There is growing interest within the reinforced concrete industry in using higher strength reinforcing steel for certain applications. This interest is driven primarily by relief of congestion, particularly in buildings assigned a high seismic design category (e.g., West Coast moment frame buildings). There are also other areas where high strength bar can help improve construction efficiencies, or—combined with high-strength concrete—allow reinforced concrete to be used in more demanding applications. The adoption of ASTM A 1035 in 2004 created the material specification that allows for 100 ksi and even 120 ksi reinforcing steel but it remains to be determined how the design codes will address it. Today, the vast majority of concrete design and construction uses Grade 60 steel, with occasional but increasing use of Grade 75. The long-term question for the industry is whether the introduction of 100 ksi or higher strength reinforcing steel is merely a new niche product to help congestion relief in seismic areas, or if the emergence of these materials is the beginning of a movement to higher strength reinforcing steel across the board.
So far, the main demand for high strength reinforcing steel has been in seismic areas on the West Coast where congestion issues, especially at beam-column intersections continue to plague rebar placing and design. A recent 31-story condominium project in Seattle proved the potential benefits of 100 ksi reinforcing steel in just such a situation. "Typical spacing of the confinement steel in the columns is 4 to 5 inches using standard Grade 60 #4 bars," says Brian Booth of Harris Rebar Seattle Inc., Tacoma, Wash. "This creates severe congestion issues at the intersection areas. By switching to #5 bars made of Grade 100 steel, the spacing increased to between 8 to 12 inches, allowing faster construction and easier placement of the concrete."
Rebar fabricators and placers in other seismic markets agree. "We can't get this material into our market fast enough," says Tony Buck of CMC Fontana Steel in the Los Angeles area. "We struggle with terrible congestion issues on projects every day. The ability to increase bar spacing and reduce bar diameter in these congested areas could really help placement time. Over the life of the project, this could add up to significant cost savings that potentially could offset any additional costs of the higher strength material."
But reinforcing steel fabricators in nonseismic areas have seen another trend. "We aren't seeing any demand for the higher strength rebar in our market area," says Greg McVeigh with Trowbridge Steel outside Washington, D.C.
In certain markets, the use of highly flowable concrete may be an alternative solution for congestion issues. A showcase project for reinforced concrete in Chicago, the Trump Tower is an example of using only conventional reinforcing steel. The building will be the second tallest in North America—only the Sears Tower rises higher—but the building was designed using mostly Grade 60 bar with limited use of Grade 75. The key to this design wasn't the reinforcing steel; it was the use of high-strength concrete, some of it self-consolidating in key areas.
Design, production, and fabrication issues
Despite the potential benefits of higher strength reinforcement, there are still a number of important questions that have to be answered from design and production to fabrication and placement. From a design standpoint, all the current codes limit the allowable design strength to 80 ksi. The higher strength steel does not have the same distinct yield point as Grade 60 material—it tends to be more brittle—requiring a new understanding of the designing and detailing requirements for the proper use of the material. Tests conducted by the Florida DOT cautioned against "blind substitution" because the ductility was inadequate despite the higher strength. According to the Florida report, "lap splices, hook embedments, etc., that are sufficient for Grade 60 will likely be inadequate" for steel meeting ASTM A 1035. The report concludes that it is not clear what yield stress should be used for detailing and goes on to conclude that "the whole approach of how to deal with reinforcing that lacks a distinct yield point, around which most codes are written, needs to be addressed."