Harald Budelmann

Strength and deformation of concrete with variable content of moisture at elevated temperature up to 90°C

Abstract It is known, that the change of mechanical properties of concrete due to elevated temperature is also influenced by the moisture content. This change was primarily studied for prestressed concrete reactor vessels (PCRV). Because a PCRV is a mass concrete structure, the results of this research cannot be transfered to slender members. To simulate drying conditions of the latter, specimens of differing initial moisture content were subject to elevated temperatures and defined climates. The results of tests reveal the differing influence of moisture on strength and modulus of elasticity. The compressive strength is partly increased, partly decreased as the moisture content grows. Tensile strength and modulus of elasticity are weakened by decreasing moisture. Also the thermal strain as function of type of aggregate and moisture content was studied. The changes are caused by the alteration of structure of cement stone and by microcracks due to incompatibility.

Non-destructive measurement toolkit for corrosion monitoring and fracture detection of bridge tendons

One of the most considerable challenges of building management is to ensure the integrity and the functionality of prestressed members during service life. But particularly corrosive and mechanical influences on the prestressing steel reinforcement strongly impair the long-term performance and durability of interior as well as of exterior tendons in concrete bridges and of other prestressed structures. The durability state and degradation of tendons can be assessed primarily on the basis of the tendons force, the corrosion state and the existence of steel fractures. For the determination of these parameters, this paper will briefly report on the application and practical verification of a measurement toolkit, which is used for monitoring, inspecting and testing of prestressed concrete elements, with main consideration of prestressed tendons in bridges. This toolbox comprises conventional inspection and several innovative measurement and monitoring techniques, which have been developed, tested and optimised by the authors in the last decade.

Verification of thermal restraint of a railways trough structure by long-term monitoring

At the central railway station of Berlin a trough structure was erected to extend the tracks from the underground station to ground level. The trough is subjected to high ground water pressure. Safety against up-lift called for the coupling of the trough with the underwater concrete slab and the back-anchorage of the structure by piles. Such coupling leads to high restraint due to hydration of the concrete and the ambient weather. Analysis required assumptions regarding the interaction between the trough and the impeding sub-structure, as well as the stipulation of load-independent design actions based on meteorological studies. The owner decided to install a long-term monitoring system in order to verify the thermal and mechanical response of the structure. However, the main goal of monitoring is the improvement of specifications for design and tender of future structures with comparable complexity. The program and the monitoring techniques will be described, and specific results will be reported. Preliminary conclusions are drawn.

Earthquake protection of colonial bell towers in colima, mexico with externally prestressed frps

ABSTRACT A methodology for the seismic vulnerability reduction of old masonry towers with external prestressing is presented. It is applied at the Colonial bell-towers of the Cathedral of Colima, Mexico, characterized for being a high seismic area (M>7.5). The 3D FE models are calibrated with experimental data and assessed through nonlinear static approaches including the seismic demand and an accurate validated masonry model. Based on an extensive parametric study on different configurations of old masonry towers, it is selected an optimal prestressing force and device. The Colonial towers are retrofitted with four prestressing devices of FRPs to convert them into a high energy-dissipative reinforced masonry. The external vertical prestressing is included at key points identified in the seismic vulnerability assessment. This technique is in compliance with the demand for architectural conservation and may be located without drilling and unbounded in order to be fully removable. The seismic performance is enhanced by increasing force, displacement, and internal confinement. It is observed an upgrading of 35% and 20% of displacement capacity. With these results it is corroborated that external vertical prestressing allows a substantial increment of ductility for seismic energy dissipation purposes.

Seismic risk mitigation of historical masonry towers by means of prestressing devices

This work presents the investigation of the efficiency of different prestressing devices as a rehabilitation measure for the seismic risk mitigation of historical masonry towers. As a first phase, the seismic vulnerability of theoretical masonry towers was assessed by means of numerical models validated with information from the literature, observed damage and behavior of these structures due to passed earthquakes (crack pattern and failure mechanisms), and mainly taking into account the engineering experience. Afterwards, the validated models were rehabilitated with different prestressing devices; analyzing the results and concluding which device or the combination of them improved in a better way the seismic performance of the masonry towers. Finally, the methodology will be applied in two historical masonry towers located in seismic areas; the medieval tower “Torre Grossa” of San Gimignano, Italy, and one of the bell towers of the Cathedral of Colima, Mexico.

Effects of a clay additive on the properties of no-slump concrete

The strength of no-slump concrete is mainly achieved by a high particle packing density (ppd), which causes a strong interlocking of the coarse aggregates. Also inert fillers like fly ash are enhancing the strength by help of interlocking of micro-sized particles (Bornemann 2005). Both the compressive strength in the green and hardened state is enhanced. However, additional water or liquifier is needed to keep a sufficient workability. Accordingly there’s often a lack of water for the cement hydration process, that is either retained by the filler or is replaced by high superplasticizer content. Anyway there is a discrepancy to be noted between high green strength (little water needed) and high compression strength of hardened concrete (more water needed). The stickiness of a filler-optimized mixture causes hindrances in manufacturing, yet another problem. Another theory, wherein green strength is explained by capillary forces between micro-sized particles (Wierig 1972) seems to fail. Capillary forces have a maximum effect on particles with diameters from 0.1-1 mm, while friction forces are dominant when particle diameters exceed 1 mm (Bornemann 2005).

Assessment of the State of Condition of Prestressed Concrete Structures with Innovative Measurement Techniques

The reliability of tensile elements is a requirement for the safety of prestressed structures. Besides many other data, the assessment of the load-carrying capacity of a prestressed concrete structure requires the knowledge of the actual force of the tendons and their corrosion state as well as the localization of fractures. Neither for grouted interior nor for ungrouted exterior tendons non destructive and reliable methods to measure the prestress and to assess the corrosion activity or to locate tendon fractures are available today. This contribution reports on actual research projects, dealing with fiber optic sensors to measure deformations and dealing with the variation of high frequency damping of steel tendons in concrete members due to corrosion. Furthermore a high-frequency-method is used to locate fracture in tendons. First applications of some of these innovative techniques on a real structure are presented: the Herrenbrücke-bridge in Lübeck. Since three winters, measurements on this prestressed concrete bridge assessing its state of condition are carried out.

Incorporation of Concrete Rehabilitation Measures into Life-Cycle Maintenance

Key components of life cycle analysis of concrete structures are models for the assessment of the actual condition and for the prediction of its further development. Durability models have to consider the different influencing effects and degradation processes throughout the entire life cycle of a structure taking into account mutual dependencies, interactions, scattering and uncertainties. If reinforced concrete structures have been repaired during their lifetime, aging and deterioration of maintenance measures - like surface coating or concrete cover replacement - also must be taken into account. Degradation mechanisms of repair measures are different and they are going to be crucial for the durability of the repair measure. Taking into account the important parameters - like bond properties between concrete and repair mortar under thermal gradients, or of the adhesion influencing parameters of surface coatings - prediction models are needed. In this contribution, first ideas are presented on how the complex dependencies of the functionality and durability of repair measures may be recorded.