Sample Reduction

Understanding aggregate sample reduction methods

In the complex process of aggregate analysis, the practice of sample reduction is crucial to achieve representative test portions and thereby ensure the accuracy and reliability of testing results. Various methods including the Rotary Sample Divider Technique, Riffle Box Sample Reduction Technique, Fractional Shovelling Sample Reduction, and Quartering Method for Sample Reduction offer diverse approaches to achieve this goal. Each method, with its unique procedure and application, ensures that the sample reduction is done effectively while maintaining the integrity of the sample and ensuring the representativeness of the test portions obtained.

Rotary Sample Divider Technique

To effectively employ a rotary sample divider in aggregate sample reduction, one must initially choose a configuration that can yield test portions within either 100% to 150% or 85% to 115% of the predetermined mass, tailored to the requirements of the specific test method. It's notable that schemes and calculations of sample reduction, akin to those used with a riffle box, are also applicable here, provided the rotary sample divider can facilitate an even division into a minimal number of subsamples. The process involves placing the aggregate sample into the divider’s hopper, initiating the rotor, and then starting the vibrating feeder once the rotor reaches its operational speed. A minimum of 35 revolutions should be completed before the sample is fully dispensed. If this criterion is not met, the operator should recombine the subsamples and repeat the procedure at a lower feed rate. In situations where the divider lacks a feeder or cannot achieve 35 revolutions, the mass of each portion should be manually verified to ensure it falls within the specified range of the targeted mass.

Riffle Box Sample Reduction Technique

Utilizing a riffle box for sample reduction involves placing the aggregate sample evenly into one of the receptacles and pouring it down the center line of the box, ensuring even distribution. For a test portion mass within 100% to 150% of a specified mass, calculations involve obtaining the specific test portion mass, determining the mass of the laboratory sample, and then computing subsequent divisions until a mass less than 1.5 times the specified test portion mass is reached. The sample reduction then depends on the achieved mass, involving either one division operation followed by quarter divisions or directly proceeding with quarter divisions. For a portion mass within 85% to 115% of the specified mass, similar steps are taken, but the sample reduction method varies. Depending on the calculated mass, the reduction can involve a three-quarter division followed by quarter divisions, directly proceeding with quarter divisions, or a one-half division followed by quarter divisions. In each scenario, after three successive quarter divisions, it's advisable to reassess the subsample mass to decide if further division is needed.

Fractional Shovelling Sample Reduction

In scenarios where a specialized sample divider isn’t at disposal, fractional shovelling serves as an alternative for partitioning a laboratory sample into several subsamples of approximately equal mass. Start by determining the mass of the laboratory sample (mL) and calculate the number (n) of subsamples needed based on the ratio of mL to the specified test portion mass (mT). The rounding off of the number of subsamples depends on the required tolerance; for a test portion mass within 100% to 150% of mT, it should round down, and for 85% to 115%, it rounds to the nearest whole number. Ensure the shovel’s capacity doesn’t exceed a mass of mL/(10n). Systematically, take n shovelfuls from the laboratory sample, assigning each to separate designated areas on the working surface. Continue this process until the entire sample is distributed. Utilize random numbers, as outlined in annexes B and D of EN 932-1:1996, to randomly select the subsample(s) to be retained for further analysis.

Quartering Method for Sample Reduction

In the quartering method, the laboratory sample is placed on a working surface and thoroughly mixed by heaping it into a cone and turning it over with a shovel to form a new cone; this process is repeated three times. Each shovelful is deposited at the peak of the new cone to ensure an even distribution and thorough mixing of different aggregate sizes. The third cone is then flattened into a uniform heap. This heap is quartered along two intersecting diagonals at right angles, and one pair of opposite quarters is discarded while the remainder is shovelled into a stockpile. This quartering process is repeated until the specified test portion mass is obtained. Implementing a plate or a quartering cross of wood or metal can be beneficial in cases where the material tends to segregate, ensuring a more efficient and accurate quartering process.