Effects of fines on the strength characteristics of mixtures

• The relationships between fines contents and stress–strain curves are studied.
• The relationships between fines contents, compressive and tensile strengths are analyzed.
• A new strength criterion of the mixtures is established.
• The failure mechanism of the mixtures and the shortcomings of the new criterion are discussed.

Mixtures composed of coarse and fine particles are complicated inhomogeneous materials and are widely encountered in nature. Mixtures with different fines contents have different strength characteristics, and the study of these characteristics is still nascent. To analyze the relationships between fines concentration and strength, 27 unconfined compressive tests and 27 radial-splitting tests with 9 different fines contents were conducted. Laboratory tests on samples with different fines concentrations indicated that the percentages of fines in the mixtures significantly impacted their strength, and 3 regions could be identified. The compressive strength of the mixture increased slowly with increasing fines content for fines concentrations of < 55% by weight, rapidly for fines concentrations of 55% to 75% by weight, and slowly again for fines concentrations > 75% by weight. The tensile strength increased slowly when the fines content was less than 45%, quickly for concentrations from 45% to 85% by weight, and then slowly for concentrations > 85%. The maximum compressive and shear strengths (95% fines concentration) were approximately 25 times greater than the minimum strengths (5% fines concentration). Meanwhile, the maximum tensile strengths were 50 times greater than the minimum values, but the strain values only differed by a factor of 3 regardless of mode (compressive or tensile). The fines had stronger effects on the strengths of the mixtures than their strains. As a whole, the ratio of different compressive-tensile strength lines varied by fines concentration. Based on the relationships between compressive and tensile strengths, a new strength criterion was established. Three additional unconfined compressive, radial-splitting, and direct shear tests were conducted to verify the rationality of the new criterion. The results indicated that values calculated by the criterion were in good agreement with the tested data. Comparison and analysis revealed that the surfaces between coarse and fine particles were weak zones. The fines influenced the strength by modifying these zones. This finding reveals how fines affect the strength of mixtures for a fixed void ratio.