Figure 13-20.—Increase of concrete compressive strenth with curing age.figure 13-20. Note the long-time gain in strength thatoccurs when proper temperature and moistureconditions are maintained.CONCRETE TESTINGSeveral tests, such as slump, air content, and weightdetermination, are necessary to determine the quality offreshly mixed concrete. In addition, strengths tests areneeded to determine whether a hardened concretesatisfies specified strength requirements. This sectionbriefly discusses those tests.Slump TestAs you know, the measure of the workability orconsistency of a concrete mix is its slump. With too littleslump, the mixture may be too difficult to work into theforms and around the reinforcing steel. On the otherhand, with too much slump, the concrete ingredientsmay segregate and excessive bleeding or migration ofwater to the top surface of the freshly placed concretemay occur. Excess bleeding increases the water-cementratio near the top surface of the concrete and results ina weak top layer with poor durability.To determine whether a freshly mixed concretesatisfies the specified requirements for slump, you mustperform a slump test. By now, you should be thoroughlyFigure 13-21.—Apparatus for air-content test.familiar with the procedures of slump testing. If not, youshould review the discussion of slump testing that is inthe EA3 TRAMAN.Air-Content Test (ASTM C 231)An air-entraining admixture is added to the concretemix so that enough air will be entrained to improveworkability and durability of the mixture, but notenough to reduce strength substantially. Air-entrainingcements may also be available for use in some militarysituations. The desired amount of air is generally from4.0 to 7.5 percent of the total mix.The equipment for determining the percentage ofentrained air is included in the boxed test kit. The basictool is the pressure type of indicator, as illustrated infigure 13-21. The equipment furnished in these kitsvaries with the manufacturers. Each kit contains thecomplete equipment for conducting the test, including adetailed instruction pamphlet and the calibrationprocedure for the particular meter. Before the air contentof a concrete mixture can be determined, the13-29
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