Optimizing Paste and Mortar Margins (α and β) to Enhance Compressive Strength in Cemented Sand, Gravel and Rock

A suitable range of paste and mortar margins (α and β) to enhance compressive strength in Rich-Mix cemented sand gravel and rock (CSGR) material for application in CSGRD construction is critical. SL 678-2014 recommends margins > 1, which are specifically designed to fill the voids within the fine and coarse aggregates with paste and mortar, respectively, while allowing some excess for workability. However, the optimum ranges of values after 1 are inadequately determined, often leading to high efforts and time-consuming trial mixes that are not economical. This study evaluates two datasets to identify the optimal ranges of α and β margins for compressive strength development in Rich-Mix CSGR, aiming to achieve the compressive strength class C18020, intended for use as cushion, protective, and seepage control layers in CSGRD. Using Pearson correlations, t-statistics, and p-values, the first dataset (7, 28, 90, and 180 days) showed weak correlations between paste margins and compressive strengths (coefficients 0.172 to 0.418, p-values > 0.05) and negligible relationships for mortar margins (coefficients −0.269 to 0.204, p-values > 0.05), affirming the contribution of other factors in the compressive strength development in CSGR. The second dataset (14, 28, 90, and 180 days) revealed significant positive correlations between paste margins and strengths at 14, 90, and 180 days (coefficients up to 0.850, p-values < 0.05). Mortar margins, however, negatively impacted strength (coefficients −0.544 to −0.628, p-values < 0.05), revealing the need to control the sand ratio. The optimal range of values was 1.05 ≤ α ≤ 1.09 and 1.15 ≤ β ≤ 1.25, with a water–binder ratio of 0.7~1.3, vibrating–compacted value (VC) of 2~8 s, and sand ratio of 18~35%. These findings highlight the significance of precise paste and mortar margin ranges in the compressive strength development of Rich-Mix CSGR.