CT 309 & CT 304 Changes

For all of you laboratory folks, Caltrans has released new versions of CT 304-Preparation for Bituminous Mixtures for Testing and CT 309-Rice Theoretical Maximum Specific Gravity of HMA.  The new versions can be found here:

CT 304-June 2011

CT 309-June 2011

I’ll summarize the changes to CT 304 in this post and in CT 309 in another post soon. 

I highly recommend reading over the new CT 304.  There are some important changes that can affect your certification procedures as well as some really helpful new sections that will be assets to your laboratory on a daily basis.  The entire format of CT 304 has been revamped to be easier to reference and understand and it is heavily focused on the mix design methods covered in Asphalt Institute’s MS-2 “Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types” as well as Superpave Series No. 2 (SP-2) “Superpave Mix Design”.

The biggest change that I noticed right away is that Caltrans has finally converted this test method from metric measurements to US measurements.  I know that my laboratory will be celebrating this change.  Caltrans has set a goal to switch all of the test methods back over to US units but it is a slow process and if you look through the current test methods they are a mixed bunch with some of them referencing metric units and some referencing US units. 

The following is a list of the other mentionable changes:

• The new format breaks out the other test methods that are referenced as well as what information should be reported. 
• The scale used for this test method must now be accurate to 0.1 g instead of 1 g.
• If you are hand mixing you must now use a sand bath on the surface of the hot plate to minimize localized overheating.
• It no longer specifies the use of CT 303 to determine “K” values and now allows you to pick any starting point for determining your optimum binder content.
• It gives equations and examples for determining the percent of asphalt by total weight of mix and how to determine the weight of asphalt required in each batch. 
• It clearly specifies temperatures for mixing and curing divided by each different type of binder.  Some are now reliant on manufacturer specifications instead of Caltrans recommended temperatures.
• It now specifies the temperature that the binder should be heated to prior to mixing and adds additional regulations.  The binder should not be heated above 375˚F at any time, it should not be kept at the mixing temperature for over 2 hours before using it, and it should not be reheated to mixing temperature more than 2 times.
• There are now temperature regulations for the mixing bowl prior to mixing.  It should not be heated to over 50˚F over the mixing temperature and not over 375˚F.
• There is now mention of buttering the bowl with a mixture that has the same binder content as the lowest batch you will be testing.
• There is an alternate procedure for curing your specimens where instead of curing at 140˚F for 15-18 hrs you can cure your specimens at 295˚F for 2-3 hours.  This is only allowed if the combined absorption of the aggregate is less than 2%.
• It is now regulated that the mixing bowl must be scraped to within ± 0.1% of the initial total batch weight.
• The section on preparation of field-mixed HMA has been greatly reduced in this version of the test method.  It now only mentions the temperatures that the HMA should be heated to for testing and adds an additional note that the mix should only be reheated to test temperature 2 times and should not be held at that temperature for more than 3 hours.  This section now only mentions loose mix and no longer talks about compacted mix.
• The specifications on molds used in this test method have been greatly expanded to not only include inside diameter and height but also specifies hardness, smoothness, and wall thickness.
• It is now required to use a rubber specimen or handful of rags to warm up the compactor before using it on your HMA specimens.  There is also now a procedure on how to warm up the compactor.
• Part 2, the compaction section of the test method has greatly changed to reflect a clearer step-by-step process and is no longer broken out by the type of test you will be using the specimen for.  There is no longer a mention of the Swell Test or Moisture Vapor Susceptibility Test and this section seems to focus more on compaction for stability and specific gravity tests now.
• It is now specified that instead of only having to bring the mix to compaction temperature before starting the test that the mix must be heated for to that temperature for 2-3 hours before compacting.
• It is now specified that the time between removing the mix from the oven and starting compaction is 1 min or less.
• The amount of tamping blows need to form the mix into semi-compacted condition has changed from recommending 20 to recommending 25.  However, this number is still based on individual observation.  There has also been a change where the recommended range based on observation has changed from 10-50 tamps to 20-50 tamps depending on the type of HMA. 
• It is now specified to put a paper disk on the compacted specimen before leveling off the load.
• It is now specified that the speed on the leveling load is to be 0.25 in/min.
• It is now specified that the specimens be returned to the oven for 15 min to 2 hours after leveling to retain temperature prior to stability testing.  In the past there was no time specified.
• There are also now four appendices which specify the procedures for preparing HMA test specimens with up to 15% RAP, Liquid Antistrip, Dry Lime, and Lime Slurry.

Some of these changes were much anticipated, many of which are already in use by most of the industry.  Others may seem like a bit of overkill but my opinion of the June 2011 version of CT 304 is positive and I'm looking forward to the new version being a very useful tool. 

Happy reading and good luck!


*UPDATE 9/22/11: Soon after posting the new version of CT 304, Caltrans pulled it off of the website so that they could re-evaluate the reporting of binder content by total weight of mix instead of the traditional dry weight of aggregates.