Category Archives: concrete

One two three ..and…… start

Dear reader , 
In the very first post , we saw the overview of what is happening in the neighborhood plot.
Right ? From the distance , we just saw that some thing was happening inside the enclosed boundary.
Today let us take a micro view of all that takes place during the construction.
Let us follow the actual sequence of the activities being taken up right from start to finish;
rather even before starting the actual construction.
Basic idea is to have an insiders perspective about the entire construction.

Overview :

There are 4 phases of construction:

1.Planning phase
2.Pre construction phase
3.Actual construction phase

    3.A:  Below the Plinth
    3.B: Above the Plinth
4.Handing over phase

1.The Planning phase :

This is most crucial phase in the entire project. The importance of planning cannot be over emphasized.

Many crucial decisions are being taken during this phase.

Some of them we have already introduced during the earlier article.
Planning starts after land acquisition is complete.
Land is in the complete possession of the present owners 
( who shall be henceforth be called as the clients )

Funding for the project has been arranged and all formalities are completed.

Statutory formalities related to the project have been completed.

The Architect & RCC consultants have been engaged and have been shown the site.

Architect has been given the initial requirements and the draft drawings have been approved by the clients.

RCC contractors have been short listed.
Material suppliers have been identified for key materials needed for RCC .

Waterproofing agency has also been approved.

Drawings have been prepared and approved for execution.

FLOOR LAYOUT  which is a given in the marketing brochure
FLOOR LAYOUT
which is a given in the marketing brochure
ACTUAL WORKING DRAWING issued at the site for construction.
ACTUAL WORKING DRAWING issued at the site for construction.

Local approving authority has  sanctioned building plans .Permission to build has been obtained.

The tube well is ready with submersible pump. Water supply for construction has been arranged .

TUBE WELL at site. A Submersible pump is being installed for regular water supply.
TUBE WELL at site.
A Submersible pump is being installed for regular water supply.

Trial pit has been excavated and soil testing has been completed.

The safe bearing capacity of 40 T/SqM has been determined at a depth of 8′ from the natural ground level

Type of foundation has been decided.

Clients need a Basement  and hence it has been jointly decided to go for a mat type of foundation which shall  be suitable for the basement
A perspective has been prepared for giving a complete view to the would be buyer.
PERSPECTIVE OF THE PROJECT which is a part of the brochure handed over to the prospective customer.
PERSPECTIVE OF THE PROJECT which is a part of the brochure handed over to the prospective customer.

Brochures have been printed with the details of the project, specifications ,

types of tenements available, carpet area statement for different units 
Board has been erected at the site with the title of the project,names of the client, architect , RCC designer, Legal adviser etc.
Clients have decided to appoint a full time  Supervisor at site and hence interviews were conducted and an experienced supervisor has been appointed and deployed at the construction site.

2.Pre– construction:

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.

WOODEN RAILING ERECTED AT SITE for center lines marking. [ Markings being verified]
WOODEN RAILING ERECTED AT SITE for center lines marking.
[ Markings being verified]

Key Materials such as cement ,sand aggregate, Tor steel, binding wire & cover blocks have been procured.

INITIAL STOCK OF ORDINARY PORTLAND CEMENT has been procured at site.
INITIAL STOCK OF ORDINARY PORTLAND CEMENT has been procured at site.

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.

MASS EXCAVATION being done with the excavator.
MASS EXCAVATION being done with the excavator.

Box type waterproofing has been completed.
Raft reinforcements have been put into position .

BOX TYPE WATER PROOFING BEING DONE for the basement raft.
BOX TYPE WATER PROOFING BEING DONE for the basement raft.

Dowel bars for the periphery walls & intermediate columns have been tied.

Cover blocks have been put .Pour card has been filled up.

The basement raft has been concreted with the help of RMC .
Side walls have been cast and box type of water proofing has been done on the external face.
the intermediate columns have been cast and roof slab of the basement has been completed.
3.A Above the plinth

4.Handing over:

Author’s note :
Starting from this post , we shall be detailing on these phases in a series of articles.
The author would like to present focused information in different articles so that the absorption of knowledge is gradual but effective.
In the subsequent articles , we shall continue our journey above the plinth.
Till then , Good bye.& Take care
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Pre-stressed concrete ? Don’t be stressed .

Overview:

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. 

What’s prestressing ?

Background:
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.

Process of building a stone arch:
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.

LOOK AT THE BEAUTIFUL STONE ARCH
LOOK AT THE BEAUTIFUL STONE ARCH
Analogy :

Hammering of crown in its position induces compression which is prestressing and this compression neutralizes the tension arising due to dead load & live loads on the arch.
Conceptually,Same process is done in concrete while prestressing.

Some important terms :
1.Tendon :
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.

Types of prestressing:
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 or post- 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.

Post-tensioned :

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.

Advantages of pre-stressed concrete over RCC :

Process of prestressing
Case study of mall
Span 12 m.
RCC beam 1.2m
Prestressed beam 0.25 m
Headroom is available

Author’s notes :

Dear Reader

Need some more time to complete this article .

Some photographs are to be added and descriptions to be completed.

Just bear with me for the time being.

Thank you so much

Confirmed Concreting ?

Reinforced cement concrete is an outstanding invention of the mankind.
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 article brings to you preliminary discussions on materials used in concrete

What is concrete ?
Concrete is a composite material prepared with stone  & 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 ,the  bond is created by  this cement paste

Coarse aggregate 1/2" to 3/4" mix .
Coarse aggregate 1/2″ to 3/4″ mix .
Artificial Sand or Crushed sand provides a good interlock between particles.
Artificial Sand or Crushed sand provides a good interlock between particles.
Main constituents:
Cement:

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).
Thus the occurrence of micro cracks on the surface of freshly laid slab is reduced if PPC is used.

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.

Coarse aggregate( Stone ):
For purpose of concrete as used in a RCC building the stone size is 1/2″ to 3/4″.

The % of flaky & elongated aggregates shall be restricted as these reduce the crushing strength.

Water:

Water used in concrete should be good potable water free from acid or alkali compounds.