Date of incident: March 2016
Notice of incident number: 2016166960008
Employer: Concrete pumping company

Incident summary

At a construction site, workers were pouring and placing concrete on a suspended slab. The concrete placing boom of the concrete pumper truck was fully extended. The outrigger of the pumper truck failed and the boom came down, striking two workers. One worker was fatally injured, and the other sustained serious, life-altering injuries.

Investigation conclusions

Cause

• Brittle fracture of the outrigger collar plate: The concrete pumper truck’s front right outrigger collar plate suffered a brittle fracture through its entire cross-section due to a pre-crack in a stationary weld zone that had been present since the concrete pump unit was manufactured. This fracture led to a second fracture in the collar plate, and the concrete placing boom subsequently fell. The boom struck two workers, causing fatal injuries in one worker and serious, life-altering injuries in the other worker.

Underlying factors

• Defective outrigger collar steel plate: The steel plate material that the manufacturer installed for the outrigger collar assembly had inadequate fracture toughness/impact energy for the intended loads. This likely resulted from improper heat treatment by the steel manufacturer.
• Faulty welds during manufacturing: During the manufacturing of the pump unit, the collar plate was faultily welded to the outrigger tunnel assembly, resulting in slag inclusions, incomplete penetration, and lack of fusion. Within 48 hours after the welding process, a 4-millimetre-deep crack developed, caused by welding residual stresses. This initial crack, in combination with the extremely low level of fracture toughness of the collar plate, resulted in the collar plate’s failure at the time of the incident.