Ready mix concrete usually goes from the mixing site to its final resting place in a mixer truck. This method of delivering concrete has been around since the 1920s, and the effectiveness of this method has contributed to its longevity. The mixer truck has a rotating mixer drum set at an angle. Inside the shell of the mixer drum is a set of blades or fins that wrap in a spiral configuration from the head to the opening of the drum. Having these spiraling fins allows the concrete to load and mix when the drum goes one direction and to discharge when the drum goes the other way.
To load raw materials into the truck, the drum has to be moving very fast in the loading direction. The concrete is mixed and loaded, then hauled to the job site with the drum rotating at between 2 and 6 rotations per minute. This helps make sure it remains at optimum consistency when it arrives at the job site.
Mixer trucks keep ready mix concrete at the right consistency even when the concrete is mixed off site, but sometimes they do all the mixing while delivering. This solves the problem of premature hardening or slump loss that can result from delays between mixing and transportation. It also allows concrete to be hauled to sites further away from the plant.
While most trucks discharge concrete at the back, front-discharge units are becoming increasingly popular. With the increased visibility that front-discharge units offer, the driver can take the truck directly onto the site and direct the discharge chute without having to rely on anyone else.
Drums are designed to be filled at between 60 and 80 percent capacity, depending whether the drum is acting as mixer or agitator. If concrete isn’t transported in time or is exposed to significant temperature changes, it may experience a change in consistency. Water can be added if the consistency is too thick, but it should be added all at once and the drum of the mixer truck should be turned at least 30 revolutions at mixing speed. Concrete should be discharged within 90 minutes and before 300 revolutions after water was added, unless the purchaser specifies otherwise. Some conditions may require air-entraining, air-reducing, or set-retarding admixtures to compensate for loss of air, high temperatures, or long delivery times. A 5 percent air entrainment, for example, traps microscopic air bubbles to help the concrete handle expansion or shrinking caused by temperature changes
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