Mechanical Properties Of Shotcrete Research Articles

A study on the sulfate erosion deterioration law and damage model of shotcrete in high geothermal tunnels

AbstractWhen building a tunnel in an environment rich in high‐temperature hot water, it is particularly necessary to pay attention to the influence of sulfate ions in underground hot water on tunnel shotcrete. In order to study the sulfate erosion mechanism and mechanical properties of shotcrete in a real high‐temperature hot water environment, this study was carried out by setting the curing temperature (20, 40, 60, and 80°C), humidity (55% RH, 95% RH), and erosion age (0, 15, 30, 60, and 90 d) as the test influencing factors; a full combination of dry‐wet cycle test was carried out, and the specimens under different conditions were analyzed macroscopically and microscopically. The results show that with the increase of the number of dry‐wet cycles, the quality of shotcrete increases first and then decreases, and the mechanical properties gradually decrease. In the early stage of erosion, the erosion product is mainly ettringite, and the macroscopic damage is aggregate spalling. In the later stage of erosion, the erosion product is mainly gypsum, and the macroscopic damage is expansion damage. Compared with standard curing, a certain degree of high temperature curing has little effect on the sulfate attack resistance of shotcrete, but when the curing temperature exceeds 60°C, the concrete is seriously damaged. Finally, by constructing the damage model of sulfate attack shotcrete, the variation of compressive strength of shotcrete with age after sulfate attack under different curing conditions was successfully predicted.

Study on the Influence of Shrinkage-Reducing Materials on the Volume Stability of Shotcrete with a Fluorine-Containing Alkali-Free Accelerator.

In contrast to ordinary concrete, shotcrete exhibits greater shrinkage deformation. The utilization of an aluminum fluoride alkali-free accelerator (AF) is prevalent in tunnel engineering, attributed to its economical nature and fast setting and hardening. Currently, there is a dearth of research on the volume stability of aluminum fluoride accelerators, and no relevant studies have been conducted on the application of shrinkage-reducing materials in shotcrete using an aluminum fluoride accelerator. With the use of an aluminum fluoride accelerator, the early strength of shotcrete is reduced, and the addition of a shrinkage-reducing agent (SRA) and super-absorbent polymer (SAP) can also lead to a further reduction in the early strength of concrete. Therefore, this study aims to investigate the influence of these two shrinkage-reducing materials (SRA and SAP) on the workability and mechanical properties of shotcrete. The primary focus of this research is to determine the better compensation effect of the two shrinkage-reducing materials. To accomplish this, the study focuses on assessing the setting time, compressive strength, and volume stability of shotcrete that has been mixed with an aluminum fluoride accelerator using SRA and SAP. Additionally, we also aim to explore the hydration and shrinkage compensation mechanism of these shrinkage-reducing materials on shotcrete by conducting a microscopic test analysis.

Mechanical properties of shotcrete with addition of sugarcane bagasse fiber and ash

Mechanical properties of shotcrete with addition of sugarcane bagasse fiber and ash

Effect of Fly Ash Content in Shotcrete on Mechanical Properties

As one of the key technologies of NATM (New Austrian Tunnel Method) construction, shotcrete has advantages such as simple, fast and adaptable technology, and becomes an indispensable modern construction method in tunnel construction. Adding some fly ash into shotcrete can reduce cracks, compensate shrinkage and improve workability. In this paper, raw materials are selected for mix design, and the mechanical properties of shotcrete with different fly ash content are experimentally studied. The test results show that the 28d compressive, flexural and tensile properties of sprayed concrete shotcrete gradually decrease with the increase of fly ash content. In order to avoid too much strength reduction and ensure structural safety, the fly ash ought to be controlled within a certain amount in the actual construction process.

Study on Mechanical Properties of Basalt Fiber Shotcrete in High Geothermal Environment.

With the development of infrastructure, there are growing numbers of high geothermal environments, which, therefore, form a serious threat to tunnel structures. However, research on the changes in mechanical properties of shotcrete under high temperatures and humid environments are insufficient. In this paper, the combination of various temperatures (20 °C/40 °C/60 °C) and 55% relative humidity is used to simulate the effect of environment on the strength and stress–strain curve of basalt fiber reinforced shotcrete. Moreover, a constitutive model of shotcrete considering the effect of fiber content and temperature is established. The results show that the early mechanical properties of BFRS are improved with the increase in curing temperature, while the compressive strength at a later age decreases slightly. The 1-day and 7-day compressive strength of shotcrete at 40 °C and 60 °C increased by 10.5%, 41.1% and 24.1%, 66.8%, respectively. The addition of basalt fiber can reduce the loss of later strength, especially for flexural strength, with a increase rate of 11.9% to 39.5%. In addition, the brittleness of shotcrete increases during high temperature curing, so more transverse cracks are observed in the failure mode, and the peak stress and peak strain decrease. The addition of basalt fiber can improve the ductility and plasticity of shotcrete and increase the peak strain of shotcrete. The constitutive model is in good agreement with the experimental results.

Application of Advanced Composite Cementitious Materials and High Performance Admixtures in High Performance Shotcrete

In this paper, the influence of liquid alkali-free accelerator and advanced composite cementitious materials on the mechanical properties, durability and workability of shotcrete was studied. The important role of the two materials in the performance improvement of shotcrete was clarified. The technical measures proposed were verified by field test. The results show that: In terms of workability, the composite use of alkali-free accelerator and functional admixture can achieve fast setting and high early strength of shotcrete, thus effectively reducing the rebound rate; In terms of mechanical properties, functional admixture can effectively improve the early mechanical properties of shotcrete, while alkali-free accelerator can ensure the long-term development of mechanical properties; In terms of durability, the use of functional admixtures can effectively improve the compactness of shotcrete, and the test results of 56d electric flux are lower than 1000C.

Improvement Mechanism of Calcium Sulphoaluminate Cement on the Early Mechanical Properties of Shotcrete at Low Temperature

Improvement Mechanism of Calcium Sulphoaluminate Cement on the Early Mechanical Properties of Shotcrete at Low Temperature

The Effects of Marble Dust on the Rheological and Mechanical Properties of Shotcrete

Recycling industrial waste materials has become an environmental and economic necessity. The utilization of these materials to develop concrete contributes not only to their disposal, but also to the preservation of the environment. This study attempted to evaluate the properties of sprayed concrete with Marble Dust (MD) as partial replacement of concrete components. For this purpose two series of concrete mixtures were prepared: the first series included six mixtures containing MD as cement replacement with percentages of 5%, 10%, 15%, 20%, 25%, and 30% by weight, the second series included six mixtures containing 5%, 10%, 15%, 20%, 25%, and 30% of MD as replacement for sand. Also, conventional concrete mixture with 100% cement and 100% sand was produced as control mixture. Rheological and mechanical properties as pumpability, build-up thickness, rebound percentage, adhesion strength, and compressive and tensile strength were studied. In general, the results indicate that use of MD improves shotcreting as an application and shotcrete’s performance.

Effects of Diethanolamine (DEA) and Glass Fibre Reinforced polymer (GFRP) on setting time and mechanical properties of shotcrete

By now, various accelerators have been used to accelerate the chemical reactions between cement and water to achieve a desired strength. This study investigates the effects of diethanolamine (DEA) as a less corrosive accelerator on the setting time and mechanical properties of fibre reinforced shotcrete (FRS). Glass fibre reinforced polymer (GFRP) was also used to improve the flexural and tensile strengths of shotcrete. In total, 76 different mix designs were prepared and tested to determine the setting time, flexural and tensile strength of shotcrete. The mixes were designed based on three influential parameters including GFRP, water/cement (w/c) ratio and the amount of DEA. According to the results, the flexural and tensile strengths of all specimens increased by adding 0.5% of the weight of GFRP. However, both tensile and flexural strengths showed an insignificant reduction by adding DEA to the mixes. The tensile and flexural strengths of specimens decreased slightly by adding DEA in the presence of GFRP. At a certain amount of DEA, the setting time increased twice with increasing the w/c ratio. The results showed a reduction in the shotcrete strength by using DEA. Thus, in the case where both the setting time and shotcrete strength are of great importance, the optimal DEA level is close to 0.3% of the dry weight of cement and 0.5% GFRP.

Research on influences of fly ash content on mechanical properties of shotcrete

Shotcrete technology is a key technology of the New Austrian tunnelling method. Owing to advantages of simple and quick process of construction and strong adaptability, it is an indispensable means of modern tunnel construction. Mixing shotcrete with appropriate amount fly ash not only can reduce the generation of cracks and compensate the shrinkage, but also can improve the shotcrete peace ability and reduce the cost of project construction. Through a given number of tests, we study on the influences of fly ash content on the mechanical properties of shotcrete, i.e., tensile, compressive, flexural strength. The results of experiments show that: the strength indexes of shotcrete decrease gradually with the increase mixing content of fly ash. When the fly ash content is more than 28%, the concrete strength has serious loss. For ordinary shotcrete, it is suggested that the optimum dosage of fly ash should be about 9%.

Durability performance of brine-exposed shotcrete in salt lake environment

Salt lake is widely distributed in Western China. The high concentrations of chlorine salt, sulfate, and magnesium salt in soil and underground water are the leading causes to the deteriorated performance of lining shotcrete and corrosion of reinforcement. This study aims to investigate the durability performance and deteriorated rules and mechanism of shotcrete lining in salt lake corrosion environment by using dry-wet (D&W) alternate method for simulating the corrosion mode of tunnel lining. A shotcrete durability experiment is performed in 5% Na2SO4 + 5% MgSO4 + 3.5% NaCl solution. Relative dynamic elastic modulus, mass change, and relative compressive strength are measured. Thereafter, the mineral composition and microscopy of corrosion products and the air-void structure parameter and ion content of water-soluble sodium, chloride, and calcium ions; acid-soluble sulfate ion; and pH value of pore solution are tested using X-ray diffraction, thermogravimetric-differential scanning calorimetry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, RapidAir 457, and electrochemical method.Acid solubility sulfate content clearly increased with the D&W cycle, and water-soluble chloride and sodium content slowly increased. Meanwhile, water-soluble calcium and concrete pH value decreased. The combination of the expansion stress and crystallization pressure from thaumasite, brucite, gypsum, ettringite, and crystallized salt enlarged the air-void number and air content in shotcrete. Moreover, web crack formation caused deterioration of the physical and mechanical properties of shotcrete. Meanwhile, normal concrete was damaged by macrocracks connected with air void and microcracks, thereby creating crystallization pressure. Drawing stress between steel fiber and shotcrete could evidently absorb the expansion stress and crystallization pressure. Consequently, steel fiber could improve shotcrete corrosion resistance.

SHOTCRETE IN MINE CONSTRUCTION

This article deals with concerns of construction of subsurface structures with the use of shotcrete as a support, the samples of mine construction with application of shotcrete have been shown, in particular projected mine with vertical well bore. The calculation method for construction of well bore with the use of shotcrete support has been stated, the major analytical relationships have been set out, a graph showing the growth of strength in a support has been generated for several sites according to the results of actual geotechnical survey. The research of properties of concrete with fast development of strength has been implemented for mine engineering and construction of transportation infrastructure. The results of experimental analysis of shotcrete with organo-mineral and chemical additives have been listed. Performance improvement of physical and mechanical properties of shotcrete has been proven when the complex active nano agent added into composition of the binder. Improved kinetics of development of strength related to accelerated admixture helps in proportioning concrete compositions for practically any firm and medium-firm grounds during construction of support of subsurface structures.

Application of carbonate precipitating bacteria for improving properties and repairing cracks of shotcrete

Shotcreting is a construction technique commonly used in many civil and mining engineering applications. Cracking is an unavoidable and inherent weakness of most cement-based construction materials including shotcrete. In this study, the effect of Bacillus Subtilis on healing and mechanical properties of shotcrete was evaluated. For this purpose, bacteria were introduced into mix design and curing solution in order to examine the effect of each approach on the compressive strength, tensile strength, permeability, porosity and healing of shotcrete specimens. The results of uniaxial compressive test showed up to 30% increase in the compressive strength of bacteria-exposed shotcrete specimens compared to control specimens. This strength improvement was 10% more in shotcrete specimens than in the conventional cast concrete specimens with the same mix design. The presence of bacteria, both in the mix design and curing solution, was found to enhance the tensile strength and decrease the water absorption and porosity of shotcrete. The maximum reduction in water permeability at 28days was observed in the specimens cured in the bacteria-containing solution. The ability of bacteria to heal the cracks created in the specimens was also evaluated in the present work. Bacterial precipitation of calcium carbonate in the pores of specimens was confirmed by SEM/XRD analysis. The results give clear evidence supporting the utility of calcium carbonate precipitating bacteria for healing the cracks and improving the durability of shotcrete.

Mechanical properties, permeability and durability of accelerated shotcrete

In this paper, the experiment is completed into two phases. In the first phase, the performance of setting time of pastes, mechanical properties of shotcrete at different hydration age and permeability of shotcrete at 90day were tested. Meanwhile, in the second phase, the durability performance of shotcrete with 10% fly ash and 0.64 vt.% steel fiber was studied under environment corrosion which included freeze–thaw cycle, accelerated carbonation test with and without bending load, sulfate attack, chloride ion diffusion and acid corrosion respectively. The results showed that, in comparison to ordinary concrete (OC) with the same mixture, ordinary shotcrete (OS) had better durability properties except to chloride ion. Thus, steel fiber (steel fiber reinforced shotcrete, SFRS) not only led to much denser microstructure in shotcrete matrix but also could significantly improve the early-age compressive and splitting tensile strength, permeability and durability of shotcrete.

Microstructure, permeability and mechanical properties of accelerated shotcrete at different curing age

Given the different hydration processes of ordinary concrete, shotcrete with low-alkali accelerator not only has short final setting time and high early age mechanical properties but also different hydration products. Shotcrete microstructure was studied at different curing ages by X-ray diffraction, thermogravimetry–differential scanning calorimetry, and scanning electron microscopy to investigate its hydration process. The pore structure and permeability of shotcrete were also tested. The mechanical properties of shotcrete were then studied. Results showed that under the effect of accelerator, the retarded action of gypsum disappeared in the cement–accelerator–water system. C3A hydrated quickly to form calcium aluminate hydrate (CAH) crystals, and a mesh structure was formed by ettringite, albite, and CAH. A large amount of hydration heat improved the hydration rate of the cement clinker mineral and the density, thus leading to high mechanical properties at the early curing age. The setting time of pastes increased with increasing water–binder ratio and fly ash (FA) dosage. Thus, the hydration process and the microstructure and morphology of the hydration products changed. Excessive water and FA increased the percentage of pores in shotcrete, which directly caused poor permeability. The permeability and compressive strength were poor when the FA dosage was 30% compared with that of 20%. Based on the investigation, a simple 0.5 power relationship between compressive strength and splitting tensile strength of mixture S2, meanwhile, there was an exponential relationship between mechanical performance and permeability or porosity, respectively.

Influence the Carbonation Resistance and Mechanical Properties of Shotcrete by Accelerated Carbonation Test

In order to investigate the carbonation resistance of shotcrete and the mechanical properties after carbonation, the accelerated carbonation test was carried out. The results indicate that the carbonation resistance of shotcrete is superior to that of normal concrete. With the increasing of carbonation depth, compressive strength and splitting tensile strength of shotcrete grew rapidly. The admixing of steel fiber can further improve the carbonation resistance, reduce the carbonation rate, and increase the splitting tensile strength of shotcrete greatly. Besides, based on analyzing the effects of construction technology and steel fiber of concrete for the carbonation resistance, a carbonation depth model for shotcrete was established. Key words: shotcrete; carbonation; steel fiber; mechanical properties

Experiment Study on Mechanical Properties of Shotcrete under Heat Injury Environment of High Ground Temperature Tunnel

With the tunnel growing more long and deep,heat injury problems have become increasingly prominent.In order to analyze how would the environment of high ground temperature impact on compressive strength of shotcrete, experiment was designed to explore the compressive strength of C25 shotcrete at 28d age by simulating environmental. The test results show that: water tank and the oven can more accurately simulate the natural hot-humid and hot-dry environment; Compared to the standard curing conditions, Whether in the hot-humid environment or hot-dry environment, the compressive strength of shotcrete is decreased. Mixed with 25% fly ash can improve the compressive strength of shotcrete in hot-humid environment.

Influence of Aspect Ratio of Macro Synthetic Fiber on Mechanical Properties of Shotcrete

Influences of moisture content and loading rate on flexural toughness were experimentally studied for fiber reinforced shotcrete (FRSC) with steel fiber or macro synthetic polypropylene fiber. According to the four-point bending test method specified in ASTM C1609 and Chinese standard CECS 13, the flexural toughness of specimens after drying for 0h, 16h, 24h and 72h in condition of (20±2)°C and (60±5)% relative humidity was tested at a loading rate of 0.05 mm/min. For specimens after drying for 24h and 72h, flexural toughness was tested at loading rates of 0.05 mm/min, 0.10 mm/min, and 0.20 mm/min respectively. With the moisture content decreasing, the flexural toughness T100,2.0, first-peak flexural strength, and residual flexural strength at prescribed deflections of FRSC exhibited decreasing tendency. The specimens with 0.5 vol% of steel fiber showed higher T100,2.0 value than that with 0.9 vol% of macro synthetic fiber. The residual strength and flexural toughness of FRSC increased with the increase of loading rate.