Concrete and Metallic Structures Activation Calculations

The objective of this report is to detail the work done during the TP IV project at EPFL. The idea is to look into the activation of concrete and metallic structures of the EPFL’s nuclear reactor: CROCUS. This study has been done by two ways: MCNP computations and TLD measurements. The results are consistent between the two methods, as both show that the concrete roof is more activated than the other part of the cavity. However, the MCNP model seems to overestimate the radioactivity of the roof when compared to the walls. Indeed, a factor four has been found between them and with the TLD’s only a factor two. Moreover, the MCNP model gives high numbers for the activity in the concrete wall. Indeed, an activity of around 3 kBq/cm3 was found for the ten first centimetres of concrete in all the walls. The mains isotopes responsible for this activity are: 60Co and 133Ba, they are both long-lived radionuclides and it would be interesting to look at they concentration with time.

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

Design of concrete girder bridges has significantly evolved during the last decades. This has been particularly relevant with respect to shear design, motivated by changes in actions and design models. As a consequence, assessing the shear strength of existing bridges leads in many cases to unsatisfactory safety levels. Furthermore, many existing bridges do not comply with current code regulations with respect to minimum amounts of shear reinforcement. This situation can lead to expensive retrofitting and strengthening of a significant number of existing bridges. The assessment of the shear strength of these bridges has thus become a significant task for structural engineers. Design codes are not always appropriate for assessing the strength of existing bridges. They propose safe models providing sufficient accuracy for design of a wide number of structures. However, these models do not account for some particularities of prestresses bridges and neglect a number of shear-transfer actions that can be relevant for their strength. This is typically the case of shear carried by the inclination of the compression chord, the increase of the stress in the tendons or the effective strength of concrete in the web of cracked prestressed girders. Accounting for more realistic approaches for these parameters may significantly increase the estimated strength of a structure with respect to code provisions, avoiding in many cases unnecessary retrofitting or strengthening. In this paper, the results of a tests campaign carried out at Ecole Polytechnique Fédérale de Lausanne on 10 prestressed concrete girders (10 meters long, 0.78 m high) are outlined. The specimens were provided with very low amounts of shear reinforcement and some of them present defective stirrup anchorage to simulate realistic conditions of existing structures. The experimental results are discussed with reference to the stress field method where the role of the different shear-transfer actions is investigated and compared to current code provisions.

fib Symposium Tel-Aviv 20132013

Zürich Stadtspital Triemli Personalhäuser – Resource assessment of structural elements

Maléna Bastien Masse, Julie Rachel Devènes, Corentin Jean Dominique Fivet, Célia Marine Küpfer

The Triemli Personalhäuser are three equal 15-story buildings located on the Zürich Triemli Stadtspital campus and erected between 1964 and 1969. Cast-in-place reinforced concrete (RC) slabs and walls form the building cores and their surrounding corridors. The rooms are arranged around these cores. The load-bearing intermediate walls, made of prefabricated masonry, support thin precast slabs used as permanent forms. A RC layer is cast over these precast slabs and connects the room slabs with the cast-in-place slabs of the corridors and cores. The self-supporting facade consists of precast RC panels. The City of Zürich plans the deconstruction of these three buildings, thus making available a large amount of RC elements composing the structure and the facades of these buildings. Little-known and rarely implemented, the reuse of concrete elements from obsolete buildings in new projects is a sustainable approach that promotes a circular economy. When reusing, the components of obsolete buildings are carefully dismantled without being crushed. They are then cleaned, possibly repaired or trimmed, and reused without many transformations in a new project, maintaining their shapes, technologies, and mechanical properties. In addition to maintaining the embodied energy and history of the reused components, reuse allows the construction industry to reduce demolition waste, greenhouse gas emissions, and material consumption. This report is a preliminary resource assessment and aims at inventorying and assessing all structural elements of the Triemli Personalhäuser, focusing on their potential value for reuse. Both precast and cast-in-place RC elements are included. They are part of the load-bearing structure or are self-supporting such as the precast facade elements. The proposed methodology allows identifying all properties needed to evaluate the potential for reuse of an element: geometry, material properties, current condition, aesthetics, accessibility, resistance, future durability and environmental impacts. After reviewing available reports and drawings on the buildings, onsite visits are carried out to complete the information and visually inspect the structural elements. During the inspection, the elements are assessed with regards to their suitability for reuse and their condition is classified into a five-grade scale. The investigations are completed with destructive and non-destructive testing of the material properties. Together, Buildings A, B and C are made up of approximately 7 000 m3 of materials constituting their load-bearing system, with approximately 2 500 m3 of precast concrete, 3 400 m3 of cast-in-place concrete and 1 100 m3 of masonry. Of this total, approximately 4% of the volume is dropped from the analysis due to the bad condition of the elements, namely the balcony slabs, the roof slab, and the external stairs. The other elements are in a good or acceptable condition and ae inventoried and analysed in detail. The inventoried elements are divided into 5 categories: (1) facade elements; (2) slab elements; (3) wall elements; (4) column elements; and (5) staircases. Each of these categories are subdivided into a certain number of element types for which a complete factsheet is prepared, including pictures, drawings and useful information on their condition. The volume and weight of each element types are given, as well as their share of the total material volume. The embodied global warming potential (in kgCO2eq) for fabrication and demolition of the elements is also calculated. The results of the investigation on material properties confirm sufficient compressive strength for all elements. The carbonation depths measured on the cores are lower than the cover thickness of the reinforcement. Thus, the concrete is not carbonated in the reinforcement areas and the risk of corrosion is kept low, insuring a good durability of the elements. This document should serve as a base for designing and planning future reuse applications for the concrete elements extracted when deconstructing the Triemli Personalhäuser. The information presented here will help the planners to prioritize the reuse strategy on the elements in the best conditions, with the largest volume share and thus with the largest embodied global warming potential.

EPFL2022

Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

Reinforcement detailing rules describe the shape, geometrical dimensions, and amount of steel to be placed in reinforced concrete structures. They allow for simple and fast designs, account for several effects neglected in the design, ensure a satisfactory behaviour under serviceability conditions, a sufficient robustness, and an adequate behaviour in case of unexpected actions. Most detailing rules provided in codes have not been updated to correspond to current manufacturing processes, material performances and scientific knowledge. They are often based on "rules of good practice" which, while satisfactory, lack a sound scientific basis and may not be needed. This is one reason why detailing rules may differ amongst countries and design codes. Some of these rules are overly conservative, in particular when evaluating existing structures, while others may neglect significant effects. Even though these rules play a major role in the economy and safety of concrete structures, little research has been performed in this domain. Several detailing rules have been identified as needing additional investigations to verify their adequacy to current practical needs and recent technological evolutions. This thesis presents a research programme on three main detailing rules: bend detailing and required mandrel diameter, anchorage of shear reinforcement with bends and hooks and minimum amount of shear reinforcement. This research aims at developing mechanical models, simplified formulas and detailing provisions, on the basis of experimental results. These were obtained using state-of-the-art measurement techniques such as Digital Image Correlation or Fibre-Optic Measurements.Bends and hooks of reinforcing bars are usually obtained by plastic bending of the bars against mandrels. Codes specify minimum mandrel diameters to avoid splitting or spalling failures that may limit the resistance of the detail. The thesis includes a research programme on the detailing of bends and the required mandrel diameter to avoid concrete failures leading to spalling of the concrete cover. A mechanical model for the design of bent reinforcement was developed. A simplifying standard bending procedure is proposed, in which details formerly requiring various bend diameters can be obtained by using a single mandrel, allowing for faster automated manufacturing of bent reinforcement.Bends and hooks at the end of the bars are efficient solutions for the anchorage of reinforcement. However, these details are relatively sensitive to the cracking state of the surrounding concrete. For shear reinforcement, spalling failure of the concrete cover for bars close to the concrete surface can occur. The thesis includes an investigation on the mechanical response and performance of bend and hook anchorages. A mechanical model and practical considerations on the activation of shear reinforcement in beams are presented.The required minimum amount of shear reinforcement in beams and slabs has been discussed for decades. It is crucial to ensure economic and safe new structures and to accurately assess existing ones. The investigation shows that the shear behaviour is strongly dependent on the amount and the post-yield response of the shear reinforcement (ductility class). For all investigated detailing rules, the implementation of these findings into codes is discussed, highlighting the consistency of the recent changes, particularly with the new generation of Eurocode 2.

EPFL2022

Assessment of Shear Strength for Existing Bridges with Low Amounts of Shear Reinforcement

Miguel Fernández Ruiz, Aurelio Muttoni, Michael Markus Rupf

fib Symposium Tel-Aviv 20132013

Zürich Stadtspital Triemli Personalhäuser – Resource assessment of structural elements

Maléna Bastien Masse, Julie Rachel Devènes, Corentin Jean Dominique Fivet, Célia Marine Küpfer

EPFL2022

Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

EPFL2022