Key Engineering Materials Vol. 919

Abstract: The present work addresses the powder bed binder jet 3D printing as an additive manufacturing process for cement-based materials in the constructions industry. Features are created through the interaction among the droplets of the liquid binding agent and the layered powder bed. The printhead movement over the powder bed at a given feed rate forms voxels and single-lines from the coalesce of successive droplets and adjacent lines are consolidated to create the designed cross-section. Here, statistical models have been developed to study the effect of printing parameters (aggregate particle size, feed rate, velocity of powder spread, pressure of the fluid and nozzle diameter) on the resultant dimension of a single printed line, using a factorial design of experiment. The hardware of the 3D printer, the physical properties of the powder blend and binder are initial constraints for designing voxels. Linear regression models of significant parameters are presented. Pressure is one of the most significant factors, it has a profound effect on the granule formation mechanism. Cubic samples printed with higher pressure level are characterized by higher residual porosities from crater channels during the printing process. The results demonstrate a fundamental understanding of the binder–powder interaction for cementitious materials which can be leveraged to determine the minimum printable feature with required dimensional accuracy, based on the chosen process parameters.

Abstract: Nature-based solutions are sustainable building materials produced recovering and enhancing agricultural biomasses which are by-products or waste of crops as, for example, rice, flax or hemp. Specifically, this research investigates the properties of hempcrete which is produced mixing lime, which acts as binder, and hemp shives, as vegetal aggregate. Hempcrete is characterized by breathability and excellent insulating properties, moreover it is a sustainable material due to the introduction of vegetal material and due to the carbonation of lime which gives further carbon dioxide sequestration. The mechanical properties of the material are largely variable and, in this research, triaxial tests have been performed to evaluate this experimental methodology as a technique applicable to evaluate the mechanical behavior of this material. The tests have been performed on samples produced with the same mix design developed by an Italian manufacturer for the production of prefabricated hempcrete blocks. These building components are used as non-loadbearing blocks, they are introduced in building envelopes or in indoor partition walls as insulating elements.

Abstract: The increasing rate of fire disaster especially in the developing countries has renewed the demand for utilization of more economically sustainable materials for built environment. In this paper, the effect of calcined Ebonyi shale (CES) incorporated as partial replacement (15%) of cement on the thermo-mechanical properties of high strength concrete were investigated. The preparation of the CES was carried out by calcining the Ebonyi shale at a temperature of 900 °C. Both raw Ebonyi shale (RES) and calcined Ebonyi shale (CES) were analyzed using scanning electron microscope (SEM) and X-ray fluorescence (XRF). After curing time of 28days, several samples were exposed to varying temperatures. A comparison of the results showed that incorporation of CES enhanced high strength properties of concrete at elevated temperature. Consequently, economical and eco-friendly mixes that reduces CO₂ emissions of the overall cement production of clinker were achieved.

Abstract: The present contribution reports the results of the experimental investigation on a sustainable lightweight cementitious composite material, in which virgin sand is partially substituted by a very fine composite powder retrieved from the manufacturing process of kitchen and sanitary ware. In order to obtain lightweight structural concrete suitable for screeds, the mixture is completed by the addition of recycled polyethylene terephthalate aggregates and lightweight glass spherules. First, physical and morphological properties of the raw powder are thoroughly investigated. Then, the mechanical response of the composite is assessed by means of three-point bending tests and uni-axial compression tests. The conglomerates exhibit promising mechanical performance, thus being regarded as possible candidate for innovative and sustainable structural applications.

Abstract: This study presents a simple one-dimensional analytical model describing the pull-out process of an elastic fibre embedded in a cement matrix, which captures the ductile behaviour of Fibre Reinforced Concrete (FRC) elements.The shear stress arising at the frictional interface between fibre and matrix during the pull-out is assumed to increase with the slippage distance, as a consequence of the growing abrasion of the fibre surface.The equilibrium conditions between the external axial load and the interfacial shear stress are imposed with reference to the undeformed configuration.The model is validated through comparison with both experimental data obtained by testing partially recycled polymeric fibres embedded in a cement matrix, and several datasets available in the literature comprising polypropylene fibres with and without silica coatings.The proposed model can properly describe the response of synthetic fibres that exhibit considerable axial elongation and slip-hardening interface behaviour.However, it may also predict the non-linear relation between the tensile load and the fibre displacement for different kinds of fibre, by setting adequately the constitutive parameters.

Abstract: The innovative sustainable technology based on natural fabric-reinforced cementitious matrix (NFRCM) is analyzed for strengthening masonry. A new frontier for composite materials is proposed as an alternative to well-known traditional technologies used to improve the seismic behavior of buildings, such as the portuguese technique ‘gaiola pombalina’, the Italian ‘baraccata house’ and the turkish ‘himis house’. Preliminary sensitivity analysis is performed on NFRCM and ‘baraccata’ numerical models. Both technologies are numerically compared. From the experimental results of in-plane incremental load test carried out by CNR-Ivalsa a numerical model is calibrated by non-linear pushover analysis to evaluate the behavior of masonry wall strengthened with natural fibers. This paper demonstrates the effectiveness of NFRCM systems for strengthening masonry as: i) a non-invasive solution without significant thickness; ii) a sustainable technology; iii) an intervention involving the whole wall surface avoiding failure local mechanisms.

Abstract: Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly used to strengthen existing concrete and masonry structures in the last decade. Two guidelines are available for the design and construction of FRCM strengthened members: ACI 549.4R (2013) and CNR-DT 215 (2018). Both these guidelines employ the effective strain, i.e. the strain at which the composite action is lost, as key parameter for the evaluation of the capacity of FRCM strengthened members. The American guideline ACI 549.4R (2013) employs the results of clevis-grip tensile tests on FRCM coupons to determine the composite effective strain. Such strain is determined by the Italian guideline CNR-DT 215 (2018) combining the results of direct shear tests on FRCM-substrate joints and of tensile test of bare fiber textile. The effective strain is strictly related to the matrix-fiber bond behavior, which can be expressed by the interface shear stress-slip relationship, i.e. the cohesive material law (CML). The effective strain is not sufficient for a full understanding of the structural response of strengthened members, since the knowledge of the CML is needed to predict important parameters such as the crack pattern or the location where debonding occurs in beams strengthened in flexure. This paper provides a simple procedure to obtain the CML from the load response obtained by direct shear tests of FRCM-substrate joints. The procedure is discussed and applied to the case of poliparaphenilene benzobisoxazole (PBO) FRCM-concrete joints previously tested by the authors.

Abstract: Old and seismically prone buildings are in need of strengthening in order to comply with the latest building codes and to prolong their service life. For over two decades fiber-reinforced polymers (FRP) have been successfully used for this purpose. However, the poor performance in high temperatures of organic matrices has led researchers to investigate the use of inorganic matrices. Consequently, textile-reinforced mortars (TRM) have been opted for strengthening, since they incorporate textiles impregnated in inorganic cementitious matrices. Lately, in order to promote sustainability and lower the high carbon emissions of cement, alkali-activated mortars, also called geopolymers, have been investigated as an alternative. Their high performance and fireproof properties have made them excellent candidates as matrices in advanced composites for strengthening. This study aims to provide an overview of research in the field of advanced composites with alkali-activated matrices used for strengthening of concrete members. Systems implementing either fiber sheets or meshes have been used so far to strengthen reinforced concrete members, indicating promising results of the new advanced composite.

Abstract: Mortar-based composites are an emerging technology for the repair and strengthening of reinforced concrete and masonry structures. In most cases, the effectiveness of the retrofitting work relies on the substrate-to-composite bond capacity but in some applications, connectors are also used to prevent debonding and improve the performance of the retrofitted structure. Indeed, the use of connectors is recommended by design guidelines and suppliers are required to test them for acceptance. The paper presents a laboratory investigation on steel reinforced grout connectors, made by rolling ultra-high tensile strength steel textiles, comprising either galvanized or stainless-steel micro cords. Tensile tests were first carried out for mechanical characterization. Pull-out tests were then performed on connectors installed in holes drilled in wall panels and injected with either cement or lime mortars. Concrete, tuff masonry, brickwork and limestone masonry were used as substrate materials. Test results are commented to analyze the effect of textile rolling on tensile response, of textile and matrix properties on pull-out strength and failure mode, as well as to highlight their significance for design purposes.