Stephan Pirskawetz

Plastic Shrinkage Cracking Risk of Concrete – Evaluation of Test Methods

Concrete pavements subjected to fast evaporation during the first hours after concrete placing are prone to plastic shrinkage cracking. In this paper the mechanisms by which evaporation of water leads to capillary pressure, and finally to shrinkage and cracking, are briefly discussed. In the main part, the paper presents results of a study on the evaluation of three different measurement setups. With two of these setups usually the efficiency of fibres in reducing plastic shrinkage cracking of concrete is assessed in Germany and Austria. The third one is an advanced setup that was developed in collaboration of BAM and Empa. It allows performing measurements of parameters that are important for the evaluation of the cracking risk.

Detection of early-age cracking due to restrained autogenous shrinkage

The influence of the mix composition on the early-age autogenous shrinkage of high-strength cement paste with and without silica fume and the resulting crack formation were investigated. The mechanisms of autogenous shrinkage were studied systematically by linking the measured deformations to hydration kinetics and to pore structure data. The time t0 which assigns the transition from the plastic state to a solid structure was determined by analysis of the measured strain rate. It was demonstrated that the autogenous shrinkage correlates reasonably well with the self-desiccation and the thereby expected capillary stresses. The crack formation was detected using acoustic emission (AE) analysis. AE measurements were performed simultaneously on hardening silica fume cement pastes with and without external restraint of deformation. The results showed that the intense autogenous shrinkage during the acceleration period of cement hydration was associated with the begin of micro cracking shortly after t0, irrespective of the existence of external restraint. The cracking was more pronounced in the case of restraint.

Joint Sealing Systems for Pavements—A New Approach Towards a Performance Related Evaluation of Capability and Durability

Joint sealants influence decisively the performance and service life of pavements although they account for only a small fraction of the total investment. Motivated by the damages observed and the resulting, increasing maintenance efforts, the Federal German Government recognizes the need for performance-evaluated joint sealing systems with improved capability (fitness-for-purpose) and durability. A literature study showed that an identification of the actual mechanical system behavior under realistic loads as well as a prediction regarding the durability (fatigue, climatic effects) of joint sealing systems are either completely lacking in most of the relevant evaluation methods or have only been incompletely addressed previously. Furthermore an imbalance between commonly used test methodologies and the actual development status of modern modified sealing materials exists, i.e., the current test methods are not effective in evaluating the performance of tailor-made products. In this paper, the authors suggest a methodology to overcome the present situation. In contrast to the existing, predominantly empirical evaluation and selection of joint sealing materials and systems for pavements, the new approach is defined by verified performance under relevant and superimposed loads. This new approach is expected to allow a more engineered joint design. In addition to the adaptation of performance-oriented material identification tests, a special focus was placed on the development and installation of a complex test facility for the investigation of the service capability and durability of joint sealing systems in building constructions in general. This paper presents an attempt at the realization of this approach for pavement joints with the help of our new joint sealant test equipment utilizing a specific, adapted load function, which comprises cyclic movements (slow and fast acting), as well as crucial climatic exposures. The test data and its interpretation are discussed. For example, the actual mechanical behavior of the various joint sealing systems as well as the relevant maximum loading of cohesive and adhesive bonds can be deduced and used to differentiate between systems. Furthermore, information gained allows discrimination of products within the various joint sealing systems. The test results will also enable numerical simulations, e.g., of different joint designs or materials by finite element analysis. The fatigue behavior is detected by analysis of cycle-dependent changes of the mechanical system characteristics. The evaluation methodology further allows investigation of the degradation mechanisms of specific system failures and, thus, enables service life prediction by reproducing the performance of the complete system under realistic conditions. Constructional defects and material flaws can be activated and detected by the performance-related test methodology, thus identifying possible corrections to material selection and application procedures. The potential of the proposed evaluation methodology is discussed for several thermoplastic and reactive joint sealing systems.

The burned shield-bearers: a detective story

Severe fires at the Friedrichshain flak bunker in May 1945 destroyed or severely damaged an important part of the collection of the Kaiser-Friedrich-Museum, Berlin, today’s Bode Museum. Fragments of two marble sculptures by Tullio Lombardo, sculptor of the Italian renaissance, have survived and are now stored in the Bode Museum. In preparation for restoration, the condition of the sculptures has been documented in detail in 2010. The study revealed that the fragments are not stable enough to exhibit them in a horizontal nor in an upright position. The intention of a further study in 2012 was to find a correlation between ultrasonic velocity and mechanical properties of burned Carrara marble as a basis for stabilization measures. The results and numerous supplementary experiments show that the current condition of the sculptural fragments can not only be explained by the exposure to fire.

Combination of Digital Image Correlation and Acoustic Emission for Characterizing Failure Modes in Strain-Hardening Cement-Based Composites (SHCC)

The tensile behavior of strain-hardening cement-based composites (SHCC) is usually investigated on macroscopic scale by means of direct tension tests or bending tests. Additionally, the micromechanical properties of the composites are often described based on single fiber tension and pull-out tests. Such investigations, performed both on macroscopic and microscopic scales, are based on ‘classical’ force and displacement measuring techniques. Advanced test methods such as digital image correlation (DIC) and acoustic emission analysis (AE) may facilitate the identification and the analysis of the failure mechanisms in SHCC, which is important for both monitoring loaded SHCC elements and further material development and optimization. In this study, these two techniques are combined to characterize the failure mechanisms of three different types of SHCC in direct tension tests. The results are related to data of stress and strain measurements. It is shown that DIC provides detailed spatially resolved and stress related strain measurements. Furthermore, it is demonstrated that AE allows for the localization of active cracks, quantification of the damage accumulation under increasing stresses, and characterization of the dominant crack bridging mechanisms and failure modes observed in the different types of SHCC.

Characterizing the crack development in strain-hardening cement-based composites (SHCC) by means of acoustic emission

The article at hand presents an investigation on crack development in three different types of strain-hardening cement-based composites (SHCC) subjected to uni-axial tensile loading. The aim of the work was to evaluate the applicability of acoustic emission (AE) measurements for determining the progressive damage within the material, as well as for differentiating the individual damage events by their origin and decisive mechanisms, such as matrix cracking, fiber pullout or fiber rupture. The acoustic emission method proved to be fully appropriate for recording and evaluating the fracture related processes in various types of SHCC. Valuable information on failure mechanisms and quantitative description of damage depending on SHCC composition was obtained and evaluated with respect to the measured stress-displacement curves and under consideration of fracture surfaces and crack patterns observed on the specimens.

Zwangsinduzierte Rissbildung in Hochleistungsbeton durch autogenes Schwinden

In diesem Beitrag werden Untersuchungsergebnisse vorgestellt, die den Einfluss von Mikrosilica auf die Selbstaustrocknung und das damit verbundene autogene Schwinden der Bindemittelmatrix von Hochleistungsbetonen beschreiben. Die dadurch hervorgerufenen Schadigungsprozesse wurden mit Hilfe von zerstorungsfreien Prufmethoden verfolgt. Dabei zeigte sich, dass eine durch auseren Zwang bedingte Mikrorissbildung durch Schallemissionsanalyse detektiert werden kann.