Pedro Serna

Influence of recycled aggregate quality and proportioning criteria on recycled concrete properties

This paper presents the results of experimental research using concrete produced by substituting part of the natural coarse aggregates with recycled aggregates from concrete demolition. The influence of the quality of the recycled aggregate (amount of declassified and source of aggregate), the percentage of replacement on the targeted quality of the concrete to be produced (strength and workability) has been evaluated. The granular structure of concrete and replacement criteria were analyzed in this study, factors which have not been analyzed in other studies. The following properties of recycled concretes were analyzed: density, absorption, compressive strength, elastic modulus, amount of occluded air, penetration of water under pressure and splitting tensile strength. A simplified test program was designed to control the costs of the testing while still producing sufficient data to develop reliable conclusions in order to make the number of tests viable whilst guaranteeing the reliability of the conclusions. Several factors were analyzed including the type of aggregate, the percentage of replacement, the type of sieve curve, the declassified content, the strength of concrete and workability of concrete and the replacement criteria. The type of aggregate and the percentage of replacement were the only factors that showed a clear influence on most of the properties. Compressive strength is clearly affected by the quality of recycled aggregates. If the water-cement ratio is kept constant and the loss of workability due to the effect of using recycled aggregate is compensated for with additives, the percentage of replacement of the recycled aggregate will not affect the compressive strength. The elastic modulus is affected by the percentage of replacement. If the percentage of replacement does not exceed 50%, the elastic modulus will only change slightly.

Chemoselective Synthesis of Substituted Imines, Secondary Amines, and β‐Amino Carbonyl Compounds from Nitroaromatics through Cascade Reactions on Gold Catalysts

Substituted imines, alpha,beta-unsaturated imines, substituted secondary amines, and beta-amino carbonyl compounds have been synthesized by means of new cascade reactions with mono- or bifunctional gold-based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, alpha,beta-unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO2 system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three-step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO2 can also be used as a bifunctional catalyst to obtain substituted beta-amino carbonyl compounds from nitroaromatics and alpha,beta-unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold-catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.

Selective Reductive Coupling of Nitro Compounds with Aldehydes to Nitrones in H2Using Carbon-Supported and -Decorated Platinum Nanoparticles

Nitrones were synthesized in high yields directly from nitro compounds, aldehydes, and H2 using carbon-decorated platinum nanoparticles. The high selectivity for nitrone synthesis contrasts that of common supported metal catalysts and corresponds to an increase from roughly 6 to 97%. The catalytic performance is tuned by precise control of the structure of the active sites and the characteristics of the support.

New Views on Effect of Recycled Aggregates on Concrete Compressive Strength

This paper presents an in-depth study on the effect that composition and properties of recycled coarse aggregates from previous concrete structures, together with water/cement ratio (w/c) and a replacement ratio of coarse aggregate, have on compressive strength, its evolution through time, and its variability. A rigorous approach through statistical inference based on multiple linear regression has identified the key factors. A predictive equation is given for compressive strength when recycled coarse aggregates are used. The w/c and replacement ratio are the capital factors affecting concrete compressive strength. Their effect is significantly modified by the properties and composition of the recycled aggregates used. An equation that accurately predicts concrete compressive strength in terms of these parameters is presented. Particular attention has been paid to the complex effect that old concrete and adhered mortar have on concrete compressive strength and its mid-term evolution. It has been confirmed that the presence of contaminants tends to increase variability of compressive strength values.

First Ultra-High-Performance Fibre- Reinforced Concrete Footbridge in Spain: Design and Construction

Abstract Ultra-high-performance fibre-reinforced concrete (UHPFRC) is defined as a cement-based composite material that combines three technical advances in concrete: high compressive strength, presence of steel fibres and flowability. Since UHPFRC is a very expensive material, in order that UHPFRC can be considered as a competitive alternative in structural applications, it is necessary to optimise the design of structural systems. This process leads to UHPFRC lightweight structures that minimise the concrete needs and give solutions more similar to steel than concrete. The innovative design of the 43,5 m span footbridge, constructed only with UHPFRC and placed over the Ovejas ravine in Alicante, Spain, is presented in this paper. The new UHPFRC footbridge has replaced the former design devised in steel due to many factors such as lower construction and maintenance costs, high durability in aggressive environment close to the sea leading to an increase in service life and its aesthetic enhancement. This paper covers several issues about the design and construction of this singular footbridge, such as design justification, structural evaluation, UHPFRC mix design, quality control and construction process at all stages. The static and dynamic load tests performed are also presented. The footbridge over the Ovejas ravine has become the first structure constructed with UHPFRC in Spain, and as far as we know, it is the first truss pedestrian footbridge made only of UHPFRC in the world.

Shear Behavior of Self-Compacting Concrete and Fiber-Reinforced Concrete Push-Off Specimens

The shear behavior of reinforced “Z”-shaped push-off specimens made with self-compacting concrete (SCC) and self-compacting fiber-reinforced concrete (SCFRC) was analyzed by means of experimental tests. Testing consisted of two phases. Firstly, specimens were precracked subjected to linear load along the shear plane. During this first phase three precracked levels were distinguished (without precrack, thin and thick precrack). Then, precracked specimens were tested under direct shear load. The shear behavior along the shear plane was analyzed by means of the crack opening and shear displacement versus shear load process. Variables were: the type of concrete (SCC, or SCFRC with different fibers contents: 40 kg/m3 or 60 kg/m3), the transversal reinforcement (TR) and the precrack width. The analysis was specially focused on the study of aggregate interlocking. The failure occurrence is better controlled thanks to the presence of fibers, the shear behavior is more ductile.

Towards a Zero‐Waste Oxidative Coupling of Nonactivated Aromatics by Supported Gold Nanoparticles

We show that gold nanoparticles are able to perform the direct oxidative coupling of nonactivated aromatics with O2 as the only co-reagent. In this reaction, the aromatic acts both as reactant and solvent. Biphenyl, for example, can be obtained from benzene with high selectivity and a turnover number (TON) of 230 per pass. Similarly, several substituted biaryls can be prepared. Pd performs only one TON and even when a second catalytic functionality is introduced, together with strong acidic conditions, TON is always lower than 100. Other catalysts require iodine for performing the reaction, leading to 2 kg of waste for 1 kg of biphenyl formed, whereas no waste is created by the oxidative coupling with gold nanoparticles.

Effect of Residual Strength Parameters on FRC Flexural Creep: Multivariate Analysis

This paper reports the multivariate analysis of experimental results from more than one hundred FRC prismatic specimens tested under sustained flexural loads for at least 90 days, collected from previously published sources. Principal Component Analysis was used to minimise the number of variables in the modelling process while compromising the minimum amount of information. The creep parameters analysed were the creep coefficients at 14, 30, and 90 days and the corresponding crack opening rates. They were related to the following factors: concrete compressive strength, residual load-bearing capacity in flexure, fibre material, and load. Multiple linear regression was used for the modelling of these relationships. Higher levels of flexural toughness were found to significantly reduce the variability of all creep parameters. Differences in fibre material were detected to introduce important differences in interaction with other factors, especially the load ratio, which was attributed to the association between the range of flexural toughness to be expected and the type of fibres used.

Influence of Fibre Reinforcement on the Long-Term Behaviour of Cracked Concrete

The influence of fibre reinforcement on the long-term behaviour of cracked concrete is analysed in this work by means of a creep test. Nine concrete mixes were prepared (7 SFRC’s and 2 conventional RCs) based on two basic mix designs. Concretes type I were conceived for structural pre-cast applications and concretes type II reproduce a general purpose. Fibre dosages and conventional reinforcements were varied to represent a wide spectrum of post-peak flexural responses. In all cases with fibre reinforcement steel fibres were used. Conventional RC specimens were reinforced with two steel rebars. In addition to the variables of mix design of concrete, there are two significant variables related to the creep test: the pre-crack opening level (CMODpn) and the stress level (I c ) sustained during the test. Creep tests were performed by applying a constant flexural load on notched pre-cracked specimens and controlling crack opening evolution. Some of the specimens developed a sudden increase of crack opening deformations during the creep test. Creep coefficients and Crack Opening Rates were calculated and analysed. Creep coefficients show significant dependence on the analysed variables. The results of this experimental campaign show that creep on SFRC specimens may be similar to a conventional RC.

Aprender de los errores. Una manera de adquirir responsabilidad ética, medioambiental y profesional

En esta comunicacion se presenta el modo en que se ha introducido la competencia transversal “Responsabilidad etica, medioambiental y profesional” en la asignatura “Materiales de Construccion y sus aplicaciones a la ingenieria civil” de segundo curso de grado, para garantizar la adquisicion de un nivel de dominio I. El mayor logro de este trabajo es tener presente la competencia transversal en todas las sesiones de trabajo en el aula a lo largo del curso, por lo que la competencia pasa a formar parte del trabajo diario del alumno. Ademas presente ventajas en cuando a la adquisicion de evidencias y facilita la evaluacion de la misma.