Looking for Trouble
How to prevent quality problems with aggregates.
by Daniel Brown, Contributor
Nobody likes problems with aggregate quality or gradations, but they happen. Sampling errors can throw off test results. A crushing plant manager can strive to overproduce and cause gradations to go out of spec. A loader operator can make mistakes. The list goes on and on. So when problems develop, how quickly are you able to respond and pinpoint the cause?
FNF Construction, an Arizona-based highway contractor, ensures aggregate quality through various steps in its production process. It owns several quarries and gravel pits, and typically crushes aggregates as project needs dictate. Stacker belts can segregate the material, so stockpile management is important. The contractor works to keep the end of the stacker belt within 5 feet of the top of pile, to prevent segregation. And FNF builds smaller surge piles at the crusher for testing purposes, says quality control manager Tom Kennedy. The smaller piles produce more accurate samples.
For new asphalt mixes, Kennedy says FNF always paves a test strip with 500 to 700 tons of hot mix. Doing a test strip debugs the hot-mix plant, establishes a flow of materials, and limits the contractor’s liability in case of a problem, Kennedy says. Aggregate gradations are checked at a minimum of three places:
At the crusher, either off the belt or from the surge piles;
At the hot plant, after a pugmill blends the aggregates but before they go into the dryer drum; and.
In the hot mix, either sampled from the truck or behind the paver.
If a problem develops with material passing one spec sieve, Kennedy first checks for a testing or sampling error. Usually re-sampling and retesting once or twice will confirm or refute the first test.
Suppose the target is 80 percent passing the 0.375-inch sieve, but suddenly the mix sample shows 65 percent passing that sieve. Kennedy begins a logical tracking process to trace down the problem. “Say I check the cold feeds at the asphalt plant, and I get 65 percent on the cold feeds,” he says. “That tells me the plant is doing its job. Simultaneously, I check those surge piles at the crusher. If I check the 1-inch-minus pile and expect to find 30 percent passing the 0.375-inch sieve — but I get 5 percent passing the 0.375-inch, that’s the problem.”
Kennedy says if an entire stockpile was crushed out of spec, the problem should have been caught during the mix design process. “You may have to do a new mix design,” he says. “Sometimes you do a product change.”
Contractors use various statistical control methods and software to troubleshoot aggregate gradation problems. David A. Bramble, a Maryland asphalt contractor with five hot-mix plants, has programmed an Excel spreadsheet to flag aggregates that are out of spec. “We keep a three-dimensional database in a spreadsheet program, and I have every gradation test that I have taken since 1988,” says G. Marshall Klinefelter, bituminous quality control manager for Bramble.
|Microsoft’s Excel program has a feature called conditional formatting, shown here with the colored shading. David A. Bramble uses this feature to spot irregularities in aggregates. Percentages passing that are on target are shown in green; numbers within plus or minus one standard deviation from average are white; percentages beyond one standard deviation on the fine side are yellow; and percentages beyond one standard deviation on the coarse side are colored orange.|
The spreadsheet lists percentages passing every sieve for the following on a given aggregate blend: the current test; the maximum (the finest it’s ever been); the minimum (the coarsest it’s ever been); the average of all tests taken to date, the standard deviation, and the average of the last five tests.
“I can watch the gradations on any given sieve, and if it gets beyond one standard deviation on a certain sieve, it encourages me to call the aggregate supplier to find out what’s going on,” Klinefelter says. Typical factors that may impact aggregate shape include crusher downtime, speeds, and components, as well as equipment operator changes.
Klinefelter hopes to catch aggregate problems before the material reaches the hot-mix plant. “But sometimes we have to adjust on the fly,” he says. Bramble might add coarse material to a blend that is too fine, or compensate in other ways.
To minimize stockpile segregation, FNF Construction keeps the stacker no more than 5 feet above the pile.
Microsoft’s Excel program has a feature called conditional formatting. On a spreadsheet of percentages passing all of the various sieves, conditional formatting will color in certain cells to show Klinefelter percentages passing that are either on target (green); within plus or minus one standard deviation from average (white); or beyond one standard deviation on the fine side (yellow); or beyond one standard deviation on the coarse side (orange). “It makes reading the data fast and accurate,” Klinefelter says.
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