When mixing concrete, it’s crucial to get the correct mix of ingredients. One of those ingredients is aggregate, small inert granular materials that influence the properties of the concrete. Before the use of Symons forms for curing, the mix of your concrete will be important to determine.
Aggregates can be chosen for a variety of reasons, including the durability and shape of the particles, one of them being size. Choosing to use a small aggregate may impact your void content and other outcomes in your project. Keep reading for more information on the use of aggregates, especially small aggregates, when mixing concrete.
Because of the major impact aggregates have on both fresh and hardened concrete, deciding to use them and which one to use should not be taken lightly. These inert fillers can compose up to 80% of concrete, giving the substance body and reducing shrinkage.
In addition to impacting the ultimate composition of your concrete, the aggregate chosen can have an economic impact on your project. Larger aggregates are usually less expensive and reduce the amount needed to meet cement requirements. Less cement means less water, which also reduces the risk of shrinkage and cracking that may cause expensive problems later on. However, there are other reasons to choose smaller aggregates that we will explore.
The most important decisions to make around aggregate are what shape and size you will use, as well as the texture as related to shape. The particle shape and surface texture will impact the properties of fresh concrete more than hardened concrete, though they matter for both.
Size specifically is related to your mix proportions and the type of work you’re doing. Aggregates are graded based on the size. A smaller aggregate is usually associated with stronger concrete, and the 20mm size fulfills this threshold for strong concrete, while 40mm is the normal strength threshold. The smaller your course aggregates are, the finer your fine aggregates should be, as this will increase the workability of your concrete.
Coarse vs. Fine Aggregates
Another way to refer to small and large aggregate is coarse vs. fine.
By definition, coarse aggregate is anything more than 4.75mm in size per piece. This may be boulders, cobblestone, or gravel. Anything below 4.75mm is a fine aggregate, including sand, silt, clay, or crushed versions of rocks. Sand is the most common small aggregate and is often called a “soft deposit” when used.
There is no maximum size for aggregate, but most structural applications will not go beyond 40mm unless it is a mass concreting project like a dam. In those cases, the size can go as high as 150mm. The term “maximum aggregate size” may appear during contracting as a reference to the smallest sieve that 100% of your aggregate could fit through.
Fine aggregates are broken down into four zones, with Zone 1 being the largest fine aggregates and Zone 4 being the smallest. This can be determined using a sieve, usually 0.6mm in size. Only 15-34% of Zone 1 aggregates can fit through this sieve, while up to 100% of Zone 4 aggregates will. These fine aggregates fill the voids left by coarse aggregates within a concrete mixture, leading to improved workability.
Other Factors to Consider
In addition to size, there are a number of other things to consider when choosing an aggregate, such as shape and texture, as well as what the concrete is intended to be used for.
Rough textured, angular, or elongated particles will require more water in order to produce workable concrete than smooth, rounded, smaller aggregates. This would also mean the cement content is increased to maintain the proper ratio of water to cement. As a general rule, flat and elongated particles are avoided or limited to about 15% by weight of the total aggregate. The unit-weight is a measure of the volume that graded aggregate and the voids between them occupy within concrete.
This void content between the particles will also impact the amount of cement paste required for the mix. For example, an angular aggregate increases the void content.
There is no size of aggregate that is the best. Instead, your project and needs will determine how these factors balance against each other.