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Materials Testing and Structural Investigations




Covermeter Survey of Bridges


Adequate cover to the steel reinforcement in a structure is important to ensure that the steel is maintained at a sufficient depth into the concrete to be well away from the effects of carbonation or aggressive chemicals. Excessively deep cover also has its own attendant problems; crack widths may be increased and the lever arm decreased.

Electromagnetic devices, more commonly referred to as covermeters, may be used for the immediate non-destructive checking of steel reinforcement in structures.

The presence of reinforcement in concrete can be detected by the influence that reinforcing steel has upon an electromagnetic field induced by the covermeter. The covermeter operates by measurement of the reluctance of the magnetic circuit.


A covermeter may be used to determine the position and direction of steel reinforcement together with the concrete cover to the bar and/or the size of the bar.

The position and direction of steel reinforcement can be determined by moving the search head of the covermeter over the surface of the structure until the meter shows a maximum deflection. At this position the reinforcing bar is below and parallel to the length of the search head. This technique can be applied to establish the pattern and spacing of reinforcing bars embedded in concrete.

Covermeters can be used to assess the concrete cover by means of correlation and interpretation of maximum deflection readings.

Depending on the type of electromagnetic device, it is possible to assess the bar size diameter through the following techniques.

If the concrete cover is known, the maximum deflection reading can be used to assess the diameter of the reinforcement bar.

If neither the cover nor the bar size is known, then be application of a series of shim tests it becomes possible to assess the bar size and the cover simultaneously.

Interpretation of Results

The accuracy of cover measurements may be taken as 15% or 5mm whichever is the greater.

Although BS5400: Part 4 :1984 may not have been applicable to the design of the structure, the values of cover given in Table 1, as reproduced below, give the current nominal covers to the reinforcement for concretes of different quality and exposure conditions.

Environment Examples Nominal cover (mm)
Concrete grade
25 30 40 50 & Over

Concrete surfaces exposed to:

abrasive action by sea water

water with pH < 4.5

Marine structures

Parts of structure in contact with moorland water

- - 65* 55

Concrete surfaces directly affected by:

de-icing salts

sea water spray

Wall & structure supports adjacent to the carriageway

Parapet edge beams

Concrete adjacent to the sea

- # 50* 40

Concrete surfaces exposed to:

driving rain

alternative wetting and drying

Wall & structure supports remote from the carriageway

Bridge deck soffits

Buried parts of structure

- 45* 35 30

Concrete surfaces above ground level and fully sheltered against all of the following:


de-icing salts

sea water spray

concrete surfaces permanently saturated by water with a pH 4.5

Surface protected by bridge deck water-proofing or by permanent formwork.

Interior surface of pedestrian subways, voided superstructures of cellular abutments

Concrete permanently under water

45 35 30 25

Table 1. Nominal cover to reinforcement under particular conditions of exposure.

+Actual cover may be up to 5mm less than nominal cover
-Concrete grade not permitted
*Air entrained concrete should be specified where the surface is liable to freezing whilst wet
#For parapet beams only, grade 30 concrete is permitted provided it is air entrained and the nominal cover is 60mm

Reproduced from BS 5400: Part 4: 1984 with the permission of the BSI

Full copies of BS 5400 can be obtained from the BSI

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