Unit Weight (ASTM C 138) - 14071_298

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entrained-air  indicator  must  be  calibrated  accurately, and the correction factor for the aggregate contained in the  concrete  must  be  determined. To perform the test, follow the procedures contained in  NAVFAC  MO-330  and  in  the  instruction  book furnished with the meter. The instruction book also describes   the   calculations   for   determining   the entrained-air  content. Unit Weight (ASTM C 138) The unit weight, or density, of concrete varies with the  amount  and  density  of  the  aggregate,  the  amount  of entrapped or entrained air, and the water and cement contents. Conventional concrete used in structures, such as buildings and pavements, has a unit weight in the range of 140 to 150 pounds per cubic foot (pcf). For other types of concrete, the unit weight ranges from 15 pcf for lightweight insulating concrete to 400 pcf for heavyweight  concrete. To  determine  the  unit  weight  of  freshly  mixed concrete, you will need a cylindrical metal measure (container)   of   either   1/10-,   1/5-,   or   1/2-cubic-foot capacity. If necessary, you should calibrate the measure before performing the test procedures. To calibrate the measure, you first determine the tare weight of the measure, and then fill the measure with water at room temperature.  Then  determine  the  temperature,  density, and weight (in pounds) of the water. To determine the density of the water, use table 13-3 and interpolate, if necessary. Next, calculate the calibration factor of the measure by dividing the density of the water by the weight of the water required to fill the measure. The ASTM procedures for determining the unit weight are summarized as follows: Table 13-3.—Density of Water 1.  Fill  the  measure  with  fresh  concrete consolidated  in  three  layers,  as  described  for  the air-content test. After each layer is rodded, tap the sides of the container 10 to 15 times with a rubber or rawhide mallet  to  remove  any  air  pockets. 2. After filling and consolidating, strike off the top surface, taking care to leave the measure level full. 3. Clean all excess concrete from the exterior of the measure. Then weigh it and determine the net weight of the concrete inside the measure by subtracting the tare weight of the measure from the gross weight of the measure  and  concrete. 4. Calculate the unit weight by multiplying the net weight of the concrete by the calibration factor for the measure. Compressive Strength Test (ASTM C 39) The  compressive  strength  of  hardened  concrete  is determined   from   compression   tests   on   standard cylindrical  specimens.  As  you  know,  compressive strength  tests  are  used  during  concrete  mix  design  to evaluate  the  performance  of  the  materials  and  to establish mixture proportions that will give the required strength. The tests are used also to control the quality of the concrete in the field. “Compressive  strength”  is  defined  as  the  average  of the strengths of all cylinders of the same age made from a sample taken from a single batch of concrete. At least two  cylinders,  or  preferably  three,  are  required  to constitute a test. So, if tests are to be made at 7 and 28 days, you will need four or six specimens. The standard specimen is 6 inches in diameter by 12 inches long and is  capped  with  a  suitable  material  to  provide  a smooth-bearing  surface  on  each  end  of  the  specimen. You learned the procedures for preparing and capping compressive strength specimens in the EA3 TRAMAN. If necessary, you should review those procedures. The following paragraphs discuss only the procedures used to   perform   compression   tests   on   the   prepared specimens. The  equipment  you  will  use  to  perform  the compression test is a compression-testing machine, having a capacity of 250,000 pounds. An example of that machine, shown with a test cylinder in place, is illustrated  in  figure  13-22. The  procedures  for  conducting  the  compression  test are as follows: 1. Prepare the testing machine by cleaning the bearing plates and, if needed, cleaning and lubricating 13-30

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