Aggregates: Aggregates for Concrete Pt 1

Aggregates are not “just dirt.” That’s a direct quote heard previously. Often, too little attention is paid to the aggregates, and they are selected for one reason: price. The reality is that aggregates, on average, will make up around 70% or more of the weight and volume of your concrete mix.

Aggregates are a fundamental part of any concrete mix, playing a key role in the final product's strength, durability, and workability.  For those working with volumetric mixers or looking to optimize their concrete production, understanding the balance between fine and coarse aggregates is essential. This post is part 1 of a series of posts on the most underrated part of your concrete mix design. Aggregates!

Fine Aggregates: What They Bring to the Table

Fine aggregates typically consist of natural sand, crushed stone, or manufactured sand that is smaller than 3/8” or 5 mm in diameter. Their main role is to fill the gaps between the larger aggregates, making the concrete mix more cohesive, and providing a smooth surface finish. The right amount of fine aggregate helps ensures proper bonding of the mix and reduces the need for excess water, which can weaken the final product.

However, the quality and type of fine aggregate can impact the concrete’s performance. For instance, excessive fines, or overly fine particles, may increase water demand, while an irregular particle shape can affect the concrete's workability. In a volumetric mixer, sand that is too fine can have packing issues in the aggregate bins and will not flow out properly.

Coarse Aggregates: The Backbone of the Mix

Coarse aggregates are larger particles, usually more than 3/8” or 5 mm in size, such as gravel, crushed stone, or recycled concrete. These aggregates provide the bulk of the concrete's volume, contributing to its overall strength and resistance to cracking under stress. The size, shape, and texture of the coarse aggregate will influence the concrete’s performance, making it essential to choose wisely.

For those using volumetric mixers, the ability to adjust the mix design on-site allows for greater control over the aggregate proportions and quality. This flexibility can be a game-changer in producing consistent, high-performance concrete. As a volumetric mixer operator, you also have to be aware of the maximum size of the aggregates.  I personally recommend a max aggregate size of 1” stone. While I have run 1 ½” rock, it is hard on the belt, gates and auger.

Finding the Right Balance

The balance between fine and coarse aggregates is critical to producing durable, workable concrete. This is what is known as total aggregate gradation. Too much fine aggregate may make the mix sticky and hard to finish, while too much coarse aggregate could result in a harsh mix that’s difficult to place and finish. Understanding the aggregate gradation, or the distribution of particle sizes within the aggregate, helps optimize the balance between strength, workability, and finishability.

Cheap is not always best. As a matter of fact, the cheapest aggregate is often the worst for your concrete mix design. This is another area that you have to find the right balance. Cheap aggregates can have too many fines (dirty), poor gradation or reactivity issues. On the flip side, you can't haul the best aggregates, for normal concrete, from 200 or 300 miles away in a competitive concrete environment. 

At Concrete Business Solutions, we work with businesses to fine-tune their mix designs, helping them select the right aggregate proportions for specific applications and performance goals. With proper training and a better understanding of how aggregates influence concrete production, you can optimize your operations and deliver a quality product every time.