Mixing and Molding Temperatures According to Tex-205-F and Tex-206-F

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TxDOT has re-written most of the Series 200 test procedures to include recycled materials and substitute binder. By using RAP and RAS, you can certainly save on costs. But, studies have shown that the liquid binder contained in recycled materials is stiffer and harder than virgin binder is. This is because the recycled binder has gone through some of the aging process. Studies further have shown that when you use around 20% RAP or any amount of RAS, that in essence bumps up the PG binder by one PG grade and we have seen that typically when recycled materials are used, it almost always guarantees higher cycles on the Hamburg Wheel Test. Because adding recycled materials stiffens the mix, TxDOT is allowing contractors to use a lower PG Grade "Substitute Binder" than what is originally called for on the plans.

We all know that heat has a direct relationship on compaction, both in the lab and in the field. Let’s first look at laboratory mixing. If you are using a lower grade binder, your mixing temperatures are lower than mixing temperatures when using a higher-grade binder. Now let’s throw RAP and RAS into the mix. What does that do to your mixing temperatures? Note 1 with Table 1 of Test method Tex-205-F now says that if you are using a Substitute Binder and you are using RAP and/or RAS, you should mix at the originally specified binder temperature. Note 2 of Table 1, written for recycled mixes when not using a substitute binder, states that if you are using RAP and/or RAS you can mix up to 325 degrees regardless of the grade of the binder.

Tex-205-F – Table 1

Table 1—Asphaltic Material Mixing Temperatures by Grade and Type

_1. If using RAP or RAS and a substitute PG binder in lieu of the PG binder originally specified on the plans, defer to the originally specified binder grade when selecting the mixing temperature.
_2. When using RAP or RAS, mixing temperature may be increased up to 325°F to achieve adequate coating.

To complicate things a little more, let’s add Warm Mix to that. Now you can drop down a grade or two in your binder, mix at the higher original specified binder temperature and now have the capability of producing the mix between 215 and 275 degrees. Which temperature should you use? Section 4.7 of Tex-205-F tells us to mix at the original specified binder temperature.

In regards to molding temperatures in the lab, which is governed by Tex-206-F, it is much the same with the exception of Warm Mix Asphalt. The following table guides us on the suggested compaction temperatures. But when using WMA and the production temperature is less than the temperature shown in Table 1, you should mold at your target discharge temperature. There is another exception to this rule and that is if you are molding WMA specimens for Indirect Tensile Strengths and Hamburg Specimens, you will cure for FOUR HOURS at 275 degrees and mold at 275.

Tex-206-F – Table 1

Table 1—Compaction Temperatures

Note: Mixtures must be compacted at the selected compaction temperature within a tolerance of ± 5°F (± 3°C).
_1. If using RAP or RAS and a substitute PG binder in lieu of the PG binder originally specified on the plans, defer to the originally specified binder grade when selecting the compaction temperature.
_2. Use the target discharge temperature when it is less than the compaction temperature shown.
 
One thing that is not yet addressed but is currently being evaluated is the scenario that if you are using RAP and/or RAS and are not using a substitute binder, should you increase the molding temperature? We already have the ability to increase the mixing temperature to 325 degrees and because recycled materials increase the stiffness of the mix, I suggest that it would be appropriate to increase the molding temperature by one grade as well; however, TxDOT has not taken an official position on that yet.

In closing, the one thing I can say with certainty is that as a Level 2 Mix Design Specialist, whatever temperature you are approved to mix and mold at, make sure you notate it on the mix design form and on the QC/QA form so everyone will have more consistent results by mixing and molding at the design temperature.

We no longer have the simplicity to select mixing and molding temperatures based solely on the grade of binder we are using so please become familiar with the new test procedures and specifications. The Hot Mix Asphalt Center has integrated those changes into our training and certification program and I urge all of you to go to the HMAC website at www.txhmac.org, click on the TxDOT logo and read the latest test procedures and specifications. There are many changes and in order to design high quality mixes and you should become familiar with these changes. If you have any questions, please do not hesitate to call us.