Tack welding reinforced bars

A common practice during the construction of reinforced concrete structures is to prefabricate assemblies of reinforced bars on site and lift them into position by a crane. The bars may be held together by twisted wires or by tackwelds. If the joints in a welded assembly have to transmit loads in service then they should be correctly designed and welded with suitable consumables and preheating of the joint region if necessary.

Full details of the correct procedure are contained in BS 7123:1989 Metal arc welding of steel for concrete reinforcement. Tack welds which are used for locational purposes only and are not required to transmit the full tensile strength of the bar should nonetheless be made to a reasonable standard. It is important that they are fit for their purpose, whether it is to locate the reinforcing steel during handling and erection, or for subsequent full strength welding as appropriate.

Unfortunately, it has been all too common in the past for anyone on site who can strike an arc to be given the task of tack welding without regard to electrode type or size or to weld quality. This can lead to unfortunate results as shown by a typical case dealt with by TWI.

A reinforcing bar assembly was being lifted by crane and transported over a deep culvert during construction of a flyover at a road junction. The failure of a number of tack welds caused the 30m long assembly to fall into the culvert in which men were working, luckily without causing any injuries.

Inspection on site by a TWI engineer showed that many of the tack welds, including those that had failed, contained longitudinal cracks. These were typical of hydrogen cracks in reinforcing bars which are initiated at the root of the weld, propagate through the heat affected zone and into the weld close to the surface.

The sizes specified in BS 7123 for tack welds in the form of lap joints are a minimum throat thickness of 4mm and a length of at least 25mm. This requirement was met by the defective welds which had been made with basic covered electrodes but the 32mm diameter BS 4449:1979 Grade 460 bars had not been preheated before tack welding. Chemical analysis showed the steel to contain 0.28%C and 1.2%Mn with a carbon equivalent of 0.51% which is the maximum allowable CE for this grade of reinforcing bar. The welds had throat thicknesses of 6mm minimum which according to BS 5135:1984 Process of welding of carbon and carbon manganese steels corresponds to an arc energy of 2.2kJ/mm.

Tack welding lap joints in 32mm reinforcing bars of the above composition, with the weld sizes used, requires preheating to 100°C according to the recommendations of BS 7123. The lack of preheat was the cause of the extensive hydrogen cracking that occurred and the decision on the welding procedure had been taken by unqualified personnel. This occurred because the tack welds were not intended to carry any load.

The advice given by TWI can be summarised briefly as follows:

1. Welding of reinforcing bars should be carried out to BS 7123. This covers tack welding for locational purposes.
2. The design of tack welded assemblies should take account of any transportation requirements including location of lifting points for safe handling.

For more information, please email contactus@twi.co.uk