When I arrived on site at 7:20am on June 6, 2012, the concrete pump truck was backed up in the alley with the pump arm looming high above the cistern. The concrete crew was finalizing the prep work. The 9” thick concrete slab is the lid of the 35,000 gallon cistern that will hold the collected rainwater for all the domestic water use at Desert Rain. The lid also serves as the parking floor for the garage.
The concrete started to spew from the large pump hose. The crew efficiently began screeding the concrete. In this process, straightedges are used to remove excess concrete and bring the top surface of the concrete to the proper, previously marked grade line This slab is level on the west end of the cistern where there are intake and mechanical hatches, then slopes gently on the east end. During the screeding process, the crew used a concrete vibrator. Right after placement, concrete contains up to 20% trapped air. The amount varies according to the mix of the ‘slump’, the placement method, size of the form and the amount of reinforcing steel used. Concrete vibration consolidates the concrete by moving the concrete particles, then removing entrapped air. Vibration helps settle the concrete and allow it to flow more readily into corners and around the rebar. This eliminates voids or ‘honeycombs’ and brings more of the paste to the surface to assist in finishing. Since concrete flows better with vibration, the mix can contain less water, providing greater structural integrity in the finished product. The screeding and vibrating crew were followed by Keith with a concrete float. The float forces the aggregate down and raises the cream (the gravel-free concrete) to the surface for finishing. The slab would be firm enough to walk on in a few hours when Keith would make cuts in the slab to relieve stress in the concrete and help control cracking.
Keith and his crew (Jeff, Chris and David) demonstrated their experience as the 90,000 pounds of concrete arrived in three, consecutive trucks and was pumped into the slab. In a little over an hour, the forms were filled, screeded and floated. The cistern, now has a lid and the garage, now has a floor. Nice work guys!