The depth of reinforcement below the surface of concrete can be measured using a covermeter. Covermeters are electro-magnetic devices consisting of a search head and a control box. Most modern equipment has digital read-out and some systems have an audible output which increases in loudness or pitch as the search head approaches the position of a reinforcing bar. Older equipment may have an analogue output in the form of a needle sweeping across a graduated scale.
To check bar locations, the measuring head is moved slowly across the surface of the concrete. The read-out is observed until a minimum is reached and the position is noted. Bars of different diameter give a different response and when measuring cover to reinforcement it is necessary to know the bar size so that the equipment can be set accordingly. If reliable drawings are not available, the concrete at selected locations should be broken out to expose the reinforcement so that the actual bar size can be measured. Difficulties can be experienced when bars are closely spaced or where bars are lapped and this should be allowed for when using the equipment and choosing inspection locations. Use of the covermeter is covered in BS 1881: Parts 201 and 204.