Rolf Breitenbücher

Influence of cyclic loading on the degradation of mechanical concrete properties

Structures like bridges, concrete pavements, cooling- and wind towers are exposed beside damaging environmental influences to cyclic mechanical loading scenarios. Therefore, the reliability of such structures depends among other influences on the degree of structural damage due to fatigue loading. In order to estimate the state of a structure it is necessary to know the development of the crucial material properties during its service lifetime.

Lifetime-Oriented Design Concepts

Structures deteriorate during their lifetimes, e.g. their original quality decreases. In terms of structural safety, this reduces the original safety margin, a process, which also can be described as an increase of structural damage. If, in such deterioration, the safety parameter decreases below the admissible safety limit, or the structural damage parameter increases beyond the admissible damage limit, then the structural service life will be terminated. If the failure safety value or the structural damage parameter both reach unity, the structure (theoretically) will fail.

Deterioration of Materials and Structures: Phenomena, Experiments and Modelling

Reliable computational prognoses of the structural integrity and serviceability throughout the lifetime of structures require the realistic consideration of the damage behaviour of the construction materials for various loading scenrios including static and cyclic loading, environmental loading processes such as moisture and heat transport, corrosion processes, freeze-thaw actions and possible interactions between these long- and short-term processes. Both, load-induced damage mechanisms such as evolving microcracks and physically and chemically induced deterioration originate from mechanical, physical and chemical processes starting at lower scales of the microstructure of the materials. Investigating and understanding these processes acting at various scales is a prerequisite for the development of adequate and suitable material models suitable for life-time oriented simulations.

Damage-Oriented Actions and Environmental Impact on Materials and Structures

Mechanical loading and ambient actions on civil engineering structures and components cause lifetime-related deteriorations. Not the rare extreme loading events are in the first place responsible for the evolution of structural degradation but the ensemble of load effects during the life-time of the structure. It is of major importance to have models at hand which adequately reflect the experienced time histories of impacts, and which can include justified predictions of future trends. Leading types of loading and load-effects with relation to mechanical fatigue as well as damages due to hygro-thermal and chemical impacts are considered in this chapter. Selected contributions from wind and temperature effects with certain meteorological characteristics as well as from traffic loads on roads and railway lines are modeled as typical examples of contributions to mechanically induced degradations of structures. A specific aspect is the permanent settlement of soil due to high-cyclic, longterm loading, for which novel representations are developed. The attack of freeze-thaw circles in different environments and of chemical impacts leading to solving, swelling and leaching processes in concrete including principle interactions are discussed as examples for the main types of non-mechanically induced degradations.

Hybrid Concrete Elements with Splitting Fiber Reinforcement Under Two-Dimensional Partial-Area Loading

In practice, concrete structures are often subjected to concentrated loads. Under such high stress concentrations, large splitting stresses are generated due to the resulting stress diffusion. In order to resist these splitting stresses, the state of the art is to place transverse steel reinforcement in zones where the most critical splitting stresses occur. An alternative approach gaining increasingly importance, is to add steel fibers to the concrete mixture. Regarding economic concerns, however, it is recommended not to reinforce the entire concrete element with steel fibers, rather in zones where high splitting stresses are expected. Based on this fact, a new design concept to manufacture hybrid concrete elements has been developed.

Development of Self-compacting Concrete with Good Shape Stability for Slipform Construction

The aim of this research project was to modify a conventional self-compacting concrete (SCC) for use in a slip-form paver. For this purpose, two alternatives were investigated: (1) thixotroping agents (concept A) that were added to the original concrete mix and (2) viscosity-increasing additives (concept B) that were intermixed shortly before the concrete was finally placed to reduce the concrete’s flowability. The investigations were performed preliminarily on pastes and mortars and only subsequently on concretes. For the modification of pastes and mortars, thixotroping additives mainly based on modified starches and modified polymers were used. For modification of the viscosity, superabsorbent polymers and naphthalene sulfonates in combination with polycarboxylate ether (PCE) were considered. The conducted investigations indicated that it is quite difficult to create slip-form SCC using thixotroping additives because the thixotropic effect is reversible and results in a low “green strength” when the concrete is mixed for a second time (e.g., during the delivery and placement process). When superabsorbent polymers and naphthalene sulfonate were added during the concrete placement, the stiffening effect was irreversible.

Influencing Parameters on the Dewatering Behavior of Annular Gap Grouts

The annular gap between segment lining and bedrock, caused by tunnel driving, must be filled instantaneously with an adequate grouting mortar. The decisive requirements for such grouts are, on the one hand, optimal flow properties lasting for several hours and sufficient stability and, on the other hand, a rapid development of shear strength immediately after grouting. The latter is normally achieved by dewatering under pressurization into the surrounding soil. Thus, two contradictory requirements are demanded for such grouts. The dewatering behavior of these grouts, which is significantly influenced by their constituents, is an important key factor. The correlation of these properties has been studied in systematic and extensive tests. The comprised grout mixtures are also performed in accordance with real design concepts to ensure an adequate application in practice. For this purpose, a test set-up was developed, simulating the conditions within annular gaps up to 18 cm in width. The amount of filterable water, including temporal effects, has been determined. Additionally, the development of shear strength by the dewatering effect has been examined at dewatered grouts. The investigations were carried out on typical grout mixtures with different variations of parameters (e.g. fly ash: additive ratio). From the results, the main influencing parameters on the dewatering behavior and development of shear strength could be defined. Consequently, well-proved mixtures can be recommended for further purpose.

Risse in Betonfahrbahndecken - Das Resultat aus Überlagerungen verschiedener Einwirkungen

Seit einigen Jahren werden Rissbildungen in Betonfahrbahndecken immer wieder mit Alkali-Kieselsaure-Reaktionen (AKR) in Verbindung gebracht. Fur diese Risse kommen jedoch verschiedene Einwirkungen in Betracht, die bei einer ganzheitlichen Betrachtung nicht vernachlassigt werden durfen. In umfangreichen Untersuchungen an verschiedenen Streckenabschnitten mit und ohne Rissen wurde sowohl anhand von Rissaufnahmen und Recherchen in den Bauwerksakten als auch anhand von einschlagigen Untersuchungen an Bohrkernen den Rissursachen und insbesondere den Einflussen einer AKR hierauf nachgegangen. Dabei zeigte sich, dass in den meisten Fallen eineUberlagerung und Interaktion verschiedenster Einflusse aus thermischen / hygrischen Zwangsbeanspruchungen, Verkehrsbelastung und / oder AKR fur die Rissbildung verantwortlich waren.

Future Life Time Oriented Design Concepts

In the following two possible applications of the lifetime control concepts proposed within this book are examplarily presented.