How to do a Concrete Cover Check

Cover Measurement for Quality Assurance

Choice of system

The most common equipment used for cover surveys are eddy current cover meters and ground penetrating radar. There are also rare occasions in which ultrasonic imaging equipment may be suitable, but it is generally not appropriate.

Pulse induction cover meters are accurate to within 1mm within their shortest scanning range, older magnetic reluctance systems are essentially obsolete having lower precision and a greater tendency to drift from a zero value. The greatest range to which a cover meter is suitable is generally 140 mm for a structure with 16 mm diameter reinforcing bars (although maximum depth of penetration is as much a function of depth as it is of bar spacings).

Ground penetrating radar [GPR] does not have as tight an accuracy range, with measurements usually being within 10 mm depending upon operator skill and calibration for the on-site materials. GPR is typically only used for cover measurements where a structures reinforcing cage is very congested or as a secondary result of surveys for reinforcing layout of complicated structures.

Evaluation of job

When conducting quality assurance checks on new structures the choice is between doing a complete survey or to reduce the scope of the survey. For precast elements with tight control it may be possible to check key points on the reinforcing cage and confirm compliance for every element, but for in-situ construction surveying complete pours is impractical.

The alternative is to treat the cover of all similar elements together as a normal statistical population then determine the number of points which need to be surveyed to provide a suitable level of confidence that the whole structure is compliant.

For example the precast beams may be one population and the cast insitu columns may be another. It may be necessary to further break down populations, such as when average cover at the base of a wall may be more consistent where the bars are tied into the structure. Near the top where they are free and retained by bar spacers only there may be greater variance or a trend to low or high cover.
Then the overall size of the population must be estimated, number of bars per square meter and overall surface area will suffice. Finally armed with information on each population the number of random tests to conduct can be calculated. ASTM E 122 offers one procedure with helpful commentary.

As an alternative to calculating sample sizes manually there may be specifications that indicate the minimum expected testing requirements. For example the Vic Roads Section 610.33, which states that for new construction every 25m2 area of concrete a 3m2 sampling region must have 10 measurements recorded using a calibrated cover meter.

Detailed guidance can be found within the DBV Guide to Good Practice "Concrete Clear Cover and Reinforcement" (Edition 2002) which provides clear and concise methodology for the treatment of results (we can provide a link to a summary of this in English for interested customers). The upcoming Z7-07 from the Concrete Institute of Australia also has guidance with regards to cover measurements on new structures.

Completion of survey

When working on site the survey can hopefully be completed as planned. If the real environment is very different to that discussed during the planning stages then it be necessary to change the scope of the job or replan. Some general "gotchas" that may be found on site.

Intervening material

Carpet, asphalt and thick coatings will all increase the apparent cover depth and should be compensated for in post processing or within the systems settings if available. Testing on the inside of curved elements will have the same effect

Limited access

With cover meters or GPR they can only detect reinforcing directly beneath the sensor, where angles and fixtures prevent scanning it may be necessary to make qualified assumptions based upon surrounding rebar layout, these must always be clearly stated in reports.

Nearby metal

For magnetic cover meters any metal close to the point of scanning will decrease the apparent cover measurement. Metallic coatings or steel fibre reinforcing will completely prevent scanning with radar or cover meter.

Reporting

Where checks are small scale it may be sufficient to train the operator make final decisions on compliance and approve the elements directly, such as with cover checks upon receipt of pre cast panels. Larger surveys will require at least data retention to confirm compliance and in some cases a formal report. A proforma should be used on site when data is being recorded manually, and even with automatic data logging notes should be taken which relate file names to survey locations as a bare minimum. A report should clearly indicate the region surveyed, if it is compliant or non-compliant, and then collected data and calculations for review if necessary. It should also clearly indicate which standards and specifications were used along with calibration information for test systems.

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