Without the reinforcement of steel rebar, buildings, bridges and walls would collapse. Engineers and contractors must know which strength rebar to use for each project, to stabilize the building and ensure public safety. Knowing which rebar to use requires an understanding of its yield strength, and setting those standards requires the skill of engineers and scientists.
Rebar is a steel bar that is used to reinforce masonry structures and concrete. It is typically made from carbon steel and is often ridged, to assist in anchoring the bar in concrete. Rebar is available in different grades of yield strength. Yield strength is a measurement that indicates when a material will give way to stress and permanently bend, which is referred to as the yield point. Before a material reaches its yield strength it will return to its original shape after stress is applied.
Rebar grades can be 40, 60 or 75 inches per pound. In a 40 grade, the minimum yield strength in pounds per square inch would be 40,000. For a grade 60 it would be 60,000, and in a grade 75 it would be 75,000. If using the metric grade, a grade 40 would be metric grade 280, which would mean a minimum yield strength in megapascals of 280. Grade 60 would be grade 420 in metric grade, which would mean the minimum yield strength in megapascals of 420. For grade 75, the metric grade would be 520, which would be minimum yield strength in megapascals of 520.
When laws were passed that required federally-funded building projects to use materials that used the metric calculations it proved to be cost prohibited to stock both metrically-sized bars and the original inch-sized bars. In 1995 the Concrete Reinforcing Steel Institute and the Steel Manufacturers Association campaigned to make changes so that a material could reflect its rating in both metric and British units. Adjustments were made, and compromises reached resulting in the metrically-sized rebar being identical to the inch-sized bars.
In the United States the size designations of rebar used to reinforce concrete are set by ASTM International, which was originally called American Society for Testing and Materials. This is a voluntary standards organization initially created by a group of scientists and engineers. It is an international organization that is a source for technical building material standards (see Resources below).
The American Society for Testing and Materials was created in 1898 when a group of scientists and engineers collaborated on a solution to the frequent rail breaks that occurred with the railroad. Their efforts resulted in the standardization of steel used in rail construction. Over the years the organization continued to take a role in creating higher building standards. Today there are over 30,000 ASTM members, which include technical experts from over 120 countries.
- Photo Credit A. Johnson
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