Not All Rocks Are The Same

The basic properties of concrete consist of aggregate, cement, water and admixtures.  There are many subcategories in each of those basic ingredients, but that gives an idea of what we use to make concrete.  Aggregates make up most of a yard of concrete, making up about 75% of the volume and when added to a binder, like cement, can (or cannot) make a very durable and strong concrete.  The following is a high level look at how important aggregates are when looking at concrete suppliers.  There are books written on the topic that don’t begin to scratch the surface of aggregates in concrete, but I will attempt to keep it concise and focus on the important information.

The rock and sand used in concrete is considered aggregate.  Because it consists of such a high percentage of the volume of concrete, it is essential that the quality of those aggregates is very good.  But what makes high quality aggregate?  I won’t bore you with the many tests that rock and sand go through before they are used but here is a list of things we look for in a high quality aggregate:

Cleanliness – rock that is mined from an area that is high in clay or silts often has issues with “cleanliness”. Silt and clay is very bad for concrete because those materials tend to soak up a lot of water making that fluidity unavailable to make the concrete workable.  To maintain workable concrete water is added creating diluted cement, or a low water to cement ratio.  Ultimately concrete strength and durability are impacted.  Another problem with “dirty” aggregates is the bond between the cement mortar and the aggregate.  If this bond is not strong, strength and durability are severely compromised.

Quality – aggregate quality testing is an important part of any mix design. The abrasiveness, strength, absorption, specific gravity, and many other properties are tested before an aggregate can be used in concrete. Each of these is important and has many effects on the chemistry of the concrete and how easy it is to achieve repeatable and consistent results.

Gradation – aggregate gradation is a term used to quantify the percentage of each size of rock there may be in a stockpile of aggregate. Not all rocks are the same size, and in fact, we don’t want them to be the same size.  A variety of different sizes fit together and a make a stronger matrix than all one size of rock.  As an example, if put ping pong balls into a glass there would be a lot of open spaces in between each ball.  If marbles, BB’s, and shot gun pellets are all added to the glass the air would be displaced much better.  It works the same way in concrete.  Large gaps in the sizes of rock in each mix cause a high demand for cement mortar significantly increasing the cost of the concrete, among other negative effects.

Aggregate quality is one of the most important factors to consider when choosing a concrete supplier.  All other ingredients of concrete are pretty much universal and are readily used by all suppliers.  Aggregate is the main difference between suppliers in any given market.  Aggregate cleanliness, quality and gradation directly impact your ability to place, finish, and cure concrete.  Most importantly, your choice in aggregate (and subsequently the concrete supplier) directly impacts your ability to make the owner happy and your ability to get the next job.