Institute of Fundamental Technological Research

Fibre concrete specimens reinforced with 2% vol. of plain and hooked steel fibres have been tested in direct tension. The fibres were of two-dimensional (2D) orientation or they were magnetically aligned (1D). Several independent systems were used to measure strain and elongation of the specimens. The failure of fibre reinforced concrete was always ductile and the specimens of 1D type shown distinct strain hardening behaviour. In the latter case it was also possible to observe the multiple cracking of the matrix.

A method is presented which makes it possible to evaluate the fibre content or to estimate the distribution of fibres in steel fibre reinforced composite materials from the analysis of X-ray pictures. The basic notion in this method is an apparent fibre spacing defined as the average spacing between the intersections of individual fibre projections upon a certain plane and an arbitrary base line drawn on that plane. Such apparent spacing may be estimated analytically, it may also be measured directly on the radiogram. The comparison of analytical and experimental data shows satisfactory agreement.

Humidity field in the concrete may be found by solving the diffusion differential equation. The discussion of possible boundary conditions was done on the example of the infinite concrete cylinder, drying upon its surface. Two proposed boundary conditions lead to very similar solutions, provided that the evaporation coefficient at the surface is high enough. A comparison with the experimental results was made. Linear diffusion equation does not describe the drying process adequately. Diffusion coefficient has been suggested to be a quadratic function of the humidity, and the corresponding nonlinear equation has been solved by the numerical way.

A basic error is determined, which may be defined from the characteristics of measuring devices used in the field of strain measurements, Well known mathematical operation is then used to obtain the formula for the error of an indirectly measured function, which is assumed to be expressed by a differential of this function.

The analysis gives the limit values of errors for given state of strain and for given position of measuring device. The deduced formulae are used to find out the errors in the measurements principal values of strain and their directions are calculated from the direct readings. The proposed conclusions are directly applicable to the experimental analysis of deformation of concrete and prestressed concrete elements.