Combined while-drilling techniques for the assessment of the fire damaged concrete cover

It is well known that the fire exposure of concrete structures lead to steep gradients of the material properties within the concrete cover. As a consequence, the assessment of the residual conditions of the structure is quite a challenging task, because the commonly available ND techniques are aimed at smoothing the effect of the inherent material heterogeneities at the scale of the coarse aggregate, which is also the significant range for the problem at issue. One promising answer is provided by the drilling resistance technique, which provides the point-by point mechanical response of concrete in terms of energy consumption or penetration rate while drilling a hole with a common hammer drill. Past experiences highlighted the features of this method. The pros are the speed of the test (in the order of 10 mm/s), the high resolution and the immediate availability of the results. The main drawbacks are the local disturbance ascribable to the hard aggregate particles and the not adequate sensitivity to low levels of damage in the material. In this paper the possibility to obtain more information from the same operation of drilling a hole is considered as a way to improve the sensitivity, reliability and range of this test. One first direction is the analysis of a sorted sample of the drilling dust, which is collected in a test tube by means of a special device. Different test tubes have been developed in order to allow a visual observation of the powder or the implementation of a thermal analysis. A second option is the visual inspection of the remaining hole by means of an endoscope fitted with a digital camera. After solving the problem of projecting and merging the images of the lateral surface of the hole into a single flat picture, the detection of local flaws, defects and colour changes becomes a viable task. Further studies are in progress in order to find other meaningful parameters to be monitored during the drilling process or other measurements to be performed in the drilled hole with specially instrumented probes. An account on these new results and a discussion on the benefits ensuing from the combined analysis of wide range of parameters is also included in the paper.