Hormigones de alta performance con fibras de acero

En este trabajo se presentan algunos resultados obtenidos en LEMIT correspondientes al proyecto de cooperación entre UNLP-LEMIT y UPC (Cataluña, España) sobre el empleo de fibras de acero en hormigones con elevados niveles de resistencia. Se evalúa el comportamiento de hormigones simples y reforzados bajo solicitaciones de compresión y flexión, incluyendo en este último caso diferentes tipos de probetas, configuraciones de ensayo, y distintos tipos y contenidos de fibras. Se discuten los cálculos de tenacidad y resistencia residual conforme los lineamientos de ASTM C 1018. Los resultados muestran cómo es posible caracterizar a los hormigones reforzados empleando probetas con distinta geometría y diferenciar el tipo o contenido de fibras utilizados en cada caso. Asimismo se observa el efecto del tipo de acero de las fibras y del contenido de refuerzo en hormigones con resistencias del orden de 70 MPa.This work presents several results obtained at LEMIT corresponding to the cooperation project between UNLP-LEMIT and UPC (Catalonia, Spain) about the use of steel fibers in concretes with high strength levels. The behavior of plain and fiber reinforced concretes under compressive and flexural loads, including in this last case different types of specimens, test setups, and different contents and fiber types is evaluated. Toughness calculation and residual strength in accordance to ASTM 1018 is discussed. Results show how it is possible to characterize fiber reinforced concretes employing specimens with different geometry, and distinguish the type or content used in each case. In the same way, the effect of the steel type of the fibers and the content of fiber reinforcing in concretes with strengths up to 70 MPa is observed

Material nonlinear analysis of steel fibre reinforced concrete beams failing in shear

Experimental research has pointed out that fibre reinforcement gives valuable contribution for the shear strength of concrete beams. To obtain data to check the validity of the formulation proposed by RILEM TC 162-TDF for the evaluation of the fibres contribution for the concrete shear strength, sets of concrete beams were tested experimentally. To simulate the behaviour of this kind of structures, a computational code was developed, based on the finite element techniques. An accurate simulation of the behaviour of structures failing in a brittle mode, such is the case of the beams failing in shear, is a true challenge in the computational mechanics domain. To reproduce with enough accuracy the fracturing process of this type of elements, a multifixed crack model is implemented. The ability of the post-cracking stress-strain diagram proposed by RILEM TC 162-TDF to simulate the crack propagation is checked. A strain softening trilinear diagram is also derived from inverse analysis using the force-deflection relationship obtained in RILEM three-point notched beam tests. Its capability to model the fracture mode I is also assessed. The numerical strategy developed is described in the present work and the appropriateness of the model is evaluated simulating some beams tested experimentally.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BSAB/291/2002-POCTI. Fundo Social Europeu (FSE) -SFRH/BSAB/291/2002-POCTI

Reología de hormigones autocompactables

El diseño y aplicación del Hormigón Autocompactable (HAC) constituye uno de los temas de mayor impacto e interés en el mundo en el campo de la tecnología del hormigón. En términos reológicos un HAC se caracteriza por poseer una tensión umbral de cizallamiento muy baja y una viscosidad plástica capaz de garantizar el transporte, llenado y consolidación del hormigón sin segregación. La caracterización reológica en estado fresco brinda información para diseñar diferentes clases de HAC y permite valorar la influencia de los materiales componentes así como el efecto de diversas variables externas. Este trabajo analiza la reología del HAC, en primer lugar se presentan los conceptos reológicos y las variables que modifican la respuesta del hormigón fresco, destacando las características particulares del HAC; en segundo término se muestran resultados correspondientes a un programa de investigación en el que se empleó un BML Viscometer 3 (CONTEC) con el fin de ponderar la influencia de factores que afectan la producción del HAC como la temperatura del hormigón, el tiempo transcurrido luego de finalizar mezclado y las condiciones de exposición luego de la elaboración; finalmente se analiza la vinculación entre los parámetros reológicos y las medidas de los ensayos de tipo ingenieril más frecuentemente utilizados (escurrimiento y embudo en V).The mix design, characterisation, and application of Self-Compacting Concrete (SCC) are undoubtedly key topics of major impact and interest in the field of concrete technology worldwide. In rheological terms, SCC is characterised by a very low yield stress and an adequate plastic viscosity to assure stability during transport, casting and later hardening of the material. The rheological characterisation in the fresh state allows obtaining information for the design of different classes of SCC, including the component materials and the effect of external variables. This work analyses the rheology of SCC; rheological concepts and the variables affecting the response of the fresh concrete are presented firstly, highlighting the specific characteristics of SCC, the second part presents the results of a research program in which a CONTEC 3 viscometer was utilised with the aim of studying the influence of different factors affecting the production of SCC, such as temperature of the material, and time and the exposure conditions after fabrication. Finally, correlations between rheological parameters and the results of frequently-used engineering tests (slump-flow and V-funnel), are analysed

Comportamento da ligação de laminados de CFRP inseridos no betão sob acções cíclicas

Com o objectivo de avaliar a influência do comprimento de aderência e da história do carregamento no comportamento da ligação de laminados de fibras de carbono (CFRP) ao betão, foram efectuadas campanhas de ensaios com três comprimentos de aderência distintos e diferentes tipos de história de carga. O CFRP é inserido em ranhuras efectuadas no betão e é fixo ao betão por intermédio de um adesivo epoxy. No presente trabalho, os ensaios de arranque em flexão efectuados são descritos e os resultados obtidos são apresentados e analisados.Bettor MBT. Cimentos Molins e Grace. Fundação para a Ciência e Tecnologia (FCT) - SFRH/BD/3259/2000. Fundo Social Europeu (FSE) - SFRH/BD/3259/2000

Creep en estado fisurado en hormigones reforzados con fibras

El fib Model Code 2010 presenta avances significativos para uso de Hormigón Reforzado con Fibras ya que no sólo considera su clasificación en función de los niveles resistentes sino que brinda criterios para el diseño estructural de elementos como losas armadas, losas sin armadura convencional, uniones de columnas, paredes y muros. Sin embargo el documento no considera casos donde las cargas sostenidas den lugar a deformaciones diferidas (fluencia). Esto genera un interés particular para las macrofibras poliméricas y se ha formado un comité de la RILEM “Creep behavior on cracked sections in FRC”. Este trabajo analiza los mecanismos que provocan deformaciones diferidas en hormigones con fibras en estado fisurado y discute en qué casos tales deformaciones adquieren relevancia. Se sintetizan resultados sobre hormigones con fibras de acero y poliméricas

Effect of aging on the fracture characteristics and brittleness of a high-strength concrete

Results of notched beam (fracture) tests on a 60 MPa silica fume concrete at the ages of 4, 10, 31 and 232 days are presented. Fracture parameters at the different ages were obtained using the size effect and cohesive crack models, which indicate that the fracture resistance (toughness and energy) decreases and the brittleness increases with the age of the concrete. This trend is attributed to the increase in the strength of the hardened cement paste and the interfaces that leads to less bond cracking and more aggregate rupture, and consequently, to more brittle failure and lower toughening.Peer Reviewe

Estimation of the modulus of elasticity for dam concrete

The modulus of elasticity of dam concrete is difficult to determine directly from tests due to the necessity for large specimens and testing machines. In order to study the applicability of simple elastic models for predicting the modulus from standard size specimens, tests were conducted on prisms of 45×45×90 cm fabricated with dam concrete (maximum aggregate of 120 mm). The tests on standard 15×30 cm cylinders were made with the mortar and wet-screened components of this concrete. It is seen that the use of the data from these components together with estimated values of the modulus of the aggregates gives reasonable predictions of the moduli of the dam concrete. This has been verified for a range of ages, from 7 to 180 days.Peer Reviewe

Bond behavior of near-surface mounted CFRP laminate strips under monotonic and cyclic loading

Near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminate strips are used to increase the load-carrying capacity of concrete structures by inserting them into slits made in the concrete cover of the elements to be strengthened and gluing them to the concrete with an epoxy adhesive. In several cases the NSM technique has substantial advantages when compared with externally bonded laminates. In order to assess the bond behavior between the CFRP and concrete under monotonic and cyclic loading, an experimental program, based on a series of pullout bending tests, was carried out. The influence of the bond length and loading history on the bond behavior was investigated. In this work the details of the tests are described, and the obtained results are discussed. Using the experimental data and an analytical-numerical strategy, a local bond stress-slip relationship was determined. A finite element analysis was performed to evaluate the influence of the adhesive on the global response observed in the pullout bending tests.The first author wishes to acknowledge the grant SFRH/BD/3259/2000 provided by FCT (Portuguese Foundation for Science and Technology) and FSE (European Social Fund). The tests were performed at the Structural Technology Laboratory of the Technical University of Catalonia, Spain, with the help of Miguel Angel Martín and Ernesto Diaz. The authors wish to acknowledge the support provided by S&P and Bettor MBT who supplied the material used in the study

Service Life and Life-Cycle Assessment of Reinforced Concrete with Fly ash and Limestone Calcined Clay Cement

Environmental impact due to the emission of carbon dioxide during concrete production can be taken care by reducing the clinker content in the cement. The clinker content can be reduced by replacing it with fly ash and limestone calcined clay. Such systems can have a potential to exhibit enhanced durability/service life when exposed to chloride and carbon dioxide. However, estimating probabilistic service life of concretes with such alternative binder systems is difficult due to the lack of quantitative estimates of the input parameters such as chloride diffusion coefficient (DCl), ageing coefficient (m), carbonation coefficient (KCO2), and chloride threshold (Clth). This paper presents the experimentally observed estimates of these parameters for the following systems: (i) 100% OPC, (ii) 70% OPC + 30% fly ash, and (iii) limestone calcined clay cement (LC3) – known as OPC, PFA, and LC3 concretes, respectively, herein. A total of three concrete mixes were designed. Also, based on these input parameters, the probabilistic service life estimates of a bridge pier and a girder made of these three concretes and exposed to chlorides and carbon dioxide are presented. For chloride ingress study, Fick’s 2nd Law of diffusion and Clth have been used. For carbonation study, a recently developed model for estimating carbonation depth (using mixture proportion) have been used. Then, the life‑cycle assessment (LCA) of these three concrete systems in terms of the CO2 emissions per unit of concrete per year of estimated service life is presented - for both chloride and carbonation induced corrosion. In chloride laden environments, the service life of the bridge pier and girder systems could be enhanced by about 10 times by using fly ash or LC3 concretes – for similar strength grade concretes. Also, the average annual CO2 emissions (during the expected service life) of PFA and LC3 concretes could be about 3 and 7 times, respectively, lower than that of OPC concretes of similar strength grade. In case of carbonation-induced corrosion, the limited experimental data indicate that the PFA and LC3 concretes could exhibit a lower service life and higher average annual CO2 emissions (during the expected service life) than OPC concretes

On the experimental study of the interface between a fiber composite lamina and concrete

In order to properly evaluate the failure characteristics and the strength of the bond between concrete and a fiber reinforced polymer (FRP) lamina, it is necessary to study the shear failure of the corresponding interface under different loading conditions. Two test configurations that permit such characterization are discussed here. The first is applicable to thin surface bonded laminas, and the test is performed using two standard concrete cylinders joined by the laminas and subjected to uniaxial tension. The second is applicable to laminates that are bonded on the surface or inserted into cuts made on the surface. The specimen is made up two blocks with a hinge between them and is loaded in bending. The results can help identify the mechanisms that could be of practical interest in structural applications.Spanish Ministry of Science and Technology - grant PB98-0293. Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/3259/2000. Fundo Social Europeu (FSE) - SFRH/BD/3259/2000. CAPES (Brazil). Bettor MBT (Spain)