Volume 26 (2018): Issue 3 (September 2018)

An additional concrete head or drop panels are not common solutions for strengthening flat slabs and have not yet been an adequately studied solution in the field of laboratory research. The investigation of drop panels in a laboratory is quite complicated, so that an examination directly at a construction site could be a better solution. Long-term measurement sensors were installed inside a concrete drop panel, which is on the bottom surface of a flat slab. Gauging was performed during various loading situations, including the loading of heavy machines, the ambient temperature load, and a uniformly distributed load. Results from the measurements are given in comparison with analytical models, which have been studied with respect to the influence of shrinkage and the bond-slip behaviour of a reinforcement.

In the past, research on the use of FRP in civil engineering has been focused on strengthening existing structures where FRP reinforcements were applied to the surface of concrete elements. Recently, the application of FRP reinforcements has been studied to replace steel reinforcements for use in areas of increased environmental loads, with a need to exclude the corrosion of the reinforcement or to ensure the electromagnetic neutrality of the individual elements of the load-bearing structure. The GFRP reinforcement ratio was verified considering failure modes in flexure and the bond of the GFRP reinforcement with concrete. Besides classical reinforcements, GFRP has also been used in prestressed variants, and the possibility of its use as permanent formwork has been verified. In terms of extending the use of non-metallic reinforcements, it is important to note the long-term exposure and possible degradation of the mechanical properties.

Considering the importance of leaking cracks in terms of serviceability and durability of watertight concrete structures, emphasis is placed on thermal movements and their effect on foundation slabs and walls. Both members are usually restrained to some degree externally and/or internally. The results indicate that restrained thermal stresses are the primary cause of early-age cracks in concrete members.

This paper offers a discussion of mitigation strategies to prevent the formation and propagation of early-age separating cracks. A FEM-based analysis was used to determine the development of stresses in walls on mat foundations in relation to the crack risk assessment.

Flat slabs represent a structural system with a typical concentration of shear forces near the vicinity of its local supports. A possible failure from punching is a dangerous phenomenon due to the brittleness and possible progressive collapse of a whole structure. An improvement in the structural behaviour of a slab-column connection provides transverse reinforcement. The amount of this reinforcement and thus its contribution to the resistance against punching has a limit, which is represented by the maximum punching capacity. This capacity can be assessed using the kmax factor or by direct verification of the strut capacity. The article deals with the results of a test campaign carried out on flat slab specimens with their transverse reinforcements designed in such a way that the crushing of the struts is the governing mode of any failure. The test results obtained allowed for an evaluation of the kmax factors and provide an answer as to whether it is possible to cover failures due to the crushing of struts by this factor.

The external bonding of carbon fiber-reinforced polymer (CFRP) strips by two-component epoxy adhesive on the concrete surfaces of buildings and bridges is a retrofitting method accepted worldwide. The gradient anchorage (GA) is an anchoring method especially developed to anchor prestressed CFRP strips to concrete elements without a need for mechanical clamping after the installation phase. This method takes advantage of the adhesives property to undergo accelerated curing when heated. The results of more than fifteen years of research on the development of the gradient anchorage at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are presented in this paper. The basic principles and application steps are explained, and the main results starting from the development of the technique up to the testing of real scale girders are described, and the new challenges posed by this innovative system are highlighted. The gradient anchorage is a valid alternative to a mechanically anchored system for prestressed FRP (P-FRP).

This paper presents the results of recent research activities on the hardening-induced stresses and the risk of cracking in very thick concrete members. These activities focus on a very large member with bloc-like dimensions. In contrast to typical member types such as ground slabs or foundation walls, bloc-like members are dominated by temperature gradients in all three dimensions (height, width and length) at the same time. Besides, the comparably high stiffness of these members leads to very low external restraints in cases of common applications. In these cases the members considered show distinct eigenstresses (residual stresses or self-balanced stresses) but only low restraint forces and moments. The assessment of the hardening-induced cracking of such members therefore requires a detailed analysis of the temperature and stress fields.

This contribution presents comprehensive investigations on the hardening-induced risk of cracking of a typical bloc-like concrete member in the form of an underwater foundation. The results of the simulation were compared with selected practical observations during the construction of the underwater foundation of a recently built bridge.

Steel bracing may be used to enhance the seismic strength of RC frames. Concentric steel bracing generally reduces ductility, which is a key component of seismic design. To overcome the problem, ductile steel brace-RC frame systems are therefore usually introduced in the form of eccentric braces. In the present study, the Off-Diagonal Bracing System (ODBS), which works as a concentric type of bracing, is investigated. In this paper the response of ODBS elements to cyclic loading is first explored and compared with those of other types of bracing such as X-bracing and inverted-V bracing systems. The time history analysis responses and cyclic hysteresis responses of a number of lowrise to mid-rise RC frames retrofitted with different types of bracing systems are then evaluated and compared. It is shown that under seismic excitation, a much reduced base shear is experienced by frames retrofitted with ODBS compared to other bracing systems. The results of time history and cyclic hysteresis response analyses also indicate a far greater energy dissipation capacity and ductility for the ODBS compared to other bracing systems. It is also concluded that ODBS performs best in lowrise frames. The out-of-plane buckling response of the ODBS is also investigated, and it is shown that a double-plated central connection can control such an adverse response.

The heating and cooling of buildings with large-scale ceiling systems nowadays is widely used in traditional as well as in new, low-energy buildings. This type of system is being employed in a building of the Civil Engineering Faculty (FCE), Slovak Technical University, in Bratislava. The building’s refurbishment in 2010 included the complete replacement of the building’s envelope. The replacement is a lightweight facade with a high percentage of transparent construction. Due to the differences between the type of envelope and the heating system, the operation of the heating system frequently causes thermal discomfort, especially during warm spring or autumn days. The aim of the measurements was to evaluate the control of the operation of the heating output, the appropriateness of the location of sensors measuring the outdoor temperature and possible improvement of the current control system to improve the heating system’s quality.