Part III :Role of good quality rebars in improving the durability of civil constructions and their endurance to natural disasters
As stated in second Part of this series of report, ASTM A615/A615M has no provisions to control higher values of yield strength (YS), minimum tensile to yield strength ratio and upper limit of tensile strength (TS). These properties of rebars are of utmost importance and that’s why American Concrete Institute (ACI) code, ASTM A706/A706M and most other international codes related to reinforced concrete design for earthquake / huricane prone zones specify a minimum and maximum YS and TS and also a minimum value for TS/YS ratio for rebars which should be > 1.25. At first sight these stringent control of strength parameters seem very perplexing. Why should be a cap at strengths? Is stronger material bad for building structures ? To answer these points we have to understand the role of rebars in concrete structures.
Most of the Caribbean regions are prone to seismic and hurricane activities and their occurrence of high to moderate intensities are very common. The energies generated by these natural calamities are transferred to the structures on the earth. The maximum effect of these energies is on civil structures. If not properly dissipated through a man made system, these structures may have catastrophic collapse of buildings and other structures causing tremendous loss of human lives and properties. Concrete structures possess very high strength in compression but practically have zero endurance to tensile stresses. The natural disasters such as earthquakes and hurricanes cause tensile stresses on structures where a simple concrete with cement, sand and gravels fail miserably. Steel reinforcement bars have good tensile as well as compressive strengths. This material when embedded in concrete substantially imparts tensile properties, namely yield strength, tensile strength and ductility to the erected structures. The yield strength of rebars which is defined as the maximum force per unit area before they start yielding due to the tensile force , is very important and useful in absorption and dissipation of forces caused by natural disasters. A major chunk of these forces transferred to the concrete structures is absorbed by steel rebars during its yielding. However, the yielding force needed to yield rebars should not be too high than the designed strength. The excess YS of rebars will prevent its yielding during the events of natural calamities and forces will be transferred to concrete causing their fragmentation in to big chunks and falling down. The tensile strength of rebars which is the maximum required force before it completely breaks in to pieces, is another very crucial and important parameter that safeguards the concrete structures in event of very high intensity of forces . A higher TS of rebars with their longer elongation properties is good mate for concrete. In view of these facts, ACI, ASTM and other standards prescribe a minimum TS/YS ratio of 1.25. If proper rebars are embedded in concrete structures the losses due to natural disasters can be minimized to a significant extent. Now most of the countries especially in Europe, Asia and North America are following building codes designed incorporating the rebars to withstand / minimize the impact of natural disasters.
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.