Figure 15-38.-Roll or thread test.fingers and the test board (fig. 15-38) with justenough pressure to roll the mass into a thread ofuniform diameter throughout its length. The rateof rolling should be between 80 and 90 strokesa minute, considering a stroke to be one completemotion of the hand forward from and back tothe starting point.2. When the diameter of the thread has beenreduced to 1/8 in., break the thread into six oreight pieces (fig. 15-39). Squeeze the piecestogether between the thumbs and fingers of bothhands into a uniform mass roughly oval in shape,and again roll out into a thread. Continue thisalternate rolling to a thread 1/8 in. in diameter,breaking, combining together, and rerolling. Dothis until the thread crumbles under the pressurerequired for rolling and the soil can no longer berolled into a thread. The crumbling may occurwhen the diameter of the thread is still greater than1/8 in. This is considered a satisfactory end point,provided the soil has previously been rolled intoa 1/8-in. thread at least once.3. Gather the portions of the crumbled soiltogether, place it in the moisture content can, anddetermine the water content from the differencein weight before and after you oven-dry it.4. Repeat the process on at least two addi-tional specimens. All three tests should agreewithin 1 percent.The plastic limit is simply the determined watercontent.Plasticity IndexThe PLASTICITY INDEX (PI) of a soil is thenumerical difference between its liquid limit andits plastic limit; that is, PI = LL – PL. The PIthat appears in figure 15-37 means plasticity index.Figure 15-39.-Roll or thread test sample, before and aftercrumbling.CONCRETE TESTINGBefore delving into the remainder of thischapter, you may find it helpful to return tochapter 7 and review the topics concerning con-crete. As you recall, in that chapter you studiedconcrete in terms of its use as a constructionmaterial, and you learned of the properties andrequirements that comprise a good concrete. Youalso know that when concrete is placed in the fieldon a construction project, the concrete used mustsatisfy certain specified requirements. It is towardsthose properties and various requirements thatconcrete testing is directed.CONCRETE TESTSIn concrete testing, as in soils testing, no singletest will provide all of the information required.Rather, there is an array of tests that must be per-formed. The following describes those tests withwhich an EA is most commonly concerned.Aggregate TestsIn order to provide the strongest and mostdurable concrete, the aggregate contained in themixture must be the best possible in terms ofgradation, shape, strength, and cleanliness.During the design of a concrete mixture, theaggregate selected for use must adequately meetthose requirements. To determine this, varioustests are performed. These include tests for15-34
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