Our last blog post covered the important role aggregates play in many phases of highway construction. As samples are taken from the job site, prepared and tested in the lab, it’s important to ensure you’re meeting quality assurance requirements. Part 2 of this three-part series will cover proper preparation of bulk field samples for lab testing as well as necessary equipment and tips.
Coarse aggregate samples must be representative of the aggregate and should be obtained as described in ASTM D75 or AASHTO T 2. After samples are collected from a stockpile, conveyor belt, bin, or other source, it’s time to prepare the specimens for splitting procedures. Aggregate and other materials sampled in the field need to be reduced to appropriate sizes for lab testing while minimizing the introduction of variability during handling. In some cases, a few particles on a sieve might affect a gradation enough to alter an interpretation of the field sample and consequently the entire site’s compliance with specifications. The appropriate bulk sample reduction method is dependent mainly on the nominal maximum size of the aggregate, the amount of free moisture in the sample and the equipment used.
Bulk sample reduction is a first step in preparing properly sized specimens for tests like sieve analysis, soundness, hardness and other physical characteristics. The focus of the reduction process is in preserving the same particle size distribution of the bulk sample in subsequent smaller fractions used for test specimens. The main reduction methods are discussed in detail in ASTM C702 or AASHTO T248 and include:
- Mechanical Splitter
- Miniature Stockpile
Mechanical Splitter – This accurate and repeatable method is the most widely accepted for reducing coarse aggregate samples to test size. Splitters are commonly available for aggregates with particle sizes up to 37.5 mm (1½ in). Make sure the sample is air dried before using a Mechanical Splitter. Place the sample in the hopper or pan and uniformly distribute it from edge to edge, ensuring that it’s well-blended. Carefully introduce the sample into the chutes, allowing the aggregate to flow freely through the openings across the entire width of the chutes and into the catch pans. Continue this process until the entire sample has been halved, taking care so the catch pans don’t overflow. One of the halves is then split into quarters and so on until the desired test sample size is obtained. There are two important factors to keep in mind at this point:
- Don’t try to create a test specimen to an exact weight. A properly divided sample is much more likely to preserve the qualities of the original material
- Ensure that the weights of two properly divided halves are similar, but size distribution will be altered if you try to make each half exactly the same weight
A Mechanical Splitter with a gated hopper allows even distribution of the sample in the hopper for more accurate and repeatable splits and reduces handling efforts. Retain unused material until after testing in case a retest is needed.
Quartering – This method is acceptable for moist or compacted aggregate or when a mechanical splitter is not available. Coarse aggregates can be damp or dry for this method. Place the bulk sample on a level surface and mix the material by turning the entire sample over with the shovel three times. With the last turn, shovel the whole sample into a conical pile by depositing each shovelful on top of the preceding one. Carefully flatten the pile to a uniform thickness and divide the flattened pile into four equal quarters with a shovel or trowel. Remove two diagonally opposite quarters, including all fine material. Brush the cleared spaces clean. Successively mix and quarter the remaining material in the same fashion as the original sample. Continue this process until the desired quantity is obtained. Save the unused portion of the original field sample until all testing is completed in case a retest is needed.
If the floor is uneven, you can use an alternate method where the field sample is placed on a canvas blanket and mixed with a shovel, or by a lifting each corner of the blanket and pulling it over the sample toward the opposite corner to roll the material. Flatten and divide the pile or if the surface beneath the blanket is too uneven, insert a pipe under the canvas and divide the pile into two equal parts. Remove the pipe, leaving a fold in the canvas between the sample halves. Slide the pipe under the canvas blanket at a right angle to the first division and dissect the two halves of the sample. Lift the pipe evenly from both ends dividing the sample into equal quarters. Remove two diagonal parts including the fine material and clean the area. Mix and quarter the remaining material until the desired sample size is obtained.
Miniature Stockpile - This is the method used for fine aggregate with free moisture on the particle surface when preparing damp, fine aggregate only. Place the bulk sample on a level surface where there will be no material loss or contamination. Mix the sample by turning the entire sample over three times with a shovel. With the last turning, shovel the entire sample into a conical pile by depositing each shovelful on top of the preceding one. If desired, the conical pile may be flattened to a uniform thickness and diameter by pressing on the apex of the conical pile with the shovel. Obtain a sample for each test to be performed by selecting at least five increments of material at random locations from the miniature stockpile using a sample thief, small scoop, or spoon.
- Straight-Edged Scoop
- Shovel or trowel (for mixing the aggregate)
- Small sampling thief, small scoop or spoon
- Be sure to choose the correct sample reduction method based on aggregate moisture content
- Distribute the sample uniformly from edge to edge when placing in hopper (Splitter)
- Control the rate at which the materials are poured through the chutes for best results (Splitter)
- Brush cleared spaces clean of fines after removing the two opposite quarters from the flattened field sample to reduce sample bias (Quartering)
- Take all five samples from different locations to ensure representative samples (Miniature Stockpile)
Stay tuned for Part 3, where we’ll cover tips for testing a prepared sample. We’ll outline some of the main tests, necessary equipment and tips for effective sieve analysis of coarse aggregate.