Friday, December 3, 2010

REINFORCEMENT

Reinforced concrete is a composite material use where one single material cannot achieve the design requirements. Concrete is strong in compression but weak in tension. Hence, so that concrete to be good in construction, their should have a strengthening in the concrete. This strengthening (reinforcing) material is known as fibers. 


CHARACTERISTICS  OF  REINFORCING   MATERIAL  IN  CONCRETE
1.     It must have tensile strength.
2.     It must be able to achieve this tensile strength without undue strains.
3.     the material must be able to easily bent into any required shape
4.     Its surface must be capable of developing an adequate bond between the concrete and the reinforcement to ensure that the required design tensile strength is obtain.
5.     It must be available at reasonable cost.

The material which meets all the above requirements is steel in the form of bars, and is supplied in two basic types.
1.     mild steel
2.     high tensile steel


STEEL BARS USE IN REINFORCEMENT

the characteristic strength of mild steel is 250 N/mm2 and 460 N/mm. the surface of mild steel provides adequate bond but the bond of high- yield bars, being more critical with the higher stresses developed ,is generally by rolling on to the surface of the bar longitudinal or transverse ribs.
The reinforcement in concrete may be simple bar or series of bars, bend to a given schedule which known as bar schedule and tied according to the reinforcement drawings with stirrups. The normal diameters of bars used at this site are 6, 10,12,16,20 and 25.
Steel bars are available in various brands in the market some are locally manufactured and some are imported brands. Whatever brand is used by the contractor he should submit test reports for tensile strength, elongation.   


4REBAR DETAILS

SECTION SIZE

UNIT WEIGHT

CROSS SECTION AREA

WEIGHT OF 12M BAR

NO OF BARS FOR A TON

R6
0.222
0.283
2.664
375
R10
0.617
0.785
7.404
135
T10
0.617
0.785
7.404
135
T13
1.04
1.327
12.48
80
T16
1.58
2.011
18.96
53
T20
2.47
3.142
29.64
34
T25
3.85
4.909
46.2
22
T32
6.31
8.042
75.72
13
T40
9.88
12.56
118.56
8

R-Mild steel                        T-High tensile steel

 

BAR BENDING

Steel bars should cut and bend to the required shape so that they are fabricate  bind and place them within the form work .Bar bending is done by skilled person and he should be given the required bar schedule prepared  according to the reinforcement drawings.
BAR SHEDULE PREPARATION

Bar schedule is a special scheduling of steel bars which includes whole the details of each bar of a particular member required for bending the bar. The general bar schedule includes following details.
·        Member
·        Bar mark
·        Type and size
·        No. of member
·        No. bars in each
·        Total no.
·        Length of each bar (mm)
·        Total length (mm)
·        Shape of bending demension
Using the above method , we find out the quantity of  necessary bars in the site.
When we order bars, at first we should make the bar schedule.

LAPPING TWO STEEL BARS

Steel bars manufactures are of 6m lengths. But in practice we would need bars longer than 6m. In such cases we have to lap two bars as no failure occurs and as it produce its original strength. in this site, the recommended lap lengthy is 50 times the bar diameter if both bars are of same diameter. When lapping two bars of different diameters, the lap length is considered as 50 times the smallest diameter.


CRANCK

Crank is a slight bending in bars at the lap so that maintains the clear cover even at the lap position.
Crank length = ( d1+d2+5)*10mm
d1 =diameter of smaller bar.
d2 = diameter of largest bar

  COVERING FOR REINFORCEMENT

The purpose of providing cover is to protect reinforcement from harmful conditions such as whether; chemical attacks, heat and electricity that may reinforcement could be corroded. Hence before concreting, we should be careful weather sufficient numbers of covers are

provided so that ensure the protection of reinforcement. Usually clear cover can be provided by using cover blocks, stool and Donuts. Donut is made of plastic.
 
Cover blocks are made using cement and fine sand motor mixture at site premises. It depends on the grade of the necessary concrete. As example, if we need a cover blocks for the grade 15 concrete, the proportion among cement sand and aggregate of grade 15 is 1:3:6, therefore we make cover blocks by using cement and sand according to 1:3.

Stools are generally used in slab reinforcement to produce required covering and stop both the top layer net and bottom layer net touching together (top bottom net separation). The stool height and type is designed according to the slab thickness. There are two types of stools
In our site, cover blocks are used as follow.
·        For columns- 40mm
·        For walls      - 30mm
·        For beams    - 35mm
·        For slabs       - 25mm


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