days), the cylinder is CAPPED on both ends with a thin layer of gypsum CASTING PLASTER or sulfur CAPPING COMPOUND. For testing, the cylinder is placed under the piston of a machine capable of applying a very high pressure (for a 6-inch-diameter cylinder with a compressive strength of about 6,000 pounds per square inch, the rupturing pressure must reach about 170,000 pounds). Pressure is applied and increased until the cylinder collapses.

FLEXURAL TEST

The FLEXURAL strength is its ability to resist a breaking force. The flexural strength of concrete is considerably less than its compressive strength. For a flexural strength test, a TEST BEAM, cast in a TEST BEAM MOLD, like the one shown in figure 3-14, is cured and then broken by a BEAM BREAKER.

The test specimen must be formed with its long axis horizontal. The concrete must be placed in layers approximately 3 inches in depth, and each layer must be rodded 50 times for each square foot of area. The top layer must slightly overfill the mold. After each layer is rodded, the concrete must be spaded along the sides and ends with a mason's trowel or other suitable tool. When the rodding and spading operations are completed, the top must be struck off with a straightedge and finished with a wood float. The test specimen must be made promptly and without interruption. Test beams should 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 CONCRETE

To compute the volume of concrete required for a concrete pad, multiply the length of the pad by its width times its thickness to get cubic feet (length x width x thickness). For example, a concrete pad is 20 feet in length by 30 feet in width and has a slab thickness of 4 inches. You first convert the 4 inches into 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 determine the volume in cubic yards of concrete required for the pad which, in this case, is 200 cubic feet.

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 200 cubic feet, requires 7.41 cubic yards of concrete.

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 percent. In the above case, multiply 7.41 cubic yards by .10 to get 0.741 cubic yards. Add the 0.741 percentage to the 7.41 cubic yards for a total of 8.15 cubic yards required for the concrete pad.

Another method for estimating concrete is shown in table 3-52 of the NAVFAC P-405 which covers the 037 rule. This is a decimal equivalent to 1 cubic yard divided by 27 cubic feet which equals .037037. This method is accurate; however, the Seabees prefer the L x W x T ÷ 27 method.

BATCHING CONCRETE

Batching is the process of weighing or volumetrically measuring and introducing into a 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, rather than by volume, because of inaccuracies in measuring aggregate, especially damp aggregate. Water and liquid air-entraining admixtures can be measured accurately 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