Preparation of Aligned Steel Fiber Reinforced Cementitious Composite and Its Flexural Behavior

Summary

This protocol describes an approach for manufacturing aligned steel fiber reinforced cementitious composite by applying a uniform electromagnetic field. Aligned steel fiber reinforced cementitious composite exhibits superior mechanical properties to ordinary fiber reinforced concrete.

Abstract

The aim of this work is to present an approach, inspired by the way in which a compass needle maintains a consistent orientation under the action of the Earth's magnetic field, for manufacturing a cementitious composite reinforced with aligned steel fibers. Aligned steel fiber reinforced cementitious composites (ASFRC) were prepared by applying a uniform electromagnetic field to fresh mortar containing short steel fibers, whereby the short steel fibers were driven to rotate in alignment with the magnetic field. The degree of alignment of steel fibers in hardened ASFRC was assessed both by counting steel fibers in fractured cross-sections and by X-ray computed tomography analysis. The results from the two methods show that the steel fibers in ASFRC were highly aligned while the steel fibers in non-magnetically treated composites were randomly distributed. The aligned steel fibers had a much higher reinforcing efficiency, and the composites, therefore, exhibited significantly enhanced flexural strength and toughness. The ASFRC is thus superior to SFRC in that it can withstand greater tensile stress and more effectively resist cracking.

Introduction

Incorporating steel fibers into concrete is an effective way to overcome the inherent weakness of brittleness and to improve the tensile strength of concrete¹. During the past decades, steel fiber reinforced concrete has been extensively investigated and widely used in the field. Steel fiber reinforced concrete is superior to concrete in terms of cracking resistance, tensile strength, fracture toughness, fracture energy, etc.² In steel fiber reinforced concrete, steel fibers are randomly dispersed, thereby uniformly dispersing the reinforcing efficiency of the fibers in every direction. However, under certain loading conditions, only some of the steel fibers in concrete contribute to the performance of the structural elements because the reinforcing efficiency of the fibers requires that they be aligned with the principle tensile stresses in the structure. For instance, when using steel fiber reinforced concrete containing randomly distributed steel fibers to prepare a beam, some of the steel fibers, especially those parallel to the direction of the principal tensile stress, will make major contribution to reinforcing efficiency, while those perpendicular to the direction of the principal tensile stress will make no contribution to reinforcing efficiency. Consequently, finding an approach to align the steel fibers with the direction of the principal tensile stress in concrete is necessary to achieve the highest reinforcing efficiency of the steel fibers.

The orientation efficiency factor, defined as the ratio of the projected length along the direction of the tensile stress to the actual length of fibers, is usually used to indicate the efficiency of the reinforcement of steel fibers^3,4. According to this definition, the orientation efficiency factor of the fibers aligned with the direction of the tensile stress is 1.0; that of the fibers that are perpendicular to the tensile stress is 0. Inclined fibers have an orientation efficiency factor between 0 and 1.0. The analytical results show that the orientation efficiency factor of randomly distributed steel fibers in concrete is 0.405⁴, while that from tests of ordinary steel fiber reinforced concrete is in the range of 0.167 to 0.500^5,6. Evidently, if all the short steel fibers in concrete are aligned and have the same orientation as the tensile stress, the steel fibers will have the highest reinforcing efficiency and the specimens will have the optimum tensile behavior.

Some successful attempts to prepare aligned steel fiber reinforced concrete have been conducted since 1980s. In 1984, Shen⁷ applied an electromagnetic field to the bottom layer of steel fiber reinforced cementitious composite (SFRC) beams during casting, and X-ray detection analysis revealed that steel fibers were well aligned. In 1995, Bayer⁸ and Arman⁹ patented the approach for preparing aligned steel fiber reinforced concrete using a magnetic field. Yamamoto et al.¹⁰ considered the orientation of steel fibers in concrete to be mainly influenced by the casting approach and attempted to obtain aligned steel fiber reinforced concrete by keeping fresh concrete flowing into the formwork from a constant direction. Xu¹¹ attempted to align steel fibers in shotcrete by spraying steel fibers from a constant direction. Rotondo and Wiener¹² sought to make concrete poles with aligned long steel fibers by centrifugal casting. These experimental studies reveal that aligned steel fiber reinforced concrete has significant advantages over randomly distributed steel fiber reinforced concrete.

Recently, Michels et al.¹³ and Mu et al.¹⁴ have successfully developed a group of aligned steel fiber reinforced cementitious composites (ASFRCs) using electromagnetic fields. In these studies, various solenoids were made to provide a uniform magnetic field for aligning steel fibers in mortar specimens of different sizes. The solenoid has a hollow cuboid chamber, which can accommodate specimens of predefined sizes. When the solenoid is connected to direct current (DC), a uniform magnetic field is created in the chamber with a fixed orientation, which aligns with the axis of the solenoid. According to the principle of electromagnetics¹⁵, magnetic fields can drive ferromagnetic fibers to rotate and align in fresh mortar. Appropriate workability of the mortar is critical for allowing steel fibers to rotate in fresh mortar. A high viscosity may cause difficulty in the alignment of the steel fibers in the mortar, while low viscosity may lead to the segregation of fibers.

This paper describes the details of the preparation of ASFRC specimens and tests the flexural properties of ASFRC and SFRC. It is expected that ASFRC has a higher flexural strength and toughness than SFRC. Thus, ASFRC potentially has advantages over SFRC in withstanding tensile stress and resisting cracking if used as cover concrete, pavement, etc.

Using the fractured specimens after flexural tests, the orientation of the steel fibers in the specimens is investigated by observing the fractured cross sections and utilizing X-ray scanning computed tomography analysis^16,17,18. The mechanical properties of ASFRCs, including their flexural strength and toughness, are reported and compared with those of non-electromagnetically treated SFRCs.

Protocol

1. Solenoid Magnetic Field Setup

NOTE: The magnetic field is generated by a solenoid with a hollow chamber. The setup is a polybutylene terephthalate (PBT) board solenoid skeleton coiled with 4-6 layers of enamel insulated copper wire and wrapped with a plastic insulating layer for protection (Figure 1). After connecting the coil to DC, the current in the coil creates a uniform electromagnetic field within the solenoid chamber with a fixed direction and constant magnetic induction intensity. Use the magnetic field to align steel fibers in fresh mortar and prepare the ASFRC specimens. In this study, we prepared 150×150×550 mm prism specimens using a solenoid with a chamber size of 250×250×750 mm.

1. Correlate the magnetic induction intensity to the electrical current of the solenoid.
   1. Connect the solenoid to DC and apply current from 0 to 10 A with a step length of 1 A. Measure and record the magnetic induction intensity in the solenoid chamber using a tesla meter.
   2. Plot the magnetic induction intensity-current curve (Figure 2), which will be used in later steps to determine the necessary current of the solenoid.
      NOTE: Carefully follow electrical safety procedures when connecting the solenoid to the power source and in all other operation procedures relevant to electrical power.

2. Workability of Fresh Mortar

1. Prepare three mortar mixes with steel fiber volume fractions 0.8%, 1.2%, and 2.0%, respectively (Table 1). The three mixes have the same matrix composition with a water to cement to sand ratio of 0.42:1:2. According to the mix ratio, weigh 0.5 kg of cement, 1.0 kg of sand, and 0.21 kg of water for workability tests.
2. Add water to the mortar mixer first. Then add the cement. Mix the water and cement for 30 s. Then mix for another 30 s, and during this 30 s of mixing, gradually add sand to the mixer. Then mix for another 60 s.
3. Test the sinking depth of the mixture using a sinking depth meter following the Chinese standard for test method of performance on building mortar (JGJ/T70-2009)¹⁹.
4. Repeat steps 2.2 and 2.3, adjusting the dosage of the superplasticizer until the sinking depth falls into the 50-100 mm range. Record the dosage of the superplasticizer that produces the desired workability and supplement it as part of the mix proportion in Table 1. Also test the specific density of the fresh mortar after the workability is achieved. The optimized dosage of a polycarboxylate superplasticizer from the aforementioned tests is 0.10% (mass ratio to cement), and the specific density of fresh mortar is 2186 kg/m³.
5. Test the viscosity of the fresh mortar using a co-axial rotational mortar rheometer (Figure 3). The rheometer has a water bath that can maintain the temperature of the sample container at 20 °C.
   1. Put 300 mL of fresh mortar mixed within the previous 5 min into the sample container.
   2. Begin the viscosity test. The probe gradually drops into the fresh mortar in the container, and the container begins to rotate. As the fresh mortar moves within the rotating container, it applies a shear force on the probe. In the process, the rheometer records the shear stress and shear rate and plots the curve of shear stress to shear rate. The slope of the curve is the viscosity of the mortar^20,21. In this investigation, the viscosity of the fresh mortar from tests is 0.82 Pas.

3. Specimen Preparation

1. Determine the magnetic induction intensity of the magnetic field and the current of the solenoid.
   1. Using the viscosity of the cement mortar determined in step 2.5.2, calculate the magnetic induction intensity of the magnetic field required for aligning steel fibers in cement mortar using Equation (1):¹³
      (1)
      where B is magnetic induction intensity, η is the viscosity of fresh mortar, l_f is the length of steel fiber, m is the mass of an individual steel fiber, r_f is the radius of steel fibers, μ is the permeability of steel fibers, μ₀ is the permeability of the vacuum, Δt is time interval, and α(t+Δt) is the angular acceleration at the next time interval. According to the viscosity and the parameters of the steel fiber used in the tests, the required magnetic induction intensity is 9.83 mT.
   2. Determine the electrical current of the solenoid required to create a sufficient magnetic induction intensity according to Figure 2 or Equation (2):¹⁴
      (2)
      where I is the required current, N is the number of solenoid turns, and L is the length of the solenoid.
      Using Equation (2), the required current is 8.3 A, while from Figure 2 it is about 8.5 A.
2. Prepare ASFRC specimens
   1. Use a 15 L mortar mixer to mix fresh mortar. For each batch, mix 7.5 L of mortar according to the mix proportions listed in Table 1. Table 1 denotes the ASFRC mixes as A-V_f, where A indicates that the steel fibers are aligned and V_f indicates the volume fraction of steel fiber. Accordingly, the SFRC mixes are denoted, for comparison, as R-V_f, where R indicates that the steel fibers are randomly distributed. The SFRC mixes are not listed in Table 1 but have the same proportions as ASFRC.
   2. Weigh the raw materials and mix the steel fiber reinforced cement mortar following routine procedures.
   3. Pour the fresh mortar into a plastic mold with clear size of 150×150×550 mm. Cast the specimens promptly after mixing to avoid losing workability. It takes around 25 min to cast one ASFRC prism from the contact between cement and water.
   4. Move the mold onto a compacting table, and switch on the compacting table for 30 s. Add more mortar as needed to ensure that that the mold is completely filled.
   5. Put the mold into the chamber of the solenoid.
   6. Switch on the solenoid and compacting table for 50 s.
      NOTE: For ordinary concrete the reasonable compacting time is around 60-120 s. In this test, it is attempted to control the total compacting time within this range. Longer compacting time may improve the alignment of steel fibers; however, it may cause over compacting and consequently the segregation (the sinking of steel fibers and coarse aggregates if there are). Less compacting time may cause poor alignment of steel fiber and unconsolidated concrete.
   7. Switch off the compacting table.
   8. Switch off the solenoid after the compacting table has stopped completely.
   9. Gently take out the mold from the solenoid and smooth the top surface of the mortar with a trowel. Avoid disturbing the steel fibers near the top surface.
3. For each mix, prepare three electromagnetically treated specimens (following steps 3.2.2-3.2.9) and three non-electromagnetically treated specimens (following steps 3.2.2-3.2.4 and 3.2.9). In the preparation of non-electromagnetically treated specimens, the total compacting time was 80 s—the same as that in the preparation of electromagnetically treated specimens.
4. Leave the specimens indoors and in their molds for 24 h. Then demold and cure the specimens in a fog room until they are used for mechanical tests.

4. Three-Point Bending Test

1. After 28 days, take out the specimens from the curing room and mark the positions for loading (A), supports (B), mid-span deflection (C), and LVDT fixing points (D) (Figure 4).
2. Place the specimen on the three-point bending test rig (Figure 4) of the MTS test machine and fix a LVDT to the mid-span using a LVDT holder on each side surface of the specimen (Figure 4).
3. Connect the LVDT to a datalog. Then set the data acquisition frequency on the control PC of the test machine.
4. Gradually raise the specimen by raising the bottom supports so that the upper loading cell of the test machine is very close to, but not touching, the top surface of the specimen.
5. Zero the initial load, mid-span deflection (LVDT), and displacement (load cell) values.
6. Start the test and apply a three-point bending load to the specimen with a displacement control at a speed of 0.2 mm/min. Record the full history of the loading and mid-span deflection of the specimen.
7. Watch the load and deformation of the specimen. After peak value, when the displacement is greater than 30 mm, stop the test. Usually, the specimen is cracked and the load is less than 1.0 kN.
8. Repeat steps 4.1-4.7 to test all specimens.

5. Steel Fiber Orientation Analysis

1. Count the number of steel fibers on the fractured section.
   1. Separate the specimens into two portions at the cracked section.
   2. Measure and record the orientation of the steel fibers on the fractured cross section of the cement mortar specimen. The orientation is the angle between a steel fiber and the axis of the specimen. Because manually measuring the orientations of steel fibers is difficult and can produce inaccurate measurements, orientations can be categorized as one of six angle ranges: 0 - 15 °, 15 - 30 °, 30 - 45 °, 45 - 60 °, 60 - 75 ° and 75 - 90 °. Record the number of steel fibers in each group, and then calculate the average fiber orientation efficiency factor of the specimen using Equation (3):
      (3)
      where η_θ is the average orientation efficiency factor of the steel fibers, l_f is the length of an individual steel fiber, n is the total number of steel fibers on the cracked section, and θ_i is the angle between a steel fiber and direction of the magnetic field applied to the specimen (in the calculation, the middle value of the angle range is adopted for all steel fibers in each group).
2. Perform X-ray computed tomography analysis.
   1. Cut a 75 mm cube from each mortar specimen.
   2. Perform the X-ray scanning of the cube using an X-ray computed tomography system. Place a specimen on the test platform and begin scanning. The specimen rotates 360 ° gradually and the machine records the attenuation of the X-rays caused by the specimen at each rotating step. The computed tomography system generates a three-dimensional digital structure of the cube.
   3. Identify the steel fibers in the digital cube structure by black and white binary processing. Then obtain the digital image describing the distribution of steel fibers.
   4. Determine the coordinates of all steel fibers by image analysis.
   5. Calculate the orientation of each steel fiber according to its coordinates.
   6. Calculate the orientation efficiency factor of the fibers using Equation (3).

Results

The flexural strengths of ASFRCs and SFRCs determined from three-point bending tests are shown in Figure 5. The flexural strengths of ASFRCs are higher than those of SFRCs for all fiber dosages. The flexural strengths of ASFRCs were 88%, 71%, and 57% higher than those of SFRCs at the fiber volume fractions of 0.8%, 1.2%, and 2.0%, respectively. These results imply that the aligned steel fiber reinforces the cementitious matrix more effectively than randomly d...

Discussion

The electromagnetic solenoid developed in this study has a chamber measuring 250×250×750 mm and cannot accommodate the full size structural elements. Although the size of the chamber limits the application of the setup, the concept and protocol proposed in this paper will inspire the further development of a full size setup for manufacturing ASFRC elements, particularly precast elements.

Achieving an appropriate viscosity of fresh mortar is essential factor for controlling the qualit...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Cement	Tangshan Jidong Cement Co., Ltd.	P×O 42.5	Oridnary Portland Cement
Sand		River sand	Fineness modulus is 2.4
Superplasticizer	Subote New Materials Co., Ltd.	PCA-III	Polycarboxylated type, water reducing ratio is 35%
Steel fiber	Tianjin Hengfeng Xuxiang New Metal Materials Co., Ltd.	Round straight	Diameter 0.5mm, length 25mm