PART XVII CONSTRUCTION, EXCAVATION AND DEMOLITION — Hopper signal device
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Where a concrete placing boom operator is unable to see and monitor the hopper on the concrete pump from every location the operator shall be present during the pumping activity, there shall be a device at the hopper for the concrete delivery truck driver and other workers to signal the pump operator in the event of a problem at the pump or hopper.
Not legal advice. Verify the cited article against the signed agreement.