Chapter 8: Conclusions
8-1
8.
CONCLUSIONS
8.1
General Conclusions
In the present work, a detailed analysis of the effective ductile behaviour of steel reinforcing bars subjected to the combined effects of seismic action (low-cycle fatigue condition) and corrosion phenomena was carried out; the work was developed inside the framework of a European research project funded by the Research Fund for Coal and Steel, called Rusteel (Effects Of Corrosion On
Low-Cycle Fatigue (Seismic) Behaviour of High Strength Steel Reinforcing Bars, 2009).
Experimental monotonic tensile and Low-Cycle Fatigue (LCF) tests were executed on a set of steel reinforcing bars representative of the actual European scenario of reinforcements’ production, following respectively the prescriptions imposed by European standard EN 15630-1:2010 and a protocol opportunely elaborated in order to evaluate the influence of different parameters (i.e. the free length of the specimen, the testing frequency, the diameter and, above all, the level of imposed deformation) on the ductile behaviour of steel rebars. The experimental tests took also into account the differences due to various production processes: TempCore, Micro Alloyed, Cold Worked and Stretched samples were considered.
LCF tests generally evidenced buckling phenomena in steel reinforcements in correspondence of the first/second cycle in compression, leading to lower values of the maximum strenght in tension or in compression respect to the ones obtained from monotonic tests. The dissipative capacity of steel reinforcements usually decreased with the increase of the imposed deformation and with the increase of the free length: for the same level of applied strain, specimens tested considering a free length equal to the spacing stirrup in low ductility class (L0=8φ) evidenced a lower dissipation of seismic energy and a more evident degradation of the samples.
As regards the influence of production process on the experimental tests’ results, the higher ductile capacity of Micro Alloyed steel respect to TempCore ones, both in terms of elongation to maximum load coming from monotonic tensile tests and in terms of dissipative capacity, evaluated as total density of dissipated energy under following tension/compression cycles, was evidenced. As an example, considering the results coming from steel rebars B400C (diameter 16 mm), the values of Agt
obtained from tensile tests were equal to 18% and 15% respectively in the case of TempCore and Micro Alloyed bars; moreover, looking at the cyclic dissipative capacity, the total dissipated energy for imposed deformation equal to ±2.5% and free length six diameters was respectively equal to 386 and 430 MPa in the case of TempCore and Micro Alloyed process.
The effective levels of deformations induced by seismic events on rebars were evaluated through the execution of numerical analyses on a set of representative r.c. case studies opportunely designed according to the actual European and Italian prescriptions (EN 1998-1:2005, D.M.14/01/2008). A new mechanical model for steel reinforcing bars embedded in the concrete, including relative slips between rebars and concrete and able to fully characterized their effective behaviour under cyclic loading actions, was elaborated.
Chapter 8: Conclusions
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The modified hardening slip model, starting from the model already presented by Braga et al. (2004), was based on simply assumptions about the slip field, the bond stress-slip relationship and the simplified stress-strain law for steel; the model so elaborated was able to correctly reproduce the behaviour of steel reinforcing bars including the fundamental contribution of the relative slips between steel reinforcements and surrounding concrete and adding, respect to the initial formulation by Braga et al. (2004), the hardening phenomena necessary for the correct evaluation of the strain demand due to seismic events.
Incremental Dynamic Analyses were executed on the bi-dimensional fiber models of r.c. case studies opportunely elaborated using the modified slip model for the constitutive law of steel reinforcements; specific natural time histories, selected to maximize the seismic requirements in terms of both energy dissipation and deformation, were used. The global seismic assessment of buildings, following the indication of actual design standards, evidenced a different behaviour between buildings designed in high and low ductility, with the activation of brittle shear mechanisms in beams and columns of the 1st floor for levels of p.g.a. more or less equal to the one adopted in the design in the case of LDC buildings. Moreover, despite what imposed during the design process according to capacity design approach, the global assessment of the structures evidenced that beams and columns of the highest floors of the buildings (4th and 5th levels) were usually not directly involved in the global collapse mechanisms, since in general, plastic hinges developed for very high values of the p.g.a. (higher than 0.40-0.45g).
The investigation of the local ductile behaviour of steel reinforcing bars allowed the individuation of the effective levels of deformation due to seismic action. In particular, two main different conditions were individuated: the first one was characterized by steel rebars able to reach high levels of deformation (up to 10%) only in tension or in compression, without the execution of complete tension/compression cycles and resulting, consequently in lower values of the dissipated energy; this condition was generally more frequent in beam elements, even if also in columns a similar situation was individuated, in relation to the effective levels of forces reached. The second condition was characterized by steel rebars able to execute complete hysteretic cycles, generally with lower values of deformations (even if some exceptions were found) but with a higher dissipative capacity in terms of energy density.
On the base of the results of numerical analyses and taking into consideration the values obtained from the experimental low-cycle fatigue tests, a protocol for the execution of LCF tests aiming at the seismic production control of steel reinforcements was elaborated, prescribing the execution of 8 or 10 complete hysteretic cycles with imposed deformation equal to ±2.5%, respectively in the case of low and high ductility class.
A detailed investigation of the mechanical properties of corroded steel reinforcing bars was moreover executed: in the current literature in fact, many works evidenced durability problems of r.c. structures in presence of aggressive environmental conditions, leading to possible corrosion phenomena on steel reinforcing bars, mainly as a consequence of chloride attack or carbonation conditions.
Experimental monotonic tensile and low-cycle fatigue tests were executed on corroded steel reinforcing bars opportunely threatened in salt spray chamber; a protocol for the execution of accelerated corrosion tests was elaborated in order to
Chapter 8: Conclusions
8-3 reproduce the effective conditions due to aggressive environmental conditions: two different exposure times were considered, equal to 45 and 90 days. The results, elaborated in terms of percentage mass loss of the corroded specimens, evidenced a strong decrease of the mechanical properties of steel reinforcing bars: in particular, high reductions of the elongation to maximum load were visible, with percentage variations up to the 50-60%, while the decreases of yielding and tensile strenght were, in general, not significative.
In particular, the most critical situation was evidenced in the case of specimens B500A (Cold Worked process), in which the Agt dropped from an initial average
value (obtained from three different reference samples) equal to 6.0% to values between 0.80% and 5.10% in relation to the different mass loss; in general, more localized (pitting) phenomena were associated to higher decreases of the ductility. Considering, afterwards, the low-cycle fatigue behaviour of corroded steel reinforcing bars, no high reduction of the dissipative capacity was evidenced for low level of imposed deformation (±2.5%), independently from the free lenght and the diameter of the specimens analyzed; on the other hand, for increasing levels of imposed strain (±4.0%) the mechanical behaviour of steel reinforcements generally changed, evidencing a more brittle behaviour with a lower number of cycles up to failure (ranging between 4 and 6) and lower values of the total density of dissipated energy, respect to the ones obtained from uncorroded bars. In general, significative degradations of the elongation to maximum load were evidenced in all the considered steel grades and processes; once again, on the other hand, Micro Alloyed steel reinforcing bars generally provided a better behaviour, with lower reduction of the Agt (up to 30%).
The comparison between the results obtained from experimental tests on corroded steel reinforcing bars and the numerical ones coming from the numerical analyses evidenced some problems, especially in the case of rebars subjected to high values of deformation due to seismic action; as an example, in the case of steel rebars B450C (diameter 16 mm) the average value of Agt coming from corroded
samples was between 4.3% and 5.7%, while the maximum deformation demand due to seismic input ranged around 8.0%-10.0%, with following potential problems in sustain the seismic requirements. As a consequence, further investigations and analyses are required.
8.2
Further developments
Other experimental low-cycle fatigue tests on corroded steel reinforcing bars will be executed in the following period, allowing the individuation of the effective seismic capacity of all the selected corroded rebars, including, for example, significative steel grades such as B450C and B500B (TempCore steel) for bars of diameter 16 mm; the results obtained from LCF tests will be compared with the data provided by reference specimens consequently evaluating the effects of corrosion attack. In relation to the severity of corrosion attack, specific Corrosion Damage Indicators
(CDI) will be defined, according to what proposed inside Rusteel project: as an
example, significative parameters for the determination and the quantification of damage may be the decrease of the mechanical properties, in terms of strenght (yielding and tensile) and, in particular, ductility (expressed in terms of Agt,
Chapter 8: Conclusions
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Specific correlations between CDI, Classes of Exposure defined in Eurocode 2 (EN 1992-1-1:2005) and Performance Index (PI), generally coinciding with the effective mechanical characteristics of bars (strenght, ductility, dissipated energy, number of cycles in the case of LCF behaviour) will be finally provided analyzing the ability of steel reinforcing bars to maintain an adequate safety level in presence of seismic actions or less.
Moreover, a further implementation of the modified hardening slip model will be executed in order to include an opportune bond-slip law including the degradation due to corrosion attack, to be adopted for the execution of numerical simulation; the so defined model will be applied to specific reinforcements in order to evaluated the effects of corrosion also on the ductility demand due to seismic event on rebars. Obviously, a specific probabilistic analysis will be required in order to establish the possible diffusion of corrosion inside a generic building, and to assign the opportune constitutive law to selected rebars.
Finally, a new European research proposal, called NewReBar (NEW dual-phase
steel REinforcing BARs for enhancing capacity and durability of antiseismic moment resisting frame) has been proposed for the investigation of the influence of
the combined effects of seismic action and corrosion on dual-phase steel reinforcing bars.
