to get 600. Finally, multiply the 600 by 0.25 to determinethe volume of concrete required for the pad which, inthis case, is 150 cubic feet.Concrete is ordered and produced in quantities ofcubic yards. To calculate the number of cubic yardsrequired for the pad, divide the cubic feet of the pad by27. This is required because there is 27 cubic feet in 1cubic yard. Therefore, the concrete pad described in theprevious paragraph, which has a volume of 150 cubicfeet, requires 5.56 cubic yards of concrete: 150 cubicfeet divided by 27 = 5.56 cubic yards.Concrete projects often present varying degrees ofdifficulty; therefore, extra concrete is required tocompensate for these difficulties. Once the total numberof cubic yards of concrete is computed, add a little extra,normally 10 percent, to compensate for waste. Tocalculate the excess needed, multiply the cubic yards by.10 (10 percent). In the above case, multiply 5.56 cubicyards by .10 to get 0.556 cubic yards. Add the 0.556cubic yards to the 5.56 cubic yards for a total of 6.116or 6.12 cubic yards required for the concrete pad.BATCHING CONCRETEBatching is the process of weighing orvolumetricall y measuring and introducing into a mixerthe ingredients for a batch of concrete. To produce auniform quality concrete mix, measure the ingredientsaccurately for each batch. Most concrete specificationsrequire that the batching be performed by weight, rathcrthan by volume, because of inaccuracies in measuringaggregate, especially damp aggregate. Water and liquidair-entraining admixtures can be measured accurate]yby either weight or volume. Batching by using weightprovides greater accuracy and avoids problems createdby bulking of damp sand. Volumetric batching is usedfor concrete mixed in a continuous mixer, and themobile concrete mixer (crete mobile) where weighingfacilities are not at hand.Specifications generally require that materials bemeasured in individual batches within the followingpercentages of accuracy: cement 1%, aggregate 2%,water 1%, and air-entraining admixtures 3%.Equipment within the plant should be capable ofmeasuring quantities within these tolerances for thesmallest to the largest batch of concrete produced. Theaccuracy of the batching equipment must be checkedand adjusted when neccessary.Mixing ConcreteConcrete should be mixed until it is uniform inappearance and all the ingredients are evenlydistributed. Mixers should not be loaded above theirrated capacities and should be operated atapproximately the speeds for which they were designed.If the blades of the mixer become worn or coated withhardened concrete, the mixing action will be lessefficient. Worn blades should be replaced and thehardened concrete removed periodically, preferablyafter each production of concrete.When a transit mixer (TM) (fig. 7-1) is used formixing concrete, 70 to 100 revolutions of the drum atthe rate of rotation designated by the manufacturer asmixing speed are usually required to produce thespecified uniformity. No more than 100 revolutions atmixing speed should be used. All revolutions after 100should be at a rate of rotation designated by themanufacturer as agitating speed. Agitating speed isusually about 2 to 6 revolutions per minute, and mixingspeed is generally about 6 to 18 revolutions per minute.Mixing for long periods of time at high speeds, about 1or more hours, can result in concrete strength loss,temperature rise, excessive loss of entrained air, andaccelerated slump loss.Concrete mixed in a transit mixer should bedelivered and discharged within 1 1/2 hours or beforethe drum has revolved 300 times after the introductionof water to cement and aggregates or the cement to theaggregates. Mixers and agitators should always beoperated within the limits of the volume and speed ofrotation designated by the equipment manufacturer.Overmixing ConcreteOvermixing concrete damages the quality of theconcrete, tends to grind the aggregate into smallerpieces, increases the temperature of the mix, lowers theslump, decreases air entrainment, and decreases thestrength of the concrete. Also, overmixing puts needlesswear on the drum and blades of the transit mixer.To select the best mixing speed for a load ofconcrete, estimate the travel time to the project (inminutes) and divide this into the minimum desirednumber of revolutions at mixing speed-70. The resultswill be the best drum speed; for instance, if the haul is10 minutes, 70 divided by 10 equals 7. With this drumspeed, the load will arrive on the jobsite with exactly 70turns at mixing speed, with no overmixing of theconcrete mix and no unnecessary wear on the7-4
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