days), the cylinder is CAPPED on both ends with athin layer of gypsum CASTING PLASTER or sulfurCAPPING COMPOUND. For testing, the cylinder isplaced under the piston of a machine capable ofapplying a very high pressure (for a 6-inch-diametercylinder with a compressive strength of about 6,000pounds per square inch, the rupturing pressure mustreach about 170,000 pounds). Pressure is applied andincreased until the cylinder collapses.FLEXURAL TESTThe FLEXURAL strength is its ability to resist abreaking force. The flexural strength of concrete isconsiderably less than its compressive strength. For aflexural strength test, a TEST BEAM, cast in a TESTBEAM MOLD, like the one shown in figure 3-14, iscured and then broken by a BEAM BREAKER.The test specimen must be formed with its longaxis horizontal. The concrete must be placed in layersapproximately 3 inches in depth, and each layer mustbe rodded 50 times for each square foot of area. Thetop layer must slightly overfill the mold. After eachlayer is rodded, the concrete must be spaded along thesides and ends with a mason’s trowel or other suitabletool. When the rodding and spading operations arecompleted, the top must be struck off with a straightedgeand finished with a wood float. The test specimen mustbe made promptly and without interruption. Test beamsshould be cured for a period of 28 days. Like cylinders,the flexural strength may be determined after 7 days,utilizing the probable 28 day strength of concrete.COMPUTING CONCRETETo compute the volume of concrete required for aconcrete pad, multiply the length of the pad by itswidth times its thickness to get cubic feet (length xwidth x thickness). For example, a concrete pad is 20feet in length by 30 feet in width and has a slabthickness of 4 inches. You first convert the 4 inchesinto feet by dividing 3 by 12 to get 0.333 feet. Next,Figure 3-14.—Test beam mold.multiply the 20 feet by 30 feet to get 600 square feet.Then multiply 600 square feet by 0.333 to determinethe volume in cubic yards of concrete required for thepad which, in this case, is 200 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 padby 27. This is required because there is 27 cubic feetin 1 cubic yard. Therefore, the concrete pad, describedin the previous paragraph which has a volume of 200cubic feet, requires 7.41 cubic yards of concrete.Concrete projects often present varying degrees ofdifficulty; therefore, extra concrete is required tocompensate for these difficulties. Once the totalnumber of cubic yards of concrete is computed, add alittle extra, normally 10 percent, to compensate forwaste. To calculate the excess needed. multiply thecubic yards by 10 percent. In the above case, multiply7.41 cubic yards by .10 to get 0.741 cubic yards. Addthe 0.741 percentage to the 7.41 cubic yards for a totalof 8.15 cubic yards required for the concrete pad.Another method for estimating concrete isshown in table 3-52 of the NAVFAC P-405 whichcovers the 037 rule. This is a decimal equivalent to 1cubic yard divided by 27 cubic feet which equals.037037. This method is accurate; however, theSeabees prefer the L x W x T ÷ 27 method.BATCHING CONCRETEBatching is the process of weighing orvolumetrically 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 concretespecifications require that the batching be performedby weight, rather than by volume, because ofinaccuracies in measuring aggregate, especially dampaggregate. Water and liquid air-entraining admixturescan be measured accurately by either weight orvolume. Batching by using weight provides greateraccuracy and avoids problems created by bulking ofdamp sand. Volumetric batching is used for concrete3-30

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