Four Tips for Lab Sampling
BY Dan Ridolfi
Sometimes samples are sent to an outside lab to perform a test you cannot perform in-house. Sometimes you’ll have a contract dispute that requires testing performed by a third party. In either case, following a few best practices will save a lot of headaches and ensure the sample you sent represents the material produced. Here are four keys for the best lab tests.
Size it Right
Test methods for the American Association of State Highway and Transportation Officials (AASHTO), the American Society for Testing and Materials (ASTM), and departments of transportation (DOTs) specify minimum sample sizes. None of them set a limit on maximum sample size. The risk to a quality test result comes from very small samples; not from a sample that’s too large.
The minimum sample size is a function of maximum aggregate size. The larger the maximum aggregate, the larger the minimum sample size. The theory behind minimum sample size is to reduce the influence of one or two larger aggregate particles on a test result.
Don’t be afraid to gather large samples. Having a large amount of material ready for testing is always a good idea, but keep worker safety in mind. To reduce the risk of injury, the size of the container shouldn’t be so large as to make it heavy when filled. Many companies limit container weight to 50 pounds or less. Check with your safety department for the recommended maximum container weight guidelines.
Obtaining large samples while limiting container weight look like competing recommendations. To accomplish both tasks successfully, thoroughly blend materials after sampling, and store samples in their individual containers. A product such as the Quartermaster™ asphalt divider from the Gilson Company helps blend larger amounts of materials (hot-mix asphalt and aggregate) and then reduce them into sizes that are safe to handle. Once blended, any container of material should be just as representative of the whole sample as the next.
Sometimes, combining and blending material can affect the sample. One of the biggest dangers to maintaining a representative HMA sample through blending and splitting is the influence of a release agent. It’s required to spray splitting devices with a release agent to prevent material from sticking to the device. Never use a petroleum-based solvent like diesel or WD40. Silicon or plant-based agents work best. They prevent adhesion while not absorbing the liquid binder from the surface aggregate.
Keep it Cool and Dry
Where and how samples are stored is a very important factor in ensuring the sample represents production. The energy and effort in labeling and storing samples should be commensurate with their importance. If a set of cores will be used for density acceptance, then they should be stored indoors in a cool environment. Prolonged exposure to heat will cause a core to break down a bit.
Leaving cores in a box in the back of a pickup truck or inside a hot shipping container is the worst way to store them, and the most common way they are damaged before testing.
HMA samples are not as sensitive to heat as cores. Storing these samples inside a shipping container is not uncommon and is done successfully. It’s never a good idea, however, to leave sample containers of HMA samples exposed to the sun. Prolonged exposure to high heat and direct sunlight will affect sample properties.
Aggregate samples are tolerant to heat, but usually the bags they are stored in are not. Aggregates should be stored in a dry environment out of the sun. The most common way aggregate samples are damaged in storage is when their bags decay after exposure to sunlight. If aggregate samples are important, be sure to keep the bags out of the sun. If they must be stored outside, a bucket is a better choice for storing them.
Stay in Control
When it is time to obtain a sample for testing, whether for your own testing purposes, quality assurance, or for dispute resolution, you should have a very good idea as to the properties of that material before sampling. Having a good, statistically based quality control program is fundamentally important in today’s environment. Without one, you’re just guessing. If you are sampling aggregates for HMA design, you need to know if that bag of aggregate you have sampled, blended and split for the lab is representative of the pile to be used in production. If you don’t, you’re setting the production target values for gradation, voids in mineral aggregate (VMA), etc., on values that may not be repeatable. If the results aren’t repeatable, expect plant shutdown and project delays.
HMA tests interrelate. The high air voids typically correlate to low binder content. On-target gradation results typically correlate to on-target VMA. These are just two examples of many more relationships. Before obtaining samples for volumetrics or for performance tests, develop a correlation between gradation and binder content and other quick turn-around properties. Make small adjustments in production to maintain consistent quality material, so no matter when a sampling milestone is reached, the material will be of sufficient quality.
Developing quality control programs for individual plants based in Lastrada™ quality control software is what we do. We help plants reduce cost and improve quality. For more information, contact Dan Ridolfi at www.lastradapartners.com.