In situ monitoring of corrosion processes by coupled micro-XRF/micro-XRD mapping to understand the degradation mechanisms of reinforcing bars in hydraulic binders from historic monuments

International audienceHistoric monuments have been partly built since antiquity with iron or steel reinforcements sealed in mortars or hydraulic binders. But the presence of chloride in the environment can weaken the structures due to the corrosion of these metallic parts, leading to the cracking of the binder. In this context, in order to better understand the first steps of these corrosion mechanisms a chemical cell was designed to operate in situ analyses of the phases precipitated when a chlorinated solution is introduced in the vicinity of the bar. The chemical and structural characterization (micro-XRF and micro-XRD respectively) was performed under synchrotron radiation at the SOLEIL-DiffAbs beamline. Moreover, complementary SEM-EDS analyses were carried out before and after the in situ cell experiment in order to determine the final localisation of the corrosion products inside the crack network. The results show that iron can spread up to 1 mm away from the metallic bar inside the pores of the binder after 44 h of corrosion. Moreover, in accordance with laboratory experiments conducted in solution in the presence of Fe2+ and Cl- ions the reaction pathways conduct to the successive formation of an intermediate Fe(ii)-Fe(iii) chlorinated green rust which transforms into ferric oxyhydroxides such as akaganeite or goethite depending on the local concentration of iron

Mechanical properties of cotton fabric reinforced geopolymer composites at 200-1000 °C

Geopolymer composites containing woven cotton fabric (0–8.3 wt%) were fabricated using the hand lay-up technique, and were exposed to elevated temperatures of 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C. With an increase in temperature, the geopolymer composites exhibited a reduction in compressive strength, flexural strength and fracture toughness. When heated above 600 °C, the composites exhibited a significant reduction in mechanical properties. They also exhibited brittle behavior due to severe degradation of cotton fibres and the creation of additional porosity in the composites. Microstructural images verified the existence of voids and small channels in the composites due to fibre degradation

Validation of the model developed by Le Roy for the shrinkage of self-compacting concrete

By means of concrete equivalent mortar (CEM) principle, concrete properties can be experimentally studied at mortar level. Based on this principle, this paper studies the influence of mix design parameters on autogenous and drying shrinkage of self-compacting concrete by means of experiments at mortar level. Mix design parameters of this study are W/C (water/cement) ratio and A/C (limestone filler/cement) ratio. The results show that autogenous shrinkage increases with decreasing W/C and A/C contrary to drying shrinkage which increases with higher ratios. Furthermore, based on mortar and paste results, it is shown that the model developed by Le Roy to predict the shrinkage of CEM and possibly of SCC, from shrinkage measured on cementing paste, is also valid for shrinkage of limestone filler based self-compacting cementious materials. Furthermore, some known results on the influence of water/cement ratio and filler/cement ratio have been confirmed and can be explained by porosity results

Influence of aggregate skeleton on shrinkage properties: validation of the model developed by Le Roy for the case of self-compacting concrete

Shrinkage behavior of self-compacting concrete (SCC) can be different from that of traditional vibrated concrete, because of different paste and aggregate volumes. For traditional concrete, shrinkage can be estimated based on shrinkage results obtained on paste level. Based on homogenization techniques, Le Roy developed a model relating the shrinkage of concrete to the shrinkage of the representative cement paste, considering a granular coefficient taking into account the elastic properties and the concentration of the aggregates. By means of an extended experimental program, the applicability of this model to the case of SCC has been verified. Furthermore, some known results on the influence of water/cement ratio and filler/cement ratio have been confirmed and can be explained by porosity results

Influence of elevated temperature on properties of radiation shielding concrete with electric arc furnace slag as coarse aggregate

The development of value-added materials from by-product of the steel-making process can promote sustainability in construction to move towards a circular economy. The use of Electric Arc Furnace (EAF) steel slag as heavyweight coarse aggregate to develop sustainable radiation shielding concrete could provide both technical and economic benefits with less environmental impact. This contribution investigates the behaviour at high temperature of a sustainable radiation shielding concrete. Thermal behaviour of EAF slag concrete was compared to another heavyweight concrete made of barite aggregates and to a normal-weight concrete. On one hand, the thermal stability of the three different aggregates was determined via simultaneous Thermogravimetry and Differential Scanning Calorimetry analyses and visual observations after 10 \ub0C/min heating. On the other hand, the evolution of thermal conductivity of concretes during heating, the residual mechanical properties and the assessment of potential spalling occurrence were investigated. Stereo-microscope and Scanning Electron Microscope pictures provide additional explanations by showing the paste-aggregate interface after heating at 450 \ub0C. The results indicated that EAF concrete displayed less strength reduction at increasing temperature. This improved behaviour is attributed to both the strong bond between the paste and the aggregates, and the better thermal behaviour of the EAF slag aggregates compared to barite aggregates

Thermo physical properties of lateritic soil bricks: influence of water content

Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Effect of high temperature on mechanical and physical properties of lightweight cement based refractory including expanded vermiculite

Four different composite mixtures with varying amounts of expanded vermiculite were exposed to high temperatures of 300, 600, 900 and 1100 degrees C for 6 h. Physical and mechanical properties including unit weight, porosity, water absorption, residual compressive strength, residual splitting tensile strength and also ultrasonic pulse velocity were determined after air cooling. Microstructures were investigated by scanning electron microscopy. Lightweight concrete with vermiculite shows a good performance at elevated temperatures. Expanded vermiculite is a significant lightweight aggregate for cementitious materials which are used for fire resistance applications. Concrete with vermiculite can be used as cement based refractory

Estudio del comportamiento de concreto asfáltico con residuos siderúrgicos como agregados

Spa: El auge de la construcción de grandes obras de ingeniería ha impulsado la demanda del acero, lo que ha generado un incremento en la producción de residuos siderúrgicos, situación que causa problemas ambientales debido a su acumulación y no disposición adecuada. Igualmente, la explotación de recursos naturales no renovables como los agregados pétreos están generando deterioro en el medio ambiente; si a esto le sumamos que las regulaciones ambientales para la explotación de estos materiales no renovables son cada vez más estrictas generando incremento significativo en los costos de producción. En consecuencia, esto lleva a un cambio de paradigma: utilizar materiales no convencionales y emplear técnicas de reciclajes de los pavimentos existentes. El uso de residuos industriales en diferentes procesos debe estar enfocado hacia el desarrollo sostenible y la protección del medio ambiente. Durante el proceso de la fabricación de acero se producen diferentes residuos entre los cuales se encuentran la escoria de horno al oxigeno (BOF), escoria de horno de arco eléctrico (EAF) y polvo de alto horno (BFD). Esta investigación analiza la conveniencia técnica del uso de la escoria BOF y escoria EAF como agregado grueso y estudia la alternativa del uso de BFD como agregado fino, para fabricar mezclas asfálticas en caliente para pavimentos, como una alternativa para mitigar los problemas ambientales derivados de la acumulación de residuos siderúrgicos y de la explotación de materiales no renovables, como la grava y la arena. Para lograr el objetivo, se analizaron once tipos de mezclas asfálticas, una mezcla con materiales convencionales y diez mezclas sustituyendo parcial (50%) y totalmente (100%) el agregado grueso por escoria BOF y escoria EAF y el agregado fino por BFD. El diseño de las mezclas se realizó con la metodología Ramcodes, la cual se basa en el principio del polígono de vacíos. Mediante ensayos se evaluaron las características del diseño preliminar y verificaron las propiedades de desempeño de cada una de las mezclas. También se estudia y analiza la posibilidad de modificar el cemento asfáltico con polvo de escoria de horno de arco eléctrico (EAFD), Para lo cual se realiza el estudio reológico añadiendo 3%, 6% y 10% de EAFD y se compara con los resultados del cemento asfáltico base. Se realizan los ensayos con el reómetro de corte dinámico DSR para determinar los parámetros G* y δ en los cementos asfáltico originales, envejecidos en el horno rotatorio de película delgada (RTFO) y en el horno de envejecimiento a presión (PAV). También se realiza el ensayo de recuperación elástica a diferentes esfuerzos MSCR, el cual mide las propiedades viscoelásticas del cemento asfáltico envejecido a corto plazo en RTFO. Los resultados de este estudio confirman el uso de escoria BOF y escoria EAF como agregado grueso e indican la factibilidad del uso de BFD como agregado fino, para reemplazar parcialmente los agregados convencionales, en la fabricación de concretos asfálticos para uso en carreteras. Respecto a las propiedades mecánicas, las mezclas cumplen con los requerimientos del INVIAS para un nivel de tránsito NT-3, como son estabilidad, flujo y propiedades volumétricas. Asimismo, las propiedades de desempeño (susceptibilidad a la humedad, susceptibilidad a la deformación permanente, módulo resiliente y fatiga) presentaron un buen comportamiento.DoctoradoDoctorado en Ingeniería y Ciencia de los Materiale