Structural analysis of a composite bridge structure combining UHPFRC and prestressed concrete

Reinforced concrete being the most applied construction material today performs very well in most applications but still lacks durability under severe environmental conditions. Especially existing structures built decades ago show degradation. Using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) to improve durability is a promising option seen the extraordinary performance of this material when applied in a composite section. Restrained shrinkage of the overlay plays an important role in this type of application. A conceptual bridge cross section combining UHPFRC and reinforced concrete has been numerically analysed. Two main parameters, tensile strength and strain hardening capacity, were varied. The analysis indicates the importance of strain hardening of UHPFRC and the influence of its tensile strength on the structural response under restrained shrinkage (deformation controlled loading) and traffic loads (force controlled loading). The results validate the concept of using a UHPFRC layer to improve the structural durability of concrete constructions.

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.

EPFL2011

Next generation synthetic fibers UHPFRC for sustainable structural applications

Amir Hajiesmaeili

The ever-increasing demand for rehabilitation of existing structures and construction of new ones, along with environmental issues of the construction sites represent a heavy burden for society in terms of economy and environment. Meanwhile, Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has proved its potential to be one of the solutions to contain the explosion of the maintenance costs, because of its extremely low permeability and outstanding mechanical properties. Even though using UHPFRC already significantly contributes to decreasing the environmental impact of rehabilitation construction sites by up to 50% when compared to using conventional concretes, further optimization of their mix design with replacement of high environmental cost components like steel fibers and clinker, would be another step towards sustainability. The main objective of this study was to further improve the already established concept of UHPFRC in two main directions: firstly, to develop new strain hardening UHPFRC mixes with reduced environmental impact by replacing the steel fibers with synthetic ones and by replacing 50% vol. clinker with cheap and widely available Supplementary Cementitious Materials, and secondly, to investigate the mechanical properties, delayed response, protective function, and environmental impact of the newly developed material for structural applications. An advanced packing density model was developed to optimize the powder mix that consisted of six different powders. An extensive experimental campaign with a focus on the tensile behavior was carried out to investigate the factors influencing the mechanical properties of the UHPFRC mixes and validate their performances. The achieved new mix named PE-UHPFRC has on average a tensile elastic limit of 7.7 MPa, a tensile ultimate strength of 11.7 MPa, a tensile deformation capacity of more than 3.5%, and a compressive strength of 120 MPa, well adapted for cast-onsite applications of rehabilitation, or strengthening associated to rebar. The delayed response of PE-UHPFRC was investigated with a focus on autogenous deformations and associated eigenstresses under full restraint conditions, using a Temperature Stress Testing Machine. The results showed more than 70% reduction in the eigenstresses in a 50 mm thick PE-UHPFRC layer when compared to those observed in conventional UHPFRC with steel fibers, which is highly beneficial for rehabilitation applications. An original capillary absorption setup was developed to investigate the protective properties of the PE-UHPFRC under tensile loading, at different strain levels ranging from 0.15 to 20â°. Before reaching the tensile elastic limit, the capillary absorption coefficient was extremely low, at 24 g/m2âh, however, a sudden increase in the capillary absorption was observed after the elastic limit under tensile load. Finally, the environmental benefits of PE-UHPFRC combined with rebar were characterized at the structural level, for the strengthening of a bridge with a span of 34 m, and compared with those of: (1) a reconstruction with prestressed concrete, and (2) a strengthening with conventional UHPFRC with steel fibers combined with rebar. The LCA results showed on average 55% and 34% decrease in the environmental impact for the strengthening method with R-PE-UHPFRC compared with the reconstruction with prestressed concrete and strengthening with conventional R-UHPFRC, respectively.

EPFL2019

Comportement et modélisation des éléments de structure en béton fibré à ultra-hautes performances avec armatures passives

Dario Redaelli

The present research improves the experimental and theoretical knowledge on the behaviour of structural elements made of ultra high performance fibre reinforced concrete (UHPFRC) with ordinary reinforcement. UHPFRC is a recently developed material with much higher mechanical properties and durability than ordinary concrete. However, it has been used so far in a limited number of structural applications. A better knowledge on the behaviour of UHPFRC in structural members is needed to be able to take advantage of its outstanding properties in structural design. One of the ways in which the structural efficiency of UHPFRC can be studied is to investigate the improvement in structural performance that is gained by using UHPFRC instead of ordinary concrete in common structural members. As an alternative, new structural shapes and structural concepts can be explored, more adapted to the specific material properties. The work presented in this thesis focuses on the first approach by considering the behaviour and modelling of UHPFRC structural members loaded in tension and in bending with axial load, and reinforced with ordinary steel reinforcement. The experimental and theoretical study carried out on tensile members contributes to the understanding of the fundamental mechanisms that control cracking of a tensile member reinforced with both fibres and bars, in service and at ultimate. A numerical model has been developed to solve the differential problem of bond in the presence of strong mechanical non linearity. This model has been used to simulate the main aspects controlling bond in a UHPFRC member. The results obtained with the model help in clarifying the differences between cracking in ordinary concrete and in UHPFRC ties. The model allows identifying the parameters that influence the strength and ductility of the tie and was used to validate simplified assumptions to model the behaviour of UHPFRC ties in service. The behaviour of columns has been studied with a theoretical and experimental approach. The results of two test series on UHPFRC and HPFRC columns are reported in the thesis. An analytical model has been developed to study the behaviour of a confined concrete member in compression. A numerical model has been implemented to simulate the non linear behaviour of ordinary concrete and UHPFRC columns. The models have been used to simulate test results reported in this thesis or taken from the literature. The numerical model has been used to evaluate the effect of variations of the mechanical behaviour of concrete on the structural behaviour of a column, with or without reinforcement. The results of those parametrical studies help understanding the influence of mechanical properties on structural response. This thesis contributes to a better understanding of the behaviour of UHPFRC members with ordinary reinforcement. The comparative approach used in the research makes it possible to show the differences between the structural behaviour of ordinary, high strength and fibre reinforced concrete. Indications are also given on the possible ways in which an effective use of UHPFRC in structural members can be achieved.

EPFL2009