Bestimmung der Tunnelinnenschalendicke mit zerstörungsfreien Prüfverfahren

Determining the thickness of tunnel internal shell with non‐destructive testing methods This report deals with the influence of non‐destructive testing methods for determining the thickness of internal tunnel shells and evaluates the results of examinations performed on a construction site. The investigations cover different measurement techniques performed on three tunnel blocks; also analyzed were the specific advantages of these techniques, the employment measurement personnel and different measurement grids used before and after ridge‐gap grouting. An essential parameter influencing the measurement accuracy is the calibration needed by the measurement devices at the site of deployment. Because the measurements were not able to unmistakably confirm the quality or success of ridge‐gap grouting through injection, these results had to be examined additionally using drill cores and endoscopy. Permitting a crystallization of essential aspects and recommendations as regards the conduction of non‐destructive tests for determining the thickness of tunnel shells, the obtained findings are to be incorporated into future projects.     

Analyse des Gefüge‐ und Feuchtezustandes in mineralischen Baustoffen mit der Mikro‐Röntgen‐3D‐Computertomografie

Condition Assessment of Microstructure and Moisture Distribution in Mineral Building Materials by Micro X‐Ray Computed Tomography. The knowledge of microstructure and moisture condition in capillary porous building materials is essential for building climate control and building durability. The 3D X‐ray computed tomography (3D‐CT) opens up new vistas for the visualization of both parameters. In this article the performance of this innovative non‐destructive testing method is demonstrated by means of two studies showing its feasibility. The first feasibility study was carried out with a drilling core taken from a fair‐faced concrete cladding which was strongly damaged by freeze‐thaw cycling. The 3D‐CT investigations have shown that it is possible to visualize the crack and moisture distribution especially interesting besides the spatial distribution of aggregates and hardened cement paste. So the method opens up new vistas for the tracing of interactions between cracks due to frost and moisture distribution without any destructive interventions. This can help to clarify the damage mechanisms in concrete due to frost which are partly not sufficient known up to now. Objective of the second feasibility study was to demonstrate not only the spatial but also the temporal distribution of the moisture in tuff during the capillary water absorption test. The 3D‐CT investigations have shown that the spatial motion of the capillary moisture front with delayed water absorption of pumice, a magmatic tuff component, can be visualized. Based on these studies this article finally shows the next steps which are necessary to improve the performance of the test method.     

Verifizierung zerstörungsfreier Prüfverfahren zur Detektion von Spannstahlschäden an Spannbetonbauteilen mit nachträglichem Verbund

Verification of Non‐Destructive Testing Techniques for Detection of Prestressing Steel Damage at Post‐Tensioned Concrete Members For an assessment of the risk potential of existing structures where in the case of prestressing steel damage the load‐bearing capacity could be significantly affected non‐destructive testing techniques which enable reliable evaluation of the condition of the prestressing steels are of utmost importance. During the demolition of the Hohenzollerndamm bridge in Berlin where damage of prestressing wires due to hydrogen‐induced stress corrosion cracking were found in former investigations parts of the girders of the bridge superstructure could be taken out. After non‐destructive investigation by means of two testing techniques based on magnetic leakage flux measurement and one technique based on electromagnetic resonance measurement the single wires were inspected in order to verify the non‐destructive testing results with respect to their reliability. From the results it can be concluded that areas with fractures of several wires could be detected by the two techniques based on magnetic leakage flux measurement.     

Beurteilung der Betonfestigkeit unter Nutzung der künstlichen Neuronalen Netze aufgrund zerstörungsfreier Untersuchungen

Determination of the Compressive Strength of Concrete using artificial Neural Network and non‐destructive Tests This paper deals with the neural identification of the compression strength of concrete on the basis of nondestructively determined parameters. A methodology for the neural identification of the compression strength of concrete has been developed. The obtained results show that Levenberga‐Marquardta artificial neural networks are highly suitable for assessing the compression strength of concrete. The results of practical verification of neural identification two reinforced concrete building structures are presented.     

Methoden zur Analyse des Bruchverhaltens hochfester Stahlfaserbetone

Methods for analyzing the fracture behavior of high‐strength steel fiber‐reinforced concretes High‐strength and ultra‐high strength fiber‐reinforced concretes are most suitable for applications with extreme mechanical loads. These extreme conditions require a ductile behavior under tensile loading, which is obtained solely by the addition of steel fibers and their working mechanism. Profound know ledge on the working mechanism of the steel fibers is necessary for optimizing this material. Usually, this knowledge is obtained by means of classical measuring techniques of destructive tests. Adopting measuring techniques from non‐destructive material testing helps to analyze and to identify the different stages of the fracture mechanism of high‐strength and ultra‐high strength fiber‐reinforced concretes in detail. The application of different non‐destructive measuring techniques is shown exemplary on tensile tests conducted on an ultra‐high strength fiber‐reinforced concrete and its applicability for analyzing the fracture behavior is discussed. The main focus is on the characterization of the relevant failure modes under tensile loading by the different measuring techniques and the comparison with classical measuring techniques (e. g. extensometer). The tensile tests have been analyzed by optical deformation measurements using digital image correlation (DIC), acoustic emission analysis (AE), and 3D computed tomography (CT).     

Peak factor estimation of non‐Gaussian wind pressure on high‐rise buildings

A vast quantity of measurements of wind‐induced non‐Gaussian effects on buildings call for the burgeoning development of more advanced extrema estimation approaches for non‐Gaussian processes. In this study, a well‐directed method for estimating the peak factor and modeling the extrema distribution for non‐Gaussian processes is proposed. This method is characterized by using two fitted probability distributions of the parent non‐Gaussian process to separately fulfill the estimations of the extrema on long‐tail and short‐tail sides. In this method, the Johnson transformation is adopted to be the probabilistic model for fitting the parent distribution of the non‐Gaussian process due to its superior fitting goodness and universality. For each dataset, two Johnson transformations will be established by two parameter estimation methods to individually estimate the extrema on two sides. Then a Gumbel assumption is applied for conveniently determining the non‐Gaussian peak factor. This method is validated through long‐duration wind pressure records measured on the model surfaces of a high‐rise building in wind tunnel test. The results show that the proposed method is more accurate and robust than many existing ones in estimating peak factors for non‐Gaussian wind pressures.     

Kraftbestimmung in Schrägseilen durch dynamische Messung

Force Determination in Stay Cables by Means of Vibration Measurements The knowledge of the actual tensile forces in the cables in the ropes or cables of cable‐stayed bridges or in external tendons is required for the examination and assessment of these elements. The determination of these forces by lift‐off tests with hydraulic jacks is connected with considerable expenditures as well as the danger of damages. But also the accuracies achieved by vibration measurements were, however, not satisfying. For high cable forces and short cables in some cases errors of up to ± 10% of the actual cable force could be determined. Therefore fast and non‐destructive methods for the determination of the forces are required. In this paper, a practicable technology to determine the cable force by means of vibration measurements is illustrated at the Viennese cable stayed bridge crossing the Vienna Donaukanal.     

Validierung zerstörungsfreier Prüfverfahren im Bauwesen

Validation of Non‐Destructive Testing Methods in Civil‐Engineering In many fields of technology validation provides the objective proof that the chosen testing method is capable to fulfill customer's demands. Especially in non‐destructive testing in civil engineering with only a few methods standardized, validation is a meaningful tool to gain the client's confidence in a certain method. The capability of a method becomes obvious and the customer can estimate the quality of the results. This becomes important if the engineer has to draw reliable conclusions based on the results and if he makes high demands on accuracy and preciseness of the results. In practical use the question may arise on what confidence level a measured thickness of a concrete member can be utilized in a static calculation or about the reliability of a pile foundation being embedded in the competent bearing layer of the subsoil after non‐destructive pile length measurement. This article gives fundamental information how to carry out a validation giving the example of thickness measurement at foundation slabs. In principle the given validation methodology can be applied to various testing tasks occurring in the field of civil engineering. In case of minor requirement concerning the accuracy and preciseness simplified estimations of the uncertainty of measurements can be carried out.     

Impact of constructional energy‐saving measures on radon levels indoors

Energy‐efficient building refurbishment has the aim of saving energy and thus reducing CO₂ emissions. Increased energy efficiency of a building often implies reduced air exchange. Together with other indoor air quality problems, this may lead to an increase in indoor radon concentration (Rn‐222). In order to investigate the extent of this problem, measurements of radon concentration in energy‐efficient refurbished and low‐energy houses (passive houses) were carried out. Track etch detectors were exposed in each type of building over a period of 1 year. A reference sample of non‐refurbished non‐passive buildings was drawn from the National Radon Database for comparison. Buildings were selected that have the same radon relevant properties and were built on comparable geological subsoil like those investigated. The reference sample was compiled in such a way that the measured values from the rooms on the ground floor of the refurbished and passive houses were each assigned a measured value from the database. The statistical analysis shows that the houses refurbished for energy efficiency have a wider distribution of radon concentrations indoors than the non‐refurbished ones. Both the mean value and the median of the radon concentration have nearly doubled in buildings refurbished for energy efficiency. The difference is statistically significant. On the other hand, there is no significant difference between the distributions of passive houses and houses not refurbished for energy efficiency.     

Anwendung der Zuverlässigkeitsanalyse auf Messungen mit zerstörungsfreien Prüfverfahren am Beispiel der Tunnelinnenschalenprüfung

Application of Reliability Theory in Case of Testing Internal Tunnel shells with Non‐Destructive Testing Methods Design rules in civil engineering are usually developed by applying the reliability theory. Consequently the safety concept of the EUROCODES is based on this theory. Since a decade reliability analysis is carried out also for durability problems. In this article it will be shown how reliability theory can be applied to measurement tasks. Non‐destructive testing of internal tunnel shells with ultrasonic echo is described as an example. The application is regulated in a guideline issued by the German Federal Ministry of Transport, Building and Urban Affairs. The realisation of data assessment and thereby achieved benefits are described. The appropriateness of non‐destructive testing methods can be quantified in certain properties in relation to the boundary conditions of the testing task.     

Das Potential der passiven Kühlung im Gebäudebestand Bürobauten – Vorschlag einer Typologie

Passive cooling of existing office buildings – proposal of a typology Passive cooling techniques use natural heat sinks (e. g. night air, ground) to dissipate excess heat while using storage effects in the building structure to modulate heat gains. Since most of the available heat sinks have a limited capacity a very low cooling load is a precondition. To assure a successful refurbishment of an office building with passive cooling techniques the question has to be discussed how suitable the buildings preconditions are. The paper discusses the building‐related and the non building‐related factors and establishes a typology. This typology allows identifying and classifying buildings according to their suitability for passive cooling. A “decision matrix” finally combines building‐related and non building‐related parameters and shows the buildings potential for passive cooling at an early stage of planning.     

Applicability of infrared thermography to the study of the behaviour of stone panels as building envelopes

Thin stone wall cladding systems used on exterior building envelopes typically consist of stone panels made of granite. There is a risk of detachment if the cement used to stack the panels is defective. There is a wide range of destructive techniques to evaluate the state of the facades, but they are scarcely practical. Infrared thermography (IRT) offers a cost-effective alternative to traditional evaluation. The sizes and shapes of the defects detected by this methodology in the cement holding the panels are presented in order to evaluate the capacity of the aforementioned technique.     

Performance of variable curvature sliding isolators in base‐isolated benchmark building

The performances of variable curvature sliding isolators like variable frequency pendulum isolator (VFPI) and variable curvature friction pendulum system (VCFPS) installed in the base‐isolated benchmark building subjected to bi‐directionally acting seven strong earthquakes have been studied. The shear type base‐isolated benchmark building is modelled as three‐dimensional linear elastic structure having three degrees‐of‐freedom at each floor level. Time domain dynamic analysis of the building has been carried out with the help of constant average acceleration Newmark‐Beta method and non‐linear isolation forces has been taken care by fourth‐order Runge‐Kutta method. The force‐displacement responses of the VFPI and VCFPS are studied under parametric variations of their key characteristics for the comparative performance evaluation. The time history variations of response characteristics and peak response evaluation criteria are also investigated for overall comparison of their performances. The performance of VFPI and VCFPS are observed both in uniform and hybrid isolation systems. The force‐displacement responses of both VFPI and VCFPS subjected to strong near‐field earthquakes show excessively large isolator displacements at higher initial radii of curvature of sliding surface. The large isolator displacements of VFPI and VCFPS can be restrained efficiently by addition of viscous fluid dampers in comparison to the increase in coefficient of friction of isolators. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluation of building materials using infrared thermography

Infrared thermography is a non-destructive testing technology that can be applied to determine the superficial temperature of objects. In this method, detectors collect infrared radiation to create a thermal image showing the superficial temperature distribution. Despite thermography’s many potentials, its application to building materials has not been greatly studied yet. The parameters that may affect measurements are not completely understood and interpreting the results becomes difficult and confusing.To assess the use of thermography, some simple experiments were carried out at the Building Physics Laboratory (LFC) of the Engineering Faculty of Porto University (FEUP). A sensibility study was performed with LFC’s equipment to evaluate how measurements are influenced by emissivity, environmental conditions, colour and reflectivity.The wetting and drying process of building materials was also studied, as water evaporation is an endothermic reaction inducing local surface cooling. And, lastly, the comfort of some interior floor coatings was evaluated by comparing thermal images obtained from the sole of a barefoot after having been in contact with different materials. The results of these experiments are presented in this paper.     

Hybrid Time‐Frequency Blind Source Separation Towards Ambient System Identification of Structures

Abstract: Ambient system identification in noisy environments, in the presence of low‐energy modes or closely‐spaced modes, is a challenging task. Conventional blind source separation techniques such as second‐order blind identification (SOBI) and Independent Component Analysis (ICA) do not perform satisfactorily under these conditions. Furthermore, structural system identification for flexible structures require the extraction of more modes than the available number of independent sensor measurements. This results in the estimation of a non‐square modal matrix that is spatially sparse. To overcome these challenges, methods that integrate blind identification with time‐frequency decomposition of signals have been previously presented. The basic idea of these methods is to exploit the resolution and sparsity provided by time‐frequency decomposition of signals, while retaining the advantages of second‐order source separation methods. These hybrid methods integrate two powerful time‐frequency decompositions—wavelet transforms and empirical mode decomposition—into the framework of SOBI. In the first case, the measurements are transformed into the time‐frequency domain, followed by the identification using a SOBI‐based method in the transformed domain. In the second case, a subset of the operations are performed in the transformed domain, while the remaining procedure is conducted using the traditional SOBI method. A new method to address the under‐determined case arising from sparse measurements is proposed. Each of these methods serve to address a particular situation: closely‐spaced modes or low‐energy modes. The proposed methods are verified by applying them to extract the modal information of an airport control tower structure located near Toronto in Canada.     

Korrosionsmonitoring und Bruchortung in Spannbetonbauwerken – Möglichkeiten und Grenzen

Corrosion Monitoring and Fracture Detection in Prestressed Concrete Structures – Possibilities and Limits Corrosive influences may affect the long‐term functionality of prestressed elements of prestressed concrete constructions and may lead to catastrophic failure of the member. Hence, the objective of the condition assessment of prestressed components has to be detecting existing defects and damages of the prestressed elements, e.g. grouting faults, corrosion and steel fractures in time. For an objectively early diagnosis of corrosion damages on prestressing elements, the Structural Health Monitoring (SHM) based on non‐destructive testing and monitoring methods is predestined. In this contribution causes of tendon corrosion in prestressed concrete (PC) bridges will first be specified, furthermore advantages and the strategy of corrosion monitoring are presented. Afterwards the state of the research and art of non‐destructive techniques and evaluation (NDT/NDE) of the corrosion state and of fracture detection of the prestressed elements in bridges with its possibilities and limits will be discussed. In this context also innovative sensors and measuring methods developed by the authors at the iBMB within the collaborative research center (CRC) 477 “Structural Health Monitoring” will be introduced.     

Zerstörungsfreier elektrochemischer Chloridentzug an der Donaubrücke Pfaffenstein: Langzeiterfahrungen über eine bauwerksschonende und verkehrserhaltende Technologie

Non‐Destructive Electrochemical Chloride Extraction on the Danube Bridge Pfaffenstein: Long‐Term Experiences about a Structure‐ and Traffic‐keeping Technology Between 2003 and 2007 in total 200 m² of corrosion active hollow box girder floor slabs were rehabilitated by a non‐destructive, electrochemical chloride extraction (ECE) within the Danube bridge Pfaffenstein that is situated in Regensburg along the highway A93. These concrete areas were chloride contaminated by a leaking drainage system from the carriageway above – up to 4% by cement mass. Hence, the reinforcement was corrosion active, but did not show considerable loss of cross‐section or concrete deterioration. After completing the ECE, where more than 28 kg of chloride could be removed, multiple potential surveys have been made about a time span of up to three years. These measurements have shown that chloride induced corrosion activity could be eliminated safely, also with some residual chloride. Both highway and a heavy traffic bearing main street, which crosses under the bridge, haven't been affected by the repair. The principle of ECE, its side effects and the experiences collected by the author since 2001 shall be discussed here.     

Peak factors of non‐Gaussian wind forces on a complex‐shaped tall building

This paper focuses on the development of peak factor formulas of non‐Gaussian wind pressure processes after reviewing the current estimation methods of non‐Gaussian peak factors. A skewness‐dependent peak factor is proposed by accounting for the contribution of skewness and kurtosis parameters in some existing Hermite moment‐based formulas. The possible correction on the upcrossing rate used in the translation process approach is also investigated. Wind tunnel pressure data on a practical 43‐story building with unusual shape is used to validate the accuracy of the skewness‐dependent peak factor by investigating various statistical properties of wind‐induced fluctuating pressure field on the complex‐shaped tall building example. Copyright © 2012 John Wiley & Sons, Ltd.     

Three‐dimensional computer‐aided finite element method retrofitting modeling and non‐destructive testing techniques for the assessment of actual existing high‐rise fire‐damaged reinforced concrete building

Concrete is generally fire resistant. A fire in a concrete structure rarely results in a serious damage as to require substantial demolition. But, loss of the utility of a building could result in serious financial consequences for the owner, which calls for immediate reinstatement. To work out proper and efficient repair strategy, however, would require a thorough investigation of the effect of fire on the structural properties of the concrete and steel; the significance which any permanent change in material characteristics may have on the future structural performance of the member; the feasibility of repairs to compensate of any unacceptable reduction in structural performance, durability, and so on; and the influence which fire exposure of individual member may have on the performance of the entire structure. These all said tasks are dependent on the complete analysis of the fire‐damaged building. Without it, no repair works estimation, extent of repair and kind of repair can be carried out for the fire‐damaged buildings. Therefore, the impeccable analysis and design is of utmost importance for repair of such buildings after preliminary investigation of the extent of fire damages to the concrete structural members. This forms the basis of this research study, which aims at detailed analysis and design of the actual existing high‐rise fire‐damaged buildings for fire retrofitting and assessment of fire damages by non‐destructive techniques. Fire damages in buildings due to explosion, accidents or by some other reasons cause severe structural damages. The structural integrity of existing buildings is now a burning issue. Analytical, theoretical and design‐cum‐construction techniques are constantly being reviewed by government agencies and engineering consultants. Therefore, researchers are delving into this matter to find the best retrofitting techniques for fire‐damaged buildings. This paper is an outcome of such detailed research studies. It covers the actual case study of existing buildings, review of existing knowledge for fire damages and their mitigation and protective design technologies, and analytical and computational techniques, which have limited research data. In this study, Extended 3D Analysis of Building Systems (ETABS) is used as software for fire retrofitting analysis, and UBC‐97 is used as a code for the fire analysis and design. The ETABS building model is verified by manual calculations as well. Copyright © 2011 John Wiley & Sons, Ltd.     

Detektion und Lokalisierung von strukturellen Schäden mit Hilfe von geführten Wellen

Detection and localization of structural damages by means of guided waves. The procedure presented serves for an automatic large‐area monitoring of steel structures by means of non‐destructive testing. Piezoceramic members are applied as a cluster to the structure under observation. These elements are used both as sensors and as actuators and initiate guided wave propagation in the structure. Damages are detected by the changed structural response. Beside other methods of signal processing the principle of beamforming is used with the sensor signals. The example of a 6 mm steel plate shows the sensitivity of the developed procedure for damage detection.