Talking Asphalt: Practicing process control – July 2013

I’m convinced that process control testing and evaluations for asphalt materials are oftentimes overlooked. A lot of testing is focused on verifying acceptable properties, and that’s a worthwhile effort. But there’s more to a balanced quality control program than that.

“Process control” is about assuring that the production process is under control – knowing your component materials, anticipating equipment wear and tear, making timely adjustments, avoiding over-corrections, etc. It’s really just doing your homework in order to produce a consistent finished product.

For the aggregate part of production, it’s knowing about changes in the quarry or pit. If the production shifts to another part of the working face, does the aggregate change? Is the specific gravity and absorption the same as the earlier material? What about clay content or particle shape? What about the other sizes or products being made? If the block plant needs material or if more ag-lime is needed, the gradation will change. Any of these considerations can influence the performance.

Think about the aggregate at the mixing plant. Is the moisture content consistent? How about the gradation, clay content, particle shape? Check to see if the aggregate stockpiles are segregated or intermixed. These conditions cause variability. Verify that adequate quantities of ingredient materials are available for a constant, uniform feed. Are the correct sizes on hand?

Next, check the other materials – RAP, additives or modifiers, anti-strip agents, etc. How much variation is there in the RAP? Make sure the asphalt binder is the correct grade. What about the warm mix process?

Now, the mixing plant and related equipment; review their operations. The mixing plant is a sophisticated collection of mechanical and electronic components – and parts do wear or fail. Any problems or changes there? Everything running consistently?

Check that the burner is set for the applicable fuel type, production rate, desired mixing and discharge temperature, etc. For batch plants (yes, they still exist), monitor the screens and hot-bins, check the mixing time and make sure the scales zero properly. For drum plants, check that the correct moisture content is being used. For all plant types, make sure the various calibrations are current, and keep an adequate level of material in the surge bins. Use proper load-out techniques and avoid/minimize segregation.

Watch for developing trends in test data. For example, if the air voids are steadily decreasing, check the aggregate gradations – particularly the P-200 content. Don’t overreact to a single test result. Look at all the available information, and run another test ASAP. If mixture adjustments are necessary, make only one adjustment at a time then retest to see how the adjustment worked. Do not over-correct.

Not all non-spec material can be avoided; things do break. But, if you do your homework and keep your eyes open, most of these situations can be avoided or minimized. Give some thought to how to better utilize your QC resources and maintain control of the production process.

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