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Exposing the aggregate in concrete is done by embedding stones, gravel or pebbles into the freshly poured mixture. The resulting material after this process is called exposed aggregate concrete (EAC).
Why use Exposed Aggregate Concrete?
There are several benefits to using EAC:
- It can act as a filter against sulphates and carbon dioxide, which would otherwise cause deleterious effects on conventional concrete.
- It provides enhanced durability of all components composed of concrete where high abrasion resistance must be present.
- It allows for more efficient heat transfer than standard concrete due to the cooling properties that accompany water evaporation on the surface.
- EAC is aesthetically pleasing and provides a more natural look than conventional concrete.
Advantages of Exposed Aggregate Concrete
There are several advantages to using EAC. EAC has increased resistance to abrasion and can also serve as a filter against sulphates and carbon dioxide, which would otherwise cause deleterious effects on conventional concrete. Exposed aggregate allows for enhanced durability of all components composed of concrete where high abrasion resistance must be present. Additionally, the cooling properties that accompany water evaporation on the surface make it possible for EAC to have more efficient heat transfer than standard concrete, which leads to improved energy efficiency when used in buildings. Furthermore, exposed aggregate concrete provides a more natural and aesthetically pleasing appearance than conventional concrete.
Disadvantages of Exposed Aggregate Concrete
Exposed aggregate concrete has increased abrasion resistance, but it also has the drawback of being susceptible to the alkali-silica reaction, which can deteriorate mortar and cement and cause structural damage. The high water absorption characteristics of exposed aggregate can also lead to freeze-thaw damage due to deterioration in winter when freezing occurs, causing expansion problems during the thawing process. This is because EAC increases the permeability of concrete, allowing moisture penetration leading to breakage. Finally, poor workmanship on the part of contractors will result in inefficient use of materials leading to higher costs.
How Exposed Aggregate Concrete is Made?
The process of making EAC begins with the examination of aggregate to ensure that it is free of organic and inorganic contaminants. It also must pass through sieves which remove any particles larger than 3/8 inch or 1/32 inch respectively for #5, #4, #3 and #2 grades of aggregate. Next, the engineer will determine whether fly ash needs to be added depending on the size and type of aggregates used and the desired characteristics for concrete. Even though the proper fly ash must be added, EAC will still have a slightly higher water-cement ratio than regular concrete, making it necessary to add more water. The mixture is then poured into formwork which creates the final product. After this step, any excess material is removed. Finally, the exposed aggregate concrete will either be allowed to dry or cured using steam, hot water or even through solar radiation if enough sunlight exists.
EAC must undergo curing for more extended periods because adding too much water can lead to large cracks in the finished product. This method requires that all free liquids be removed from the surface and edges of the structure before applying a sealant, which prevents further water loss and allows movement to be controlled through de-moulding. The de-moulded form is then moved to an area on the job site where it will dry.