Wednesday, October 19, 2011

CT 207 Changes-September 2011

Caltrans has released a new version of CT 207: Method of Test for Determining Specific Gravity and Absorption of Fine Aggregate.  The new test method can be found on the Caltrans website and here: CT 207.

  • The test method has changed drastically so be sure to read through this one and not just my notes.  Here is a list of the major changes:
  • The test method’s scope now includes the bulk specific gravity (oven dry) and apparent specific gravity and is no longer specifically for use in portland cement concrete.
  • Your balance is now required to have a minimum capacity of 2000g instead of 1000g.
  • The test method has been converted from metric to English units.
  • The test method no longer specifies that your volumetric flask has to be calibrated to 0.15 mL at 23˚C but instead only specifies that it have a mark to indicate a volume of approximately 500 mL.
  • A water tank is now specified in the apparatus section w/ a constant temperature of 73 ± 3˚F and enough depth to maintain the water level above the 500 mL mark on your flask.
  • It is now required to wash the sample over a #200 sieve screen and dry back to constant weight before beginning the test method.  Previously there was no wash process involved.
  • The sample must be soaked for 24 ± 4 hrs now instead of the previously specified minimum of 15 hours.
  • After decanting the water from the sample, the sample is to be spread on a nonabsorbent surface.  In the past it was recommended to spread it on a porous (in other words absorbent) surface. 
  • It is now specified to stir frequently to insure homogeneous drying and to break up clumps.  Stirring is now heavily encouraged and explained. 
  • How the tamping occurs is now explicitly explained including that each drop should start about 0.2in above the surface of the fine aggregate and that you should clear the material away from the base of the cone before you lift it.
  • There is now a table with photos that explains what the material should look like after lifting the cone.  (VERY USEFUL)
  • The test procedure now specifies the sample to be 500 ± 10 g instead of 500 g.
  • When introducing water to the flask with the SSD sample, it now says to fill the flask to approximately 90% capacity vs. “almost to the 500 mL mark” in the old test method.
  • It now notes that manually rolling the flask could take 15-20 minutes, although it is not specified as required.
  • It now specifies that when filling the flask to the calibrated capacity, you should measure to the bottom of the meniscus.
  • The oven-dry mass is no longer found by weighing a split of the SSD sample.  Now you will use the same material sample that you had in the flask.
  • You no longer weigh the flask empty; all of the weights in this test method are in the flask so you will weigh the flask with 500 mL of water in it as well as with water and the SSD sample.  Because of this change, the calculation has changed slightly as well.
  • The test method now gives you calculations for not only Bulk Specific Gravity (SSD) and Percent Absorption, but also Bulk Specific Gravity (Oven-Dry) and Apparent Specific Gravity.
  • There is now an example test result summary at the end of the test method. 
This new version of the test method is much more descriptive and well formulated.  It should come as no surprise to you but this test method is notorious for creating disputes between contractors and Caltrans.  Hopefully with this new version there will be less problems and all labs will be able to obtain similar results.

My only complaints with this version are as follows.  The majority of the test method species weight in grams but the weight of the tamping rod is specified in ounces. Secondly, in step 4 of the test method, it says to fill the flask to its "calibrated capacity" but it would be much less confusing if it read "to the 500 mL mark".  Finally, the biggest issue that I have is with the photos on the table.  I think this table and the photos will do a great deal of benefit for the testing community but the third photo, the one that the technicians really need to see, is blurry whereas the other two are perfectly clear.  It would have been very helpful to have a clear photo here.

Overall I'm excited about the new test method and hope that it helps in obtaining better correlation with our local districts' Caltrans labs.  Happy Reading!

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