Issue 51

D. Falliano et alii, Frattura ed Integrità Strutturale, 51 (2020) 189-198; DOI: 10.3221/IGF-ESIS.51.15 190 I NTRODUCTION n the field of lightweight concretes, one of the most promising materials that most effectively achieves thermal insulation, acoustic absorption and fire resistance is foamed concrete [1]-[4]. The ingredients of foamed concrete are water, cement, fine sand, additives (if any) and preformed foam, the latter providing a porous microstructure that is responsible for the lightweight characteristics. The density range that can be obtained with foamed concrete is very wide, from 300 kg/m 3 to 2000 kg/m 3 . In particular, foamed concretes with densities up to 1000 kg/m 3 are primarily used to exploit their insulating properties for non-structural applications, while structural elements can be realized with densities exceeding 1400 kg/m 3 . Similar to other cementitious materials, in the low-density range foamed concrete has poor mechanical strength. For this reason, different strategies have been proposed in the relevant literature to improve the mechanical strength of foamed concrete at low densities and to limit its brittle behavior, such as addition of fly ash and silica fume [5], introduction of short fibers of different nature [6]-[9] or placement of bi-directional grid reinforcement [10], [11]. Regardless of the reinforcement adopted, the mechanical characteristics of such lightweight cementitious materials is affected by the mix design (water-to-cement ratio, quality and characteristics of the aggregates, etc.) as well as by the quality of the preformed foam and by the nature of the foaming agent used for its generation [12]-[14]. On the other hand, studies dealing with the fracture behavior of foamed concrete are relatively few [15]-[17], and confined to classical foamed concrete. Previous studies by the authors demonstrated that the fracture behavior of foamed concrete is markedly affected by the curing conditions [18], [19], especially at the lower densities. Another interesting aspect concerns the development of enhanced variant of lightweight concretes that incorporate by-products or slags such as electric arc furnace slag [20]-[22] or foundry slag [23], which has motivated the present research work. This paper aims to extend the knowledge on the mechanical characteristics and the fracture behavior of foamed concrete when slags are introduced in the mix design as micro-aggregates. In particular, the foamed concrete variant considered in this contribution is prepared with the addition of biochar, which is the solid waste of the thermochemical conversion of biomass. The choice of the use of biochar derives from previous studies that have shown significant increases in terms of both flexural strength and fracture energy of traditional cement-based composite materials [24]. Moreover, the foamed concrete of this experimental campaign is prepared with a viscosity enhancing agent (VEA) such that the resulting mix has high cohesion and viscosity at the fresh state (green strength), and is termed “extrudable foamed concrete” [2], [10], [25]. As a first step of this experimental campaign, standardized prismatic specimens were prepared and tested according to UNI EN 196-1, in order to determine the optimal process to introduce the biochar particles into the foamed concrete matrix (among three possibilities: mixed with cement, dispersed into the hydration water or dispersed in the pre-formed foam). Once the optimal preparation technique has been established, prismatic notched specimens were prepared and tested according to JCI-S-001 standards, namely three-point bending tests in CMOD (Crack Mouth Opening Displacement) mode, in order to evaluate the fracture energy. These notched specimens, tested at 28 days, were prepared with dry density of 1600 kg/m 3 , and different contents of biochar (as percentage with respect to the cement weight) in addition to reference specimens without biochar, and two different curing conditions (water and air). Comparison between specimens with biochar additions against reference specimens has been performed in terms of flexural and compressive strength values, load-CMOD curves, fracture energy and related ductility. S PECIMEN PREPARATION AND TESTING CONDITIONS he first step of the experimental campaign is focused on the preparation procedure in relationship to the biochar addition. Indeed, three different introduction modalities are evaluated. In the first modality, the biochar is directly added in the cement at the dry state. After a proper mixing of the powders, water and VEA are added. Once a homogeneous paste is obtained, the preformed foam is finally added in order to achieve a target dry density of 1600 kg/m 3 . In the second modality, the biochar is added after the mixing of cement, VEA and water. After obtaining a homogeneous mix including the biochar, the pre-formed foam is finally added as in the first modality. In the third modality, the biochar is mixed with the foam. After generating the pre-formed foam, the biochar is added in the foam and the micro-aggregates are dispersed through a vertical mixer. At this stage, the foam with biochar and the cementitious paste are mixed together. The comparison between the three preparation modalities is made in terms of stability of the mix and density at the fresh state. It has been found that the first two modalities exhibit no significant differences, whereas in the third modality (biochar added in the foam) percentages of biochar ൒ 0.5% of cement weight lead to an instability of the foam with coalescence phenomena and a resulting material having densities higher than the target dry density (hence higher than the density I T