Influence of pore water on the wave propagation in concrete

Table of contents

1. 1. 1. Project data
      2. Report in the annual report 2020
      3. Report in the annual report 2019
      4. Report in the annual report 2018
      5. Report in the annual report 2017

Project data

Titel | Title Untersuchung des Einflusses von Porenwasser auf die Wellenausbreitung in Beton bei hohen Belastungsgeschwindigkeiten | Investigation of the influence of pore water on the wave propagation in concrete at high loading rates Förderer | Funding Bundesministerium für Wirtschaft und Energie (BMWi) Zeitraum | Period 09.2017 – 11.2020 Leiter | Project managers Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach, Prof. Dr.-Ing. habil. Ulrich Häußler-Combe Bearbeiter | Contributor Dipl.-Ing. Oliver Mosig

The presented project was funded by the German Federal Ministry of Economic Affairs and Energy (BMWi, project No. 1501553) on the basis of a decision of the German Bundestag.

Report in the annual report 2020

FASTER, STRONGER, WATER?

Concrete exhibits higher strength under fast, dynamic loading than under slow, static loading. This behaviour is known as the strain rate effect and has been the subject of research for many decades. The possible causes of this increase in strength are seen in various phenomena, but most of these have only been demonstrated qualitatively.
In the context of this research project, the extent to which pores present in concrete
and the pore water contained therein influence the propagation behaviour of stress waves in concrete was investigated, and this can be seen as a further cause of the strain rate effect.

In heterogeneous media, such as concrete, the propagation of load waves results in partial reflections and partial transmissions of the wave at the interfaces of the individual constituents due to the different material properties. As a result of the countless reflections and transmissions inside the concrete, transient, i.e. time-dependent, stress states prevail and not the static equilibrium. Within the scope of the project, numerical simulations with differently introduced model pores were carried out. Parameters such as the medium, the diameter or the number of pores were varied in different loading scenarios. In particular, the question was investigated to what extent existing pore water influences the reflection and transmission behaviour compared to the air-filled pore.

It was shown that measurable strength increases at the pore edge are reduced under dynamic loading compared to those under static loading. As a result of the notch stresses, the strength of the concrete is locally exceeded and microcracks form, which can be interpreted as the beginning of the fracture process. The medium of the pore – air- or water-filled – plays a subordinate role, so that the changed material impedance due to the presence of pore water cannot be used as a further cause of the strain rate effect.

In addition to the numerical investigations, experimental compression tests were carried out on defined water-saturated concrete specimens to determine the global influence of pore water on the static and dynamic compressive strength of the concrete. It is shown that both the static and the dynamic compressive strength of concrete decrease in magnitude with increasing pore water content and are therefore independent of the loading rate.

Report in the annual report 2019

TROUBLEMAKERS IN CONCRETE

Cement, aggregates and water – due to its composition, concrete is a prime example of heterogeneity. The cement acts as a binder for the aggregates; the chemical setting processes are initiated by the addition of water. After hardening, a solid, heterogeneous mixture of substances is present - a kind of artificially produced rock consisting of a cement matrix, the aggregates as well as air and water pores. The result is a strongly discontinuous distribution of stiffness at the meso level, with stress peaks at the boundary surfaces of the constituents under loading. This is comparable to the notch stress effect in mechanical engineering.

The aim of this research project is to investigate the influence of these impurities – especially the water pores – on the propagation of pressure waves in concrete at high loading rates.

In heterogeneous media, the propagation of such load waves results in partial back reflections and partial transmission of the wave due to the different material properties. Inside the material, there are transient, time-dependent stress states and no static equilibrium. It is assumed that due to reflections and transmissions, the mean stress inside the heterogeneous concrete is lower than the stress applied from the outside. Due to the material heterogeneity in combination with the high loading rate, a higher measured strength of the concrete can result – a possible cause of the so-called strain rate effect.

In order to be able to make qualitative and quantitative statements, numerical simulations are carried out with several models in which the pores and impurities are analyzed. Parameters such as the medium, the diameter or the number of pores are varied in different load scenarios. In addition, compression tests are carried out on defined water-saturated concrete specimens to determine the global influence of pore water on the static and dynamic compressive strength of concrete. This shows that the static, as well as the dynamic compressive strength of the concrete, decreases in the same way with increasing degree of saturation.

Report in the annual report 2018

WATER

Neben Zement und Zuschlag bildet Wasser einen Hauptbestandteil des Betons. So werden die Eigenschaften des Betons bei der Herstellung wesentlich durch das Verhältnis von Wasser und Zement, den sogenannten w/z-Wert, gesteuert.

Nach der Erhärtung des Betons liegt das Wasser in unterschiedlichen Zuständen vor: zum einen in Form von chemisch und physikalisch gebundenem Wasser in den Calcium-Silikat-Hydrat-Phasen des Zementsteins, zum anderen verbleibt ein restlicher Teil als freies Wasser in der Porenstruktur des Betons. Die Umgebungsbedingungen, wie beispielweise Regenschauer, Luftfeuchtigkeit und Temperatur, steuern dabei die Wassersättigung der Poren im Beton.

Ziel dieses Forschungsvorhabens ist es, den Einfluss des freien Porenwassers auf die Ausbreitung von Druckwellen im Beton bei hohen Belastungsgeschwindigkeiten zu untersuchen. Dazu werden Betonprobekörper unterschiedlich lange wassergelagert, um definierte Sättigungsgrade, von trocken bis voll wassergesättigt, zu erzielen. Erste Ergebnisse zeigen, dass die Betondruckfestigkeit mit steigendem Wassersättigungsgrad abfällt. Dieses Phänomen konnte sowohl im statischen als auch im dynamischen Belastungsfall in gleicher Weise beobachtet werden. Die Mechanismen, welche zur Festigkeitsreduktion führen, scheinen demnach unabhängig von der Belastungsgeschwindigkeit zu sein.

Im Vergleich zu wassergesättigten Proben versagen die trockenen Betonproben explosionsartiger, mit einer erhöhten Riss- und Bruchstückanzahl. Es wird angenommen, dass das freie Porenwasser die innere Reibung und Verzahnung an den Rissoberflächen herabsetzt. Die Bruchflächen gleiten somit besser aneinander ab und mindern die Druckfestigkeit des Betons. An den Rissspitzen wirkt das Wasser als eine Art Keil und beschleunigt die Rissbildung. Durch die Verwendung unterschiedlich geformter Betonprobekörper konnte außerdem festgestellt werden, dass im Vergleich zu schräg verlaufenden Rissen bei parallel zur Belastungsrichtung verlaufenden Rissen ein höherer Festigkeitsabfall zu beobachten ist.

Das diesem Bericht zugrunde liegende Vorhaben wurde mit Mitteln des Bundesministeriums für Wirtschaft und Energie aufgrund eines Beschlusses des Deutschen Bundestages unter dem Förderkennzeichen 1501553 gefördert.

Report in the annual report 2017

WAVES IN CONCRETE

Plane impact, earthquakes, and explosions – such extraordinary loads are critical to the design and construction of buildings, for example power plants or airports. To ensure and assess the safety of these structures, understanding of the material and damage behaviour of concrete under rapid, dynamic impact is crucial.

Concrete has higher strengths under dynamic stress than in the quasi-static case. This phenomenon has been known for over 100 years, as the so-called ‘strain rate effect’. In addition to inertial effects due to the rate-dependent growth rate of the microcracks, the pore water is considered as another cause of this increase in strength. The viscous behaviour of the free water between the crack surfaces creates capillary forces that counteract crack growth. Also, the pores and their water content have a significant influence on the propagation of the impact wave.

To describe the wave propagation at these discontinuities, artificial pores are to be introduced into concrete specimens and loaded dynamically in the split-Hopkinson bar. Number, size, position, and water content of the artificially generated pores are to be varied. To reduce the influence of other discontinuities, such as aggregate’s grain sizes and air pores, a very fine-grained, well-compacted concrete is used.

Also, special attention must be paid to the measuring technology and especially its arrangement. Since the stress wave passes through the specimen within a few microseconds, a high sampling rate of the strain gauges is required. The partial transmission, reflection, and superimposition of the wave play a role as well. The position of the strain gauges thus determines what part of the wave is measured. In combination with numerical simulation, the experimental results are verified to gain insights into the influence of water-filled pores on the behaviour of concrete under impact load.

The presented project was funded by the German Federal Ministry of Economic Affairs and Energy (BMWi, project No. 1501553) by a decision of the German Parliament.