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Densifiers
Unit Weight (ASTM C 138) - 14071_298

Engineering Aid 2 - Intermediate Structural engineering guide book
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Figure 13-20.—Increase of concrete compressive strenth with curing  age. figure  13-20.  Note  the  long-time  gain  in  strength  that occurs   when   proper   temperature   and   moisture conditions  are  maintained. CONCRETE  TESTING Several tests, such as slump, air content, and weight determination, are necessary to determine the quality of freshly mixed concrete. In addition, strengths tests are needed  to  determine  whether  a  hardened  concrete satisfies  specified  strength  requirements.  This  section briefly  discusses  those  tests. Slump Test As you know, the measure of the workability or consistency of a concrete mix is its slump. With too little slump, the mixture may be too difficult to work into the forms and around the reinforcing steel. On the other hand, with too much slump, the concrete ingredients may  segregate  and  excessive  bleeding  or  migration  of water  to  the  top  surface  of  the  freshly  placed  concrete may occur. Excess bleeding increases the water-cement ratio near the top surface of the concrete and results in a weak top layer with poor durability. To determine whether a freshly mixed concrete satisfies  the  specified  requirements  for  slump,  you  must perform a slump test. By now, you should be thoroughly Figure  13-21.—Apparatus  for  air-content  test. familiar with the procedures of slump testing. If not, you should review the discussion of slump testing that is in the EA3 TRAMAN. Air-Content Test (ASTM C 231) An air-entraining admixture is added to the concrete mix so that enough air will be entrained to improve workability  and  durability  of  the  mixture,  but  not enough  to  reduce  strength  substantially.  Air-entraining cements may also be available for use in some military situations. The desired amount of air is generally from 4.0 to 7.5 percent of the total mix. The  equipment  for  determining  the  percentage  of entrained air is included in the boxed test kit. The basic tool  is  the  pressure  type  of  indicator,  as  illustrated  in figure 13-21. The equipment furnished in these kits varies  with  the  manufacturers.  Each  kit  contains  the complete  equipment  for  conducting  the  test,  including  a detailed  instruction  pamphlet  and  the  calibration procedure for the particular meter. Before the air content of   a   concrete   mixture   can   be   determined,   the 13-29







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