Position: Finally, multiply the 600 by 0.25 to determine the volume of concrete required for the pad which, in this case, is 150 cubic feet by a concrete batch mixer . Concrete is ordered and produced in quantities of cubic yards. To calculate the number of cubic yards required for the pad, divide the cubic feet of the pad by 27. This is required because there is 27 cubic feet in 1 cubic yard. Therefore, the concrete pad described in the previous paragraph, which has a volume of 150 cubic feet, requires 5.56 cubic yards of concrete: 150 cubic feet divided by 27 = 5.56 cubic yards. Concrete projects often present varying degrees of difficulty; therefore, extra concrete is required to compensate for these difficulties. Once the total number of cubic yards of concrete is computed, add a little extra, normally 10 percent, to compensate for waste. To calculate the excess needed, multiply the cubic yards by .10 (10 percent). In the above case, multiply 5.56 cubic yards by .10 to get 0.556 cubic yards. Add the 0.556 cubic yards to the 5.56 cubic yards for a total of 6.116 or 6.12 cubic yards required for the concrete pad.
BATCHING CONCRETE
Batching control is the process of weighing or volumetricall measuring and introducing into a concrete batch mixer the ingredients for a batch of concrete. To produce a uniform quality concrete mix, measure the ingredients accurately for each batch. Most concrete specifications require that the batching be performed by weight, rathcr than by volume, because of inaccuracies in measuring aggregate, especially damp aggregate. Water and liquid air-entraining admixtures can be measured accurate]y by either weight or volume. Batching by using weight provides greater accuracy and avoids problems created by bulking of damp sand. Volumetric batching is used for concrete mixed in a continuous batch mixer, and the mobile concrete mixer (crete mobile) where weighing facilities are not at hand.
Specifications generally require that materials be measured in individual batches within the following percentages of accuracy: cement 1%, aggregate 2%, water 1%, and air-entraining admixtures 3%. Equipment within the batching plant should be capable of measuring quantities within these tolerances for the smallest to the largest batch of concrete produced. The weighing accuracy of the batching equipment must be checked and adjusted when neccessary. Mixing Concrete Concrete should be mixed until it is uniform in appearance and all the ingredients are evenly distributed. Concrete batch mixers should not be loaded above their rated capacities and should be operated at approximately the speeds for which they were designed. If the blades of the mixer become worn or coated with hardened concrete, the mixing action will be less efficient. Worn blades should be replaced and the hardened concrete removed periodically, preferably after each production of concrete. When a transit batch mixer (TM) (fig. 7-1) is used for mixing concrete, 70 to 100 revolutions of the drum at the rate of rotation designated by the manufacturer as mixing and batching speed are usually required to produce the specified uniformity. No more than 100 revolutions at mixing and batching speed should be used. All revolutions after 100 should be at a rate of rotation designated by the manufacturer as agitating speed. Agitating speed is usually about 2 to 6 revolutions per minute, and mixing and batching speed is generally about 6 to 18 revolutions per minute.
Mixing for long periods of time at high batching speeds, about 1 or more hours, can result in concrete strength loss, temperature rise, excessive loss of entrained air, and accelerated slump loss. Concrete mixed in a transit mixer should be delivered and discharged within 1 1/2 hours or before the drum has revolved 300 times after the introduction of water to cement and aggregates or the cement to the aggregates. Concrete batch mixers and agitators should always be operated within the limits of the volume and speed of rotation designated by the equipment manufacturer. Overmixing Concrete Overmixing concrete damages the quality of the concrete, tends to grind the aggregate into smaller pieces, increases the temperature of the mix, lowers the slump, decreases air entrainment, and decreases the strength of the concrete. Also, overmixing puts needless wear on the drum and blades of the transit batch mixer. To select the best mixing and batching speed for a load of concrete, estimate the travel time to the project (in minutes) and divide this into the minimum desired number of revolutions at mixing speed-70. The results will be the best drum speed; for instance, if the haul is 10 minutes, 70 divided by 10 equals 7. With this drum batching speed, the load will arrive on the jobsite with exactly 70 turns at batching speed, with no overmixing of the concrete mix and no unnecessary wear on the weighing.
