Introduction

The international community is actively engaged in addressing diverse climate changes, particularly the global warming induced by greenhouse gas emissions. Commencing with the United Nations Framework Convention on Climate Change in 1992, substantial endeavors have been undertaken to mitigate greenhouse gas emissions, exemplified by the legally binding Kyoto Protocol, enacted in 1997 with a focus on developed countries and their responsibilities regarding emissions. The Paris Agreement, implemented in 2015, emerged as a successor to the Kyoto Protocol, representing a significant stride in climate change response. Additionally, the Intergovernmental Panel on Climate Change (IPCC) released a Special Report on Global Warming of 1.5°C in 2018, providing crucial insights. In consonance with global initiatives, South Korea has expressed its commitment to achieving carbon neutrality by 2050. The nation has articulated a comprehensive vision outlining the pathways to realizing carbon neutrality within the same timeframe [1].

Simultaneously, on a global scale, the construction industry is witnessing an intensified focus on concrete. In September 2020, the Global Cement and Concrete Association (GCCA) introduced the Carbon Neutrality Concrete 2050 Climate Pledge, aligning itself with the overarching global objective of fostering a carbon-neutral future [2]. Concurrently, there is a notable expansion of low-carbon concrete certification systems, particularly in environmentally progressive nations. European and American countries are progressively implementing and advocating for low-carbon concrete certification systems, aiming to streamline the production and adoption of low-carbon concrete within domestic contexts. In South Korea, initiatives are underway to address global environmental challenges and achieve carbon neutrality, as evidenced by the implementation of an environmental performance label certification based on environmental product declarations (EPD). The increasing prevalence of South Korea’s concrete certification instances featuring EPDs, crucial in mitigating 70% of greenhouse gases within building structures, is experiencing a rapid surge [3].

Meanwhile, Off-Site Construction (OSC), also referred to as modular construction, is emerging as a viable solution to persistent challenges in the construction industry, including labor shortages, safety incidents, and quality assurance issues [4]. OSC entails the prefabrication of structures and facilities in controlled factory environments, followed by transportation to the construction site for on-site assembly and installation. Prefabricating structures in advance holds the potential to minimize unnecessary materials and processes, resulting in reduced carbon emissions and construction waste [5, 6]. Precast concrete, recognized as a prominent method within the OSC paradigm, is anticipated to offer advantages in terms of safety, quality, and carbon footprint reduction [7].

Therefore, this study aims to develop a carbon emission evaluation system for precast concrete products, a secondary product of concrete, in response to the carbon reduction technology demands for concrete, a key material in the construction industry.

Background

Precast Concrete

Precast concrete denotes a derivative product of concrete, formed by casting and curing concrete reinforced within reusable molds or frameworks. Manufactured in controlled factory environments, modular concrete products remain unaffected by weather and seasonal conditions. After production, these products are transported to construction sites, where they are assembled in place and integrated with on-site concrete for application. The modular nature of precast concrete allows for design standardization, while its production in a controlled environment ensures uniform quality, minimizing cracking and enhancing overall product quality. Furthermore, production in a temperature-controlled setting reduces the impact of temperature, resulting in air savings compared to on-site casting. Mechanization in both production and construction processes maximizes efficiency. When considering the entire lifecycle of precast concrete, cost savings can be anticipated due to reduced labor, air savings, diminished formwork and scaffolding requirements, and other factors, contributing to economic benefits. Additionally, factory-based prefabrication minimizes material waste, leading to reduced construction waste. The modularization of the production process also reduces facility and machinery operation, thereby expecting a reduction in carbon emissions during production [7, 8, 9].

As depicted in Figure 1, as of 2017, the estimated sales revenue of South Korea’s precast concrete construction market stands at approximately 620 million dollars, constituting a 0.7% market share in the overall construction market [8]. The market share of the precast concrete construction method is expected to continue growing, with an anticipated increase in demand for research and technologies related to carbon reduction in precast concrete, addressing various environmental concerns in the future [10].

Figure 1.

Annual Sales Status and Growth Rate of Precast Concrete Market in South Korea [8].

Status of South Korea’s EPD Certification

EPD certification system serves as a framework for quantifying the environmental impacts of products or services throughout their entire lifecycle, encompassing raw material extraction, manufacturing, transportation, use, and disposal. This certification is achieved through a comprehensive life cycle assessment, providing detailed environmental performance information. By transparently disclosing environmental impacts, the system aims to incentivize environmental improvements. Additionally, products certified with EPD certification, meeting criteria such as having a carbon footprint value below the maximum allowable carbon emissions or exceeding the minimum reduction rate, can obtain low-carbon product certification. In South Korea, as of October 2023, 448 companies and 2,040 products have obtained EPD certification, while 189 companies and 627 products have received low-carbon product certification [3].

Recent years have witnessed a notable increase in EPD certifications for concrete products. The revision of certification standards by the Green Building Certification (G-SEED) in 2016, which provides scoring benefits for the application of construction materials with EPD certification or low-carbon product certification in buildings, has contributed to the proliferation of the development and certification of construction materials, including concrete. The number of concrete products obtaining EPD certification increased from 13 in 2018 to 392 in 2020. Furthermore, the growing interest in OSC systems that consider Environmental, Social, and Governance (ESG) factors has led to an increased demand for EPD certification for architectural precast concrete products. Although precast concrete products offer flexibility in carbon reduction mix improvements and manufacturing line changes compared to ready-mix concrete products, research and performance related to carbon reduction products are lacking. In addition, the absence of a systematic approach to the entire life cycle assessment, including emission coefficients and evaluation scenarios, makes it challenging for manufacturing companies to actively respond to these challenges.

Precast Concrete Carbon Emission System

Overview

The assessment of carbon emissions in precast concrete involves a quantitative evaluation system that measures energy and carbon emissions throughout the pre-manufacturing, manufacturing, transportation, and disposal stages. By scrutinizing the environmental impact in the assessment outcomes, the goal is to identify opportunities for implementing carbon reduction strategies.

This study is dedicated to meeting the precision of carbon emissions at the level of the South Korea Ministry of Environment-certified carbon emission software [11]. The objective is to formulate a comprehensive precast concrete life cycle carbon emission evaluation system for designing and monitoring carbon-reduced products. The developed system is applied in evaluation cases, and the results are subjected to analysis.

From the perspective of the manufacturing process of precast concrete, with a distinction of materials at each stage, the pre-manufacturing stage involves inputs of primary materials—concrete and reinforcement—along with supplementary materials such as blending agents and water. In the manufacturing stage, inputs encompass the energy and water required for precast concrete generation. The transportation stage accounts for the transport of materials from the pre-manufacturing stage and the distance covered during the transportation of the product after the manufacturing stage. Finally, in the disposal stage, various types of waste are generated during the manufacturing of precast concrete. This study emphasizes the four stages: pre-manufacturing, manufacturing, transportation, and disposal, as illustrated in Figure 2. Quantities of raw materials input, energy consumed, transportation distances, and other relevant factors are recorded at each stage. Ultimately, in the evaluation stage, the total carbon emissions of the input product, along with results comprising stage-specific and material-specific carbon emissions, are presented.

Figure 2.

Process Chart of a Precast Concrete Carbon Emissions Evaluation System.

Evaluation Methodology

(1) Pre-manufacturing Stage

In assessing carbon emissions during the pre-manufacturing stage, the materials to be input can be broadly categorized into concrete and steel materials. Concrete materials comprise coarse aggregate, fine aggregate, cement, admixtures, and water (product water) as subcategories. The carbon emissions during the pre-manufacturing stage were cumulatively calculated by multiplying the input quantity of each raw material involved in precast concrete production [12]. Carbon emissions for each material in this stage were evaluated by applying emission coefficients from Korean Life Cycle Inventory Database (LCI DB) [13].

(2) Manufacturing Stage

To evaluate carbon emissions during the manufacturing stage, input the amount of energy sources used and the quantity of water consumed in the manufacturing process. Input the usage of electricity, LPG, LNG, and other energy sources from the production of concrete to the completion of precast concrete. The quantity of water input in this stage refers to the steam water used during precast concrete generation and the wash water used during cleaning, differing from the water input in the pre-manufacturing stage. Carbon emissions during the manufacturing stage were cumulatively calculated by multiplying the energy and water usage by the quantity produced. Carbon emissions in this stage were also evaluated by applying emission coefficients from Korean LCI DB [13].

(3) Transportation Stage

To evaluate carbon emissions during the transportation stage, input the distance traveled for the transportation of raw materials before production and the distance covered by the finished precast concrete product to the construction site. Calculate carbon emissions cumulatively by multiplying the total transportation distance by the quantity produced. Carbon emissions for transportation distance in this stage were evaluated by applying emission coefficients from Korean LCI DB [13].

(4) Disposal Stage

In assessing carbon emissions during the disposal stage, input the quantity of all types of waste generated during the precast concrete manufacturing process. Carbon emissions in this stage were cumulatively calculated by multiplying the quantity of waste emissions by the quantity produced. Carbon emissions in this stage were also evaluated by applying emission coefficients from Korean LCI DB [13].

System Configuration

The precast concrete carbon emission evaluation system is developed using Microsoft Office Excel, as depicted in Figure 3. Evaluators input quantities and emissions directly for each stage to perform carbon emission assessments.

Figure 3.

Configuration of Precast Concrete Carbon Emission Evaluation System (Pre-con).

(1) Information Input Sheet

The Information Input Sheet is designed for entering fundamental details about the precast concrete production company, including the registration of a photo of the product undergoing evaluation. It includes input fields for product name, application, evaluator’s name, and production company address.

(2) Pre-manufacturing Stage Input Sheet

The Pre-manufacturing Stage Input Sheet involves the input of quantities for coarse aggregate, fine aggregate, cement, admixtures, water, and steel materials. The basic unit for materials is kg, and upon input, it undergoes conversion to the base unit. This value is then multiplied by the emission coefficients corresponding to each material type, subsequently reflected in the evaluation results.

(3) Manufacturing Stage Input Sheet

In the Manufacturing Stage Input Sheet, the quantities of energy and water used during manufacturing are entered. The unit for energy is applied appropriately based on the type, while water is categorized into steam and wash water, with a basic unit of kg. Input quantities are converted to the base unit, multiplied by the emission coefficients associated with the selected types of energy and water, and reflected in the evaluation results.

(4) Transportation Stage Input Sheet

The Transportation Stage Input Sheet involves the input of transportation means and distances. The unit for transportation distance is km, and emission coefficients are applied based on the type of transportation means. These coefficients are then multiplied by the distance traveled both before and after manufacturing up to the construction site.

(5) Disposal Stage Input Sheet

In the Disposal Stage Input Sheet, various waste materials generated during both pre-manufacturing and manufacturing, such as waste concrete, waste iron, and vapor, are entered. The unit for waste disposal is kg, and upon selecting waste types, emission coefficients are applied, contributing to the overall results.

(6) Evaluation Results Sheet

The Evaluation Results Sheet presents the previously entered basic information along with various calculated outcomes. It itemizes carbon emission quantities for each stage, providing the total emissions for pre-manufacturing, manufacturing, transportation, and disposal. Regarding material-specific carbon emission quantities, the sheet displays emissions for each type of raw material, energy, and waste selected for each stage. Additionally, it includes the maximum allowable carbon emission, one of the criteria for low-carbon product certification set by the Korea Environmental Industry & Technology Institute. This information enables a comparison of the product’s carbon emissions against the standard, offering insights into any disparities. The cumulative mass contribution, based on the quantity of raw materials input during the pre-manufacturing stage, serves as a reference for mix design during precast concrete production.

Case Study

Evaluation Target

The evaluated precast concrete production company is situated in Cheongju and has an annual production capacity of 900,000 m³ of precast concrete. This company operates a total of five factories, and the selected facility for evaluation produces the most diverse range of precast concrete products. The company specializes in the manufacturing of various precast concrete products, encompassing walls, slabs, columns, beams, and more.

Evaluation Method

In this study, the precast concrete carbon emission assessment system was employed to evaluate carbon emissions. The assessment focused on the carbon emissions of precast concrete products manufactured by an actual precast concrete production company in the year 2020.

During the pre-manufacturing stage, the assessment considered the quantity of raw materials (i.e., coarse aggregate, fine aggregate, cement, admixture, water, steel material) input into the precast concrete products. In the manufacturing stage, the assessment was based on energy usage and water input for the precast concrete products. The transportation stage involved evaluating carbon emissions based on transportation distances for raw materials, finished products, etc., throughout all preceding stages. In the disposal stage, the assessment aimed to evaluate carbon emissions based on the waste generated, including wasted concrete, steel bars, and lecterns, during the manufacturing process of precast concrete products.

Evaluation Results

Table 1 refer to the results of the case study conducted in this study. As depicted in Table 1, during the pre-manufacturing stage, steel materials demonstrated the highest carbon emissions, followed by cement. In the manufacturing stage, electricity served as the primary energy source, and water, not differentiated between fresh and reclaimed water, was included in the pre-production stage, resulting in zero carbon emissions during this stage. Throughout the transportation stage, various materials, including cement, admixtures, reinforcing steel, crushed aggregate, crushed stone, and evaporative loss inhibitor, were transported for the product’s fabrication. The carbon emissions from transportation appeared relatively low. In the disposal stage, waste concrete, waste reinforcing steel, and waste prestressing strands were discharged, with waste concrete exhibiting the highest carbon emissions.

Conclusions

This study aims to develop a carbon emission evaluation system for precast concrete products, a secondary product of concrete. The following conclusions were drawn:

1.This study developed a quantitative carbon emission evaluation system for precast concrete products, categorizing the production process into pre-manufacturing, manufacturing, transportation, and disposal stages.

2.In the case study, the evaluation of carbon emissions revealed that the pre-manufacturing stage exhibited the highest carbon emissions, with steel materials demonstrating the highest emission at 19,722.443 kg-CO₂. In contrast, the transportation stage displayed the lowest carbon emissions among all stages, at 0.252 kg-CO₂.

3.The study proposes that the developed carbon emission evaluation system could offer technical support for carbon analysis in precast concrete production companies, fostering practical technological advancements in the manufacturing of low-carbon products.

4.The utilization of the proposed precast concrete carbon emission evaluation program allows for the assessment of carbon emissions at each stage—pre-manufacturing, manufacturing, transportation, and disposal. Establishing a database for secondary concrete products could serve as foundational data for predicting carbon emissions and designing for carbon reduction in OSC based industries in the future.