1,721,010 research outputs found
Estimation of Steel Rebar Strength in Existing Concrete Bridges
No full textTo design a retrofit and/or maintenance protocol for existing reinforced concrete bridges, the assessment of rebar steel strength is generally required. The current methodology consists of uniaxial tensile tests performed on bar segments extracted from a structural element. Nevertheless, in several situations (e.g., the assessment of bridges in service), this traditional method cannot be used. Hence, a new simplified approach is introduced herein. It consists of the so-called “strength-for-age curves,” which relate the average strength of steel to the year of construction. Such curves are statistically computed from a database stored in the Department of Structural and Geotechnical Engineering of Politecnico di Torino (Italy). As a result, the yield and tensile strength values experimentally measured from rebar in two existing bridges in Northern Italy, built in 1930 and 1975, respectively, are correctly predicted by the proposed model
Lo stato di salute dei ponti in c.a. - da valutare con la massima “confidenza” anche in assenza di indagini per la caratterizzazione meccanica dei materiali
No full textResidual crack width in RC and R/FRC ties subjected to repeated loads
No full textDurability of reinforced concrete (RC) structure is mostly related to the ability of concrete cover to protect the embedded rebar from corrosion initiation and propagation. As cracks due to loads or rheological phenomena are almost inevitable, the geometry of crack pattern in service is a key parameter that needs to be evaluated in plain and fiber-reinforced R/FRC members. In fact, not only the direct ingress of aggressive agents, such as oxygen and water, is a function of crack width, but also concrete carbonation and the chloride ion ingress are accelerated by the presence of wide cracks. Furthermore, the use of fiber reinforced concrete requires detailed investigations, in order to define the relationship between durability and crack width even in presence of cyclic loads. Accordingly, in this research project, crack width is measured by using traditional mechanical strain gauges and a new device based on the optical conoscopic holography. The latter allows the non-contact measure of crack profile, at the end of each loading cycle, both in plain and fiber-reinforced ties subjected to sets of repeated loads. As a result, contrarily to crack width at the peak of load, the width of residual cracks is not always reduced by the presence of fiber, and this could affect the durability of RC and R/FRC structures
A numerical displacement-based approach for the structural analysis of cable nets
Full text linkA new displacement-based approach is proposed herein to predict the behaviour of pre-tensioned cable nets subjected to vertical loads. The cables are contained in horizontal plane and have a single degree of freedom in each node, where loads are applied. The model is based on the equilibrium equations of an infinitesi mal cable-element, which are solved by considering the catenary equation under the hypothesis of (a) zero bending stiffness, (b) linear elastic behaviour of materials, (c) small deformation, and (d) the existence of perfect hinges in each node. More precisely, a finite-difference numerical procedure is introduced in order to evaluate nodal displacements related to a set of applied vertical forces. The effectiveness of the proposed approach is then assessed by comparing the numerical results with those obtained by other models found in the current literature. Finally, the proposed approach is used to design new and more efficient insulating panels for the green house technology
Mechanical performances of mortar prisms and concrete slabs incorporating rubber aggregates
No full textTo reduce the environmental impact of cement-based composites used by the construction industry, stone aggregates can be substituted by unconventional aggregates, such as the rubber from end-of-life tyres. This substitution strategy can be applied to both mortars and structural concretes. Nevertheless, each application requires a specific particle size distribution, to the point that the maximum aggregate size of concrete can be 10 times larger than that of mortar. Consequently, the mechanical response under bending actions, ruled by the fracture energy, can drastically change with the structural dimension. This size-effect is particularly evident in the experimental analyses performed herein, which include three point bending tests on mortar prisms and concrete slabs. As a result, the fracture toughness always increases in presence of rubber, and in the case of large particle size the increment of ductility can lead to the increment of the flexural strength
Mechanical properties of concrete made with fluff
No full textA detailed investigation on the use of automotive shredder residues, the so-called fluff, as an alternative aggregate of structural lightweight concrete, is the subject of the present paper. Specifically, a new granulated fluff, obtained through a granulation process already used to treat returned concrete, substitutes the traditional gravel made with expanded clay. Slump values are measured with the slump cone test on fresh concrete, whereas the depth of penetration of water under pressure, and the uniaxial compressive tests as well, are performed on hardened concrete cylinders. As a result, a new parameter, herein called “inconsistency parameter”, is introduced and used to define both the mechanical properties (i.e., the strength and ductility) and the workability of the lightweight concretes made with virgin or plastic waste aggregates. According to the required structural performances in service, the optimal value of the inconsistency parameter can be defined as a function of both the water/cement ratio and the content of the granulated fluff
Ecological and mechanical performances of ultra-high- performance fiber-reinforced cementitious composite containing fly ash
No full textAn experimental campaign, performed on different types of ultra-high-performance fiber-reinforced cementitious composite (UHP-FRCC)—made with four replacement rates (0, 20, 50, and 70%) of cement with fly ash and cured for 1, 4, and 13 weeks—is described in this paper. Specifically, 72 cylinders were tested to measure the compressive strength and Young’s modulus of elasticity; stress-strain relationships were obtained from 72 dumbbell-type specimens subjected to uniaxial tension, and 12 beams, tested in four-point bending, provided the moment-curvature diagrams. The best UHP-FRCC was selected through an eco-mechanical analysis, capable of combining the mechanical performance with the environmental impact of concrete. When the ultimate bending moment of a beam is the functional unit of this analysis, the higher the replacement rate of cement, the better the beam performance, although material properties and structural ductility show opposite trends
Influence of Portland cement alkalinity on wool-reinforced mortar
No full textNatural wool is a good insulating material, both thermally and acoustically. With the increase in demand for the usage of waste materials, other applications have been found, such as the use of wool as a fibre reinforcement in mortars and concretes. Unfortunately, wool, like other natural organic materials, dissolves in alkaline environments and, consequently, the performance of wool reinforcement cannot be guaranteed for a long time. To address this issue, three series of wool-reinforced mortar beams, with various contents of alkalis in cement, were investigated. The chemical compatibility and the effects of alkalinity on mechanical performance were investigated by testing the beams under three-point bending and, subsequently, by analysing the microstructure of the mortars using energy-dispersive X-ray spectroscopy. The results showed that the lower the alkalinity of the cement paste, the better the resistance of wool fibres in the cementitious matrix, thus guaranteeing larger post-cracking residual stresses in the wool-reinforced mortars
The use of wool as fiber-reinforcement in cement-based mortar
No full textFiber-reinforced cementitious mortars are widely used in the construction industry. Indeed, the fracture toughness in tension increases with the volume and the aspect ratio (i.e., the ratio between length and diameter) of the fibers, which are generally made with polymeric (e.g., polyethylene, polyvinylchloride, etc.) or inorganic (e.g., glass, carbon, etc.) materials, or with steel. Also vegetal fibers, such as bamboo and hemp, have been used in the last decades to reinforce mortars. Besides, with the aim of introducing animal fibers, the use of wool as fiber-reinforcement is investigated for the first time in the present paper. According to UNI EN 196-1-2006, three point bending tests have been performed on small beams made, respectively, with plain mortar, and mortar reinforced with 1% in volume of wool. To compare the performances with mortars containing vegetal fibers, also beams reinforced with hemp have been tested. In some tests, wool and hemp are previously treated with atmospheric plasma in order to modify the nano-metric properties of the fiber surface. As a result, both the flexural strength and the ductility increase when wool, treated or not, is added to cementitious mortars. Similarly to hemp, wool does improve the mechanical and ecological performances of the mortars, and creates a link between textile and construction markets
The carbon footprint of normal and high-strength concrete used in low-rise and high-rise buildings
Full text linkTo reduce the mass of CO2 released into atmosphere by the construction industry, the performance strategy can be adopted. It is based on the use of High-Strength Concrete (HSC) in alternative to Normal-Strength Concrete (NSC). Such concretes are herein considered to design the reinforced concrete structures of three buildings, having 14, 30 and 60 floors, respectively. For each building, the structural analyses, carried out for four classes of concrete (i.e., C25, C40, C60 and C80) in accordance with Eurocode 2, provides different dimensions of the structural elements. In other words, the amount of CO2, released in the atmosphere due to the production of the structural materials, is a function of both concrete strength and height of the building. As a result, the minimum impact of low- rise buildings occurs when the structural elements are made with NSC. Conversely, only when HSC is used to cast the structural elements of tall buildings, can the carbon footprint be effectively reduced
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