Wet-mix Shotcrete Research Articles

Flexural Behaviour of Reinforced Shotcrete beams Comprehending Waste Plastic Fiber

Abstract Wet-mix shotcrete is often used as a placement method in tunnelling and ground support. However, to date, only a limited number of studies have been identified the roles of waste plastic fiber (WPF) on wet-mix shotcrete mixtures. This experimental study show the flexural performance of reinforced shotcrete concrete members (beams) having waste plastic fiber, which may be considered as a new study. In order to achieve that, a manufacturing of wet-mix shotcrete machine has been developed to product special wet-mix shotcrete that will be used to cast reinforced shotcrete concrete members containing waste plastic fiber. Extensive attempts were done in this project to generate a special wet-mix shotcrete combinations using locally sourced waste materials like beverage bottles. The qualities of WPF shotcrete concrete (SC) were investigated in terms of fresh, hardened, mechanical, and bending behaviour, with extensive results analysis. Five SC formulations (0.25, 0.5, 0.75, 1.0, and 1.25) percent WPF content, as well as the control shotcrete (SC0.00), were used in the experiments. In addition, the flexural behaviour of SC beams casted from the same waste materials was investigated. The results revealed that all SC beams had almost similar flexural behaviour when compared to the creation of crack patterns, as well as the ductility index and stiffness. The maximum ductility index was 2.29 for SC0.25, while the minimum stiffness was 1.31 for SC 1.25 beam. The flexural resistance of SC beams show in beams deflection state, the primary crack with presence the waste plastic fibers was small, because of the resistance of plastic fibers to tensile stresses happening at a moment of growth the crack. It also shows that adding WPFs to SC up to (Vf=1%) results in a rise in the loads that cause initial cracks when compared to beams made with reference mix.

Radiation shielding properties of shotcrete containing different aggregates

In this study, the physical and mechanical properties of wet mix shotcrete produced using crushed stone aggregate (CS), ferrochrome slag aggregate (FS), and barite (B) aggregate have been investigated. Furthermore, the radiation shielding properties have been determined experimentally, theoretically, and through simulation. A control sample was produced using 50 % CS and 50 % FS. Subsequently, shotcrete groups were produced by substituting B aggregate at replacement rates of 25 %, 50 %, 75 %, and 100 % instead of FS. The mechanical properties such as density, ultrasonic pulse velocity (UPV), compressive strength, flexural strength, and splitting tensile strength of the produced spray concrete groups were investigated. Additionally, experimental radiation shielding measurements were conducted using a high-purity germanium (HPGe) detector system with emissions from four different radioactive sources (133Ba, 137Cs, 60Co, and 22Na) spanning nine different gamma energies ranging from 276.5 to 1332.5 keV. As a result of the experimental measurements, the linear attenuation coefficients (μ) of the shotcrete samples were obtained. A proportional increase in μ was determined with increasing barite content. Using the experimental mass attenuation coefficient (μ/ρ) results, half-value layer (HVL) and tenth-value layer (TVL) thicknesses, as well as mean free path (MFP) values, were calculated. An increasing B aggregate ratio did not have a significant effect on the μ/ρ value, while an increase was observed in the HVL, TVL, and MFP values. Moreover, the WinXCOM interface was employed for theoretical calculations, and the FLUKA Monte Carlo code was utilized for simulation calculations. The obtained data were compared with experimental results, revealing a good agreement between them. Based on the obtained data, it was concluded that increasing the B aggregate ratio resulted in improvement in mechanical properties compared to the control sample, and additionally, it was determined that B aggregate-added shotcrete is an effective gamma radiation shielding material.

Investigation of ferrochrome slag utilization in wet mix shotcrete: Mechanical strength and radiation shielding performance

In this study, wet-mix shotcrete specimens were produced by substituting natural aggregate (NA) with ferrochrome slag aggregate (FS). Volumetrically, the amount of NA was reduced by 25%, 50%, 75% and 100% and a corresponding amount of FS aggregate was added. The physical, mechanical, durability and radiation shielding properties of the specimens were analysed. The gamma radiation performance was determined using energies 133Ba, 137Cs, 60Co and 22Na (ranging from 276.5 keV to 1332.5 keV) and a high-purity germanium HPGe detector system. The neutron radiation performance of the shotcrete specimens was obtained using FLUKA simulation. The WinXCOM interface was used for theoretical calculations, while the FLUKA Monte Carlo code was used for simulation calculations, and the data obtained were compared. It was observed that shotcrete mixtures having up to 50% ferrochrome slag aggregate showed a significant increase in mechanical properties. In radiation shielding performances, linear attenuation coefficient (μ), half-value layer (HVL), Tenth-value layer (TVL), mean free path (MFP) and radiation protection efficiency (RPE) parameters increased with increasing ferrochrome slag aggregate ratio, while mass attenuation coefficient (μ⁄ρ) values remained almost constant. It was experimentally determined that the 5 cm thickness shotcrete produced had RPE values of 75% and 50% at low (276.4 keV) and high (1332.5 keV) energies, respectively. It was concluded that the use of 50% ferrochrome slag aggregate has a positive effect on the mechanical properties of shotcrete mixtures. Additionally, ferrochrome slag-reinforced shotcrete is a good gamma and neutron radiation shielding material.

Influence of materials and nozzle geometry on spray and placement behavior of wet-mix shotcrete

Shotcrete finds extensive applications in civil engineering, from repair and rehabilitation to new constructions. Nevertheless, shotcrete construction frequently suffers from placement issues, such as rebound and overshooting. These challenges can result in reduced operational efficiency, increased material wastage, diminished construction quality, and potential workplace hazards. It is worth noting that these placement issues are intrinsically linked to the spray characteristics of shotcrete. These characteristics are contingent upon various factors, including the material's flowability and viscosity, as well as the nozzle geometry. This research experimentally characterized the spray behavior of wet-mix shotcrete. High-speed cameras and image analysis techniques were utilized to capture and characterize wet-mix shotcrete on a full-scale spraying setup. The study experimentally determined the key placement performance parameters such as placement, rebound, and final build-up thickness and correlates the placement performance of shotcrete with the spray characteristics. The findings reveal that wider spray opening angles, smaller droplet sizes, and higher velocities contribute to improved material deposition, reduced rebound, and increased build-up thickness. The study also assesses the impact of nozzle geometry and rheology of the shotcrete mixture. Increased flowability led to slightly better spray behavior and placement. Higher material viscosity produced a more consistent spray which enhanced shotcrete performance. Multi-air jet guns, where compressed air directly contacts droplets, can enhance shotcrete spray efficiency.

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.

Rheological analysis of wet-mix shotcrete with various viscosity-modifiying admixtures

Shotcrete must be suitable for the process of transport (pumping) and application (spraying) on site. Therefore, it is necessary to obtain a suitable consistency and rheology for its placement. This paper evaluates the influence of various viscosity modifying admixtures (VMAs) on the rheology and thixotropy for application in wet-mix shotcrete. Six VMAs were used and classified into three groups, according to their composition: additives based on silica, phyllosilicates, and polymeric additives. These rheology modifiers were studied in depth in mortars, and the values of the yield stress (τo) and plastic viscosity (μ) of the material were obtained, as well as the thixotropy (hysteresis areas), which represents the energy required for the structural recovery of the fluid. To obtain these parameters, the fluid was tested under dynamic state with a laboratory rheometer, applying a ramp of shear rates. Additionally, the consistency in mortars was determined by obtaining the flow table diameter on the flow table test. This evaluation was carried out in mortars with different contents of superplasticizer (SP). All this information made it possible to evaluate the influence of SP in combination with each of the VMAs, obtaining a workability box, determining the hysteresis areas and verifying which combinations obtained a rheological behaviour superior to the control mix (without VMAs). The results described were correlated with the rebound index obtained in the shotcrete mixes performed on-site. The thixotropy in mortars and the values of the rebound rate on site led to the most accurate correlation that allowed the selection of the most effective VMAs for use in shotcrete. Lastly, both combined results (laboratory and on-site) permitted a process that facilitated analysis of the wet-mix shotcrete to be designed and optimised.

Motive Characteristics and Adhesion-Rebound Mechanism of Single Aggregate Based on Shotcrete

The theory of viscoelastic collision mechanics is used to explore the rebound and adhesion mechanisms of single aggregates in shotcrete at the meso-level in this study. Abrasive balls with different radiuses were used, fully considering the characteristics of the aggregates and the sprayed wall, to build multifactor physical models for the shotcrete rebound of the aggregate. The movement trajectories, velocities, and energy dissipations of abrasive balls are analyzed, and the shotcrete-rebound mechanical model of a single aggregate based on wet-mix shotcrete is proposed. The results of the study show that there are obvious differences in the rebound velocity of the aggregate in different collision forms, and the viscoelastic properties of aggregates affect the jet stability of wet-mix shotcrete. According to the quantitative and qualitative analyses, the energy loss during the collision-rebound of the aggregate is inversely proportional to the rebound velocity and proportional to the rebound resistance. The functional relationship between the resistance F and the particle radius R during the rebound and adhesion of aggregate can be expressed as F=CR1/5, where C is a constant.

On the porosity of low-clinker shotcrete and accelerated pastes

Although the number and size of interconnected pores have been identified as the most important aspects of concrete microstructure, comprehensive datasets on shotcrete porosity and pore size distributions are still scarce and their key controls are poorly investigated. In this study we investigate the effects of the spraying process, setting accelerator addition and mix design on the microstructure of real-scale dry- and wet-mix shotcrete and hand-mixed and sprayed accelerated pastes. A newly proposed deconvolution analysis of the pore size distributions, measured by mercury intrusion porosimetry, offers increased precision in determining the critical and median pore diameter parameters. In total >50 samples were analysed. Results show that the dry-mix shotcrete exhibits a shift towards coarser pore sizes (∼100–1 μm) than wet-mix shotcrete. Combinations of different supplementary cementitious materials are favourable for producing wet-mix shotcretes with refined pore structures. The addition of setting accelerators, up to 10 wt-% of binder mass, and the spraying process cause systematic variations in the pore volume and pore structure of (sprayed) paste and shotcrete.

Spatial distribution characteristics and microscopic mechanisms for enhancing mechanical properties of MWCNTs in recycled coarse aggregate shotcrete

This study aimed to investigate the spatial distribution characteristics of multi-walled carbon nanotubes (MWCNTs) in recycled coarse aggregate shotcrete (RCAS) and the influence of the dosage of MWCNTs on flow characteristic, mechanical properties and its microscopic mechanism. In this paper, 100% of natural coarse aggregate (NCA) was replaced by recycled coarse aggregate (RCA). Multiple nano-characterization techniques were adopted, and the wet-mix shotcrete technology and a combined device for variable addition of MWCNTs suspension were used. The experimental results showed that with the increase of the dosage of MWCNTs, the flow characteristic of RCAS in fresh state decreased. In terms of mechanical properties, the compressive strength, splitting tensile strength and flexural strength of RCAS all showed a trend of increasing first and then decreasing, when MWCNTs was 0.09 wt%, the flexural strength and splitting strength of RCAS reached the peak value. When the dosage of MWCNTs was 0.03 wt%, 0.06 wt% and 0.09 wt%, the quantity distribution of MWCNTs in the RCAS was average and centered. Appropriate dosage of MWCNTs (0.06 wt% or 0.09 wt%) had a positive effect on improving the compactness of interface transition zone (ITZ) of the RCA, optimizing the internal pore size distribution of RCAS, accelerating the hydration reaction rate and increasing the yield of hydration products.

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.

Experimental study of a wet mix shotcrete for primary tunnel linings—Part I: Evolution of strength, stiffness and ductility

A comprehensive experimental program on the mechanical, hygral and thermal behavior of a modern wet mix shotcrete is presented. The present part I deals with the evolution of the uniaxial compressive strength, splitting tensile strength, Young’s modulus, and specific mode I fracture energy. The results are compared to empirical relations used in some shotcrete models. Part II deals with creep and shrinkage of shotcrete as well as with the thermal and hygral material behavior. Together, part I and part II of this study lay the foundation for the thorough calibration and validation of material models for a wet mix shotcrete.

Experimental study of a wet mix shotcrete for primary tunnel linings—Part II: Shrinkage, creep, thermal and hygral behavior of shotcrete

Part II of the study of a wet mix shotcrete focuses on creep and shrinkage as well as on the thermal and hygral behavior. It deals with the time-dependent mechanical behavior due autogenous shrinkage, combined autogenous and drying shrinkage, and basic creep and combined basic and drying creep for moderate and high levels of sustained compressive stress, applied at different shotcrete ages. Furthermore, the evolution of the temperature due to hydration and the degree of hydration, the evolution of the porosity and the water content of sealed specimens, and the desorption isotherm are determined, providing additional material data for shotcrete models.

Influence of Colloidal Silica and Silica Fume on the Rheology and Mechanical Properties of High-Performance Shotcrete

Wet-mix shotcrete technology is widely used worldwide. One possible approach to developing this technology is the use of various admixtures to improve its pumpability and shootability. However, the commonly used slump test is insufficient to evaluate the capabilities of fresh shotcrete, because they do not assess the influence of multiple factors on its workability and placement properties. The purpose of this paper is to investigate and determine the effect of colloidal silica with a particle size of 10 nm (with 2%, 3%, and 4% replacement) on high-performance shotcrete from a rheological and mechanical perspective. In addition, to compare these indicators, silica fume is used with a 4% and 7% mixing rate. Rheological parameters such as torque viscosity and flow resistance of fresh concrete were evaluated using an ICAR rheometer, and the measured values were correlated with pumpability and shootability. The results of the study demonstrate that torque viscosity decreases and flow resistance increases with the increase in the percentage of colloidal silica. Similarly, silica fume increases flow resistance and reduces torque viscosity. These findings demonstrate the possibility of obtaining high-performance shotcrete with improved pumpability and shootability by employing more colloidal silica and silica fume. It was also established that these supplementary cement materials improve the strength and durability of shotcrete.

The hydration of fast setting spray binder versus (aluminum sulfate) accelerated OPC

The (early) hydration mechanisms of two different binder systems used for shotcrete were investigated: the so far almost unexplored low sulfate binder (spray binder), used in the field of dry-mix shotcrete; and ordinary Portland cement, accelerated by aluminum sulfate, widely used for wet-mix shotcrete. The basis for the fast setting of the spray binder is the rapid dissolution of C3A and the subsequent formation of flaky CO3-AFm phases. Thereby induced high aluminum concentrations in the pore solution lead to a blockage of alite dissolution during the first hours of hydration. At later stages, higher amounts of portlandite are formed in the dry-mix, compared to the wet-mix system. The lower calcium availability for portlandite formation in the wet-mix system is explained by an enhanced formation of C–A–S–H phases with a higher Ca:Si ratio. Additionally, wet-mix systems show lower porosity and higher compressive strength after 1 d of hydration and beyond.Graphical abstract

An Optimization Method for Mix Proportion of Wet-Mix Shotcrete: Combining Artificial Neural Network with Particle Swarm Optimization

Wet-mix shotcrete has been widely used to support roadways during the excavation of underground mining. In practice, the mix proportion of wet-mix shotcrete plays a key role in later successful application. To obtain an optimal mix proportion, a large number of trial physical experiments should be carried out. Therefore, in this paper, a new ANN–PSO model is proposed to obtain the mix proportion of wet-mix shotcrete quickly, precisely and economically. The artificial neural network (ANN) model was used to establish the objective functions for particle swarm optimization (PSO) optimization, while the PSO was adopted to optimize mix proportions of wet-mix shotcrete to achieve optimal objectives. This hybrid model was applied to optimize mix proportions of wet-mix shotcrete in the Jinchuan mine. The results revealed that the ANN model yielded a mean relative error (MRE) of 2.755% and an R2 of 0.980, indicating an excellent prediction to establish the reasonable objective function. Additionally, PSO spent less than 60 s obtaining an optimal mix proportion of wet-mix shotcrete required by the mine. Consequently, this ANN–PSO model can be used as an efficient design guide to facilitate decision making, prior to the construction phase.

Flow ability and Mechanical Properties of Shotcrete concrete incorporated with Waste Plastic Fibers

Polyethylene terephthalate (PET) fiber is a green-friendly fiber that is capable of enhancing the mechanical properties of wet-mixing shotcrete. The main purpose of this study is to see how varied volumes of waste plastic fibers (WPF) affect the flowability and mechanical properties of wet-mix shotcrete. For this aim, a variety of experimental tests based on WPF content were chosen. Fresh and mechanical tests included slump, T500, density, compressive strength, and splitting strength were applied. The results shown a improved in shotcrete performance as the WPF content increased. Among all fitting correlations, density and compressive strength revealed the strongest linear ship association. Due to greater interlocking between WPF and concrete matrix, WPF was a major use in enhancing splitting tensile strength. WPF had the most influence on splitting strength, with 23–31 percent, 7–23 percent, and 6–38 percent for 7, 14, and 28-day, respectively.

자연 섬유와 광물성 혼화재를 혼입한 스케이트 파크용 습식 숏크리트의 역학적 특성 및 내구성

본 연구는 스케이트 파크용 습식 숏크리트 개발을 위하여 자연섬유와 광물성 혼화재를 혼입한 습식 숏크리트의 역학적 특성과 내구성으로 염소이온침투저항성, 소성수축균열, 마모저항성을 평가하였다. 공기량과 슬럼프는 각각 6.0±1.5 %, 130±20 mm를 만족하여 펌핑성과 붙임성을 확보할 수 있을 것으로 판단된다. 압축강도는 모든 변수에서 고성능 숏크리트의 기준인 35.0 MPa 이상으로, 휨강도는 4.5 MPa 이상으로 측정되었다. 강도측면에서는 자연섬유의 혼입보다는 광물성 혼화재의 혼입이 보다 큰 영향을 미친 것으로 확인되었다. 염소이온침투저항성은 자연섬유의 혼입으로 오히려 통과전하량이 증가하여 염소이온 침투저항성이 감소하는 것으로 나타나, 광물성 혼화재의 추가적인 혼입이 필요하다는 것을 확인하였으며, 이를 통하여 높음에서 낮음의 수준까지 염소이온 침투저항성을 향상시킬 수 있었다. 소성수축에 의한 균열 폭은 평균을 기준으로 자연섬유의 혼입으로 약 80 % 수준으로 저감시킬 수 있었으며, 광물성 혼화재의 혼입에 따른 영향은 뚜렷하게 나타나지는 않았다. 마지막으로, 마모 깊이는 모든 변수에서 2.0 mm 이하로 측정되어 마모저항성이 매우 우수한 것으로 나타났다. 자연섬유의 혼입으로 약 63 % 수준으로, 광물성 혼화재의 혼입으로 약 53 % 수준으로 마모 깊이가 저감되는 것을 확인하였다.

Workability, strength and toughness properties of different types of fiber-reinforced wet-mix shotcrete

The main focus of this study is to investigate the effects of macro steel (ST), forta-ferro (FF) and polyamide (PA) synthetic fibers on the slump, compressive, splitting tensile, flexural strength, flexural toughness and rebound rate of wet-mix shotcrete. The toughness capacities of fiber reinforced shotcrete (FRS) mixes was also evaluated in accordance with the JSCE SF-4 and CECS 13-2009 standards. The fiber volume fractions added into the shotcrete mixes are 0.25%, 0.5%, and 1%. The test results showed that when the fiber ratio is increased from 0.25% to 1%, slump values for all the FRS mixes decreased significantly and the highest decrease occurred for the PAFRS mixes. The ST fibers are highly more effective than PA and FF synthetic fibers to enhance the strength and toughness capacities of concrete due to ST fibers have higher tensile strength and outstanding geometrical shape. The CECS 13-2009 and JSCE-SF4 standards are sufficient to calculate the toughness capacities of FRS specimens up to their limit deflection values. However, if higher deflection values were used by these two standards, the significant contribution of all fibers in the inelastic region would be more realistically taken into account.

Sulfate durability and leaching behaviour of dry- and wet-mix shotcrete mixes

Shotcrete is a material frequently used in underground constructions such as tunnels, where Ca leaching and sulfate attack are important durability issues. In this study, two test methods were employed to investigate the sulfate resistance and leaching behaviour of dry- and wet-mix shotcretes in sulfate solutions on a time-resolved basis. Tests on powdered samples showed the dissolution of AFm, portlandite and C-A-S-H, subsequently followed by the precipitation of calcite and ettringite. The extent of sulfate expansion of drill cores corresponded to the chemical response of the powder materials, particularly on the reactivity of Ca- and Al-containing hydrated cement phases. The use of high-purity metakaolin (7–10 wt%) in the shotcrete binder significantly decreased the sulfate durability, while high amounts of other SCMs such as blast-furnace slag and silica fume (>20 wt%) had a positive effect on the durability of shotcrete.