Dec 31, 2018
It’s in the Production
BY Sandy Lender
At an industry meeting in 2018, a contractor bemoaned the high cost of aggregate materials. He especially hated it when incoming rock was watered down to increase its weight, thus the price of actual product received in the truck. He was tired of being charged for water.
Of course the concept of an aggregate supplier risking a business relationship with such poor business practices startled me. Whatever dust control measures a site has in place, it should be common knowledge that aggregate suppliers work to keep moisture out of materials just as hard as asphalt mix producers do. All segments of the construction industry have stormwater runoff regulations to which they must adhere, thus keeping rainwater and condensation from contaminating stockpiles is to each company’s benefit. Delivering wet material to any mix-production customer wouldn’t drive up an aggregate supplier’s profit—it would alter liquid percentages in all types of mixes during their production, resulting in the contractor/customer seeking more reliable material elsewhere.
Here’s my point: Look at your job mix formula. In a perfect world, your lab team submits a glorious JMF that the letting agency applauds and accepts, and your plant team produces a mix (or two) within spec for the duration of the project.
We can design a mix with a tight spec band for each of the aggregates to be used and we can program the plant controls to feed the exact weight of each material to meet the accepted JMF. But the real world is not a constant. If you discover the material from Stockpile A built from material delivered by Supplier ABC has 25 percent moisture content, you’re going to see a discrepancy on the feed belt and in the mix. Suddenly the amount of binder needs to change. The drying time and temperature is in flux. I dare say it’s all wacky.
If you can build Stockpile A with material delivered by Supplier XYZ and find it has a more consistent 4 percent moisture content, you’re going to stick with the supplier who has better quality control. You will also look at your own quality control, won’t you? You’ll cover the labeled and clearly divided stockpiles from which you pull the tested, on-spec material that goes into the PWL mix. You’ll train your loader operator to scoop material from the dry part of the pile. You’ll set controls to detect moisture content variations and so on.
Because the real world throws variables such as rainy days, lost truck drivers, new loader operators, failed sensors, broken weigh scales, or any number of oddities at you, plant operators have to stay on their toes during hot-mix asphalt production. The JMF may dictate perfect parameters to blend the most perfect performing pavement known to your DOT, but that doesn’t mean the stars aligned for that perfect production last night.
Instead of counting on the JMF to do all the work, make sure you’re ready for something unexpected to happen. Make sure you know what to do if a truckload of aggregate arrives with a waterfall cascading out the tailgate. (Turn that mess around and send it back.) Make sure you know what to do if a belt scale alarm goes off. Common sense, best practices not only keep contractors from grumbling about the bottom line at industry conferences, these good practices keep the JMF on target.