Assessing the performance of earth building materials: a review of recent developments

After being almost abandoned in Europe at the end of the Second World War, raw earth is currently regaining the interest of civil engineers and architects worldwide. Raw earth (unfired earth) displays very interesting thermo\u2010hygro\u2010mechanical properties, which can contribute to the reduction of the environmental impact of buildings not only during construction but also during service life. Nevertheless, one of the main reasons preventing dissemination of raw earth into mainstream construction practice is the lack of commonly agreed protocols for assessing engineering performance. In this context, the RILEM Technical Committee 274\u2010TCE is critically examining current experimental procedures to propose appropriate testing methods that could be adopted as standards. The present paper summarizes the main challenges faced by the committee and describes some of the existing procedures for measuring the engineering properties of earth materials. The main issue identified by the committee is that laboratory protocols do not accurately reproduce field conditions. The representativeness of laboratory samples is also questionable due, for example, to different degrees of material homogeneity with respect to the field. Finally, the paper identifies some possible routes to reduce the discrepancies between laboratory testing and field conditions in relation to the thermo\u2010hygro\u2010mechanical characterization of earth materials

Laboratory investigation of hygrothermal monitoring of hempconcrete walls

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

Are Deicing Salts Necessary to Promote Scaling in Concrete?

International audienceThe main purpose of the present study is to investigate the role of the material parameters such as permeability, thermal diffusivity, and pore size distribution on the mechanical behavior of cementitious structures submitted to frost action, such as surface scaling. An experimental device, in which a cement paste specimen is exposed to freezing-thawing cycles under a thermal gradient, has been developed. The experimental results show that under high thermal gradient (up to 1.5°C/mm), skin damage can occur without a saline layer in contact with the frozen surface. This can be explained and quantified in the framework of poromechanics. The model is based on the coupling between liquid-ice crystal thermodynamic equilibrium, liquid water transport, thermal conduction, and elastic properties of the different phases that form the saturated porous material. It eventually predicts that a less permeable sample is more susceptible to damage by surface defacement, which explains the observed experimental result

Investigation of water to ice phase change in porous media by ultrasonic and dielectric measurements

International audienceThe main objective of this paper is to study the evolution of the ice content of porous media submitted to sub-zero temperatures by dielectric and ultrasonic measurements. Dielectric measurements are made by a capacitive sensor-based apparatus. The amount of ice formed within the tested sample is estimated from the global dielectric constants of the sample and of all the phases that form the tested composite material. On the other hand, ultrasonic measurements are based on the evolution of the ultrasonic wave velocity through the tested sample during a freezing-thawing cycle. These two methods lead to very close results and appear to be cheaper alternatives to low temperature calorimetry. The ice content curves are analyzed with the help of thermoporometry concepts in order to characterize the pore size distribution. Results appear to be complementary to mercury intrusion porosimetry ones. Moreover the commonly observed hysteresis of the ice content during a freezing-thawing cycle is investigated with respect to material microstructure

Hawking radiation of massive modes and undulations

We compute the analogue Hawking radiation for modes which posses a small wave vector perpendicular to the horizon. For low frequencies, the resulting mass term induces a total reflection. This generates an extra mode mixing that occurs in the supersonic region, which cancels out the infrared divergence of the near horizon spectrum. As a result, the amplitude of the undulation (0-frequency wave with macroscopic amplitude) emitted in white hole flows now saturates at the linear level, unlike what was recently found in the massless case. In addition, we point out that the mass introduces a new type of undulation which is produced in black hole flows, and which is well described in the hydrodynamical regime.Comment: 37 pages, 8 figures, published versio

Earth as construction material in the circular economy context: practitioner perspectives on barriers to overcome

The need for a vast quantity of new buildings to address the increase in population and living standards is opposed to the need for tackling global warming and the decline in biodiversity. To overcome this twofold challenge, there is a need to move towards a more circular economy by widely using a combination of alternative low-carbon construction materials, alternative technologies and practices. Soils or earth were widely used by builders before World War II, as a primary resource to manufacture materials and structures of vernacular architecture. Centuries of empirical practices have led to a variety of techniques to implement earth, known as rammed earth, cob and adobe masonry among others. Earth refers to local soil with a variable composition but at least containing a small percentage of clay that would simply solidify by drying without any baking. This paper discusses why and how earth naturally embeds high-tech properties for sustainable construction. Then the potential of earth to contribute to addressing the global challenge of modern architecture and the need to re-think building practices is also explored. The current obstacles against the development of earthen architecture are examined through a survey of current earth building practitioners in Western Europe. A literature review revealed that, surprisingly, only technical barriers are being addressed by the scientific community; two-thirds of the actual barriers identified by the interviewees are not within the technical field and are almost entirely neglected in the scientific literature, which may explain why earthen architecture is still a niche market despite embodying all the attributes of the best construction material to tackle the current climate and economic crisis

Effect of carbonation on the hydro-mechanical properties of Portland cements

International audienceWe evaluate experimentally the effect of carbonation on the hydro-mechanical properties of Portland cement. Samples were carbonated at 90 °C and 28 MPa under wet supercritical CO2. Two types of carbonation features were achieved, either the samples were homogeneously carbonated or they displayed sharp carbonation fronts. Using a tri-axial apparatus, the static elastic moduli and the mechanical strength were measured at in-situ pressure conditions (28 MPa) and showed a degradation of the mechanical properties of the samples where a carbonation front prevailed. Water and gas permeabilities were measured and showed that the samples with a carbonation front exhibit a stress sensitive permeability. P and S elastic wave velocities were measured to evaluate dynamic (ultrasonic range, 1 MHz) elastic moduli. The use of an effective medium theory approach enabled us to characterize the density and distribution of cracks within the samples. This approach outlines that the samples which developed a carbonation front were damaged