This paper aims to calculate and analyze the spatial and temporal variations in the groundwater flow quantity in Nam Co Lake based on the water balance principle. The results show that a large amount of groundwater was gradually lost and that, groundwater loss decreased from 1.9 billion m³ to 1.5 billion m³ from the period of 1980–1984 to 1995–2009. The comparative analysis in the current study indicates that the decrease in the groundwater index has a strong linear relationship with the temperature of the ground surface on the Tibetan Plateau, with a correlation coefficient as high as 0.92. Moreover, environmental variations such as large-scale engineering construction projects and increases in water storage may have played dominant roles in the sudden changes in the water quantities of plateau lakes (e.g., Nam Co Lake) during the periods of 1990–1995 and 2000–2009. The increased water levels resulted in reduced groundwater losses, which may lead to the substantial expansion or gradual shrinkage of the Qinghai–Tibet Plateau lakes over short periods of time. The results of this study provide an important reference for studying the mechanisms of lake water level changes on the Qinghai–Tibet Plateau.

Optimizing binders by using industrial waste and then adjusting microstructure has potential to develop low production cost and low carbon emission ultra-high performance concrete (UHPC) to meet different practical engineering requirements. This study first investigates the influence of silica fume, cenosphere and fly ash on paste mixing time, flowability, flexural and compressive strength of UHPC by using three-factor and four-level orthogonal test. Moreover, the effect of fly ash and slag content were further discussed, and the modification mechanisms of binders were revealed by analyzing pore structure, interface and characteristics of hydration products. The results show that silica fume and cenosphere are the most sensitive factors affecting paste mixing time and flowability of UHPC, respectively. The sensitivity of binders on compressive and flexural strength of UHPC can be ranked according to silica fume > fly ash > cenosphere and cenosphere > fly ash > silica fume, respectively. The binders’ composition for low-carbon UHPC with compressive strength grade of 120 MPa is that cement: cenosphere: silica fume: fly ash equals to 1:0.33:0.33:0.56, and the total amount of fly ash reaches 40%. Meanwhile, the 28d compressive/flexural strength decreases and the flowability increases with the increase of fly ash content, resulting from pore volume with size of 5–50 nm and Ca/Si ratio on the interface between steel fiber and matrix increase. The flowability of UHPC with slag is reduced and the cumulative pore volume with size of 50 nm–5 um pores is increased due to the influence of slag morphology, but the porosity and interfacial Ca/Si ratio is reduced because of the high pozzolanic activity of slag, leading to similar compressive and flexural strength with that of UHPC with fly ash.

Owing to the excellent strength and durability, ultra-high-performance concrete (UHPC) has been used for fabricating large-scale and important infrastructures. However, mix proportion of UHPC is still the core factor influencing its workability, strength, cost and energy resource consumption. Based on this, the amount of cementitious materials, water-binder ratio, and the content of steel fibers were matched to obtain UHPC with required workability and strength according to three-factor five-level orthogonal range analysis considering the interaction of these three parameters. Experimental results show that the water-binder ratio and steel fiber content is the primary factor to guarantee the fluidity/compressive and flexural strength of UHPC, respectively. For developing UHPC with compressive strength grade of 150 MPa and flexural strength higher than 50 MPa, the amount of cementitious materials (including cement, silica fume, cenosphere, and fly ash) and the content of steel fibers should be higher than 1000 kg m⁻³ and 2.5 vol.%, and the corresponding water-binder ratio is equal to 0.16. When the aim is to fabricate UHPC with compressive strength grade of 120 MPa and flexural strength higher than 40 MPa, the water-binder ratio can be increased but should be lower than 0.20 with the increasing amount of cementitious material, and the volume fraction of steel fibers should be higher than 1.5 vol.%. High steel fiber content and water-binder ratio all easily coarsens the microstructure and pore structure of UHPC, and this phenomenon cannot be compensated by using high amount of cementitious materials. It should be adjusting the matching degree of amount of cementitious materials and water-binder ratio to obtain a slurry with appropriate fluidity and cohesiveness, and then content of steel fibers can be selected to perform without adverse effects.

： As a new type of modified fiber, model steel fiber has both the appearance and function of steel fiber and the advantages of synthetic fiber. In order to make up for the mechanical properties of rubber concrete, the basic mechanical properties of C30 model steel fiber content reinforced rubber concrete, including compressive strength, splitting tensile strength, flexural strength, and static elastic modulus were studied. The results show that: the rubber particles decrease concrete strength, and the lower rate of compressive strength is greater than the bending and splitting tensile strength, but the toughness of concrete is enhanced. The 6-18mesh with10% vol. was chosen. The model steel fiber changed the damage state of rubber concrete, producing only micro cracks in the compressive, splitting tensile and flexural failure. When the model steel fiber content is 8 kg/m 3 , 28 d flexural strength increases by 25.8%, the ratio of compressive strength to flexural strength and the static elastic modulus is reduced by 6.9% and 5.3%.

The particle size and dosage of quartz sand significantly affect the bleeding and segregation of UHPC mixtures, thereby influencing their strength and durability. However, the maximum particle size of quartz sand is a key focus of existing research, and the influence sensitivity of particle size gradation and dosage for UHPC performance are not clear. Based on this, this study systematically investigates the effects of particle size gradation and dosage of quartz sand on the slump flow and strength of UHPC, and the grey relational analysis method is employed to identify the sensitive particle size fractions. The results show that the compressive and flexural strength of UHPC with quartz sand mixing ratio of 40:40:20 and 40:50:10 is significantly improved at both 7 d and 28 d curing ages compared with those using coarse, medium, and fine quartz sand individually. The order of particle size affecting slump flow, flexural and compressive strength of UHPC is 0.6–1.18 mm > 0.3–0.6 mm > 1.18–2.36 mm > 0–0.075 mm > 0.075–0.15 mm > 0.15–0.3 mm. The key measure for enhancing strength and ensuring workability of UHPC lies in the proportion of quartz sand with particle size of 0.6–1.18 mm, which needs to be above 40% to serve a filler and framework function. When coarse, medium, and fine quartz sand mixing ratio equals to 40:40:20, the dosage increases lead to the decrease in UHPC slump flow, and as the quartz sand dosage varies from 900 kg/m³ to 1500 kg/m³, the 28 d compressive strength of UHPC first increases and then decreases. It is recommended to use a quartz sand dosage of 1050 kg/m³ ± 50 kg/m³ and mixing ratio of 0.6–1.18 mm quartz sand larger than 40% to produce UHPC exhibiting slump flow larger than 600 mm and compressive strength of 120–150 MPa. The findings provide important guidance for the preparation and performance regulation of UHPC.