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Hochfeste und ultrahochfeste Stahlfaserbetone eignen sich aufgrund ihrer Eigenschaften für den Einsatz bei extremen mechanischen Beanspruchungen. Das für diesen Einsatz erforderliche duktile Bruchverhalten unter Zugbeanspruchung wird ausschließlich durch die Zugabe von geeigneten Stahlfasern und deren Wirkung in der Zementsteinmatrix erreicht. Für eine gezielte stoffliche Optimierung des Systems sind fundierte Kenntnisse zur Wirkungsweise der Stahlfasern notwendig, die bei bisherigen Untersuchungen lediglich auf Basis von klassischen Messverfahren zerstörender Prüfungen gewonnen wurden. Durch das Einbeziehen von Methoden der zerstörungsfreien Materialcharakterisierung können das Bruchverhalten hochfester und ultrahochfester Stahlfaserbetone orts- und zeitaufgelöst untersucht und die einzelnen Phasen des Schädigungsprozesses identifiziert werden. Anhand der vorliegenden Ergebnisse von Zugversuchen eines ultrahochfesten Stahlfaserbetons wird der kombinierte Einsatz von Methoden der zerstörungsfreien Materialprüfung gezeigt und deren Anwendbarkeit in der Analyse des Bruchverhaltens diskutiert. Das Hauptaugenmerk liegt dabei auf der Charakterisierung der ausschlaggebenden Versagensmodi unter Zugbelastung durch die einzelnen Verfahren und dem Vergleich zu klassischen Messmethoden (z. B. Extensometer). Die Zugversuche wurden dazu parallel durch optische Verformungsanalyse mittels Bildkorrelation (DIC), Schallemissionsanalyse (SEA) und 3-D-Computertomografie (CT) begleitet.
The tensile behavior of strain-hardening cement-based composites (SHCC) is usually investigated on macroscopic scale by means of direct tension tests or bending tests. Additionally, the micromechanical properties of the composites are often described based on single fiber tension and pull-out tests. Such investigations, performed both on macroscopic and microscopic scales, are based on ‘classical’ force and displacement measuring techniques. Advanced test methods such as digital image correlation (DIC) and acoustic emission analysis (AE) may facilitate the identification and the analysis of the failure mechanisms in SHCC, which is important for both monitoring loaded SHCC elements and further material development and optimization. In this study, these two techniques are combined to characterize the failure mechanisms of three different types of SHCC in direct tension tests. The results are related to data of stress and strain measurements. It is shown that DIC provides detailed spatially resolved and stress related strain measurements. Furthermore, it is demonstrated that AE allows for the localization of active cracks, quantification of the damage accumulation under increasing stresses, and characterization of the dominant crack bridging mechanisms and failure modes observed in the different types of SHCC.
Alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. However, slag-based mortars, in particular, are prone to shrinkage and associated cracks. In this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. This study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. Direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. Mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process.
Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.
Bending beams and slabs are typical examples for structural elements used for reinforced concrete structures such as bridge girders, T-beams and bridge decks. Their strength related failure modes at maximum loading can be divided into bending and shear failure. The failure of beams loaded in bending can occur with or without indication. Therefore, conventional design concepts aim on failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexural compression failure. These indicating factors can also be used for Structural Health Monitoring (SHM) of civil infrastructure systems (e.g. bridges) to identify structural changes. In this context, non-destructive testing (NDT) methods offer different techniques for measuring deflections or crack formation and opening. However, profound knowledge on the determining failure modes of bending beams and their detection by NDT methods is required for the reliable application of SHM. Different NDT methods have been used in this study for analysing the load-bearing behaviour of a reinforced concrete beam in bending. The different measuring techniques are briefly described and their applicability is discussed by means of experimental results. For this purpose, the load-bearing behaviour of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by optical NDT and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending beam was equipped with two single-mode (SM) sensor fibres. One fibre served as Distributed Optical Fibre Sensor (DOFS), whereas the other fibre contained Fibre Bragg Grating (FBG) sensors. In addition, optical deformation measurements using Digital Image Correlation (DIC) and Stereophotogrammetry (SP) were conducted.
Damage mechanisms analysis of reinforced concrete beams in bending using non-destructive testing
(2019)
Beams that are loaded in transverse direction are one of the main structural elements used in reinforced concrete structures. Bending beams are used, for example, in bridge girders or joists. The strength related failure modes at maximum loading can be divided into bending and shear failure that could occur with or without indication. Conventional design concepts used in practice for designing structural elements subjected to bending aim at failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexure-compression-failure. These indicators can also be used to identify structural changes of civil infrastructure systems (e.g. bridges) using non-destructive testing methods for Structural Health Monitoring (SHM). However, profound knowledge on the determining failure modes of bending beams is required for the reliable application of structural health monitoring. Therefore, resulting effects of different thermal and mechanical loads on the response of a structure are investigated by means of a bridge demonstrator. The results of this investigation are used for developing a monitoring system. Different non-destructive testing (NDT) methods have been used for analyzing the deformation behavior of a reinforced concrete beam in bending. The suitability of the different measuring techniques will be discussed by means of the obtained experimental results. For this purpose, the deformation behavior of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by the NDT methods and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending tests have been accompanied by Acoustic Emission analysis (AE), optical deformation measurements using Stereophotogrammetry (SP) and Digital Image Correlation (DIC). The conducted tests have shown that the three-dimensional detection of deformations and resulting strains with optical techniques is beneficial for analyzing the damage mechanism of loaded structures and related crack formation and propagation. Optical techniques can be supplemented by acoustic emission testing which gives detailed information on crack formation and progressing damage. The combination of both techniques offers the application as efficient monitoring system for larger structures.
The bond between polymer fibers and the surrounding cementitious matrix is essential for the development of concrete reinforcement. The single fiber pull-out test (SFPT) is the standard characterization technique for testing the bond strength. However, the different phases of debonding cannot be distinguished by the SFPT. This study investigates the debonding of different polymer fibers from the surrounding cementitious matrix with a modified SFPT and proposes methods to change the SFPT setup to generate more valuable information on the debonding mechanism. The SFPT was equipped with linear variable differential transformers (LVDT), digital image correlation (DIC) and acoustic emission (AE) analysis. The results demonstrate that the modified SFPT allows a better understanding of the different phases of debonding during fiber pull-out. Furthermore, bond strength values calculated by different methods reveal that the chemical bond of the investigated polymers is not different as reported by previous studies. Deformation measurements performed using LVDTs and DIC are suitable measuring techniques to characterize the debonding mechanism in SFPT. A correlation between recorded AE and debonding phases was not found.