With an ever-increasing interest in reducing plant process water, producers are looking for more efficient methods to solve the shift-ending drum cleanout. So when a question inquiring about water reduction ideas was posted on the ARI Web site, many producers took time to offer some production tips.
"I agree, excess wash water at the end of a shift is an important industry problem that needs to be addressed," says one respondent. "We don’t use quite 800 gallons of water, yet reducing the amount we do use is becoming a priority ... One concern is using the leftover wash water the next morning in spec concrete. Most of our larger commercial work schedules first round deliveries."
Note that this concern about meeting specifications is currently being addressed by a special ASTM C-9 subcommittee. The topic of mix water requirements has been on the table for years with no conclusive resolution. The subcommittee met in Denver in June and some guidelines may emerge from that session.
Specific concerns have to do with reusing water that has been treated with a cement stabilizing agent and what effect any residual chemicals might have on the concrete. As sometimes happens, practices in the field get ahead of efforts to quantify and standardize procedures. Noting the desirability of reducing water waste, the Florida DOT did a study to determine what effects the practice of reusing chemically stabilized wash water might have on concrete strength and performance. The final report, issued in October 2000, confirms the general acceptability of this practice. It also notes various benefits beyond reducing water use, including eliminating the need for waste water disposal and the associated slurry ponds and settling pits. Furthermore, it says this practice reduces EPA concerns related to wash water.
Returning to the producer’s point of view, another respondent to the Aggregate Research Industries Web forum posting shared this positive experience. "I have successfully used Delvo with 50 gallons of water in mixers after the last load of the day for well over 12 years now with excellent results." The key, he continues, is getting drivers to buy into the program and do it right, namely, "running the [mixture] back and forth enough times to get all of the blades covered. It takes a bit of practice but is well worth it." He also notes that keeping track of which trucks are washed out this way enables plant operators to hold back the proper amount of water each morning to avoid getting wet loads. He adds, "Also, it has been my experience that finishers actually like the first load of the day, due to the extra cement fines in the mix."
Several manufacturers offer hydration stabilizers for use in recycling mixer wash water. These include Boral Material Technologies (Boral DS: www.boralmti.com), Fritz-Pak Corp. (Mini Delayed Set: www.fritzpak.com), Grace Construction Products (Recover: www.graceconstruction.com) and Master Builders (DELVO: www.masterbuilders.com).
In addition to using these hydration stabilizers in wash water, many drivers carry pre-portioned packages with them. In the event of unexpected delays, being able to dose the concrete with such a product can save the load-or even save the drum.