Harald S. Müller

A model law for the notch sensitivity of brittle materials

Abstract In this paper a model law for the notch sensitivity of brittle materials, for instance hardened cement paste, mortar or concrete is presented. This model law shows that notch sensitivity is a necessary however not a sufficient condition for the applicability of linear elastic fracture mechanics. The model law indicates that notch sensitivity of a brittle material decreases with increasing fracture toughness, decreasing tensile strength and decreasing specimen size. The model law explains the increase of the net failure stress of notched specimens with increasing notch depth after passing through a minimum. Such behavior frequently has been observed in experiments on hardened cement paste, mortar and concrete specimens. Results of flexure tests on notched and unnotched hardened cement paste specimens and concretes of various sizes are in accord with the model law.

The role of material engineering within the concept of an integrated water resources management

By means of a case study, the successful implementation of a rheologically optimised cement-based mortar for the construction as well as for the rehabilitation of rain water cisterns is presented in this paper. The material was developed within the scope of a German–Indonesian joint project [“Integrated Water Resources Management” (IWRM)], funded by the German Federal Ministry of Education and Research. Comprehensive rheological investigations are presented which provide the database for the optimization of the mortar with regard to its intended range of application. For the selection of the source materials, special emphasis was placed on the ready availability at low cost. The rheological properties of the fresh mortar allow an easy workability by hand while the hardened mortar shows a durable and tight appearance at the same time. The developed material can be used as a coating for walls, floors and ceilings of cisterns, for the local rehabilitation of damaged areas only or even as a construction material for complete new cisterns. The future multiplication of the IWRM project results within the region was assured by a local capacity development when the presented material concept was applied in practise in Indonesia for the construction of sustainable rain water cisterns in Gunung Kidul.

Water penetration into micro-cracks in reinforced concrete

Cracks play a decisive role for durability and service life of reinforced concrete structures. In the contribution very fine micro-cracks, which are usually neglected in durability design were induced mechanically into steel reinforced mortar and concrete elements. It turned out that even finest micro-cracks are immediately water filled whenever the surface comes in contact with liquid water. lt can be concluded that micro-cracks have also to be taken into consideration in any realistic service life design. The penetration depth of micro-cracks in concrete plays a decisive role for service life. The role of micro-cracks for durability can be significantly reduced by water repellent treatment of the cracked surface. Further investigations will tell us to which degree and in which environment self-healing may reduce the influence of micro-cracks for durability of reinforced concrete. (A)

Long-Term Safety of Well Abandonment: First Results from Large Scale Laboratory Experiments (COBRA)

A long-term safe and reliable abandonment of wells is a crucial prerequisite for a secure abandonment of underground storage sites, while considering the long-lasting environmental impact, e.g. leakage through wells. To study the impact of the cementation on the tightness of wells, both, the cementation of a well during completion and abandonment are investigated in Full-scale laboratory experiments. Different autoclaves from small to Full-scale have been developed to test cementations under various conditions and to perform long-term tests under in situ conditions of a CO2 storage site. The experiments show that the surface-texture (e.g. roughness) of the drilled well has a significant influence on the formation of mud-channels—for rough surfaces, up to 75 % of the serrations consist of non-displaced mud and only 25 % are well hardened cement, creating possible leakage pathways. Even under idealized cementation conditions using a cement recipe characterized by a very low shrinkage, micro-annuli are formed. These micro-annuli are connected throughout the whole oil-field casing in the Full-scale experiments for which the widths of the micro-annuli in the order of 10–20 µm could be deduced. Along the micro-annuli the cement is carbonated due to the flow of CO2-bearing fluid. The fluid flow could also be verified by a Spatial Time-Domain-Reflectometry (TDR) setup embedded in the cementation, which was successfully tested as an in situ monitoring system. In the Full-scale experiments, chemical reactions in the system casing—cement—rock—fluid were examined. The geochemical analyses during and after the experiment show variations in pH, conductivity and chemical composition of the brines, which are well described by an interplay of corrosive processes and precipitations of carbonate minerals.

New Types of High Performance Concretes – Potentials for Innovations in Concrete Construction

Among the new types of high performance concretes this paper deals with self-compacting concrete (SCC) and ultra high strength concrete (UHPC). Both types of self-compacting concrete, the powder type SCC and the stabilizer type SCC, are presented. Differences in the mix design and composition as well as the properties in the fresh and in the hardened state are indicated. While most of the properties of SCC are roughly similar to normal concrete (NC) – apart from the self-compacting property – this holds by far not true for UHPC. Not only the mechanical properties but also the durability of UHPC deviates significantly from normal concrete. In addition, it is shown that UHPC is more sustainable than normal concrete if the ecological impact is considered in relation to the performance of the concrete.

Kinetics of Drying Shrinkage and Creep: An Experimentally Based Code-Type Approach

The paper presents the results of creep tests conducted at temperatures ranging from 20 to 70 °C at 65% RH on concrete samples preconditioned at several environmental relative humidities. During the conditioning of the samples as well as during the execution of the creep tests, miniaturized humidity sensors were employed to measure the relative humidity in the concrete pores. The measured creep and shrinkage deformations are the higher, the higher the initial moisture content of the samples, and additionally, the higher the rate at which concrete losses moisture due to a dry ambient environment. The rate of change of the mean relative humidity of the samples was found to scale in a unique manner the rate of drying shrinkage, independent of the moisture content. Based on the mean relative humidity measured in the concrete samples, simple mathematical formulations to predict basic and drying creep are proposed. Assuming that basic creep is influenced by the moisture content, the evaluation of the model suggests that drying creep, drying shrinkage and mean relative humidity follow the same kinetics.

The PACE-1450 Experiment – Investigations Regarding Crack and Leakage Behaviour of a Pre-stressed Concrete Containment

The subject of the project “PACE-1450 – Experimental Campaign” was to complement the variety of research programs dealing with the leakage behaviour of concrete containments of nuclear power plants under accidental conditions. The test campaign has been run with a specimen of realistic dimensions, reinforcement and pre-stressing. Supplementary investigations on the crack pattern of the used specimen are underway. Leakage tests have been performed under different temperature and pressure conditions with varying media mixtures of air and steam. This paper summarizes the results of the air leakage investigations at different temperatures.

Sicherheit durch Beton : Schutz vor Explosion, Brand und Risikostoffen : 13. Symposium Baustoffe und Bauwerkserhaltung, Karlsruher Institut für Technologie (KIT), 16. März 2017

Durch eine Veranderung von Gefahren mussen Sicherheits- und Schutzvorkehrungen stetig angepasst werden. Hierzu gilt es die weitlaufigen Einsatzmoglichkeiten des Werkstoffs Beton als schutzender Baustoff zu optimieren und weiterzuentwickeln. Der vorliegende Tagungsband vermittelt das Wissen uber die Schutzfunktionen von Betonbauteilen gegen Explosion, Brand und Risikostoffe. Neben betontechnologischen Optimierungen werden konstruktive Aspekte an praxisnahen Beispielen ausgehend dargestellt.

Self-healing of Cracks in Strain Hardening Cementitious Composites Under Different Environmental Conditions

Strain Hardening Cementitious Composites (SHCC) may be characterized by their high strain capacity, which is obtained by adding fibers to the cement-based fine mortar. PVA fibers are often used. When a crack is formed because of imposed strain, fibers bridge the new crack surfaces and hinder wide opening. In this way multiple crack formation is observed instead of widening of few existing cracks. Crack formation is one of the most frequent reasons for corrosion of steel reinforcement and of limited service life of reinforced concrete structures. Water and aggressive aqueous solutions can migrate deep into concrete via existing cracks. Initially it was believed that micro-cracks in SHCC are so fine that they will not transport liquids by capillary action. But by means of neutron radiography it could be shown that even very fine cracks are very quickly filled with water by capillary action. From the water filled cracks water migrates further into the porous structure of hardened cement paste. From these results it follows that formation of fine cracks as such is not sufficient to protect concrete from deep penetration of water and aggressive aqueous solutions. Surface treatment with a water repellent agent can solve the problem. But self-healing of fine cracks may turn out to be an inherent protection of concrete. The influence of self-healing on capillary absorption was studied experimentally. It can be shown that if pre-strained SHCC with micro-cracks is exposed to high relative humidity capillary absorption is gradually reduced. If pre-strained SHCC is stored in water or in saturated aqueous Ca(OH)2-solution, self-healing is accelerated and capillary absorption reaches finally values of the material without cracks. Hence, increased durability of SHCC is not primarily due to the fineness of cracks but rather due to accelerated self-healing.

Bauwerkserhaltung - Instandsetzung im Beton- und Stahlbetonbau : 12. Symposium Baustoffe und Bauwerkserhaltung, Karlsruher Institut für Technologie (KIT), 10. März 2016

Die Instandhaltung von Betonbauwerken ist von zentraler Bedeutung fur deren nachhaltigen und wirtschaftlichen Betrieb. Eine geeignete Instandhaltungsplanung erfordert jedoch Kenntnis der zu erwartenden Einwirkungen auf das Bauwerk, uber die Leistungsfahigkeit der Baustoffe und die Moglichkeiten der Instandsetzung. Der vorliegende Tagungsband vermittelt dieses Wissen. Die Entwicklung angepasster Instandsetzungskonzepte wird diskutiert und die gangigen Instandsetzungsprinzipien werden vorgestellt.