Other topics

H – 14 Strength of drilled concrete cores

Full title: Influence of reinforcement on the compressive strength of concrete core samples

Investigations of core samples provide a cost-efficient way to gain numerous important insights into an existing structure. Despite the possibility of locating reinforcement non-destructively, it is often unavoidable that core samples contain reinforcement when extracted. The standard DIN EN 12504-1, which governs the testing of core samples in practice, specifies that, if present, the diameter and location of the reinforcement must be documented. However, the effects on test results are not described in further detail, nor are they quantified using correction factors.

As part of a student's work, the effects that the diameter and location of reinforcement may have on the material behavior of concrete core samples under compressive loading are to be investigated in greater depth. In addition to a literature review, the thesis will be supplemented by numerical and/or experimental investigations. The aim is to develop recommendations for the evaluation of test results from short- and long-term compression tests (compressive strength, modulus of elasticity, and creep behavior). Depending on the interests and orientation of the student, the focus may be on numerical simulation, experimental investigations, or a combination of both.

The thesis comprises the following work packages:

• Literature review on normative recommendations/requirements as well as the current state of knowledge regarding the effects of reinforcement in concrete core samples on material behavior
• If applicable, numerical simulation of small-scale concrete cylinders under compressive loading with varying reinforcement content and/or location
• If applicable, development of a test concept, execution of tests, and evaluation of results from laboratory tests on concrete cylinders with varying reinforcement content and/or location
• Development of recommendations for the testing of concrete core samples containing reinforcement

The specific tasks will be agreed upon individually with the student.

Contact person:
Dipl.-Ing. Jenny Pech
+49 351 463-34665

H – 13 Cracks and absorbed water in drilled concrete cores

Full title: Studies of potential crack formation and water absorption as a result of core sampling and further wet processing

Core-drilled concrete samples provide a cost-effective way of obtaining a range of important knowledge about an existing structure. Core drilling is therefore one of the most important methods of obtaining samples from existing structures for structural analysis. There are two methods available: adding water in wet drilling reduces the generated frictional heat and allows the drill dust to be removed. It can be assumed that some of the drilling water is absorbed by the concrete through capillary action. The same applies to further processing steps such as grinding and sawing with the addition of water. In the more complex dry drilling process, however, the concrete dries out as a result of the generated frictional heat. A change in the original moisture condition is therefore to be expected with both methods. In addition, there are indications in literature and practice that microcracks can occur in the drilled core during the drilling process.

As part of a thesis (project and/or diploma thesis/master's thesis), the water absorption and crack formation resulting from sample extraction and processing are to be investigated using various existing concrete samples that will be made available during the course of the work. The aim is to develop recommendations as to whether and, if so, how these influences can be evaluated for structural engineering investigations on the drill core.

The work consists of the following work packages:

• Presentation of the current state of knowledge on the influence of core samples as a result of various sample extraction methods and influencing parameters (impact factors, duration of exposure, concrete properties)
• Development of a test concept for investigating the aforementioned influencing factors
• Supervision, implementation, and evaluation of tests in the laboratory
• Development of recommendations for taking into account crack formation and water absorption as a result of sample extraction

The specific tasks will be agreed upon individually with the student.

Contact person:
Dipl.-Ing. Jenny Pech
+49 351 463-34665

H – 12 Creep behaviour of old concrete

Existing structures that no longer meet current and future requirements often experience higher service and live loads as a result of conversion or strengthening measures. The additional compressive stresses lead to a renewed creep process in the existing concrete. These creep deformations must be taken into account in the planning stage using suitable prediction models in order to be able to quantitatively estimate losses in prestressing force, deflections, redistribution of internal forces, and other deformation-related changes in the structure. However, the currently valid normative creep models are based entirely on tests with comparatively young, previously unloaded concrete and are therefore not reliable for predicting creep deformations at high load ages. The very few creep tests available on old concrete show that creep deformations are not negligible even at very high concrete ages and are underestimated by normative creep models.

In this context, the Institute of Concrete Structures is currently conducting investigations into the creep behavior of old concrete. Student work can make a decisive contribution to the knowledge gained in the project by examining sampling and measurement methods, material and component behavior, and defect influences in detail through research work and laboratory tests.

Possible topics currently include:

• Development of model approaches for the creep of old concrete from literature
• Investigations into the influence of errors in creep tests on drilled core samples
• Effects of overestimating and underestimating the creep of old concrete (e.g., as a result of load increases in an existing structure or when reusing components)
• Shrinkage and swelling of old concrete
• Determination of compressive and tensile stresses in concrete structures using the drill core hole and ring core methods
• Recovery measurement of concrete after very long periods of loading
• Literature research on prestressed precast concrete elements in the GDR

The specific task will be agreed individually with the student. It is possible to design the task in such a way that the thesis builds on the project work.

Contact person:
Dipl.-Ing. Jenny Pech
+49 351 463-34665

H - 11 Extending the service life of prestressed railway concrete sleepers

The ballast superstructure with prestressed concrete sleepers is by far the most common type of superstructure in the German railway network. Sleepers are an integral part of the railway superstructure, transferring forces from the rail to the ballast while ensuring the track's position. Approximately 2.7 million old sleepers are removed and new ones installed on the DB network every year, which is disadvantageous both economically and in terms of sustainability. Therefore, the aim should be to extend the service life of the sleepers. The following topics arise within the scope of this project:

• Literature research on the historical development of prestressed concrete sleepers (dimensioning, earlier sleeper types and existing studies on service life).
• Comparison of sleeper dimensioning according to DIN EN 12320 and UIC 713, and the specifications in EC 2. This will be followed by a parameter study on the influence of deviations between the regulations.
• A parameter study on the influence of different boundary conditions on the load-bearing behavior/design of sleepers (condition of the ballast, condition of the rail, influence of the stiffness of the superstructure, etc.).
• Simulation of the load-bearing behavior of different types of sleepers, from manufacture to end of service life.

Contact Person:
Elias Will, M.Eng.
+49 351 463 35467

H - 10 Nachrechnung und Bewertung historischer Bemessungen von Festen Fahrbahnkonstruktionen

Die Feste Fahrbahn wird allgemein als bevorzugte Option für den Bau neuer Hochgeschwindigkeitsstrecken betrachtet. Die über 50-jährige Entwicklung dieser Technologie hat zu zufriedenstellenden Ergebnissen bei den derzeit eingesetzten Bauformen geführt. In der Entwicklungsgeschichte haben sich jedoch nicht alle Varianten als dauerhaft bewährt – in einigen Fällen kam es zu Schäden. Im Rahmen von Instandhaltungsmaßnahmen sind daher statische Nachrechnungen bestehender Konstruktionen erforderlich. Dafür müssen auch historische Bemessungsmethoden analysiert und nachvollzogen werden. Daraus ergeben sich folgende Themen und Aufgabenstellungen für Projekt- und Abschlussarbeiten (Master und Diplom), die individuell mit den Studierenden abgestimmt werden können:

• Literaturrecherche zur historischen Bemessung der Festen Fahrbahn (Deutschkenntnisse erforderlich)
• Parameterstudie zur Bemessung der Festen Fahrbahn gemäß DIN EN 16432-2 und Betonkalender 2015
• Nachrechnung historischer Feste-Fahrbahn-Konstruktionen
• Vergleichsstudie: Bemessung der Festen Fahrbahn nach DIN EN 16432 im Vergleich zu anderen Normen (z. B. Eurocode oder Model Code)

Contact:
Franz Grützmacher
+49 351 463-40412

H - 9 Numerical study of loading scenarios on slab track structures

Slab track is generally considered the preferred option for the construction of new high-speed lines. Over 50 years of development of this technology has produced satisfactory results for the designs currently in use. However, not all roadway designs have aged equally well during the development period. Some exhibit numerous damages or are in a condition that makes replacement investment worth considering. Therefore, it is of interest to determine the causes of damage to such structures. Possible impacts will be identified and compared to find out a potential cause of the damages.             

This thesis deals with the following work packages:

• Comprehensive literature review on design of and loading scenarios on slab track structures
• Performing rough hand calculations to estimate the magnitude of the reations
• Investigation of possible modeling of the slab track in relation to the reations
• Determination and comparison of the results of the individual actions in the governing condition at selected points incl. determination of the maximum and minimum combinations of the actions and interpretation of the results

This investigation is carried out by means of software.

Contact:
Franz Grützmacher
+49 351 463-40412

H - 5 Influencing parameters for acoustic signals' transmission

Full title: Investigation of different influencing parameters for the transmission of acoustic signals in acoustic emission analysis (SEA)

Acoustic signals are everywhere in our life. In everyday life they can be processed by our brain to get the needed information, e.g. from a conversation. In construction, they can also be used in combination with various measurement techniques for building diagnostics. Sound emission analysis (SEA) is a non-destructive testing method that can draw conclusions about a change in the condition of the structure based on the analysis of the sound waves emitted by the structure.

For a successful building diagnosis, the quality of the measured signals is crucial, which can be influenced by many parameters. These parameters can basically be divided into three groups, the sound source in the building component, the propagation path of the acoustic signals from the source to the sensor, and the measurement system. While the first two depend on the material properties of the component, parameters from the last group, e.g. the coupling of the measurement sensor, can be influenced by the measurement setup. A good coupling not only ensures a good signal quality, but also the reproducibility of the measurements.

Within the scope of the project work, various influencing parameters such as coupling means, the contact pressure of the measuring sensor with respect to the sensor coupling are to be investigated by means of tests on small-format specimens.

Details of the task will be specified during the processing time.

Contact:
Ronghua Xu
0351 463-33776