Conceptual Frameworks to Explain Reverse Size Effect in Intact Rocks Considering End and Annulus Damage

Contrary to the conventional size effect on rock strength, the reverse size effect refers to a rise in the unconfined compressive strength of small cylindrical rock specimens with an increase in the diameter. The current study attempts to explain this phenomenon by building upon the existing end surface damage explanation (flaws/imperfections generated during specimen preparation) and proposing new damage frameworks. The frameworks considered mainly differ in how damage/flaws are spatially distributed within the specimen. Specifically, damage within an annulus around the specimen circumference and combinations of end and annulus damage were tested, and their ability to match the observed trends of porosity versus specimen diameter and elastic modulus versus specimen diameter in Gosford Sandstone was statistically assessed. In the data set considered, Gosford Sandstone exhibited a drop in porosity and an increase in elastic modulus with a rise in specimen diameter within the reverse size effect regime. The results indicated that the end damage framework is unable to explain these trends, and annulus damage has a far greater influence on porosity and elastic modulus due to the geometrical relationship between annulus thickness and annulus volume. Lastly, typical trends in prepeak damage threshold and dilation angle for Gosford Sandstone specimens in the reverse size effect regime were explained in the context of the proposed annulus damage frameworks. To further support the frameworks, analysis of data for a different rock type (high porosity limestone) is also presented.