Experiment on Magnetic Compaction to Reduce Bugholes in Free-Form Concrete Panels

Abstract

Free-form concrete panel (FCP) molds require precise manufacturing because each mold demands a unique shape. Therefore, automation technology for producing these molds is being developed. However, when concrete is cast in a free-form mold and subjected to the impact of compaction to distribute it, deformation occurs in the precisely designed form. Consequently, free-form molds are often produced manually, which results in bugholes on the surface of FCPs. These bugholes lead to quality issues in the panels, including deterioration in aesthetics and strength. This study aims to develop a magnetic compaction technology that installs an object which rotates due to magnetic force inside a free-form mold and applies magnetic force from the bottom to perform compaction according to the free form. By comparing a control group using the existing manual FCP production method and an experimental group using magnetic compaction, strength was measured and bughole incidence was verified. As a result, although the experimental group was subjected to rotary motion, no material separation or deterioration in strength occurred. Furthermore, a similar standard deviation of 62.23 mm² and a mean difference of 187.42 mm² were observed between the control group and the experimental group. The results of the t-test showed that at a 95% confidence level, the t-value was −16.35 and the p-value was 0.00. This confirms that the incidence of bugholes was reduced in the experimental group where magnetic compaction was applied. This research may contribute to reducing the occurrence of bugholes in existing free-form concrete panels and securing both aesthetics and strength.

1. Introduction

Free-form structures differ from existing structures in that their exterior surfaces are embodied with various curves. Due to these characteristics, free-form structures serve as landmarks around the world and have the effect of generating economic and social profits. In line with this, various studies are being conducted on free-form construction materials, design, construction, etc., and from these, this study has set free-form concrete panels as its research scope. FCP characteristics are as follows. Because the exterior surface of a free-form structure consists of a huge curved surface, it is not easy to produce all at once, so the massive curved surface is divided into easily producible panels such as FCPs. The divided panels are designed in three dimensional space using Catia, Revit, Rhino, etc., and to implement this, a free-form panel mold is produced [1]. Free-form panels have different curved surfaces and curvatures depending on the position they are used in, so the mold is custom produced. In this process, if precision deteriorates or the strength, flexibility, etc., of the materials used is insufficient, quality deterioration and construction errors such as cracks in the panel’s curvature and misalignment of the joint connection occur, as shown in Figure 1.

For existing free-form structures, materials such as metal, glass, Fiber-Reinforced Plastic (FRP), Glass-Fiber-Reinforced Concrete (GFRC), and Ultra-High-Performance Concrete (UHPC) are used [3,4]. Among these, concrete is suitable as a free-form panel material because it has good compressive and tensile strength and is advantageous for controlling cracks and demonstrating ductility capacity. However, manufacturing FCPs involves custom production of free-form molds, resulting in high production costs. Additionally, used free-form molds are difficult to reuse and are treated as construction waste, causing problems such as disposal costs and environmental pollution. To resolve this issue, research is being conducted on the development of equipment that automatically manufactures FCPs, as shown in Figure 2. This equipment consists of components that implement the upper and lower parts of the mold and controls the side form. It creates the curved surface of a free-form mold by moving the rod using CNC (computer numerical control) technology according to the automatically entered design form. FCPs are produced by casting concrete inside the completed free-form mold.

However, there are limitations to compaction work in the process of manufacturing free-form concrete panels using FCP automatic production equipment. Compaction involves repeatedly applying vibration or impact to discharge air from inside the mold when filling it with concrete. However, when impact, including vibratory compaction, is applied to a free-form mold with precise curvature, precision deteriorates, leading to deformation of the mold’s form. This results in errors in the panel and creates bugholes on the panel’s surface. Bugholes deteriorate the durability and aesthetics of the panel and are a factor in deteriorating quality [6]. Yun (2022) implemented a free-form mold through FCP automatic production equipment but produced FCP by handwork. This caused the air inside the mold not to be discharged externally, resulting in bugholes, as shown in Figure 3 [1].

This study intends to use magnetic force as a method to reduce the occurrence of bugholes in FCP. In the aforementioned introduction, the risks arising from the failure to compact during free-form concrete panel production were explained. Following this, through a literature review, precedent studies to reduce bugholes, as well as equipment, and precedent studies related to free-form concrete panel production are analyzed. Based on the analyzed studies, the magnetic compaction experimental methodology of this study is presented. To verify this, an experimental group and a control group are defined, and a magnetic compaction experiment is conducted for comparative analysis between the two groups. The comparative analysis items measure the compressive strength and flexural strength of the experimental and control groups produced through the experiment. In this study, the objective is to reduce the incidence of bugholes occurring on the surface; therefore, bugholes that occur internally are not included in the scope of the research. To assess the incidence of surface bugholes, a 3D scan is conducted. Subsequently, a t-test is used to determine whether the occurrence of bugholes has been reduced. This process is shown in Figure 4.

2. Literature Review

Concrete bugholes refer to surface pores created when air trapped in concrete moves to the surface in contact with the newly cast concrete mold. Bugholes are broadly divided into four types: entrapped air voids and entrained air voids, which occur because there is no moisture inside, and gall pores and capillary pores, which contain moisture that cannot undergo a hydration reaction with cement. These bugholes are related to the permeability and diffusion of external harmful ions and are a factor that weakens durability. In particular, when there are many micropores, such as bugholes, the amount of irrecoverable deformation increases after drying shrinkage or creep [7]. However, suppressing the occurrence of such bugholes is not easy. During the manufacturing process of concrete produced using composite materials, air is inevitably generated from the internal structure. If the amount of air inside the concrete is excessive, strength deteriorates and cracks occur, but a proper amount of air provides excellent resistance to freezing [8,9]. Therefore, by mixing bubbles consisting of air or other gases inside the concrete, foam concrete, autoclaved aerated concrete, and aerated concrete are used [10]. However, the scope of this study is limited to bugholes arising from the materials and mixing ratios used in free-form concrete panels. Currently, there are no clear standards regarding free-form concrete, and although various studies on free forms are being conducted, research on bugholes is insufficient [11]. Therefore, the literature review intends to analyze precedent studies regarding bugholes in general concrete and apply them to the area of free-form concrete.

Generally, besides ordinary Portland cement, high alumina and calcium sulfoaluminate have been used for concrete to improve foam concrete’s early strength and reduce setting time in Portland cement and rapid-hardening Portland cement [12,13,14]. However, for free-form concrete panel production, concrete such as UHPC (Ultra-High-Performance Concrete) and GFRC (Glass-Fiber-Reinforced Concrete) is used [15]. In fundamental studies, it has been analyzed that mortar, instead of concrete, has been used [16,17]. Therefore, in this study, with the objective of checking for the occurrence of bugholes in free-form concrete panels, mortar will be used.

To check for bughole incidence, studies were conducted using a variety of methods. Among these, Xradar used ground-penetrating radar (GPR) and ultrasound pulse velocity (UPV) to measure the occurrence of bugholes in the beam and girder during the concrete’s casting phase. At this time, internal bugholes were verified with the criteria for bugholes set to at least 2 mm [18].

The change in the structure of bugholes in concrete due to the elution of calcium was analyzed. For 3 months, at 2-week intervals, the change in the macropore size distribution of the surface was measured using the image analysis method compliant with the ASTM C457 test method. Bugholes at least 10 μm in size were measured on the surface of concrete that had undergone an underwater curing process for 28 days [19,20].

Research was conducted on a system that identifies bugholes in concrete cross-sections through 2D-to-3D technology. Numerous bugholes in concrete specimens are randomly collected as 2D data, and as the frequency of occurrence increases, intersections between bugholes occur. The 2D size distribution of the intersection between bugholes is determined by a probability distribution. The 3D size of the bughole is calculated based on the 2D statistics collected. This is the implementation of the 2D size in the image as 3D data. Such a method was simplified into a system, and the resulting bugholes were identified by defining bugholes of various sizes [21].

This study’s objective is to reduce the occurrence of bugholes. To verify this, it was necessary to define the criteria for calculating the number and size of bugholes. Based on the analysis of precedent studies, the criteria for calculating the number and size of the bugholes in this study are summarized in Figure 5. Figure 5 expresses the difference in the size of the bughole when viewed from the side and top. Although the bughole was generated with the same size R, the sizes of the bughole observed from the side and top were measured differently, as $r^1,r^2 \mathrm{and} r^3$. Therefore, a free-form concrete panel is produced through experimentation, and form data are measured through 3D scanning. Bughole incidence will be measured by calculating the number and size of bugholes observed from the top view of the form data.

Previous studies to reduce bugholes were analyzed. Autogenous shrinkage caused by bugholes in concrete results in cracks. Additionally, a fatty-acid-based self-shrinkage reducing agent was applied to the concrete, and an SEM (Scanning Electron Microscope) image was taken. The results confirmed that the voids were minimized as the internal moisture disappeared [22]. From a material standpoint, a permeable sheet of fibrous material was used in the formwork to reduce the moisture that causes bugholes. Lee (2012) conducted an experiment of pouring concrete into a Euroform permeable type with water holes. As a result, the surface roughness of the concrete to which the water-permeable sheet was applied was measured to be about 50 to 200 μm. This indicates that the concrete surface is smooth, and unlike general surfaces, it was determined that no bughole phenomenon occurred [23]. However, it is incorrect to use surface roughness as a criterion to confirm the occurrence of bugholes. For surface roughness, surface height is judged to be an important factor. However, bugholes must be judged based on the area occupied by the bugholes relative to the total surface area, not the depth [24]. Therefore, in this study, the area occupied by bugholes out of the total area will be measured to confirm the bughole incidence.

3. Definition and Experimentation of Concrete Magnetic Compaction Methodology

This study aims to measure the improvement in strength and the incidence of bugholes through experiments to verify the performance of magnetic compaction in free-form concrete panels. The experiment is designed to assess the performance of magnetic compaction by classifying a control group and an experimental group and comparing the results. Both groups are produced with a W/C ratio of 38%, which is used in free-form concrete panels [17]. The control group refers to the existing free-form concrete panels. The existing free-form concrete panels were manufactured using handwork to cast and disperse concrete into the mold during the production process. This manual process can influence the design of the precise free-form mold, making it difficult to disperse concrete elaborately inside the mold. Therefore, the control group is manufactured using handwork without any additional compaction. The experimental group is formed using the magnetic compaction method presented in this study, without any additional compaction. The comparison between the control and experimental groups aims to verify the performance of magnetic compaction. The magnetic compaction process, as shown in Figure 6, involves placing a free-form mold on top of the magnetic controller and positioning a PTFE spinbar in the center. After casting the concrete, the spinbar is rotated and moved according to the mold’s form to disperse the concrete. At this time, the longer the compaction time with the spinbar, the more the fluidity and compressive strength may decrease. Therefore, the operating time is kept within 10 s [25].

The dimensions of the control and experimental groups were produced to a square form of $400\times 400\times 1600 \left(\mathrm{mm}\right)$, in accordance with KS L ISO 679 [26]. This experiment was a basic experiment on the performance of magnetic compaction and was conducted in a form without free-form curves. Since magnetic compaction was not used in the control group, an iron mold made of existing metal was used. When the experimental group used metal materials, it interfered with the transmission of magnetic force. Therefore, a mold made of PLA filament was produced using a 3D printer. The 3D printer used in this study is the MASTER-S produced by NEXTOP. It utilizes the FDM (Fused Deposition Modeling) method to melt and layer plastic filament to create the output. The characteristics of the equipment are shown in

Table 1. MASTER-S 3D printer specification.

Hardware	Exterior	Steal, Acryl, ABS
	Bed material	Aluminum PCB
	Bed maximum temperature	110 °C
Nozzle	Size	0.4 mm
	Maximum temperature	260 °C
	Layer thickness	0.06 mm~0.3 mm
		(recommend 0.1 mm~0.2 mm)
Software	Program used	Cura
	Supported operating system	Windows/Mac OS/LINUX
	Supported file	STL/OBJ/DAE/AMF

. The bed temperature of the 3D printer was set to 55 °C, and the nozzle temperature was set to 210 °C. The details of the control and experimental groups are summarized in

Table 2. Characteristics and mixing ratio of control group and experimental group.

Classification		Control Group		Experimental Group
Compaction methods		Handwork		Magnetic compaction
$\mathrm{Dimensions}(\mathrm{mm}$)		$400\times 400\times 1600$		$400\times 400\times 1600$
Mold ingredients		Metal materials		PLA filament
W/C (%)	C (g)		S (g)		W (g)
38%	250		95		250

.

The control group involved casting concrete into an iron mold and dispersing it by hand without any additional compaction. In the experimental group, the manufactured mold was installed on top of the magnetic controller, and the spinbar was positioned at the center of the mold. The magnetic controller used was the MS-500 from INTLLAB, and the specifications of the equipment are shown in

Table 3. Magnetic controller specification.

Specifications
Input	$180.09\times 130.05\times 44.96{\mathrm{mm}}^3$
Mixing speed	$0~3000\mathrm{rpm}$
Overall dimensions	$180\mathrm{mm}\times 130\mathrm{mm}\times 45\mathrm{mm}$
Working table dimensions	$130\mathrm{mm}\times 130\mathrm{mm}$
Table material	316 Stainless steel plate
Weight	350 g

.

At this time, the spinbar could be moved within the mold by magnetic force, but it was secured with thread for smoother control. Concrete was then cast while the spinbar was slowly rotated at the maximum speed of 3000 rpm, moving back and forth three times from the center of the mold to both ends. Afterward, the thread was pulled to remove the spinbar from inside the concrete, and the experiment was concluded by curing both the control and experimental groups. This experimental process is shown in Figure 7.

4. Analysis of Magnetic Compaction Experiment Results

The magnetic compaction performance to be verified in this experiment is as follows.

(1)

Whether materials separate and strength deteriorates due to the spinbar’s rotational movement

(2)

Whether bughole incidence is reduced through magnetic compaction.

Firstly, the concrete dispersion ability of the spinbar was verified. This involved comparing the compressive strength and flexural strength of the control group that performed manual labor and the experimental group that performed magnetic compaction on days 1, 3, and 7. Figure 8 shows the data graph of the compressive and flexural strengths measured for the control and experimental groups. As a result, the compressive strength (a) of the control and experimental groups was measured to be similar, but that of the experimental group was superior. Flexural strength (b) is an essential characteristic in realizing free forms. There was a difference of about 0.4 MPa in the strength of the control group and the experimental group on the first day. However, the difference gradually increased, with a difference of 1.1 MPa on the third day and 2.6 MPa on the seventh day. The magnetic compaction experiment method was expected to deteriorate strength due to rotational movement. However, the compressive strength was similar to that of the existing free-form panel production method without deterioration. Additionally, the flexural strength was further improved compared to the existing method. This means that the concrete dispersion ability of the magnetic compaction method is improved compared to that of the existing free-form panel production method and that compaction is performed without material separation or strength deterioration.

To analyze the bughole incidence, the criteria presented by precedent studies were referred to. In order to measure the incidence of bugholes, the area of holes occurring on a smooth concrete surface was selected as bugholes. In this study, the bughole incidence was measured through the sum of the area of bugholes generated on the concrete surface through 3D scanning. Bugholes vary in size and are mostly composed of small holes and a few large holes. In order to calculate the area for this, the shape of the bugholes was assumed to be circular in this study. In the past, the occurrence of bugholes was visually confirmed, but this has low accuracy. Therefore, in this study, accuracy was ensured by overlaying the shape measured by 3D scanning with the design shape to measure the bugholes that occur. Also, the data used to confirm bughole incidence should be similar. In this study, nine test specimens each in the control group and experimental group were divided into eight zones. The total bughole area developed in each zone was then used to collect data. These criteria are the same as in

Table 4. Criteria for measuring bughole incidence [23].

Previous Studies Criteria	Applied Criteria
Bughole selection	Select the holes identified through 3D scanning as bugholes
Bughole size	Assume that the bughole size is circular to calculate the area of the void
Method for measuring bughole incidence	Measure bughole by overlapping scan data measured by 3D scan with design data
Use of bughole data	9 test specimens in the control group and the experimental group are divided into 8 zones, and similar group amounts of data are utilized

.

To check the bughole incidence, the surfaces of the control and experimental groups were scanned using a 3D scanner. In this study, Proto 3000’s 3D scanner, Go! SCAN SPARK was used. This is an optical scanner and its specifications are listed in

Table 5. Go! SCAN SPARK specification.

Hardware
Size	$89\times 114\times 346{\mathrm{mm}}^3$
Weight	$1.25\mathrm{kg}$
Scan range	$390\times 390{\mathrm{mm}}^2$
Software
Program used	VXelements
Support file	STL/TXT/WRL/X3D/X3DZ

.

Using VXelements, a scan analysis program, the scan data acquired through 3D scanning was overlaid with the CAD design form file to calculate the number and diameter of bugholes that arose on the upper surface. To compare the control group and the experimental group, the test specimens were divided into eight equal parts from Zone 1 to Zone 8, each measuring $40 \mathrm{mm}$ wide and $20 \mathrm{mm}$ high, as shown in Figure 9.

To calculate the number of bugholes applicable to each zone, bugholes at least −0.2 mm in depth and bugholes visible to the naked eye were calculated from the overlapping scan data. Since the bughole diameters have different dimensions, the diameters were measured assuming they were circular. At this time, the bugholes with a diameter of under 2 mm were calculated as circles with a diameter of 1 mm. The bugholes 2 mm to under 3 mm in diameter were calculated as circles with a diameter of 2 mm. Bugholes that fell outside the specified range were calculated as circles with the measured diameter. This process was performed to measure the area of the bugholes that occurred in the total area, as shown in Figure 10.

The magnetic compaction presented in this study is a method that disperses concrete while the spinbar rotates by the magnetic force operating from the bottom of the mold. However, the spinbar’s rotating compaction method does not externally discharge internal air but generates air internally in the concrete. Therefore, a total of 18 control and experimental groups were divided into Zones 1 through 8, and the bughole incidence was measured.

Table 6. Descriptive statistics of bughole incidence.

	N	Mean	Std. Deviation	Std. Error
Control group	72	34.27	32.90	3.88
Experimental group	72	221.73	91.57	10.79

represents the descriptive statistics for the bughole incidence of the control group and experimental group test specimens. The average of the two groups is 34.27 mm² and 221.73 mm², respectively, and a similar standard deviation of about 62.23 mm² was shown. The mean difference between the two groups was measured at 187.46 mm² and to verify its statistical significance, a t-test was conducted.

The hypothesis of this study is that the incidence of bugholes will be reduced in FCPs compared to magnetic compaction. Therefore, to prove this, the hypothesis of the t-test was set to the presence or absence of a difference in bughole incidence between the two groups (control group and experimental group).

Table 7. t-test results.

t	df	Sig (2-Tailed)	Mean Difference	Std. Error Difference	95% Confidence Interval of the Difference
					Lower	Upper
−16.35	142	0.00	−187.45	11.46	−210.12	−164.79

represents the t-test results, and at a 95% confidence level, the t-value is −16.35 and the p-value is 0.00. In the two-tailed test, the null hypothesis may be rejected. This verified that in the test specimens upon which magnetic compaction was conducted, bughole occurrence was reduced.

5. Conclusions

In this study, a magnetic compaction technology was developed to reduce bughole incidence in free-form concrete panels due to limitations in compaction performance. Magnetic compaction technology enables free movement according to free forms while conducting compaction. However, it was necessary to verify the performance of magnetic compaction, including strength deterioration and internal air generation from the rotational movement of the spinbar. To do so, two experiments were performed. A control group using the existing free-form concrete panel production method and an experimental group using magnetic compaction were classified. Between the two groups, the compressive strength and flexural strength were measured on days 1, 3, and 7. The compressive strength differed by 0.2 MPa on the first day, 3.6 MPa on the third day, and 0.7 MPa on the seventh day, showing relatively comparable values. The flexural strength differed by 0.4 MPa on the first day, 1.1 MPa on the third day, and 1.4 MPa on the seventh day, with the strength of the experimental group being higher. This means that the magnetic compaction method improved the concrete dispersion ability compared to the existing free-form panel production method and that material separation and strength deterioration did not occur.

To check the bughole incidence, nine form data sets for each group were obtained through 3D scans. To verify the performance of magnetic compaction, the bughole incidence of the two groups was compared by dividing them into eight zones and confirming the number of bugholes that occurred in each zone. The collected form data were overlaid with the design forms, allowing for a comparative analysis by examining the positions and numbers that had a depth of at least 0.2 mm between the two groups. At this time, the diameters of the bugholes were measured, and their areas were calculated assuming they were circular. Based on the collected bughole incidence data from the control group and the experimental group, a t-test was conducted to determine whether the difference between the two groups was statistically significant. As a result, at a 95% confidence level, the t-value was −16.35, and the p-value was 0.00, proving that there is a significant difference between the two groups in a two-tailed test.

However, a plastering process is required after compaction work, and experiments need to be conducted on various parameters (working time, strength of the magnetic force, and changes in strength due to aging). Additionally, since materials like UHPC are used in the production of free-form concrete panels, it is necessary to select an appropriate magnetic force based on the materials used. Future studies aim to develop magnetic compaction technology applicable to free-form concrete panels through experiments on the aforementioned parameters. Finally, the goal is to develop automated magnetic compaction technology that can be applied to existing free-form automatic production equipment. This magnetic compaction method is expected to reduce the occurrence of bugholes in existing free-form concrete panels, secure aesthetics and strength, and further improve the quality of FCPs.