1,720,980 research outputs found
A simple optimized foam generator and a study on peculiar aspects concerning foams and foamed concrete
Full text linkThis paper presents a study on peculiar aspects influencing foams and foamed concrete properties, starting from the foam generation up to the compressive strength of the lightweight and ultra-lightweight cementitious material. In particular, after a brief introduction on foam stability, this research work shows a simple and inexpensive foam generator used to produce the commonly used foams in concrete. The significant influence of the air pressure value, of nature and concentration of the foaming agents on density as well as the percentage drainages of the foams produced are therefore discussed. The results show that foams generated with the protein foaming agent have more suitable characteristics to produce foamed concrete, thanks to the significantly longer lifetime compared to foams produced with the synthetic foaming agent. The latter are characterized by very high drainage values even after a few minutes from their generation. Foams are then used to make lightweight (target dry density equal to 600 kg/m3 and 800 kg/m3) and ultra-lightweight (target dry density of 400 kg/m3) foamed concretes that show interesting results in terms of stability also when foams with high drainages are employed. The study provides explications of the differences between the compressive strength of lightweight foamed concrete obtained with foams generated using protein and synthetic foaming agents. Then, the significant influence of the increase in concentration of protein foaming agent on the compressive strength of ultra-lightweight foamed concretes is presented
Improving the flexural capacity of extrudable foamed concrete with glass-fiber bi-directional grid reinforcement: An experimental study
Full text linkA wide experimental campaign of 40 three-point-bending tests on small-scale foamed concrete beams in the low-to-medium density range (400–800 kg/m3) is presented. The considered “extrudable foamed concrete” can keep its dimensional stability at fresh state. In order to increase the mechanical characteristics while preserving lightweight properties associated with the low densities, bi-directional grids of glass fibers are placed close to the bottom external face of the beams. Additionally, the use of short polymer fibers embedded within the cementitious matrix is investigated. Different specimen characteristics are explored, including: three curing conditions, three target dry densities, and two fiber contents. The presence of bi-directional grids increases the bending strength values for all the examined conditions: for 400 kg/m3 the increase is up to 1700%, and for higher densities is, on average, 175%. The curing condition affects the failure mode: specimens cured in water with densities higher than 600 kg/m3 exhibit a premature failure of the grid reinforcement without separation from the concrete substrate, whereas in those cured in air and cellophane typical bond failures with detachment occur. Only for densities of 800 kg/m3 the further addition of short polymer fibers produces non-negligible flexural strength increase of 31%, on average
3D-printable lightweight foamed concrete and comparison with classical foamed concrete in terms of fresh state properties and mechanical strength
No full textThis paper presents a novel type of foamed concrete that is termed “3D-printable lightweight foamed concrete” (3DP-LWFC). Unlike classical lightweight foamed concrete (C-LWFC), this novel material is able to keep its shape at the fresh state due to enhanced consistency and viscosity. This peculiarity lends itself to being implemented in automated extrusion production process and 3D printing applications without the use of formwork, which is particularly convenient in the building industry. These unique fresh state properties of 3DP-LWFC are demonstrated through a specific extrusion test conceived and used in this experimental campaign, and highlighted by comparison with results related to C-LWFC. Despite the remarkably different behavior of the novel material at the fresh state, the mechanical strength of 3DP-LWFC is even slightly higher than C-LWFC. This is demonstrated through a wide experimental campaign focused on the compressive and flexural strength of 3DP-LWFC, which includes different dry densities, curing conditions, cement types, water/cement ratios. Additionally, the effect of the mixing conditions on the mechanical strength of 3DP-LWFC, in particular the rotational speed of the mixer during the preparation of the paste, is also analyzed and discussed. It is found that the increase of mixing intensity from 1200 rpm to 3000 rpm resulted in a considerable increase of mechanical strength values of 3DP-LWFC, up to more than 70% for the compressive strength and up to around 100% for the flexural strength
Structural foamed concrete: preliminary studies for applications in seismic areas
Full text linkThe experimental research presented in this contribution highlights the possibility of producing foamed concretes with target dry
densities of 1550±50 kg/m3 and 1750±50 kg/m3 for the use in structural applications, thanks to compressive strengths greater than
25 MPa. The lower structural weight compared with ordinary concrete suggests the idea of using this material in seismic areas to
exploit its advantages in relation to inertial forces. However, the reduced elastic modulus compared with ordinary concrete of equal
compressive strength must be considered. In addition to demonstrating the beneficial effect of reducing the maximum diameter of
the fine sand used to produce the foamed concrete, this contribution also shows how the behavior of a reinforced concrete frame
changes (increase in the main vibration mode and decrease of the maximum shear at the base of the frame) if the foamed concrete
presented in this study is used instead of ordinary concrete of equal compressive strength
Key factors affecting the compressive strength of foamed concrete
Full text linkThis contribution aims to highlight, from an experimental point of view, the key factors affecting the compressive strength of foamed concrete. An experimental campaign has been conducted on a broad group of cubic specimens made of foamed concrete under compression tests at 28 days. In addition to the obvious influence of the density on the achievement of the compressive strength, other factors have been studied. In particular, three different curing conditions, three foaming agents with either synthetic or protein nature, two different cement types, and three water/cement ratios have been included in this experimental investigation. As a result of this experimental campaign, it has been found that the not only the density, but also the foaming agent and the water/cement ratio play a major role in the strength achievement of the foamed concrete. It is also demonstrated that the combination of the foaming agent with a particular water/cement ratio is a crucial parameter affecting the compressive strength of this material
Strategies to increase the compressive strength of ultra-lightweight foamed concrete
Full text linkFoamed concrete is a special lightweight concrete characterized by the presence of a stable preformed foam in the mix proportion that give rise to the development of a system of air voids in the material. Its physical and mechanical properties are strongly influenced by microstructural properties, in turn linked to various parameters such as the amount of foam, the presence of mineral additions or chemical additives, the characteristics of the mixing process and so on. Since ultra-lightweight foamed concrete is characterized by excellent functional properties (thermal insulation, sound absorption, fire resistance) but very poor mechanical properties (compressive strength), in this contribution three different ways to improve the compressive strength of this material without worsening its lightness are discussed. More specifically, the three different strategies (improve the consistency of the fresh lightweight cementitious paste through the addition of a viscosity enhancing agent, add silica fume and increase the rotational speed of the vertical mixer) lead to appreciable improvement in the compressive strength of the produced foamed concrete. The most significant increases in compressive strength are associated with the addition of silica fume in the mix, in proportion to 10% of the cement weight
Compressive and flexural strength of fiber-reinforced foamed concrete: Effect of fiber content, curing conditions and dry density
No full textAn extensive experimental campaign was carried out to investigate the mechanical strength of fiber-reinforced lightweight foamed concrete. The considered foamed concrete was prepared with a viscosity enhancing agent that increases the cohesion and consistency of the cement paste at the fresh state (extrudable foamed concrete). The flexural strength was evaluated on almost 60 small-scale prismatic beam specimens, and the compressive strength on 100 cubic specimens in accordance with two different testing standards for comparative purposes. The effects of three different curing conditions (air, cellophane and water), three target dry densities (400, 600, 800 kg/m3), three fiber contents (0.7%, 2.0%, 5.0%), and the presence of an additional glass-fiber-reinforced-polymer (GFRP) mesh in the tensile zone of the beams (besides the short fibers) were analyzed. The polymer fibers increased the flexural capacity of the beams, especially for the low-density specimens and for the higher contents of fibers (2.0% and, above all, 5.0%), but had a negligible influence on the compressive strength. The additional GFRP mesh combined with the short fibers improved the flexural capacity considerably in all the examined conditions, turning out to be the best strategy for obtaining high mechanical strengths associated with low densities typical of ultra-lightweight concrete elements
Confinement effect of different arrangements of transverse reinforcement on axially loaded concrete columns: An experimental study
Full text linkConcrete behaves as a brittle material due to its low inherent tensile strength, but it can also exhibit a markedly ductile behavior when coupled with transverse reinforcement like steel stirrups. The role of stirrups is to enhance confinement effect, to restrain the lateral expansion of concrete, thus modifying the concrete stress-strain constitutive law and enabling higher compression strains and higher ductility. This paper focuses on the confinement effect induced by different arrangements of transverse reinforcement on axially loaded concrete columns. An experimental campaign has been carried out, comprising 18 concrete columns with two different mechanical strengths and reinforced with three different layouts of stirrups, namely typical closed square hoops, closed stirrups with additional cross ties, and a novel type of stirrups involving rectangular hoops with additional restraint plates, the latter offering an enhanced diffused confinement action and limiting extensive spalling of the cover concrete. Formation and propagation of longitudinal micro-cracks are reduced with the novel type of diffused stirrups and a moderate-to-high increase of ductility is observed. However, the beneficial effects induced by diffused stirrups are more pronounced in medium-strength concrete and almost negligible in low-strength concrete that collapses due to a brittle cracking failure without involving the confinement action of the transverse reinforcement
Investigation on the Rheological Behavior of Lightweight Foamed Concrete for 3D Printing Applications
No full textUnlike ordinary concrete, lightweight foamed concrete (LWFC) has the benefit of decreasing the self-weight of constructive elements while ensuring an efficient thermal insulation and acoustic absorption as well as high fire resistance. A novel version of LWFC has been recently developed by the authors, with the unique property of “extrudability” in a wide density range, meaning that its production process can be carried out without formworks and exploiting innovative 3D printing technologies. The present contribution is focused on the rheological behaviour of this innovative extrudable LWFC (ELWFC). The rheological behaviour in terms of yield stress of the ELWFC is studied via a rotational rheometer in two different modalities, namely constant shear rate and increasing shear rate. Comparison of the rheological behaviour between ELWFC and classical LWFC is also presented. Additionally, the dimensional stability of the cementitious paste at the fresh state having a given yield stress is assessed through an extrusion test. In particular, the experimental investigation is focused on a target dry density of 800 kg/m3, which is identified as a good compromise between insulating features and mechanical strengths. The experimental results show that the considered ELWFC, characterized by a zero slump in the extrusion test, has a yield stress of around 150 Pa (constant shear rate) and 130 Pa (increasing shear rate)
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