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- 2010 (13) (entfernen)
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- Englisch (13)
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- Concrete (13) (entfernen)
Surface wave velocities over the sonic frequency range (<20 KHz) were measured on concrete specimens undergoing various cycles of loading and unloading. Acoustic Emission test (AE) was conducted simultaneously to monitor the microcracking activities. The sonic surface wave velocity was found to be highly stress-dependent. The observed changes in surface wavespeed are repeatable and follow a particular trend. By measuring the wave velocities in both loading and unloading phases, the effects of stress and stress-induced damages could be distinguished. The observed trend could be explained by a combination of acoustoelasticity and microcracking theories.
Today modern reprocessing techniques lead to materials, which bear the potential to be used as aggregates or additives in mineral building materials. Because of highly sophisticated reprocessing techniques residues with relatively stable composition over a certain range and relatively defined properties can be produced. At the same time, new German and European regulations demand a waste management to decrease waste volume stored at landfills to save natural resources and to enhance sustainable development by recycling of diverse residues. This paper shows the activities of the working group 'Saving Resources by Material Recycling'.
The delamination of a plaster layer, which covers a concrete structure, can be detected by active thermography. In this paper, a systematic series of measurements is described, where the gap between plaster and concrete was varied. These investigations were compared to FEM-simulations. The experimental and simulated data were in good agreement. It was found that the used experimental setup yields only marginal differences between fixed and lose plaster in the thermal behavior. However, significant characteristics within the temperature decays could be resolved, which are clearly related to the air gap.
The changes in the sonic surface wave velocity of concrete under stress were investigated in this paper. Surface wave velocities at
sonic frequency range were measured on a prismatic concrete specimen undergoing several cycles of uniaxial compression. The
loading was applied (or removed) gradually in predefined small steps (stress-controlled). The surface wave velocity was measured
at every load step during both loading and unloading phases. Acoustic Emission (AE) test was conducted simultaneously to
monitor the microcracking activities at different levels of loading. It was found that the sonic surface wave velocity is highly stress
dependent and the velocity-stress relationship follows a particular trend. The observed trend could be explained by a combination
of acoustoelasticity and microcracking theories, each valid over a certain range of applied stresses. Having measured the velocities
while unloading, when the material suffers no further damage, the effect of stress and damage could be differentiated. The slope
of the velocity-stress curves over the elastic region was calculated for different load cycles. This quantity was normalized to yield a
dimensionless nonlinear parameter. This parameter generally increases with the level of induced damage in concrete.