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Comparison of carbonation depths measured on in-field exposed existing r.c. structures with predictions made using fib-Model Code 2010
No full textThe fib-Model Code for Service Life Design, which is referenced within Model Code 2010, considers different deterioration mechanisms of concrete structures. In particular, it proposes a physical model for the assessment of the carbonation depth in time, which, for existing structures, requires data, such as the type of cement and the water-to-cement ratio of the concrete, that are often unavailable. In the paper the theoretical results obtained with the fib-model are discussed and compared with experimental data obtained during an extensive campaign carried out on cast-in-place uncracked concretes of in-field exposed existing r.c. structures from a highway infrastructure. This comparison has highlighted the key role played not only by the environment, but also by the quality of the concrete through the inverse effective carbonation resistance of concrete, R^-1_NAC;0, on the evolution of the carbonation depth in time. The direct measurements of the carbonation depth on existing r.c. structures allows the inverse effective carbonation resistance of concrete to be determine d and correlated to the concrete mean compressive strength at 28 days obtained from compressive tests on cores taken from the investigated structures
Deflections due to shear
No full textIn general, deflection should be calculated taking into account both the bending and shear contributions. According to Eurocode 2 it is possible to ignore the shear contribution, when the shear contribution is smaller than 10% of the bending contribution. Model Code 2020 does not give any information. As shown in the literature, this limit can be overcome not only in case of deep beams, but also in case of beams whose span-to-depth ratio is smaller than 12. However, shear deformation in cracked sections is often neglected because there are no suitable models for everyday applications. In this section, in order to study shear deformation of RC beams with thin web, an experimental campaign was carried out at the Politecnico di Torino on seven RC beams with the same transversal I-section 600 mm high and 400 mm wide, width-to-web thickness ratio of 4 and span-to-depth ratio of about 12, reinforced with two reinforcement ratios (0,5% and 0,8%). Three testing schemes were considered, that means: simply supported beams, subjected to two symmetrical loads; simply supported beams, subjected to one symmetrical load; simply supported beams, subjected to one anti-symmetrical load. From the experimental observations it results that shear does not produce only a transversal deformation, as assumed in Saint-Venant theory, taken into account through the determination of the shear strain. Nevertheless, in a cracked RC beam, shear produces also longitudinal deformations of the tension and compression chords due to the truss behaviour of cracked RC beams. From this effect on the deformations of these chords, it also results the influence of shear on the mean curvature. On the basis of the experimental observations, three calculation methods are proposed, for the analysis of the effect of shear on the deformation of RC beam, presenting via via an higher level of approximation, that means refined method, simplified method, graphical method. Finally, on the basis of the above graphical method, when a greater precision is not required, in case of RC beams subjected to uniform load, a simplified application rule is proposed for a quick check of cases where the shear contribution can be ignored
Experimental analysis of the effective pre-stress in large-span bridge box girders after 40 years of service life
No full textThe rheological properties of concrete and pre-stressing steel have a significant influence on the deformation and pre-stress losses of p.c. members and thus affect their long-term deflection and service behaviour. Long-span concrete bridge girders can be particularly sensitive to pre-stress losses as the dead load is large compared to the total load. Some observations of excessive long-term deflections have been made of long-span pre-stressed concrete box girder bridges over the last few years. This has led some researchers to question the reliability of the available theoretical models to provide correct long-term predictions. The main results of experimental tests conducted on 9 concrete bridge box-girders with a span of 35. m are presented in this work. The girders, which were part of an Italian motorway bridge dismantled after 40. years of service life, were tested up to failure with a specially built test frame. The mean deformation measured over five zones of each beam allowed the effective stress in the pre-stressing tendons to be evaluated for a total of 45 zones. Finally, the experimental data have been compared with theoretical predictions obtained by applying both the rules of an international standard (Eurocode 2) and a refined analytical model that takes into account the presence of two types of concrete with different properties as well as the various stages of the construction and pre-stressin
Strength, stiffness and ductility of r.c. beams strengthened with FRP sheets
No full textIn the last decades, the rehabilitation of civil structures has become increasingly an important challenge in the construction market. Among the various methods used to strengthen and repair r.c. beams, wet lay-up systems of fibre reinforced plastic (FRP) sheets represents an effective and convenient method. The advantages of bonding FRP sheets to the concrete derive from the light weight due to low density, the high mechanical characteristics in terms of strength and stiffness, the high resistance to corrosion and chemicals and the easiness to adapt to the shape of the structural member. Experimental and theoretical investigations show that good improvements in the strength and stiffness of r.c. beams can be obtained, although in the presence of a limited ductile behaviour. This is also connected to the failure mode of the beams. In this context, the paper presents the main results of an experimental campaign aimed at studying the behaviour, in service and at collapse, of r.c. beams strengthened with mono-directional carbon fibre reinforced polymer (C-FRP) sheet
Modelling of the cracking behaviour of prestressed concrete structures
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