Concrete | EPFL Graph Search

Concrete is a composite material composed of aggregate bonded together with a fluid cement that cures over time. Concrete is the second-most-used substance in the world after water, and is the most widely used building material. Its usage worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminium combined. When aggregate is mixed with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts with the water through a process called concrete hydration that hardens it over several hours to form a hard matrix that binds the materials together into a durable stone-like material that has many uses. This time allows concrete to not only be cast in forms, but also to have a variety of tooled processes performed. The hydration process is exothermic, which means ambient temperature plays a significant role in how long it takes concrete to set. Often, additives (such as pozzolans or superplasticizers) are in

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

The transfer of forces through rough surface contact in concrete

Max Tirassa

Aggregate interlocking is a phenomenon occurring when the opposite sides of a concrete crack slide relative to each other. Due to surface roughness, protruding material from one crack-side can engage with the opposite one and thus exchange forces. This affects the load-carrying capacity for structures where the failure is determined by the development of large cracks, like beams and slabs without transverse reinforcement. In such cases, aggregate interlocking contributes significantly to the various shear-transfer actions, allowing forces to traverse cracks. Aggregate interlocking has been studied for several decades, leading to various experimental and theoretical approaches. However, a number of aspects remain unclear, like the development of new cracks from the initial one and the influence of certain concrete properties on the interlocking forces (e.g. aggregates, crack surface roughness). The present research project comprises three scientific publications focusing on the fundamentals of interlocking and force transfer across concrete cracks and other interfaces related to concrete structures. It is based on the results of an extensive experimental campaign performed with an arrangement allowing to apply precise kinematics upon material discontinuities. The tested specimens include pre-cracked concrete prisms subjected to simultaneous crack opening and sliding and steel-to-concrete interfaces characterized by simple geometries (spheres and rebar-surfaces). Particular attention is given to the roughness properties of the tested cracks and interfaces, several of which were scanned at high resolution. The experimental results are used to develop a new model for estimating aggregate interlocking forces as a function of crack kinematics. The model is based on the approach originally introduced by Li and Maekawa (1987) with the Contact Density Model and allows estimating the contact properties using 2D crack-profiles. Depending on the crack width, two contributions to force transfer are considered. For contacts occurring in wide cracks, contact forces are calculated using an elastic-plastic material law, whereas for small crack opening the effect of residual material soundness is considered. The model can be applied on surfaces corresponding to tests with various failure modes, thus considering different levels of roughness. Finally, the bond between steel rebar-ribs and concrete is investigated using steel-to-concrete interface specimens, and several similarities with aggregate interlocking are discussed. The previously introduced model is extended to the case of interlocking rebar-ribs, where it can estimate the bond and confinement stresses and the decrease in bond strength for cases where cracks develop parallel to the rebars. The thesis ends with an extensive appendix containing details on the performed tests which can be of interest for future researchers.

EPFL2020

Time-dependent strength of concrete in compression and shear

Darko Tasevski

Sustained load actions are permanently present in concrete structures, as for example self-weight and dead loads on bridges or soil pressure on cut-and-cover tunnels. These actions may increase throughout a structure's lifetime, for instance after refurbishing or in the case of addition of new elements. It has been experimentally observed that under sustained load actions, the response of concrete may be significantly different than the response under rapid loads. Traditionally, sustained actions are treated differently at serviceability limit states (for instance delayed deflections) than at the ultimate limit state (as the potential failure under sustained load). The distinction is given by separating the time-dependent deformation due to linear creep from the strength reduction due to nonlinear creep. Although quite practical, this approach is not sufficient to understand the phenomenon of strength reduction under sustained loads and the potentially beneficial effects of the delayed response of concrete. The present thesis introduces a comprehensive approach for the long-term analysis of concrete structures, allowing to consistently investigate the response both at serviceability and ultimate limit states. The first part of the thesis aims at establishing a theoretical framework for the evaluation of the time-dependent uniaxial compressive strength of concrete under sustained loads. Based on this work, it is demonstrated that the strength reduction is more severe under a constant sustained load than in the case of loads increased with a low loading rate. In addition, both the mechanical approach and the conducted tests show that the detrimental effect of sustained loading is associated with a potentially beneficial increase of deformation capacity at failure. Within this framework, typical engineering design situations are addressed, such as the case of application of a variable action after a period of sustained loading. The time needed for the continued cement hydration to overcome the detrimental effect of sustained loads is also investigated. The second part of the thesis aims at verifying whether the shear strength of members without transverse reinforcement is influenced by the action of sustained loads. It could be shown that there is no marked decrease of the shear strength for longer durations of application of the load or very low loading rates compared to typical shear tests. This behaviour is supported by detailed observations performed by means of digital image correlation, allowing to quantify the contribution of the various potential shear transfer actions. This work has resulted with several proposals for improvement of codes of practice.

EPFL2019