|Asphalt Crumb-Rubber Blending|
here was a time when rubberized asphalt cement (RAC) was somewhat of a novelty item for many producers and departments of transportation (DOTs). Although the technology has been around since the 1960s, rubberized pavements have been assigned an experimental status for many decades. Gradually, however, state DOTs are realizing the benefits of using RAC in their mix designs, and today the technology is being used more widely in Arizona, Texas, Florida, and California..
In California specifically, RAC pavements moved far beyond being labeled as experimental or novel in October 2005 when Governor Arnold Schwarzenegger signed into law California Assembly Bill 338. This bill requires the California Department of Transportation (Caltrans) to gradually phase in the use of RAC on state highway construction and repair projects. By January 2013, the bill aims for 35 percent of all projects constructed by Caltrans to include RAC.
That commitment made by the State of California to rubberized pavements has many hot-mix asphalt (HMA) producers looking for efficient and cost-effective ways to incorporate ground tire rubber (also called crumb rubber) into their mixes. Sacramento-based Teichert Materials is one of those companies.
The eighth licensed contractor in the state of California
Teichert Materials is one division of the parent company, A. Teichert and Son. This family-owned firm traces its history all the way back to 1887 when Adolph Teichert—a German immigrant—started his own business building walkways, cellar floors, sidewalls, fencing, and other projects in the Sacramento area. The company has California State Contractor’s License #8, the oldest active contractor’s license in the state.
Today, Teichert is made up of two main operating companies: Teichert Materials, which includes aggregates, ready-mix concrete, and precast-products divisions; and Teichert Construction, which handles asphalt paving, mass grading, concrete curbs and sidewalks, and utility installation.
An operating arm in the Teichert Materials company is known as Teichert Aggregates. This group produces HMA products and generally purchased RAC from a third-party contractor. But as rubberized asphalt pavements are now specified in an increasing number of projects statewide— and especially in the Sacramento area—the company has opted to begin making its own RAC. In order to do that, they had to install a mixing system.
The asphalt-rubber mixing systemTeichert Aggregates serves a large market area in California, ranging from Yuba City in the north to Vernalis in the south and from Esparto in the west to Martis Valley in the east.
"We add crumb rubber to our mixes in all of those areas except Martis Valley," explained Paul Mercurio, production manager for Teichert Aggregates. "At our Sacramento plant, we decided to further endeavor into the business by making our own blend of crumb rubber."
The company opted to purchase an asphalt-rubber mixing system from CEI Enterprises. The system includes two main components: a Portable Mixing Unit and a Portable Reaction Tank.
Here’s how the process works:
Starting up and learning the process
Representatives from the company that supplied the system— CEI—assisted the personnel at Teichert Aggregates with the installation and startup of the new asphalt-rubber mixing system. The system was connected to a new drum-mix plant at the Sacramento location.
"We actually took delivery on the asphalt-rubber mixing system at the end of last year’s season, but initially we didn't use it except to hold conventional liquid AC," said Mecurio. "It was this season that we started processing the crumb rubber in the system.
"In the beginning, we had a few issues—getting valves and a feeder operating smoothly, fixing a computer glitch, and optimizing the burner while trying to get it going right away. But CEI was there. Their crew took care of us. They got it all dialed in. And it turned out to be a good startup— and a good blending system."
Looking back, CEI delivered the kind of service that Teichert had specifically expected when they first set out to make this critical equipment-buying decision.
"Teichert is a careful, conservative company, and they knew exactly what they wanted and needed," said a CEI spokesperson. "They wanted to stay with an experienced manufacturer that could supply training, service, and long-term equipment support. They also wanted to comply with all California regulations on emissions and with all Caltrans specifications. And they wanted the highest continuous production rate to the plant that was available."
When the asphalt-rubber mixing system is in full operation, it is capable of continuously running at least 30 tph (27 tonnes per hour) of mixed rubberized asphalt to the HMA plant. The assumed blend ratio is 20 percent ground tire rubber by weight to 80 percent virgin liquid AC by weight, but this ratio can be raised or lowered by the operator depending on Caltrans specifications.
Blending control is achieved by using a fully automated PLC (programmable logic controller) system. The blending-control keypad and display allow entry of the desired size and ingredient content. All motors involved in the blending process are controlled by variable-frequency drives, driven by signals from the PLC. The CEI system operated by Teichert Aggregates features push-button controls, a blending control panel, and a display—all of which are panel mounted. There is also a fold-up canopy that provides operator comfort. As another option, controls can be inside an air-conditioned room.
The Portable Mixing Unit includes a circulating hot-oil heater thatwas specified with a low NOx natural gas-fired burner. All of the system's pumps and all of the other components are isolated from the hot-oil system by valves to allow for easier maintenance or repair.
The Portable Reaction Tank features a total capacity of 24,000 gal. (90,850 L). The unit operated by Teichert Aggregates features a triple-compartment design that allows for asphalt-rubber storage in three compartments, divided into 8,000 gal. (30,283 L) each. This maximizes storage and allows for more continuous production: the HMA plant can draw rubberized asphalt from one tank while the product ages in another tank, and meanwhile a third tank can be initiating the blending process for an entirely new batch of rubberized asphalt.
With this new technology at hand, Teichert Aggregates is now able to produce its own RAC for the Caltrans projects that occur in the vicinity of their Sacramento plant. Mercurio foresees that this is going to be an important feature in the years ahead for the company’s daily operations.
"Caltrans is encouraging the use of rubberized asphalt," Mercurio said. "And Sacramento County is also being very proactive in using rubberized asphalt. It's really a great product."