Maintaining measurement uniformity
Calibration of strain gauged columns is central to maintaining measurement uniformity of concrete cube testing machines, which are used throughout the construction industry.
Concrete cube testing machines, verified using strain gauged columns, are used throughout the construction industry to ensure quality of concrete production and therefore structural safety. NPL provides a UKAS-accredited strain gauged column calibration service to provide traceability for these testing machines.
Concrete cube testing
The construction industry, when pouring concrete, needs to ensure that the strength and conformity of the mixture is suitable for the purpose it is being used for.
The strength and conformity of the concrete is measured by testing samples of the concrete mixture, which have been moulded into standard size cube moulds and allowed to fully set. Once set, the moulded concrete cube is placed centrally in a compression testing machine in which the force required to crush is measured. When testing highlights non-conformity, this allows the user to take remedial action prior to further construction work.
Measurement of the correct concrete strength requires the force to be evenly applied to the cube. The ability of the machine to apply an evenly distributed force is certified using a strain gauged column.
Strain gauged columns
A strain gauged column is a vertical metal cylinder with four strain gauge bridges evenly spaced around the circumference halfway along its length. The strain gauged column is dimensionally measured on a surface table and then placed centrally in the NPL 3 MN compression testing machine, specific forces are applied, and the output readings from the four strain gauge bridges are monitored. The differences between these readings show how uniformly the load was applied to the ends of the strain cylinder.
Strain gauged column calibration
UKAS-accredited concrete testing laboratories must have their strain gauged columns calibrated every two years. NPL calibrates strain gauged columns in accordance with BS EN 12390-4:2019.
The strain ratio measurements of the strain gauged column are performed in NPL’s 3 MN reference compression machine, which has a long history of stability, both for the accuracy of the applied forces and the behaviour of the machine frame.
The calibration of the strain gauged column includes the measurement of the height, top and bottom diameter, flatness, and parallelism of the end faces. NPL has the facilities to refurbish the end faces of a strain gauged column if necessary.
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