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Customers reach out daily asking if we have specific sizes of aggregates. In our experience, the most requested size of coarse aggregate has been ¾”. But what does ¾” actually mean? In this article, we will discuss what aggregate size actually means.
Let’s go over the definitions of size related to construction aggregates. The definitions sited in specifications are published in ASTM C125, “Standard Terminology Relating to Concrete and Concrete Aggregates”. There are two definitions in this standard that can help explain a specific aggregate size.
“Maximum Size” is defined as the smallest sieve opening through which the entire amount (or 100%) of the aggregate sample is REQUIRED to pass. Pay attention to the word “required”. This means that a specification written for 100% passing the 1” sieve means the entire aggregate sample HAS to pass through the 1” sieve.
“Nominal Maximum Size” is defined as the smallest sieve opening through which the entire amount of the aggregate is PERMITTED BUT NOT REQUIRED to pass through. In this definition, we’re paying attention to the last four words. Using a similar example from above, this would mean that a specification would most likely be written as 100% passing the 1” sieve and 90-100% passing the ¾” sieve. In this example, the ¾” sieve would be the nominal maximum size because the entire sample HAS to pass through the 1” sieve but the entire sample is also allowed to pass through the ¾” sieve.
ASTM and the American Concrete Institute (ACI) both use the nominal maximum size definition, e.g. identifying a #67 as ¾” to #4. The Federal Highway Administration (FHWA) Superpave asphalt pavement mix design defines nominal maximum sieve size as one standard sieve size larger than the first sieve to retain more than 10 percent. Meaning if the ¾” sieve retains 8% and the ½” sieve retains 15%, then the ¾” is the nominal maximum size. The FHWA then defines the maximum size as one size larger than the nominal maximum size. Similar to the concrete aggregate definitions.
This is where it can get confusing, frustrating, and possibly lead to incorrect products being used for project applications. If one asks for a ¾” aggregate product, are they saying the entire sample HAS to pass through the ¾” sieve or are they asking for an aggregate that is PERMITTED TO but doesn’t HAVE TO pass through the ¾” sieve?
Let’s go back to the ASTM #67 aggregate. This is arguably the most common coarse aggregate used in ready-mix concrete in Minnesota. This aggregate is also used as coarse filter aggregate on many projects. The gradation is 100% passing the 1” sieve and 90-100% passing the ¾” sieve. This aggregate is often purchased and referred to as ¾” aggregate.
Any way you look at aggregate sizing, it is imperative that the message is conveyed; is it maximum size or nominal maximum size? For certain construction applications, the aggregate size can directly impact the performance of the materials.
Should you have questions regarding this topic, reach out to us.
