Date of incident: March 2021
Notice of incident number: 2021182170010
Employers: Property owner; contractor; concrete pumping company; concrete pump repair company; engineering inspection and testing company

Incident summary
At a residential construction site, three people (who did not meet the definition of workers under the Workers Compensation Act) were placing concrete for foundation walls for a new house. Also on site were two workers employed by a concrete pumping company. One worker operated a concrete pump truck, equipped with a 38 m boom. The other operated a concrete mixer truck. During pumping operations, a component on the concrete pump boom suddenly underwent a catastrophic failure and the boom fell onto two of the people placing concrete under it. The two people sustained fatal injuries.

Investigation conclusions

Causes

• Brittle fracture of turning column. The turning column on the concrete pump’s boom suffered a brittle fracture through its entirety due to a shallow crack or localized stress in a fillet weld between the column and the lower horizontal assembly plate. At the time of the incident, the boom was at its maximum possible extension. The critical load placed upon it by this position is believed to have triggered the sudden brittle cleavage fracture. The entire boom fell to the ground.
• Poor fracture toughness of shaft’s steel. The steel shaft of the turning column was found to have extremely poor fracture toughness (ability to resist further cracking once an initial crack is started). The manufacturer’s mill certificate form for this steel had a field to record the steel’s fracture toughness, but the field was blank. The owner of the machine and the people who inspected and repaired the machine were not aware of this property.
• Inadequate repair. The concrete pump’s boom was seriously damaged in a misadventure four months prior to the fatal incident. The significance of the damage to the turning column was not adequately identified when weld repairs were made to the turning column following the misadventure. Important aspects of the repair were not adequately assessed or carried out. Any crack on the turning column shaft should have been considered very serious, given the brittle properties of the shaft’s steel and the extent of the damage to the boom. The worker who made the repairs was not provided adequate information or instruction about the steel or the required repair.

Contributing factors

• Inadequate manufacturer’s information regarding inspection and repair. The existing poor fracture toughness property of the turning column had not been communicated to individuals involved in the inspection or repair of the part. There was not adequate manufacturer’s information about inspection and repair or criteria for removal from service following misadventure, overloading, or damage to the turning column.
• Inadequate engineering inspection and certification following misadventure. The engineering inspection of the concrete pump’s boom was incorrectly identified as an annual inspection rather than as a certification after repair. No instructions for repair were provided and no inspection was made following the repair.
• Inadequate communication of history of concrete pump boom. Individuals responsible for making decisions about the inspection and repair of the concrete pump’s boom did not have a thorough understanding of the history of the boom, including how it was damaged and the extent of the damage. The details of misadventure were not included in the concrete pump’s inspection and maintenance log.
• Inadequate safe work practices. The people involved in the placing of concrete with the concrete pump did not follow established industry safe work practices such as the use of personal protective equipment and avoidance of working directly under the boom.

Other safety issue

• Inadequate pre-use inspection of boom. Although the concrete pumping company’s truck operators performed daily pre-use inspections on the concrete pump truck, the checklist they used to do so did not have any fields for the boom structure or the concrete pump apparatus. These components should have been part of the pre-use inspection protocol. Such an inspection would have been visual and confined only to locations visible to the operator. Therefore, since the failure location was hidden, this safety issue was not causal to the incident.

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