This study aims to verify the effectiveness of fiber reinforcing with and without cement on settlement controlling of subgrade models, and to investigate the effect of fiber reinforcement on the load-settlement behavior of subgrade models. To this end, laboratory subgrade model tests were conducted under different static vertical loads. Three subgrade models composed of different fillers were constructed in a rigid concrete tank, and the internal earth pressures and settlements at different depths were measured through an earth pressure cell and settlement plate. Results show that the fiber-reinforced model keeps a slight difference to the unreinforced model in terms of earth pressure distribution under lower applied surface pressure. However, the earth pressure at various locations under each surface pressure was obviously lower than that of the other two models due to the combined effect of fiber and cement. In addition, for the unreinforced subgrade model, the 60 cm settlement domain was restricted within 40 cm depth through fiber-cement and fiber reinforcing, and the total settlement under 100 kPa was decreased by 48.5% and 30.8%, respectively. Moreover, reinforced models present with different settlement deformation features. The inflection points, after which the rate of settlement decreased with increasing applied surface pressure, were observed in the pressure-settlement curves. Under 200 kPa, the fiber-cement and fiber reinforcement decreased the total settlement of the unreinforced model by 61.4% and 34.7%, respectively. The greater applied surface pressure, the more efficient was fiber-cement reinforcing in settlement controlling.

Considering the current requirement for high temperatures and the significant energy consumption in the preparation of geopolymer-based cements, this paper presents a study on the compressive strength of metakaolin-based geopolymers containing various low-calcium fly ash admixtures, prepared at room temperature (25 ± 2 °C). The physical properties and microstructure of the geopolymers were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The type of alkaline cations, phase transformation, evolution of characteristic functional groups, and hydration characteristics of the microstructures were analyzed, and the hydration mechanism is discussed. The experimental results indicated that the fly ash content had a more significant impact on compressive strength than the alkaline cation type (Na⁺/K⁺). The optimal formulation (20% fly ash with 20% KOH activator) reached a compressive strength of 76.70 MPa at 28 days, which was around 6% higher than that of the NaOH-activated counterpart (72.34 MPa). Crystalline phase analysis in the transformation of mullite and microstructure analysis indicated that the increase in compressive strength could be attributed to the effective filling of the matrix interface by chemically inert fillers and the dense N-A-S-H and C-(A)-S-H multi-dimensional gel structures. These experiments prove the feasibility of using fly ash and metakaolin to prepare geopolymer materials with high compressive strength at room temperature.

To investigate the dynamic stability of natural subgrade filler (NSF) and fiber-binder reinforced subgrade filler (RSF) under cyclic load after freeze–thaw (FT) cycles, a triaxial test was conducted to determine the correlation between cumulative plastic strain (CPS) and the quantity of loading cycles, as well as the evolution law of dynamic strength and critical dynamic stress (CDS) with different FT cycles. The CPS change in the NSF and RSF shows three states (stable, critical, and destructive) with increasing vibration times. However, both fillers have different failure forms, and the curve shapes of the CPS with loading cycle quantities before and after failure are also different. With the number of FT cycles increasing, the requisite dynamic stress threshold for NSF specimen failure decreases continuously. After three FT cycles, the anti-cumulative deformation ability of the NSF decreases by approximately 32%. The anti-cumulative deformation abilities of the NSF after seven and nine FT cycles, respectively, are similar. The amelioration measures could significantly enhance the FT resistance of the NSF. After zero, one, three, five, seven, and nine FT cycles, the requisite dynamic stress threshold for the RSF to reach destruction is increased 1.52, 1.89, 1.98, 2.32, 2.2, and 2.45 times, respectively, compared to that of the NSF. A mechanical model of critical dynamic stress of the NSF and RSF that considers the FT cycle was obtained using a multivariate nonlinear regression method.