is 10 minutes, 70 divided by 10 equals 7. With this drum speed, the load will arrive on the jobsite with exactly 70 turns at mixing speed with no overmixing of the concrete mix and no unnecessary wear on the equipment. If the concrete cannot be discharged immediately, the operator should turn the drum at the minimum agitating speed of 2 revolutions per minute. When the transit mixer arrives at the project having used the minimum amount of mixing turns, the operator is able, if necessary, to delay discharging the concrete. Delay is limited by the maximum of 300 rotations allowed.

REMIXING CONCRETE. - Concrete begins to stiffen as soon as the cement and water are mixed. However, the degree of stiffening that occurs in the first 30 minutes is not usually a problem; concrete that is kept agitated generally can be placed within 1 1/2 hours after mixing.

Fresh concrete left to agitate in the mixer drum may be used; if upon remixing, it becomes sufficiently plastic to be compacted in the forms.

Under careful supervision a small amount of water may be added to remix the concrete provided the following conditions are met: (1) maximum allowable water-cement ratio is not exceeded, (2) maximum allowable slump is not exceeded, (3) maximum allowable mixing and agitating time (or drum revolutions) is not exceeded, and (4) concrete is remixed for at least half the minimum required mixing time or number of revolutions.

Adding too much water to make concrete more fluid should not be allowed because this lowers the quality of the concrete. Remixed concrete can be expected to harden quickly. Subsequently, a cold joint may develop when concrete is placed next to or above the remixed concrete.

Mobile Concrete Mixer Plant

The trailer-mounted mobile concrete mixer plant (fig. 3-16) carries cement, sand, and coarse aggregates in divided bins mounted on the unit. The cement is carried in a separate bin located across the rear of the unit, and the sand and aggregate are carried on each side of the unit. Water is carried in a single tank mounted in front of the aggregate bins and is pumped to the mix auger. Sand and aggregates are accurately proportioned by weight and simultaneously dropped with a mixture of cement from the material feed system into the charging end of the mix auger/conveyor at the rear of the unit. At this point, a predetermined amount of water enters the mix auger. The action of this combined auger and paddle homogenizer mixes the ingredients and water rapidly, thoroughly, and continuously to produce a continuous flow of uniformed quality concrete. The material mixing action is a continuous process that can proceed until the aggregate bins are empty. On the other hand, mixing and delivery may be stopped at any time and then started again at the will of the operator. This permits production to be balanced to the demands of the placing and finishing crews and other job requirements.

Operators assigned to the "crete mobile" must thoroughly read and understand the technical manual before operating the plant.

The following are the mobile concrete mixer plant cautions and warnings:

Follow all preventive maintenance procedures.

Do not allow any foreign matter in the cement bin.

Do not allow particles larger than 1 1/2 inch in the aggregate bin.

Do not allow the waterlines and flowmeters to freeze with water in them.

Do not run the water pump dry.

Do not continue to operate the machine if the hydraulic oil temperature exceeds 190°F.

Wash out the auger within 20 minutes of the last use.

Never attempt to operate the unit while in motion.

Never attempt to repair the machine while in operation (always turn the power source off).

Keep your entire body clear from all moving parts.

Never attempt to walk on top of the aggregate bin to cross from the cement bin to the water tank (use the ladder).

Never walk or stand under the auger.

Never climb inside the aggregate bin (use a small pole to dislodge any aggregate that has bridged). Never enter the cement bin while in operation (there are moving parts inside the bin).