Lothar Stempniewski

Numerical analysis of high speed concrete fragmentation using a meshfree Lagrangian method

The topic of this paper is the fragmentation of concrete due to explosive loading. A meshfree Lagrangian method called smooth particle hydrodynamics (SPH) is used in the numerical simulation. A macroscopic constitutive law for concrete is implemented in the SPH-Code. It is shown that the SPH-method is able to simulate the damage of concrete slabs under contact detonation. The numerical results are compared with the data from two tests. The good agreement between them suggests that the SPH-program can predict the correct maximum pressures as well as the damage of the concrete slabs. Finally the fragment distributions of the tests and the numerical calculations are compared.

Distributed strain measurement with polymer optical fibers integrated into multifunctional geotextiles

Fiber optic sensors based on polymer optical fibers (POF) have the advantage of being very elastic and robust at the same time. Unlike silica fibers, standard PMMA POF fibers can be strained to more than 40% while fully maintaining their light guiding properties. We investigated POF as a distributed strain sensor by analysing the backscatter increase at the strained section using the optical time domain reflectometry (OTDR) technique. This sensing ability together with its high robustness and break-down strain makes POF well-suited for integration into technical textiles for structural health monitoring purposes. Within the European research project POLYTECT (Polyfunctional textiles against natural hazards) technical textiles with integrated POF sensors, among others sensors are being developed for online structural health monitoring of geotechnical structures. Mechanical deformation in slopes, dams, dikes, embankments and retrofitted masonry structures is to be detected before critical damage occurs. In this paper we present the POF strain sensor properties, reactions to disturbing influences as temperature and bends as well as the results of the different model tests we conducted within POLYTECT. We further show the potential of perfluorinated graded-index POF for distributed strain sensing with increased spatial resolution and measurement lengths.

Experiments on concrete under shock loading

On detonations or hypervelocity impacts concrete structures are loaded with shockwaves. These shockwaves cause a steep increase in pressure within the wavefiont which propagates at high velocity. For a numerical simulation of such process a constitutive material law F = f (E, E, T) is needed which adequately describes concrete under these conditions. As basis for such a constitutive law tests on concrete under explosive loads were performed and different material parameters were investigated. We report our approach to measure material parameters during tests with different explosive charges on concrete and our model tests for demolition work. We got first experience in using a new temperature sensor, called the atomic layer thermopile, to measure the temperature raise caused by the adiabatic compression. This sensor was developed for performance measurement of high energetic laser pukes, The mode of operation of this sensor is based on the temperature g-adient between the sensor surface and the base of the device. Very fast response time is possible, because there is no need of getting a complete heat transfer from the ambient concrete temperature to the whole sensor. Responsible for the output voltage proportional to the temperature rise is the thermal Seebeck-effect which can be splitted in a longitudinal and a transversal component due to the orientation of the layers. For stress measurement manganin gauges and for strain measurement ordinary strain gauges were used. For the application of these methods at high velocity loads in concrete special encapsulations are needed in order to save the functionality and to provide fast rise times of the output signals. Results of the measurements will be shown and remaining problems will be discussed.

Shaking Table Tests of a Full-Scale Two-Storey Pre-Damaged Natural Stone Building Retrofitted with the Multi-Axial Hybrid Textile System “Eq-Grid”

This chapter reports about seismic testing on a full-scale two-storey building made of broken natural stones. The dynamic behavior for two stages (a) undamaged and (b) pre-damaged, repaired and strengthened was analysed. All tests took place on the 7 × 5.6 m2 shaking table of Foundation EUCENTRE in Pavia, Italy. The used strengthening technique “eq-grid” was developed over 10 years at the Karlsruhe Institute of Technology (KIT). It consists of a multi-axial hybrid fibre textile embedded in a mortar matrix. In the first years 1:2 scale single masonry walls were quasi statically and/or pseudo-dynamically tested at the KIT lab to identify the adaptability of a mortar-textile-composite for strengthening different kinds of masonry structures including soft lime stone bricks and high strength natural stones. In the end a complete new strengthening system “eq-grid” was developed. To check the results of the conducted wall tests a full-scale two storey masonry building made of high strength natural stones was constructed at Foundation EUCENTRE. The building was uniaxially shaken at different levels of the same earthquake record. In the first stage the unstrengthened building was damaged in such a way that collapse was nearly reached. In the following stage the pre-damaged test sample was repaired and retrofitted using “eq-grid”. As done in practice, the system was applied only on the outer surface of the building. The comparison of the measured and observed dynamic behavior showed a significant increase of the maximum acceleration and ductility. In the end a much better overall dynamic performance of the pre-damaged retrofitted building was achieved. These results fully confirmed the research work based on single wall tests.

Numerical Simulation of Air-Steam Leakage Behaviour of Reinforced Concrete Walls

The concept of double wall containments without liner has been developed during the last years as an alternative to reinforced concrete containments with steel liner. The leakage behaviour of prestressed concrete containment walls has been investigated with several test setups at different levels of both complexity and representation. Most of the experimental test setups showed a large variation of obtained results. The influence of steam condensation inside the cracks was taken into account by only a few experiments (Herrmann et al. 2002, Granger et al. 2001). For a better understanding of the leakage behaviour of cracks through reinforced concrete walls which are subjected to air-steam mixtures at elevated temperatures and pressures it is necessary to perform numerical simulations. A 2D Finite Element Model consisting of a fluid model and a structural model has been used to calculate the leakage behaviour of cracked concrete walls with the Finite Element Program ADINA. Further details of the structural model can be found in (Niklasch et al. 2005). In every time step the fluid model and the structural model are solved iteratively.

Simulation of Shock Wave Loaded Concrete with Discrete Cracks

This work presents a method to simulate the behavior of concrete under a high dynamic load. A discrete crack model with a cohesive crack zone is used to describe the fragmentation of the concrete. The aim of the presented research is the simulation of blasting of concrete. The results of these calculations will be compared with experimental results.

Vulnerability And Risk Analysis For Earthquake Prone Communities

Summary The paper presents some results of the seismic risk subproject implemented by the Center for Disaster Management and Risk Reduction Technology (CEDIM). The main attention is paid to the vulnerability analysis of the existing building stock of communities in Germany, which is conducted in terms of the European Macroseismic Scale (EMS-98). Damage probability matrices and vulnerability curves are constructed for the vulnerability classes A, B, C, D, which are representative for the existing building stock. Vulnerability composition models are constructed for communities of different size. In combination with the seismic hazard input and estimated values at risk they allow analyzing possible damage and losses from potential future earthquakes.

The SPH/MLSPH Method for the Simulation of High Velocity Concrete Fragmentation

The topic of this paper is the application of the SPH/MLSPH-method to high velocity concrete fragmentation. After a short review of the SPH/MLSPH method, a beam under static concentric loading and linear elastic material behavior is considered to show the advantage of MLSPH in opposite to SPH. A constitutive law for concrete taking into account the dynamic strength increase under high velocity loading is briefly proposed. Finally, the application of the SPH/MLSPH-Code in conjunction with this constitutive law for concrete onto two concrete slabs under contact detonation is discussed. The SPH and MLSPH-results obtained with different particle number and smoothing lengths are compared with the experimental results.

Impact On Liquid Filled Containers

The study summarizes experiments and theoretical investigations to explore the behavior of liquid filled containers under medium speed impact. Thin-walled cylindrical steel containers with different wall thickness were impacted by gun projectiles. To investigate the major difference of the container failure modes empty and water filled containers were used. In the case of the water filled containers the impact damages ranged from simple perforation of Front and rear wall to a catastrophic front wall rupture which started at the rim of the impact hole and an additional perforation of the rear wall. The development of the pressure distribution in the water starting at the point of penetration was followed through the container to the opposite wall and back to the point of entry of the projectile. Stresses and strains in the containment were registered. Also the impact velocity and the geometry of the container cause different failure modes. The primary and the reflected pressure fields interact with the wall creating specific crack patterns. Theoretical investigations by means of the Finite Element Method helped to explain the experimental findings.

Seismic Isolation And Site Effects, An Inter-disciplinary Investigation At The Seismic Test Site INCERC, Bucharest, Romania

The Collaborative Research Center 461 (CRC 461) Strong Earthquakes: A Challenge for Geosciences and Civil Engineering (Germany) and the Romanian Group for Strong Vrancea Earthquakes (RGVE) work on a multidisciplinary attempt towards the mitigation of the impact of the next strong Vrancea earthquake (Romania). The recently installed Multidisciplinary Seismic Test Site INCERC (MSTS) located in the eastern part of Bucharest serves as focus to study the entire sequence relevant in engineering seismology including structural dynamics, seismology, soil mechanics, and engineering geology. Within this co-operation, the complete chain for seismic hazard and seismic risk analysis and can be checked and verified. For this purpose a base isolated test building was constructed, where acceleration will be measured at the base and at the top. Additionally. free field accelerometers in various depths were installed. Thus. free field accelerations and the response of the test building can be measured simultaneously. One of mainly objectives, among others. is to investigate the complete dynamic system, incorporating the base isolated test building and the soil. under seismic loading. Transactions on the Built Environment vol 57,