Jaroslav Halvonik

Punching Resistance of Flat Slabs with Openings – Experimental Investigation

Flat slabs are commonly used structures in contemporary architecture. Although their common use there is still problem in design of these structures. The openings adjacent to a column are often used for plumbing and such a position of the openings increases shear stresses in the flat slab near the column. This paper deals with experimental work focused on the punching shear resistance of the flat slab specimens with openings adjacent to column compared to the flat slab specimens without openings. The opening influence is determined experimentally and by using models for the assessment of punching resistance from relevant standards and codes. The material properties of concrete and reinforcing steel were obtained from the laboratory tests.

Bond Behaviour between GFRP Reinforcement and Concrete Using a Pull-Out Test

Corrosion of steel reinforcement is one of the most often deterioration reasons of RC structures. At present, the corrosion of steel reinforcement can be avoided by using non-metallic reinforcement from composite materials, especially in structures that are exposed to extreme environmental environment. These materials are durable and non-conductive. They are composited from two materials: fibres and matrix. The most commonly used FRP (Fiber Reinforced Polymers - FRP) reinforcement are glass fibre reinforced polymers (GFRP). The bar surface can be e.g. sanded, wrapped, with helically wound ribs. The bond between concrete and reinforcement is one of the basic requirements for the composite action of both materials. The transfer of forces between the steel reinforcement and the concrete is provided by the following mechanisms: adhesion, friction and mechanical interlocking. The bond between GFRP reinforcement and concrete is different and it is ensured by friction and mechanical interlocking of the rebar surface. The chemical bond does not originate between GFRP reinforcement and the surrounding concrete, so adhesion does not contribute to transfer of the bond forces. Some few test methods are used to determine the bond between GFRP reinforcement and concrete. The pull-out test were used to determine the bond behaviour between GFRP rebars and concrete. This paper describes the preparation, process, results and evaluation of the pull-out tests.

Experimental Verification of Concentrated Load Transverse Distribution in Shear

An ability of reinforced concrete slabs to distribute concentrated load in transverse direction is an important property for their verification. Several methods for the assessment of transverse distribution (effective width) were proposed but they were not incorporated in the current standards. This paper deals with an experimental analysis of the effective width for shear verification of slabs subjected to concentrated load. Shear resistance of the slab specimens subjected to concentrated load obtained from tests has been compared with resistance of the slab strips loaded by uniform shear. Based on this check, the effective width has been evaluated. Experimental results were also compared with results of models introduced in relevant standards.

Secondary Moments due to Prestressing with Different Bond at the Ultimate Limit State

Abstract Secondary effects of prestressing develop in statically indeterminate structures (e.g., continuous beams) due to the restraint of deformations imposed by hyperstatic restraints. These effects may significantly influence internal forces and stresses in prestressed structures. Secondary effects are influenced by the redundancy of a structural system, which raises the question of whether they will remain constant after a change in the structural system, e.g., due to the development of plastic hinge(s) in a critical cross-section(s) or after the development of a kinematic mechanism, or if they will disappear when the structure changes into a sequence of simply supported beams. The paper deals with an investigation of the behavior of continuous post-tensioned beams subjected to an ultimate load with significant secondary effects from prestressing. A total of 6 two-span beams prestressed by tendons with different bonds were tested in a laboratory with a load that changed their structural system into a kinematic mechanism. The internal forces and secondary effects of the prestressing were controlled through measurements of the reactions in all the supports. The results revealed that the secondary effects remained as a permanent part of the action on the experimental beams, even after the development of the kinematic mechanism. The results obtained confirmed that secondary effects should be included in all combinations of actions for verifications of ultimate limit states (ULS).

Experimental Investigation of the Maximum Punching Resistance of Slab-Column Connections

Abstract 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 Maximum Punching Capacity of Flat Slabs

Two ways how to determine maximum punching resistance of flat slabs with shear reinforcement are currently used. The first way is verification of the concrete strut capacity at the column periphery defined as VRd,max. The second limit is defined as kmax multiple of the punching shear resistance without shear reinforcement VRd,c. The values of kmax are proposed usually in between 1.4 and 2.0. Results of the experimental tests are presented in the paper that were focused on above mentioned limits, whether failure of the struts can precede any other form of punching failure that is limited by kmax*VRd,c. Two experimental slab samples reinforced with high amount of shear reinforcement that increased punching capacity above capacity of the concrete struts were tested together with two slab samples cast without shear reinforcement. Comparison has shown that punching resistance of flat slab with shear reinforcement has been 1.7 times higher than resistance without shear reinforcement. While some standards allow for use kmax value of 1.9 in this case. This indicates that limits based only on the kmax factors may overestimate actual maximum punching shear resistance.

Shear Resistance of Slabs Subjected to Concentrated Loads

Shear resistance of reinforced concrete members without shear reinforcement is a discussed topic because of its brittle character of failure. Bridge slabs subjected to concentrated loads have the ability to distribute local loads in transverse direction, which contributes to their shear resistance. Verifications used in the past (based on theory of allowable stresses) provided much higher shear resistance than currently valid EC2. This often leads to requirements of shear reinforcement in areas where it wasn’t necessary in the past. This also brings up the question of reliability of existing structures.Multiple experimental campaigns at several European universities have been carried out in past years with the intention to verify the real resistance of slabs subjected to concentrated loads. The process of selecting and calibration of a new model for verification is also in progress. This paper deals with analysis of experimental results based on currently valid and new models and their comparison.

Shear Assessment of Concrete Bridge Deck Slabs

The transitions from old STN standards to Eurocode standards brought several problems into bridge design and assessment. Shear reinforcement is now often required even in concrete members, which were previously allowed to be built without it. Moreover, assessment of existing reinforced concrete bridge structures often shows their insufficiency in shear capacity, which means that they should be strengthened or replaced. Work on new generation of Eurocodes is currently in progress and current model for shear assessment should be replaced by a new (and more precise) one. This paper deals with the problem of shear assessment of concrete bridge according to current standard and also according to the new shear models that are under consideration.

Secondary Effects of Prestressing at ULS on Hyperstatic Structures

Secondary (parasitic) effects of prestressing develop in hyperstatic structures (continuous beams) due to restraining of imposed deformation by hyperstatic restraints. These effects may, in some case, significantly influence internal forces and stresses in prestressed structures. Internal forces due to the secondary effects should be included in design combinations for verification of both ultimate and serviceability limit state. Because secondary effects are influenced by structural system, there is a question how they will change after changing of the structural system e.g. due to development of plastic hinge (s) in a critical cross-section (s) or after development of kinematic mechanismThis article describes an experimental program at Slovak University of Technology in Bratislava, Department of concrete structures and bridges and its results. Program were focused on investigation of behavior of continuous post-tensioned beams with significant secondary effects of prestressing subjected to ultimate load. Together six, two span beams were tested, with maximum load changing structural system into kinematic mechanism. Secondary effects of prestressing were detected by measurement of reactions in all supports, further there were measured displacements in the quarters of both spans and strains in critical sections of the beams.

Safety Evaluation of EC2 and MC 2010 Models for Assessment of Punching Resistance of Footings

Model for assessment of punching resistance in Eurocode 2 has been taken over from Model Code 1990. The model is fully empirical and was developed in 70s based on the experimental results obtained on flat slab specimens mainly loaded under axis-symmetric conditions. Therefore some doubts are soaring about applicability of the model for assessment of slab members exposed to the different conditions. Spread footings and foundation slabs represent such members. Growing number of experiments carried out on the footings last two decades allowed us to check safety of current model and compare it with the safety of the new models e.g. mechanical model based on the Critical Shear Crack Theory (CSCT) published in Model Code 2010 or amendments of EC2 model proposed by German group.