To supply uniform, high-quality concrete, producers must keep ready mix trucks in prime operating condition and follow good mixing and delivery practices. The Truck Mixer Manufacturers Bureau (TMMB) provides standards for truck mixers. Concrete should be completely mixed in the truck for 70 to 100 revolutions, and discharged within 1 « hours or before the drum has reached 300 revolutions. Problems such as head packs and cement balls may occur and are usually a result of problems with batching and mixing. They can have adverse effects on mix performance. Typically, a rear-discharge truck can place concrete into forms about 4 feet high. A front-discharge truck can place a flowable mix into a 6 «-foot-high wall.