A series of large-scale direct shear tests were carried out to study the stress -strain relationship of the interface between the spoil mixture and concrete under different roughness conditions. The results showed that roughness significantly affects the shear strength properties and dilatancy characteristics of the interface. Under different roughness conditions, the shear stress ratio and the normal deformation of the interface tend to be stable after larger shear strain, and the interface presents the characteristics of a critical state. With the increase of shear strain, the void ratio of the interface shows the law of transformation from the initial void ratio to a certain stable void ratio. Based on the void ratio prediction formula of the interface, the relationship between roughness and critical state parameters was established, and the interface state parameters were introduced into the hyperbolic model. Finally, a state -dependent hyperbolic model of the interface considering the roughness was established. Importantly, the model can well reflect the shear stress -strain relationship of the interface under different roughness conditions.

Featured Application This research paper encourages repurposing waste material for ground improvement. The results of this study contribute towards a greater understanding of the strength and durability performance of treated soils under normal, fluctuating, and adverse moisture conditions.Abstract Expansive soil underlying structures pose a significant risk to the integrity of superstructures. Chemical soil stabilization can be used to strengthen soils due to the cost and impracticality of mechanical approaches. Waste materials such as recycled gypsum and rice husk ash have been considered alternatives because of their sustainable and economic advantages. A combination of these additives was used to address the high absorption of gypsum and the lack of cohesion of the pozzolan. The study assessed the short-term and long-term performance of expansive soil treated with recycled gypsum and rice husk ash under normal and fluctuating moisture conditions. Direct shear tests indicated ductile and compressive soil behavior with improved shear strength. A good approximation of stress-strain response was made with a modified hyperbolic model for treated soils that exhibited strain hardening and compressive volumetric strain. Durability and water immersion tests were performed for samples after varying curing periods and cycles of capillary soaking to assess the behavior when exposed to varied environmental conditions. Samples under the modified durability test experienced significant strength loss, with decreasing compressive strength as curing durations increased. Specimens in the modified water immersion test experienced significant strength loss; however, it was determined that curing durations did not contribute to the change in the strength of the sample. Expansion index tests also determined that the treatment effectively mitigated expansivity and collapsibility in all samples. Despite improvement in shear strength and expansion potential, further investigation is needed to enhance the durability of soil treated with gypsum and rice husk ash.

The volume of shield tunnel spoil (STS) is very large, its effective management is difficult, and it even causes environmental pollution. In this study, to achieve its recycling, a novel controlled low strength material (CLSM) was prepared by utilizing high fine-grained STS as partial aggregates instead of sand, and its engineering performance was thoroughly evaluated. In the process of mix proportion design, key parameters such as the STS-tototal aggregate ratio (TS/TA), foam agent content (F), water-to-binder ratio (W/B), binder-to-total aggregate ratio (B/TA), and fly ash-to-cement ratio (FA/C) were employed. Workability aspects (i.e., flowability, bleeding rate, and setting time) and physical and mechanical properties (i.e., unconfined compressive strength and density) were evaluated. Additionally, the pH of bleeding and leachate, as well as the impact of foam agent content on CLSM properties, were examined. The findings revealed that an increase in the TS/TA ratio was associated with a decrease in flowability, density, and compressive strength, as well as an extension in setting time. The CLSM, with a flowability range of 150-300 mm, exhibited a bleeding rate below 2%, setting times between 3.6 and 6.1 hours, 28-day strength ranging from 1.06 to 3.24 MPa, and fresh density ranging from 1810 to 2060 kg/m3. Generally, these results met the required specifications, although the fresh density was slightly lower. The pH results indicated that the CLSM is non-corrosive. Furthermore, our investigation highlighted the substantial influence of foam agent content on flowability and setting time. An increase of 0.1 parts per thousand in the proportion of foam agent within the total aggregates resulted in a flowability increase of 2.1-2.6 mm and a setting time increase of 4.25-4.99 minutes. Therefore, it is feasible to utilize high fine-grained STS in the production of CLSM.

PurposeThe purpose of this paper is to check the feasibility of using biomaterial such as of Phragmites-Australis (PA) in cement paste to achieve sustainable building materials.Design/methodology/approachIn this study, cement pastes were prepared by adding locally produced PA fibers in four different volumes: 0%, 0.5%, 1% and 2% for a duration of 180 days. Bottles and prisms were subjected to chemical shrinkage (CS), drying shrinkage (DS), autogenous shrinkage (AS) and expansion tests. Besides, prism specimens were tested for flexural strength and compressive strength. Furthermore, a mathematical model was proposed to determine the variation length change as function of time.FindingsThe experimental findings showed that the mechanical properties of cement paste were significantly improved by the addition of 1% PA fiber compared to other PA mixes. The effect of increasing the % of PA fibers reduces the CS, AS, DS and expansion of cement paste. For example, the addition of 2% PA fibers reduces the CS, expansion, AS and DS at 180 days by 36%, 20%, 13% and 10%, respectively compared to the control mix. The proposed nonlinear model fit to the experimental data is appropriate with R2 values above 0.92. There seems to be a strong positive linear correlation between CS and AS/DS with R2 above 0.95. However, there exists a negative linear correlation between CS and expansion.Research limitations/implicationsThe PA used in this study was obtained from one specific location. This can exhibit a limitation as soil type may affect PA properties. Also, one method was used to treat the PA fibers.Practical implicationsThe utilization of PA fibers in paste may well reduce the formation of cracks and limit its propagation, thus using a biomaterial such as PA in cementitious systems can be an environmentally friendly option as it will make good use of the waste generated and enhance local employment, thereby contributing toward sustainable development.Originality/valueTo the authors best knowledge, there is hardly any research on the effect of PA on the volume stability of cement paste. Therefore, the research outputs are considered to be original.

In regions with seasonal frozen soil, mechanical properties of soil are impacted by freeze-thaw cycles, which influence the shear resistance of soil-concrete interface in geotechnical engineering. For evaluating shear properties at the soil-concrete interface, freeze-thaw cycles and direct shear experiments were conducted in this research. The stress-displacement curves, shear strength and parameters of the interface were analyzed in relation to freeze-thaw cycles, while the influences by moisture contents and normal stresses were considered. Results show that the curves related to shear stresses and displacements at the interface are strain-hardening, and shear properties gradually deteriorate with repetitive freezing and thawing. The shear strength is positively related to normal stresses, and it increases by approximately 250% while normal stress varies from 100 to 400 kPa. However, it is negatively correlated with growing moisture contents and freeze-thaw cycles. The reduction in shear strength is about 21%-25% after freeze-thaw cycles, along with a decrease in cohesion ranging from 14% to 20% and for angle of internal friction it reaches at 14%-24%. Moreover, an improved hyperbolic model based on the logistic function and hyperbolic model was established to evaluate shear properties at the interface under freeze-thaw cycles, providing a theory base for engineering construction in seasonally frozen soil regions.