Publication Date:
2022
Abstract:
Structural behavior of reinforced concrete elements strongly depends on the interaction between the reinforcing bars and the surrounding concrete, which is generally referred as “bond in concrete”. In service conditions, the reinforcement-to-concrete bond governs deformability through the tension stiffening of concrete surrounding the bar as well the crack development and crack width. At Ultimate Limit State, bond governs anchorage and lap splices behavior as well as structural ductility.
When plain (smooth) bars were used, the steel-to-concrete bond was mainly associated with “chemical adhesion/friction” that is related to the surface roughness of the rebar. As steel strengths increased the need to enhance interaction between steel and the surrounding concrete was recognized, and square twisted rebars, indented rebars or, later on, ribbed rebars came into the market, the latter being the type of deformed bar most commonly adopted since the 1960/70s.
When ribbed rebars became widely used, several research studies started worldwide for better understanding the interaction between ribs and the surrounding concrete. Researchers evidenced the development of micro-cracks (due to the wedge action of the ribs) towards the external face of the structural element. If confinement is provided by the concrete cover, by transverse reinforcement or by an external transverse pressure, the full-anchorage capacity is guaranteed and a pull-out failure occurs, with crushing of concrete between the ribs. On the contrary, with lesser confining action, a splitting failure of bond occurs; the latter may provoke a brittle failure of the lap splice or, in some cases, of anchorages.
However, after many years of research studies on bond-related topics, there are still several open issues. In fact, new materials entered into the market, as concrete with recycled aggregates or fibre reinforced concrete; the latter, having a kind of distributed reinforcement into the matrix (the fibres), provides a better confinement to the wedge action of the ribs.
In addition, concrete and steel strength continuously increased over the years, causing changes in the bond behavior due to differences in mechanical properties of materials but also to the different concrete composition at the interface with the steel rebar causing a different bond behavior. Moreover, the lower water/cement ratio of these high-strength concrete makes the bleeding phenomena less evident, changing the concrete porosity in the upper layers of the structural element and thus making the current casting position parameters no-longer reliable. Finally, concrete with recycled aggregates are becoming more important in a market that is looking forward to a circular economy.
As such, all the experimental results and database that allowed the calibration of bond rules now present in building codes for conventional concrete, may be not be representative of these new types of materials nowadays adopted in practice.
Furthermore, after more than 50 years of service life, structural elements may not satisfy the current safety requirements for several reasons, including material degradation (with particular reference to steel corrosion) or increased loads, by also considering the seismic actions that were non considered by building codes at the time of the original design. The structural assessment of existing structures requires proper conceptual models and new approaches for evaluating the reliability of existing structures by also considering the remaining expected service life. In addition, specific rules for older materials, as plain smooth bars, should be revised for a better assessment of old structures. Last, but not least, interventions in existing structures may require new
When plain (smooth) bars were used, the steel-to-concrete bond was mainly associated with “chemical adhesion/friction” that is related to the surface roughness of the rebar. As steel strengths increased the need to enhance interaction between steel and the surrounding concrete was recognized, and square twisted rebars, indented rebars or, later on, ribbed rebars came into the market, the latter being the type of deformed bar most commonly adopted since the 1960/70s.
When ribbed rebars became widely used, several research studies started worldwide for better understanding the interaction between ribs and the surrounding concrete. Researchers evidenced the development of micro-cracks (due to the wedge action of the ribs) towards the external face of the structural element. If confinement is provided by the concrete cover, by transverse reinforcement or by an external transverse pressure, the full-anchorage capacity is guaranteed and a pull-out failure occurs, with crushing of concrete between the ribs. On the contrary, with lesser confining action, a splitting failure of bond occurs; the latter may provoke a brittle failure of the lap splice or, in some cases, of anchorages.
However, after many years of research studies on bond-related topics, there are still several open issues. In fact, new materials entered into the market, as concrete with recycled aggregates or fibre reinforced concrete; the latter, having a kind of distributed reinforcement into the matrix (the fibres), provides a better confinement to the wedge action of the ribs.
In addition, concrete and steel strength continuously increased over the years, causing changes in the bond behavior due to differences in mechanical properties of materials but also to the different concrete composition at the interface with the steel rebar causing a different bond behavior. Moreover, the lower water/cement ratio of these high-strength concrete makes the bleeding phenomena less evident, changing the concrete porosity in the upper layers of the structural element and thus making the current casting position parameters no-longer reliable. Finally, concrete with recycled aggregates are becoming more important in a market that is looking forward to a circular economy.
As such, all the experimental results and database that allowed the calibration of bond rules now present in building codes for conventional concrete, may be not be representative of these new types of materials nowadays adopted in practice.
Furthermore, after more than 50 years of service life, structural elements may not satisfy the current safety requirements for several reasons, including material degradation (with particular reference to steel corrosion) or increased loads, by also considering the seismic actions that were non considered by building codes at the time of the original design. The structural assessment of existing structures requires proper conceptual models and new approaches for evaluating the reliability of existing structures by also considering the remaining expected service life. In addition, specific rules for older materials, as plain smooth bars, should be revised for a better assessment of old structures. Last, but not least, interventions in existing structures may require new
CRIS type:
7.1 Curatela
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