Identifying a reinforcing bar’s method of manufacture has been made easier. Pacific Steel has introduced a new bar marking pattern for its reinforcing steel made using the quench and tempered (QT) production technique.
This will allow users to clearly identify quench and tempered reinforcing bar from that made using the micro-alloy (MA) technique. MA and QT refer to different manufacturing methods.
While both methods can produce reinforcing bars with the same strength and ductility, the ability to weld, rebend or thread is vastly different. Essentially, by following some basic rules as found at Pacific Steel’s web site, only Seismic 500E MA, or Seismic 300E reinforcing steel should be welded, rebent or threaded.
The markings are in response to concerns about the correct handling of reinforcing steel on-site. Feedback from the industry was that while engineers are specifying a particular type of steel, the specified steel is not always being used on site, according to Pacific Steel Group general manager Alan Pearson.
“This is a cause for concern because QT bar has more limitations in its use than MA reinforcing bar, and has implications on the safety of structures,” Mr Pearson says. The most common method worldwide for producing high strength reinforcing steel is to quench the red hot steel immediately after the last stage of hot rolling.
The surface of the bar is quenched to give a very hard layer which is then tempered to strong, but more ductile steel by the residual heat from the centre of the bar. This is called the Quench and Self Tempered (QST) process or, more commonly, Quench & Tempered (QT). It is important to note that it is the combination of high strength outer casing and ductile inner core working in conjunction with one another which provides the necessary mechanical properties.
The less common method of producing high strength reinforcing is to add special alloying elements — commonly vanadium — at the steel-making stage. The bars undergo no quenching, but rather air cool after they have been rolled.
This is called the Micro-Alloying (MA) process. The MA reinforcing bar, unlike the QT bar, has a homogenous cross section in terms of chemistry, crystal structure, strength and ductility.
It is a more expensive process, due to the need for the microalloys, but the end product has benefi ts which, in many cases, justify the additional cost. “The new markings are designed to make it easier for those individuals working on-site to ensure they are using the correct steel as specifi ed by engineers.
QT steel is more limited in its applications than MA reinforcing steel, and it’s critical that the two types of steel are not used interchangeably,” Mr Pearson says. “Our new bar markings are designed to minimise the risk of the wrong type of steel being used for the job.
If imported bar is being used on site we urge project managers to check the method of manufacture before any welding, re-bending or threading is undertaken.” The New Zealand standard does not currently require the method of manufacture to be marked on the bar.
However, this is subject to a proposed amendment to the reinforcing bar specification. For more information on different types of reinforcing steel visit the new-look Pacific Steel web site at www.pacificsteel.co.nz