Henderson Thomas » Ireland » Structural Investigations

Non-Destructive Hardened Concrete Inspection

NDT Hardened Concrete Inspection without destruction of the tested part

HTA's Non destructive testing (NDT) process evaluates Hardened Concrete for differences in characteristics without destroying the tested part. We at HTA ensure that the part can still be used upon competion of the inspection.

NDT Hardened Concrete detail

Using both electromagnetic cover meters and ground penetrating radar technology HTA can carry out cover meter surveys to detect the following :-

Minimum cover readings based upon a designated grid system (normally 500mm or 250mm grid) to ascertain which reinforcing bars within a test area are most likely at risk from corrosion due to reduced levels of concrete cover. Readings require calibration against an actually exposed reinforcing bar which is normally required for other NDT testing.

Spacings of reinforcement and changes in size of reinforcement can also be obtained by the technology discussed above. One major issue is if you require to know the definitive diameter and type of reinforcement this cannot be determined by NDT techniques alone. As discussed above a calibration breakout of the reinforcement in question is required.

Half-cell potential surveys can identify areas where reinforcement corrosion is most likely to occur without having to remove the cover concrete.

A quick and easy test method to carry out on reinforced concrete. Using the exposed reinforcement as discussed above, the techniques require a high impedence voltmeter (calibrated) a half cell and connections to the reinforcement. Readings are recorded in millivolts (mV). Generally the more negative the reading the higher probability of corrosion occurring at the time of the survey.

However there are certain limitations and possible causes of error, which need to be taken into consideration. In HTA’s experience the most common is saturated concrete. Results are normally presented as a contour plot, and the ASTM C876:2017 gives criteria for interpreting results.

Concrete is an alkaline material. The alkalinity produces a protective film around the embedded reinforcement within, which will protect the reinforcing bar from corrosion.

The protective film surrounding the reinforcement can be broken down by a number of processes. One of them being due to the alkalinity of the concrete being reduced due to the absorption of carbon dioxide gas, known as carbonation. The carbon dioxide gas ingressing into concrete produces a reduction in the protective alkalinity of the concrete, and given the right conditions will allow the bars to actively corrode.

The depth of carbonation can be determined by spraying an indicator solution (1% phenolphthalein solution) on to a freshly fractured face of concrete.

The solution will react with an alkaline concrete to form a pink colour, however changes to a low pH will produce a clear result. An example is given in the attached photo.

Depth of carbonation survey

The highly alkaline conditions at the reinforcement/concrete interface produces a passive film on the surface of the reinforcement. Free chloride ions can locally breakdown this protective film and allow corrosion to occur. It is important to ascertain the source of the chlorides, normally either cast in at the time of construction or ingressed after construction (from de-icing salts or from marine exposure).

Chloride contents are normally expressed as a percentage by mass of concrete. Conversion to percentage by mass of cement can be made by an assumption of cement content or by actual chemical analysis. Determination of chloride levels is carried out by extraction of incremental drillings ensuring no contamination of samples occurs (accreditation by UKAS a must).

A guide to the risk of corrosion due to chloride content in concrete is given in TRRL Research Report 93 (PR Vassie) and from the Concrete Society TE60, stating the following:-

Chloride % by weight of cement Risk of corrosion
> 1.0 High
0.4 - 1.0 Moderate
< 0.4 Low

Care must be taken when discussing chloride contents if high levels of carbonation occur at the same time.

NDT Hardened Concrete Inspections - HTA

Visual, general and principal inspections of concrete structures, highways & non-highways

For almost 20 years, HTA has successfully carried out numerous Non Destructive (NDT) Hardened Concrete Inspections to highways, non-highways and concrete structures across the UK.

This has included Ultrasonic Pulse Velocity Testing to both highway related and non-highway related.

Concrete inspections undertaken:

  • Visual
  • Crack
  • Delamination (Hammer Tap)
  • Covermeter
  • Half-cell potential
  • Resisitivity
  • Linear Polarisation readings
  • Ultrasonic Pulse Velocity (PUNDIT)
  • Carbonation depth
  • Extraction of dust / core samples for laboratory analysis