First Water Injection WMA Method Approved by Caltrans

The long awaited news of a water injection warm mix asphalt method being approved is finally here.  Caltrans has now added Astec Inc.'s Double Barrel Green technology to its list of approved methods for WMA jobs.  This is a much anticipated approval by such companies as Granite Construction who have invested a considerable amount of capital in Astec Double Barrel Green asphalt plants all over the state.  Information on the Astec Double Barrel Green technology can be found on their website here.

Astec's Double Barrel Green System
Source: www.astecinc.com 

The technology, which Astec rolled out in June of 2007, is an apparatus that can be added easily to any of Astec's double barrel drum plants and can also be retrofitted onto several other types of hot plants.  What makes the Astec system desirable to many producers is that it does not require an expensive additive to produce warm mix asphalt.  All you need is the physical system and water.  As the virgin asphalt bitumen passes through the apparatus, water is injected at a metered rate.  The water and bitumen mix to form microscopic steam bubbles, in other words, a foamed product which causes the liquid asphalt to expand in volume and become less viscous.

Viscosity is a measure of the resistance of the fluid, or in other words how freely it flows.  An example of a highly viscous liquid is honey whereas a lower viscous liquid is water.  Cold asphalt bitumen is typically a highly viscous liquid until it is heated.  Once it starts to cool back down it becomes highly viscous again which is why hot mix asphalt becomes harder to lay down as it cools.  The foaming technology of the Astec system allows the bitumen to have a lower viscosity without as high of a temperature.  This makes it possible to produce the material at a lower temperature and/or have workability of the asphalt mix for longer.  After a time period the steam bubbles in the asphalt mix begin to evaporate, leaving only the asphalt bitumen behind.  This is typically as you compact the pavement and the temperature of your mix drops below 212 degrees F., leaving you with what looks like a traditional hot mix asphalt pavement.

Just like with other WMA mixes, the paving crew will likely have to change some of their rolling patterns to account for the differences in the mix viscosity.  Typically rollers can be used immediately instead of having to wait until the mat has cooled down.

Caltrans's approval of the Astec Double Barrel Green technology means that on Caltrans jobs where warm mix is either an option or a requirement, contractors are able to use this product without a lengthy approval process through Caltrans.  For more information on the approval process see my previous post here.  The list of Caltrans approved products can be found at: Approved Products.

This is a big win for producers across the state.  One of the benefits of WMA that has been proposed to producers is that producing WMA will reduce fuel consumption at the plant and lower operating costs.  With chemical additive WMA, this cost savings can be eaten up by the additional cost of the additive.  With a water injection system, once you've paid for your capital expenditure for the system, the cost of the added water is negligible.

It has been rumored that water injection WMA may not be as effective as chemical additive WMA but with a water injection technology finally added to Caltrans's approved products list you have to assume that there is not enough of a difference to warrant fear in investing in such a technology.