A competent RCC contractor has been finalized after several rounds of negotiations and the contract has been awarded.Architectural plans ,foundation details and the Center Line plans have been issued by the Architect.
RCC contractor has mobilized on site.
labor hutments have been erected and labour have mobilized.
Cement godown & water tank constructed .
Plot has been properly enclosed with corrugated sheets on the boundary.
A security agency has been contracted for detailing the round the clock security for the site.
Center line Railing has been done and footings center lines have been marked.
RCC consultant has inspected and approved the center line markings.
Plinth level has been decided by the architect and marked on the wall of site office.
Key Materials such as cement ,sand aggregate, Tor steel, binding wire & cover blocks have been procured.
Concrete mix has been finalized based on the actual samples of materials taken from the site.
3.Actual construction :
3.A Below the plinth
Mass excavations have been completed with a hired excavator.
The black cotton soil which is not suitable for back filling has been transported outside.
The actual sub strata or the foundation level has been approved by the RCC consultant at site.
Line out has been given for the leveling course and PCC has been completed for the entire basement raft.
The concrete is strengthened by adding reinforcement bars. But RCC has its own limitations such as higher depths for increased spans. Pre-stressed concrete is a modern day marvel which is a boon to the construction technology .Whenever we think of a large span of beam or slab, invariably , we think Pre-stressed sections.
this article is here to introduce you with this wonderful topic.
We already know that any flexural member shall have both compression and tension . Whereas,the concrete is quite strong in compression, it is quite weak in tension and hence steel bars are provided in concrete to take care of the tensile forces. This combination is known as Reinforced cement concrete.
Prestressing means inducing compression in the concrete members so that when that element is subjected to tensile stress , the tension gets neutralized by the compression induced beforehand.
Analogy to pre-stressing:
All of us appreciate the Arch in a building.Beauty of the arch is beyond words. An arch is a flexural member which has only compression in the entire cross section and most important , there is no tension.
You shall ask how is this feat achieved ?
Let us start from the basics.
A temporary semicircular formwork is first built at the opening where the arch needs to be built. Well dressed stones are then joined with cement mortar .The masonry is started from the lower ends and is slowly taken higher up on the curves.
When both sides reach the apex point,a stone which is known as the key stone or crown is hammered in the narrow gap .
This stone is in the shape of a wedge .The forceful insertion of the crown stone in the narrow gap induces compression in the entire cross section.
Now the temporary staging is gradually removed and we have a fully
self- supporting Arch in position.
Hammeringofcrowninitspositioninducescompressionwhichisprestressing and this compression neutralizesthetensionarisingduetodeadload & liveloadsonthearch. Conceptually,Sameprocessisdoneinconcretewhileprestressing.
Some important terms :
It is the bunch of steel wire which is subject to measured tension.This tension when transferred to concrete , creates an equal & opposite compression in the concrete.
2.Sheathing: It is the sleeve in which the tendons are located. This is used in post-tensioned elements
3.Jacket: This is where both ends of the tendons are secured .
4.Jack : The pulling mechanism which induces measured tension in the tendon
5.Grouting : the high strength cement slurry which is injected under pressure in the sheathing.
By virtue of the fact as to whether the prestressing is done before or after casting of concrete, the prestressing is known as Pre-tensioned orpost-tensioned. Pre-tensioned:
The tendons are laid lengthwise ,at the specific location in the member to be pre-stressed. the measured tension is applied to the tendon. Casting is done in the member with the requisite grade of concrete. As the concrete sets, the tendon is released from one end . The tendon tends to go back to its original length as it is released. The surrounding concrete resists this movement and in turn gets compressed.
The tendons are laid in the sheathing ,at the specific location in the slab or beam to be pre-stressed. Casting is done in the member with the requisite grade of concrete. After the final setting of concrete ,the measured tension is applied to the tendon by pulling from one end . After pulling, the stretched tendon is secured in the jacket. The tendon tends to go back to its original length as it is released and hence exerts pressure on the Jacket .jacket in turn compresses the concrete.
Processofprestressing Casestudyofmall Span 12 m. RCCbeam 1.2m Prestressedbeam 0.25 m Headroomisavailable
Author’s notes :
Need some more time to complete this article .
Some photographs are to be added and descriptions to be completed.
It is not not possible to think of a building where the RCC has not been used in this form or that.
All crucial elements of a structure such as Footings,beams,columns & slab are cast in RCC.
RCC is a carefully crafted creation of concrete and steel bars .
This articlebringstoyou preliminary discussions on materials used in concrete
Whatisconcrete? Concreteisacompositematerialpreparedwithstone &sand which is bonded by cement.
A chemical reaction which is called exo-thermic, takes place in cement in the presence of water .
The cement paste gets solidified in some time which is known as the setting time.
Whereas stone & sand are inert materials ,thebond is created by this cement paste
Either OPC or PPC can be used.
Ordinary Portland cement is available in grades 43 & 53 and is mostly used for all RCC works.
Portland Pozzolona cement which is of grade 33 can also be used.
Only thing which needs to be stressed here is that the PPC has a delayed setting time as compared to OPC and hence the rate of strength gain with respect to time is less.
Thus the striking off period has to be increased by at least 50% if PPC is used.
Thus the project completion period shall extend due to delay in De-shuttering activity.
But the main benefit of using PPC is that it is economical as compared to OPC .
The heat evolved is also lesser as compared to OPC ( being of higher grades). ThustheoccurrenceofmicrocracksonthesurfaceoffreshlylaidslabisreducedifPPCisused.
Fine aggregate ( Sand):
It can be either River sand or natural sand.
Maximum silt percentage of silt in the sand should not be more than 6%.
Due to shortage of sand, now a days, crushed sand which is also known as the artificial sand is being used.
The microscopic view of natural sand shall show rounded particles whereas that of the crushed sand shows angular particles.
The rounded particles are responsible for workability whereas the angular particles give good interlocking between particles.
Therefore a judicious combination of natural sand & crushed sand are recommended to use both of these properties.
For purpose of concrete as used in a RCC building the stonesizeis1/2″to3/4″.
The % of flaky & elongated aggregates shall be restricted as these reduce the crushing strength.
Water used in concrete should be good potable water free from acid or alkali compounds.