Effect of adhesive thickness and concrete strength on FRP-Concrete Bonds

The use of fiber-reinforced polymer (FRP) composites for strengthening, repairing, or rehabilitating concrete structures has become more and more popular in the last 10 years. Irrespective of the type of strengthening used, design is conditioned, among others, by concrete-composite bond failure, normally attributed to stress at the interface between these two materials. Single shear, double shear, and notched beam tests are the bond tests most commonly used by the scientific community to estimate bond strength, effective length, and the bond stress-slip relationship. The present paper discusses the effect of concrete strength and adhesive thickness on the results of beam tests, which reproduce debonding conditions around bending cracks much more accurately. The bond stress-slip relationship was analyzed in a cross section near the inner edge, where stress was observed to concentrate. The ultimate load and the bond stress-slip relationship were visibly affected by concrete strength. Adhesive thickness, in turn, was found to have no significant impact on low-strength concrete but a somewhat greater effect on higher strength materials.

Discrimination of AE Signals From Friction and Concrete Cracking in a Reinforced Concrete Frame Subjected to Seismic Trainings

This paper shows the preliminary results of the development and application of a procedure to filter the Acoustic Emission (AE) signals to distinguish between AE signals coming from friction and AE signals coming from concrete cracking. These signals were recorded during the trainings of an experiment carried out on a reinforced concrete frame subjected to dynamic loadings with the shaking table of the University of Granada (Spain). Discrimination between friction and cracking AE signals is the base to develop a successful procedure and damage index based on AE testing for health monitoring of RC structures subjected to earthquakes.

Análisis bibliométrico de las publicaciones científicas españolas en la categoría Construction & Building Technology de la base de datos Web of Science (1997-2008)

En este trabajo se analizan las publicaciones procedentes de instituciones españolas recogidas en las revistas de la categoría Construction & Building Technology de la base de datos Web of Science para el periodo 1997-2008. El número de revistas incluidas es de 35 y el número de artículos publicados ha sido de 760 (Article o Review). Se ha realizado una evaluación bibliométrica con dos nuevos parámetros: Factor de Impacto Ponderado y Factor de Impacto Relativo; asimismo se incluyen el número de citas y el número de documentos a nivel institucional. Entre los centros con una mayor producción científica destaca, como era de prever, el Instituto de Ciencias de la Construcción Eduardo Torroja (CSIC), mientras que atendiendo al Factor de Impacto Ponderado ocupa el primer lugar la Universidad de Vigo. Por otro lado, sólo dos revistas Cement and Concrete Research y Materiales de Construcción aglutinan el 45.26% de toda la producción científica española, con 172 trabajos cada una de ellas. En cuanto a la colaboración internacional, destacan países como Inglaterra, México, Estados Unidos, Italia, Argentina y Francia

La introducción del hormigón armado en España: razón constructiva de su evolución

España se incorporó a la técnica del hormigón armado con más de dos décadas de retraso respecto a Francia o Alemania. En 1890, en Europa se construían ya estructuras de hormigón armado de cierta envergadura y complejidad. En España hubo que esperar hasta 1893 para la primera obra en hormigón armado, que fue un sencillo depósito descubierto en Puigverd (Lérida), ejecutado por el ingeniero militar Francesc Macià con patente Monier. En 1898, de la mano de Hennebique, se empezó la construcción de los dos primeros edificios con estructura de hormigón armado en España. Fueron dos obras puntuales, con proyectos importados de Francia, pero necesarias para introducir de manera definitiva el material. En paralelo, en París, se estaban edificando en hormigón armado la mayoría de los pabellones de la Exposición Universal de 1900. En el cambio de siglo, las construcciones de hormigón armado habían alcanzado ya la madurez proyectual y técnica en Europa. A pesar de la incorporación tardía, se puede constatar por las obras ejecutadas que en un periodo corto de tiempo, entre 1901 y 1906, se alcanzó en España prácticamente el mismo nivel técnico y constructivo que tenían el resto de los países que fueron pioneros en el empleo del hormigón armado. El desarrollo e implantación de una técnica constructiva no es un proceso lineal, y son muchos los factores que intervienen. Las patentes tuvieron una gran importancia en el desarrollo inicial del hormigón armado. Estas ofrecían un producto que funcionaba. Las primeras estructuras de hormigón armado no se calculaban y se construían siguiendo una reglamentación, se compraban. Y el resultado de esa “compra” solía ser, en la mayoría de los casos, satisfactorio. Las patentes vendían sistemas estructurales cuyo funcionamiento estaba corroborado por la experiencia y la pericia de su inventor. Esta investigación parte de la hipótesis de que las patentes sobre cemento y hormigón armado depositadas en España entre 1884 y 1906 fueron uno de los factores que proporcionaron a los técnicos y a las empresas españolas una pericia constructiva sólida en el empleo del hormigón armado. En este trabajo se aborda el estudio del proceso de introducción del hormigón armado en España desde una perspectiva fundamentalmente técnica, incorporando las patentes como una de las razones constructivas que explican su rápida evolución y generalización en un periodo de tiempo breve: 1901-1906. En este proceso se contextualiza y analiza una de las figuras que se considera fundamental en los primeros años del hormigón armado en España, la del ingeniero Juan Manuel de Zafra y Estevan. Esta tesis analiza las patentes de hormigón armado desde el punto de vista estadístico y constructivo. Desde ambas perspectivas se verifica la hipótesis de partida de esta investigación, concluyendo que las patentes fueron una de las razones constructivas de la evolución del hormigón armado en España y de su rápida implantación. ABSTRACT Spain incorporated the reinforced concrete technique more than two decades after France and Germany. In central Europe reinforced concrete structures of considerable size and complexity were being built in 1890, while in Spain it was not until 1893 that the first work, a simple open air water tank, was implemented in Puigverd (Lleida) by the military engineer Francesc Macià with a Monier patent. In 1898 the construction of the first two buildings with reinforced concrete structure in Spain started, with the guidance by Hennebique. They were two isolated cases with projects imported from France, but playing a key role to definitively introduce the material in Spain. In parallel, in Paris, most of the pavilions of the 1900 World Expo were being built in reinforced concrete. At the turn of the century reinforced concrete buildings had reached maturity both as a technology and as a design practice. Despite the late assumption of the material, the works carried out in the very short period between 1901 and 1906 clearly show that Spain reached practically the same technical and constructive level as the other pioneering countries in the use of reinforced concrete. The development and implementation of a constructive technique is never a linear process, there are many factors involved. The patents offered a successful product. Initial reinforced concrete structures were not calculated and built according to regulations, they were bought. And this purchase in most cases was satisfactory for the required use. Patents sold structural systems whose performance was supported by the experience and expertise of its inventor. The hypothesis of this research is based upon the assumption that the cement and concrete patents registered in Spain between 1884 and 1906 were one of the factors that provided Spanish technicians and companies with a solid constructive expertise in the use of reinforced concrete. This investigation studies the introduction of reinforced concrete to Spain from a predominantly technical perspective, incorporating patents as the constructive reason for the rapid evolution and spread in such a short period of time: 1901-1906. Along the way, the role of engineer J. M. de Zafra, generally considered a key agent in the initial years of reinforced concrete in Spain, is contextualized and analyzed. This dissertation analyzes the patents of reinforced concrete from a statistical and constructive point of view. From both perspectives the hypothesis of this research is verified, concluding that patents were one of the constructive reasons for the development of reinforced concrete in Spain.

Influence of cement properties in the reaction rate and mechanical behavior of concrete with high fl y ash content

The use of fly ash (FA) as an admixture to concrete is broadly extended for two main reasons: the reduction of costs that supposes the substitution of cement and the micro structural changes motivated by the mineral admixture. Regarding this second point, there is a consensus that considers that the ash generates a more compact concrete and a reduction in the size of the pore. However, the measure in which this contributes to the pozzolanic activity or as filler is not well defined. There is also no justification to the influence of the physical parameters, fineness of the grain and free water, in its behavior. This work studies the use of FA as a partial substitute of the cement in concretes of different workability (dry and wet) and the influence in the reactivity of the ash. The concrete of dry consistency which serves as reference uses a cement dose of 250 Kg/m 3 and the concrete of fluid consistency utilized a dose of cement of 350 Kg/m 3 . Two trademark of Portland Cement Type 1 were used. The first reached the resistant class for its fineness of grain and the second one for its composition. Moreover, three doses of FA have been used, and the water/binder ratio was constant in all the mixtures. We have studied the mechanical properties and the micro-structure of the concretes by means of compressive strength tests, mercury intrusion porosimetry (MIP) and thermal analysis (TA). The results of compressive strength tests allow us to observe that concrete mixtures with cements of the same classification and similar dosage of binder do not present the same mechanical behavior. These results show that the effective water/binder ratio has a major role in the development of the mechanical properties of concrete. The study of different dosages using TA, thermo-gravimetry and differential thermal analysis, revealed that the portlandite content is not restrictive in any of the dosages studied. Again, this proves that the rheology of the material influences the reaction rate and content of hydrated cement products. We conclude that the available free water is determinant in the efficiency of pozzolanic reaction. It is so that in accordance to the availability of free water, the ashes can react as an active admixture or simply change the porous distribution. The MIP shows concretes that do not exhibit significant changes in their mechanical behavior, but have suffered significant variation in their porous structure

Study of the damage evolution of the concrete under freeze-thaw cycles using traditional and non-traditional techniques

Some experiments have been performed to investigate the cyclic freeze-thaw deterioration of concrete, using traditional and non-traditional techniques. Two concrete mixes, with different pore structure, were tested in order to compare the behavior of a freeze-thaw resistant concrete from one that is not. One of the concretes was air entrained, high content of cement and low w/c ratio, and the other one was a lower cement content and higher w/c ratio, without air-entraining agent. Concrete specimens were studied under cyclic freeze-thaw conditions according to UNE-CENT/TS 12390-9 test, using 3% NaCl solution as freezing medium (CDF test: Capillary Suction, De-icing agent and Freeze-thaw Test). The temperature and relative humidity were measured during the cycles inside the specimens using embedded sensors placed at different heights from the surface in contact with the de-icing agent solution. Strain gauges were used to measure the strain variations at the surface of the specimens. Also, measurements of ultrasonic pulse velocity through the concrete specimens were taken before, during, and after the freeze-thaw cycles. According to the CDF test, the failure of the non-air-entraining agent concrete was observed before 28 freeze-thaw cycles; contrariwise, the scaling of the air-entraining agent concrete was only 0.10 kg/m 2 after 28 cycles, versus 3.23 kg/m 2 in the deteriorated concrete, after 28 cycles. Similar behavior was observed on the strain measurements. The residual strain in the deteriorated concrete after 28 cycles was 1150 m versus 65 m, in the air-entraining agent concrete. By means of monitoring the changes of ultrasonic pulse velocity during the freeze-thaw cycles, the deterioration of the tested specimens were assessed

The influence of sulfuric environments on concretes elaborated with sulfate resistant cements and mineral admixtures. Part 2: Concrete exposed to Magnesium Sulfate (MgSO4) = Estudio de la influencia de los medios con presencia de sulfatos en hormigones con cementos sulforresistentes y adiciones minerales. Parte 2.Hormigones expuestos a sulfato magnésico (MgSO4)

The present work studies the resistant of the concrete against magnesium sulfate (MgSO4) and compare the results with values obtained previously of the same concretes exposed to sodium sulfate (Na2SO4). Thus, it is possible analyze the influence of the cation type. To that end, four different concrete mixes were made with sulfur resistant cement and mineral admixtures (silica fume, fly ash and blast furnace slag). The concretes were submerged for different period in magnesium sulfate (MgSO4). After that, different tests were carried out to define mechanical and microstructural properties. The results obtained were compared with reference values of concretes cured in calcium hydroxide [Ca(OH)2]. According to the results, the concrete with blast furnace slag presented the best behavior front MgSO4, meanwhile the concretes with silica fume and fly ash were the most susceptible. The resistance of the concrete with blast furnace slag could be attributed to the characteristics of the hydrated silicates formed during the hydration time, which include aluminum in the chemical chain that hinder its chemical decomposition during the attack of magnesium. The magnesium sulfate solution was most aggressive than sodium sulfate solution. El presente trabajo estudia la resistencia de hormigones al ataque de sulfatos provenientes de sulfato magnésico (MgSO4) y compara estos valores con resultados previos de los mismos hormigones atacados con sulfato sódico (Na2SO4). De esta manera se estudia la interacción del catión que acompaña al ion sulfato durante su afectación a la matriz cementicia. Para lo anterior, se diseñaron cuatro dosificaciones empleando cementos sulforresistentes y adiciones minerales (humo de sílice, ceniza volante y escoria de alto horno). Los hormigones se sumergieron, por distintos periodos de tiempo, en disolución de sulfato magnésico (MgSO4) de concentración 1M, para después realizarles ensayos mecánicos y a nivel microestructural. Los valores obtenidos se compararon con los obtenidos en el hormigón de referencia curado en hidróxido cálcico. El hormigón con escoria de alto horno presentó el mejor comportamiento frente a MgSO4, siendo las mezclas de humo de sílice y ceniza volante las más susceptibles. La resistencia del hormigón con escoria se atribuye a las características de los silicatos hidratados formados durante la hidratación, los cuales incorporan aluminio en las cadenas impidiendo su descomposición ante un ataque por magnesio. El medio con sulfato magnésico mostro una mayor agresividad que el medio con sulfato sódico.

Study of the Influence of Fines on a Self-Compacting Concrete

The paper presents the results of the tests of a self-compacting concrete made with fines which include Portland cement and three fillers: hornfels, limestone and metakaolin, in a weight proportion between 23% and 45% of the admixtures. The first mix proportions were designed with a high proportion of Portland cement (720-750kg/m3), and are compared to those having a smaller content of cement and more fillers. The results obtained show that the limestone filler percentage should be higher than the hornfels one, and both of them significantly higher than that of the metakaolin so as to facilitate the fluidity and self-compactability. AIso, the higher proportion of fillers causes a rounded porosity in the mixing which has a bearing on better compressive strength results.

Experimental and numerical analysis of concrete Slabs subjected to Blast

This paper summarizes the research activities focused on the behaviour of concrete and concrete structures subjected to blast loading carried out by the Department of Materials Science of the Technical University of Madrid (PUM). These activities comprise the design and construction of a test bench that allows for testing up to four planar concrete specimens with one single explosion, the study of the performance of different protection concepts for concrete structures and, finally, the development of a numerical model for the simulation of concrete structural elements subjected to blast. Up to date 6 different types of concrete have been studied, from plain normal strength concrete, to high strength concrete, including also fibre reinforced concretes with different types of fibres. The numerical model is based on the Cohesive Crack Model approach, and has been developed for the LSDYNA finite element code through a user programmed subroutine. Despite its simplicity, the model is able to predict the failure patterns of the concrete slabs tested with a high level of accuracy

Experimental and numerical study of bond response in structural concrete with corroded steel bars

Corrosion can affect the bond between reinforcing bars and concrete and hence the transfer of longitudinal stresses. Although a number of experimental studies on bond failure have been conducted in recent years, the findings have diverged rather widely, due primarily to differing test conditions. The present paper reports on an experimental programme consisting of eccentric pull-out tests run on corroded steel bars in specimens subjected to accelerated or natural corrosion. An axisymmetric bi-dimensional FE model with finite deformations initially developed to study bond mechanics with sound steel bars, has been enhanced to consider bond effects in corroded steel bars. The model simulation is compared to some of the experimental results for corroded and sound bars and the findings are analysed.

Bond Modelling of prestressed concrete during the prestressing force release

This paper presents an analytical model for simulating the bond between steel and concrete, in precast prestressed concrete elements, during the prestressing force release. The model establishes a relationship between bond stress, steel and concrete stress and slip in such concrete structures. This relationship allows us to evaluate the bond stress in the transmission zone, where bond stress is not constant, along the whole prestressing force release process. The model is validated with the results of a series of tests and is extended to evaluate the transmission length. This capability has been checked by comparing the transmission length predicted by the model and one measured experimentally in a series of tests.

Hydration of C3S, C2S and their Blends. Micro- and Nanoscale Characterization

This study forms part of wider research conducted under a EU 7 th Framework Programme (COmputationally Driven design of Innovative CEment-based materials or CODICE). The ultimate aim is the multi-scale modelling of the variations in mechanical performance in degraded and non-degraded cementitious matrices. The model is being experimentally validated by hydrating the main tri-calcium silicate (T1-C3S) and bi-calcium silicate (β-C2S), phases present in Portland cement and their blends. The present paper discusses micro- and nanoscale studies of the cementitious skeletons forming during the hydration of C3S, C2S and 70 % / 30 % blends of both C3S/C2S and C2S/C3S with a water/cement ratio of 0.4. The hydrated pastes were characterized at different curing ages with 29 Si NMR, SEM/TEM/EDS, BET, and nanoindentation. The findings served as a basis for the micro- and nanoscale characterization of the hydration products formed, especially C-S-H gels. Differences were identified in composition, structure and mechanical behaviour (nanoindentation), depending on whether the gels formed in C3S or C2S pastes. The C3S gels had more compact morphologies, smaller BET-N2 specific surface area and lesser porosity than the gels from C2S-rich pastes. The results of nanoindentation tests appear to indicate that the various C-S-H phases formed in hydrated C3S and C2S have the same mechanical properties as those formed in Portland cement paste. Compared to the C3S sample, the hydrated C2S specimen was dominated by the loose-packed (LP) and the low-density (LD) C-S-H phases, and had a much lower content of the high density (HD) C-S-H phase

Bond modelling of prestressed concrete during the prestressing force release

This paper presents an analytical model for simulating the bond between steel and concrete, in precast prestressed concrete elements, during the prestressing force release. The model establishes a relationship between bond stress, steel and concrete stress and slip in such concrete structures. This relationship allows us to evaluate the bond stress in the transmission zone, where bond stress is not constant, along the whole prestressing force release process. The model is validated with the results of a series of tests, considering different steel indentation depths and concrete covers and is extended to evaluate the transmission length. This capability has been checked by comparing the transmission length predicted by the model and one measured experimentally in two series of tests.

Estudio de hormigones reciclados no estructurales fabricados con árido reciclado mixto : propiedades mecánicas y expansión debida al contenido de sulfatos

El objetivo principal de este trabajo de investigación es estudiar las posibilidades de utilización del árido reciclado mixto para un hormigón reciclado en aplicaciones no estructurales, justificando mediante la experimentación la validez para esta aplicación, tanto del árido reciclado como del hormigón reciclado. Esta tesis se centró en los aspectos más restrictivos y limitativos en la utilización de los áridos mixtos en hormigón reciclado, basándose tanto en la normativa internacional existente como en los resultados obtenidos en los estudios bibliográficos consultados. La primera tarea realizada fue la caracterización completa de las propiedades del árido reciclado mixto, recogiendo especialmente los siguientes aspectos: granulometría, contenido de finos, absorción y densidades, composición del árido reciclado, índice de lajas, coeficiente de Los Ángeles, partículas ligeras y contenido de sulfatos. De este estudio de los áridos reciclados, se han destacado relaciones entre las propiedades. Las diferentes correlaciones permiten proponer criterios de calidad de un árido reciclado mixto para un hormigón reciclado. Se ha elegido un árido reciclado mixto entre los estudiados, de características límite admisibles, para obtener resultados conservadores sobre el hormigón reciclado fabricado con él. En una segunda etapa, se ha realizado un estudio de dosificación completo del hormigón reciclado, evaluando la consistencia del hormigón en estado fresco y la resistencia a compresión del hormigón en estado endurecido y se ha comparado con las mismas propiedades de un hormigón convencional. Se ha analizado la capacidad de absorción del árido conseguida con los métodos de presaturación empleados y en función de su estado de humedad, para poder evaluar las relaciones agua/cemento totales y efectivas del hormigón. Se ha estudiado el efecto de estos dos parámetros tanto en la consistencia como en la resistencia del hormigón reciclado. Finalmente, se ha estudiado el hormigón fabricado con un 50% y 100% de una partida de árido reciclado mixto de calidad admisible y se han ensayado las siguientes propiedades: consistencia, resistencia a compresión, resistencia a tracción indirecta, módulo de elasticidad dinámico, cambios de longitud, porosidad abierta y microscopía. Para analizar el efecto de los sulfatos, se han añadido artificialmente cantidades de yeso controladas en el hormigón reciclado. Se fabricaron hormigones con dos tipos de cemento, un cemento CEM I 42,5 R con elevado contenido de C3A, que debería dar lugar a expansiones mayores y un cemento con adiciones puzolánicas CEM II A-P 42,5 R, que atenuaría el comportamiento expansivo en el hormigón. Los resultados finales indican que la utilización del árido reciclado mixto en proporciones de hasta un 50%, permiten cubrir la gama de resistencias más exigentes dentro del hormigón no estructural. El contenido de sulfatos puede variar desde un 0,8% hasta un 1,9%, según el tipo de cemento y la proporción de sustitución del árido natural por árido reciclado mixto. Tanto en el caso del árido reciclado como en el hormigón, se ha realizado un estudio comparativo entre el conjunto de datos recopilados en la bibliografía y los obtenidos en este estudio experimental. En varias propiedades del hormigón reciclado, se han comparado los resultados con las fórmulas de la Instrucción EHE-08, para establecer unos coeficientes de corrección a aplicar a un hormigón reciclado con fines no estructurales. The main objective of this investigation work is to study the possibilities of using recycled mixed aggregate for a recycled concrete in non structural applications, justifying by means of experimentation both the validity of the recycled aggregate and recycled concrete. This thesis focused on the most restrictive and limiting aspects in the mixed aggregate use in recycled concrete, on the basis of the international standards as well on the results obtained in the bibliographic studies consulted. The first task achieved was the complete charcaterization of the mixed recycled aggregate properties, specially the following aspects: grain size analysis, fines content, absorption and densities, recycled aggregate composition, flakiness index, Los Angeles coefficient, lightweight particles and sulphate content. From this study, correlations between the properties were highlighted. The different correlations make possible to propose quality criterions for recycled mixed aggregate in concrete. Among the recycled aggregates studied, one of acceptable characteristics but near the limits established, was chosen to obtain conservative results in the recycled concrete made with it. In a second step, a complete recycled concrete mix design was made, to evaluate concrete consistency in the fresh state and concrete compressive strength in the hardened state and its properties were compared to those of a control concrete. The aggregate absorption capacity was analized with the presaturation methods achieved and in function of its state of humidity, to evaluate the total and effective water/cement ratios. The effect of these two parameters, both in consistency and compressive strength of recycled concrete, was studied. Finally, the concrete made with 50% and 100% of the elected recycled mixed aggregate was studied and the following concrete properties were tested: consistency, compressive strength, tensile strength, dynamic modulus of elasticity, length changes, water absorption under vacuum and microscopy. To analize the effect of sulphate content, some controlled quantities of gypsum were artificially added to the recycled concrete. Concretes with two types of cement were made, a cement CEM I 42,5 R with a high content of C3A, that would lead to major expansions and a cement with puzzolanic additions CEM II A-P 42,5 R that would lower the expansive behaviour of concrete. The final results indicate that the use of mixed recycled aggregate in proportions up to 50% make possible to cover the overall demanding strengths within the non structural concrete. Sulphates content can range between 0,8% and 1,9%, in function of the type of cement and the proportion of natural aggregate replacement by mixed recycled one. Both in the case of recycled aggregate and concrete, a comparative study was made between the data coming from the bibliography and those obtained in the experimental study. In several recycled concrete properties, the results were compared to the formulas of Spanish Instruction of Structural Concrete (Instruction EHE-08), to establish some correction coefficients to apply for a non structural recycled concrete.

Cohesive-frictional modelling of bond and splitting action of prestressing wire

This work shows a numerical procedure for bond between indented wires and concrete, and the coupled splitting of the concrete. The bond model is an interface, non-associative, plasticity model. It is coupled with a cohesive fracture model for concrete to take into account the splitting of such concrete. The radial component of the prestressing force, increased by Poisson’s effect, may split the surrounding concrete, decreasing the wire confinement and diminishing the bonding. The combined action of the bond and the splitting is studied with the proposed model. The results of the numerical model are compared with the results of a series of tests, such as those which showed splitting induced by the bond between wire and concrete. Tests with different steel indentation depths were performed. The numerical procedure accurately reproduces the experimental records and improves knowledge of this complex process.