In the rapidly developing modern society, many raw materials, such as crushed limestone and river sand, which are limited, are consumed by the concrete industry. Naturally, the usage of waste materials in concrete have become an interesting research area in recent years, which is used to reduce the negative influence of concrete on the environment. Hence, this paper presents the development of a sustainable lightweight wet-mix shotcrete by replacing natural coarse gravel with a kind of byproduct, nut shell (walnut). Fibers made from dumped polyethylene terephthalate (PET) bottles were mixed in the composite to improve the properties of the lightweight wet-mix shotcrete. The initial evaluation of the fresh concrete mixed with different volume fraction of walnut shell was carried out in terms of its performance capacities of mechanical properties (i.e., tensile and compressive strength), pumpability and shootability (i.e., slump, pressure drop per meter and rebound rate) and the results were compared with plain concrete. With increase of walnut shell, compressive and splitting tensile strength of casting concrete decreased, while slump and pressure drop reduced slightly. Additionally, appropriate dosage of walnut shell can improve the shootability of fresh concrete with low rebound rate and larger build-up thickness. In the second series tests, polypropylene (PP) fiber and multi-dimension fiber were also mixed in composite for comparative analysis. After mixing fibers, the splitting tensile strength had obtained marked improvement with slight reduction of compressive strength, along with acceptable fluctuation in terms of pumpability and shootability. Furthermore, relation of density and compressive strength, relation of rebound and density, build-up thickness and relation of compressive and splitting tensile strength were discussed. This study found wet-mix shotcrete incorporating PET fiber with walnut shell of about 35% coarse aggregate replacement could be used for roadway support as lightweight shotcrete per requirements of mine support.

Wet shotcrete can reduce dust and improve supporting strength, however, safe and efficient pipage is a key technical part of wet shotcrete process. The paper studied the pipe flow law of wet shotcrete based on lubrication layer by build the experimental pumping circuit of wet shotcrete that can carry out a number of full-scale pumping tests. The experimental results show there was a linear relationship between pressure loss and flow rate. Combined with the Buckingham rheological equation, the computing equations of the yield shear stress and plastic viscosity were deduced through linear regression. A simple analytical method allowing for a rough estimation of the pumping pressure was proposed and used when considering the lubrication layer of wet shotcrete in pipes. In addition, two kinds of particulate distributive models were established along the time axial to analyze the formation of lubrication layer which is related with particles migration. By computational fluid dynamics simulation, the lubrication layer thickness of different mix proportions was estimated. A new method for measuring the thickness of lubrication layer was proposed to verify it by binarization processing. Finally, according to the comparative analysis of experiments, simulation and computed value, it can be seen that the lubrication layer plays a key role in the process of wet shotcrete flow and with the increase of lubrication layer thickness pipe pressure declines gradually.

The membrane electrode assembly (MEA) plays an important role in the proton exchange membrane fuel cell (PEMFC) performance. Typically, the structure comprises of a polymer electrolyte membrane sandwiched by agglomerate catalyst layers at the anode and cathode. Optimization of various parameters in the design of MEA is, thus, essential for reducing cost and material usage, while improving cell performance. In this paper, optimization of MEA is performed using a validated two-phase PEMFC numerical model. Key MEA parameters affecting the performance of a single PEMFC are determined from sensitivity analysis and are optimized using the response surface method (RSM). The optimization is carried out at two different operating voltages. The results show that membrane thickness and membrane protonic conductivity coefficient are the most significant parameters influencing cell performance. Notably, at higher voltage (0.8 V per cell), the current density can be improved by up to 40% while, at a lower voltage (0.6 V per cell), the current density may be doubled. The results presented can be of importance for fuel cell engineers to improve the stack performance and expedite the commercialization.

A pressure transmitter was installed at a specific position in a concrete conveying line to disclose the pressure drop when compressed air was conveyed during concrete spraying. A statistical analysis of the pressure at different positions was undertaken. Experimental results demonstrated that in the accelerate zone of horizontal conveying of concrete in the line, the pressure drop mainly occurred during the acceleration, collision, and friction processes. The momentum equation was introduced during the experiment, which interpreted the pressure drop caused by the accelerated conveying of concrete. The theoretical equation was corrected based on the results of theoretical experiments by introducing the value of α, and the experimental results were then optimized, thus obtaining an approximate model of pressure drop during the conveying of concrete. In addition, experimental results were compared with a model equation that showed the reliability of the proposed model. Research conclusions are of great significance to regulate the pressure drop in the conveying line of concretes, to design working parameters of concrete spraying devices, and to predict the ultimate distance for the conveying of concrete.

In order to reduce the dust hazard of shotcrete, the paper analyzed the mechanism of dust production from the perspective of various dust sources and developed the plunger‐type wet spraying machinery and wet mixer which can be applied in the small‐section tunnel for reducing dust on the root cause. The dust concentration between dry‐mix shotcrete and wet‐mix shotcrete was compared and analyzed. The results showed that the average total dust concentration and respirable dust concentration in the process of dry spraying were 99.4 mg/m³ and 52 mg/m³, respectively, while 9.8 mg/m³ and 4.08 mg/m³ in the wet‐mix shotcrete process, and the average dust concentration decreased by more than 90% after applying new wet‐mix shotcrete outfits. The clean zone of dust was divided according to the dust distribution. Finally, the dust suppression technology was optimized by using an accelerator and adjusting wind pressure and spraying angle. Results indicated that the application of the accelerator reduced the dust production efficiently; moreover, when the wind pressure was 0.2 MPa with 1 m spraying distance and 90° angle of spraying, the dust concentration and rebound reached the minimum.

Air entraining agent (AEA) can bring a lot of microbubbles into fresh concrete for improving its fluidity; however, high fluidity has adverse effect on the shootability of wet-mix shotcrete. In order to solve the contradictory issue, the paper developed a new type of green switch air entraining agent (GSAE) that can improve both the pumpability and shootability. The single-admixture and combination tests containing foaming ability and surface tension were performed with Deer agitator and automatic tension meter. The new AEA was developed with two constituents A and B. A was prepared withSapindus mukorossi(S-1), dodecyl trimethyl ammonium chloride(1231), and polyacrylamide. B was prepared with lauryl sodium sulfate (K12) and silicone oil. The mass mix proportion was S-1 : 1231 : polyacrylamide : K12 : silicone oil = 1 : 0.33 : 0.2 : 0.33 : 0.47. The application method of GSAE proposed that A was premixed with fresh concrete and then B was added at nozzle. Experimental investigation showed that the optimal mixing amount of GSAE was 0.1%–0.2% relative to cement. All performance measured of wet-mix shotcrete with 0.12% GSAE met the first-grade product requirements of the China National Standard. Compared with the conventional type of AEA, the proposed GSAE is capable of effectively improving pumpability and shootability.