Part II: Current scenario on quality of reinforcement bars used in Caribbean region vis-à-vis international
It is a well known fact that steel rebars are embedded in concrete structures to improve their modulus of elasticity and strength. Steel bars with proper chemistry, microstructure and surface treatments impart significant improvement in the above properties of the concrete. During the last two decades a phenomenal change in quality of reinforcement bars has taken place. Let us examine whether our (Caribbean regions) changes are in tune with the rest of the world.
Keeping in view the economical aspects, durability, endurance to natural calamities (earthquakes, tsunami, hurricanes etc) and concerns for the carbon emission, significant modifications are being suggested and implemented on standards and code of practices for different sectors. The construction sector is not an exception to this. A survey on trend of use of reinforcement bars (rebars) in construction sectors of Caribbean countries indicates that in majority of the cases plain / deformed steel bars conforming to the ASTM 615A are being used. The steel rebars conforming to this standard are suitable for use in least demanding applications. The Caribbean region is very prone to seismic and hurricanes activities and therefore needs special precautions during erection of their buildings, bridges and other structures. To cope up with these natural disasters, ASTM and many standards of other countries came out with specific standards for manufacture and applications of rebars in seismic prone areas. ASTM A706/A706M is the standard for rebars for use in seismic prone areas. Let us examine the difference between ASTM A615/A615M (currently being used in Caribbean regions) and ASTM A706/A706M (improved version on rebars) standards:
|ASTM A615 / A615M (420 grade)||ASTM A706 / A706M (420 grade)|
%P (Maximum) 0.06
Steel Chemistry % Maximum)
C=0.30; Mn =1.50; P= 0.035; S=0.045; Si=0.50
YS = 420 MPa
TS = 620 MPa
% Elongation = 9
Bend test requirements =3.5d-5d
YS Min. Max.
420MPa 540 MPa
TS 550 MPa -
% Elongation 10-14
Bend test requirements =3d-4d
The above table shows that the chemistry and mechanical properties of rebars are more strictly controlled for A706 / A706M than A615/A615M. These properties greatly affect the weldability and endurance to seismic / hurricane activities.
The majority of the countries of the world have now switched over to rebars conforming to ASTM A706 /A706M for designing of their civil structures. This standard ensures improved endurance to natural disasters, improved weldability as well as improved resistance to corrosion and pitting. A controlled chemistry of steel can be achieved only when steels are produced by blast furnace route or by electric arc furnace method after controlled purification of the molten metal. These methods of production of steel rebars reduce the casting defects such as voids, inclusions and tramp elements namely sulfur, phosphorus etc . Survey indicates that the rebars rolled from the ingots produced from iron scraps and melted in induction furnaces are very common in application in Trinidad and Tobago. The cost of production of rebars, by this route, is considerably cheaper than the other methods and is available at lower prices. Unfortunately the rebars produced by induction furnace route have many defects such as nonstandard chemistry, inclusions, and voids etc and difficult to meet the standard specifications. The general public unaware of these facts go for low cost rebars easily available in the market. Use of such rebars in columns, roof castings, bridges and foundations are big concerns for safety and durability.
Stay tuned for the next part of this series which will focus on the role of good quality rebars in improving the durability of civil constructions and their endurance to natural disasters.
Part I: How effective are our reinforcements?
What you need to know about steel rebarsAmongst the vast numbers of man-made invented composites, the reinforced concrete comprising of cement, water, sand, gravel and steel, perhaps is a composite material that had maximum impact in revolutionizing the quality of life of human being. After curing, the concrete attains appreciably high strength in compression but remains very poor in tension, flexure, shear and torsion. It was an accidental discovery when a French gardener named Joseph Monier in 1867 observed that a phenomenal improvement in durability and strength of his flower pots took place after iron nails were embedded in castings made by Portland cement, sand and water. This discovery led to a series of developments in designing of rebars to achieve stronger and longer lasting concrete structures. It was a long journey for the currently popular TMT steel rebars to reach to its optimum properties imparting improved life to concrete structures. Now TMT rebars are used all over the world for erection of bridges, flyovers, high and low rising buildings, pavements, concrete roads, canals, dams etc. These rebars are produced by quenching and tempering the process having composite microstructure with ductile core area and harder outer rim. This dual structure imparts strength as well as ductility to TMT rebars. They are free from any distortion of crystals of steels and therefore are more resistant to corrosion with desired ratio of tensile and yield strengths. The absolute carbon content is also kept at lower side in such types of rebars which improves their weldability.
Figure 1 : Cross section of TMT bars is composite microstructure. The core area is soft pearlite – ferrite and the outer rim is harder tempered martensite microstructure
Unfortunately production process of TMT type of rebars needs special mills with very strict control of quenching and tempering parameters. This increases their cost of production. Many companies still produce rebars without quenching and tempering treatments and market them in many countries. It is not possible to differentiate between TMT and non TMT bars just by looking from naked eye. The users unaware about the quality of such rebars use them in their construction work. As shown in the following figure 2, both TMT and normal bars (non TMT) have ribs and it is not possible for consumers to differentiate between them. It can be differentiated only after looking in to the rim formed on the cross section of rebars. TMT bars develop a circular rim around the cross section but non TMTs do not exhibit such a rim.
Figure 2 : Digital photographs of CTD, TMT and non TMT bars
How to find out whether the rebars are TMT or non TMT
It is very simple. Cut a small piece of the bar. Grind its cross section area on emery paper to remove scratches. Dip the cross section in acidified solution of alcohol for 30 seconds (98% alcohol + 2% nitric acid). Remove the bar from the solution and let them get dried. If a circular ring appears along the periphery of its cross section, it is TMT bar. The photograph of cross section of TMT bar after testing is shown in the above figure 2.
Part II of this series of write up will describe the “Caribbean vis-à-vis international scenario on use of rebars for reinforcement of concrete structures”