High Performance and the Importance of Capacity
Many standards and certification schemes require a certain tensile strength of rebar products (as rebar couplers and mechanical anchorages). Often, product information and specification contains a reference to a certain strength.
- To select the right product performance, it is important to see the correlation between strength and ductility
- It is necessary to look at the performance of the complete assembly: rebar + rebar product
- A good way to illustrate this context is a stress-strain curve
Almost every rebar heat exceeds the characteristic stress and strain requirements, which are specified in the rebar standards. Therefore, “actual” values of strength and strain are higher than the “specified” values. Products with a certain capacity are combined with real rebar – having “actual” values. In such a combination, the capacity of the rebar meets the capacity of the product (coupler, anchorage etc.). Anchorages and splices with lower strength than the rebar will be the weak link and fail before the possible or even the specified strength and strain is reached. This limits the ductility of the reinforcement. The failure mode will not be ductile, but a sudden rupture. Reduced ductility of the reinforcement reduces the robustness of the entire structure in extraordinary load situations (like overload, fire, earthquakes, accidents or attack).
A capacity level for a rebar product that equals the specified rebar tensile strength is not sufficient to guarantee the development of the specified ductility of the reinforcement.
100% continuity – no weak link – high performance
Projects of great importance for the society and structures with a high danger potential are to be designed and build robust. Examples are infrastructure projects, buildings gathering many people and structures in the energy sector. Ductility is one of the principal ways to achieve robustness. To achieve that, it is necessary for rebar products to maintain a high ductility of the complete assembly (rebar + rebar product), a capacity above the specified rebar tensile strength.
The ultimate level is the development of the real existing, “actual” strength and ductility of the rebar, i.e. 100% continuity – no weak link – high performance! This should be required for structures with high demands to robustness.
The development of the actual ductility (or at least a large portion of it) can be specified:
- for headed bars by referring to ISO 15698, category B3
- for mechanical couplers by requiring the development of Agt,spec outside the splice or a higher strength than the just the specified (e.g. 130% fy,spec)