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- 8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen (20)
- 7 Bauwerkssicherheit (10)
- 7.4 Baustofftechnologie (4)
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- 7.1 Baustoffe (2)
- 7.3 Brandingenieurwesen (2)
- 5 Werkstofftechnik (1)
- 5.2 Metallische Hochtemperaturwerkstoffe (1)
- 5.5 Materialmodellierung (1)
On the background of the need for reuse of building rubble and the saving of natural resources, a European Union research project was performed covering concrete technology and durability aspects. A very special precondition was the use of large-scale processed building rubble with unknown origin and the total replacement of aggregates above 2 mm. Comprehensive tests were carried out on the properties of the starting material from different processing plants and fresh concrete, particularly the interdependence of water addition and wor ability. Investigations on hardened concrete properties included strength devel-opment, creep and shrinkage, modulus of elasticity, capillary absorption, freezing and thawing resistance, and carbonation behavior over a long period. The results demonstrate that the industrial production of a high-grade, durable concrete is possible. The project is a promising contribution for sustainable development.
Rapid and accurate non-destructive evaluation (NDE) techniques are needed to assess the in-place condition of concrete structures. However the time and effort required to perform NDE tests using conventional surface-mounted contact sensors hinder rapid evaluation of full-scale structures. The authors are cooperating in research to develop rapid and robust methods that collect mechanical wave signals from concrete structures and to characterize defects in concrete
using that signal data. The suitability of surface waves and laser-based, non-contact wave detection techniques for this purpose are examined here. After a brief introduction to non-contact sensing, an experimental set-up is described and results from experimental tests are presented. Mechanical surface waves in a concrete slab specimen are generated with several different conventional sources. The ability of a laser vibrometer to detect wave propagation from concrete that does not have any reflective treatment (normal concrete surface) is demonstrated, allowing the generation of ultrasonic images. Test results from a concrete specimen containing a subsurface defect are then presented. The obtained ultrasonic images illustrate some limitations of using surface waves to detect subsurface defects in concrete, although approaches to improve the results are proposed.
An advanced quantitative approach of pulse-phase-thermography for nondestructive
testing in civil engineering is described in this contribution. The
characteristic frequency of the maximum phase-contrast between defects and sound
areas is used as a means for the characterization of its depth.
The new approach is tested in the laboratory on concrete structures with defects of
polystyrene. The surfaces of the structures were heated with IR-radiators for varying
time periods.
The presented investigations were funded by the Deutsche Forschungsgemeinschaft
(DFG) and were carried out in co-operation with the Technical University of Berlin
(TUB).
The active approach for non-destructive-testing in Civil Engineering (NDT-CE) with infraredthermography (IR), developed at the Federal Institute for Materials Research and Testing (BAM), is described in part one of this contribution. The active IR-approach is based on the principles of impulse- (IT) and pulse-phase-thermography (PPT). The concept is a direct result of a research project at the BAM in cooperation with the Technical University of Berlin (TUB) and was funded by the Deutsche Forschungsgemeinschaft (DFG). In the second part results of the new thermography-scanning system (thermo-scanner) of BAM on debonding of multi-layered structures, especially carbon fibre reinforced plates (CFRP) on concrete are presented. Both, the thermo-scanner developed at BAM, and the investigations carried out, are part of the
EU-Project Sustainable Bridges. In the third part of the paper the characteristic frequency of the optimum phase-contrast as a means for the characterization of defects depth in CE by means of PPT is introduced. A new quantitative approach is tested and discussed on concrete structures with defects of polystyrene and gas concrete.
Pitting corrosion of carbon steel and iron in chloride containing alkaline environment is already well investigated, although a couple of questions are not clarified satisfactorily. The critical chloride content for steel in concrete and the critical chloride to hydroxide ion ratio for steel in alkaline solutions is still a major concern, considering chloride induced corrosion problems of steel in concrete. Another not satisfactorily clarified point is the repassivation of carbon steel in consequence of cathodic polarization. In the recent literature some indications are shown that repassivation occurs after a certain time of polarization. GLASS [4-9] and PEDEFERRI [1-3] draw the conclusion that the ennoblement of open circuit potentials (OCP) indicates re-passivation effects due to cathodic polarization. SAGÜÉS [10] carried out potentiodynamic repassivation tests and draws the conclusion that the repassivation potential Er is nearly independent of the solution composition, but influenced by severity of corrosion attack and is in the potential range from - 0.8 V to - 0.4 V vs. Saturated Calomel Electrode (SCE). The transferability of SAGÜÉS results to questions concerning cathodic protection (cp) is questionable, as potentiodynamic polarization tests (PPT) give no information on changes induced by long time polarization.
Therefore, the investigations discussed in this paper aim to clarify, to what extend the re-passivation of carbon steel due to cathodic polarization occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step the corrosion state of five nominal equal test specimens was determined by electrochemical techniques, i.e. electrochemical noise measurements (ENM) and electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarization resistance respectively as well as evaluating potential and current noise signals, the specimens were polarized cathodically. Impedance data were recorded previous to, during and after polarization. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and diffusion properties. The results show that reduction of oxides and oxygen diffusion during cathodic polarization has strong impact on the systems behavior.