1) When purchasing rebars for an RCC structure, what are the things to consider?
Ans. Things to be considered are-
i. Grade of the rebar, meaning whether the rebar is of 300 grade OR 400/400W grade OR 500/500W grade. Grade denotes the yield strength of the rebar.

Grade

Yield Strength (MPa)

Ultimate Tensile Strength
(MPa)

Minimum standard requirements

 

 

Grade 300

300

330

Grade 400/400W

400

440

Grade 500/500W

500

550

 

 

 

Source: BDS /ISO 6935-2:1991(E)     

 

 

ii. % Elongation

Grade

Elongation
(%)
Gauge Length= 5D

Minimum standard requirements

 

Grade 300/300W

16

Grade 400/400W

14

Grade 500/500W

14

 

 

Source: BDS /ISO 6935-2:1991(E)     

 



iii. Bend Test Report

Sample Bend Test Report:

 

Diameter of the bar
(mm)

Mandrel Diameter
(mm)

Is fracture/crack visible under normal vision after the bend test at 180 degree?

Test Result

Standard: BDS ISO 6935-2:2006

8

25

No

Satisfactory

 

 

 

 

10

32

No

Satisfactory

 

 

 

 

12

50

No

Satisfactory

 

 

 

 

16

80

No

Satisfactory

iv. Chemical Composition Analysis

Sample chemical composition analysis:

Chemical Composition, %

SN

Size
(mm)

C

Si

Mn

P

S

Cr

Mo

Ni

Cu

V

Ceq

Standard:
BDS ISO 6935

0.24 Max

0.65 Max

1.70 Max

0.055 Max

0.055 Max

 

 

 

 

 

0.52 Max

1

8

0.17

0.15

0.87

0.017

0.016

0.15

0.00

0.00

0.25

0.00

0.39

2

10

0.18

0.12

0.79

0.027

0.004

0.11

0.00

0.00

0.42

0.00

0.38

3

12

0.18

0.09

0.78

0.026

0.007

0.12

0.00

0.00

0.39

0.00

0.38

4

16

0.19

0.14

0.82

0.029

0.007

0.07

0.00

0.01

0.42

0.00

0.39

5

20

0.17

0.24

0.88

0.017

0.009

0.13

0.00

0.02

0.36

0.00

0.41

6

25

0.18

0.20

0.94

0.042

0.023

0.13

0.00

0.02

0.47

0.00

0.43

7

32

0.16

0.23

0.90

0.016

0.016

0.11

0.00

0.03

0.39

0.00

0.40

 

Note: For any project, particularly big projects, one type of grade should be used. DO NOT mix two or three types of grades in a single RCC work.

2) What is the role of steel reinforcement (rebar) in concrete structures?
Ans. Concrete is predominantly a compression resisting element. It has no tension resisting capability. Steel has excellent tension resisting ability and this ability is called tensile strength. The higher the tensile strength of the steel, the better is its tension resisting ability. In reinforced concrete structures, concrete and steel work together as a single unit. When a part of a building is under load, either tension or compression, concrete and steel work together to resist that load. In the figure below, a vertical load is acting on the beam. Under this load the concrete cracked, but the reinforcing steel resisted the load and held the concrete together.

3) What does 'W' denote in 500W grade?
Ans. The 'W' denotes the weldibility of the steel product. Weldibility depends on the carbon equivalent of the product.
4) How to identify an original TMT bar?
Ans. TMT Bars will always have the characteristic outer ring. This outer ring is absent in non-TMT bars.

The characteristic outer ring can be revealed by conducting a simple test-called the nital-etch test.
Two samples are produced, one from a conventionally hot-rolled 60 grade bar (without the TMT process) and the other from a TMT bars.
The samples are rubbed with nital (5 percent nitric acid in alcohol) solution at ambient temperature for a few seconds. The sample of TMT bar will reveal a darker outer ring with a lighter core, whereas the sample of 60 grade will remain unaltered.

Microscopic constituents of the conventionally hot-rolled 60 grade bar

Note: In conventional hot rolling, the rolled bars cool normally in air and form ferrite-pearlite structureT

Microscopic constituents of the TMT bar


Note: This unique combination of a strong tempered outer layer and a highly ductile core gives TMT bars their unmatched resilience and reliability. The tempered martensite layer is formed by the TMT process.
5) How should an ideal TMT bar look like for civil construction?
Ans. An ideal TMT rebar should have a uniform tempered martensite priphery which is clearly visible after the nital etch test. Depending on the size and grade, the hardened periphery will be about 20-30% of the bar cross-sectional area in an ideal TMT bar.

6. Why ductility is so important for reinforcing bars?

Ans. Consider two types of steel re-bar. Type 1- Grade 500 with zero ductility and Type 2-Grade 500 with some ductility.

For Type-1, Yield stress is equal to Ultimate Tensile Stress (UTS) which is equal to 500 MPa.
But for Type-2 the Ultimate Tensile Stress is greater than the Yield Stress i.e. UTS is greater than 500MPa.
Thus when a load generating a stress just greater than 500 MPa is applied on both the types, Type-1 will fracture immediately but Type-2 will deform and withstand the load. The ductile property of Type-2 allows it to carry the additional load.