Two workers struck by concrete placing boom when pumper

WorkSafeBC Investigations WorkSafeBC (Workers’ Compensation Board of B.C.) Page 1 of 13 NI number: 2016166960008 www.worksafebc.com

Two workers struck by concrete placing boom when pumper truck outrigger failed

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.

Purpose of this report The purpose of this online incident investigation report is to identify the causes and contributing factors of this incident to help prevent similar incidents and to support preventive actions by industry and WorkSafeBC. This online version is not the official WorkSafeBC report. It has been edited to remove personal identifying information and to focus on the main causes and underlying factors contributing to this incident.

Notice of incident information Number: 2016166960008 Outcome: Fatal × 1, Injuries × 1 Core activity: Concrete pumping Region: Lower Mainland Date of incident: March 2016


Table of Contents 1 Incident details ......................................................................................................... 3

1.1 Employer ....................................................................................................................... 3 1.2 Work process ................................................................................................................ 3 1.3 Sequence of events ........................................................................................................ 3

1.3.1 Pre-incident events ............................................................................................ 3 1.3.2 Incident ............................................................................................................. 4

1.4 Equipment ..................................................................................................................... 5 1.4.1 General .............................................................................................................. 5 1.4.2 Inspection of equipment .................................................................................... 5 1.4.3 Set-up of the pump unit..................................................................................... 6

1.5 Post-incident observations ............................................................................................ 6

2 Findings ................................................................................................................... 8 2.1 Set-up location .............................................................................................................. 8 2.2 Outrigger collar plate .................................................................................................... 8

2.2.1 Fractures in the collar plate ............................................................................... 9 2.2.2 Metallurgical analysis ....................................................................................... 9 2.2.3 Fracture analysis ............................................................................................. 10

2.3 Concrete pumper truck inspections ............................................................................. 11

3 Conclusions ............................................................................................................ 12 3.1 Cause ........................................................................................................................... 12

3.1.1 Brittle fracture of the outrigger collar plate ................................................... 12 3.2 Underlying factors ...................................................................................................... 12

3.2.1 Defective outrigger collar steel plate .............................................................. 12 3.2.2 Faulty welds during manufacturing ................................................................ 13

4 Health and safety actions ........................................................................................ 13 4.1 WorkSafeBC ............................................................................................................... 13


1 Incident details

1.1 Employer The employer is a concrete pumping company that employs approximately 20 workers, including pump operators, mechanics, and concrete placers. The firm operates a fleet of approximately 12 concrete pumper trucks (trucks with concrete pump units mounted on them) as well as one small line-pump unit. The employer had been hired to perform the concrete placing and finishing services for a multiple-unit townhome development. The employer had provided concrete pump units to this worksite four times before the incident date. This was the first time at this site for the concrete pumper truck involved in the incident. The actions, training, and experience of the workers at the site were not relevant to the causation of this incident.

1.2 Work process The construction site was the second phase in a multiple-unit townhome development. The townhomes were to be built on top of a concrete suspended slab with underground parking below. At the time of the incident, the workers were placing and finishing the concrete for the suspended slab. In preparation for the construction of the building components, formwork is constructed to hold the concrete. Once the formwork is complete, concrete is delivered to the site by way of concrete delivery trucks. These trucks back up to the pump unit, which is located on a concrete pumper truck. The concrete delivery trucks offload their concrete by way of a chute into a hopper that is located at the rear of the pump unit. Using remote pump controls, the pump operator then pumps the concrete from the hopper through a concrete placing boom, which is extended in five sections, and then through a rubber extender hose (called a whip) to where it is intended to be placed. Workers (concrete placers) on the suspended slab rake, spread, and level the concrete once it is placed. Other concrete delivery trucks wait nearby and back into place one after another to maintain a continuous supply of concrete until the pour is completed.

1.3 Sequence of events

1.3.1 Pre-incident events

On the morning of the incident, the pump operator arrived on site to set up approximately an hour before the scheduled start time. The pump operator began the set-up of the pump unit by parking the concrete pumper truck on a ramp and placing several pieces of wood (dunnage) on the ground in preparation for setting the outrigger bases. The outriggers and support pads are used to distribute and support the weight of the concrete pumper truck and to level the entire unit out. Once all four outriggers were deployed, the pump operator fully extended the placing boom horizontally across the work area that was to be poured (see Figure 1). He then installed a whip to extend the reach of the pump unit and raised the boom into the air.


Figure 1: Diagram of the overall work area. The black rectangle represents the approximate location where the concrete pump unit was set up. The blue outline indicates the suspended slab that was being poured. The red circles indicate the approximate locations of the placer and the crew foreman at the time of the incident. The first concrete delivery truck arrived on site, backed up to the rear of the pump unit, and began discharging concrete into the hopper of the pump unit. The pump operator began pumping concrete to the southeast corner of the suspended slab. Shortly afterward, the second concrete delivery truck arrived and waited for the first truck to be emptied. The pump operator was positioned on the slab, where he could see the work being done while operating the remote pump controls. The crew foreman was near the end of the placing boom (as shown above in Figure 1), manoeuvring the whip and placing the concrete onto the slab. The placer was nearby, spreading the concrete out as it was placed. Workers reported that the concrete pour was going well.

1.3.2 Incident

The incident occurred approximately 40 minutes after the pour started, while the boom was fully extended over the front right outrigger of the truck. Witnesses stated that at the time of the incident, the boom was approximately 4 to 5 feet (1.2 to 1.5 metres) above the heads of the placer and the crew foreman. While the concrete was being placed, the front right outrigger failed, which caused the concrete pump unit to tip over and the boom to drop suddenly (see Figure 2). Workers reported what was described as a “creaking” sound and shaking of the boom just before it fell. These are normal sounds and actions when pumping concrete through a fully extended placing boom. As the boom fell, it struck the placer and the crew foreman. The placer was fatally injured, and the crew foreman sustained serious, life-altering injuries.

N


Figure 2: View of the collapsed outrigger and boom. The third section of the boom came to rest on the edge of a previously poured concrete wall, thereby preventing the boom from crashing through the suspended slab and the formwork.

1.4 Equipment

1.4.1 General

The equipment involved in the incident included a concrete pump unit owned by the employer and manufactured in South Korea. The pump unit was mounted on a truck chassis. The boom comprises five sections. During travel on roadways, the boom is in the folded-up position and secured on the top of the concrete pumper truck. Once on site, the boom is unfolded with the use of hydraulics and positioned where needed.

1.4.2 Inspection of equipment

Workplace safety requirements specifically related to concrete pumping are set out in sections 20.26.1 to 20.54 of the Occupational Health and Safety Regulation. Also, section 20.26.3 of the Regulation incorporates by reference certain sections of CSA Standard Z151-09, Concrete pumps and placing booms. One of the incorporated sections is section 5.2, which requires two types of regular inspections of concrete pumps:

• Frequent inspections, conducted at intervals ranging from daily to monthly or as recommended by the manufacturer

• Periodic inspections conducted every certain number of working hours and at least once per year

Section 5.2 specifies the systems and components to be covered in each type of regular inspection and who may conduct frequent and periodic inspections. Section 20.47(2) of the Regulation requires that some aspects of a periodic inspection — including inspection of the placing boom and structural support system, as well as a visual weld inspection — be done by a professional engineer.


An engineering firm had routinely done the periodic inspections of the concrete pumper truck involved in the incident. This firm provides engineering and certification consulting services for a variety of cranes and other machinery, including concrete pumper trucks. The engineering firm completed a periodic inspection of the pump unit about two months before the incident, as shown by a decal affixed to the inside of the driver’s door of the concrete pumper truck. In addition to the inspection decal, the firm produced a written inspection report titled “Regular Concrete Pumper Inspection,” which was dated, signed, and stamped by a professional engineer about two weeks after the inspection.

1.4.3 Set-up of the pump unit

The pump unit was set up with the use of four outriggers. The two front outriggers extended linearly, while the two rear outriggers swung out into position. A safety label depicting the pump unit and outrigger configuration was attached to the inside of the door where the pump valves were located on the passenger side. The label showed the wrong configuration (see Figure 3). Specifically, the safety label showed a pump unit with two swing-out-type outriggers on the front and two telescoping outriggers on the rear. This label had been affixed by the manufacturer.

Figure 3: Safety label on the pump unit, depicting the wrong outrigger configuration.

1.5 Post-incident observations The day after the incident, the concrete pumper truck was taken to a secure location for material testing of the pump unit’s failed components by an independent engineering firm contracted by WorkSafeBC (not the firm that had previously inspected the pump unit). As shown in Figures 4 and 5, the outrigger collar plate had fractured (torn and ripped), causing the front right outrigger to fail and the boom to fall.


Figure 4: The fractured outrigger collar plate. The inset photograph shows the outrigger collar plate intact.

Figure 5: Close-up view of the fractured collar plate.


In addition, it appeared that the metal plate and welds had failed (see Figure 6).

Figure 6: Close-up view of the collar plate. The red lines in the inset photograph show the fillet weld failures on the right side of the outrigger tunnel assembly.

2 Findings

2.1 Set-up location Following the incident, the ramp location where the pump unit was set up was examined and measured. This location had been used in the recent past for setting up other concrete pump units. The ground supporting the outriggers showed no evidence of failure or instability. Immediately following the incident, WorkSafeBC commissioned a survey contractor to take detailed measurements of the ground location and concrete pump unit. The ground in this area sloped 4 degrees 1 minute (4°1'), or 4.017°, toward the front of the truck, and sloped crosswise 0°55' (0.917°). The truck had a lengthwise slope to the front of 1°3' (1.056°) and crosswise of 0°56' (0.943°). The manufacturer permits a 3° operational limit. Although the set-up location was on a slight slope, it was determined that the concrete pump unit was properly levelled with the outriggers. Statements given following the incident also indicated that the pump unit was appropriately set up for the work being done and was operating within its capacity.

2.2 Outrigger collar plate An independent engineering firm contracted by WorkSafeBC examined and conducted testing on the front right outrigger’s metal components. This analysis included conducting a metallurgical failure analysis and other similar testing to determine the mechanism of the failure of the outrigger collar plate.


Additionally, testing was conducted on other critical components of the pump unit. This was to enable engineers to determine if those other components had similar characteristics to the metal and strength properties of the failed section of the front right outrigger collar plate.

2.2.1 Fractures in the collar plate

The collar plate was 2 inches (51 millimetres) thick. Figures 7 and 8 show the locations of the fractures in the collar plate.

Figure 7: Diagram showing the size and dimensions of the fractured collar plate. The red lines show the fracture locations.

Figure 8: Fractured outrigger collar plate reassembled to show the fracture locations.

2.2.2 Metallurgical analysis

Laboratory examinations included taking samples of the failed structural components to determine material properties and fracture modes, and a fracture toughness analysis of the failed component. The fractured collar plate pieces were analyzed using a direct reading optical emission spectrometer. The collar plate that failed was a low-alloy, medium-carbon steel plate containing manganese, chromium, and molybdenum, but no nickel. Therefore, it did not meet any North American grade of steel.


WorkSafeBC investigators obtained the mill test certificates (a certification of the chemical composition and mechanical properties of the steel plate) from the supplier of the concrete pump. The supplier imports concrete pumps from South Korea and ships them to various locations throughout the world. The mill test certificates from the supplier were presented as being for this pump unit, which came from the steel manufacturer in South Korea. The steel components that were tested from other areas of the pump unit were successfully matched — by chemical composition and mechanical properties — to the steel plates that were originally ordered for the manufacturing of this pump unit. Analysis shows that the steel of the fractured collar plate is very brittle. The independent engineer’s report states that during laboratory testing, the steel showed “very brittle behaviour at +10°C with transitioning to ductile [malleable] behaviour just beginning at +20°C. These temperatures are considered very high for full brittle fracture behaviour of a quench and tempered low alloy steel. Full ductile behaviour was observed at +96°C, which again is considered to be very high.” The only component on the pumper truck that exhibited brittle behaviour at warm temperatures (+10°C) was the fractured collar plate on the front right outrigger. The other tested components — including the left side outriggers and outrigger collar components — have a chemical composition and strength properties, including impact energy levels (brittleness), that correspond with those on the mill test certificates. WorkSafeBC investigated previous owners and insurance records to identify any previous damage and repair work that might have been done on this outrigger assembly. No evidence was found of any replacement or repair work involving the outrigger collar plates. The low fracture toughness/impact energy of the collar plate material most likely resulted from improper heat treatment by the steel manufacturer — in particular, too slow of a cooling rate during quenching. This condition was only found in the material of the collar plate that failed.

2.2.3 Fracture analysis

The fractures shown in Figure 8 (above) were analyzed. The engineer’s report states the following (see Figure 9 below):

The [fracture A] side surface is shown [on the left] in [Figure 9]. The fracture is brittle in appearance and contains several weld defects at the fracture origin. The origin area is a node for several plate welds and contains slag inclusions, incomplete penetration, and lack of fusion…

The [fracture B] side surface is shown [on the right] in [Figure 9] after rust removal. This fracture also has a brittle appearance and a single fracture origin (shown at arrow).

Figure 9: The fracture A side of the collar plate is shown on the left. The fracture B side is shown on the right.


Analysis shows that fracture B occurred first, followed by fracture A. A close-up view of both fracture sides shows slag and lack of fusion at the base of the weld at the fracture origins. Slag is a by-product of welding, a deoxidized piece of material; the foreign material trapped in the weld decreased the strength and provided the starting location of the cracks. Lack of fusion is incompletely fused spots within the weld between the two materials to be joined — no full adhesion, no joining of the two plates welded together. The slag and lack of fusion resulted in reduced strength of the welds and created the starting location for the cracks. The origin area of the fracture on the fracture B side shows no evidence of progressive fracture that would be caused by cyclic loading in service; in other words, the cause of fracture B was not fatigue. Fracture B began as a pre-crack that was 4 millimetres (just over 1⁄8 inch) deep. This pre-crack was caused by welding residual stresses and probably occurred within 48 hours of the collar plate being welded to the outrigger tunnel. The engineering report explains that the pre-crack existed for months or years before the incident:

The pre-crack that formed the brittle fracture origin was sectioned, polished, and etched (2% Nital) [a solution of alcohol and nitric acid] to reveal the nature of the cracking. The crack formed at the toe of the weld in the heat-affected zone. This is a location most susceptible to cold cracking (often delayed by up to 48 hours) during the welding process. The crack origin area is filled with oxide, indicating it has been exposed to the atmosphere for a relatively long period of time (months or years).

The initial crack influenced the development of additional cracks in the plate. The material would have likely taken the stress imposed on it by the operation of the pump unit. Thick black oxide layers at the fracture cross-sections indicate that the collar plate had significant-sized cracks for months or years. These cracks, in combination with the extremely low level of fracture toughness of the plate, resulted in the failure of the plate under load at the time of the incident. The low impact energy/fracture toughness of the steel plate accelerated the time frame of the failure. A critical stress calculation that included the 4-millimetre pre-crack showed that the collar plate would suffer a sudden brittle fracture at a stress level of 45,000 PSI, which is less than 35% of its measured tensile strength (breaking strength) of 131,500 PSI. The independent engineering report states: “The collar should have been able to support the load… had the pre-crack not been present.”

2.3 Concrete pumper truck inspections WorkSafeBC investigators examined the concrete pumper truck’s inspection certification records for approximately six and a half years before the incident. These records consist of a checklist form that was completed by a technician for the engineering firm that had done the inspections. As part of the checklist form, the pump unit’s safety label was listed as an inspection item. This particular pump unit’s safety label was of the wrong configuration that showed a pump unit with two swing-out-type outriggers on the front and two telescoping outriggers on the rear (shown above in Figure 3). This inaccuracy was not noted or addressed in the inspections. Following each inspection, the form was signed, stamped, and sealed by an engineer. The engineering firm provided no photographs or field notes in relation to the inspections done in the three years prior to


the incident. The engineering firm advised WorkSafeBC that photographs are taken during inspections only if there is a problem to be repaired.

The independent engineering firm that conducted the post-incident testing concluded that the pre-crack present in the collar plate’s welded joint could not have been detected visually or with magnetic particle inspection techniques prior to the collar plate failure occurring unless significant dismantling of the outrigger assembly was done. The pre-crack could likely have been detected with ultrasonic testing techniques if the joint configuration was known (from the original manufacturer’s drawing) and a calibration standard with the exact joint configuration existed.

Although this concrete pumper truck had been inspected periodically through its operational history, these certification inspections were inadequate and not completed as required by the Regulation and CSA Standard Z151-09. The requirements include having the physical inspection done by a professional engineer and having the documentation to reflect that the inspection was conducted in accordance with the CSA standard. These two requirements were not met.

Although these inspections were not conducted in accordance with the CSA standard, the investigation determined that this was not a factor in the incident.

Additionally, these inspections were not conducted according to the engineering firm’s own post-incident recommendations to the company that conducts periodic repairs to some of the employer’s concrete pump units. In an opinion letter dated a few days after the incident, the engineering firm that did the pre-incident inspections recommended removing all external panels that restrict inspections of all outrigger box welds. This document also states that it may be necessary to remove the outriggers to conduct an effective inspection. These procedures were not conducted when this pump unit underwent its last certification inspection.

3 Conclusions

3.1 Cause

3.1.1 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.

3.2 Underlying factors

3.2.1 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.


3.2.2 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.

4 Health and safety actions Employers, workers, or others in industry may have taken measures to prevent a recurrence of this type of incident. WorkSafeBC examines recommendations from incident investigations to see what can be done to prevent similar incidents.

4.1 WorkSafeBC At the time of the incident, WorkSafeBC generated notice of incident 2016166960008, detailing the facts collected immediately after the incident.

In October 2016, WorkSafeBC published WorkSafeBC Bulletin WS 2016-02 titled “Concrete pumper trucks: Incident results in one death, one serious injury.” This bulletin is available at worksafebc.com. It reminds the concrete pumping industry to:

• Follow manufacturer’s instructions for operating and maintaining outriggers and booms on concretepumper trucks.

• Regularly inspect all welds and stress points on outriggers and booms.• Position outriggers according to manufacturer’s instructions and based on soil stability.

Copyright

© 2017 Workers’ Compensation Board of British Columbia. All rights reserved. WorkSafeBC (Workers’ Compensation Board of B.C.) encourages the copying, reproduction, and distribution of publications to promote health and safety in the workplace, provided that WorkSafeBC is acknowledged. However, no part of this publication may be copied, reproduced, or distributed for profit or other commercial enterprises or may be incorporated into any other publications or product without written permission of WorkSafeBC.

Documents

Two workers struck by concrete placing boom when pumper