Severin Häfliger

Last Name

Häfliger

First Name

Severin

ORCID

0000-0001-7355-0976

Organisational unit

09469 - Kaufmann, Walter / Kaufmann, Walter

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Publications 1 - 10 of 10

Load‐deformation behavior of locally corroded reinforced concrete retaining wall segments: Experimental results
Häfliger, Severin; Kaufmann, Walter (2023)
Structural Concrete
Local reinforcement corrosion damage reduces the load-bearing capacity of reinforced concrete structures and, even more severely, their deformation capacity. This problem is of particular concern for cantilever retaining walls, whose loading is dominated by earth pressure and hence, depends on the wall deformations. With a limited deformation capacity at the ultimate limit state due to the locally corroded reinforcement, the earth pressure may not drop to its reduced value typically assumed in design, and simultaneously, the structural resistance may be severely impaired by the cross-section loss. Load redistributions are impeded since retaining walls are statically determined vertically and typically segmented longitudinally. This increases the risk that affected structures collapse, exhibiting a brittle failure. The situation is aggravated by the fact that the wall deformations prior to failure are too small to be detected by conventional monitoring, as indicated by a previous study. To improve the basis for quantifying the related risks and the magnitude of prefailure deflections, this study investigates the load-deformation behavior of cantilever retaining walls affected by local pitting corrosion, focusing on (i) the influence of the corrosion pit distribution among different reinforcing bars on the load-bearing and deformation capacity and (ii) the interdependence of corrosion, reduced deformation capacity and deformation-dependent loading. To this end, eight large-scale experiments on retaining wall segments were conducted in the Large Universal Shell Element Tester (LUSET), simulating the lower part of a 4.65-m-tall cantilever retaining wall. Five specimens contained initial damage (pitting corrosion simulated by a spherical mill). In the remaining three specimens, artificial corrosion damage was induced during the experiments. For two of the latter specimens, the loading was adapted in real-time control depending on their deformation to simulate the decreasing earth pressure. These are the first large-scale hybrid tests in the field of corrosion research to our knowledge. The experiments confirmed that the ultimate load and the corresponding deformation strongly differ depending on the corrosion pit distribution, even among specimens with equal mean cross-section loss. Furthermore, it was found that the deformation increase due to corrosion damage depends on the loading and, hence, on the compaction of the backfill. The observed deformation increase ranged between 0.8 and 1.4 mm per meter height at 40% crosssection loss, with loose soil causing a larger deformation increase. The load transfer between the damaged and undamaged reinforcing bars was found to take place in the first two crack elements above the construction joint. Local bending moments occurred in the reinforcing bars in the vicinity of the corrosion pits due to the shift of the center of gravity of the bar at the pit. Fiber optic strain sensing allowed visualizing the bending moment decrease in the embedded part of the damaged bars as a consequence of a lateral bearing pressure.
Influence of quasi-static strain rate on the stress-strain characteristics of modern reinforcing bars
Häfliger, Severin; Fomasi, Sara; Kaufmann, Walter (2021)
Construction and Building Materials
While it has been known for decades that even for quasi-static loading, increments in strain rate lead to increased stresses in the inelastic range, this effect is often ignored. However, accurate knowledge of the stress-strain characteristics of reinforcing bars is an indispensable prerequisite for the safe design of new structures and the realistic assessment of the structural safety of existing ones, and the strain rate dependency of the reinforcing steel characteristics should thus be accounted for in many situations. An exemplary case is quality control: Reinforcing bars produced today are periodically tested to check their conformity with specifications, determining their stress-strain characteristics in standard tensile tests. However, the applied quasi-static strain rates may vary considerably, but are not commonly reported. Hence, the results are subject to considerable uncertainty. Another relevant case is the structural safety assessment of existing structures affected by local corrosion of the reinforcement: Their cross section (and hence, stiffness) varies considerably along the bar axis and consequently, the strain rate in corroded sections is significantly higher than in non-corroded sections, leading to higher yield stress and tensile strength. This study investigates the effect of quasi-static strain rates on the stress–strain characteristics of modern reinforcing bars based on a comprehensive experimental campaign. In four series of experiments, 41 tensile tests on three different types of reinforcing bars were conducted, applying strain rates between 0.004 ‰/s and 1.0 ‰/s. Compared to the static stress, an increase of up to 8% in the dynamic stress was observed, depending on the type of reinforcing bar. Based on these observations, a simplified model for the strain rate dependency was developed and validated against experimental data, showing excellent agreement.
Load-deformation behaviour of reinforced concrete structures affected by local corrosion
Häfliger, Severin (2023)
A growing number of ageing structures is affected by pitting corrosion due to the ingress of chlorides or structural defects such as honeycombs. The resulting local damage reduces their load-carrying capacity and – even more pronouncedly – their deformation capacity due to the corresponding strain localisation. The latter is particularly critical for statically indeterminate structures whose structural safety relies on plastic load redistributions or for structures whose main loading is deformation-dependent, such as the earth pressure in the case of retaining walls. In fact, many design rules in current codes are based on the lower bound theorem of the plasticity theory (though often implicitly, e. g. by neglecting initial internal or external restraint stresses), which requires a sufficient deformation capacity as commonly available in uncorroded elements. These rules are, however, no longer applicable to locally corroded structures unless their deformation capacity is carefully assessed. Unfortunately, and despite much research conducted over the past decades, no mechanically consistent, generally applicable assessment strategies in case of local corrosion exist. This thesis addresses this knowledge gap by investigating the influence of local corrosion on the load-deformation behaviour of reinforced concrete structures, focusing on the practical case of corroding cantilever retaining walls. A comprehensive series of tensile tests on artificially damaged bare reinforcing bars revealed the influences of (i) strain rate, (ii) varying microstructural layers over cross-section, and (iii) the pit geometry on their load deformation behaviour. Whereas the varying strain rate (i) along the bar axis tends to increase the tensile strength at the corrosion pit, it is potentially reduced for increasing cross-section loss in modern reinforcing bars exhibiting a varying microstructure (ii) over the cross-section, as it is characteristic for quenched and self-tempered (“Tempcore”) reinforcing bars. Depending on the pit geometry (iii), the apparent uniaxial tensile strength and the deformation capacity in the pit and in its vicinity increase due to a triaxial stress state. This effect counteracts strain localisation and leads to a significantly higher deformation capacity of affected bars than assumed by common strain localisation models. A series of large-scale tests on cantilever retaining wall segments with artificially damaged reinforcing bars confirmed a pronounced influence of the effective corrosion distribution among the reinforcing bars, as anticipated based on a preliminary theoretical analysis: the load-carrying and deformation capacity of structures containing many slightly corroded bars differs significantly from that of structures with only a few but severely corroded bars, even if the total cross-section loss is equal in both cases. Hence, merely indicating the mean cross-section loss is inappropriate to conclude on the load-deformation behaviour of a structure. Two hybrid tests, where the corrosion damage was increased at simultaneously decreasing load simulating the earth pressure, revealed that the deformation increase caused by an increasing cross-section loss is very limited even for considerable damage (approximately 1 mrad rotation for 40% cross section loss). Deformations might notably increase only very close to failure, which challenges the successful application of monitoring systems relying on deformation measurements. Finally, a mechanically consistent model enabling the reliable assessment of the structural safety and the load-deformation behaviour of locally corroded reinforced concrete structures was developed: the Corroded Tension Chord Model. In its basic version, this model combines the effects of tension stiffening and strain localisation. Based on the experimental observations, it was enhanced to account for the effects of a triaxial stress state at the corrosion pit, considering axisymmetric damage. The model predictions of the experimental results are very promising, with the comparison indicating an additional softening effect – exceeding that of the triaxial axisymmetric stress state – at the corrosion pit, probably caused by superimposed bending stresses due to unilateral corrosion. The deformation capacity of the specimens was thus clearly less impaired than predicted by established strain localisation models.
New conceptual approach combining the probabilistic nature of localised rebar corrosion and the load-deformation behaviour
Yilmaz, Deniz; Häfliger, Severin; Kaufmann, Walter; et al. (2021)
fib Symposium Proceedings ~ Capacity Assessment of Corroded Reinforced Concrete Structures
There is a need for sound engineering models and concepts taking into account the damage mechanisms of chloride-induced corrosion with respect to the load-bearing behaviour of reinforced concrete structures. In this paper, we present a novel conceptual approach combining the physical-electrochemical processes of chloride-induced corrosion initiation/propagation with the mechanical aspects of load-deformation behaviour. A particular focus lies on the stochastic nature of localised corrosion and the relevance of the location of corrosion for the load-bearing behaviour, which is considered with the Corroded Tension Chord Model (CTCM). We present a numerical implementation of this concept, applied to a case study of a bridge deck cantilever slab.
Application of distributed optical measurements to structural concrete experiments
Häfliger, Severin; Mata Falcón, Jaime; Kaufmann, Walter (2017)
SMAR 2017 Proceedings
This paper explores the combined application of digital image correlation (DIC) and high-resolution fibre optical (FO) measurements to structural concrete elements. These novel distributed optical instrumentation techniques allow measuring virtually continuously and without affecting the mechanical behaviour (i) displacements on the concrete surface and (ii) elastic as well as plastic strains in reinforcing bars, respectively. The results of an experimental study consisting of two reinforced panels subjected to diagonal tension show the ability of DIC to successfully detect complex crack patterns and their kinematics. The reinforcement strains recorded by FO correlate perfectly with the position and opening of the cracks. The results prove the great potential of DIC and FO to measure phenomena like bond, crack behaviour and shear transfer that hardly ever have been directly measured in detail until now. Transforming the large amount of collected data into useful structural information is an important challenge for these techniques.
Combined application of distributed fibre optical and digital image correlation measurements to structural concrete experiments
Mata Falcón, Jaime; Häfliger, Severin; Lee, Minu; et al. (2020)
Engineering Structures
The combined application of distributed fibre optical strain measurements on reinforcing bars and digital image correlation (DIC) measurements on the concrete surface has a great potential to increase knowledge in many fields of structural concrete. This paper explores the advantages of these measurement techniques for concrete tests and the key aspects to be considered in order to obtain reliable measurements suitable for quantitative analysis. The uncertainty of DIC analysis is highly dependent on the test conditions and user carefulness, and should be assessed for each test. A procedure to quantify the DIC uncertainty in large scale structural tests is presented, showing that it is highly dependent on the quality of the calibration. Comparative tests on distributed fibre optical strain measurements with different fibre coatings show that polyimide-coated fibres capture properly high strain gradients and, therefore, should be used when instrumenting reinforcing bars in RC specimens. Moreover, the measuring noise was found to be dependent on the absolute strain level. Combined plots of crack kinematics and reinforcement strains, stresses and forces are shown for the results of a series of two concrete panel tests subjected to diagonal tension. Crack locations predicted by both measurements match perfectly in these experiments.
Experiments on locally corroded retaining wall segments and their assessment with the Corroded Tension Chord Model
Häfliger, Severin; Kaufmann, Walter (2021)
Capacity Assessment of Corroded Reinforced Concrete Structures CACRCS (2020) Proceedings
Localised corrosion reduces the strength and, even more pronouncedly, the deformation capacity of reinforced concrete structures. These effects are particularly critical in hyperstatic systems and structures subjected to deformation dependent loading, as they may cause premature brittle failures limiting moment redistribution and load reduction, respectively. This paper presents the results of eight largescale experiments investigating the load-deformation behaviour of retaining wall segments with varying localised corrosion damage. The experiments included two hybrid tests exploring the interaction of deformation capacity and deformation dependent load. The Corroded Tension Chord Model was used to model the structural behaviour, revealing excellent agreement with the experimental data.
Influence of cross section loss on the stress-strain characteristics of corroded quenched and self-tempered reinforcing bars
Häfliger, Severin; Kaufmann, Walter (2021)
Construction and Building Materials
Many ageing reinforced concrete structures are affected by severe damage of reinforcing bars due to corrosion. Accurate knowledge of the stress-strain characteristics of the installed reinforcing bars is an indispensable prerequisite for the realistic assessment of the structural safety of such structures. The reduction in cross section does not only reduce the load bearing capacity of the reinforcing bars, but in case of hot-rolled, quenched and self-tempered (QST, process branded as “Tempcore®” or “Termex®”) reinforcing bars also changes their structural characteristics. Since these bars exhibit three distinct microstructures in core, transition zone and outer annulus, each with different mechanical characteristics, the overall stress-strain behaviour of a corroded QST reinforcing bar depends on the composition of its residual area, which varies significantly with ongoing corrosion. As QST reinforcement is widely used in concrete structures worldwide, this effect is of high importance for any structural safety assessment of structures affected by corrosion. This study investigates the influence of microstructure on strength and ductility of QST reinforcing bars subjected to axisymmetric cross section loss. In a comprehensive experimental campaign, 31 tensile tests on QST reinforcing bars with continuously reduced diameters were conducted, exploring the contribution of the concentric layers of the cylindrical cross section to the overall behaviour. Experimental results indicate a pronounced variation of strength and ductility over the cross section. Based on these observations, a simplified model was developed for the relationship between loss of cross section and mechanical characteristics. A validation shows excellent agreement between model and experimental data.
Modelling the load-deformation behaviour of lap splices with the Tension Chord Model
Häfliger, Severin; Kaufmann, Walter; Thoma, Karel (2022)
Engineering Structures
Lap splices are part of virtually every structure made of reinforced concrete. As loads need to be transferred safely along these discontinuities, numerous studies focused on their strength. However, even though they are traditionally placed also in highly loaded regions of elements, which are potentially undergoing plastic deformations, little attention has been paid to their load-deformation behaviour and deformation capacity. This study presents a sound mechanical model based on the established Tension Chord Model to assess the loaddeformation behaviour of lap splices. A thorough analysis of the load transfer mechanism reveals that the major part of the load is transferred at the beginning and the end of a lap splice. Therefore, the lap splice acts as a double reinforced tensile element over a considerable length, which drastically reduces its deformation capacity to less than half of the minimum value expected for adjacent parts. This especially needs to be addressed in performance-based design methods, where the deformation demand is compared to the existing deformation capacity. The theoretical results are validated with a recently conducted experimental campaign, exhibiting excellent agreement of model and experimental data. Besides a comprehensive analysis of the influencing parameters, a simplified modelling approach for practical applications and design recommendations for new structures are presented in this publication.
Corroded Tension Chord Model (CTCM) for concrete structures with locally corroded reinforcement
Häfliger, Severin; Yilmaz, Deniz; Angst, Ueli; et al. (2020)
Many ageing RC structures suffer from severe localised corrosion of the reinforcing bars, particularly if exposed to chlorides. This damage affects strength and ductility: The cross-section loss of the reinforcing bars reduces the load carrying capacity, and the highly localised damage results in a localisation of deformations, which may severely impair the structure’s deformation capacity. A recent study at ETH Zurich investigated these effects on a sound mechanical basis, extending the established Tension Chord Model by corroded crack elements. This paper introduces the modelling concept of the resulting Corroded Tension Chord Model (CTCM) and presents the results of its application to structural elements with several corrosion spots. It reveals a significant decrease of the deformation capacity. Finally, an outlook on ongoing validation experiments is given.

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