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90-6.02 MACHINE MIXING <br /> • Concrete mixers may be of the revolving drum or the revolving blade type,and the mixing drum or blades shall be <br /> operated uniformly at the mixing speed recommended by the manufacturer. Mixers and agitators that have an accumulation <br /> of hard concrete or mortar shall not be used. <br /> • The temperature of mixed concrete, immediately before placing, shall be not less than 10°C or more than 32°C. <br /> Aggregates and water shall be heated or cooled as necessary to produce concrete within these temperature limits. Neither <br /> aggregates nor mixing water shall be heated to exceed 65°C. If ice is used to cool the concrete,discharge of the mixer will <br /> not be permitted until all ice is melted. <br /> • The batch shall be so charged into the mixer that some water will enter in advance of cementitious materials and <br /> aggregates. All water shall be in the drum by the end of the first one-fourth of the specified mixing time. <br /> • Cementitious materials shall be batched and charged into the mixer by means that will not result either in loss of <br /> cementitious materials due to the effect of wind, in accumulation of cementitious materials on surfaces of conveyors or <br /> hoppers,or in other conditions that reduce or vary the required quantity of cementitious material in the concrete mixture. <br /> • Paving and stationary mixers shall be operated with an automatic timing device. The timing device and discharge <br /> mechanism shall be interlocked so that during normal operation no part of the batch will be discharged until the specified <br /> mixing time has elapsed. <br /> • The total elapsed time between the intermingling of damp aggregates and all cementitious materials and the start of <br /> mixing shall not exceed 30 minutes. <br /> • The size of batch shall not exceed the manufacturer's guaranteed capacity. - <br /> • When producing concrete for pavement or base, suitable batch counters shall be installed and maintained in good <br /> operating condition at job site batching plants and stationary mixers. The batch counters shall indicate the exact number of <br /> batches proportioned and mixed. <br /> • Concrete shall be mixed and delivered to the job site by means of one of the following combinations of operations: <br /> A. Mixed completely in a stationary mixer and the mixed concrete transported to the point of delivery in truck agitators <br /> or in nonagitating hauling equipment(central-mixed concrete). <br /> B. Mixed partially in a stationary mixer,and the mixing completed in a truck mixer(shrink-mixed concrete). <br /> C. Mixed completely in a truck mixer(transit-mixed concrete). <br /> D. Mixed completely in a paving mixer. <br /> • Agitators may be truck mixers operating at agitating speed or truck agitators. Each mixer and agitator shall have <br /> attached thereto in a prominent place a metal plate or plates on which is plainly marked the various uses for which the <br /> equipment is designed, the manufacturer's guaranteed capacity of the drum or container in terms of the volume of mixed <br /> concrete and the speed of rotation of the mixing drum or blades. <br /> • Truck mixers shall be equipped with electrically or mechanically actuated revolution counters by which the number <br /> of revolutions of the drum or blades may readily be verified. <br /> • When shrink-mixed concrete is furnished, concrete that has been partially mixed at a central plant shall be <br /> transferred to a truck mixer and all requirements for transit-mixed concrete shall apply. No credit in the number of <br /> revolutions at mixing speed will be allowed for partial mixing in a central plant. <br /> 90-6.03 TRANSPORTING MIXED CONCRETE <br /> • Mixed concrete may be transported to the delivery point in truck agitators or truck mixers operating at the speed <br /> designated by the manufacturer of the equipment as agitating speed, or in nonagitating hauling equipment, provided the <br /> consistency and workability of the mixed concrete upon discharge at the delivery point is suitable for adequate placement and <br /> consolidation in place, and provided the mixed concrete after hauling to the delivery point conforms to the provisions in <br /> Section 90-6.01, "General." <br /> • Truck agitators shall be loaded not to exceed the manufacturer's guaranteed capacity and shall maintain the mixed <br /> concrete in a thoroughly mixed and uniform mass during hauling. <br /> • Bodies of nonagitating hauling equipment shall be constructed so that leakage of the concrete mix, or any part <br /> thereof,will not occur at any time. <br /> • Concrete hauled in open-top vehicles shall be protected during hauling against rain or against exposure to the sun for <br /> more than 20 minutes when the ambient temperature exceeds 24°C. <br /> • No additional mixing water shall be incorporated into the concrete during hauling or after arrival at the delivery <br /> point,unless authorized by the Engineer. If the Engineer authorizes additional water to be incorporated into the concrete,the <br /> drum shall be revolved not less than 30 revolutions at mixing speed after the water is added and before discharge is <br /> commenced. <br /> • The rate of discharge of mixed concrete from truck mixer-agitators shall be controlled by the speed of rotation of the <br /> drum in the discharge direction with the discharge gate fully open. <br /> Contract No. 10-OM7104 <br /> 82 <br />