The influence of casting position and disturbance induced by reinforcement on the structural concrete strength

It is well known that control specimens used to assess the concrete strength of new structures have different casting and curing conditions than those of actual structures. Notably, after pouring of the concrete and before its hardening, a number of phenomena such as concrete bleeding and plastic settlement occur, influencing the in-situ strength with respect to that of small and homogeneous control specimens (cubes or cylinders). In addition, the development of these phenomena and their structural implications are influenced by the presence of reinforcing bars, disturbing the settlement and bleeding of fresh concrete. In this paper, these aspects, with particular emphasis on the effective structural strength, are investigated by means of a testing program performed with refined measurement techniques such as tomography and Digital Image Correlation. On that basis, consistent design rules are derived to correct the strength of control specimens in order to calculate the resistance of a structural concrete member.

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

Traditionally, the concrete strength is measured on cubes or cylinders having normalized dimensions, suitable vibration and curing conditions and their strength is assessed in laboratory under fast loading rates. However, the in-situ strength of structural elements can considerably differ from that of a small and homogenous control specimen due to a number of issues. Notably, phenomena taking place during the consolidation process of fresh concrete may affect the compressive resistance of tall members as well as the bond strength of reinforcing bars located in top layers. During concrete consolidation, water migrates in the direction of the free surface while concrete settles downwards, phenomena referred as concrete bleeding and plastic settlement respectively. Under these circumstances, a decrease in the concrete properties near the upper surface is observed as well as a development of cracks and voids surrounding horizontal reinforcing bars, causing potential disturbances on the compressive stresses and affecting the mechanical engagement between bars and concrete. In addition, the response of structural concrete may differ from that of material samples due to non-uniform stress states, the material brittleness, the cracking induced by imposed strains, the rheological response of concrete and the presence of embedded disturbances. As a result, the strength measured in material samples needs to be corrected with strength reduction factors to ensure suitable structural analysis. In this thesis, an in-depth investigation is performed on the different phenomena affecting the compressive and bond strength of structural members. These aspects are assessed by means of several testing programmes instrumented with refined measurements techniques such as tomography and Digital Image Correlation. An extensive experimental programme comprising 76 column and prism tests was carried out to evaluate the influence of casting position, loading direction and bar disturbances on the compressive resistance of structural elements. The detailed measurements performed at the fresh and hardened state resulted in the proposal of consistent design rules accounting for the investigated phenomena. Focus was also given on the influence of material brittleness and the implications of internal stress redistributions on the structural response of reinforced concrete columns and compression zones of members in bending. The pertinence of the investigations were validated based on more than 400 column tests collected from the literature. The implications of casting conditions on pull-out and spalling failures were also assessed by means of 137 pull-out tests on reinforcing bars presenting variable diameter, concrete cover, casting height and embedded length. The investigations resulted in the proposal for a physically-consistent approach, evaluating the pull-out resistance as function of casting conditions and reinforcement characteristics. Finally, the phenomenon of cover spalling was examined with respect to the action of a radial inner pressure, as originated by bond engagement or associated to the volumetric expansion of corroded reinforcement. The mechanisms inducing spalling were analysed by means of a comprehensive experimental programme comprising 56 specimens instrumented with Digital Image Correlation. A mechanical model is eventually proposed to evaluate bond-related cases of cover spalling.

EPFL2021

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

Fatigue of reinforced UHPFRC members and monitoring of fatigue action effects on bridges

Bartlomiej Wojciech Sawicki

The objective of this thesis is to develop knowledge and methods to prevent fatigue failure of bridges. The Fatigue Limit State is investigated from both the demand (S) and resistance (R) side. The S-part focuses on reduction of uncertainties through monitoring of fatigue action effects: â¢Analysis of 28-month-long high-frequency monitoring of a slabâs portion of a prestressed concrete road bridge demonstrated that the stress ranges due to traffic loading and temperature action can be of similar magnitudes. Furthermore, they should be treated together as their combination can be fatigue-relevant. â¢The monitoring duration influences the reliability of results, which are further extrapolated to obtain the Cumulative Fatigue Damage for the total service duration of a structure. To quantify the associated uncertainty, the monitoring duration-dependent Cumulative Damage correction Factor (CDF) was calibrated. The minimum recommended monitoring duration is 100 days, and for this observation period the correction factor Î³CDF=4 for massive structures and Î³CDF=20 for temperature-sensitive elements should be applied. After one year of monitoring Î³CDF can be reduced to 1.3 and 2.5 respectively. The correction factor is significantly smaller than the one obtained with method suggested by Eurocode, leading to Î³CDF=20. The R-part of this thesis concentrates on a structural response and fatigue resistance of reinforced UHPFRC (R-UHPFRC) and was based on experimental testing of full-scale R-UHPFRC beams. The results can be applied to both new structures and elements strengthened with R-UHPFRC layer: â¢It is demonstrated that after loading-unloading cycles, due to modified mechanical properties of a part of UHPFRC element which entered into strain-hardening domain, the distribution of stress in the cross-section is re-arranged, influencing the global structural response. Two important fatigue-relevant conclusions are drawn: I)the stress range in the rebar is much lower than calculated using initial material properties; and II)the portion of UHPFRC is subjected to tensile-compressive rather than to tensile-tensile fatigue stress. â¢The fatigue phenomenon of R-UHPFRC member is observed in detail using Distributed Fibre Optics sensing. The strain variation during the fatigue process in both UHPFRC and rebar remained stable for most of the experiment. The rapid increase of strain range occurred at around 90% of the test duration. During the last 1% of the fatigue test, the increase of strain in the reinforcement bar took place while the increase of the beam deflection range ensued during last 1â° of test duration. Importantly, the failure of reinforcement bar is identical with failure of the member. â¢Fourteen beams were exposed to constant amplitude fatigue in four-point bending. It was the largest experimental campaign on R-UHPFRC ever executed. Two-level fatigue verification was proposed: I)global verification using normalized minimum and maximum load levels and the modified Goodman diagram, contrary to previous methods based on the maximum load and load range; and II)local verification of stress-range in reinforcement bar using the standard S-N curves. The research work presented in this thesis brings new knowledge in both fatigue demand (S) and resistance (R) of both R-UHPFRC structures and reinforced concrete structures strengthened using UHPFRC. More economic solutions for structures can be obtained, leading to both financial and environmental savings.

EPFL2021

On the compressive and bond strength of reinforced concrete as structural properties

Francesco Rafael Alberto Moccia

EPFL2021

Fatigue of reinforced UHPFRC members and monitoring of fatigue action effects on bridges

Bartlomiej Wojciech Sawicki

EPFL2021

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

Frédéric Monney

EPFL2022