There are occasions when it becomes necessary to verify the condition of concrete pavement slabs. Voiding can occur beneath the slab due to washout, slab movement or subsidence, and this can lead to rapid degradation of the slab. This becomes an important issue in heavily trafficed areas such as airfield aprons and warehouse floors and concrete roadways, where safety is a concern.

Testconsult has developed several non-destructive techniques to check for voiding in concrete slabs and these can be used singly or in conjunction with each other:

Impedance Testing

The mechanical impulse test or impedance test was originally developed for the non-destructive testing of deep foundations, although it has increasingly been applied to test concrete structures over the past ten years.

The method is based on measurement of the dynamic impedance, or mobility of a point on the structure to a transient force.

The structure is excited by a blow from an instrumented hammer containing a load cell which monitors force, and a velocity transducer monitors the response of the structure. The two signals - force and velocity - are recorded and converted to a frequency response. The velocity spectrum is then divided by the force spectrum to produce a mobility or mechanical admittance plot, which is the characteristic signature of the structure at the point of test. The slab mobility at low frequencies and the general shape of the response curve are a function of the concrete quality and the slab support.

The method has been applied to concrete highways and runway slabs, and can detect problems associated with slab movement either side of transverse joints. This also extends to monitoring of slabs repaired by grouting to improve their seating. Factory and warehouse floors, tunnel and sewer linings are other applications. Impedance testing will locate voiding, poor support, delamination and poor quality concrete.

Trials of this technique were carried out on a stretch of concrete highway in Oklahoma, USA, in 1989, during which the effectiveness of grouting up voids beneath undowelled slabs was monitored. Impulse readings were taken before and after grouting and in many areas it was found that the slab support had deteriorated. When slabs were lifted in selected positions, it was found that pressure pads had effectively jacked up the slab in grouting positions! A more suitable grouting procedure was then adopted to prevent this. This method was also used extensively on the French side of the Channel Tunnel to assess the effectiveness of the grouting operations behind the tunnel linings. Small battery-powered units were specially built for the contract.

Impulse Radar

Impulse radar can also be used for checking concrete slabs. As well as detecting voiding beneath the slab, it has the advantage of being able to look for voiding and steel work within the slab. The main limitation is that if reinforcement spacing is too close together, the signal is unable to penetrate beyond. Look in the geophysics section for further details and applications of Impulse Radar.

Confirmation

It is always a good idea to confirm the findings of NDT methods as mentioned above by a visual examination. This can be done unobtrusively by drilling a small diameter hole through the slab and examining the suspected void with a flexible videoimagescope. The thickness of concrete can be used to correlate radar images and improve accuracy.