Bonding effect on the evolution with curing time of compressive and tensile strength of sand-cement mixtures

Soil-Cement mixtures are used in several geotechnical applications and consist of artificially structured materials with stable fabric due to the presence of artificial bonds, which are the hydrated cement minerals coating the aggregates (soil particles). An extensive experimental study was performed on samples of an artificially cemented silty sand prepared by adopting three different dosages of cement and different water-cement ratios for each dosage. Realistic values for the dosages and water-cement ratios were adopted considering soil-cement mixtures performed by compaction or by grout injection. The unconfined compressive strength (UCS) and indirect tensile strength (ITS) of the mixtures were measured for different curing times. The values found were related to the water-cement ratio adopted for the mixing process, because of the dependency between the porosity of cement and the geometry of the bond minerals observed in electron scanning microscope images. The bond geometry varied considerably depending on the amount of water used in the preparation, particularly in the case of low cement dosages. The definition of degree of bonding was adopted to describe the evolution along curing time of UCS and ITS considering the values measured after 3 days of curing as reference points. This degree of bonding is useful for the definition of constitutive models for artificially structured materials. This parameter was indirectly related with the bond geometry; that is, the paths of strength evolution found were similar for samples with dispersed bonds. The improvement of tensile strength and compressive strength are correlated through a constant, independent of the dosage and water-cement ratios adopted for the mixture. Several authors have found identical values for this constant when studying compacted sand-cement mixtures. The particular structure resulting from the preparation process adopted does not affect this constant, however it affects the maximum UCS and ITS achieved for each cement dosage studied. This maximum depends on the water-cement ratio, and therefore this ratio is an important design parameter to be considered when prescribing compacted or grouted solutions.