Design for punching of prestressed concrete slabs

Prestressing in flat slabs helps in controlling deformations and cracking under service loads and allows reducing the required slab thickness, leading thus to more slender structures and being therefore an economic solution for long spans. However, as a consequence of the limited thickness of these members, punching is typically governing at ultimate limit state. Investigations on the topic of punching shear strength have shown that the presence of prestress in flat slabs has a number of potential beneficial effects, namely the vertical component (force) carried by inclined tendons, the in-plane compression stresses and the bending moments developed near the supported region. The approach provided by codes of practice for punching design in presence of prestressing may however differ significantly. Some neglect the influence of the introduced bending moments due to prestressing and the sections at which deviation forces of the tendons are considered may be located at different distances from the edge of the supported region. In this paper, the influence of prestressing on the punching shear strength of members without shear reinforcement is investigated by using the fundamentals of the Critical Shear Crack Theory. On that basis, and accounting also for 65 tests available in the scientific literature, the suitability and accuracy of a number of design codes, such as Model Code 2010, Eurocode 2 and ACI 318-11, is investigated and compared.

Structural behavior of multifunctional GFRP joints for concrete structures

Thomas Keller, Florian Riebel

A multifunctional all-FRP joint has been developed for the transfer of bending moments and shear forces in thermal insulation sections of concrete slab structures used in building construction. Tensile forces from moments are transferred by horizontal GFRP bars, while a pultruded cellular GFRP element transfers the compression forces. The shear forces are transferred by inclined GFRP bars and the webs of the GFRP element. The new joint considerably increases energy savings for buildings due to the low thermal conductivity of GFRP materials. The quasi-static behavior of the joint at the fixed support of cantilever beams was investigated. Two parameters were studied: shear- or moment-dominated loading mode and concrete strength. Results show that the all-FRP joint does not play a critical role at the ultimate limit state. Ductile failure occurs through concrete crushing. The GFRP bars lead to a significant improvement in joint performance compared with similar joints comprising steel bars. Higher concrete strength does not, however, significantly improve the ultimate load. [All rights reserved Elsevier]

2009

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

EPFL2012

Monitoring and modelling of construction materials during hardening

Marco Viviani

During the past decade, the science of concrete behavior has made much progress and many useful results are now incorporated in technical codes such as those governing mix design, quality of mix and post pouring verification. The stage of Very Early Age (VEA) concrete begins with pouring and ends with hardening. Behavior at this stage influences safety and durability of structures. Concrete is a cast-in-place hardening material. Other hardening materials, such as epoxy mortars have recently raised interest. Techniques used for concrete can potentially be applied to these new composite materials. The overall objective of this thesis is to develop a methodology for predicting mechanical properties and for separating important phenomenon in hardening materials, such as commercial concrete and mortar, to be ultimately applied in real time and in situ. A literature survey has shown a lack of knowledge in the areas of measurement techniques at VEA, prediction of compressive strength, separation of the autogenous deformation and values for the thermal expansion coefficient. This lack of knowledge is especially clear for cement-based materials hardening at varying temperatures. Few VEA data are available and few tools are capable of monitoring VEA parameters. A new device has been constructed to monitor deformations and help identify phenomenological contributions to the total deformation. The design of this device takes into account aspects such as usability in the field and reliability in practice. More than 80 embedded fiber optic sensors and 100 specimens were used for development and testing. Several special sensors have been designed and tested so that their features are adapted to the characteristics of the hardening material at VEA. An important part of this work has been the development of procedures where VEA data are interpreted and reduced in order to determine the parameters that describe the evolution of the degree of reaction, as well as mechanical properties, and allow for the decoupling of values for the total deformation into parts that are related to different physical phenomena. Concluding, this methodology allows accurate prediction of the compressive strength evolution in term of degree of reaction using VEA data carried out within the first 72 hours of the hardening material life. The methodology also leads to straightforward determination of values for the thermal expansion coefficient and the separation of values for autogenous and drying deformation. Furthermore, pilot testing on epoxy mortar demonstrates that the methodology is not limited to cement-based materials Finally, the results of this thesis have the potential to make further contributions to scientific research and to strategies for quality control and safety during construction.