Understanding Exposed Aggregate Better

Exposing aggregate is often done in driveways or on patios, mainly when an exposed aggregate concrete floor will be painted. Concrete with exposed aggregate (called “parging” by masons) is composed of individual rounded stones (pebbles) and/or crushed stone bound together with Portland cement and water. The surface may not be as durable as other types of concrete, and for this reason, it is usually covered with a thin coating of portland cement and water before it sets. Parging is frequently used to cover exterior horizontal surfaces such as siding on the house but can also be applied over concrete for an exposed aggregate look. Unfortunately, when the concrete has been parged, there are more opportunities to be scratched, dented or damaged. 

Exposed aggregate is a construction material composed of large pieces of rock embedded in mortar or cement. It is usually used for paving roads and garden paths but can also be found as some composite materials under the name exposed mixed aggregates. A similar appearance on walls gives them an antique appearance referred to as a rustic finish. The aggregate may not be visible on all surfaces; frequently, some areas are left unflagged (unglazed), producing a lighter colour overall than fully flagged (glazed) concrete. Flag refers to the side/top surface with the exposed aggregate showing through the thin mortar joint overlay that covers most, if not all, of the concrete surface. 

To achieve the best appearance with exposed aggregate floors, surfaces should be slightly roughened by sandblasting or scarifying before applying a patching material over the surface. This is usually done after the floor has been laid but can be done at any stage during slab-on-grade installation. A thin coat of portland cement and water will seal the concrete from staining, help it set faster, blend in better with other materials used to create an overall design effect for interior or exterior use. Sometimes aggregates are dyed in different colours to give a speckled effect that is natural-looking. Glazed surfaces are watertight and do not require sealing. They also tend to wear more evenly than flagstone surfacing because they are much harder to scratch, chip or gouge. 

 A significant advantage of exposed aggregate is that it can look like natural flagstone, mainly if the aggregate is irregularly shaped. In addition, the strength and hardness of concrete are often comparable to many types of natural stone used for flagstones. However, care must be taken to ensure that the resulting surface does not become slippery when wet, particularly if polished to a gloss finish. 

There are also some disadvantages when it comes to exposed aggregate: 

• The aggregate may not blend well with surrounding materials.
• The surface may become slippery when wet.
• Because water is required for setting and curing, in some situations (including underfloor heating), this can lead to problems in winter if the ground becomes frozen or saturated.

Remember, before applying an exposed aggregate finish over a concrete slab, the area must be adequately prepared for this type of application. This means that imperfections such as surface discolouration, laitance or other minor blemishes must be repaired by scarifying and patching the concrete slab before applying any finish. 

To achieve the best results, it is also recommended that you hire a professional concrete contractor to do the job for you. The professionals will be familiar with all of the proper techniques and methods to ensure your exposed aggregate finishes look their best.