The interaction between short fibres and concrete in the post-cracking phase influences crack spacing and width in the composite. In order to perform analysis of deformation of a composite and the fibre displacement at the crack, single fibre was examined. A two-fibre model considering the distribution of fibre length and incomplete bonding was developed. Numerical analysis reveals that two-fibre model analysis is believed to be more accurate than that obtained from the single-fibre analysis. Comparing the solution of the single-fibre and two-fibre system shows that the latter gives a greater fibre displacement at the crack. The study was performed in the Department of Structural Engineering, year 2009/2010.

Concrete is one of the most popular construction materials in the world. The concrete properties depend from many factors. One of them is aggregates quality, which can be defined as an impurity level. The most unwanted materials in fine aggregates are clay and soil particles. These particles can impure aggregates due to irregular sands in quarry and during the digging process. In the present paper were estimated how impurity impact the properties of concrete. During the research the aggregates were impured with clay and soil particles. The change of fresh concrete workability, hardened concrete density, compression strength, water absorbability and frost resistance were evaluated. The results show that aggregates impurity with clay and especially with organic materials (soil) is very dangerous. The concrete workability, frost resistance is decreasing. The clay and soil have a different effect on hardened concrete density and compression strength. Soil decreased both parameters while well mixed clay increased these characteristics. However, generalizing all results, can be stated, that keeping the same level of concrete workability it is necessary to raise water and cement ratio and it will give strength loss effect.