Abstract

Introduction

Material and Methods

Results and Discussion

Conclusions

References

Abstract

To explore the effect of multi-scale polypropylene fiber (PPF) hybridization on the mechanical properties of roller-compacted concrete (RCC), the early-age (3, 7, 14, 28 days) compressive strength, splitting tensile strength and uniaxial tensile test of RCC reinforced with micro-, macro- and hybrid polypropylene fibers were investigated. Then, the tensile stress–strain curve of polypropylene fiber-reinforced roller-compacted concrete (PFRCC) and the corresponding tensile parameters were obtained. The uniaxial tensile constitutive equation of PFRCC and fiber hybrid effect function was also proposed. Finally, the enhancement mechanism of fiber hybridization on mechanical properties of RCC was analyzed. The results indicated that the strength and toughness of PFRCC improved with the incorporation of PPF, showing obvious plastic failure characteristics of PFRCC. Before curing the concrete for 7 days, micro-PPF played a major role in strengthening RCC, while macro-PPF played a major role in reinforcing concrete after that. Moreover, the tensile strength and toughness indexes of multi-scale PFRCC performed the best, indicating the positive hybridization of three types of PPF. The proposed PFRCC uniaxial tensile constitutive equation and fiber hybrid effect function based on existing researches were also well matched with the experimental results

Introduction

Roller-compacted concrete (RCC) is commonly used in a variety of road pavements, pavement bases and et al. (Lopez-Uceda et al., 2016), which is known for saving cement, early high strength and reducing construction time. However, RCC also includes some disadvantages, such as low tensile strength, poor crack resistance and toughness, limiting its development in engineering structures (Bayagoob & Bamaga, 2019; Lam et al., 2018). Previous studies (Ahmad & Umar, 2018; Benouadah et al., 2017; Marushchak et al., 2019; Yoo & Banthia, 2016) have shown that the addition of fibers can effectively control the formation and development of cracks in concrete, and greatly increase the strength and toughness of concrete.

In practical engineering applications, there are many kinds of fibers, among which steel fiber and polypropylene fiber (PPF) are the most popular (Hussain et al., 2020; Madhkhan et al., 2012). Steel fiber has high elastic modulus and stiffness, which is widely used for its good strengthening and toughening effect on concrete. However, its development is limited by its deficiencies of high cost, poor corrosion resistance and inconvenient construction. PPF is often divided into micro-PPF and macro-PPF according to the size. Micro-PPF can restrain the early cracks of concrete significantly, but it has no obvious reinforcing and toughening effect on concrete. Macro-PPF, also known as the candidate material of steel fiber, has comparable reinforcing and toughening effect with steel fiber (Yin et al., 2015). Many researches showed that the hybrid fiber-reinforced concrete can make different fibers complement each other and promote the mechanical properties of the concrete matrix (Turk et al., 2021; Yao et al., 2003).

1- Introduction

2- Experimental program

3- Discussion of results of DIC on FRP materials

4- Conclusions

References

Abstract

Accurately measured strains are critical when investigating the application of Fiber Reinforced Polymer (FRP) materials, but traditional mechanical strain measurement methods have several critical drawbacks related to the installation process and the recording capabilities of the devices. FRP materials typically used in the civil engineering field feature large asymmetries and heterogeneity originated from the manual installation procedures, as opposed to the highly controlled FRP fabrication methods used in other fields that result in more homogeneous materials. The feasibility of using an optical full-field Digital Image Correlation (DIC) technique for measurement of strain fields on FRP materials used in the civil engineering industry has been investigated and the level of error in the DIC method when using more traditional methods was determined. The main advantage of using DIC over more traditional methods, which is the capacity of DIC to measure full field strains instead of strains at isolated points, has been demonstrated by providing exemplar measurements of various specimens of FRP materials. The reported strain fields are examples of what was obtained during an experimental campaign to understand the behavior of FRP anchors and other materials. The main conclusions drawn from the observation of those strain fields are discussed.

Introduction

Determining operational limits and safe working strains, as identified in international guidelines such as ACI 440.2R [1] and CNR-DT 200 [2], is paramount to the application of Fiber Reinforced Polymers (FRP) in civil engineering. One of the most common applications of FRP materials in civil engineering is as Externally Bonded Reinforcement systems (EBR-FRP) to strengthen existing structures. Accurate measurement of the FRP strains when under load in such applications provides insight and understanding of the load distribution within the FRP material. When conducting tests, a number of traditional instruments are available for the measurement of displacements and strains, including Linear Variable Differential Transformer devices (LVDT) or foil Strain Gauges (SG). These instruments have been in use for decades, but feature some significant drawbacks as the installation of the instrumentation is typically invasive, expensive, cumbersome or timeconsuming. But the most significant deficiency of these devices is that they only measure displacements or strains at specific, isolated points. Spatial strain fields allow both general strain distributions and localized strain concentrations to be observed. Spatial strain field measurements are critical when investigating EBR-FRP systems, where the FRP materials are non-homogenous and highly anisotropic as opposed to more traditional metallic or cementitious materials. This effect is exacerbated by “low-tech” in situ manufacturing methodologies typically involved in the installation of EBR-FRP systems. The manufacturing approaches typically used in EBR-FRP systems result in low fiber volume fractions, which is defined as the ratio between the volume of fibers and the total volume, and in large variations in fiber orientation both in-plane and through thickness due to the lack of molding/compression force. Modern full-field imaging techniques such as Digital Image Correlation (DIC), thermograph and particle image velocimetry enable the determination of strain fields during testing.

1- Introduction

2- Experimental rotor/AMB system

3- Strain measurement system

4- Strain measurement calibration

5- Dynamic results

6- Conclusions

References

Abstract

Active magnetic bearing systems provide contact-free rotor levitation, which allows near frictionless rotation and high rotational speeds when they operate normally. Under certain operational conditions, a transient fault, overload condition, or disturbance may occur. Touchdown bearings or bushings are therefore implemented in such systems to prevent contact between rotor and stator laminations. If the rotor makes contact with a touchdown bearing, the rotor dynamics may become transient or persistent in contact. Appropriate control strategies through the magnetic bearings to restore contact-free rotor operation may extend the life of touchdown bearings, and minimise operational downtime. To achieve this, an understanding of the contact dynamics is required, together with the relationship between contact and magnetic bearing forces. In this paper, rotor/touchdown bearing contact conditions are investigated experimentally using an active magnetic bearing system with a flexible rotor. Design methodology is presented for a measurement system capable of providing rotor/touchdown bearing contact related data, based on strain measurement. Strain induced contact signals are calibrated against applied magnetic bearing forces. The frequency dependent behaviour of the active magnetic bearing system is considered using evaluated force and phase measurements. The measurement data represent system identification for the potential of active magnetic bearing force-based contact control.

Introduction

The ability of Active Magnetic Bearings (AMBs) to provide contactless rotor levitation with vibration control has a number of benefits, including high-speed rotor operation, the elimination of the need for lubrication, long life and reliability, and energy efficiency due to the low friction losses. However, passive bearings are also used in AMB systems as Touchdown Bearings (TDBs) to prevent potentially damaging rotor/magnetic bearing stator contact from occurring. TDBs support the rotor while the AMB system is not operating, but also limit excursions of the rotor during operation, such as in cases of power loss, component failure, rotor mass loss, or other significant external disturbances. Such cases may cause rotor/TDB contact. Since TDBs are functional components of AMB systems, it is crucial to understand their influence on rotor dynamic behaviour in contact cases, in order to minimise potential damage. This is particularly important as TDB life and condition impacts directly on the reliability of AMB systems.

Concrete were prepared by replacing natural gravel (N) with spontaneous combustion gangue (S) or rock gangue (R) at intervals of 25%. Mechanical tests were carried out and the microstructural features of the concrete were characterized. Failure modes and strength of different concrete are various, and the compressive strength of the concrete meets the design requirements of C30 strength grade concrete. The axial compression failure mode of R coarse aggregate concrete (RAC) was shear failure. As the S replacement percentage increased, the axial compression failure mode of S coarse aggregate concrete (SAC) transformed from shear failure to longitudinal splitting failure. The ITZ of SAC is the best; as the replacement percentage increased, the peak stress and slope of the rising section of the stress-strain curves of RAC and SAC decreased, and the peak strains increased. Finally, compressive stress-strain curve prediction models for coal gangue coarse aggregate concrete were suggested.

Introduction

China is a large country with coal as its main energy source. The annual output of coal gangue solid waste, which is produced during coal production, amounts to hundreds of millions of tons. A large amount of coal gangue is stored for a long time, and such storage can destroy the ecological balance and harm the environment. Statistics show that among the more than 2100 counties in China, extensive coal gangue accumulation has been reported in more than 1200 counties. Currently, the amount of accumulated coal gangue is approximately 4.5 billion tons, and there are more than 1600 coal gangue mounds that occupy approximately 15,000 ha of land; moreover, the quantity of gangue accumulation is increasing at a rate of 1.5–200 million tons per year [1,2]. With the rapid development of infrastructure construction in China, raw materials such as sand and stone, which are used in concrete, the most common engineering materials, must be urgently supplied. In the summer of 2018, the prices of sand and stone in the Pearl River Delta (China) area exceeded 300 RMB/ m3 , resulting in a price increase of 700 RMB/m3 for C30 ready-mixed concrete, which has caused panic in the industry [3,4].

The primary objective of the study presented herein is to evaluate the efficacy of the strain-based liquefaction triggering evaluation procedure implemented using a pragmatic variant of the procedure originally proposed by Dobry et al. [11]. Liquefaction is a phenomenon that results from the contractive tendencies of loose to medium dense soils when sheared. For saturated cohesionless soils, this tendency results in the transfer of the overburden stress to the pore fluid, with the commensurate increase in pore water pressure and decrease in effective confining stress. Liquefaction has occurred in most major earthquakes and has caused significant damage to infrastructure (e.g., Cubrinovski and Green [8]; Cubrinovski et al. [9]; Green et al. [15]; Olson et al. [33]; Stringer et al. [40]; among many others). The most widely used procedure for evaluating liquefaction triggering potential is the simplified stress-based procedure originally proposed by Whitman [44] and Seed and Idriss [37]. This procedure is semi-empirical and has undergone periodic updates as a result of findings from new laboratory studies and/or the collection and analysis of additional field case history data (e.g., Youd et al. [46]; Cetin et al. [6]; Idriss and Boulanger [16]). Inherent to this procedure is the quantification of the seismic demand imposed on the soil expressed in terms of cyclic shear stress. Despite the popularity of the stress-based procedures, multiple studies have shown that excess pore water pressure better correlates to cyclic strain than to cyclic stress (e.g., Fig. 1) (e.g., Martin et al. [25]; Dobry et al., [11]; Byrne [3]). The reason for this is the relative movement of soil particles, which is requisite for excess pore water pressure generation, relates to the induced strain, regardless of amplitude of the stress applied to soil. As a result, Dobry et al. [11] proposed a strain-based liquefaction triggering evaluation procedure. Although the Dobry et al. [11] procedure generally received a positive reception by liquefaction researchers, it has failed to be adopted into practice. One reason for this is likely the requirement to perform strain-controlled cyclic laboratory tests on undisturbed and/or reconstituted specimens. This is in contrast to the simplified stress-based procedures wherein in-situ test metrics are the primary parameters used to evaluate liquefaction potential, with laboratory index tests and grain size distribution analyses having supporting roles if their performance is deemed necessary (e.g., use of measured fines content, FC, versus apparent FC in conjunction with the Cone Penetration Test, CPT, stressbased simplified procedure). Herein an alternative approach to implementing the Dobry et al. [11] strain-based procedure is proposed which circumvents the need for performing strain-controlled cyclic laboratory tests. Per this procedure, a strain-based numerical excess pore pressure generation model is used in lieu of developing analogous relationships from laboratory tests. The soil parameters required to implement the procedure include: relative density (Dr), secant shear modulus (G), and grain size distribution characteristics of the soil (i.e., FC and coefficient of uniformity: Cu); note that focus herein is on soils that are susceptible to liquefaction (i.e., non-plastic soils) and thus Plasticity Index (PI) is not needed. These required parameters are not too different from those required to implement the stress-based simplified procedures and can be estimated using simple relationships or conservative assumptions.

Reinforced concrete (RC) structures have been extensively used in multistory and high-rise buildings due to their advantages of excellent structural integrity, durability and high economic efficiency. During their service periods, RC structures may be subjected to strong earthquake ground motions. The strain rates of reinforcing steel and concrete materials at critical sections of RC structural members may reach as high as 1 s1 . Fu et al. [1] and Bischoff and Perry [2] systematically reviewed the compressive behavior of concrete at high strain rates, while the strain rate effect on the tensile strength of concrete was reviewed by Malvar and Rose [3] and Thomas and Sorensen [4]. It was reported that the compressive and tensile strength of concrete can be obviously increased at the strain rates induced by seismic loading. Moreover, the tensile strength of concrete is more susceptible to increase than the compressive strength. In addition, the mechanical properties of reinforcing steels under seismic strain rates were also examined by many researchers through dynamic loading tests [5–12]. The experimental findings indicated that the yield strength and ultimate tensile strength of reinforcing steels increase linearly with the logarithmical increase of strain rate, and the effect of strain rate on the lower strength reinforcing steels is more significant as compared with that on the higher strength reinforcing steels. In current literature, many investigations are focused on the effects of loading rates on the seismic behavior of RC members. The dynamic tests of simply supported and doubly reinforced beams were performed by Bertero et al. [13] to assess the influence of loading rate on the behavior of RC beams. It was found that the stiffness before the first yielding of reinforcing steel for the beams under the higher loading rate increases about 10% as compared with that for the beams under the lower loading rate. Significantly increases in the initial yield strength were also observed, but the effect of loading rate on the ultimate strength, flexural failure mode, measured strain, curvature and deflection ductility factors of the beams is not evident. The similar conclusions were obtained by Kulkarni and Shah [14], who carried out the experimental tests of seven pairs of RC beams subjected to static and high loading rates. It was also reported in the aforementioned studies that the failure mode of the RC beams may shift from brittle shear failure at the lower strain rate to the ductile flexural failure at higher strain rates. Otani et al. [15] performed a series of tests on four pairs of cantilever RC beams under static and dynamic loads to study the potential effects of strain rate. The specimens were tested in the vertical position and loaded horizontally using a dynamic actuator. The results showed that the seismic strain rate increased the flexural resistance of RC beams by 7%–20%. Li and Li [16] studied the dynamic behavior of simply supported RC beams with different shear spans through dynamic tests, in which the influence of loading rate on the bearing capacity, ductility, stiffness, failure mode and energy absorbing of beam specimens were systematically analyzed.