Shotcrete Process Research Articles

Evolutionary mechanism and particle characterization analysis of dust produced by wet-mix shotcrete in tunnels

Wet-mix shotcrete, an essential form of concrete support, has been widely applied in underground constructions such as tunnels and mines. This study analyzed the gas jet structure and particle velocity differential equations in the wet-mix shotcrete process and physicochemically characterized collected dust samples. Based on field measurements and laboratory analyses, we established an experimental platform and a numerical simulation model for wet-mix shotcrete to investigate dust generation characteristics. The results showed the following: 1) The particles generated during wet-mix shotcrete primarily consisted of cement particles, exhibiting a spherical and blocky shape with a relatively regular morphology. However, their surfaces possessed numerous burrs and pores, resulting in poor wettability. 2) Suspended aerosol particles mainly comprised particles below 10 µm, exhibiting a unimodal particle size distribution. Smaller particles showed a higher abundance, with particles below 2 µm accounting for over 95% of the total particles. 3) The dust generation during wet-mix shotcrete was significantly influenced by the compressed gas flow rate and material viscosity. An increase in the gas flow rate or material viscosity led to higher production of fine particles. This study elaborated the mechanisms underlying particle generation during wet-mix shotcrete, providing valuable insights for wet-mix shotcrete dust control techniques.

The effects of nozzle diameter and length on the resulting strand properties for shotcrete 3D printing

Additive manufacturing in construction (AMC) enables new design methods and strategies within the construction industry. In particular, Shotcrete 3D Printing (SC3DP) offers a high degree of design freedom by enabling the deposition of concrete at variable layer orientation based on a wet-mix shotcrete process. However, the mechanical properties and geometry of the printed layers are dependent on the material and process parameters used. In this context, the effects of air and concrete flow rates, path planning parameters, and material parameters have been investigated in previous research. The presented study investigates the influence of the nozzle geometry on the resulting strand properties, e.g. strand geometry, layer bond strength, and compressive strength, to evaluate nozzle diameter and length as control parameters for the SC3DP process. Experimental investigations were performed with fixed nozzle diameters between 10 and 30 mm and nozzle lengths ranging from 100 to 200 mm. The results show a significant influence of the nozzle diameter on the resulting strand geometry as well as the mechanical properties. Finally, concepts for a nozzle with a controllable outlet diameter were developed and evaluated.

Activation and Application of Wet Shotcrete Concrete with Coal Gasification Slag Aggregate

By analyzing the shortcomings of the existing dry shotcrete process in the coal mines and the shortage of natural sand and gravel resources, it is proposed to use coal gasification slag to replace all natural sand and gravel in the preparation of wet shotcrete. Coal gasification slag has alkali-activated cementitious activity, and its strength as an aggregate in cement mortars still shows significant growth after 28d of age. First of all, the gasified slag aggregate cement mortar was mixed with calcium carbide residue to further stimulate its activity, and the effect law of different amounts of calciumcarbide residue on mortar strength was studied. The results showed that the mortar strength increased continuously with the increase of calcium carbide residue admixture, and the mortar strength increased slowly after the admixture reached 8%. With 8% admixture, the flexural strength at 90d age increased by 58.9%, and compressive strength increased by 13.6% compared with 28d age. MIP, XRD and SEM microscopic analyses showed that the incorporation of CCR promoted the alkali-activated cementation reaction of the gasified slag aggregate, eliminated the oriented aggregation of calcium hydroxide at the cement-aggregate interface, improved the interfacial bonding, and significantly reduced the percentage of harmful pores (50-200 nm) and more harmful pores (>200 nm) in the mortar, which explained the microscopic mechanism of the significant increase in mortar strength. Then the mining wet shotcrete was prepared with coal gasification slag as aggregate and calcium carbide residue as cementing material, and the 28d compressive strength reached 34.8MPa, which significantly improved the concrete strength and ensured the stability of the coal mine roadway support.

On the significance of accelerator enriched layers in wet-mix shotcrete

The application process, which gives shotcrete its name is a robust and established method, dating back to the beginning of the 20th century. Since then, the spraying process has been significantly enhanced. However, during the last decades no major technical changes have been made. In this study the wet - mix shotcrete process including the dosing of accelerator was investigated. For this, we monitored the concrete and accelerator pressure with 5 sensors in the pumps and pipes, and analysed the accelerator distribution in the hardened shotcrete matrix. The recorded pressure fluctuations clearly indicated that the pumping of the concrete with a double-piston pump led to flow pulsations. The pressure along the accelerator pipes, controlled by a peristaltic pump, was not steady either. However, the accelerator flow pulsation had a higher frequency than that of the concrete flow. This misalignment led to changes in the accelerator to concrete ratio during the spraying process. The impact of these incongruent concrete and accelerator flows on the resulting hardened shotcrete was visually analysed with the use of 0.02 % uranin as fluorescent tracer added to the accelerator. The tracer distribution showed that changes in the accelerator/concrete ratio led to the formation of ‘accelerator layers’, layers with higher accelerator concentrations in the hardened shotcrete. These layers show differences in chemistry, mineralogy and open porosity compared to the rest of the shotcrete matrix. The presence of accelerator enriched layers can have detrimental effects on the shotcrete properties, especially affecting the durability and mechanical performance. In consequence, we recommend a revision of the shotcrete process to eliminate these inhomogeneities.

Dust pollution during shotcrete process in high-altitude tunnel based on numerical simulation

The surrounding rock needs shotcrete support after drilling and blasting excavation in the tunnel; the high concentration of dust generated in the process will endanger workers’ occupational health. Therefore, to ensure the cleanness and safety of the tunnel construction process, a full-scale model of the tunnel was established based on field data of a high-altitude tunnel of the Sichuan-Tibet railway. The dust production mechanism is summarized by combing the whole process of shotcrete. The Computational Fluid Dynamics (CFD) method was used to study the diffusion and transport of dust under different conditions. The grey relational analysis was applied to investigate the correlation values of the influencing factors on dust diffusion in the shotcrete operation area. The results show that the dust generation mechanism of shotcrete includes the sudden change of particle velocity in the jet area leading to escape and particle impact dust generation, where fine dust is easily dispersed in the tunnel. During continuous dust production, the dust concentration is higher near the wet spraying machine and on the backflow side of the working face. Increasing the air supply volume and shortening the distance between the air duct and the working face is conducive to diluting the dust concentration in the tunnel. In the high-altitude environment, the dust concentration in the tunnel decreases, the diffusion distance becomes smaller, the settlement proportion of dust particles increases, and the risk of secondary pollution increases. The simulation results and the field measurement data are consistent, which can provide theoretical support for the construction site dust control.

Application of Discrete Event Simulation in Estimating Productivity of Shotcrete Method in Divider Wall Construction

Discrete-event simulation (DES) techniques are widely used in modeling complex environments where interaction between time-dependent resources and system processes occur in sequences, such as manufacturing ecosystems or production lifecycles, providing an alternative to traditional work measurement. In this study, labor productivity of using shotcrete process in constructing wall dividers for a simple construction project is estimated using a discrete-event simulation (DES) software incorporating project data on process durations, setup time, and machine mean time to failure (MTTF) as model inputs. The simulated productivity rate of using shotcrete to construct a 1000-sq.m. wall in replications of 200, 500, and 1000 is 1.59 man-hours/sq.m., compared to computed rate of 1.33 man-hours/sq.m. using the Program Evaluation and Review Technique (PERT). The total waiting time with respect to the simulation time considering setup, process-in-waiting, and downtime is 48.9% for a single 8-hour shift per workday arrangement. It is shown that by doing multiple 8-hour shifts the resource utilization of the shotcrete equipment can be improved by an average of 43.8% per additional shift bringing the total waiting time to 9.21% for a continuous 8-hour shifts in a single workday. The methodology aims to provide a better baseline estimate of productivity as it takes in to account historical data as well as waiting time arising from resource limitations.

Special Features of Using Universal Technological Sets of Small-sized Equipment for Renovation of Buildings

Nowadays, old utility lines are the most critical issue of society. Emergency condition of these utility lines results in their destruction and pollution of ecosystem and imposes threat to life of people residing within risk area. This article offers variants of mobile universal sets of small-sized equipment that allows restoration of destructed and emergency buildings through wet-mix shotcreting process. The universal nature of these sets of equipment allows performance of entire scope of renovation works from preparation of damaged surfaces to applying anti-corrosive coatings. Upon the use of such sets of equipment, it is possible to prepare and lay construction mixtures of different purpose directly at the place of works. Combination of process operation in time enhances the efficiency of equipment and reduces power consumption.

Development of a Shotcrete Training Simulator for Civil Works

Santa Barbara Foundation Labour School decided to develop a training simulator for the teaching of the shotcrete process before the practice with the machine. Thanks to this, students will have a wider experience when using the real machine.

Research on Dust Suppression Technology of Shotcrete Based on New Spray Equipment and Process Optimization

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.

Wet Shotcrete Process Using a New Set of Small-Sized Equipment for Concreting Formless Computer Simulation

The article discusses the new all-purpose small equipment process usage. On example of the proposed equipment process analysis was maiden computer simulation. Basic machine determining dependences unevenness values are given - concrete pump hose and the value of its reliable operation. Full automation of wet shotcrete working process controlling algorithm developed.

Pipe flow of pumping wet shotcrete based on lubrication layer.

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.

Air-void structure, strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent

It is well known that the air-void structure of hardened concrete has substantial effects on the mechanical properties and durability of concrete. In this study, laboratory evaluations were conducted to quantify the effects of air-entraining agent (AEA) and silica fume on the air-void characteristics of wet-mix shotcrete (WMS) before and after shotcreting process. For this purpose, a high-resolution image analyzer capturing elaborate graphical layouts of air-void structure using the linear transverse method was employed. Also, this study examined the effects of air-void characteristics, such as air content and spacing factor, on the strength and permeability of WMS. Based on the findings of this study, it can be concluded that: (1) shotcreting process considerably reduces overall air contents in WMS; (2) incorporating AEA with a 4.5% silica fume replacement ensures both satisfactory spacing factor and good retention of small entrained air bubbles even after shotcreting, which may improve the freeze-thaw and scaling resistance; (3) the compressive and flexural strengths of WMS were reduced as the air content increased and average spacing factor decreased; and (4) the air content affected the permeability of WMS, but no consistent correlation was found between spacing factor and permeability.

Effect of equipment on spray velocity distribution in shotcrete applications

It is well known that the velocity of particles exiting the nozzle plays a central role on rebound and consolidation in the shotcrete placement process. With this in mind, the impact velocity distribution was investigated using a high-speed imaging system with a full-scale shotcrete spray generated using two dry-mix nozzles and one wet-mix nozzle. The incident velocity was found to be at its maximum in the central region around the spray axis and decrease toward the peripheral regions following a Gaussian-type function. Further analysis using normalized parameters shows that these velocity profiles, which were obtained experimentally, follow a similar function that can be used to compare the effect a given nozzle, or equipment, has on impact velocity distribution. In light of this work, innovative and unique characterization tools are proposed to further support equipment optimization and to improve the understanding of the overall shotcrete process, and especially rebound.

고로슬래그 미분말이 혼입된 자기충전 및 숏크리트용 ECC의 개발을 위한 시멘트풀 레올로지 제어

An engineered cementitious composite produced with slag particles (Slag-ECC) had been developed based on micromechanical principle. Base grain ingredients were properly selected, and then the mixture proportion was optimized to be capable of achieving robust tensile ductility in the hardened state. The rheological design is performed in the present study by opti- mizing the amount of admixtures suitable for self-consolidating casting and shotcreting process in the fresh state. A special focus is placed on the rheological control which is directly applicable to the construction in field, using prepackaged product with all pulverized ingredients. To control the rheological properties of the composite, which possesses different fluid properties to facilitate two types of processing (i.e., self-consolidating and shotcreting processing), the viscosity change of the cement paste suspensions over time was initially investigated, and then the proper dosage of the admixtures in the cement paste was selected. The two types of mixture proportion were then optimized by self-consolidating & shotcreting tests. A series of self-consolidating and shotcreting tests demonstrated excellent self-consolidation property and sprayability of the Slag-ECC. The rheological properties altered through this approach were revealed to be effective in obtaining Slag-ECC hardened properties, represented by pseudo strain-hard- ening behavior in uniaxial tension, allowing the readily achievement of the desired function of the fresh Slag-ECC. These ductile composites with self-consolidating and shotcreting processing can be broadly utilized for a variety of applications, e.g., in strength- ening seismic resistant structures with congested reinforcements, or in repairing deteriorated infrastructures by shotcreting process.

Mechanical Performance of Sprayed Engineered Cementitious Composite Using Wet-Mix Shotcreting Process for Repair Applications

, V. 101, No. 1, January-February 2004.MS No. 02-473 received December 11, 2002, and reviewed under Institute publicationpolicies. Copyright © 2004, American Concrete Institute. All rights reserved, includingthe making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including authors’ closure, if any, will be published in the November-December 2004

Aditivos para concretos de projeção

A crescente utilização de procedimentos automatizados para a instalação de concretos refratários, como bombeamento e projeção, deve-se ao entendimento simultâneo das solicitações de cada técnica de instalação e dos fatores que determinam a natureza reológica destes materiais. No caso dos concretos de projeção, importante ainda é avaliar a ação dos aditivos (floculantes, aceleradores de pega, etc.) normalmente utilizados para auxiliar a consolidação e adesão do material na superfície aplicada, reduzindo o desperdício (rebote) durante a aplicação. Contudo, os procedimentos convencionais de análise, baseados em ensaios de consistência ou em análises pós-cura, não permitem simular a influência dos aditivos na projeção do material. Neste trabalho, foi desenvolvida uma nova técnica experimental baseada na reometria de concretos, a qual foi utilizada na análise de aditivos comerciais, além de outros que teoricamente possuem potencial para auxiliar a consolidação da aplicação de concretos por projeção. Os resultados comprovaram a eficácia da técnica de ensaio desenvolvida e possibilitaram a identificação de novos aditivos que elevam a eficiência da projeção.

AlpTransit—safety in construction as a challenge: health and safety aspects in very deep tunnel construction

After a brief presentation of the AlpTransit projects (Gotthard and Lötschberg base tunnels) special aspects of safety and health protection are discussed: access via deep shafts, rock pressure, ingress of water and mud, ventilation, cooling, fire protection, escape routes and rescue. Medical aspects of climate and ventilation are discussed as dust and diesel soot emissions, blast fumes and hazards resulting from the shotcreting process. The preparation of safety concepts and comments on the planning and drawing up of safety measures are presented.

The Effects of Polysaccharide Gum Additives on the Shotcrete Process

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Bond Strength of Reinforcing Bars in Reinforced Shotcrete

There is not yet a consensus on the quality parameters for the steel-concrete bond for shotcrete reinforced elements or whether or not shotcrete is comparable in performance to regular cast-in-place concrete. This paper presents the results of a series of lab experiments on the bond strength of lap-spliced beams cast using the wet shotcrete process: the American Concrete Institute bond strength equations were used to evaluate the performance of shotcrete beams. The beams were compared to cast-in-place concrete beams of the same dimensions, steel configurations, and approximately the same concrete compressive strength. The bond strength of beams repaired using the wet process was also examined. The study includes the analysis of the performance of a new type of shotcrete admixture which enhances the steel-concrete bond. The results reveal that the bond strength is slightly lower for bottoms cast shotcrete, but that the top bar bond strength is increased considerably for the shotcrete beams, when compared to regular cast-in-place concrete, due to the reduced segregation and bleeding generated during the shotcrete process. When water soluble polymers are added to shotcrete, the performance of shotcrete as compared to control shotcrete is superior due to improved rheological behavior of concretes containing pumping aid and sag resistance additives (PASRA).