It is mentioned that increasing the amount of cement decreases the compressive strength due to raising the capillary porosity
The problem is that when you increase the amout o cement, you also increase the amount of fine particles. This draws the increment of capillary porosity and also the hydration heat which brings along shrinkage creaks. All of these provoke the decreasing in compressive strenght.
Typically the selected cement content should be in the range of 450 to 550 kg/m3, for high-strength grades, such as f'c=65 MPa to 100 MPa. Adding higher than 550 kg/m3 cement contents impacts different performance aspects of concrete including strength reduction and extensive autogenous shrinkage.
It is proven by the literature that higher porosities results in lower compressive and tensile strengths. Hasselman, Ryshkewithch Schiller and Balshin have provided different models to represent porosity–strength correlation. As a matter of fact, each 10% increase in porosity results in 15% to 25% decrease in strength.
Capillary porosity and porosity size in general depends on cement particles size distribution, and packing ability of cement. Cement fineness and percent of retain on the 45 ms sieve are factors contributing to this matter, however, in most cases and irrespective of cement particle size distribution, a significant high content of cement in mix-design adds too much fine particles in binder paste which leads to a higher than expected porosity. In such cases substitution a portion of cement with SCMs may benefit mix design, if the early age strength (
Mostly agree with comments above. The main reason for the reduction in strength is related to the hydration process itself. More than necessary addition of the cement will increase temperature during hydration process of the concrete, which leads to the formation of micro-cracks leading to the increase in the pores. Therefore, insted of increasing the strength, it will be reduced.
I agree with the comments above mentioned. In my opinion there are different concomitant reasons responsible for strength decrease:
- cracks due heat of hydration;
- cracks due to autogenous shrinkage;
- cracks due to hydraulic shrinkage.
Moreover I suggest to change aggregates moving to basalt stone for instance and in order to bridge the micro cracks add metallic or non metallic fibers.
another suggestion: reduce the max size of aggregates in order to eliminates stress concentration at the interface paste-aggregates.
luigi coppola
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