František Girgle

Experimental Verification of Behavior of Glass and Carbon Fibers in Alkali Environment

The paper discusses durability of glass and carbon fibers which are exposed to alkali environment of concrete. Mainly glass fibers are affected by the high alkali environment with pH of concrete exceeding 12.0. The degradation of fibers is reflected by decreasing of its tension strength, while the rate of decreasing is depended on exposition time and temperature. The paper briefly presents results of a large experimental investigation which should specify an influence of alkali ions concentration in a solution on reduction of the mechanical properties of fibers. Reference samples were immersed in a bath with distilled water and the others were stored at ambient air only.

Bond Behavior of FRP Bars in Concrete

This article deals with the bond of internal composite (FRP) reinforcement with different surface treatments. Bars with additional sand-coating exhibit different behavior due to different transmission of forces in relation to bars with ribbed surface. To compare the bond behavior of these types of reinforcement, an experimental program consisting of several sets of pull-out tests was designed. This test configuration was chosen primarily for simplicity, comparability with previously published results, and especially easy modification for assessment the effect of concrete cover thickness. The paper presents the results confirming the direct influence of the surface treatment and also the position of the reinforcement on the resulting bond behavior.

Influence of Alkalinity and Ambient Temperature on Long-Term Properties of GFRP Reinforcement

The article deals with the current problem of determining long-term reliability of non-metallic reinforcement in concrete structures. The alkaline environment of concrete with a pH higher than 12.0 affects the glass fibres degradative, whereas this degradation presents by reduction of their mechanical characteristics, resulting in a decrease in the tensile strength of the whole composite. The article summarizes the results of the ongoing experimental program so far, which aims to quantify this influence.

An Engineering Heat and Mass Transport Model Utilized for Concrete at Fire

Mathematical models which can sufficiently describe processes proceeding in concrete during fire play an important role when assessing a load bearing capacity of concrete structures. In this paper a coupled heat and mass transport model based on principles of conservation of mass and energy is presented. A 2D computational solver using FEM was created to find three unknowns – a temperature, an amount of free water and a magnitude of pore pressure in concrete cross section during rapid heating. These variables, mutually connected through state equation, are necessary not only for determination the load bearing capacity of concrete element but also for prediction of concrete spalling. A parametric study of numerical results influenced by material properties of concrete (e.g. permeability, thermal conductivity) and initial conditions (e.g. relative humidity, temperature) is introduced and discussed in the end.

Comparison and Review of Concrete-to-Concrete Interface Shear Resistance According to Major Design Codes

The paper focuses on interface shear behavior, more precisely on shear resistance of the interface between lightweight and regular concrete layers cast at different times without shear reinforcement crossing the interface. Different approaches according to most of major design codes (Eurocode, ACI and Model Code 2010 among other) are briefly reviewed, with emphasis on difference between variables resp. material and cross-section characteristics, on which interface resistance depends. Furthermore, two sets of experiments were carried out: direct shear test on Z-type specimens and slant shear test slightly modified to enforce adhesive failure. Obtained results are compared with theoretical values.

The Effectiveness of Strengthening of Reinforced Concrete Column Confined by FRP Wrapping

The article deals with the theme of strengthening of reinforced concrete columns by confining with carbon composite fabric. Further, this article discusses an influence of used design approach on a final efficiency of strengthening. Due to the dispersion of resulting values of characteristic properties of confined concrete it is quite difficult to qualify the accuracy of the proposed solutions by using the available design approaches. The aim of the performed experimental work was to quantify the factors that influence the strengthening efficiency of the confining of centrically loaded elements, as well as to compare the variance of the theoretically obtained results with the experimental values.

Long-Term Mechanical Properties of Fiber Reinforced Polymer Bars Exposed to Alkali Environment: Experimental Investigation

This paper deals with actual issues concerning the design and the utilization of modern composite reinforcement (FRP) in concrete structures. These advanced composite materials are, especially if the whole life cycle of the structure is considered, gradually becoming a convenient alternative to ordinary steel reinforcement. The structure reinforced with FRP reinforcement (as well as the structure reinforced with steel reinforcement) has to be designed with regard to sufficient endurance, serviceability and durability. The long-term material properties of FRP reinforcement embedded in concrete, which are influenced by temperature, load magnitude and ambient environment, must be considered during design of the structure. A high alkali environment of concrete with pH higher than 12.0 acts mainly on glass fibres which degrade and their mechanical properties are reduced consequently. The used matrix creates a barrier which insulates the bearing fibres against alkali ions attack. The main objective of the paper is therefore to describe behaviour of composite as a whole. The experimental approach and results which were reached during the tests are also presented. An effort was to specify the impact of alkali environment on the long-time properties of developed reinforcement which could be used in durable concrete structures.

Strengthening of Short Circular Columns Using FRP Materials - Determining the Effect of Confinement

The paper focuses on strengthening of loadbearing reinforced concrete vertical members by composite materials. These modern materials are used more and more in the last years. Usage of the FRP fabrics as confinement has several advantages, however there are some uncertainties regarding the design (eg. determining the compressive strength of confined concrete and limit strain of confined concrete). An assessment of influence of the confinement on the elements (its load bearing capacity and ductility) is necessary for design. There is a large number of analytical formulas, standardized approaches and regulations, which specify the resulting influence on the confinement. The paper draws attention to the considerable dispersion between those obtained results.The paper presents an analysis of the influence of reinforced concrete columns strengthening, which are confined with FRP fabric. The obtained theoretical results were compared with a real behaviour of several specimens of confined concrete columns. The short reinforced concrete circular columns were used as the test specimens. The height-diameter ratio of the columns was set as 2:1, therefore an influence of slenderness to loadbearing capacity of the column can be neglected. Consequently, the influence of confinement can be maximized and the dispersion of results according to different methodologies can be emphasized.