The Importance of Aggregates in Concrete

From the Blog


Why should anyone care about concrete aggregates?
In addition to defining the thermal and elastic properties and dimensional stability of the concrete, they also make up between 60 to 75 percent of the total volume of concrete. So, if you compare making concrete with making a cake, aggregates are as important as flour. Here is a quick visual illustration of the basic ingredient ratio that make up most concrete:

Concrete Ingredient Ratio Pyramid


What are aggregates?
They are simply inert filler made of granular materials like sand, gravel, or crushed stone that are mixed water and Portland cement to make concrete. There are two main categories of aggregate: fine and coarse. Fine aggregates are made of sand and crushed stone that can pass through a 3/8-inch sieve. Coarse aggregates refers to any particles with a diameter ranging between 3/8 and 1.5 inches. It is also

Where do aggregates come from?
They mostly come from rivers, lakes and seabed. First, rocks are crushed, then screened and finally washed until various sizes are obtained (also referred to gradation). Recycled concrete also makes a good source of aggregate for soil-cement and new concrete.

Proper proportioning and mixing of ingredients are essential to achieving the right type of concrete for the job. Not enough cement paste in the mix to fill the voids between the aggregates will produce rough and porous surfaces. A mix with too much cement paste will produce a smooth surface, easily pourable but the concrete will be prone to more shrinking and will cost more.
For example, a typical mix will have between 10-15% cement, 60-75% aggregate and 15-20% water.

How to choose the type or size of the aggregate mixture?
To produce a more workable concrete, it is recommended to use smooth and rounded aggregate instead of rough angular or elongated aggregate. A good source of smooth aggregate are natural sands and gravels dug from riverbed or seashores.  Angular aggregates created from crushed stones yield a higher surface-to-volume ratio with a better bond quality but will require more cement paste to obtain a workable mix.

Water-cement ratio:
It is the weight of the mixing water divided by the weight of the cement. Why is this formula important? To obtain high quality concrete, you need to lower the water-cement ratio as much as possible without losing workability of fresh concrete.

Matching concrete pumps with aggregate sizes:
Some concrete pumps only process smaller while others work best using larger aggregate material. For Example, a larger pump like the Putzmeister Katt Kreter uses a larger aggregate material at 1 1/2 inch. Whereas a slightly smaller pump like a Schwing BPA 500 uses only one inch aggregate.
For smaller jobs, you may only need a small towable trailer pump like a Mayco C30 which pumps half inch aggregate.

One important matter if you want a low slump concrete and pumping over 100 feet +, the slump will reduce at least 20% to 30%, due to pressure and heat. If you want a 3″ slump, you need to start out with at least a 5″ -6″ slump going into the hopper.

So next time you are doing a concrete job, think about aggregates and their different properties because they will give you the most flexibility to meet your design and construction requirements.

Do you have any questions about concrete mix or concrete pumps? Call Dick at United Equipment Sales at (503)283-2105 and if you are looking for new or used pumping equipment, do check his special deals.

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