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.