In planning a 19-story building, I want to use varied concrete grades—lower for slabs and beams, higher for columns (e.g., M50 for columns, M25 for beams and slabs). How much can I differ between these grades while ensuring structural strength and avoiding issues like cold joint formations at beam-column joints? I've heard about a maximum difference of 5N/mm^2, but I'd like clarification with references and reasons for this choice.
Differential shrinkage could be a major issue. If the building is prefabricated, then no issues. Else it is a tough job, to maintain different grades, at site. Furtehr, there will be every chance of mix-up of concrete batches. Finally, contribution of higher concrete strength for columns, is not that effective.
The permissible grade difference between concrete mixes in a multistory building is typically governed by structural design codes and standards. In many cases, it is advisable to consult the local building code or structural engineering standards applicable in your region.However, as a general guideline, a reasonable approach is to limit the difference in concrete compressive strength (grade) to ensure compatibility and structural performance. A common recommendation is to have a maximum difference of 10 to 15 Mpa between adjacent concrete elements.
Based on analysis results considering selected grades of concrete for column and beam/sab, the compatibility of sizes of column and beam/slab to be work out needs to be check for construction point of view. Also beam-column junction for this much variable grades needs to be look very carefully performance wise.
When I was working in a G+45 tower made by LnT, the difference between concrete grade was 5 mpa, for vertical members like RCC column and RCC wall, M50 grade was adopted while for horizontal members like beam and slab M45 was used
In the construction work of multi-storey buildings, we do not recommend using differences in concrete strength of more than 10 MPa, while ensuring that the concrete strength of the columns is homogeneous and of the same strength so that they are not considered weak points.
Normally, you can use varied concrete grades - lower for slabs and beams, higher for columns. However, please note that beam-column and slab-column joints shall satisfy some requirements for transfer column axial force through floor system. The requirements are shown with explanation in ACI 318 (chapter 15) and AS3600.
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