Abstract: At approximately 2:00 p.m. on May 3, 2002, Employee #1 was working with three other employees clearing concrete out of a concrete pump hose. Employee #1 and his coworkers worked for NMN Construction, DBA Largo Concrete Inc., located at 1801 Edinger Avenue, Suite 200 in Santa Ana, California and were working on a construction site at 13700 Marina Point Drive in Marina Del Rey, California. The hose was being used on the 16th floor of a new condominium complex. Once pumping was complete the 5-in. hose had to be backflushed to prevent the concrete from hardening inside of it. Due to the height of the floor being poured the hose held between 3 and 4 cubic yards of wet concrete, weighing about 16,200 pounds. In order to accomplish the backflush, a steel fitting, that weighs about 15 pounds and contains chicago air valve fittings and pressure relief fittings, was attached to the distal end of the hose and a wet foam ball was inserted into the end of the hose. Then air pressure was introduced into the hose by way of the chicago (crowsfoot) fitting at approximately 125 psi. This was intended to force the residual concrete back down the hose. This accident occurred when the foam ball quit moving. The employees attempted to remove the backflush fitting without first opening the pressure relief valve on the hose and the hose exploded. Employee #1 was hospitalized.