Preparation of Test Samples

CBR Test Equipment and Tools - 14070_277

Figure 13-11A.Front of data sheet (DD Form 1212) for California bearing ratio test. - 14070_279

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motorized) type of jack. Three proving rings are available with capacities of 2,000, 5,000, and 7,000 pounds, respectively. The surcharge weights are used to approximate (within + 5 pounds) the expected weight of the pavement and base in the field. The tripod attachment (when fitted with a dial indicator) and the swell plate are used to measure the expansion, or swell, of the material in the CBR mold. Other items needed to perform the test are equipment and tools, such as a balance or scale, a CBR mold, a 10-pound tamper, mixing bowls, spoons, spatulas, a soaking tank or bucket, and moisture canisters. Preparation of Test Samples When a bearing-ratio test is made of a compacted sample, you will use the 6-inch-diameter CBR mold with a 2 1/2-inch spacer disk in the mold beneath the sample. The use of the spacer reduces the depth of the sample to 4 1/2 inches. The use of another size spacer will result in volume and compactive effort changes that may not meet ASTM or other recognized standards for the CBR test. The method of preparing the test specimens and the number of specimens depend upon such factors as the type of airfield or road and the soils encountered at the site. The soil sample should be tested in the laboratory at a density comparable to the density required at the construction site. There are situations where moisture conditions are favorable and the subgrade will not accumulate moisture approaching a saturated condition. In these cases, samples should be tested at a moisture content that approximates actual moisture conditions expected during the time the road or airfield is used. In all other conditions, the samples are laboratory tested in a saturated condition. The saturated condition is attained by soaking the sample. First, place the sample in the mold and compact it. The compactive effort used and the number of compacted samples required depend upon the soil type, weight and type of field compaction equipment, and other job conditions. Normally, compactive efforts of 12, 26, and 55 blows per layer (for five layers) are used in each of three successive compaction tests. The 10-pound tamper is used for compacting the samples. After compacting the sample, trim it and remove the base plate and spacer disk. Then place a piece of filter paper over the trimmed or struck-off top of the sample and place the base plate over this top. Turn the mold over and set it in a bucket on the base plate. The bottom of the sample, which was next to the spacer disk during compaction, is now uppermost. Apply the appropriate 13-12 Figure 13-10.—Apparatus for soaking a CBR test sample. number of surcharge weights needed to approximate the expected in-place weight of the pavement and base. One 5-pound surcharge weight is equivalent to 3 inches of overlying material. Then set in place the tripod attachment, dial, and swell plate, as shown in figure 13-10. Immerse the mold and the sample in water in the bucket and leave them to soak for about 4 days. An initial reading of the tripod dial is made when the sample is first placed in the water. Then, at the end of the soaking period, the dial is read again to determine the amount of swell. A swell in excess of 3 percent of the initial height of the specimen is considered to be excessive. After making the final reading of the dial, you remove the sample and mold from the water and allow them to drain for about 15 minutes before conducting the penetration test. Penetration Test In the penetration test, the bearing capacity of a soil is determined by measuring the extent to which the sample, placed in a mold, is penetrated by a penetration piston. The sample (in the CBR mold) is placed in the loading press, as shown in figure 13-9. The piston is placed on top of the material, and a proving ring is placed between the top of the piston and the top of the loading press. As the jack is cranked upward, the dial in the center of the proving ring records the pressure being applied to

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