Extrusion is a process that consists in forcing a formable material to pass through a die having the cross-section of the part to be obtained. This way of processing is used with conventional and fibre-reinforced cement-based materials to fabricate various construction elements such as panels, pipes and roadside curbs. Recently, with the development of digital fabrication methods and especially 3D concrete printing by selective deposition, the extrusion techniques have experienced a significant increase in interest.
 This letter describes the screw and ram extrusion techniques and their applications in construction industry. Furthermore, the underlying mechanisms involved during extrusion flow are delineated and the roles of rheological and hydro-mechanical behaviours (the latter one in a soil mechanics sense) in defining the extrudability – ability of being extruded – of the cementitious materials are highlighted. Finally, specific points such as flow-induced anisotropy of fibre reinforced cementitious materials or surface defects are addressed.

The pH of concrete-based material is a key parameter for the assessment of its stability and durability, since a change in pH is usually associated with major types of chemical degradation such as carbonation, leaching and acid attacks. Conventional surface pH measurements with potentiometric flat surface electrodes have low spatial resolution, whereas optical pH visualization with indicator dyes (phenolphthalein) only indicates the areas with higher or lower pH than the pKa of the indicator. In this regard, it is key to develop wide-range imaging systems, enabling accurate and spatially resolved determination of pH variability for an advanced knowledge of degradation mechanisms. This contribution presents the enhancements made for a high-resolution optical pH imaging system based on fluorescent aza-BODIPY indicator dyes. The measurement range was increased to 6 pH units (pH 6.5 to pH 12.5) by a combination of two indicator dyes. Moreover, background scattering effects were sufficiently eliminated. With the improved sensor foils steep pH gradients (up to 3 pH units within 2 mm) were successfully recorded in various concrete specimens using a macro lens reaching a resolution of down to 35 µm per pixel.

The cooling test or Thermal Stress Restrained Specimen Test (TSRST) simulates fully restrained pavements, as they occur in field for laboratory assessment of the thermal cracking resistance of asphalt mixtures. In the TSRST, cryogenic stress builds up due to cooling and prevented shrinkage until the tensile strength of the specimen is exceeded and the specimen fails by cracking. By carrying out TSRST various viscoelastic parameters, e.g. relaxation, evolution of tensile stresses, and tensile strength can be analyzed. Thus, a comprehensive view on the low temperature performance is possible. Standard TSRST is controlled by setting the cooling rate of the air within the chamber at a fixed value, e.g. -10°C/h. In thermodynamics, the actual cooling rate of objects is not only influenced by the cooling but also by external conditions like humidity, air velocity, radiation condition, etc. A current study investigates the impact of additional cooling parameters rather than just the air cooling rate. Two test machines of the same manufacturer that differ in the year of production and the setup of the climate chamber are compared. An initial wide scatter of test results from the two devices could be explained by taking thermodynamics into account and the reproducibility could be significantly enhanced.

Salt crystallization is accepted to represent one of the major causes for the degradation of building and ornamental stone. As such, it has attracted the attention of researchers, who over the years have progressively unraveled most mechanisms involved in salt damage. Despite this, many questions subsist about how to quantitatively predict damage or its progression, and in particular how to relate performance on site to that in laboratory tests.
 In this context, a new RILEM TC has been started with the objective of defining laboratory tests that deliver more reliable predictions of field behavior. One deliverable of this TC, is to provide a theoretical insight into this question based on recent progress on the understanding of salt damage. This paper presents a summary of this work, highlighting key aspects relating to crystallization pressure, chemo-mechanics and transport. Implications are more specifically discussed in relation to existing accelerated tests in an attempt to better define the type of field exposure that they may best represent.
 A simple conceptual model for the development of salt damage is introduced. During an initial “induction” phase, transport of ions and accumulation of salt in the porous materials occurs without causing detectable damage until a critical point, termed “damage onset” is reached. Beyond this point, during the “propagation phase”, the material degrades increasingly. The implications of these two phases are discussed in relation to the selection of appropriate salt weathering tests and conservation interventions.

The science of rheology is increasingly used to describe the properties of fresh cement paste. Compared to standard workability tests, rheological properties allow for more fundamental investigation, more precise phenomenological description of flow properties and serve as input for numerical simulations. Standard commercially available rheometers are typically used to perform those measurements. However, the results of the measurement depend on the geometry, testing procedure and a number of potential artefacts which may occur. This technical letter describes the most common techniques and procedures used to assess the rheological properties of cement paste, as well as challenges during measurements and actions to counter these challenges.

In the global will of reducing fossil energy consumption and greenhouse gas emission in the building sector, the use of bio-based insulating materials is gaining interest thanks to their profitable properties and their suitability for the renovation of ancient buildings made out of unconventional materials. However, such materials are still lacking of characterization, and more precisely of on-site evaluation, as no complete measurement protocol is available. The starting point to fill this gap would be to set-up a protocol for whole building instrumentation, and this paper is investigating questions arisen in that goal, and more precisely regarding the impact of sensor locations on the assessment of key parameters. For that purpose, instrumented polystyrene and hemp concrete wallets of dimensions 0.9 × 0.9 × 0.1 m3 with well-known thermal and hydric characteristics are tested within a double climatic chamber. The impact of temperature sensor locations and implementations are tested through indirect estimation of the thermal conductivities of the materials composing the wallets. The effect of the hygrothermal processes on the measurement of thermal performance is also investigated through the analysis of the wall global transmittance. These results finally allow to provide some recommendations concerning the on-site instrumentation of hemp concrete walls.

The effective utilization of pseudo-ductile materials like Fibre Reinforced Concrete (FRC) depends on the incorporation of suitable material parameters in appropriate design approaches. A design methodology has been developed for slabs-on-grade addressing various failure patterns, and giving a performance requirement as the design output. This opens up the choice of fibres, allowing the use of combinations of fibres to suit the service requirements. In this context, the current study explores the use of hybrid combinations of conventional steel fibres (SF) and a genre of corrosion-resistant amorphous metallic fibres (AMF) that have the ability to significantly enhance the flexural strength of concrete, even at relatively low dosages. It is shown that AMF, when used in combination with SF, results in a synergistic response with respect to toughness; mixes with 15 kg/m3 of SF and 20 kg/m3 of AMF exhibit about 50% higher characteristic flexural strength and more than double the characteristic equivalent flexural strength than the mix with 15 kg/m3 of SF alone, in the concrete considered here. Consequently, when FRC with a hybrid combination of fibres (AMF+SF) was considered in design, a significant reduction in thickness of slab was possible, in comparison to FRC with only SF.