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Article

Study on Controlled Low-Strength Materials Using Ultra-Rapid-Hardening Cement and Stone Sludge for Backfill and Subbase Application in Road Excavation and Restoration Work

by
Jongwon Lee
and
Cheolmin Baek
*
Department of Highway and Transportation Research, Korea Institute of Civil Engineering and Building Technology, 283 Goyangdae-ro, Ilsanseo-gu, Goyang-si 10223, Republic of Korea
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(1), 46; https://doi.org/10.3390/buildings14010046
Submission received: 10 November 2023 / Revised: 9 December 2023 / Accepted: 18 December 2023 / Published: 22 December 2023
(This article belongs to the Special Issue Advancements in Cementitious Materials: Exploring the Latest Trends and Future Outlook)
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Abstract

:
A significant amount of stone sludge is generated as a by-product during the production of crushed stone aggregate, and most of it is disposed of in landfill as waste. In order to recycle this stone sludge, this study evaluated a controlled low-strength material (CLSM) using ultra-rapid-hardening cement and stone sludge for application as backfill and subbase material for road excavation and restoration work. In addition, considering the limited construction time of excavation and restoration work in urban areas, backfill and subbase materials must simultaneously satisfy conditions of fluidity, workability, quick curing time, and certain levels of strength. Therefore, in this study, CLSM was manufactured according to various mixing ratios and flow, slump, and compressive strength tests with age were evaluated. Additionally, the change trend in the microstructure of the CLSM with age was analyzed. Through indoor experiments, the optimal mixing ratios for backfill and subbase CLSM were determined, and field applicability and performance of field samples were evaluated through small-scale field construction. It was concluded that CLSM, which contains a large amount of stone sludge, can be sufficiently applied as a backfill and subbase material for excavation and restoration work if appropriate admixtures are adjusted according to the weather conditions at sites.

1. Introduction

A large portion of recent road pavement construction in South Korea has been carried out to repair damaged pavement in urban areas. In particular, small-scale excavation and restoration work on urban roads is constantly increasing as the deterioration of underground utilities progresses. Excavation and restoration work in city centers is mainly performed at night due to traffic control restrictions, and same-day excavation and restoration are enforced in principle. Therefore, insufficient compaction occurs due to the insufficient time for sufficient compaction of the backfill layer and subbase layer during the restoration process. This eventually leads to sagging of the restored section and damage to the pavement layer. To solve these problems, backfill materials that have the properties of high flowability, self-compaction, short curing time, and minimum strength for re-excavation are required. Furthermore, it is necessary to investigate mix designs with ultra-rapid-hardening cement or quick-setting agents and mix designs with maximum aggregates to reduce the amount of cement used. However, existing research attempting to resolve these issues is currently insufficient [1].
In the early days of the construction industry, most concrete was produced using river aggregate. However, as river aggregate became scarce, crushed aggregate from quarries began to be used. Currently, crushed aggregate accounts for the largest proportion of aggregates used in concrete. Quarries produce crushed coarse aggregate and crushed fine aggregate through dry and wet processes. The stone sludge generated through these processes was found to account for approximately 5% of raw stone [2,3]. Based on Korea’s crushed aggregate production, the annual stone sludge production is estimated to be more than 15 million tons. Stone sludge can be recycled, but due to transportation and disposal costs, most of it is illegally landfilled or dumped within construction sites [4,5,6].
In Europe, it was reported that stone sludge generates approximately 5 million tons annually and this amount accounts for about 40% of granite and marble production [7,8]. The environmental pollution caused by large amounts of stone sludge waste has been indicated. Landfills, in particular, have been linked to major contamination of farmland, biota, surface water, and groundwater in countries with less restrictive environmental regulations [9].
To address these problems, many studies have been conducted. In the early 2000s, basic research was conducted on the flowability and strength as a function of water content, admixture ratio, and other parameters, using stone sludge as an admixture [10]. In 2010, various studies were conducted to expand its use by proposing a concrete production and mix design replacing stone sludge with cement and aggregate particulates smaller than 0.08 mm [11,12,13]. Furthermore, various studies on cement [14,15], mortar [16], concrete [17], gypsum mortar [18], artificial aggregate [19], and asphalt mixtures [20] utilizing stone sludge have shown that the chemical composition of stone sludge directly affects the mechanical properties of final products. Recent studies have succeeded in producing concrete by replacing 10–15% of cement and fine aggregate with stone sludge. They also suggest that stone sludge could be a sustainable waste management option as it enables recycling of natural resources and is a low-carbon material [21,22,23].
Meanwhile, many road cave-ins, such as sinkholes, which have recently become an issue, are caused by water leaks due to aging and poor construction and management of underground pipes such as sewage and water pipes. Accordingly, in order to solve problems such as difficulty in securing compaction of backfill for underground structures, various studies are being conducted to develop and put into practice controlled low-strength materials (CLSMs), which allow self-filling and self-compaction. CLSMs are made by adding cement or cementitious materials to mud prepared by mixing soil from construction sites and water [24].
Ling et al. examined 115 reports related to CLSM for backfill and found that the materials used to produce CLSM varied across countries. They reported that the use of different materials has a significant impact on CLSM research and field applications [25]. In particular, as CLSM-related research has become more active, more types of industrial waste for CLSM have been researched. Zhang et al. applied fly ash and coal gangue as filler materials and reported that when the ratio was 14:5:1 for gangue, fly ash, and cement, the fluidity of the filler was good and compressive strength was sufficiently developed [26]. Chen et al. evaluated CLSM using coal industry by-products (coal gangue, fly ash, bottom ash, gasification slag, desulfurized gypsum) and cement and showed that it met the criteria of the American Concrete Institute Committee 229 [27]. In particular, in Japan, there are many reports on the characteristics of CLSM using by-products such as low sludge aggregate and glass cullet [28,29,30]. Horiguchi et al. developed a CLSM by using stone sludge, sludge ash, and sewage. After verifying the mechanical performance of the fabricated CLSM and using it as an actual backfill material for construction, they concluded that sewage, sludge ash, and stone sludge can be used as materials for new CLSMs [31].
In South Korea, as part of technological development toward practical applications of fluidized backfill material for sewer pipes using site-excavated soil generated during construction, a study presented a basic formulation range of CLSM that can respond to site soil by considering engineering properties such as flowability, material separation resistance, early strength, and re-excavation strength [32]. Lee et al. analyzed the physical properties of CLSM by type of sandy clayey excavated soil and mixing factors. The results showed that the mixing conditions needed to meet flowability and early compressive strength conditions varied even within the same soil classification. This is mainly due to the particle size distribution and fine particle content within the same classification. They reported that the maximum W/B required to meet the flowability and early strength of CLSM utilizing sandy clayey excavated soil was 300% [33]. Kim et al. evaluated the characteristics of CLSMs and derived the optimal mix design for fluidized backfill material for sewer pipes using site-excavated soil generated during construction. Then, they built a batch plant for on-site production to evaluate the re-excavatability and quality sustainability of CLSMs after on-site construction [34].
As a result of reviewing the existing literature, it was found that most CLSM studies were aimed at applications of backfill and did not include stone sludge or used only a small amount. However, in urban road excavation and restoration work, the time for sufficient compaction of not only the backfill layer but also the subbase layer constructed on top of it is limited, causing the problem of sagging after construction. Therefore, the development of backfill and subbase materials that do not require compaction and cure quickly is required. To this end, this study developed and evaluated CLSMs as backfill and subbase materials suitable for urban road excavation and restoration work. In addition, a high percentage of stone sludge was applied to protect natural resources and expand the recycling of industrial by-products. Performance evaluation and microstructure analysis were performed on CLSM for backfill using stone sludge and ultra-fast-hardening cement and on CLSM for subbase by adding coarse aggregate according to various mixing ratios. The field applicability of CLSMs with the derived optimal mixing ratio was evaluated through a small-scale field construction.

2. Materials and Methods

2.1. Stone Sludge

Stone sludge collected from Ewha Aggregate, an aggregate manufacturing plant in Gyeonggi-do South Korea, was used for this study. As a result of evaluating the basic properties, 100% of the stone sludge was found to pass through a 5 mm sieve and 47.54% passed through a 0.075 mm sieve. The maximum dry density was 1.694 g/cm3 and the water content of the stone sludge showed an average of 37.09%, as shown in Table 1.

2.2. Cement

Ordinary Portland cement from domestic company “Hanil” was used in this study and Table 2 shows the physical properties and chemical composition ratio of this cement.

2.3. Ultra-Rapid-Hardening Cement

For the development of backfill material utilizing stone sludge, ultra-rapid-hardening cement manufactured by domestic Company J was used in this study. The physicochemical properties are presented in Table 3. The ultra-rapid-hardening cement used in this study belongs to the category of specialty cement, which develops a strength of 30–40% of its ultimate strength (4 MPa) in less than 4 h of aging.

2.4. Coarse Aggregate

A crushed granite aggregate was used for coarse aggregate. The particle size was between 5 mm and 25 mm, and Table 4 shows the physical properties of the coarse aggregate.

2.5. Admixtures

In this study, superplasticizer and retarder were used to improve the workability of the CLSMs and to ensure adherence to working time constraints. Table 5 and Table 6 present the physicochemical properties of the superplasticizer and retarder.

2.6. CLSM for Backfill Material

2.6.1. Mix Proportion

Based on the results of the properties of the backfill material in the preliminary study and the economic feasibility, a mix design of the backfill material using ultra-rapid-hardening cement and stone sludge was created as shown in Table 7 [24]. The OPC-1 mix using normal Portland cement was included for comparison with the mix using ultra-rapid-hardening cement. For URHC-1, 2, and 3, the quantity of admixture was adjusted to achieve excellent workability and adhere to working time constraints through flow measurement. Furthermore, the compressive strength of the backfill material after 2 h, 4 h, and 1 day was measured to observe the initial strength development and the compressive strength at 7, 14, and 28 days was measured to examine the strength-increasing trend. The test also included the case of the URHC-4 mix, where the amount of stone sludge was increased to reduce the quantity of cement at 300% W/B.

2.6.2. Flow Test

The flowability of CLSMs in this study was evaluated according to ASTM D 6103 (Standard Test Method for Flow Consistency of Controlled Low Strength Material) [35]. Figure 1 shows the fabrication and measurement of a CLSM flow test specimen.

2.6.3. Compressive Strength Test

ASTM D 4832 (Standard Test Method for Preparation and Testing of Controlled Low Strength Material (CLSM Test Cylinders)) [36] was applied to measure the compressive strength of the CLSMs. Cylindrical specimens, measuring 100 mm in height and 50 mm in diameter, were prepared using a 2:1 cylinder-type mold. The testing temperature was 20 ± 5 °C. The CLSM specimens and testing setup are shown in Figure 2.

2.7. CLSM for Subbase Material

2.7.1. Mix Proportion

The mix proportion for the subbase layer utilizing stone sludge and coarse aggregate was derived from the CLSM mix for backfill material. The mixing quantity of coarse aggregate was 40 Vol.% of the total volume of stone sludge and coarse aggregate. The mixing equipment is shown in Figure 3, and the mix proportion is shown in Table 8.

2.7.2. Slump Test

To examine the workability of CLSM for the subbase layer using stone sludge, coarse aggregate and ultra-rapid-hardening cement, a slump test was conducted according to KS F 2402 (Test Method for Concrete Slump) [37] as with the slump test for concrete. The test setup is shown in Figure 4.

2.7.3. Compressive Strength Test

The compressive strength was measured using cylindrical specimens of ϕ 100 × 200 mm, as shown in Figure 5, according to KS F 2405 (Test Method for Compressive Strength of Concrete) [38]. Specimens were tested at 20 ± 5 °C by measuring 2-h, 4-h, and 1-day compressive strength values to observe initial strength development. Furthermore, 7-, 14-, and 28-day compressive strength values were measured to examine strength-increasing trends.

2.8. Microstructure Analysis

2.8.1. SEM and EDS

The microstructure of CLSMs containing stone sludge was analyzed using scanning electron microscopy (SEM). The equipment used was a Merlin Compact (Carl Zeiss, Oberkochen, Germany) equipped with an in-lens detector and various signal processing functions. Additionally, the elemental composition of the surface of CLSM particles was analyzed using energy-dispersive X-ray spectroscopy (EDS).

2.8.2. XRD

Materials generated from the hydration reaction of CLSMs were analyzed using X-ray diffraction (XRD). The equipment used was Bruker’s 1 Dimensional LYNXEYE detector and a D8 Advance diffractometer (Billerica, MA, USA). Samples were scanned in the 2θ range of 5°–95° with a step size of 0.01° and a time of 1 s per step. Qualitative analysis was performed by obtaining XRD patterns for CLSM specimens and standard specimens under identical conditions.

3. Results

3.1. CLSM for Backfill Material

3.1.1. Flow Test Result

The flow test results of the CLSM for backfill material utilizing ultra-rapid-hardening cement and stone sludge are as shown in Figure 6. The OPC-1 with normal Portland cement did not show any decrease in flow with the mixing time. However, in the case of the URHC-1 mix using ultra-rapid-hardening cement with no admixture, the flow rate tended to decrease after 5 min of mixing time. In contrast, URHC-2, 3, and 4 showed a very sharp decrease in flow rate when the mixing time exceeded 10 min. URHC-2 and URHC-3 showed a difference of about 80 mm, depending on the quantity of superplasticizer used. Moreover, both mix proportions showed similar trends of decreasing flow with mixing time. It was determined that with an increasing quantity of superplasticizer, the flow increased due to the separation of particles resulting from the increased free water, among the free water, adsorbent bed water, and recharge water that affected the flow in the cement matrix [39]. The flow of URHC-4 was measured lower than the other mix proportions. This is thought to be due to the reduced free water and increased adsorbent bed water resulting from the relatively increased stone sludge in the URHC-2 and URHC-3 mixes, which can have a negative impact on flowability [40]. Using a retarder of 0.2% by weight of ultra-rapid-hardening cement resulted in a mixing time of about 10 min. Thus, it is necessary to increase the retarder to adhere to time constraints according to the work situation, and the superplasticizer needs to be increased to improve the work performance.

3.1.2. Compressive Strength Test Results

Figure 7 shows the compressive strengths by age of CLSM for backfill material utilizing ultra-rapid-hardening cement and stone sludge. OPC-1, which used ordinary Portland cement as a binder, did not cure until 4 h, making it impossible to demold the specimen, and strength measurement was possible from 1 day of age. The compressive strength was low at 0.84 MPa even at 28 days of age due to the relatively high W/B. When ultra-rapid-hardening cement was used, all mix proportions showed an initial strength higher than 0.14 MPa, which is sufficient for open traffic during the backfilling of buried pipes [41]. The compressive strength values of URHC-1, 2, and 3 were rapidly increased from 2 h to 7 days after the initial strength measurement. At 28 days of age, they exhibited a more moderate increase in strength with values of 2.01, 1.98, and 1.96 MPa, respectively. The compressive strength tended to decrease as the amount of superplasticizer increased. Although the fluidity increased due to the increase in free water caused by the superplasticizer, the free water consumed in the CLSM hydration reaction for backfill material was left as pores. This led to a relatively large volume of pores, reducing its strength. Adding a retarder can inhibit the hydration reaction of ultra-rapid-hardening cement for a while. However, when the hydration reaction was initiated after some time, the final compressive strength did not significantly change due to the exothermic action of hydration by chemical reaction. Regarding compressive strength, the increase in superplasticizer decreased the compressive strength, but the difference was insignificant, and the effect of retarder was insignificant [42]. URHC-4 with an increased amount of stone sludge at the same W/B showed excellent strength properties in every mix proportion. This is because although the amount of binder was relatively reduced, the increase in stone sludge lowered the flowability, and the strength increased because the hydration reaction was initially active. Thus, considering the flowability and strength properties of backfill material mixed with ultra-rapid-hardening cement and stone sludge, the most adequate mix proportions were thought to be URHC-3 and URHC-4 with a W/B of 300%, 0.3% superplasticizer (wt./B), and 0.2% retarder (wt./B) using ultra-rapid-hardening cement as the binder.

3.2. CLSM for Subbase Material

3.2.1. Slump

The target slump of the CLSM for subbase layers utilizing ultra-rapid-hardening cement, stone sludge, and coarse aggregate was set at 170 ± 10 mm considering the flowability and workability. The slump test results showed that 20 min of mixing time was required for URHC-5 to achieve the targeted flowability. The slump could not be measured before achieving the target slump due to increased flowability, as shown in Figure 8, and material separations occurred partially. URHC-6, which reduced the quantity of binder and increased the quantity of stone sludge, took around 5 min to mix. A sharp deterioration in flowability occurred about 10 min after achieving the target slump, as shown in Figure 9. Thus, pouring should be performed immediately after adequate mixing.

3.2.2. Compressive Strength Test

To analyze the compressive strength of CLSM for subbase layers utilizing ultra-rapid-hardening cement, stone sludge, and coarse aggregate, the specimens were prepared with the target slump. Figure 10 shows the results of the compressive strength test by age. Both URHC-5 and URHC-6 mixes, which included 40% of coarse aggregate, showed compressive strength values of more than 0.9 MPa at 2 h and more than 1.0 MPa at 4 h. This ensured a compressive strength of 0.7 MPa for the following process (asphalt layer paving). For the URHC-6 mix, the quantity of stone sludge was increased to reduce the unit amount of binder. Thus, it showed a greater compressive strength because the amount of water initially added was relatively reduced and the hydration reaction was active at an early stage, shortening the mixing time to achieve the target slump. Therefore, URHC-6 is thought to be adequate as a CLSM mix for subbase layers to complete pipeline construction in the city center targeted in this study within the specified time, considering both workability and compressive strength.

3.3. Microstructure Analysis Results

3.3.1. SEM and EDS Analysis Results

CLSM specimens were taken at 2 h, 1 day, 7 days, 14 days, and 28 days of age and analyzed by SEM and EDS. Figure 11 and Figure 12 show the results of microstructure analysis. The CLSM that used ultra-rapid-hardening cement as a binder initially showed a large quantity of ettringite and dense microstructures. Furthermore, as the age increased, the density of internal microstructures increased because the hydration reaction with the cement became more active. In the early stage, a large quantity of ettringite was observed. The mechanism of strength development of ultra-rapid-hardening cement is that in the presence of SO3, Ca2+ ions eluting immediately after contact with water and Al3+ ions eluting from calcium aluminate react to produce calcium aluminate hydrate (CaO·Al2O3·nH2O). Then, this reacts with gypsum in the cement to produce ettringite (3CaO·Al2O3·3CaSO4·32H2O), which hardens rapidly, resulting in early strength development [43]. Consequently, a large quantity of ionic components such as Al, O, Ca, and S were detected in the 2 h EDS mapping as shown in Figure 11. Furthermore, the high fineness compared to ordinary cement is highly reactive when mixed with water, causing an active hydration reaction. As a result, the reaction that produces the hydrate of ettringite occurs more rapidly. This suggests that the flow value was measured lower and the compressive strength was higher compared to ordinary cement. The mix proportion with ultra-rapid-hardening cement showed a large amount of ettringite and C–S–H and C–A–H gels at 7 days of age as shown in Figure 12. In addition, on the Ca(OH)2 surface, thin sheet-shaped C–S–H gel appeared widely distributed. This indicates that as Ca(OH)2 was consumed, secondary ettringite was formed and C–S–H and C–A–H gels were generated at 7 days of age. At 28 days of age, as C–S–H gel and C–A–H gel were generated in large quantities around the ettringite nucleus, it was confirmed that the internal structure was stabilizing by filling the micropores of CLSM using ultra-fast-hardening cement and stone sludge.

3.3.2. XRD Analysis Results

Figure 13 shows the XRD analysis results of a sample collected at 28 days of CLSM using stone sludge and ultra-rapid-hardening cement. The peak of SiO2 became more evident in the mix using stone sludge. The CLSM with ordinary Portland cement showed very low peaks except for ettringite. This is thought to be due to the relatively high W/B, which inhibited the smooth hydration reaction. The CLSM with ultra-rapid-hardening cement confirmed ettringite, C–S–H, and C–A–H gel hydrate, as shown by SEM. In particular, the ettringite peak clearly appeared in the mix using ultra-rapid-hardening cement. This is determined to be due to the hydration reaction characteristics of ultra-rapid-hardening cement, as described in Section 3.3.1. Furthermore, Al2O3 in ultra-rapid-hardening cement and SiO2, which is the main component of stone sludge, cause pozzolanic reactions with CH in the cement matrix, and this accelerates the hydration reaction compared to ordinary cement [44,45]. As a result, mixing proportions with ultra-rapid-hardening cement may produce more C–S–H and C–A–H. The C–S–H and C–A–H are generated through pozzolanic reaction and they are attached to the surface of stone sludge, and the density within the CLSM matrix increases, thereby reducing harmful voids [46].

4. Evaluation of Field Applicability for CLSMs

4.1. Site Construction Overview

A field construction evaluation was carried out by applying the optimal mix proportions of URHC-3 and URHC-6 derived from the indoor experimental evaluation. This process is shown in Figure 14. The mixer used for the on-site production and pouring of CLSMs was a piece of dedicated CLSM equipment developed as part of this study.
As shown in Figure 15, before pouring the CLSM for each mix, a slump test and a flow test were conducted to characterize the CLSMs, and specimens were fabricated in the field. In addition, to evaluate the constructability of the upper layer after construction of CLSM material, the change in hardness of the CLSM layer was measured through a soil penetrometer test and a Kelly ball test.

4.2. Field Evaluation Result

4.2.1. Evaluation of Mechanical Properties

The flow test result of CLSM for backfill satisfied the ASTM D 4832 standard [36] of 200 mm or more, as shown in Table 9. Furthermore, the slump test result of CLSM for subbase layers showed a target slump of 170 ± 10 mm, as shown in Table 10. In addition, both CLSMs were found to have excellent workability during field construction. As shown in Table 9, the compressive strength of CLSM for backfill was greater than 0.6 MPa after 2 h. The compressive strength of CLSM for subbase layers was larger than 1.0 MPa after 2 h as shown in Table 10. The field test results showed lower flowability and increased compressive strength compared to the indoor test. This difference occurred because of the change in the water content due to the use of a large amount of stone sludge and the performance difference between the mixer used in the indoor test and the mixer used in the field test [47,48].

4.2.2. Review of Subsequent Process Initiation of CLSMs

To examine the possibility of subsequent processing of backfill material, an evaluation was conducted with a soil hardness meter in accordance with ASTM D 6024, Standard Test Method for Ball Drop on Controlled Low Strength Material (CLSM) to Determine Suitability for Load Application [49], and Tokyo Metropolitan Construction Bureau Quality Standards for Fluidized Treated Soil [50]. The construction area was divided into four zones, and each zone was evaluated every 10 min. The results are summarized in Table 11 and Table 12.
As a result of the hardness characteristics of the backfill CLSM over time, by using a soil hardness meter, penetration was measured at 4.28 mm at 50 min after pouring, meeting the standard value (more than 3 mm). The penetration of subbase CLSM was measured to be 4.45 mm after 40 min, about 10 min earlier than the backfill CLSM, meeting the standard (more than 3 mm). As a result of the Kelly ball test, the backfill CLSM and subbase CLSM values were found to be 73.63 mm and 71.43 mm, respectively, about 1 h after pouring, so both CLSM met the standard (75 mm or less).

5. Conclusions

In this study, the engineering properties and microstructures of CLSMs for backfill and subbase layers using ultra-rapid-hardening cement, stone sludge, and coarse aggregate were analyzed. Additionally, small-scale field construction and testing were performed to evaluate the field applicability of CLSMs. The following conclusions were drawn through this study:
(1)
For the backfill CLSM using stone sludge and ultra-rapid-hardening cement, the addition of superplasticizer and retarder is inevitable considering the working environment of urban centers. Considering the flowability and early compressive strength, the most adequate mix proportion for backfill CLSM is URHC-3 with a W/B of 300%, 0.3% (wt./B) superplasticizer, and 0.2% (wt./B) retarder, using an ultra-rapid-hardening cement as the binder;
(2)
For the subbase CLSM using ultra-rapid-hardening cement, stone sludge, and coarse aggregate, it should be poured immediately after appropriate mixing due to the decrease in flowability with mixing time. Therefore, URHC-6 is judged to be suitable for subbase CLSM mixes used to complete pipe construction in urban areas within a given time, considering both workability and compressive strength;
(3)
The microstructure analysis results showed that the primary hydration reaction of CLSM for backfill and subbase layers provided primary initial strength by generating ettringite. Subsequently, the internal structure of the CLSM was stabilized and the strength was enhanced as C–S–H and C–A–H gels were generated around SiO2, the main component of the stone sludge, and the primary ettringite nucleus;
(4)
As a result of small-scale field construction of CLSMs, it was found that they were sufficiently applicable to the field in terms of constructability and performance. However, compared to the indoor test, fluidity was lowered and compressive strength was increased. This is because the moisture content of stone sludge and the performance of the mixer used in the field are different from those in the laboratory. Therefore, the quantity of admixture used and the water content of the stone sludge need to be verified through a water content experiment, considering weather conditions before mixing.

Author Contributions

Conceptualization, C.B.; methodology, J.L. and C.B.; validation, J.L. and C.B.; formal analysis, J.L. and C.B.; investigation, J.L. and C.B.; resources, J.L. and C.B.; data curation, J.L. and C.B.; writing—original draft preparation, J.L.; writing—review and editing, C.B.; visualization, J.L.; supervision, C.B.; project administration, C.B.; funding acquisition, C.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant No. 22POQWB152342-01).

Data Availability Statement

The data presented in this study are available in the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Flow test.
Figure 1. Flow test.
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Figure 2. Specimens and test setup for compressive strength test.
Figure 2. Specimens and test setup for compressive strength test.
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Figure 3. Mixing of CLSM.
Figure 3. Mixing of CLSM.
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Figure 4. Slump test.
Figure 4. Slump test.
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Figure 5. Compressive strength test.
Figure 5. Compressive strength test.
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Figure 6. Flow test result of CLSM for backfill material.
Figure 6. Flow test result of CLSM for backfill material.
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Figure 7. Compressive strength test result of CLSM for backfill material.
Figure 7. Compressive strength test result of CLSM for backfill material.
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Figure 8. Slump test of URHC-5.
Figure 8. Slump test of URHC-5.
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Figure 9. Slump test of URHC-6.
Figure 9. Slump test of URHC-6.
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Figure 10. Compressive strength test result of CLSM for subbase material.
Figure 10. Compressive strength test result of CLSM for subbase material.
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Figure 11. EDS mapping result of CLSM using URHC.
Figure 11. EDS mapping result of CLSM using URHC.
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Figure 12. SEM image of CLSM using URHC.
Figure 12. SEM image of CLSM using URHC.
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Figure 13. 28-day XRD results for different binders.
Figure 13. 28-day XRD results for different binders.
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Figure 14. Photographs of the field construction of the CLSMs.
Figure 14. Photographs of the field construction of the CLSMs.
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Figure 15. Photographs of the field test of the CLSMs.
Figure 15. Photographs of the field test of the CLSMs.
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Table 1. Water content of the stone sludge samples.
Table 1. Water content of the stone sludge samples.
DivisionSample-1Sample-2Sample-3
Wet soil (g)525802731
Dry soil (g)383582536
Water (g)142220195
Water content (%)37.0837.8036.38
Average water content (%)37.09
Table 2. Characteristics of ordinary Portland cement.
Table 2. Characteristics of ordinary Portland cement.
Density
(g/cm3)		Fineness
(cm2/g)		Chemical Composition (%)
SiO2Al2O3Fe2O3CaOMgOSO3Ig.Loss
3.14349221.14.643.1462.82.812.132.18
Table 3. Characteristics of the ultra-rapid-hardening cement.
Table 3. Characteristics of the ultra-rapid-hardening cement.
Density
(g/cm3)		Fineness
(cm2/g)		Chemical Composition (%)
SiO2Al2O3Fe2O3CaOMgOSO3Ig.Loss
3.0050703.0223.190.4546.150.4324.141.24
Table 4. Coarse aggregate properties.
Table 4. Coarse aggregate properties.
Density
(g/cm3)		Absorption
(%)		Unit Weight
(kg/m3)		Fineness
Modulus		Sound
(%)
2.720.7416647.033.09
Table 5. Properties of superplasticizer.
Table 5. Properties of superplasticizer.
IngredientpHDensity (g/cm3)Evaporation
Residual Rate (%)
Polycarboxylate5.50.584
Table 6. Properties of the retarder.
Table 6. Properties of the retarder.
IngredientpHDensity (g/cm3)Purity (%)
Tartaric acid 7.0~8.50.3195
Table 7. Mix proportion.
Table 7. Mix proportion.
Test IDW/B
(%)		Binder TypeWeight Unit (kg/m3)Weight Composition
(%/B)
WaterBinderStone SludgeP.C.R.T.
OPC-1300Cement594250580--
URHC-1Ultra-
rapid-hardening cement		594250580--
URHC-20.200.20
URHC-30.300.20
URHC-45032367520.300.20
Table 8. Mix proportions.
Table 8. Mix proportions.
Test IDW/B
(%)		Binder TypeWeight Unit (kg/m3)Weight
Composition (%/B)
WaterBinderStone SludgeAggregateP.C.R.T.
URHC-5300Ultra-
rapid-hardening cement		4892064784780.300.20
URHC-6394185588588
Table 9. Flow and compressive strength test result of CLSM for backfill.
Table 9. Flow and compressive strength test result of CLSM for backfill.
NO.Flow (mm)Compressive Strength
(MPa, 2 h)
13600.66
23700.67
33600.63
Average363.30.65
Table 10. Slump and compressive strength test result of CLSM for subbase layers.
Table 10. Slump and compressive strength test result of CLSM for subbase layers.
NO.Slump (mm)Compressive Strength
(MPa, 2 h)
11651.14
21611.01
31601.12
Average1621.09
Table 11. Soil penetrometer test result of backfill material.
Table 11. Soil penetrometer test result of backfill material.
NO.Soil Penetrometer (mm)
10 min20 min30 min40 min50 min60 min
11.22.13.13.74.25.2
21.32.03.33.94.35.3
31.32.83.43.74.14.9
41.42.23.54.14.55.5
Average1.302.283.333.854.285.23
NO.Kelly Ball (mm)
10 min20 min30 min40 min50 min60 min
1130.8123.1100.595.482.270.5
2131.5120.9107.298.087.975.3
3132.6128.196.693.788.874.7
4130.9120.2103.390.184.574.0
Average131.45123.08101.9094.3085.8573.63
Table 12. Soil penetrometer test result of subbase material.
Table 12. Soil penetrometer test result of subbase material.
NO.Soil Penetrometer (mm)
10 min20 min30 min40 min50 min60 min
11.32.73.54.24.96.2
21.52.73.64.85.66.7
31.62.43.84.55.36.3
41.62.63.44.35.55.9
Average1.52.63.584.455.336.28
NO.Kelly Ball (mm)
10 min20 min30 min40 min50 min60 min
1125.8108.595.282.473.969.5
2126.3115.499.585.174.670.2
3132.6121.3104.286.478.173.5
4128.5118.5103.890.879.872.5
Average128.3115.93100.6886.1876.671.43
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Lee, J.; Baek, C. Study on Controlled Low-Strength Materials Using Ultra-Rapid-Hardening Cement and Stone Sludge for Backfill and Subbase Application in Road Excavation and Restoration Work. Buildings 2024, 14, 46. https://doi.org/10.3390/buildings14010046

AMA Style

Lee J, Baek C. Study on Controlled Low-Strength Materials Using Ultra-Rapid-Hardening Cement and Stone Sludge for Backfill and Subbase Application in Road Excavation and Restoration Work. Buildings. 2024; 14(1):46. https://doi.org/10.3390/buildings14010046

Chicago/Turabian Style

Lee, Jongwon, and Cheolmin Baek. 2024. "Study on Controlled Low-Strength Materials Using Ultra-Rapid-Hardening Cement and Stone Sludge for Backfill and Subbase Application in Road Excavation and Restoration Work" Buildings 14, no. 1: 46. https://doi.org/10.3390/buildings14010046

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