Papers by Author: Francesco Tondolo

Abstract: Corrosion of steel in concrete is the most detrimental source for reinforced concrete structures. Among the other effects, corrosion deteriorates the interface between steel and concrete. An axisyimmetric numerical model able to simulate the structural behaviour of bond between steel and concrete is here presented. The model is based on the thick-walled cylinder theory so that bond performance is a direct consequence of radial pressure acting on the rebar and coming from the surrounding elements and corrosion. Both elastic and inelastic contribution of concrete is also taken into account. The model is used to simulate pull-out tests present in literature. The results show the effectiveness of the model and is also compared with an empirical model already proposed in literature.

Abstract: Bond between steel and concrete plays an important role in the structural behaviour of reinforced concrete structures. In the present paper, bond tests on r.c. specimens in presence of corrosion are performed in order to evaluate the effect of corrosion on bond mechanism. The results of the tests on concrete specimens with RILEM type shape, reinforced with ribbed steel bars of 12 and 16 mm of diameter, with and without confinement, in presence of an increasing level of corrosion up to 20%, are presented. An electrochemical procedure using a current density of 200 µA/cm² is employed to produce corrosion. The test results show the effect of oxidation of the reinforcement at the interface with concrete and consequent surface cracking, the change of performance in terms of bond and the change of failure mechanism.

Abstract: The design of concrete structures exposed to environmental attack requires serious attention for concrete durability. Early age cracking due to autogenous deformations should be avoided.In this work the study of the structural effects of hydration heat and rheological behaviour of a massive concrete casting is presented. The object of the study is a skyscraper foundation slab. Aim of the work is the numerical simulation of what occurs to the structure during the hardening, in order to avoid unforeseen autogenous cracking and therefore the choice of a tailor-made concrete mixture able to fulfil the performance criteria.Non-linear finite element coupled thermal and mechanical analyses have been performed taking into account: hydration heat generation and dispersion, dimension and sequence of the casting, evolution of concrete mechanical properties in time during the hardening reaction, creep and differential shrinkage.

Abstract: Bond between steel and concrete is surely one of the most important aspects in reinforced concrete (r.c.) structures. The structural behavior in general and the formation of the resistant mechanisms in particular, for r.c. structures depends on bond. Normally bond performance has always been related to the relative rib area; in the recent years, a continuous discussion on code specifications regarding the lowering of the limits for the relative rib area (bond index) is under way. The previous values were based on a wide experimental campaign. In general the latter should guarantee a good level of bond in all the common situations. Moreover building industry increased the use of rebar straightening by means of machines used to straighten and shape rebar in coils for use on construction sites. The straightening produces a damage on the ribs that modifies their geometrical properties, essentially due to the action of the rolling line in which a permanent deformation is induced. In the present work, the experimental results of 60 pull-out tests, carried out according with the RILEM testing method, on ribbed reinforcing bars with different reduced rib profile systematically reproducing the straightening process of coil-reinforcing bar produced are shown. Three different diameters are considered: 8, 12 and 16 mm. The behavior in terms of bond strength response is also compared to the Code provisions and then discussed.

Abstract: In the present work, an experimental campaign on reinforced concrete elements subjected to simultaneous loading and degradation due to corrosion is presented. The specimens are loaded in tension and the cycling action is designed in order to simulate in terms of stress peak and stress variation, the effect of the passage of vehicles under fatigue design situations. The environmental action is simulated by means of corrosion induced with an electrochemical process; the corrosion rate is chosen in order to obtain about 27 years of equivalent carbonation attack within 25 days of test. In the mean time, 6.5·106 loading cycles are achieved. In addition to those standard tests, also other static tests were conducted in order to have a comprehensive analysis of the deterioration processes. Evolution of transversal crack due to loading and of longitudinal cracks due to corrosion is monitored. They can be related to the internal bond condition between steel and concrete. Results highlight the differences in terms of corroded and uncorroded specimens, static and cycling test and also different loading amplitude. Finally it can be observed as the growing of the damage is significantly different when a cycling action, combined with a chemical attack, is present.

Abstract: Corrosion modifies the steel-concrete interface in reinforced concrete structures. Moreover in structures subjected to cyclic load, a simultaneous mechanical deterioration due to the load is present. Both phenomena can lead to an evolution of cracks width during the service life. In order to evaluate the crack openings increase, an experimental campaign on reinforced concrete ties subjected to simultaneous loading and corrosion have been realized. Transversal crack opening, initially due to loading and longitudinal corrosion cracks evolution is monitored. Results highlight the differences in terms of corroded and uncorroded specimens, static and cycling test and also different loading amplitude. Finally it can be observed as the growing of the damage is significantly different when a cycling action, combined with a chemical attack is present.

Abstract: Bond between steel and concrete in reinforced concrete structures plays a fundamental role. The stress transfer mechanism depends on the condition of the contact surface between the two materials, the mechanical characteristics of concrete near the rebar and on the available level of confinement. Corrosion of reinforcing bars in concrete structures modifies those three factors. Because of corrosion, on the rebar surface a granular oxide layer is present and with its expansion it generates a significant radial pressure; consequently tensile stresses grow till cracking of the concrete cover with a subsequent reduction of the confinement effect. Moreover the presence of a mechanical action modifies the resisting mechanism producing an increasing damage. In this study, a model is presented for the numerical simulation of experimental tests on r.c. ties subjected to mechanical action; furthermore some considerations on reinforced concrete ties subjected also to corrosion effect are reported. From those analyses it is possible to estimate a modified bond-slip law between the reinforcing bars and the concrete, in order to take into account the level of damage.

Abstract: Corrosion modifies the steel-concrete interface in reinforced concrete structures. The efficiency of the connection between the two materials is reduced and the structural behavior both in service and in ultimate condition is affected. Moreover in structures subjected to cyclic load, a simultaneous mechanical deterioration due to the load is present. In this work an experimental analysis on reinforced concrete structures under both cyclic load and corrosion of reinforcing bars is presented. Three couples of reinforced concrete ties are connected in series and subjected to the same stress variation in order to produce the cracking conditions and to activate the bond mechanism. However, while one of the two reinforced concrete ties is only subjected to cyclic load, the second one is also corroded using an accelerated electrochemical corrosion process. The simultaneous effect of the cyclic load and corrosion is evaluated monitoring the crack opening on the structures during the test and by means of visual inspection of the sample. The test results show the correlation between the mechanism of bond and the average level of stresses for an amplified stress range.