % by wt. Of cementitious materials.
QUALITY CHECKING OF FINE AGGREGATES..
If correction of bulk age of sand is not done, concrete will be under sanded. It will be harsh and of low workability and more porous. The yield of concrete per bag of cement will also be less.
If silt content is more than 7% by volume either the sand to be rejected or it should be washed . Excess silt effects the strength of concrete
FM shall range from 2.6 to 3.6% for concrete, up to 1.6% for plastering and up to 3% for masonry
NEED OF MIX PROPORTIONING:
Because of large no. of ingredients being used in concrete in different proportions , it is very very essential to go for batch mixing
Each material used should be of minimum variation in quality and quantity
FACTS OF RMC:
All over world 40% of total cement produced is used in RMC while in India it has reached 8% now.
RMC started in India in 1980 and presently has 450 plants compare to 3700 in USA
60% of concrete in Japan is RMC .
Highlights of Batching Plant:
Capacity ranges from 30 to 240 Cubic Meters
The latest generation plants can store as many as 10,000 different concrete mix designs.
Batching plants have either pan type or twin shaft type mixers with manual control mode ,Semi-automatic or fully automatic control .
The batch capacity ranges from 0.5 to 3.0 Cum
The central plants are equipped with laser sensors to sense moisture in sand.
The storage bins are generally used of 3 types viz
- Star bins
- Elevated bins
- Silo bins
Transit Mixer:
The capacity ranges from 3 to 12 cubic meter.
The agitating speed is between 2 and 6 revolutions per minute with the mixing speed of 4 to 16 revolutions/minute.
The mixing drum and spirals are made of highly wear resistant steel with a high percentage of chromium(0.8%) and nickel (0.6%).
Concrete Pump:
Concrete pumps used in RMC plants are usually with German technology
Pumps are available in different sizes and capacities
Trailer mounted and placer boom pumps are used for pumping concrete
It facilitates smooth pumping of concrete without segregation
Advantages of RMC:
Uniform and consistent quality of concrete
Fast delivery of concrete to speed up construction
Relieves consumer from sourcing and storage of raw materials
Prevents wastage & theft of raw materials at site
Environmental friendly product
Ready to serve concrete irrespective of time.
Various grades of concrete with consistency.
We can produce various special concretes like SCC,LWC and color concrete etc
New Technology in RMC:
Transparent Concrete :
New innovations like Transparent Concrete are bound to rule the concrete manufacturing.
These concrete blocks have equal strength as that of conventional concrete.
It is prepared by using considerable amount of Glass Fiber in the mix design.
Coloured Concrete:
Though not very popular in India, use of coloured concrete is permanent alternative to painting or tile fixing abroad.
It is made by adding desired colour pigments in the batch of RMC itself.
Self Compacting Concrete:
Technology evolves to give better services to the customer.
Self Compacting Concrete (SCC) is one such boom to retail/micro customer who doesn’t have concrete compacting facility at site
SCC is type of concrete that itself flows through the steel reinforcement & gets sufficient compaction at its own.
It is achieved by the use of refined aggregates & special admixtures.
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