Jiří Witzany

Stress State Analysis and Failure Mechanisms of Masonry Columns Reinforced with FRP under Concentric Compressive Load

The strengthening and stabilization of damaged compressed masonry columns with composites based on fabrics of high-strength fibers and epoxy resin, or polymer-modified cement mixtures, belongs to novel, partially non-invasive and reversible progressive methods. The stabilizing and reinforcing effect of these fabrics significantly applies to masonry structures under concentric compressive loading whose failure mechanism is characterized by the appearance and development of vertical tensile cracks accompanied by an increase in horizontal masonry strain. During the appearance of micro and hairline cracks (10−3 to 10−1 mm), the effect of non-pre-stressed wrapping composite is very small. The favorable effect of passive wrapping is only intensively manifested after the appearance of cracks (10−1 mm and bigger) at higher loading levels. In the case of “optimum” reinforcement of a masonry column, the experimental research showed an increase in vertical displacements δy (up to 247%), horizontal displacements δx (up to 742%) and ultimate load-bearing capacity (up to 136%) compared to the values reached in unreinforced masonry columns. In the case of masonry structures in which no intensive “bed joint filler–masonry unit” interaction occurs, e.g., in regular coursed masonry with little differences in the mechanical characteristics of masonry units and the binder, the reinforcing effect of the fabric applies only partially.

The application of carbon composites in the rehabilitation of historic baroque vaults

The stabilization and reinforcement of damaged barrel vaults with lunettes over an arcaded walk, applying composite strips based on high-strength carbon fibers and epoxy resin, was performed during the restoration of a historic monastery. The application of reinforcing composite strips in the soffit of damaged barrel vaults was preceded by relatively extensive experimental research and theoretical analyses. This method significantly reduces the interventions into and the degradation of the original historic structure (surface application) and is reversible.

Strength assessment of historic brick masonry

AbstractThe identification of residual service life of a structure is an exceptionally demanding task in the case of reconstructed or newly modernised buildings. The identification of residual service life requires the study and knowledge of the mutual interaction of a building with its external environment, in particular, the time variable effects and impacts which lead to degradation processes and phenomena affecting and, in the absolute majority of cases, degrading the physical, mechanical and other properties of materials and structures. The article presents the results of in-situ and laboratory research of strength parameters of masonry from the start of the 20th century applying destructive and non-destructive tests. Besides, a probabilistic model and a procedure for the determination of masonry strength are described. It appears that the probabilistic approach leads to a design value by ca 5% higher than the deterministic approach.

THE EFFECT OF DEGRADATION PROCESSES ON THE SERVICEABILITY OF BUILDING MATERIALS OF HISTORIC BUILDINGS

The article presents an analysis of degradation processes and partial results of an experimental research into materials and structures exposed to the effects of external environments with an emphasis on the effects of moisture and chemical degradation processes on major mechanical properties of sandstone.

Stabilization and Strengthening of Historic Buildings' Stone Masonry Columns

The experimental research of failure mechanism of stone columns made of coursed masonry of regular sandstone blocks and coursed masonry of irregular (freestone) blocks under concentric compression and the research of the performance of non-reinforced as well as CFRP-reinforced stone columns completed to-date pointed out the necessity of a different approach to the assessment of the load-bearing capacity, or residual load-bearing capacity, of masonry composed of stone blocks.

The Response of Masonry Barrel Vaults to Repetitive Static and Dynamic Loads and Example of Rehabilitation of Historic Barrel Vaults

The article will present the main results of experimental and theoretical research into non-reinforced barrel vault constructions and barrel vault construction reinforced with composites based on high-strength fibres and epoxy resin, or special polymer cement mortar. Vaulted constructions of historic and heritage buildings are extremely sensitive to deformations of the supporting construction, and their response to seismic effects may often be accompanied by cracking and mechanical vault failures. The results of research and study into the dynamic behaviour of barrel vaults brings new knowledge applicable in the prevention of vault failures in regions with an increased intensity of natural or technical seismicity, for the identification and localisation of failures using e.g. MAC or COMAC criteria. It can also be valuable in their serviceability assessment and service life extension. The second part of the article presents the results of theoretical and experimental analysis and a practical example of the stabilisation and reinforcement of 16 extensively damaged barrel vaults (with a 3.05 m span) with large lunettes situated over the cloister in the Premonstrate Monastery in Tepla (built in the 16th century), located in a seismically active region of West Bohemia.

Comparison of the Degree of Consolidation of Historic Plaster Layers by Means of Lime Hydrate Nanosuspensions

Historic masonry is usually coated with multiple plaster layers, exposed to long-term extreme loading (climatic effects, elevated moisture levels, salt crystallization and recrystallization, biological effects, etc.). As a consequence of the above negative cyclic effects, degradation processes are triggered off leading to a loss in plaster cohesion with the substrate masonry and successive falling-off of surface layers. The strengthening and rehabilitation of plasters with degraded binder components, impaired structure, insufficient adhesion of individual layers of plaster to the masonry surface, with elevated salt contents in the pore system and surface crusts represent a demanding task in terms of the stabilization of such degraded plasters and coatings. Consolidation (i.e. enhancement of mechanical characteristics) belongs to the basic objectives of the restoration of historic plasters, its principle is to return binder into the material structure, fill in existing hollow spaces, heal cracks, etc. Degraded lime plasters are strengthened with consolidating agents applied in the form of solutions, e.g. lime water, organosilicates or lime nanosuspensions, etc. The lime nanosuspension is composed of lime hydrate nanoparticles, dispersed in an alcoholic medium. Due to the deposition of calcium carbonate in the degraded material, the bonds are re-strengthened and the material consolidated. The article presents partial results of theoretical and experimental research addressing potential consolidation of historic plaster layers with lime nanosuspensions prepared by the synthesis of Ca(OH)2, which were successively modified and enriched with other components.

Physical and mechanical characteristics of building materials of historic buildings

The article presents partial results of laboratory research into physical and mechanical characteristics of materials most commonly used as walling units in masonry structures of historic and heritage buildings. Core boreholes and specimens for the laboratory research of selected characteristics were sampled from accessible places of historic buildings, which had not been restored or reconstructed. The results of the research brought new knowledge about the unreliability (variance) of the properties of historical, mainly natural building materials, and, at the same time, pointed out the need for further research and extension of knowledge necessary for the assessment of residual physical and mechanical characteristics of historic masonry structures.

Demountable Frame Structure with an Elastically Embedded Diaphragm

Precast reinforced concrete demountable column system with elastically mounted diaphragms inserted into frame sections developed within the TACR grant project [1] was verified by a pseudo-dynamic tests in the UTAM AV CR experimental facility. The experimental verification and theoretical analysis was aimed at identifying the stiffness characteristics and dynamic response of the “frame” section, including the evaluation of the effect of demountable joints while mounting the diaphragm on beams using elastic (rubber) bearings. The experimental verification was performed for three different diaphragm to beam connections made by a rigid screw connection and by a connection with elastic bearings with two different stiffness values. During the first phase of the experimental verification the structure was exposed to pseudo-dynamic loading by a hydraulic jack controlled by deformation or by amplitude and corresponding frequency of the jack. The absolute deformations of the structure as well as relative deformations between individual structural elements were monitored by means of linear deformation sensors. The force necessary for reaching these deformations was also monitored. In the second phase of the experimental campaign the natural frequencies and dampening characteristics were identified. Based on the obtained values, the stiffness and dynamic characteristics of the frame structure with three different types of the stiffening diaphragm connections were identified and were compared with numerical model. Theoretical analysis and results of experimental research proved the satisfactory resistance of the proposed multi storey building system.

EXPERIMENTAL RESEARCH INTO DYNAMIC PROPERTIES OF MASONRY BARREL VAULTS NON-REINFORCED AND REINFORCED WITH CARBON COMPOSITE STRIPS

The study of dynamic behaviour of vaults of historic buildings reveals new knowledge which can be used for the local analysis and stabilisation and rehabilitation designs of damaged vaulted structures. The analysis of the results of dynamic loading brings objective background material for the identification and localisation of failures according to MAC or COMAC criteria (1, 2) and the assessment of serviceability and structural reliability of vaulted structures of historic buildings.