The paper presents selected issues related to the load carrying capacity of joints between concretes cast at different times. The most important factors affecting the shear resistance, such as: surface roughness (profile), shear reinforcement ratio, concrete strength as well as the aggregate composition are discussed, including results of previous experimental studies conducted on push-off specimens and composite reinforced concrete beams. The differences in behaviour and shear resistance of contacts between ordinary concretes, lightweight aggregate concretes and recycled aggregate concretes are presented. Principles of interface design in the light of codes of practise: AASHTO-LRFD, ACI 318-19, EN 1992-1-1 and prEN 1992-1-1 were described. The theoretical predictions were compared with 184 results of experimental tests on push-off specimens. It has been found that most of the procedures allow for a safe estimation of the load carrying capacity of interfaces – with and without shear reinforcement. However, the obtained results were mostly conservative (depending on the considered design procedure, ratio of the experimental to theoretical load carrying capacity lies in range 1.51÷2.68). This may indicate that the description of shear transfer mechanism between concretes cast at different times is still imperfect and need to be improved.

The paper discusses the principles of the Critical Shear Crack Theory (CSCT) in terms of the punching shear analysis of flat slabs made from lightweight aggregate concretes. The basic assumptions of the CSCT were discussed, explaining the differences with regard to the calculation of ordinary concrete flat slabs, relating mainly to the adopted failure criterion associated with ultimate slab rotation. Taking into account the observations and conclusions from the previous experimental investigations, it was confirmed, that contribution of lightweight aggregate particles in the aggregate interlock effect should be ignored, due to possibility of aggregate breaking. However, the analysis of the profile of failure surface confirmed, that particles of the natural fine aggregate increase the roughness of the surface and should be included by formulating failure criterion for LWAC slabs.
The theoretical load-rotation relationships were compared with the results of measurements, confirming good agreement in most cases. The theoretical ultimate rotations were lower on average by about 11% than the experimental ones. The analysis of 57 results of the experimental investigations on punching shear of LWAC slabs made from various types of artificial aggregates showed a very good agreement with predictions of the CSCT. The obtained ratio of the experimental to theoretical load was 1.06 with a coefficient of variation of 9.1%. The performed parametric study demonstrated a low sensitivity of the correctness of the CSCT predictions to a change in a fairly wide range of parameters such as: the longitudinal reinforcement ratio, concrete compressive strength and concrete density.

The aim of the paper is to present the possibilities and limitations of using the Digital Image Correlation systems. In order to assess the measurement inaccuracies the measuring volume 1250 × 1100 mm was analysed using two cameras with sensor resolution 6 megapixels. It was stated very good accuracy of the line segment length change. It causes that observation of crack widths can be considered as precisely. Some practical information concern how determine the compatibility between crack width measured traditionally and by using DIC are given. In the second part of the paper the results of the tests concerning capacity of interface between two concrete casting at the same time were presented. Use of the optical measurement system Aramis enables the analysis of the deformation, determination of failure mode of the tested specimens and limit displacement between edges of the interface.

In the paper the problem of strengthening of flat slabs against punching shear was discussed. Selected methods verified on the basis of experimental tests such as increasing size of the support, applying post-installed shear reinforcement or increasing the main reinforcement by installing additional steel flat bars, were presented. The previous studies demonstrated, that the last method allows for an increase in punching shear resistance of up to 90%, depending on the longitudinal reinforcement ratio. The example of the application of such strengthening technique in the real structure was described. The use of steel flat bars located in the vicinity of the columns and additionally anchored to the slab made possible to compensate for the load capacity deficiencies that occurred due to execution errors (lowering of the main reinforcement within the support zones).