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 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
- 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 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.
Where concrete is exposed to air, carbonation of the cement matrix will gradually advance from the surface. This is a slow process in good quality concrete and is of little consequence unless embedded ferrous metal i.e., reinforcing steel is present in the carbonated zone.
This is not necessarily a problem with plain concrete, but it can remove the protective passive layer surrounding the ferrous reinforcing steel. The passive layer, which requires a pH of 10.5 to form , is characterized as an alkaline film of oxide around the steel that acts as a barrier to oxygen and water reaching the steel. If oxygen and moisture are available at the depassivated steel surface, corrosion of the steel may then occur, consequently disrupting the concrete due to expansive nature of the corrosion products (rust).
Terracore me sure carbonation depth in the field using the phenolphthalein test method, applied to cored or extracted samples, in accordance with EN 14630.
Where concrete is partially carbonated and has not reached the reinforcement, coatings can be applied. These inhibit the carbonation front progression and the alkalinity of the concrete can redistribute to improve the passivity.
Where the carbonation front has reached the reinforcement and conditions for corrosion exist, other repair methods such as cathodic protection, must be contemplated.
Ground Penetrating Radar (GPR) testing
Where destructive testing of concrete, such as coring, is not an option, GPR testing may be utilized to locate and ascertain the presence of steel reinforcement within a given structural element. GPR is a method that uses radar pulses to produce images of the subsurface so customers can safely carry on with their projects without any surprises. Apart from the presence of steel fabric, GPR may also detect
obstructions in buried concrete such as water and power line , voids in concrete and fibre-optic lines GPR imaging
devices also detect variation in the composition of the ground material with no digging or excavation being necessary.
At Terracore, we carry out a multitude of other specialised concrete investigation testing, assisting our clients in developing a tailor-made testing program specific to their project and budgetary requirements, inclusive of reporting and recommendations for repair works, whenever this is necessary.
First & Second Edition
Terracore Ltd has over 30 years experience in drilling on-shore and off-shore, locally and abroad. We can drill from the smallest hole of 1mm up to 300mm in diameter vertically, horizontally and at any angle. Some of the services include borehole drilling, micro-piles, W.S.C approved water wells, rock bolting, bore holes for deep electrodes, downhole installations, and more. We can also drill in very confined spaces with our specially designed rigs to provide the best service with no damage to existing structures.