Development of Standard MIG Welding Procedure for Cage Welding at NSH, Singapore

by Srimanta Sam, B N Sen, Eng Poh Tzan, Yap Leong Soon

Publisher - Tata Steel Ltd., NatSteel Holdings Pte Ltd

Category - General Novel

In Reinforced Concrete Construction, off-site assembly of reinforcement bars that are transported for subsequent installation on site before concrete casting is gaining popularity. This not only reduces construction time but also offers other advantages like improved on site labour productivity, avoidance of site congestion, better quality of caged reinforcement, reduced wastage etc. Many such assemblies use MIG spot welding between transverse rebars, which is superior to tying with wires in retaining the shape of the cage till fixing/casting on site. However, a problem often encountered during stacking, lifting, transportation or fixing is the sporadic failure of weld joints. The wide variation in stacking conditions as well as lifting mechanisms commonly deployed on sites does influence the nature of load on the joints. In case of such failures, rework of the assembly jeopardizes the construction schedule and adds unnecessary cost. Apparently, no known standard guideline/specification is available for welding such cages as strength of these welds is not part of the structural engineering design. A study was made to map the weld joint characteristics vis-à-vis strength from the current welding practices for cage welding at NatSteel, Singapore with various combinations of diameters of rebar welded together. In 3 combinations of diameters studied, it was observed that weld length has the most influence on the strength of the joints, and the depth of weld penetration beyond a threshold value does not offer any significant benefit. The direction of force relative to the weld on the rebars also determines the strength of the welded joints. These findings will be used in determining the points & orientation of welds within a reinforcement cage.

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