Sampling Methods and the Running Average New Ulm Quartzite Quarries routinely performs sieve analysis tests or "gradations" for all commonly produced products. Test samples are taken directly off of production belts. Occasionally samples are obtained from inventory stockpiles. A running average of the test results, (percent passing on each sieve), is calculated on an ongoing basis. N.U.Q.Q. will run 15 to 75 sieve analysis tests for each MNDOT specification product every production season. The test results for gradation are generally updated on our web site once a month during the production season. If some products indicate a 'Date of Last Test' for a prior year (or years), then chances are the product is currently being sold from inventory produced the previous year(s).
The Sample Standard Deviation In addition to the individual tests and the running average, a sample standard deviation is calculated for each running average value on each sieve. The standard deviation value or "variance," represents the relative degree of variability of gradation test results. A high value indicates that the test results "jump around" a lot or fall over a wide range. A low value points to a consistent or non-varying pattern of results with very little deviation from the running average. It's important, however, to consider the number of tests in the sample; a large number of test results will produce a more meaningful standard deviation value.
The Production Working Range The standard deviation or variance values are also used to develop the Production Working Range. The Production Working Range is a set of gradation limits and is similar to the MNDOT gradation limits but much "tighter." For example, the MNDOT gradation limits for CA-5 concrete aggregate--on the 3/8" sieve-- is no more than 60% passing and not less than 30% passing, a spread of 30 percentage points. For example, in 1999 the N.U.Q.Q. Production Working Range for CA-5--on the 3/8 " sieve-- was not more than 46% passing and not less than 40% passing, a spread of only 6 percentage points. The numeric values of the Production Working Range are the running average values plus the sample standard deviation, (upper range limit), and minus the sample standard deviation (lower range limit).
The Moving Average Value of the Last Four Tests For some products N.U.Q.Q. also calculates a moving average of the most recent four gradation test results on each sieve. Moving average values are calculated by adding the values for the most recent test results with the values for the previous three consecutive test results and dividing these sums by four. Moving average values are calculated each time a gradation test is run and the results are compared with the N.U.Q.Q. Production Working Range. Customers may use this information when establishing QA/QC working ranges for Job Mix Formulas. This information is intended to be a guide, contractors should exercise their independent judgement when preparing Job Mix Formula submittals.
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