A Technical Publication on Rebar Processing

The fabrication of reinforcing steels, into shapes suitable for fixing into the concrete formwork, is normally performed in the European Countries by specialist reinforcement fabricators. Very little reinforcement is cut and bent on-site in Europe today. Although the cutting and bending of reinforcement appears relatively straightforward, the specialist reinforcement fabricator is well equipped to do so in a consistently accurate manner and, by working within a defined set of quality management criteria provides an important link in the reinforcement supply chain. The accuracy of cutting and bending operations is vital to ensure proper fit on site, and to maintain required lap lengths, anchorage lengths and cover. This article describes the cutting and bending operations, to give the reader an understanding of the processes involved. In India IS: 2052 specifies scheduling, dimensioning, bending and cutting of steel reinforcement for concrete.

Bent shapes are defined by means of standard shape codes. When non-standard shapes are required, guidance is given by the structural detailer as to, how these should be dimensioned and is specified in appropriate drawings. For each in standard shape, the standard gives the overall length of bar used, as a function of the principle dimensions.

The format for the Bar Bending Schedule in which the requirements for cut and bent reinforcements are specified is available in IS: 2502 (Code of practice for bending and fixing of bars for concrete reinforcement). Bar Bending Schedule (BBS) are seldom supplied to contractors by incorporating the same in the structural drawings. In majority of cases the detailed construction drawing is supplied to the contractors and they make the BBS. In the European countries schedules are delivered to the fabrication shops through e-mails by means of commercially available software packages. This is saving significant time for the fabricators, and increasing the accuracy of information. In some cases, this electronic schedule information can be linked directly to the computer control system of the production equipment. There are also significant benefits for designers and contractors in terms of reduced errors, fewer queries, and easier change procedures.

Fabrication Processes

The basic fabrication processes consist of cutting and bending the reinforcing steel. The actual processes employed in the bending and cutting of reinforcing steel depend principally on the form of material being processed, whether bar or coil. In India, reinforcing bar is supplied to the fabricator from the mill in bundles of straight lengths or with one bend commonly known as hairpin bend. Hairpin bend though facilitates transport of the rebars, requires straightening manually and thus entails additional cost. Stock lengths in India are normally 12m long but suppliers frequently supply in random length varying between 9m to 13m. Nonstandard lengths are also supplied, usually subject to a minimum tonnage requirement.

Bar is fabricated by cutting on shear lines, and bending on power bending machines. Reinforcing bar, in coil form, is also supplied from the mill in a continuous length, often in a spooled form. Coil is normally available in sizes of 8-12mm. Coil weights may typically be one to two tonnes, so that for the s m a l l e r diameters, the coil can be several kilometres in length. 16mm diameter in coil form may also be obtained through import route for fabrication into desired shapes (Exhibit -1).

Coil can be unwound and cut to length in a single operation, called either decoiling or straightening. Coil can also be straightened, bent and cut to most shapes, in continuously operating machines commonly referred to as Automatic Link Benders (or ALB's).

In India Tata Ryerson is the only rebar processor utilizing ALB's for directly processing coiled rebars thereby reducing the wastage to a negligible amount. This kind of processing facility at Tata Ryerson's Faridabad facility has created great interest and relief amongst large construction companies in India. Architects and Structural Engineers were longing to see the automated rebar processing and they are also relieved to see the start of offsite processing of rebars in India by Tata Ryerson Limited. ALB's process coil directly to bent shapes. The machines incorporate a roller straightening system, a shear and a bending table (Figure 5). ALB's increase efficiency of fabrication, compared to conventional cutting and bending.

Cutting of straight bars

Bars will normally be cut to the required length in machines called Shear Lines. These machines normally consist of an unscrambler, where bundles of steel are split and loaded into the shear line, a roller table which presents the bars to the cutting shears, a hydraulically activated set of shear blades, a run-out table to take the cut bars, and pockets into which the cut lengths of steel are deposited. In an automated line, the different cuts required from each mill length can be programmed into the machine, improving efficiency and accuracy, and decreasing wastage. The minimum length that can normally be processed by a shear line is around 600mm. The cutting tolerances (as per SP: 34 of BIS) are as follows:

Dimension

 

Tolerance

For bent bars

< 75 Cm.

+3 mm-5 mm

 

> 75 <150

+5 mm-10 mm

 

> 150<250

+6 mm-15 mm

 

> 250

+7 mm-25 mm

For Straight Length

All Lengths

+25mm-25mm


The maximum tolerance specified above in IS: 2502 and SP-34 for cutting is easily achieved by these machines.
 

Bending

Bending of reinforcing bar in large projects are invariably carried out on power bending machines however the age old practice of manual bending is still being practiced in small and medium sized projects leading to time and cost overrun.

Power bending

machines are normally driven by electric motor, through a gearbox with a number of selectable speeds. Bending is achieved by placing the bar across a rotating turntable (Exhibit 2). The bending mandrel is placed at the centre of the turntable. A carrier pin, placed off-centre of the table deforms the bar continuously as the turntable is rotated, whilst the back end of the bar is restrained, by a resistance roller or stop. All rollers are normally free rotating, so as not to offer resistance to the bar, which could cause stretching. Benders may be single headed or double headed. The latter are used to speed up production of shapes having bends at both ends. During bending, the machines incorporates additional overbend to take care of the elastic return of a bent bar (a function of the grade and diameter).

Automatic Link Benders

ALB's process coil directly to bent shapes. The machines incorporate a roller straightenin g system, a shear and a bending table (Exhibit 5). ALB's increase efficiency of fabrication, compared to conventional cutting and bending. Because of the continuous nature of the operation, these machines are very useful to minimize wastage of steel and is very cost effectives, since input in coil form comes cheaper than the straight lengths. Moreover straight lengths may randomly vary between 11m to 13 m leading to uncertainty in wastage.

Production Cost

Production costs (Cutting , Bending, transport, assembly and fixing) may vary depending on the product , preparation, design of reinforcement, from 4 to 5 man-hours per tonne to 150 man-hours per tonne. In factory processed rebars this can vary between 2 to 3 man-hours depending on product and the diameter to be processed. The bent bars are bundled (Exhibit 3) and dispatched (Exhibit 4)

Developments

Although the basic processes of cutting and bending have not changed significantly, reinforcing fabricators are increasingly seeing the benefits of increased automation, computer control, and electronic data interchange. A recent innovation is the development of carpet reinforcement (Exhibit 6), consisting of a series of reinforcing bars welded to a thin steel strip. This product can be rolled up like a carpet, and then simply unrolled on site for very rapid fixing. It is claimed that these systems can give 80-90% reduction in fixing costs, and a 20-40% material saving. These systems are suitable for reinforcement of many types of slabs.



 

 

Joining Profile

TIG Welding

What is it? Tungsten inert-gas (TIG) welding, the third of the Big Three (the others are Torch and MIG) is the cleanest and most precise, but also the most expensive. In one regard it is very like MIG welding: an arc is struck between a non-consumable tungsten electrode and the work piece, shielded by inert gas (argon, helium, carbon dioxide) to protect the molten metal from contamination. But, in this case, the tungsten electrode is not consumed because of its extremely high melting temperature. Filler material is supplied separately as wire or rod. TIG welding works well with thin sheet and can be used manually, but is easily automated. Both penetration and deposition rates are much less than those of MIG welding, but precise control of the weld is easier.

Joint Type: All joint geometries.

Design Notes: Because the heating is de-coupled from the filler supply, greater control of weld conditions is possible. Thus, TIG welding is used for thin plate and for precision assemblies, made of almost any metal. Clean surfaces and well- prepared joints are important. It is principally used for thin sections and precisely made joints.

Technical Notes: TIG welding produces very high quality welds on metals such as aluminum, magnesium, titanium, stainless steel, and nickel: cast iron and mild steel are also easily welded. The arc is started by a high frequency AC discharge to avoid contaminating the tungsten electrode; it is subsequently maintained by a DC or a square wave AC which gives greater control of penetration.


 

Attributes of TIG Welding
Size of joint Unrestricted
Maximum thickness, mm 0.2-10
Unequal thickness? Limited
Join dissimilar materials? No
Impermeable? Yes
Processing temp., oc 600-2000


The Economics: The equipment is more expensive and less portable than Torch, and a higher skill level is required of the operator. But the greater precision, the wide choice of metals that can be welded and the quality of the weld frequently justify the expense.

Typical Products: TIG welding is one of the most commonly used processes for dedicated automatic welding in the automobile, aerospace, nuclear, power generation, process plant, electrical and domestic equipment markets. The Environment TIG welding requires the same precautions as any other are welding process: ventilation to prevent inhalation of fumes from the weld pool, and visors or colored goggles to protect the operator from radiation.

Competing Processes: Torch and MIG welding.

 

New way of Joining Rebars

Automatic gas pressure is a relatively new process for joining concrete-reinforcing bars. This unique system is being adopted in advanced countries. This process is increasingly being adopted to eliminate scrap generation at construction sites.

Source: Tata Ryerson Limited