Properties of a magnetic concrete core transformer for application in wireless power transfer systems

Construction and Building Materials 227 (2019) 117041

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Properties of a magnetic concrete core transformer for application in wireless power transfer systems Kyle A.T. Edwards, Suhaib H. Al-Abed, Seyedsaeid Hosseini, Nicholas A. Brake ⇑ Department of Civil and Environmental Engineering, Lamar University, Beaumont, TX 77710, United States

h i g h l i g h t s
Composite steel – magnetic concrete cores were cast and tested under a sine wave.
Ferritic inclusions can reduce the flux losses between steel rod components.
The flux volume fraction percolation threshold is approximately 10%.
Interfacial composite flux leakage is less than 10% of composite peak flux.
A relative permeability of 28 can be achieved with 19% ferritic content and steel rod.

a r t i c l e

i n f o

Article history: Received 23 February 2019 Received in revised form 15 September 2019 Accepted 18 September 2019

Keywords: BH hysteresis magnetic concrete wireless power transfer flux percolation magnetic permeability concrete transformer dynamic wireless charging concrete pavement

a b s t r a c t A combination of ferrimagnetic and ferromagnetic inclusions were blended within a diamagnetic Portland cement matrix to increase bulk relative permeability for possible application in wireless power transfer. The objective of this study is to quantify the interactive flux exchange coupling between ferritic inclusions of different composition, shape, size, volume fraction, core geometry, and coil winding configuration. Cores made of cement mortar and various combinations of ferrimagnetic and ferromagnetic inclusions were cast and the permeability was quantified. A total of 21 different mix combinations were considered. ANOVA was used to assess the level of significance of each individual inclusion and their interaction with each other with respect to relative magnetic permeability. X-Ray diffraction, thermogravimetric analysis, and mechanical compression tests were also completed. The results suggest that powder and fibrous ferritic inclusions at a volume fraction greater than 10% can reduce flux leakage, reduce hysteresis core loss, and significantly increase relative permeability. A steel reinforced magnetic concrete CI core can achieve a peak flux of 100 mT and relative permeability of 28 when the cement matrix contains 20% volume fraction of ferritic fiber and powder inclusions. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Wireless Power Transfer (WPT) systems have the potential to transform the transportation and energy industries by enabling embedment of battery charging systems to dynamically or statically charge electric vehicles, extending their travel distance. Wireless power transfer can be achieved either via magnetic resonance or magnetic induction coupling [1]. In the latter, a primary coil is energized with an alternating current generating a magnetic field and magnetic flux that induces a voltage in a secondary coil. The efficiency will depend on several variables, including, but not limited to, coil geometry and mutual inductance, coil to coil spacing, and the magnetic permeability of the media between the coils. ⇑ Corresponding author. E-mail address: [email protected] (N.A. Brake). https://doi.org/10.1016/j.conbuildmat.2019.117041 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Enhancing the magnetic permeability of the media within or between the transmitting coil can increase system efficiency. If the WPT devices are embedded within large buildings or other structural systems, cement or concrete media could be the natural relatively low-cost embedding material. Fig. 1 shows one application of magnetized WPT system embedded within a highway system for the charging of a moving or stationary vehicle. The relative magnetic permeability of concrete, however, is similar to that of air (~1), which limits the power transfer between coils. Portland cement is a relatively low-cost binder [2] and is considered to be a weak ferromagnetic that contains traces of ferrous iron oxide which increases magnetic permeability [3]. Calcium silicates account for approximately 75% cement composition, and after the cement is hydrated, most of the products, though primarily calcium-based, have both crystalline Calcium Hydroxide (CH), and amorphous calcium silicate hydrate (C-S-H) [4]. The hydration

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WPT system embedded in magnetizable concrete

Receiving Coil Transmitting Coil in Magnetizable Concrete Exisng Pavement Fig. 1. Rendering of prototype cement encased WPT pad overlay system.

products within the mortar matrix are not naturally permeable and contain diamagnetic polymerized silicates [5,6] and small traces of ferromagnetic iron oxide [7]. The inclusion of additional ferritic material is necessary to attain the levels of permeability required to positively enhance the efficiency of the wireless power transfer device [8] and provide structural protection [9]. Ferritic materials have large magnetic domains that can be realigned with an external magnetic field to effectively increase the permeability between primary and secondary coils [6]. Fig. 2 shows how the discrete ferritic particles in a surrounding diamagnetic cementitious matrix can be magnetized after subjection to an external applied field. H. In magnets composed of relatively large spherical particles (that allow for multiple magnetic domain formations within the particle) and fibers embedded into a non-magnetic media, the ferritic inclusions will undergo coherent and incoherent exchange interaction under an external magnetic field through surface point contact or simply due to local proximity. The exchange between local ferritic clusters can increase the internal magnetic field below the surrounding surface in the magnet where the external field is applied. The larger the particle size and fiber diameter, generally, the lower bulk coercivity in the magnet and lower energy incoherent exchange modes will be activated in lieu of higher energy coherent 180-degree modes. In small particle systems, increasing packing factor and particle surface roughness will also tend to

Core

Fiber

Particle

Fiber Aligned Field H

decrease the coercivity of the bulk matrix. Fiber inclusions serve as connectors to the various population of particle cluster networks present in the bulk media; although the presence of fibers will tend to increase the coercivity because of the preferred alignment along the long axis of the fiber [6]. Most studies related to ferritic modification of cement and concrete are limited to applications in i) magnetic shielding of external fields and ii) modification of fresh and hardened properties using magnetic water or magnetically soft inclusions. In shielding applications, magnetite aggregate inclusions within concrete have been shown to increase shielding efficiency of neutron and gamma rays [10] and mechanical strength properties [11]. Carbon fiber additions can also improve shielding efficiency in the GHz range and the effectiveness is influenced by carbon composition and fiber aspect ratio [12]. Concrete containing steel fiber inclusions and steel reinforcement (aligned normal to a magnetic field) can decrease power transmission due to the lateral field deflection through the steel [13]; effectively reducing the bulk permeability [14]. The concrete matrix can be magnetized, however, with the addition of coarse and fine ferritic inclusions between 90 and 95% of total concrete mass (or approximately 75% by volume) to achieve a bulk permeability of approximately 40 [15]. Moreover, embedding a WPT coil/core system in a roadway with media having a relative permeability of 30 or greater can increase the wireless power transfer system efficiency [16] and the coil to coil power transfer output efficiency by upwards of 50% [17], and significantly reduce battery charging time. Nair and Ferron studied the fresh rheological properties of cementitious systems containing carbonyl iron powder and found that the viscosity of the cement paste can be manipulated when supplied with an external magnetic field without significantly impacting the cement hydration products or compressive strength [3,7]. Treating the concrete mixing water with a magnetic field prior to casting [18,19] or applying a magnetic field during the application of a load [20] has also been reported to significantly increase the concrete compressive strength.

1.1. Research objective and scope

Particle

Fig. 2. Magnetization of ferromagnetic multi domain particles, fibers and ferritic core.

Although there has been some research related to concrete magnetics, there is general lack of information related to magnetic permeability and the exchange coupling effects between small powder and fibrous ferritic inclusions, and large embedded

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magnetic steel cores. In order to optimize mix designs for use in WPT applications, there is need to quantify and better understand this exchange coupling mechanism between small and large ferritic inclusions. Small inclusions may be used to surround embedded ferritic cores within concrete systems to improve power transfer efficiency. In this study, both ferritic inclusions at various dosages and combinations are blended within a diamagnetic cement mortar matrix and tested at various ages, and in different core geometries and coil winding configurations to characterize the peak flux, differential and impedance relative magnetic permeability, flux losses, and hysteresis power losses. The data is then used to develop a general linear model to predict peak flux as a function of inclusion type, volume fraction, core geometry, and coil winding. The evaluated inclusions include magnetite powder, 434 stainless-steel fibers and wool, and an A36 steel magnetic core. The BH hysteresis is obtained for the different mixes and compared to a control cement mortar mix containing no magnetic inclusions. X-Ray diffraction, thermogravimetric analysis, and compressive strength tests, were also completed for quality control.

Table 2 Chemical composition and physical properties of cementitious materials.

Table 1 Chemical composition of ferritic inclusions. Composition (%)

Magnetite Powder

434 Stainless-Steel

A36 Steel

Al2O3 C Cr Cu Fe Fe3O4 Mg Mn P S SiO2 TiO2 Zn

0.10 – – – – 98 0.90 0.30 – – 0.050 0.25 0.10

– 0.042 16 – 83 – – 0.65 0.020 0.025 0.36 – –

– 0.26 – 0.20 99 – – 0.75 0.040 0.050 – – –

FA

Al2O3 CaO Fe2O3 SiO2 MgO SO3 Na2O K2O LOI

4.2 64.8 3.5 20.7 1.2 2.8 0.14 0.47 2.7

18.5 14.0 4.7 53.3 3.6 0.6 1.0 1.3 0.2

Fineness (m2/kg) Specific Gravity

410 3.15

355 2.55

Material Magnetite Powder Silica Sand SS Fiber A36 Steel Core SS Wool a

The material is comprised of cement mortar which consists of Type I Portland cement (PC), Class F fly ash (FA), a 150-mm silica sand, and mixed with distilled water at a water/(binder + magnetite powder) ratio of 0.27. A Type F ASTM C494 [21] polycarboxylate-based superplasticizer (SP) was used to maintain a consistent workability across the various mixes. The relative density of the SP was 1.09. Different combinations and dosages of magnetic inclusions were then added to the base mixture. The magnetic inclusions included magnetite powders, chopped 434 stainless-steel fibers (SS Fiber), 434 stainless-steel wool (SS Wool), and a 6.25 mm diameter A36 steel core (steel reinforcement serving as a magnetic core). The chemical compositions of the magnetic inclusions are provided in Table 1. The chemical compositions of the PC and FA are summarized in Table 2, and the physical properties of the magnetic inclusions are provided in Table 3. Note the fiber is loosely graded and sized during processing so a range a sizes and diameters are provided which was quantified using a Keyence VH Z500 R optical microscope [22]. The ASTM A36 steel core has the following mechanical properties: a) tensile yield strength: 250 MPa, b) elongation: 23%, c) elastic modulus: 200 GPa, and d) Poisson’s ratio: 0.26. Fig. 3 shows images of the constituents that comprise the various mortar mixes. The mix proportions by mass of PC for each mix are provided in Table 4.

PC

Table 3 Physical and magnetic properties of individual constituents.

2. Method and materials 2.1. Material

Composition (%)

Mean Particle Size (lm) a

31 150a 30-80a, 500-5300b 30-90c 6250a 120a

Density (g/cm3) 5.2 2.7 7.7 7.8 7.7

Diameter, bLength, cAspect Ratio.

Mixes M4 – M5, and M11 – M17 were used to test the effects of individual inclusions on the relative magnetic permeability of the material. M18 – M29 were used to determine the effects of different combined magnetic inclusions. The volumetric fraction of each magnetic inclusion is also provided in Table 4 and shown in the parenthesis as (% Vol). 2.2. Mixing and curing The dry powders (PC, FA, sand and magnetic powders) were placed into a mechanical mixer and first dry blended for one minute. The SS Fiber was then added to the dry powders and mixed for one additional minute. If the mix did not contain SS Fiber, the dry powder was blended for a total of two minutes. The water/SP solution was then added to the dry blend and mixed for an additional two minutes to enable proper dispersion of the constituents. The fresh mix was then cast into a 3D printed PLA plastic mold that was coated with a release agent and wrapped with masking tape to prevent leaking. For mixes containing the A36 steel core (6.25 mm diameter), the molds were initially filled with mortar to one-third of the mold depth. The steel core was then placed directly on top of the existing mortar along the centerline of the core. The remaining mortar was then placed to fill the entire the mold. In mixes that did not contain SS fibers, the core was suspended in place and attached to an external support as shown in Fig. 4a. In mixes containing SS Fiber, the higher viscosity prevented core settlement and did not require additional support. In mixes containing SS Wool and steel core, the steel core was wrapped in the SS Wool prior to placement as illustrated in Fig. 4b. The molds were filled to one-third the depth prior to placing the core/SS Wool combination, then filled and leveled. After casting and leveling, the specimens were vibrated on a vibrating table for 30 s. The specimens were then placed in a sealed container and cured at ambient room temperature (~22 °C) for 3 days. The specimens were then removed from the molds, labeled, and placed back into air sealed container until testing. Each specimen has a square cross-section measuring approximately 3:84  104 m2 , (19 mm  19 mm), and has an average centerline length of approximately 0:294 m. The three different core geometries: i) CI, ii) Ring, and iii) Rectangular, considered in this study are shown in Fig. 4c.

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Fig. 3. Mortar constituents: (a) PC, (b) FA, (c) Silica Sand, (d) Magnetite Powder, (e) SS Fibers, (f) SS Wool, (g) A36 steel core.

Table 4 Mortar mix proportion.

a

Specimen

PC

Water

FA

Sand

SP: % binder mass

Magnetite (% Vol)

SS Wool (% Vol)

SS Fiber (% Vol)

Steel Core

Control M4 M5 M11 M13a M14b M15 M16 M17c M18 M19 M20 M21 M22 M23 M24 M25 M26 M27 M28 M29

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 1.0 1.0 1.3 0.6 1.0 0.7 0.6 0.6 0.7 0.6 0.9 0.6

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.50 0.30 0.60 1.80 0.70 0.70 0.40 0.70 0.70 0.50 0.50 1.50 0.40 0.50 0.40 0.15 0.20 0.30 0.20 0.50 0.15

– – – 1.04 – – – – – 1.40 1.42 2.69 0.11 1.42 0.54 0.10 – 0.56 0.10 1.25 0.10

– – – – 0.05 0.05 – 0.10 0.10 – 0.08 0.20 0.06 0.08 0.06 0.05 0.05 0.06 0.06 0.07 –

– 0.44 0.93 – – – – – – 1.84 1.87 2.36 1.40 1.87 0.14 0.13 0.12 0.74 0.66 0.16 0.13

No No No No No No Yes No No Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes

(10%)

(10%) (10%) (15%) (1%) (10%) (5%) (1%) (5%) (1%) (10%) (1%)

(0.35%) (0.35%) (0.70%) (0.70%) (0.38%) (0.70%) (0.38%) (0.38%) (0.38%) (0.38%) (0.38%) (0.38%) (0.38%) (0.38%)

(3%) (6%)

(9%) (9%) (9%) (9%) (9%) (1%) (1%) (1%) (5%) (5%) (1%) (1%)

This specimen contained a single layer of SS wool. bThis specimen contained two layers of SS wool. cThe SS wool in this specimen was cut to 50 mm lengths.

2.3. Thermogravimetric analysis A thermogravimetric analysis (TGA) was completed to quantify the development of hydration products within a reference mix

with no magnetic inclusions and test mixes with magnetic inclusions. The formation of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) in the mortar matrix was quantified. A TA Instrument SDT Q 600 DSC/TGA was used to conduct the test.

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2

1 1

Steel core external supports, 2PLA reusable 3D printed mold

(a)

1

2

1

A36 steel core wrapped with SS Wool, 2PLA C-core mold

(b)

(i)

(ii)

(iii)

(c) Fig. 4. (a) Steel core support system. (b) SS wool-wrapped A36 steel core in C-I core mold. (c) Concrete (i) C-I core, (ii) ring core, and (iii) rectangular core.

The hardened sample was first ground into a fine powder using a mortar and pestle. Approximately 15 mg of the pulverized material was then placed in a crucible and inserted into the DSC/TGA. The specimens were tested from room temperature (~25 °C) to 1000 °C at a temperature ramp rate of 10 °C/min. The TGA and

DTA profiles showed phase changes between 415 and 465 °C (CH dehydroxylation), and 76–126 °C (C-S-H dehydration). The change in mass at each of these temperature ranges was then determined using equations (1–2) to determine the quantity of CH and C-S-H present in the corresponding sample.

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 CHm MT1  M T2 100 ðH2 OÞm ðM PC þ MFA ÞM T2

 CSH CSHm MT1  M T2 100 ð%Þ ¼ PC þ FA ðH2 OÞm ðM PC þ MFA ÞM T2 CH ð%Þ ¼ PC þ FA







ð1Þ ð2Þ

Ph ¼

where CHm is the molar mass of CH (74 g), H2Om is the molar mass of H2O (18 g), MT1 is the mass of the sample at the initial temperature where the mass loss and phase change is observable, MT2 is the mass of the sample at the end of the mass loss temperature range and (MPC + MFA) is the mass fraction of cementitious material (PC and FA) in the sample (~0.6 for the control mix and ~0.3 for M18) [23]. 2.4. X-ray diffraction analysis X-ray Diffraction (XRD) analysis was also completed using a Thermo Scientific ARL Equinox 100 X-ray diffractometer which uses a 50 W Micro-focus Cobalt-calibrated X-ray tube and a rotating sample holder. Each hydrated sample was ground into a fine powder using a mortar and pestle. The un-hydrated dry constituents were analyzed in their as-received state. The samples were placed in a sample holder, lightly compacted and leveled, and placed on a rotating stage within the diffractometer. Each sample was scanned for 300 s to obtain the XRD profile. The profiles were then imported into Match! analysis software to smoothen raw data and identify crystalline phases. 2.5. BH hysteresis and magnetic permeability The testing apparatus pictured in Fig. 5a was used to obtain the relationship between the magnetic flux density (B) and magnetic field (H) for the various specimens. A schematic of this apparatus is provided in Fig. 5b. Fig. 5c shows the coil and core configuration of the testing apparatus. The coil was made using enameled copper wire (22 awg) and wound over the core. The variable AC power supply supplied the circuit with a 60 Hz alternating current ranging from 0 to 7 A. The magnetizing force was determined using equation (3). The voltage was measured over the 0.2 X resistor downstream of the coil to calculate the current, i, in the primary circuit. The induced voltage in the secondary coil was then fed into an integrator to determine the magnetic flux using equation (4) [24,25]. Where N1 and N2 are the number of turns in the primary and secondary coil, respectively, A is the cross-sectional area of the concrete core, V is output voltage induced in the secondary coil circuit, and l is the centerline length of the C-I core. The voltage in the primary circuit across the resistor and the integrated voltage in the secondary circuit were recorded by a Digilent Discovery 2 data logger at an acquisition rate of 5 kHz for 2 s (WaveForms 2015).

H¼

N1 i l

B¼

1 N2 A

ð3Þ Z

t

Vdt

ð4Þ

0

The BH hysteresis was then obtained and used to determine the peak magnetic flux, Bm, at a corresponding peak magnetizing force, Hm, (evaluated at each BH loop), the resulting impedance permeability, lz, (Eq. (5)), and the differential relative permeability, lr, (Eq. (6)) [24]; where lo is the permeability of free space (1.257  106).

lz ¼ lr ¼

Bm

lo H m DB

l o DH

The hysteresis power loss per unit volume, Ph, was also calculated by integrating the BH hysteresis over a given input source period with equation (7) [26].

ð5Þ

ð6Þ

1 T

Z

T

H 0

dB dt dt

ð7Þ

where T is the period of the input voltage sinusoidal wave source (T = 0.017 s @ 60 Hz). Each specimen was tested a minimum of two times and the resulting BH hysteresis was recorded. Three different core geometries: i) CI, ii) ring, and iii) rectangular, three different winding configurations: i) half-length 100 turn primary – 100 turn secondary (100/100 H), ii) uniform 200 turn primary – uniform 100 turn secondary (200/100 U) [25] and iii) quarter length 800 turn primary – 400 turn secondary winding (800/400 Q), and one input source waveform: 60 Hz sine wave were considered in the study.

3. Results 3.1. XRD analysis X-ray diffraction was completed on mixes M18 and Control to identify crystalline phases in the magnetizable cement mortar matrix. The dry constituents of the mixes were also analyzed to form baselines for the hydrated mixes. Fig. 6 shows the diffractograms for the samples. Specimen M18 (containing magnetite), exhibits a similar pattern to that of the Control, with the identified phases of Fe3O4, Cr, Mullite, SiO2 phases coming from the magnetite, stainless steel fibers, Fly Ash and Silica Sand, respectively. The ferritic inclusions are not observed to alter the hydrated calcium and silica based crystalline phases and appear to mostly serve as inert filler. 3.2. Thermogravimetric analysis A thermogravimetric analysis (TGA) was conducted on the control mix and M18. The specimens were tested at 90 days. The CH and C-S-H content of the tested mixes were compared and analyzed. Fig. 7 shows the TGA and differential temperature (DTA) plots of each specimen. The endothermic peaks occurring due to C-S-H dehydration were observed between 83 °C and 126 °C for the control mix, and 81 °C and 124 °C for M18. The endothermic peak resulting from CH dehydroxylation occurred between 425 °C and 464 °C for the control mix and 415 °C and 453 °C M18. The temperature ranges were consistent across the three specimens and were within the temperature ranges observed in the literature for hydrated Portland cement [27]; where C-S-H dehydration occurs between 60 °C and 105 °C, and CH dehydroxylation occurs between 440 °C and 475 °C. The mass of the specimens within the temperature ranges T1 and T2 were recorded and used to determine the amount of dehydroxylated CH and dehydrated C-S-H using equations (1– 2), respectively. Fig. 8 shows the CH and C-S-H contents from the 90-day TGA as a percentage of dry cementitious material content (PC + FA). The CH and C-S-H quantities in the mixes containing the ferritic inclusions are found to be within observable ranges of hydrated Portland cement [4]. The small decrease in hydrated C-S-H content in the mixes containing ferritic inclusions can be attributable to a filling effect; where the inclusions provide a physical barrier between the cement particles and the hydrating fluid, thus lowering the number of possible hydration sites. The ±1% variation in CH across samples was relatively negligible but may also have been a result of physical interference of the magnetic inclusions.

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1

60 Hz Variable AC Power Supply, 2Resistor, 3Coil and Core, 4Resistor, 5Integrator, 6Data Acquisition Device, 7Digital Oscilloscope.

(a)

(b)

(i)

(ii)

(iii)

(c) Fig. 5. Testing apparatus. (a) Photograph of test setup. (b) Test circuit, (c) Coil and core configuration: (i) CI and (ii) ring core with half-length 100 turn primary and 100 turn secondary windings, and (iii) CI with compact quarter-length 800 turn primary and 400 turn secondary windings.

3.3. BH characteristics Fig. 9 shows the BH hysteresis for specimens M22, M15 and M18, respectively, tested at an age of 365+ days. Specimen M22

contains ferritic inclusions and no steel core; M15 contains a steel core but no fiber or powder ferritic inclusions; and M18 contains both ferritic inclusions and a steel core. The magnetizing force, H (x-axis) is plotted against the corresponding magnetic flux, B for

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Fig. 6. XRD plots of 28-day hydrated mixes and unhydrated individual ferritic inclusion and cementitious constituents.

both the CI 100/100 H (Fig. 9i) and CI 200/100 U (Fig. 9ii) core samples. The dashed line represents the maximum slope of the side of the final dynamic BH hysteresis loop (differential permeability) [24]. M22 exhibits relatively lower peak magnetic flux at corresponding peak magnetization force than M15 and M18-that contain the steel core. M18 has the highest permeability and yields the highest peak magnetic flux of the three samples shown in Fig. 9. The uniformly wound core with 200 turns on the primary and 100 turns on the secondary (CI 200/100 U) yields significantly higher magnetic flux than the half wound CI core (CI 100/100 H) for all three specimens. The uniformly wound cores minimize the flux leakage through the specimen. The squareness of the BH hysteresis is also observed to be lower in the uniformly wound cores containing the steel rod. The M22 specimen begins to saturate at approximately 6 mT and 30 mT for the CI 100/100 H and CI 200/100 U specimens, respectively; the M15 specimen begins to saturate at approximately 25 mT and 70 mT for the CI 100/100 H and CI 200/100 U specimen, respectively; and the M18 specimen begins to saturate at approximately 35 mT and 100 mT for the CI 100/100 H and CI 200/100 U specimens, respectively. At a peak magnetizing force of approximately of Hm = 4000 A/m, the specimens containing high volumetric fraction of powder and fiber ferritic inclusions, M18, increased the peak magnetic flux by nearly 30 mT compared against specimen M15; which is approximately equal to the flux produced in specimen M22 containing high volume fraction of ferritic inclusions, but no steel bar; indicating minimal flux leakage within the composite M18 sample. Fig. 10 shows summary of the magnetic flux, and the differential and impedance permeability reported at Hm = 1500 A/m for

each CI 100/100 H sample tested at 365+ days. As expected, the control specimen containing no ferritic inclusions registered the lowest peak magnetic flux (~0.25 mT). Specimen M19 yielded the highest peak magnetic flux (36 mT) at Hm = 1500 A/m. The specimens containing a steel core along with some other source of ferritic inclusion displayed significantly higher peak flux. Fig. 11 shows the hysteresis power loss (W/m3) for each of the CI 100/100 H samples at varying magnetic flux (Fig. 11a); and 20 mT (Fig. 11b) tested at 365+ days. The hysteresis power loss follows a similar trend to the peak magnetic flux and permeability data: the more permeable specimens (M19, M18, M20) yield significantly lower hysteresis power loss at 20 mT. The samples containing a steel rod also exhibit significantly lower hysteresis core loss at comparable levels of magnetic flux. There appears to be a flux percolation ferritic volume fraction threshold at or near 10%. At low volume fractions (/,ferritic ~ 1–3%), the ferritic inclusions slightly increase core loss and/or decrease magnetic permeability (M24, M25, M29) relative to the M15 control sample. At moderate volume fractions (/,ferritic ~ 5%), the core loss is moderately impacted (M27, M26). Note that although M26 has, collectively, an approximate ferritic volumetric fraction of 10%, the individual volume fraction of each constituent is at the moderate level: steel fiber (~5%) and magnetite (~5%); and does not reduce the core loss relative to the control M15. When the individual volume fraction of each constituent is at, or greater than, 10%, the core loss is reduced and the permeability is increased; suggesting effective flux percolation through the cement media. Below this flux percolation threshold, there is most likely significant flux leakage occurring

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Fig. 7. TGA and DTA plot of specimens at 90 days: (a) Control, (b) M18 at 90 days.

Fig. 8. 90-Day CH and C-S-H content as a percentage of cementitious material content.

within the matrix which subsequently leads to increased core loss and decreased permeability (relative to the control sample M15). Fig. 12 shows the peak magnetic flux, Bm, produced at a peak magnetizing force, Hm = 1500 A/m for M15, M18, and M22 at different concrete age, and different coil/core geometry. Fig. 12a shows the effects of age on the CI 100/100 H core. The younger aged concrete (<14 days) yields slightly higher magnetic flux. The reduction in the magnetic flux in the 365+ day samples can be caused by either magnetic aging-which may occur in higher carbon content steels (>0.0015%) [28], or increased hydration production of diamagnetic media in the matrix. The losses from magnetic aging of the steel is not likely to be a contributing factor here because the steel was not subjected to high temperatures to reach levels of significant aging [29]. The more likely contributing factor is the increase in diamagnetic force from increased hydration and production of polymerized silica units of C-S-H, crystalline units of CH, and carbonation products of CaCO3; all of which have larger diamagnetic susceptibility than water. Hydrated mortar contains

9

a high concentration of diamagnetic constituent (between 75% and 100%) that have negative molar magnetic susceptibility (vm * 106 mol/cm3): H2O: 12.6, Al2O3: 37.0, CH: 22.0, CaCO3: 38.2, CaO: 15, SiO2: 29.6 [30]. Fig. 12b shows the effect of core geometry and coil winding. The CI core with the quarter length 800–400 winding (CI 800/400 Q) yields the lowest magnetic flux which is expected since the winding only covers a small percentage of the core. The 100/100 halflength rectangular (Rectangular CI 100/100 H) and ring (Ring CI 100/100 H) core samples yielded similar magnetic peak flux, but significantly higher peak flux than the CI 100/100 H and Q core. The reduction in flux occurs because within the CI core, there is a 19 mm gap between the tip of the steel rod within the C core and the edge of the steel rod within the I core. Within the rectangular and ring core, the steel rod is continuous around the centerline length. This 19 mm gap most likely creates a region for the flux to leak and/or fringe. The peak flux in the control specimen M15 CI 100/100 H core is 22 mT and in the rectangular core it is 41 mT. The flux leakage near or at this region can be reduced, however, with the addition of the ferritic inclusions. M18, which contains both the ferritic inclusions (~ 20% by volume) and the steel rod, the CI 100/100 H core yields a flux of 36 mT; 14 mT higher than the control M15 sample and only 5 mT lower than the M15 CI 100/100 rectangular core; suggesting that the flux is percolating through the ferritic cement media, effectively linking the steel rod between the C and I cores, and effectively reducing flux leakage and fringing. The M18 rectangular 100/100 H core, however, yields a peak magnetic flux 10 mT greater than M18 CI 100/100 H, which indicates some flux leakage still exists in the CI core; but significantly lower than the flux leakage in M15 (19 mT). The flux leakage is reduced by nearly 50% when the ferritic media is placed within the cement media; where the core-geometry flux leakage reduction is calculated as follows: (DBm,Rec-CI, M15  DBm,Rec-CI, M18)/ DBm,Rec-CI, M15. Flux leakages related to coil winding are also reported. Fig. 12b shows the peak magnetic flux for the uniformly wound CI cores at a peak magnetizing force of Hm = 1500 A/m. Winding sample M15 uniformly (CI 200/100 U) was able to reduce flux leakage and fringing within the 19 mm gap between the steel rod edges; increasing the flux to 36 mT, but slightly lower than the partially wound rectangular core (40 mT) with the continuous steel rod. Adding the ferritic inclusions increased the peak flux to 46 mT in the uniformly wound CI 200/100 U core in M18. The increase in the sample without the steel bar, M22, was most impacted by the uniform winding; where the peak flux increased to 16 mT (CI 200/100 U) from 10 mT (CI 100/100 H). The flux leakage related to coil winding also seems to be mitigated by the addition of the ferritic inclusions. The difference in flux between the M15 CI 100/100 H and 200/100 U specimen is 14 mT and the difference in the M18 CI 100/100 H and 200/100 U specimen is 10 mT. The peak flux in the composite M18 specimen containing the steel rod and ferritic cement matrix in the rectangular core is 46 mT and in the CI 200/100 U core it is 47 mT. The peak flux in the M15 rectangular core is 40 mT and in the CI 200/100 U core it is 35 mT. The peak flux in the M22 rectangular core is 10 mT and in the CI 200/100 U core is 16 mT. If the peak flux in the M15 sample is added to the peak flux in the M22 samples, the aggregate peak flux, is 50 mT in the rectangular 100/100 H core and 51 mT in the CI 200/100 U core; which is 4 mT higher than the measured peak flux (or 8% higher than the measured peak flux) in the composite M18 sample for both the rectangular 100/100 H and CI 200/100 U cores, respectively. The 4 mT reduction in flux measured in composite M18 sample can be attributed to possible leakage or fringing associated with interfacial interactions between the steel rod and the cement matrix as the individual core components saturate. This is consistent with the results in Fig. 12e that show the

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K.A.T. Edwards et al. / Construction and Building Materials 227 (2019) 117041

(i)

(i)

(i)

(ii)

(a)

(b)

(c)

(ii)

(ii)

Fig. 9. B-H hysteresis and relative magnetic permeability of CI core (i) 100/100 H, and (ii) 200/100 U specimens: (a) M22, (b) M15, (c) M18.

composite relative impedance permeability (25) is slightly lower than the aggregated impedance relative permeability (28). Irrespectively, the flux losses (8% of peak flux) and relative impedance

permeability (12% lower than aggregated flux) attributable to composite interfacial interaction are relatively small. The differential permeability, however, which is measured in the linear range of

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K.A.T. Edwards et al. / Construction and Building Materials 227 (2019) 117041

30

40

Impedance Permeability

Differential Permeability

Magnetic Flux

35

25 w/ steel rod

w/o steel rod

30 25

15

20 15

10

B (mT)

μr

20

10

5

5

0 M19

M20

M18

M21

M26

M28

M23

M15

M25

M27

M29

M24

M17

M22

M5

M16

M4

M14

M11

Control

M13

0

Fig. 10. Peak magnetic flux, impedance and differential permeability of CI 100/100 H core samples at H = 1500 A/m tested at 365+ days (concrete age).

8000

Control M5 M29 M28 M19

7000

Ph (W/m3)

6000

M13 M16 M27 M26

M11 M22 M25 M21

M14 M17 M15 M20

M4 M24 M23 M18

5000 4000 3000 2000 1000 0 0

5

10

15

20

25 Bm (mT)

30

35

40

45

(a) 4

φ,ferric < 10 %

3.5

φ,ferric > 10 %

3

Ph (kW/m3)

2.5 2 1.5 1 0.5 0 M29

M27

M25

M15

M23

M28

M26

M21

M20

M18

M19

(b) Fig. 11. Hysteresis power loss of concrete (a) CI 100/100 H core at varying magnetic flux and (b) CI 100/100 H core at magnetic flux of 20 mT.

the BH hysteresis (below saturation), does not appear to be affected by the interfacial flux losses. The aggregated differential permeability between M22 and M15 (28.5) is equal to the composite relative differential permeability in M18 (28.5) in the uniformly wound CI 200/100 U cores. Fig. 12c and d show the hysteresis core loss comparison between the CI 100/100 H and CI 200/100 U cores. The CI 200/100 U hysteresis core losses are 2–3 times lower in the CI 200/100 U core samples for both M15 and M18.

3.4. Analysis of variance ANOVA was used to quantify the flux exchange coupling between the ferritic inclusions, core types, and core windings. All 21 mixes were considered in the analysis. The parameter estimates are shown in Table 5. The results from Table 5 confirm the results reported in Figs. 10–12. The addition of fibrous and powder ferritic inclusions within the surrounding diamagnetic matrix increased the bulk

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K.A.T. Edwards et al. / Construction and Building Materials 227 (2019) 117041

365+ days

40

< 14 days

60

B (mT)

B (mT)

30 20

CI - 800/400 Q Ring 100/100 H CI 200/100 U

CI 100/100 H Rectangular 100/100 H

40

20 10 0

0 M15

M18

M22

M15

M18

(a) 20

4

15

3 Ph (kW/m3)

Ph (kW/m3)

M22

(b)

M22 200/100 U

10

M15 200/100 U M18 200/100 U M22 100/100 H

5

CI 200/100 U

CI 100/100 H

2 1

M15 100/100 H

0

M18 100/100 H

0

M15 0

50

B (mT)

100

M18

(d)

(c) 30.0

CI 200/100 U mr

M22

150

CI 100/100 H mr

CI 200/100 U mz

CI 100/100 H mz

25.0

μr

20.0 15.0 10.0 5.0 0.0 M15

M18

M22

(e) Fig. 12. (a) Effects of specimen age for CI 100/100 H core, and (b) effects of coil/core geometry, (c) hysteresis power loss of concrete at varying flux (d) effects of coil winding on hysteresis power loss, and (e) effects of coil winding on differential and impedance permeability.

permeability and is shown to depend on the volumetric fraction of the inclusions. Chopped 50 mm steel wool (b = 5.604) is more effective in increasing the magnetic flux than continuously wrapping the steel wool around the core (b = -2.702). This is most likely due to the reduction in mix workability and paste consistency. The steel wool is bulky which impeded the smaller steel fiber and magnetite inclusions from flowing uniformly throughout the core mold when wrapped continuously in the core mold. Chopping the wool down to 50 mm length, improved the workability and mix consistency which also increased peak flux. The steel fiber inclusions increased the flux more significantly in the CI core (b = 0.937) than the ring (b = 0.829) and rectangular core (b = 0.650); confirming the steel fiber inclusions and magnetite (b = 0.347) create a flux link between the discontinuity in the steel rod leading to significantly higher flux gains in the CI sample at volume fractions greater than 10%. The steel fibers increase the flux more significantly than the magnetite powder. Ferritic grade 430 stainless steel bars have relative permeabilities between 530 and 960 at room temperature [31]. Grade 430 fibrous stainless-steel shavings or wool, on the other hand, will have lower permeability and have higher

coercivity than the larger stainless-steel inclusion due to both size and shape anisotropy [6]. However, some percentage of the fibers will tend to align in the direction of the applied magnetic field which will serve to increase permeability and decrease hysteresis power loss. The sizes of both the SS Wool and SS Fiber are much larger than that of the magnetite powder, which would allow the inclusions to increase flux and permeability more significantly. In addition, fine magnetite powder has been reported to only have a permeability of approximately 4 [7], so this result is not entirely unexpected. The larger fibers will have higher permeability irrespective of the shape anisotropy over the smaller spherical magnetite particle. The addition of the steel rod was found to increase the permeability most significantly in the Ring (b = 0.000) and Rectangular core (b = 3.15) compared with the CI core (b = 19.166) due to the discontinuity of the steel rod between the C and I core. The CI 800/400 Q coil configuration yielded significantly lower flux and the effects of the magnetic inclusions (steel fiber: b = 0.454; magnetite: b = 0.155) and steel rod (b = 29.432) were not as significant as in the 100/100 H winding indicating significant

13

K.A.T. Edwards et al. / Construction and Building Materials 227 (2019) 117041 Table 5 ANOVA parameter estimates. Parameter Estimates Parameter

b

Std. Error

t

Sig.

95% Confidence Interval Lower Bound

Upper Bound

Intercept

37.43

3.096

12.089

<0.001

31.138

43.722

Wool Length (Continuous) * Wool (/)

2.702

2.564

1.054

0.299

7.912

2.508

Wool Length (50 mm) * Wool (/) Core Type (CI) * Wind (H) * SS Fiber (/) Core Type (Rec) * Wind (H) * SS Fiber (/) Core Type (Ring) * Wind (H) * SS Fiber (/)

5.604 0.937 0.650 0.829

2.589 0.219 0.496 0.496

2.164 4.28 1.309 1.669

0.038 <0.001 0.199 0.104

0.342 0.492 0.359 0.18

10.866 1.382 1.659 1.838

Core Type (CI) * Wind (H) * Magnetite (/) Core Type (Rec) * Wind (H) * Magnetite (/) Core Type (Ring) * Wind (H) * Magnetite (/)

0.347 0a 0a

0.172 – –

2.017 – –

0.052 – –

0.003 – –

0.696 – –

Core Core Core Core Core Core

Type Type Type Type Type Type

(CI) * Wind (H) *Steel Bar (No) (CI) * Wind (H) * Steel Bar (Yes) (Rec) * Wind (H) * Steel Bar (No) (Rec) * Wind (H) * Steel Bar (Yes) (Ring) * Wind (H) * Steel Bar (No) (Ring) * Wind (H) * Steel Bar (Yes)

38.618 19.166 31.893 3.150 34.513 0a

3.326 3.331 6.269 4.379 4.486 –

11.611 5.753 5.088 0.719 7.694 –

<0.001 <0.001 <0.001 0.477 <0.001 –

45.377 25.936 44.632 5.749 43.629 –

31.859 12.396 19.154 12.049 25.397 –

Core Core Core Core

Type Type Type Type

(CI) * Wind (CI) * Wind (CI) * Wind (CI) * Wind

(Q) * SS Fiber (/) (Q) * Magnetite (/) (Q) * Steel Bar (No) (Q) * Steel Bar (Yes)

0.454 0.155 38.880 29.432

0.233 0.175 3.375 3.401

1.95 0.884 11.521 8.654

0.059 0.383 <0.001 <0.001

0.019 0.201 45.738 36.343

0.927 0.511 32.021 22.52

Core Core Core Core

Type Type Type Type

(CI) * Wind (CI) * Wind (CI) * Wind (CI) * Wind

(U) * SS Fiber (/) (U) * Magnetite (/) (U) * Steel Bar (No) (U) * Steel Bar (Yes)

1.304 0a 31.903 2.030

0.496 – 6.269 4.379

2.626 – 5.089 0.464

0.013 – <0.001 0.646

0.295 – 44.642 10.929

2.313 – 19.164 6.869

a Parameter is set to zero because it is redundant. R2 = 0.967 (Adjusted R2 = 0.949).

winding-related flux leakage in the quarter length winding specimens. 3.5. Compression strength While the primary focus of this research was to quantify the permeability of magnetizable mortar, mechanical compression tests were carried out for quality control [32]. The 28-day compressive strength for specimen M19 (highest permeability; high magnetite and SS Fiber volume fraction) was 29.9 ± 2.1 MPa. The 28-day compressive strength of the Control specimen was 65.3 ± 1.5 MPa. 4. Conclusion The addition of ferritic material inclusions to a mortar matrix has been observed to significantly increase the peak magnetic flux and relative permeability and decrease hysteresis core loss. The ferritic inclusions can increase flux percolation through a cementitious media and reduce flux leakage and fringing between discontinuous ends of an embedded steel rod. In summary, this research led to the following conclusion:
The peak magnetic flux generated by a concrete CI core will decrease slightly with time: peak flux at an age of 365+ days is ~90% of the flux generated at 14 days.
The flux percolation volume fraction threshold for fibrous and powder ferritic inclusions is approximately 10%. At volume fractions below this threshold, flux, permeability, or core loss are not significantly impacted. At volumetric fractions near 20%, the core loss can be reduced by 50%, and the peak flux can be increased by 30% to 100% relative to a control (depending on the coil winding).


Manufacturing a core with a continuous connection of the steel rod along the centerline length (Rectangular or Ring core) with a uniform winding will minimize flux leakage. The ferritic inclusions bridge the discontinuous edges of the steel rod in a CI core and decrease flux leakage and fringing by approximately 50% when the ferritic volume fraction is at or above 20%. The ferritic inclusions also decrease flux leakage in partially wound coil configurations.
Composite flux losses at H = 1500 A/m are less than 10% of the peak flux generated by the composite core near saturation. Below saturation (in the linear range of the BH hysteresis), the interfacial core losses are negligible and do not affect the differential relative permeability of the composite core.
A magnetic flux of approximately 100 mT and relative differential permeability of 28 at a magnetizing force of H = 4000 A/m is achievable in a ferritic concrete CI core containing a A36 steel rod and 20% ferritic steel fiber + magnetite inclusions.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The research reported in this paper was sponsored by Lamar University through the Research Enhancement Grant. The contents of this report reflect the views of the writers, who are responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the views of Lamar University.

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