Quality by Terracore

Quality in Construction and writer

STRUCTURAL TESTING OF EXISTING STRUCTURES

PART 1

Inevitably, concrete must sometimes be repaired. The best attempt at a repair is likely to fail if incorrect information exists about the existing strength and condition of the structural concrete or is missing. Likewise, the extent of repair and the type of material to be used for the repair depends on an accurate determination of the element’s current strength and durability. Several tests exist that may help designers gauge such parameters. Some of the most common include:

Compressive Strength Testing through Concrete Coring

To assess the in-situ compressive strength of the concrete in situations where a) there is little or no information available about the concrete used or b) there is doubt about recently supplied concrete, the taking of cores tends to be the main course of action in many situations.

The in-situ compressive strength, as measured by a core, will in most cases be lower than the compressive strength obtained from standard cast cylinder/cube specimens taken from the same concrete. In-situ compressive strengths obtained are a function of both the concrete supplied and the workmanship employed in its placing, compaction and curing. The procedures for the sampling and testing of cored specimens in EN 12504-1[2] are as follows:

Cores are to be at least 75mm diameter (locally 100mm is the normal diameter) except where not practical and then not less than 50mm. The diameter must also be at least 3.5 times the maximum aggregate size. As soon as the cored sample has been extracted it needs to be enclosed in a sealed container to prevent any exchange of moisture between the specimen and the environment until it is prepared and tested. This is to ensure the concrete remains in the same state as it exists in the structure.

The specimen tested should not contain;

• concrete from within 30mm of any surface.

• concrete from within the top 50mm or 20% (whichever is greater) of the lift in sections where height or depth of the element is not more than 1.5m.

• concrete from the top 300mm of the lift where the height or depth is 1.5m or more. It may be difficult to remove 30 or 50mm from the specimen in the case of thin sections e.g. composite decks.

Part 2 will be available on the next edition of the MCCM Magazine