Experimental study of the formation and collapse of an overhang in the lateral spread of smouldering peat fires

Smouldering combustion is the driving phenomenon of wildfires in peatlands, and is responsible for large amounts of carbon emissions and haze episodes world wide. Compared to flaming fires, smouldering is slow, low-temperature, flameless, and most persistent, yet it is poorly understood. Peat, as a typical organic soil, is a porous and charring natural fuel, thus prone to smouldering. The spread of smouldering peat fire is a multidimensional phenomenon, including two main components: in-depth vertical and surface lateral spread. In this study, we investigate the lateral spread of peat fire under various moisture and wind conditions. Visual and infrared cameras as well as a thermocouple array are used to measure the temperature profile and the spread rate. For the first time the overhang, where smouldering spreads fastest beneath the free surface, is observed in the laboratory, which helps understand the interaction between oxygen supply and heat losses. The periodic formation and collapse of overhangs is observed. The overhang thickness is found to increase with moisture and wind speed, while the spread rate decreases with moisture and increases with wind speed. A simple theoretical analysis is proposed and shows that the formation of overhang is caused by the spread rate difference between the top and lower peat layers as well as the competition between oxygen supply and heat losses

Self-ignition of natural fuels: can wildfires of carbon-rich soil start by self-heating?

Carbon-rich soils, like histosols or gelisols, cover more than 3% of the Earth's land surface, and store roughly three times more carbon than the Earth's forests. Carbon-rich soils are reactive porous materials, prone to smouldering combustion if the inert and moisture contents are low enough. An example of soil combustion happens in peatlands, where smouldering wildfires are common in both boreal and tropical regions. This work focuses on understanding soil ignition by self-heating, which is due to spontaneous exothermic reactions in the presence of oxygen under certain thermal conditions. We investigate the effect of soil inorganic content by creating under controlled conditions soil samples with inorganic content (IC) ranging from 3% to 86% of dry weight: we use sand as a surrogate of inorganic matter and peat as a surrogate of organic matter. This range is very wide and covers all IC values of known carbon-rich soils on Earth. The experimental results show that self-heating ignition in different soil types is possible, even with the 86% inorganic content, but the tendency to ignite decreases quickly with increasing IC. We report a clear increase in ambient temperature required for ignition as the IC increases. Combining results from 39 thermostatically-controlled oven experiments, totalling 401 h of heating time, with the Frank-Kamenetskii theory of ignition, the lumped chemical kinetic and thermal parameters are determined. We then use these parameters to upscale the laboratory experiments to soil layers of different thicknesses for a range of ambient temperatures ranging from 0 °C to 40 °C. The analysis predicts the critical soil layer thicknesses in nature for self-ignition at various possible environmental temperatures. For example, at 40 °C a soil layer of 3% inorganic content can be ignited through self-heating if it is thicker than 8.8 m, but at 86% IC the layer has to be 1.8 km thick, which is impossible to find in nature. We estimate that the critical IC for a ambient temperature of 40 °C and soil thickness of 50 m is 68%. Because those are extreme values of temperature and thickness, no self-heating ignition of soil can be expected above the 68% threshold of inorganic content. This is the first in-depth experimental quantification of soil self-heating and shows that indeed it is possible that wildfires are initiated by self-heating in some soil types and conditions

The exoskeleton technology as a solution to seismic adjustment of existing buildings

The high seismic vulnerability of the Italian territory and its ancient building heritage require attention regarding planning inter-ventions on existing buildings. In fact, they show both structural and technological design deficiencies mainly due to the period of construction, the lack of design to withstand horizontal forces and the type of material used, which is mainly masonry and reinforced concrete. Therefore, it is extremely difficult to intervene with solutions of seismic improvement or adjustment also concerning the economic point of view. This work suggests an advanced approach to address the problem employing the innovative concept of an external self-supporting steel system: The exoskeleton technology. It has been applied to guarantee the seismic adjustment of an existing structure that aims at reaching higher safety targets as well as new aesthetic and sustainable features. Due to the overcoming of lifespan limit of 50 years, the explored residential construction no longer complies with the current technical standards; the issue has been solved connecting the two structures by a non-dissipative rigid link to create a coupled system whose floors show an in-plane rigid behaviour while maintaining separated their response to seismic actions. Initial explanations of the internal and the outer constructions advance the dynamic analysis, which allows to highlight the main seismic properties of the whole model such as frequencies and periods of vibration, floor displacements and shear forces. Following outcomes do not just focus on how the exoskeleton can take base and floor shear forces but also the way it manages to strongly reduce displacements and deformations of the primary building, so that it can bear earthquake actions preventing not only collapse but also reducing non-structural elements damage

The exoskeleton: A solution for seismic retrofitting of existing buildings

An exoskeleton is an external steel self-supporting system rigidly linked to an existing building that need to be safeguarded against seismic actions in order to comply with the current technical standards. Its application can guarantee an innovative seismic adjustment that combines structural and safety goals with sustainable properties. The present study deals with the performances of the developed coupled system under seismic actions when a suitable exoskeleton structure is applied to a real construction. It is designed with an in-plane rigid behaviour at each floor and a non-dissipative rigid link connects the primary building to the external structure. Early descriptions of the inner and the external constructions forerun the dynamic analysis, which allows to understand seismic response of the system especially in terms of frequencies and periods of vibration, floor displacements, stiffness and shear forces. Ensuing outcomes highlight the capability the exoskeleton has in taking base and floor shear forces as well as in reducing displacements and deformations of the primary building, so that it is protected from a potential earthquake collapse

Experimental measurement of particle size effects on the self-heating ignition of biomass piles: Homogeneous samples of dust and pellets

Biomass can become an important fuel source for future power generation worldwide. However biomass piles are prone to self-heating and can lead to fire. When storing and transporting biomass, it is usually in the form of pellets which vary in diameter but are on average in the order of 7 mm. However, pellets tend to break up into smaller particles and into dust down to the µm size. For self-heating, size of particles is known to matter but the topic is poorly studied for biomass piles. This work presents an experimental study on the self-heating ignition behaviour of different particle sizes of wheat biomass. We study for the first time homogeneous samples from the dust scale to pellet diameter size, ranging from diameters of 300 µm to 6.5 mm. Experiments are done in an isothermal oven to find minimum ignition temperatures as a function of sample volume. The results are analysed using Frank-Kamenetskii theory. For the homogeneous biomass samples studied, we show that particle diameter variation does not bring a large change in self-heating ignition behaviour. The present work can be used to help quantify size effects on biomass ignition and help address the safety problems of biomass fires

An experimental set-up for cyclic loading of concrete

Innovative cementitious composite materials are drawing considerable interest due to their substantially improved mechanical properties as compared to ordinary cement-based materials. Their enhanced ductility is promising and particularly suited to structural applications under severe dynamic loading conditions. Cyclic response is essential to understand the effects of loading and unloading on the material, as well as to understanding how it behaves in the transition from tension to compression. It is also fundamental to identify its properties in terms of energy dissipation and strain-rate sensitivity. This paper presents the first part of an ongoing research project which aims to develop the constitutive relationship in innovative cementitious composites and its numerical implementation. Results from this research will facilitate the investigation of the ductility and durability of existing buildings. In this paper, an experimental set-up for uniaxial cyclic loading is described. It was developed to study reversed cyclic compression/tension loadings of innovative cementitious composites. To set the cyclic loading process, cylindrical specimens of concrete were tested. All the tests were performed on a Zwick testing machine with 50 kN load cell. The machine was customised with accessories specifically designed to meet test requirements, avoiding instability and bending moments during the alternating phases of uniaxial compression and tension. Strain gauges were used to measure lateral deformations. The customized machine has shown good performance so far. In order to test specimens with a higher number of cycles and a higher loading rate, improvements to the machine are currently under development. These tests will allow greater insight into the ductility of innovative cementitious composite materials

The use of Biochar to reduce the carbon footprint of cement-based

The organic waste management is a most current topic, because its processing and degradation it is responsible for emissions of methane and other greenhouse gases, leading to serious environmental problems. Limited oxygen thermochemical processes, such as pyrolysis or gasification, have demonstrated the energy recovery potential of the treated biomass and its environmental benefits. However, the solid part of the process -Biochar- it is considered as a waste, as only its coarse ash can be used as soil improvers. Nevertheless, several researchers have explored its potential application as green filler in order to reduce the carbon footprint both of cement production and cement-based construction materials. In this work, Biochar microparticles were used both as a filler inside the cement paste and mortar composites and as a substitute for the cement powder inside the mixes. Based on previous work, this investigation has a twofold objective: To understand the full influence of the use of an optimized percentage of Biochar (2% with respect to the weight of the cement) either as a filler in the mixture or as a substitute for cement, while guaranteeing an improvement in the strength without losing ductility. The results showed that 2 wt% of Biochar's particles are sufficient to increase the strength and toughness of the cement and mortar composites and, in place of the cement in the mixture, can maintain the mechanical properties equal to those of the reference samples

New Concepts for Next Generation of High Performance Concretes

AbstractWith the development of high-speed railway, long span bridges and high-rise buildings, new concretes need to increase strength and toughness. Adding fibers to concrete matrix has been long recognized as a way to enhance the energy absorption capacity and crack resistance of the plain concrete. In recent years, particular attention has been paid to the distribution of fibers: very small and well dispersed fibers may control the microcracks in the matrix from the very beginning of their opening and particularly high deformability of the composite may be obtained [3–5]. Carbon nanotubes (CNTs) used as reinforcing fibers has been also explored [6–8], the functional effect of their addition in a concrete equals to the one obtained with the addition of fibers. CNTs also provide a better ductility and an increase of the fracture energy. However, agglomeration and the relative high price seem to limit their application in cement based composite materials [14]. In this work, the potential beneficial effects of carbon micro/nanoparticles addition to cement pastes for improving the mechanical properties of the resulting composites has been investigated [15]. Pyrolyzed polyethylene beads (CNBs) and coconuts shells (Cocos nucifera, CCNs) were produced at Politecnico di Torino and characterized by Raman spectroscopy, thermogravimetry and scanning electron microscopy (SEM). When added to cement paste, up to 0.08 wt%, both materials were effective in increasing the cement matrix compressive strength and toughness. From SEM observations it is evident that the presence of these small particles disturb the propagation of microcracks, which has to deviate from its trajectory and has to follow the carbon nano/micro-particles contour. This mechanism increases strongly the fracture surface during the test performed by imposing the monotonic increment of crack opening. Crack and crack pinning are the mechanisms which can explain the increase of toughness in the composite samples