Thierry Chotard

Application of ultrasonic testing to describe the hydration of calcium aluminate cement at the early age

Abstract Nondestructive and in situ characterisation techniques, such as ultrasonic measurements, permit to follow cement hydration at the early age from a few minutes to a few hours after mixing. The technique reported in this paper is based on measurements in reflection modes. Results concerning an aluminous cement, Secar71, are presented (water-to-cement weight ratio (W/C): 0.3 and 0.4; temperature of measurement: 20°C; duration: 0–24 h). Information deduced from ultrasonic measurements (longitudinal wave velocity, reflection coefficient) associated with other data obtained from X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric (TG) measurements enable to propose a qualitative description of hydrations chronology. The sensitivity of these ultrasonic parameters to hydrates formation and structuring is underlined.

Application of X-ray computed tomography to characterise the early hydration of calcium aluminate cement

Abstract Techniques that monitor in situ the setting and hardening of calcium aluminate cements (CACs) are of interest to the engineering community. This paper focuses on one non-destructive technique, X-ray computed tomography (CT). The early hydration of the cement can be followed from a few minutes to a few hours after mixing. This technique is based on both X-ray absorption measurements and qualitative observations in chosen axial cross-sections. Results concerning an aluminous cement, Secar 71, are presented (water-to-cement weight ratio: 0.33; duration: 0–24 h). Information deduced from these measurements clearly show that a strong variation of X-ray absorption values occurs during the early age of hydration. Both quantitative and qualitative analyses of CT data allow a chronology of cement setting to be proposed.

Properties of cellulosic fibre reinforced plaster: influence of hemp or flax fibres on the properties of set gypsum

In the last few years, eco friendly materials have become an important part of the building materials market. Natural fibres are already used in various types of materials, like plastics, concrete and lime-based products. They demonstrate different attributes like the combination of good mechanical, thermal and acoustic properties that allow these types of materials to be used for different applications. The main drawback associated with plaster is its brittleness, especially under tensile stress. Therefore, it is interesting to investigate different methods that could potentially enhance the mechanical properties of plaster. Adding fibres to gypsum to obtain a composite material is one way to improve the behaviour of the product, especially after the failure of the matrix. The aim of this work was to the study the effects of adding natural fibres, namely hemp and flax fibres, on the setting time of plaster and the mechanical properties of the composite matrix. It was shown that hemp delayed the setting of plaster, unlike flax. The initial and final setting times almost doubled when hemp was added in a plaster matrix, whereas flax fibres did not drastically change them. Different chemical treatments of hemp were tested and the impact on the setting time was measured. The setting times of both composites made with hemp and flax were reduced once the fibres were treated (25–40% reduction), compared to the setting time of the calcium sulphate hemihydrate alone. The mechanical properties of the composite materials are also discussed. The behaviour of plaster was modified from brittle to a non-linear one when fibres were added, and even at small levels of addition, flax fibres allowed slightly higher values of flexural strength to be reached.

Characterization of composite materials by ultrasonic methods: modelization and application to impact damage

This paper presents some ultrasonic methods to detect and to characterize defects, possibly obtained after damage caused in composite materials. Firstly, a two-dimensional ultrasonic cartography, performed section by section, at different positions from the impact point, allows the participation of the delamination mechanisms which took part through the thickness of a pultruded glass fiber-reinforced plastic composite beam, to be analyzed. A very good agreement has been found with destructive testings. Furthermore, some examples are given on the use of an ultrasonic propagation model which has been developed. This model permits optimum experimental configurations to be determined, by the use of transmission and reflection coefficients or of Lamb waves. In addition, experimental and modelized time signals have been compared.

Mechanical properties of hemp-lime reinforced mortars: influence of the chemical treatment of fibers

In addition to being an environmental-friendly material, hemp fibers are also inexpensive reinforcements in thermoplastics or concrete composites, due to their intrinsic mechanical, thermal, and acoustic properties. However, chemical treatments of fibers are required to enhance the matrix/fiber interface. In this article, the influence of the addition of hemp fibers chemically treated or not, on the mechanical properties of a lime composite was investigated. Mechanical properties were evaluated by ultrasonic pulse echography and four-point bending test associated with acoustic emission analysis. The physical and chemical surface modifications of the hemp fibers were monitored with gas chromatography and scanning electron microscopy. It appears that chemical modification of the fiber surface degrades the amorphous materials present in the fiber structure, which results in an increase in the surface reactivity and also improves the mechanical properties of the composites.

Characterisation of early stage calcium aluminate cement hydration by combination of non-destructive techniques: acoustic emission and X-ray tomography

Abstract Techniques that monitor in real time the setting and hardening of calcium aluminate cements (CAC) are now of interest to the engineering community. This paper focuses on the association of two in situ techniques, acoustic emission (AE) and X-rays computed tomography (CT). These non-destructive techniques make it possible to follow the cement hydration in the early stage from a few minutes to a few hours after mixing. Results concerning a calcium aluminate cement paste, Secar71, are presented (water to cement weight ratio: 0.33; duration: 0–24 h). Both the acoustic emission (AE) signals and X-rays absorption values were recorded and analysed. Information deduced from experiments clearly show that a close correlation exists between the variations of the X-ray absorption values and the AE activity during the early stage of hydration. Based on the experiments, the combined analysis (quantitative and qualitative) of both AE and CT data make it possible to propose a relationship between the experimental characteristics recorded during the tests and the different mechanisms taking place during the early hydration of a cement paste. A chronology of cement setting is also proposed.

Residual performance of scarf patch-repaired pultruded shapes initially impact damaged

This paper presents an experimental investigation on the residual mechanical behaviour of patch-repaired composite pultruded structures initially submitted to low-velocity impact loading. It reports detailed results about static, fatigue and impact tests performed on different glass/polyester impact-damaged structures repaired by low-cost and manual techniques. All the tests were conducted at room temperature. It appears that, firstly, for all the studied structures, the initial static strength is completely recovered. Secondly, for two types of pultruded structures, the fatigue crack-growth life does not recover its initial values but compared with the damaged specimen, the lifetime is significantly enhanced. Residual performances of both undamaged and repaired specimens seemed to be influenced by the profile geometry (in fatigue bending tests) and especially by the open/closed characteristic of the structures. Carefully designed, external scarf patch repairs can recover more than 85% of the undamaged mechanical behaviour, depending on the type of residual applied loading.

Experimental determination of TSAI failure tensorial terms Fij for unidirectional composite materials

Composite materials are being used more and more for components of large thickness. For this kind of component, two-dimensional failure criteria do not seem to be adequate to describe the mechanical behaviour of these structures. This paper presents an experimental determination (without reference to damage mechanisms) of the different terms of the strength tensors of a three-dimensional polynomial failure criterion for two different composite materials (graphite/epoxy T300/914 and glass/epoxy M10). The considered third dimension leads to some experimental difficulties in the determination of out-of-plane parameters F3, F33, F44, and Fij. A new testing fixture (double compression fixture), new experimental conditions (Iosipescu shear test) and a new specimen shape (through-the-thickness tension test) are proposed to solve these problems. Finally, the two strength tensors Fi and Fij are totally determined to meet the requirements of engineers and designers.

On the mechanical behaviour of pultruded sections submitted to low-velocity impact

An experimental investigation has been carried out to study the dynamic behaviour of composite profiles made by pultrusion. Two different types of glass/polyester pultruded beams were subjected to low-velocity impact loading with several incident kinetic energy ranges. The resulting data from the drop-weight tests provide specific information about the effect of the impactor velocity, the impactor mass and the impactor nose radius, all of which have a great influence on the impact response of these profiles. A big-impactor (BI) strike produces less delamination than a small-impactor (SI) one. It appears that damage localisation has a great influence on the mechanical response of these profiles. An experimental study of the sequence of damage mechanisms during impact loading is also carried out. A methodology based on combined accelerometer/strain-gauge signal analysis is performed to identify the damage sequence which occurs over the impact duration. The post-impact vibration has a notable influence on the internal damage growth (cracking and delamination). The proposed damage sequence is strongly supported by an experimental deductive method.

Thermo-elastic behaviour of a natural quartzite: itacolumite

Itacolumite is a particular type of sandstone constituted of quartz grains, with an interlocked microstructure and large intergranular decohesions. This article is devoted to the study of the thermo-elastic behaviour of this material during thermal cycles between 20 and 800 °C. This was made by using an ultrasonic pulse-echo measurement technique of Young’s modulus E, coupled to thermal expansion experimentation and to acoustic emission. An unusual evolution of elastic properties versus temperature is found, with strong irreversible effects around the temperature of the α–β transition of quartz. A damage parameter, representing the fractional number of cracks, is calculated with a Kachanov type formulation, by using the experimental data for itacolumite and the values of E, obtained from literature for an hypothetic ideal quartz polycrystal. The evolution of this parameter versus temperature is explained by internal stresses that are developed by both thermal and elastic effects in quartz grains.