Hans-Wolf Reinhardt

Permeability and self-healing of cracked concrete as a function of temperature and crack width

Tests have been carried out on a high-strength concrete establishing permeability and self-healing behaviour of cracked concrete as a function of temperature between 20 and 80 °C and crack width between 0.05 and 0.20 mm. The results show a considerable increase of water transport with temperature. Theoretical prediction on the basis of thermodynamics showed reasonable to good agreement between theory and experiment.

Stress Drop and Stress Redistribution in Concrete Quantified Over Time by the b-value Analysis

Stress redistribution and stress drop often coincide with fracture processes. The two experiments that were chosen for our b-value analysis generate failure processes with the corresponding stress drops and stress redistributions. Using acoustic emission analysis, the acoustic waves emitted from the different fracture areas are recorded. The cumulative frequency-magnitude distribution, which leads to the b-value, was originally used in seismology to characterize earthquake populations. Concerning acoustic emissions, the b-value decreases with increasing stress levels. Therefore, it was possible to determine the start of the arise of macro-cracks over time using the b-value analysis. Furthermore, a possible application in a monitoring procedure is suggested.

Non-destructive evaluation of reinforced concrete structures

Part one discusses deterioration of reinforced concrete and testing problems with chapters on topics such as key issues in the non-destructive testing of concrete structures, when to use non-destructive testing of reinforced concrete structures, deterioration processes in reinforced concrete, modelling ageing and corrosion processes in reinforced concrete structures, components in concrete and their impact on quality, and predicting the service life of reinforced concrete structures. Part two reviews classical and standard testing methods including microscopic examination of deteriorated concrete, the analysis of solid components and their ratios in reinforced concrete structures, the determination of chlorides in concrete structures, and investigating the original water content of reinforced concrete structures.

Experimental determination of KIIC of normal strength concrete

The double-edge notched specimen made of normal strength concrete has been subjected to compressive loads. It is shown that a shear crack develops at the notch tip. Visual observation and evaluation of displacement measurements have proven that this specimen allows pure mode II testing. It was possible to determine fracture toughness KIIc. Recommendations for the testing are made.RésuméUne éprouvette entaillée sur deux côtés opposés en béton de résistance normale a été soumise à des charges de compression. On démontre que la fissure de cisaillement se développe à la pointe de l’entaille. L’observation visuelle et l’évaluation des mesures de déplacement montre que ce type d’éprouvette permet des essais en mode II pur. Il a été possible de déterminer la résilience à la rupture KIIc. On donne des recommandations pour des essais.

Nichtlineares Bruchmechanik-Maßstabsgesetz für Fichte bei Zugbeanspruchung senkrecht zur Faserrichtung

ZusammenfassungEs wird über Untersuchungen zur bruchmechanischen Größengesetzmäßigkeit von fehlerfreiem Fichtenholz bei Modus I Rißöffnungsbeanspruchung im RL-Rißsystem berichtet. Die Versuche wurden mit einseitig gekerbten, geometrisch weitgehend 2D-ähnlichen SENB-Biegeproben durchgeführt. Die Prüfkörper-Querschnittshöhend variierten von 10 bis 320 mm, die Kerblängen betrugen konstant das 0,6fache der Querschnittshöhe. Die Gesamtheit der Versuchsergebnisse, ließ sich gut durch ein stetiges, nichtlineares Bruchmechanik-Maßstabsgesetz nach Bazant approximieren, während die Weibull-“Weak-Link”-Hypothese, bedingt durch den bruchmechanischen Versagensmechanismus, eher unzutreffend ist. Der festgestelle Maßstabseffekt entsprach bei den größeren Prüfkörpern (d≥160 mm) nahezu exakt dem Maßstabsgesetz der linearen Bruchmechanik (LEFM). Bei zunehmend kleineren Größen war übereinstimmend mit den Annahmen der nichtlinearen Bruchmechanik (NLFM), das heißt infolge des zunehmenden Einflusses der dehnungsentfestigenden Bruchprozeßzone, eine abgeschwächter Maßstabseffekt zur verzeichnen. Der erhaltene NLFM-LEFM-Übergangsbereich kann durch eine von der Versuchskonfiguration unabhängige bruchmechanische Kenngröße. die Sprödigkeitsziffer β∼10, beschrieben werden.AbstractA report is given on investigations, concerning fracture mechanics based size effect of clear spruce wood loaded in mode I in RL-crack system. The tests were performed with single notched, geometrically almost 2D-similar SENB bending specimens. The beam depths,d, varied from 10 to 320 mm, the notch lengths were constant 0,6 timesd. The entity of test results could be approximized well by means of a steady nonlinear fracture mechanics size effect law by Bazant, whereas the Weibull weak link hypothesis, according to fracture mechanics based failure, is rather inapt. The received size effect conformed nearly exactly to the size effect law of linear fracture mechanics (LEFM) with the larger pecimens (d≥160mm). Along with, increasingly smaller sizes a reduced size effect was abtained agreeing well with assumptions of nonlinear fracture mechanics (NLFM), i.e. growing influence of softening fracture prozess zone. The experimental transition region between NLFM and LEFM can be specified via brittleness number β∼10, a fracture mechanics parameter independent of individual test configuration.

Effect of Superabsorbent Polymers on Durability of Concrete

The chapter deals with various aspects of durability influenced by SAP. Although the compressive strength is reduced with the addition of SAP it could be shown that the water and oxygen permeability remain the same or are improved. Due to the generation of air-filled pores by SAP the frost resistance with deicing salts is considerably superior to reference concrete without SAP. It is comparable to concrete with airentraining agents. Finally, chloride migration is also reduced depending on the type of SAP.

Materials and joints in timber structures

Description based on online resource; title from PDF title page (ebrary, viewed October 11, 2013).

Structural Health Monitoring with Wireless Sensors to Enhance Sustainability in Structural Engineering

Acoustic emission techniques (AET) are an alternative monitoring method to investigate the status of a bridge or some of its components. It has the potential to detect defects in terms of cracks occurring during the routine use of bridges. A monitoring system for large structures is developed based on a new kind of MEMS sensors as well as conventional sensors requiring just very low power. Besides the sensors the recording and analysis technique needs powerful algorithms to handle and to reduce the immense amount of data. Systems developed for structural health monitoring use embedded network techniques along with wireless communication and MEMS. Algorithms to reduce the amount of data and to enable the localization of damage areas by array techniques have to be developed. The paper is giving an overview about data processing and analysis techniques needed for wireless sensor networks with respect to structural health monitoring. The focus is especially in the field of acoustic emission techniques where often a large amount of data must be processed.

Dynamic Fracture of Concrete–3D Numerical Study of Compact Tension Specimen

The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, and ductility, and, second, there are inertia forces activated. Both influences are clearly demonstrated in experiments. For concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend on loading rate. Moreover, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of the crack propagation of the concrete compact tension specimen. The rate sensitive microplane model is used as a constitutive law for concrete. The strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. The results of the study show that the fracture of the specimen strongly depends on the loading rate. For relatively low loading rates there is a single crack due to the mode-I fracture. However, with the increase of loading rate crack branching is observed. Up to certain threshold (critical) loading rate the maximal crack velocity increases with increase of loading rate, however, for higher loading rates maximal velocity of the crack propagation becomes independent of the loading rate. The critical crack velocity at the onset of crack branching is found to be approximately 500 to 600 m/s.

Characterization of Fresh and Early Age Concrete Using NDT

The paper reviews a few methods which are suitable for the continuous measurement of the stiffening and hardening behavior of mortar and concrete. These are the dielectric method, the nuclear magnetic resonance method, the ultrasonic methods as wave-reflection method and wave-transmission method. The first two methods detect mainly the distribution of water in the sample as function of hardening time, while the other two methods rely on the development of the mechanical properties. The measurement principles are discussed and testing devices are described.