Coring Hardened Concrete
The coring of hardened concrete elements is a regular necessity and is done for many reasons. Often, a post or scaffold attachment needs adding to a structure, fence posts or railings are occasionally added to the top of a concrete casting or any number of other reasons could raise the requirement for drilling hardened concrete. In particular the need to confirm the compressive strength of an element necessitates drilling core samples.
Vertical drilling can be undertaken on concrete bases and floors with a standard hydraulic rig but horizontal or angled samples are taken using a smaller unit bolted to the element. In both cases water cooled diamond drilling is used with a core barrel of the appropriate diameter. This is determined by the test sample height to diameter ratio which is 2:1. Hence, a 200mm diameter core barrel needs to be used only on elements greater than 400mm thick. For thinner elements a smaller diameter is required. A minimum of three samples are usually taken and in the laboratory they are either ground or capped to ensure the top and bottom surfaces are parallel. The samples are cured in a water bath for a period and then crushed in a compression testing machine. The resultant average failure load is converted into an equivalent concrete cube strength to be compared to the design requirement. This process is often carried out on structural elements where samples taken at the time of placement failed to reach their target strength at 28 days.
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