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.