Strength Of Reactive Powder Concrete Research Articles

Compressive Strength Study on Reactive Powder Concrete with 30% Quartz Sand and Variations in Fly Ash Composition as Partial Substitution of Cement

The concrete industry is considered environmentally unfriendly and unsustainable due to the significant consumption of natural materials. Currently, the industry predominantly uses Portland cement as its main ingredient, leading to an increase in Portland cement production. However, the use of fly ash can help make the concrete industry more sustainable in the future. Fly ash can be used as a partial replacement for Portland cement in concrete production. This study aims to determine the effect of fly ash variations on the compressive strength of reactive powder concrete. The research method used is experimental. The concrete mix design includes 30% quartz sand and fly ash variations of 0%, 5%, 10%, 15%, 20%, and 25%. The compressive strength test specimens are cylindrical with a diameter of 7.5 cm and a height of 15 cm. The resulting test specimens have a compressive strength of more than 41.4 MPa, thus qualifying as high-strength concrete. The compressive strength test results for fly ash variations of 0%, 5%, 10%, 15%, 20%, and 25% are 62.62 MPa, 66.27 MPa, 75.59 MPa, 68.78 MPa, 66.21 MPa, and 63.70 MPa, respectively.

Compressive stress-strain relationship of steam free reactive powder concrete at ultra-low temperatures

To explore the applicability of reactive powder concrete (RPC) as a material for LNG storage tanks, the mechanical properties of RPC at ultra-low temperatures were investigate in this paper. Forty-two axial compressive strength tests were conducted on RPC prisms at 20 °C ∼ −165 °C. The influence of temperature and steel fiber on the compressive stress-strain relationship of RPC at ultra-low temperatures was evaluated. The results showed that the axial compressive strength of RPC increases at low temperatures, with a compressive strength increase of over 70 % at −165 °C compared to room temperature. The increase in compressive strength of RPC at ultra-low temperatures is significantly greater than that of ordinary concrete. A relation for predicting the axial compressive strength development of RPC at ultra-low temperatures was also proposed. Steel fibers can increase the toughness of RPC at ultra-low temperatures to a certain extent, and effectively improve the failure mode of RPC, allowing the damaged RPC to maintain its integrity. This work provides a basis for RPC applicability as LNG storage tanks.

Mechanical properties of steel fiber RPC, basalt fiber RPC, and hybrid fiber RPC: A review of research progress

AbstractBased on an analysis of the literature on fiber‐reinforced concrete (FRC), especially the development and formation of various types of FRC combined with reactive powder concrete (RPC) in recent years, this paper systematically expounds the research progress, particularly the mechanical properties, of steel fiber RPC, basalt fiber RPC and hybrid fiber RPC. This paper concludes the improvement in the mechanical properties of RPC under common curing methods, elaborates on the influence of steel fiber on the compressive, tensile, and flexural strength of RPC, and outlines the principle of fiber enhancing the mechanical properties of RPC. Additionally, it discusses the research progress in basalt FRC, and further examines the impact of basalt fiber on the mechanical properties of RPC. Referring to the research on single fiber reinforced RPC, the mechanical properties of hybrid fiber RPC are analyzed and summarized; based on the characteristics of steel fiber and basalt fiber, research on steel‐basalt hybrid fiber RPC is proposed.

Experiments and analytical model for axial strength of FRP-reinforced reactive powder concrete circular columns under axial compression

Although various experimental studies have been performed in the literature to explore the behavior of glass fiber reinforced polymer (glass-FRP) reinforced conventional concrete columns, no study focused on the axial performance of glass-FRP-reinforced reactive powder concrete (RPC) columns (GRPC columns) subjected to compression. Therefore, the current struggle investigates the behavior of nine GRPC columns through experimental testing, finite element analysis (FEA), and theoretical models on the circular samples with 250 mm diameter and 1000 mm length under axial compression. Steel fibers are added to RPC to minimize brittleness and to improve the strength of RPC. Moreover, a parametric study is planned to utilize the suggested FEA model for exploring the effects of various variables of GRPC columns on axial structural performance. The test findings showed that the GRPC columns become more ductile with the addition of fibers and a reduction in the vertical distance between the glass-FRP hoops/ties. The failure of GRPC columns occurred in the middle region of the samples with the delayed spalling of the concrete cover. The suggested analytical model based on the 600 data points presented higher accuracy than previous theoretical models. The recommended FEM for the axial capacity and associated axial shortening of examined samples portrayed errors of 3.25% and 4.65%, respectively. Considering the axial contribution of glass-FRP longitudinal bars and the transverse confining mechanism of FRP stirrups, the recommended theoretical model for the axial capacity of GRPC column samples performed well for the test findings of the examined samples with R2 = 0.842, MSE = 139.559, MAE = 195.538, and RMSE = 200.756.

Intelligent data-driven compressive strength prediction and optimization of reactive powder concrete using multiple ensemble-based machine learning approach

In recent years reactive powder concrete (RPC), also known as ultrahigh-performance concrete, emerged as one of the most efficient building materials due to its ultrahigh compressive strength (usually greater than 150 MPa), ductility, and durability. The existing literature presents a wealth of experimental data exploring the intricate interactions among the various components within RPC mixtures, emphasizing the requirement for a potent and efficient model to optimize, simulate, and enrich the understanding of RPC mixture characteristics. This study presents a notable attribute utilizing a multilayer stacked model that harnesses the potential to harmoniously seamlessly integrate and synergize various machine learning (ML) algorithms. An extensive dataset from reliable resources in the literature was used to develop a new ML model for predicting the compressive strength of RPC. It was found that there is a strong positive correlation between the RPC’s compressive strength and the dosage of silica fume, fiber dosage, and age of concrete. In contrast, water content and dosage of GGBFS were observed to have a low correlation with the compressive strength of the RPC. The most significant features were age, fiber dosage, silica fume content, cement content, HRWR dosage, fine aggregate content, and water content. Further, in this study, predictive efficacy identifies the stacking algorithm as optimal, followed by XGBoost, RF|ETR, and KNN. Notably, R2 strengths vary: stacking achieves 0.96, XGB sustains 0.954, RF and ETR hold 0.95, while KNN reaches 0.77.

The Working Performance and Mechanical Strength of Reactive Powder Concrete with the CO2 Curing Method on the Surface of Secondary Aluminum Ash

Secondary aluminum ash (SAA) is a common waste that, without reasonable treatment, results in pollution to the environment. A large amount of CO2 is emitted by human activities every day. If the CO2 cannot be treated in a timely manner, it will accelerate the greenhouse effect and pollute the environment. The CO2 curing on the surface of SAA can reduce excess CO2 emissions while improving the performance of the SAA. The application of CO2-cured SAA can simultaneously consume the emitted CO2 and solidify the SAA. In this article, the effect of CO2-cured secondary aluminum ash on the rheological properties, the initial setting time, the flexural strength (ft), the compressive strength (fcu) of reactive powder concrete (RPC), and the corresponding dry shrinkage rate (DSR) are investigated. Meanwhile, the capillary water absorption, the chloride ion migration coefficient (CMC), and the carbonization depth of RPC are determined. Scanning electron microscope (SEM) and the X-ray diffraction spectrum curves are selected to reveal the mechanism of the macro performance. Results indicate that CO2-cured secondary aluminum ash can increase the fluidity and decrease the plastic viscosity of fresh RPC. The initial setting time is increased by the CO2 curing. CO2-cured secondary aluminum ash can increase the ft and fcu by (0%~26.3% and 0% to 68.7%), respectively. The DSR is increased by adding secondary aluminum ash with an increasing rate of 0% to 91.3%. The capillary water absorption of RPC increases in the form of a linear function. The CMC and the carbonization depth of RPC are decreased by adding the CO2-cured secondary aluminum ash with decreasing rates of 0%~46.7% and 0%~45.7%. The CO2-cured secondary aluminum ash can make the hydration more compact and increased increase the hydration products (Ca(OH)2).

Investigation of the effect of different grading on the mechanical properties of reactive powder concrete

In this study, 11 designs with sand-to-cement ratios of 4.5, 4.75, 5, 5.25, 5.5, 5.75, 6, 6.25, 6.5, 6.75, and 7 were used to investigate the impact of increasing the sand-to-cement ratio in reactive powder concrete. Laboratory results indicate a decrease in compressive and bending strength, following an increase in the ratio of sand to cement. Sample S7 with the highest ratio of sand to cement has the lowest bending strength and compressive strength at the ages of 1, 7, 14, 28, 42 and 56 days.In the second process, 1 % polymer resin was added to the sand-to-cement ratio from the first step. By analyzing the results, it was observed that the addition of 1 % of polymer resin causes a decrease of 40.8 % in compressive strength at the age of 1 day. The biggest one-day pressure drop is S6.75 with a drop of 68.1 %. As the samples age, adding 1 % of polymer resin increases the compressive strength at 11 sand-cement ratios, so that the 90-day compressive strength of the S4.75 sample reaches 59.7 MPa. The addition of 1 % polymer resin also increased the bending strength by 9.1 %. After examining the results of the first and second steps of the study, the sand to cement ratios of 5.5, 6, 6.25, 6.5 and 7 were selected as optimal and suitable samples. In the third step of the study, coarse aggregate with percentages of 10, 20, 30, 40 and 50 was added to the selected samples. The results showed that the addition of coarse aggregate caused a decrease of 41.1 % in the compressive strength of reactive powder concrete at the age of 1 day, compared to the control sample. In the third step, the maximum compressive strength at the ages of 28, 42, 56 and 90 days for sand-cement ratios of 5.5, 6, 6.25, 6.5 and 7 corresponds to the sample containing 50 % of coarse aggregate. The best samples of the third step in flexural strength are those with 50 % gravel, which increases flexural strength with increasing sand-cement ratio. On the other hand, with the addition of coarse aggregate, an increase in flexural strength was observed compared to the control sample of reactive powder concrete. In the fourth step of the study, 1 % of polymer resin was added to the samples of the third step. The results of this step show that, like the third step, the maximum compressive strength of the ages of 28, 42, 56 and 90 days for the sand-cement ratios of 5.5, 6, 6.25, 6.5 and 7 for the sample with 50 % coarse aggregate. In the fourth step, by increasing the ratio of sand to cement, samples with a lower percentage of sand have higher bending strength.

Study on Mechanical Properties and Microstructure of Basalt Fiber Reactive Powder Concrete

In order to promote the wide application of reactive powder concrete (RPC) in practical engineering. In this paper, RPC was prepared using conventional and economical natural river sand instead of quartz sand and economical and environmentally friendly basalt fiber (BF) instead of steel fiber, and the macroscopic properties of basalt fiber reactive powder concrete (BFRPC) with different fiber content, such as flowability, failure mode, compressive strength and splitting tensile strength were studied, and the strength calculation formula of BFRPC was established based on the mechanical property results. The microscopic morphology and structure of BFRPC were characterized by scanning electron microscope (SEM) and Image Pro Plus (IPP) image processing software. The results show that BF has a small effect on the compressive strength of RPC, while it has a significant increase on the splitting tensile strength. When BF content is at 2 kg/m3, the 28-day compressive strength reaches 95.2 MPa and splitting tensile strength reaches 7.78 MPa. Compared with the RPC with BF of 0 kg/m3, the BFRPC shows an improvement in its 28-day compressive strength by 25.70% and an increase in its splitting tensile strength by 83.92%. According to the microscopic analysis, reasonable fiber content can optimize the internal microstructure of BFRPC, but excessive BF content will produce agglomeration and overlap, resulting in strength loss. Based on the gray correlation analysis method, it was concluded that the particle area ratio and pore fraction dimension were the most correlated with the mechanical properties of BFRPC. In addition, the feasibility and applicability of the BFRPC strength calculation formula were summarized. This research results of this paper provides a valuable reference for the further research and promotion of BFRPC.

Experimental and numerical studies on the fire resistance of prestressed reactive powder concrete beams

The prestressed reactive powder concrete beam (PRPCB) is an advanced structural member that can take full advantage of the ultra-high compressive strength of reactive powder concrete (RPC) and the high tensile strength of tendons. The fire safety of PRPCBs has been mainly studied at the material level, with relatively few experimental studies being conducted on the structural member level with the principal reason of serious fire-induced spalling. This paper proposed an innovative curing method to prevent RPC spalling under high stress and extreme heating rates. A total of eight full-scale beams were tested in ISO 834 fire, considering the parameters of the cover thickness of tendon, load ratio, partial prepressing ratio (PPR), and bonded and unbonded prestress classifications. Cross-sectional temperatures, mid-span deflections, effective prestressing, crack patterns, failure modes, and fire resistances of the beams were obtained. Subsequently, fine thermal–mechanical coupling nonlinear finite element (FE) models were developed and verified, and an extensive parametric study was performed to simulate the fire responses of PRPCBs. The results indicated that the load ratio, cover thickness, and sectional dimensions notably affected the fire safety of PRPCBs, and PRPCBs fabricated as under-reinforced beams at ambient temperature may fail as lightly reinforced beams under fire exposure. According to these parametric studies, Eurocode 2: BSEN 1992-1-2 might overestimate the fire resistance of PRPCBs with small sectional dimensions by up to 3–15%. Therefore, fire resistance suggestions for PRPCBs are presented in this paper.

Investigating mechanical properties and microstructure of reactive powder concrete blended with sulfoaluminate cement

Super absorbent polymers, expansion agents, and shrinkage reduction agents have been used to improve the self-shrinkage of ultra-high-performance concrete (UHPC), but they may degrade the mechanical properties of concrete. Sulfoaluminate cement (SAC) has been proven to improve the compressive strength and self-shrinkage of UHPC. However, few studies have focused on the role of SAC in the tensile strength and pore structure of reactive powder concrete (RPC), a type of UHPC. In our study, we used compressive strength and splitting tensile strength tests, isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to determine the mechanical properties and microstructure of RPC modified by SAC. The results showed that the addition of SAC effectively enhanced the strength of RPC, especially the early strength. Specifically, using an 8 wt% SAC caused an 871.4 % increase in compressive strength and 600% improvement in tensile strength after 8 h of curing. The tensile strength exhibited a strong positive linear correlation with the compressive strength. The rapid hydration of SAC promoted the generation of ettringite in RPC, which significantly increased the cumulative heat release and improved the strength of RPC at an early age; however, its effect was not significant on the later strength. The key reasons for the improvement in the mechanical properties of RPC by SAC modification were pore refinement and aggregate-paste interface improvement. Thus, incorporating SAC into RPC significantly enhance its early-age compressive and tensile strengths, which contributes to accelerate the formwork turnover of RPC components and operational efficiency of the stacking area.

Elevated Temperatures Impact on the Mechanical Behavior of Reactive Powder Concrete Containing Hybrid Fibers

Reactive Powder Concrete (RPC) is a combined material that will allow the concrete industry to enhance material consumption, produce economic benefits, be more durable, and be more environmentally friendly. Due to the highly dense microstructure of RPC, it performs poorly at high temperatures due to the formation of high pore pressure, which causes material breakdown. This paper demonstrates the outcomes of an investigational study of the behavior of six RPC mixes incorporating hybrid steel and plastic waste (polypropylene) fibers after being subjected to high temperatures of up to 500 ºC. Residual compressive strength, residual splitting strength, and density were performed for all the RPC mixes. The results demonstrate that the drop in the compressive strength of RPC is greater than the reduction in their corresponding splitting strength. Furthermore, within a temperature range of about 500 ºC, this mechanical strength showed a noticeable decrease. The decrement of splitting strength ranged between about 8% to 33% . Steel and polypropylene fibers also improve the residual strength of RPC samples. On the basis of these results, a mix containing mixed fibers (25 % steel and 75 % polypropylene) is recommended as appropriate for extreme temperature applications.

The Influence of Ferronickel Slag as a Replacement of Quartz Sand upon Compressive and Flexural Strength of Reactive Powder Concrete

This paper aims to evaluate mechanical behavior (compressive and flexural strength) of reactive powder concrete (RPC) with ferronickel slag (FNS) as quartz sand replacement. Standard laboratory tests were performed i.e. sieve analysis to determine the particle size distribution of the FNS, slump and density tests as well as compressive and flexural tests on the RPC. Promising results were obtained in this study. An increased consistency and density were observed upon increasing the FNS content. The utilization of FNS could lead to a compressive strength value of 86 MPa at 28 days. A higher strength enhancement in the early curing days (7 days) was observed than in the longer curing days (28 days), when FNS mixes were compared to reference mixes. The flexural strength result demonstrated two different phenomena at two different ages, where there was a slight rise at 7 days and relatively no increment at 28 days when FNS mixes were compared to the reference mixture.

Temperature Effect On Reactive Powder Concrete Using Sillimanite As Fine Aggregate

This study examines the use of sillimanite as a fine aggregate in the production of temperature-resistant reactive powder concrete. The impact of high temperatures on the mechanical strength of reactive powder concrete with High Alumina cement is investigated. The effectiveness of utilizing glass powder and sillimanite on the mechanical properties of RPC at high temperatures is investigated in this research. The samples were heated in the muffle furnace to the desired temperatures and then tested for their residual compressive strength, flexural strength and split tensile strength. The residual values of compressive strength, flexural strength and split tensile strength were measured at the temperature of 270C to 800°C. The weight loss of the specimens after exposure to the elevated temperatures was measured and the values showed enhanced resistance to the high temperature effects. The results demonstrate the greater contribution of glass powder and sillimanite towards the significant improvement of high temperature strength of reactive powder concrete than those made with normal quartz sand as fine aggregate.

Behavior of reactive powder concrete containing recycled glass powder reinforced by steel fiber

AbstractEnvironmental sustainability is described as one that avoids the depletion or deterioration of natural resources, while also allowing for the preservation of long-term environmental quality. By practicing environmental sustainability, we may assist to guarantee that the requirements of today’s population are satisfied without risking the capacity of future generations to meet their own needs in the future. Engineers in the field of concrete production are becoming increasingly interested in sustainable development, which includes the utilization of the locally available materials in addition to using the agricultural and industrial waste in construction industry as one of the possible solutions to the environmental and economic issues. This study investigated the effect of partial substitution of cement with recycled glass powder (0, 15, 20, and 25%) by weight of cement at various ages (on compressive strength) after determining the optimal ratio of replacement. This optimal ratio is used to study its effect on some mechanical properties (such as flexural strength, absorption, and dry density) of reactive powder concrete containing 1% micro steel fiber (SRPC), and furthermore, utilizing steam curing for 5 h at 90°C after hardening the sample directly. Reactive powder concrete (RPC) has been designed with the use of the local cement, silica fume, and super plasticizer with a water/cement ratio of 0.20 in order to achieve a compressive strength of 137.09 MPa at the age of 28 days. When recycled glass powder replacement (20%) was utilized, the findings revealed that the compressive strength of RPC improved by 4.2%, the flexural strength increased by 15.3%, the dry density increased by 0.61%, and the absorption was reduced by 32% at 28 days after the test results were compared to the reference mix.

Preparation of low-cost reactive powder concrete using waste steel fibres recycled from scrap tires

The application of reactive powder concrete (RPC) was not wide. One of the main problems was that copper-coated steel fibres (CSF) were expensive. In order to prepare low-cost RPC and resource utilisation of waste steel fibres recycled from scrap tires (WSF), this paper investigated the fluidity and strength of RPC containing WSF. The results indicated that the fluidity of RPC containing WSF was worse than that of RPC containing CSF. But there was not much difference in the strength of two kinds of RPC. In order to optimise performance of RPC containing WSF, the influence of mineral admixtures on the fluidity and strength of RPC was discussed. The results indicated that the optimum silica fume dosage was 20%, and the optimum fly ash and limestone powder dosages were 5%, respectively. Through this study, it was completely feasible to prepare RPC containing WSF. The use of WSF can reduce the cost of RPC.

Mechanical Properties of Reactive Powder Concrete with Coal Gangue as Sand Replacement.

Coal gangue (CG) represents a huge amount of industrial solid waste in China, and usually is used as a coarse aggregate to produce low-strength coal-gangue-based concrete. In this paper, in order to prove the possibility to obtain a higher-strength concrete with a higher CG utilization rate, reactive powder concrete (RPC) with coal gangue as a sand replacement at different replacement ratios was studied. RPC samples were prepared by replacing natural river sand (RS) with CG sand at different CG/RS weight ratios from 0–100% at intervals of 25%. Mechanical tests were carried out, and the microstructure features of RPC samples at 28 days were characterized. The test results showed that strong back shrinkage of strength existed. On days 7 and 14, the flexural strengths of samples with CG/RS replacement ratios of 0–75% fluctuated around the mean value. Strengths of samples with a CG/RS replacement ratio of 100% dropped off. However, on day 28, the flexural strengths of samples with CG were all lower than the strengths of samples on days 7 and 14. The flexural strengths and compressive strengths of the RPC with a CG/RS replacement ratio of 100% on day 14 were 14.09 MPa and 37.03 MPa, respectively, which decreased to 6.42 MPa and 28.44 MPa, respectively, on day 28. Compared with natural river sand, CG sand reduced the working performance, compressive strength, and flexural strength of RPC. Microscopic analysis showed that on day 28, increasing the CG replacement ratio could inhibit cement hydration, weaken the interface transition zone, and lead to the degradation of the RPC’s performance. Modification of CG sand would be helpful to obtain higher-strength concrete.

Research into Carbon Dioxide Curing’s Effects on the Properties of Reactive Powder Concrete with Assembly Unit of Sulphoaluminate Cement and Ordinary Portland Cement

Excessive emissions of carbon dioxide can lead to greenhouse effect thus destroying the ecological balance. Therefore, effective measures need to be taken to reduce the emission of carbon dioxide. In this study, the influence of carbon dioxide curing on the mechanical strength and NaCl freeze-thaw deterioration of reactive powder concrete (RPC) with the assembly unit of sulphoaluminate cement and ordinary Portland cement was investigated. The ratio of sulphoaluminate cement ranged from 0% to 100% by the total mass of cement with the curing age ranging from 1 d to 28 d. The mechanical strength of RPC with 50% ordinary Portland cement and 50% sulphoaluminate cement containing the polypropylene fibers ranging from 1% to 4% by volume of RPC were investigated. Moreover, the following mass and mechanical strength loss rates, the carbonation depth, the chloride ion migration coefficient and the relative dynamic elastic modulus (RDEM) during NaCl freeze-thaw cycles were determined. Finally, the scanning electron microscope (SEM) and X-ray diffraction were applied in investigating the carbonation process of RPC. Results showed that the addition of sulphoaluminate cement could improve the mechanical strength of RPC at low curing age (lower than 7 d). However, when the cuing age reached 7 d, the sulphoaluminate cement demonstrated negative effect on the mechanical strength. Moreover, the carbon dioxide curing led to increases in the mechanical strength and when ordinary Portland cement was added the enhancing effect was more obvious. Furthermore, the carbon dioxide curing could effectively improve the resistance of NaCl freeze-thaw cycles and increase the carbonation depth. Finally, the increasing dosages of polypropylene fibers were advantageous to the mechanical strength and the resistance of NaCl freeze-thaw cycles. From the researching results of the microscopic performance, the carbon dioxide curing could improve the compactness of hydration products and reduce the content of calcium hydroxide especially at the curing age of 3 days.

Improvement of mechanical and transport properties of reactive powder concrete using graphene nanoplatelet and waste glass aggregate

In this study, the use of graphene nanoplatelet (GNP) and waste glass aggregate (WGA) was investigated to improve the mechanical and transport properties of reactive powder concrete (RPC), which is an ultra-high strength concrete type. The co-addition of 0.14% GNP and 30% WGA resulted in a significant increase in the compressive strength of RPC from 192.9 to 258.9 MPa and flexural strength from 16.2 to 38.7 MPa. In the study, the most significant improvement (138.6%) was obtained for the flexural strength values of RPC. Considering the strength aspects, the relevant amounts of GNP and WGA enable production of RPC ensuring a conducted electrical charge of less than 100 Coulombs; that is, with negligible chloride ion permeability. Accordingly, the highest improvement (59.8%) after flexural strength was found for the resistance of the specimens against chloride ion penetrability. In conclusion, the findings of this study show that eco-efficient production of RPC with superior mechanical and durability characteristics can be achieved using GNP and WGA, with lower cement consumption per unit value of these studied characteristics, reaching 59%.

Bond behavior between reinforcing bar and reactive powder concrete

AbstractTo study the bond–slip behavior between reinforcing bars and reactive powder concrete (RPC), eccentric pull‐out tests of 20 groups of bonding specimens were conducted, and the variables in the test specimens included RPC strength, cover thickness, bond length, and reinforcing bar diameter. The effects of various parameters on the bond stress and peak slip were analyzed. A formula for calculating the average bond stress and between the reinforcing bar and RPC were established, and the bond–slip relationship between reinforcing bar and RPC was determined. The results indicated that the bond length had the most significant effect on the bond performance, followed by the strength of concrete, cover thickness, and reinforcing bar diameter. When the bond length increased from 3d to 4d, the average bond stress decreases by 13.2% to 23.7%. When reinforcing bar diameter increased from 16 to 20 and 25 mm, the average bond stress increased by 6.58% and 15.9%, respectively. The average bond stress increased with increase of the cover thickness. When the RPC strength increased from 105.2 to 124.8 MPa, the average bond stress increased by 7.0% to 13.2%.

Experimental investigation on sing fiber pullout behaviour on steel fiber-matrix of reactive powder concrete (RPC)

A new single fiber pullout testing method was applied to characterize the interfacial bond performance, which includes the pullout load-slip relationship, pullout energy, bond strength, and fiber utilization ratio, of the steel fiber-matrix in reactive powder concrete (RPC). The bond properties and bond strengthening mechanism were investigated from the perspective of certain parameters, including the fiber type, fiber embedment angle, silica fume content, magnetic nanoparticles content, and curing time. The microstructure of the steel fiber and longitudinal morphology of the tunnel after the pullout test were determined by scanning electron microscope (SEM). In terms of the results, the bond strength of the end-hooked steel fiber is much stronger than that of the straight steel fiber. With the increasing of embedment angle, the ultimate pullout load and pullout energy of straight steel fibers increase, and the interfacial bond strength increases. It is found that the incorporation of silica fume can effectively enhance the matrix strength. In addition, RPC containing 20%∼30% silica fume shows significant improvement in pullout behavior. The appropriate curing age contributes greatly to the hydration reaction and the matrix strength of RPC. Steaming curing for 2 d can effectively improve the interfacial bond performance. A content of 1% magnetic nanoparticles enhances the pullout behavior of straight steel fibers with no brass coating, because the maximum pullout load and pullout energy increase by 33.6% and 79.0%, respectively. The microstructural observations confirm the conclusions regarding the bond mechanism drawn from the pullout tests. In conclusion, the performance of the matrix, the characteristics of the fiber-matrix interface area and the friction of the steel fiber in the tunnel can influence the bond performance of the fiber-matrix interface of RPC.