O efeito do fogo em concretos pré-moldados reforçados com fibras de aço para túneis: da microestrutura à simulação numérica.

The occurrence of fire in fiber reinforced concrete structures is one of the main concerns regarding the use of this material. Although fire is a deleterious event, limitations are found in studies focused on evaluating the effect of elevated temperatures on the mechanical properties of the composite, as well as in terms of extrapolating the results to design. In this context, this thesis aims to understand the tendencies in the mesoscale behavior of steel fiber reinforced concretes (SFRC) after temperature exposure and simulate the fire-related stability condition of tunnel sections built with this material. The changes in the microstructure of steel fibers, cementitious matrix, and fiber-matrix interfacial transition zone were evaluated as a function of temperature. Later, the effect of elevated temperatures on the tensile strength of fibers; bond-slip behavior of fibers embedded in the cementitious matrix; and the compressive, tensile, and post-crack parameters were evaluated. Furthermore, the composite was exposed to a large-scale fire test to experimentally determine the distribution of temperatures and the post-fire tensile properties of the material. At last, a numerical model was developed to compute the effect of a fire and thermal spalling on the bending capacity of reinforced concrete, fiber reinforced concrete, and hybrid solutions. The thermal spalling model was based on a simplified approach that shutdown the layers spalled. The results show that the the post-crack parameters ffts and fftu were more considerably influenced by the properties of the fiber-matrix ITZ than the bulk matrix properties. In terms of fftu, no statistically significant change was observed for T <= 300 °C, which as explained based on the mineralogical changes in the fiber-matrix ITZ and the changes in the pull-out kinetics due to the expansion of iron oxides and shrinkage of the cement paste. Moreover, the bond-slip mechanism have shown to prevail, without fiber rupture, up to ~600 °C. Considering the 11 numerical simulation without thermal spalling, FRC and RC-FRC solutions have shown to be less sensitive than the RC30 solution, while the increase in concrete cover in RC50 solution considerably mitigated the reductions in terms of bending capacity. Nevertheless, the increase in concrete cover has negligible influence on mitigating the reductions caused by thermal spalling, being the FRC and RC-FRC the least sensitive solutions due to the diffuse reinforcing capacity of fibers in the cross-section. In this sense, the advances acquired in this thesis supported a tool for assessing the bearing capacity of tunnel sections exposed to fire with and without thermal spalling and contributed to improving the safety conditions for this type of structure.A ocorrência de incêndio em estruturas feitas com esse compósito é uma das principais preocupações relacionadas ao uso deste material. Embora incêndios sejam eventos deletérios, limitações são encontradas no estudo das elevadas temperaturas nas propriedades mecânicas do compósito, bem como em termos de extrapolação destes resultados para o projeto estrutural. Nesse contexto, esta tese tem por objetivo compreender as tendências comportamentais do concreto reforçado com fibras de aço (CRFA) após a exposição a elevadas temperaturas e simular numericamente as condições de estabilidade durante incêndios em túneis constuídos com esse material. As alterações microestruturais das fibras de aço, da matriz cimentícia, e da interface fibra-matriz foram avaliadas em função da temperatura. Posteriormente, o efeito das elevadas temperaturas foi avaliado por meio de ensaios de mesoescala na resistência à tração das fibras de aço; aderência e deslizamento das fibras embutidas na matriz cimentícia; na resistência à compressão e tração da matriz; e nas propriedades pósfissuração do concreto com fibras de aço. Além disso, o compósito foi exposto a um ensaio de incêndio de larga escala a fim de verificar a distribuição interna de temperaturas gerada e o efeito do incêndio nas propriedades mecânicas de pós-fissuração do material, o que foi posteriormente utilizado para validação do modelo numérico. Por fim, um modelo numérico foi desenvolvido a fim de prever os efeitos do incêndio e da ocorrência de fragmentação térmica na capacidade resistente de seções de concreto armado, concreto com fibras, e híbridos de concreto armado com fibras. O modelo de fragmentação térmica foi concebido de forma simplificada através do desligamento das camadas afetadas. Os resultados mostram que os parâmetros ffts e fftu foram mais influenciados pelas propriedades da interface fibra-matriz do que pelas propriedades da matriz cimentícia. Em termos de fftu, não houve alteração significativa para T <= 300 °C, o que foi explicado pelas alterações mineralógicas na zona de transição fibra-pasta e nas mudanças na cinética de arrancamento devido ao processo expansivo do óxido de ferro e retração da pasta de cimento. Considerando a simulação numérica, soluções de CRF e RC-CRF se mostraram menos sensíveis que a solução RC30, enquanto o aumento de cobrimento mitigou as reduções em termos de capacidade à flexão. Entretanto, o aumento do cobrimento não favoreceu a mitigação das reduções causadas pela fragmentação explosiva, sendo as soluções de CRF e RCFRC as opções menos sensíveis devido a capacidade de reforço difuso das fibras. Nesse sentido, os avanços gerados nesta tese subsidiam uma ferramenta de verificação da capacidade à flexão de seções de túneis expostas ao fogo com e sem fragmentação explosiva, contribuindo para aprimorar as condições de segurança deste tipo de estrutura

Assessment of the post-fire residual bearing capacity of FRC and hybrid RC-FRC tunnel sections considering thermal spalling

A design-oriented numerical model for the analysis of RC, FRC, and RC-FRC tunnel sections exposed to fire with different spalling parameters is presented. The numerical model is conceived in two steps: the first is the determination of the temperature field in the cross-section exposed to fire according to the spalling parameters; and the second is the determination of the bearing capacity of sections based on the thermal field in the section. At last, a parametric study was conducted to evaluate the effect of the fire curve, the spalling parameters, the reinforcement type, and the rebar’s concrete cover on the bearing capacity of the sections. The results showed that the use of FRC as total or partial substitution to RC mitigates the fire-related reduction in the bearing capacity of the sections. Moreover, increasing the RC concrete cover is beneficial only if thermal spalling is avoided. When thermal spalling occurs, the FRC and RC-FRC solutions yielded the lowest reductions in the bearing capacity among the reinforcement solutions tested.Peer ReviewedPostprint (published version

Influence of polypropylene microfibre (PPMF) dispersion procedure on fresh and hardened rendering mortar properties

This study was carried out to evaluate the influence of a polypropylene microfibre (PPMF) dispersion procedure on fresh and hardened state properties of rendering mortars. Specimens prepared with two different PPMF dispersion procedures were evaluated comparatively with reference specimens prepared without adding PPMF. Changes in the fresh properties were monitored using flow table, squeeze flow, and air-entrainment tests. The hardened state was characterized by capillary water absorption, air-permeability, dynamic elastic modulus (E), tensile strength according to the Brazilian test, and porosity by the Archimedes immersion method. Results show that the fresh mortar properties were not affected by dispersion procedure and all hardened mortar properties were statistically similar, except for the dimensional variation of the specimens. The study also shows that adding polypropylene microfibres in a dispersed form was more effective in terms of controlling total drying shrinkage than directly adding fibres to the cementitious matrix (as recommended by the manufacturer) or the reference mortar

Assessment of the bearing capacity reduction of FRC elements subjected to fire

The use of Fibre Reinforced Concrete (FRC) in structural applications with higher responsibility (e.g., TBM-bored tunnels; bridges; offices and residential structural elements) is increasing. In this regard, the residual structural response during and after a fire event is an issue of paramount importance which should be researched so that practitioners can provide safe and reliable designs. For this purpose, a FE thermal and structural model has been developed to predict the behaviour of FRC cross-sections exposed to fire. The model is capable to provide for any fire curve (at any exposure time): (1) the magnitudes of the thermal-dependant parameters of the concrete; (2) the temperature and stress-strain distributions along the cross-section and (3) the M-N envelopes for the ULS design. The constitutive equation of the FRC for steel fibres to simulate the full-tensile stress-strain (crack width) relationship has been derived considering the results of an experimental program carried out at the Universidade de São Paulo (Brazil) within the framework of an extensive research project devoted to characterizing the mechanical behaviour of FRC subjected to fire. This constitutive equation allows considering the progressive degradation of the FRC post-cracking response for different temperature levels. The model together this constitutive equation has been used to perform a parametric study oriented to precast FRC segmental linings considering different FRC classes, fire curves and thicknesses of the segments. The residual bending moment capacity, stress-strain distributions and other relevant results are presented and analysed aiming at understanding the cross-sectional effects of this accidental load on FRC elements.The authors thank to the Spanish Ministry of Economy, Industry and Competitiveness for the economic support in the scope of the project SAES (BIA2016-78742-C2-1-R). This work was also supported by the Fundação de Apoio ao Instituto de Pesquisas Tecnológicas (FIPT) [New Talents Program, Grant No. #01/2017 (Ramoel Serafini)] and by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) [Grant No. #2015/25457-9 (Dimas Alan Strauss Rambo)].Postprint (published version

EFFECT OF SPECIMEN SHAPE ON THE COMPRESSIVE PARAMETERS OF STEEL FIBER REINFORCED CONCRETE AFTER TEMPERATURE EXPOSURE

This study investigated the effect of specimen shape (cylindrical and cubical) on the compressive strength and elastic modulus of steel fiber reinforced concrete after exposure to the temperatures of 150, 300, 450, and 600 °C. Results show that the compressive strength and elastic modulus of the composite significantly reduce with the increase in temperature, independent of the specimen shape. Additionally, a significant difference in the compressive strength and elastic modulus conversion factors for cube-cylinder was verified with the increase in temperature. This study contributes to the limited amount of studies regarding the effect of elevated temperatures on steel fiber reinforced concretes and shows that the elevated temperatures may have a significant effect in the conversion factors for cube-cylinder

Bond-slip response of steel fibers after exposure to elevated temperatures: experimental program and design-oriented constitutive equation

This study aimed to evaluate the effect of elevated temperatures on the bond-slip behavior of hooked-end steel fibers. A total of 180 pullout specimens were tested in post-cooling conditions using a double-sided pullout test with multiple embedded fibers for target temperatures between 25 and 750 °C. Results proved that the bond strength significantly increases for temperatures up to 450 °C, and drastically decreases for temperatures of 600 and 750 °C. The contribution of hooks reduced with temperature and is negligible for temperatures higher than 600 °C, while the fiber-matrix frictional interaction seems to improve for all temperatures evaluated. A temperature-sensitive constitutive equation that allows simulating the bond-slip behavior of hooked-end steel fibers is proposed and its suitability confirmed through a numerical model.The authors would like to thank the Institute for Technological Research (IPT) and its foundation (FIPT) for their financial and institutional support through the New Talents Program (Grants #N.01/2017 and #N.01/2018). This work was also partially supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) [Grant # 305055/2019-4 (Antonio Domingues de Figueiredo) and Grant # 310401/2019-4 (Luís A.G. Bitencourt Jr.)]. The authors would also like to thank the researchers Ph.D. Renata Monte (USP), MSc Priscila Rodrigues Melo Leal (IPT), and Eng. Tiago Haddad Marum (USP) for the technical contributions that improved the quality of this work.Peer ReviewedPostprint (author's final draft

Design-oriented assessment of the residual post-fire bearing capacity of precast fiber reinforced concrete tunnel linings

This study presents a meso-scale experimental program and employs a numerical approach to determine the bearing capacity of steel fiber reinforced concretes (SFRC) tunnel linings after fire exposure. First, the effect of temperature on the mechanical properties of SFRC was determined through a refined experimental campaign. Additionally, the suitability of the bending test and the DEWS test to assess the post-fire tensile properties of the SFRC was verified. Then, a thermo-mechanical model was implemented to assess the changes in the bearing capacity of SFRC for tunnels built with TBM technology. Results show that the thermo-mechanical model properly estimated the temperature distribution and the mechanical properties as a function of the duration of fire (t) and depth (z). The bearing capacity of the SFRC segments exposed to the ISO 834 and HFC fire curves were comparable when the condition tISO=2tHFC was satisfied. Additionally, a greater bearing capacity reduction was numerically observed when the compressive region of the cross-section is affected by fire. The results obtained aid in the definition of appropriate rehabilitation operations, classifying the degree of damage sustained by the structure, and provides a procedure for designers regarding the effect of fire on SFRC structures.Peer ReviewedPostprint (author's final draft