Exposed Aggregate Concrete is a chosen option for many when looking at alternatives for Driveway's, Patio's and some decorative features. To fully appreciate the old world look that can be created you need to understand the materials that are used to produce the given effect.
Colour oxides are often used to highlight the paste surrounding the aggregate and give a mood in keeping with the projects theme. Colour is only added at an approximate dose rate of between 2 and 5% per weight of cement. (This amount can change to your personal requirement). Due to the nature of colour some natural variation will occur throughout the project given how colour attaches itself to cement particles, the water demand between loads, the weather and the way the product is handled.
Over a period of time expect some lightening to occur from natural salts being released from the concrete; also known as efflorescence.
A commercial surface retarder is used to slow the setting of the cement paste on the top surface allowing it to be easily removed by a contractor once the concrete underneath has set. The contractors experience is paramount as to the application of product and the variables which influence the final product.
The paste, which glues the aggregate, fines and sand together, is a mixture of water and cement. Plain concrete normally has design strength of 17.5 mpa. As the top surface is removed in exposed aggregate concrete we recommend a 20 mpa standard, an increased strength to improve on normal design thus decreasing the risk of delamination of stones.
Due to the high abrasion of chip a high number of exposed aggregate concretes are made using rounded pebble. Dependant upon availability Concrete Suppliers will purchase rounded pebble from a number of sites. Unlike quarried rock, rounded pebble will have a range of aggregate type, organic matter, shell and coal to name a few. Their presence can be from nil upwards dependant on not only to geographic location but also the change within the quarry. These impurities will become a feature within your finished concrete and can include but not limited too wood, dying vegetation, pumice, shell and iron pyrites.
Some aggregates contain particles of iron pyrites, which form ferrous sulphide. This can react with the cement paste and oxidise to form iron hydroxide that is brown in colour. This can cause surface staining.
Air contributes to approximately 5% by volume within a concrete mix, however up to 12% can be trapped as a result of the mixing process. In most cases a percentage of air is expelled by the concrete placer. In trowelled concrete while still present the air is hidden under the hardened smooth surface, where as with exposed aggregate concretes where the top paste is removed it can be visible in the form of bubble like craters.
Etching is the process where diluted acid is migrated onto the concrete to take excess cement paste off the aggregate, take away any early whitening from efflorescence and give a more uniformed appearance over the project. Most contractors will include etching within a full contract. This can happen as early as 3 days after the concrete has been poured and up to three weeks later.
Variation highlights may appear between loads of concrete this is caused by not only the slump of the older concrete verses the fresh concrete but also the change in set times between loads from, ground and air temperature as it changes through the day, the exposure rate and the timeliness of the retarder application. Various types of cracking (also see NZRMCA "Cracking in Concrete Floors") and exposure will occur from weather conditions, design depth (recommended minimum 100mm), placement and handling of the product.
Please talk to your contractor or placer as to risks and how they can be minimized. Ask to view some examples of the contractors work.