The ultimate target of this project is the development and the introduction of automatic image analysis techniques into the petrographic analysis techniques into the petrographic analysis methods for monitoring and assessing the quality of concrete constructions and cementitious building materials.
Automated image analysis methods have been developed to quantify the W/C ratio and the homogeneity of the cement paste. Measurements on cement paste samples and on concrete samples confirmed the direct relationship between the W/C ratio and the mean grey value measured on thin sections. Measurements on samples with different amounts of different types of sand indicate that the effective W/C ratio of concrete is strongly influenced by the specific surface of the sand. Two new homogeneity coefficients, one describing local (multifractal concept) and the other one describing global homogeneity (statistical analysis) have been defined. A comparison between the measurements of these coefficients and an expert classification gave satisfactory results.
A procedure has been established to investigate quantitatively the cement microstructure using polished sections and SEM-techniques. To improve the quantitative aspect, more methods have to developed using thresholding techniques and new concepts have to be introduced to describe the disorder of the microstructure. The results indicate that the fractal dimensions range from 1.44 to 1.71 but although the specimens have different compositions, the shapes of their structure are characterised by similar fractal dimensions. The fractal dimension is therefore not useful as a means of differentiating pore structure from BSE images of cement based materials.
New procedures have been developed to characterise the air void system using automated image analysis techniques. The different measurement procedures have been evaluated on concrete samples covering a wide range of air content and specific surface. In parallel the air void structure has been evaluated by traditional point count methods as described in ASTM C457 on thin sections. The results indicate that air void analyses on thin sections do not provide sufficiently accurate results. First of all, the measured air content is apparently too low. Secondly, the area provided by two thin sections yields a higher scatter of the measured parameters than if one plane section is measured.
Image analysis procedures have been developed to characterise quantitatively the presence and the morphology of cracks and to recognise and to quantify crack patterns. Therefore, a sample catalogue in four copies of thin sections illustrating different types of cracks in concrete has been prepared.
The grain size distribution of aggregates in hardened concrete has been determined using automated image analysis techniques.
The tasks of this project are :
- to prepare guidelines, rules and a sample catalogue for the petrographical assessment of the durability parameters in order to assure the use of uniform procedures and evaluations and to contribute to the pre-normative work in the EC
- to develop, evaluate and standardiz methods in order to measure and to quantify parameters which until now are described qualitatively by petrographical means
- to introduce a fractal approach to characterize the microstructure of the cement paste
- to develop methods in order to recognize, to describe and to quantify complex patterns (for example crack patterns) with the aid of mathematical morphology
- to develop the necessary procedures for an automatic image analysis system to measure and evaluate all the parameters resulting from the forementioned methods.
It is expected that this research will result in a fast, objective and uniform method for the quality control of new structures and in a method for the assessments of defects and damage and its causes in alread existing structures.