We are currently working on an automatic measurement of concrete porosity ensuring optimal observation conditions.

The porosity of concrete is an important parameter for the implementation of coating preparation of interior floors. The "drop test" is to place a drop of water on the surface of a concrete and observe its disappearance (Technical Specification Book 3469, CSTB, July-August 2003).

Currently, the measurement is performed manually. It gives a high degree of uncertainty because its result is highly dependent on the level of illumination, the amount of liquid deposited, the appreciation of the user ...

To improve the accuracy of the test, it is essential to ensure optimal observation conditions. To do this, we propose a completely automated measurement with image processing to obtain much information about the nature of the observed sample. The instrument in development will place several drops simultaneously, allowing also to extract information on the homogeneity of the material.

Figure 1: Observation of the disappearance of a drop of 50 L a) representation of the surface of the drop as a function of time, b) reference image before the deposition of the drop c) image of the drop in maximum spreading and its subtraction with the reference image, d) image of the drop to 15% of its disappearance and subtraction with the reference image.

With this automated processing the slabs are easily distinguishable by the criteria selected. The spreading time and the disappearance time (t1 and t2 in Figure 1-a) or the mean pore size over time are all useful information in order to accurately characterize a slab.

Currently in the testing phase, we quantify the influence of various parameters to select the optimal observation conditions, the ultimate goal being to determine objective criteria for building a relevant test protocol.