With the growing worldwide demand for new buildings and structures, there has been a corresponding drive for high-quality construction materials. Rebar, or reinforcing bar, is a high-demand material used to improve the tensile strength of concrete in structures. Because rebar and concrete have similar thermal expansion properties, it is the material of choice to reinforce structures. Rebar is often made from ridged carbon steel and comes in various sizes and grades. Today, innovations in materials and coatings are producing higher strength and more versatile rebar options, while posing new challenges for testing.
There are multiple challenges with rebar testing, and well-recognized, exacting standards to meet. One of the biggest concerns with testing rebar is the non-standard shape of the specimens. Rebar is often stored in large coils and then cut to size. So, samples need to be straightened before testing, and they may retain some curvature. The surface of the bar is ridged for better adherence to the concrete, which is good for structural stability, yet from a testing perspective, this uneven surface creates some challenges for gripping the specimen.
Another gripping challenge is created by the coatings that may be used. While coated rebar may offer better protection against corrosion, it is a slippery surface to grip properly. Additionally, some end-use applications require the rebar to be bent to create the necessary structural shapes within the concrete, so it must be tested for bend strength as well. One more concern when testing rebar is the tendency for violent failures that need to be planned for to minimize potential damage to the system and testing personnel.
Rebar is available in various sizes and grades. The grades correspond to minimum yield strength, for example, Grade 280 would have a minimum yield strength of 280 MPa; similarly, Grade 40 would have a minimum yield strength of 40,000 psi. Size is measured in diameter and offered in standard increments. The size and grade of the rebar test specimen will determine the test machine’s required force capacity. Higher grades of rebar are becoming more popular because a smaller amount of higher-strength rebar can be used to create the intended structural stability.