Figure 7-1.-Transit mixer.equipment. If the concrete cannot bedischarged immediately, the operatorshould turn the drum at the minimumagitating speed of 2 revolutions perminute. When the transit mixer arrives atthe project having used the minimum amountof mixing turns, the operator is able, ifnecessary, to delay dischargingtheconcrete. Delay is limited by the maximumof 300 rotations allowed.Remixing ConcreteConcrete begins to stiffen as soonas the cement and water are mixed.However, the degree of stiffening thatoccurs in the first 30 minutes is notusually a problem; concrete that is keptagitated generally can be placed within 11/2 hours after mixing.Fresh concrete left to agitate inthe mixer drum may be used if uponremixing it becomes sufficiently plasticto be consolidated in the forms. Undercareful supervision a small amount of watermay be added to remix the concreteprovided the following conditions are met:(1) maximum allowable water-cement ratiois not exceeded, (2) maximum allowableslump is not exceeded, (3) maximumallowable mixing and agitating time (ordrum revolutions) are not exceeded, and(4) concrete is remixed for at least halfthe minimum required mixing time or numberof revolutions.Adding too much water to makeconcrete more fluid should not be allowedbecause this lowers the quality of theconcrete. Remixed concrete can be expectedto harden quickly. Subsequently, a coldjoint may develop when concrete is placednext to or above the remixed concrete.Mixer CleaningAfter the load of concrete isdischarged from the mixer, the operatorshould wash off all excess concrete in themixer drum and blades, the discharge chuteopening, and the discharge chute before ithas a chance to harden. Spraying 15 to 25gallons of water into the drum while it isrotating will clean the inside of the drumas well as remove all grout which may havecollected in the water nozzle duringdischarge. A washdown hose is provided onthe mixer to clean areas accessible fromthe outside. A clean mixer produces a moresatisfactory mixing and discharge ofconcrete.At the plant, flush a minimum of150 to 250 gallons of water, dependingon the size of the mixer, into the drum.With the flush water in the drum, rotatethe drum in the mixing direction for afew minutes, then discharge the flushwater at the maximum drum rpm. Completethe cleaning of the outside of themixer, particularly around the dischargeend.7-5
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business