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TechTalk with Mel

More about Aggregates

In my TechTalk titled “Hey, Check out Those Aggregates”, I discussed some of the basic requirements for aggregates. Recognizing that aggregates comprise 60% to 75% of the concrete's volume, it is imperative that they be well-graded, hard, strong, durable, clean, chemically stable and meet ASTM C33. In this article, I will comment on a few other factors that affect the strength and workability of concrete mixes.

Aggregate particle shape and texture can greatly influence concrete mixes. I am frequently asked about which aggregate shape is better – round or crushed. The shape of round aggregates reduces friction within the matrix, so the mix flows better, which increases slump. This decreases the amount of water required to make the mix workable, so concrete strength increases (remember – the lower the w/c ratio, the better the concrete). Concrete finishing is also improved with round sand particles.

Surface texture of aggregates also affects friction in the fluid mix, as well as bond between the paste and aggregates. Smooth aggregates have the same affect as do round particles – friction is reduced, slump increases, water can be reduced, therefore, strength increases. Bond, however, is reduced with smooth aggregates, which has the affect of reducing strength. On the other hand, the rougher texture of crushed aggregates improves bond between the paste and aggregates. Both round and crushed aggregates can perform effectively, as long as they are well-graded, and the mix has been designed correctly.

How do we define CA (Coarse Aggregates) and FA (Fine Aggregates, sand)? What is FM (Fineness Modulus)?

The generally accepted definition of CA are aggregates larger than a No. 4 (0.187”) sieve, while FA are those aggregates that pass a No. 4 sieve. Often, there is confusion about the terminology in describing maximum aggregate size. Maximum aggregate size is the smallest sieve opening through which 100% of the aggregate must pass through. The nominal maximum size of aggregate is the smallest sieve opening through which the major portion of the aggregate must pass.

FM reflects the mean size of an aggregate, and is a measure of how coarse the aggregate is.  It is an important indicator for sand.  FM is calculated by adding the cumulative percentages of the weight retained on each sieve size, and dividing that total by 100.  The lower the FM, the finer the sand stockpile.  Too large a variation in sand FM can greatly affect concrete strength and workability.

Other ASTM C33 (and AASHTO M6) requirements for FM state that:

  • There must not be more than 45% of fine aggregate retained between any two consecutive sieves
  • FM must be between 2.3 and 3.1, and not vary more than 0.2 between loads
  • The amount passing a 100 sieve is not used for calculating FM.  Everything passing 100 is considered to be pan

The bulk density (unit weight) of an aggregate is the weight of aggregate required to fill a specific unit volume.  For example, we refer to aggregates as having a weight expressed in lbs/cu. ft.  Well-graded aggregates decrease the amount of voids, which increases density.  Keep in mind that variations in the unit weight will result in either over-yielding or under-yielding of concrete batches.

Aggregates are a critical ingredient in your concrete mixes – take the time to understand them, and use them properly.




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