Carbonation Behavior of an Aged Reinforced Concrete Building in Seoul

Abstract

This study assessed the carbonation-related durability of an existing reinforced concrete building in Seoul scheduled for demolition to examine the level of durability performance commonly assumed for building structures. The compressive strength of concrete core specimens was compared with the estimated compressive strength derived from the rebound hammer, showing similar overall trends despite noticeable scatter, indicating that rebound testing can serve as a supplementary indicator when interpreted with caution. Carbonation depth measurements revealed that indoor locations tended to exhibit the greatest carbonation depths, likely reflecting higher CO₂ concentrations associated with occupancy and daily activities, as well as indoor ventilation and moisture conditions. For exterior walls, orientation affected carbonation progress; carbonation depths were greater on the southwest-facing wall than on the northwest-facing wall, suggesting that higher solar radiation may promote drying and facilitate CO₂ diffusion, thereby accelerating carbonation. When the carbonation rate coefficients were compared under similar compressive strength conditions, the southeast-facing wall exhibited a coefficient approximately 1.1 times greater than that of the northwest-facing wall. These results indicate that carbonation cannot be explained by strength alone and highlight the importance of incorporating exposure-related factors (e.g., solar radiation, drying, rainfall, and shielding) into carbonation behavior assessment.

1. Introduction

In general, the durability of concrete structures deteriorates over time owing to the influence of the external environment as their service life increases, and this deterioration can lead to a reduction in structural performance [1]. The major factors affecting the durability of concrete structures include reinforcement corrosion caused by concrete carbonation and chloride attack, freeze–thaw damage, and sulfate attack [2,3,4].

In many cases, these deterioration phenomena arise from coupled interactions among multiple mechanisms, ultimately leading to the corrosion of the reinforcement. Moreover, because accelerated laboratory tests are often adopted for time and analytical convenience, it is extremely difficult to quantify the acceleration factor relative to the actual environmental conditions experienced by the concrete structures. This is because many deterioration processes that reduce the durability of reinforced concrete structures are governed by moisture-mediated transport, and the transport and reaction rates of deterioration agents are strongly influenced by the moisture state [5].

Therefore, to quantitatively predict the deterioration of concrete structures, it is necessary to identify the governing deterioration mechanisms by relating the actual environmental conditions surrounding the structure to moisture characteristics within the concrete.

In metropolitan areas such as Seoul, high energy consumption associated with industrialization and high population density leads to substantial greenhouse-gas emissions; accordingly, elevated atmospheric CO₂ concentrations have been reported [6]. In general, carbonation in concrete is known to proceed more rapidly as the CO₂ concentration increases [7,8,9]. Accordingly, carbonation-induced durability deterioration in reinforced concrete structures is expected to be more pronounced in metropolitan areas than in smaller cities.

To enhance the safety and service life of reinforced concrete structures, Durability Design of Concrete Structures (KDS 14 20 40) [10] was revised in February 2021. In the revised code, the minimum specified compressive strengths are set according to exposure conditions to ensure adequate durability of structural concrete. Notably, “carbonation” was added as an exposure condition. While carbonation can densify the cementitious microstructure and thereby improve strength and durability [11], carbonation in conventional reinforced concrete has generally been recognized as a degradation mechanism because the carbonation of Ca(OH)₂, one of the hydration products, reduces internal alkalinity and can depassivate embedded steel reinforcement, leading to corrosion. Accordingly, concrete exposed to the atmosphere is required to meet a minimum specified compressive strength of 30 MPa to ensure carbonation resistance. Consequently, the required compressive strength has increased compared with the 24 or 27 MPa classes commonly used in existing building structures (e.g., apartment buildings), which contributes to higher construction costs.

However, research evaluating durability by extracting concrete cores from actual aged buildings and examining their in situ deterioration state remains limited. This lack of field-based evidence hinders the quantitative linkage between real environmental exposure and deterioration progress. Accordingly, core-based investigations of existing structures are needed to support more reliable durability assessments and designs.

In this study, carbonation, one of the most critical deterioration mechanisms affecting the durability of reinforced concrete, was evaluated in aging reinforced concrete structures scheduled for demolition to examine the level of carbonation resistance typically achieved by concrete widely used in building structures.

2. Experimental Methods

2.1. Investigated Building

An overview of the investigated building used for the durability assessment in this study is presented in

Table 1. Overview of investigated buildings.

Item	Description
Location	Yeongdeungpo-gu, Seoul
Structure type	Reinforced concrete structure
Completion date	December 1983
Area	72, 90 m2
Finishing materials	Exterior: plaster mortar + water-based paint Interior: plaster mortar + wallpaper or tile Staircase: plaster mortar + oil-based paint

. The elevation and architectural plan of the building are shown in Figure 1 and Figure 2, respectively. The target building was a reinforced concrete apartment complex located in Seoul. It was constructed in December 1983 and had been in service for approximately 39 years at the time of investigation. The complex consists of five buildings (Buildings 101–105). Buildings 101–103 are 10-story buildings, whereas Buildings 104–105 are 11-story buildings; all buildings have a corridor-type layout. In this study, two buildings (Buildings 101 and 103) were selected from the five buildings for detailed investigation.

As part of the baseline survey, it is essential to review and analyze documents produced and archived during construction and maintenance, such as records of the specified compressive strength of concrete, mix proportions, and repair history. However, except for the original drawings, this information could not be confirmed.

Regarding finishing materials, plastering mortar was applied both outdoors and indoors. A water-based exterior wall paint was applied over the plastering mortar on the exterior surface, and wallpaper was installed in most indoor areas. Tiles were also observed in some kitchens. In the staircases, oil-based paint was applied over the plastering mortar.

The investigation items and sampling locations are listed in

Table 2. Concrete core extraction.

Factors	Location of Core Extraction
Compressive strength	Partition wall Exterior wall facing northwest 1st floor Exterior wall facing southeast 1st floor Exterior wall facing northwest 2nd floor (Building 101) Exterior wall facing southeast 2nd floor (Building 103)
Carbonation depth	Partition wall Kitchen Staircase Exterior wall facing northwest 1st floor Exterior wall facing southeast 1st floor Exterior wall facing northwest 2nd floor (Building 101) Exterior wall facing southeast 2nd floor (Building 103)
Schmidt hammer	Same as the location of the core specimens for compressive strength

. The tests included measurements of compressive strength and carbonation depth. Compressive strength was measured using core specimens tested with a universal testing machine and was compared with the compressive strength estimated from Schmidt hammer rebound numbers measured at the same locations. The sampling locations were divided into interior and exterior areas. Interior cores were obtained from partition walls, kitchens, and staircase walls. For exterior sampling, cores were initially planned to be extracted from the south- and north-facing sides to compare the influence of drying conditions associated with solar radiation on carbonation. However, because the building orientation was rotated by approximately 45° from the true south–north axis, exterior cores were extracted from southeast- and northwest-facing sides.

2.2. Climate Data Analysis

Climate data for Seoul were analyzed for the period from January 1983 to July 2023 using the Korea Meteorological Administration (KMA) Weather Data Open Portal [12]. For the CO₂ concentration, the average CO₂ level in Seoul for 2019–2020 was obtained and analyzed using the greenhouse-gas monitoring dataset provided by the Seoul Metropolitan Government Research Institute of Public Health and Environment [13].

2.3. Core Sampling

As shown in Figure 3, four concrete core specimens with a diameter of 100 mm were extracted from each sampling location. These cores were used for compressive strength testing, and the remaining core was used to measure the carbonation depth.

2.4. Compressive Strength

The compressive strength of the core specimens was measured in accordance with KS F 2422 (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete) [14]. Prior to testing, the top and bottom surfaces of each specimen were ground to ensure planarity. Compressive strength tests were then performed, and the measured values were corrected for the height-to-diameter ratio, in accordance with the KS correction factors presented in

Table 3. Correction factors for the compressive strength of core specimens with different height-to-diameter ratios.

H/D Ratio	Correction Factor
2.00	1.00
1.75	0.98
1.50	0.96
1.25	0.93
1.00	0.89

.

2.5. Strength Estimation Using Rebound Hammer

For the rebound testing, a Schmidt hammer was applied 20 times at each core extraction location, and the average round number was obtained. The compressive strength was estimated by applying an age-dependent correction factor (see

Table 4. Value of age coefficient α_n.

Age (Day)	28	50	100	150	200	300	500	1000	3000
αn	1.00	0.87	0.78	0.74	0.72	0.70	0.67	0.65	0.63

) to the strength estimation equations proposed by the Japan Society of Materials Science (Equation (1)) [15]. The Tokyo Metropolitan Building Materials Inspection Institute (Equation (2)) [16] and the Architectural Institute of Japan (Equation (3)) [17]. The final estimated compressive strength was considered as the average of the three estimates.

F_c = 13R₀ − 184,

(1)

F_c = 10R₀ − 110,

(2)

F_c = 7.3R₀ + 100,

(3)

Here,

F_c: estimated compressive strength (kgf/cm²).

R₀: corrected rebound number.

2.6. Measurement of Carbonation Depth

Carbonation depth was measured on the core specimens extracted from each location by spraying a 1% phenolphthalein solution, as shown in Figure 4. Spraying phenolphthalein solution immediately after core extraction may distort the carbonation depth owing to residual drilling powder on the specimen surface. Therefore, the core surfaces were first cleaned and then dried for 1 day before a 1% phenolphthalein solution was sprayed to measure the carbonation depth.

The carbonation coefficient was evaluated based on the measured carbonation depths. The carbonation depth and coefficients were compared according to the measurement location, exposure condition, and finishing type. In addition, the relationship between the compressive strength (i.e., corrected core strength or rebound-hammer-based estimated strength) and carbonation coefficient was examined.

3. Results

3.1. Climate Data Analysis

The results of the climate data analysis for Seoul, obtained from the KMA Weather Data Open Portal and the monitoring dataset provided by the Seoul Metropolitan Government Research Institute of Public Health and Environment, are presented in

Table 5. Results of climate data analysis.

Division		Average Value
Temperature		12.72 °C
Warm periods (June~August)	Average temperature	24.63 °C
	Min temperature	21.23 °C
	Max temperature	28.80 °C
Cold period (December~February)	Average temperature	−0.57 °C
	Min temperature	−4.26 °C
	Max temperature	3.61 °C
Atmospheric pressure		1006 hPa
Relative humidity		63.13%RH
Monthly precipitation		119.14 mm
Wind speed		8.79 m/s
Monthly solar radiation time		179.96 h
CO2 concentration		443.8 ppm

. The monthly variations in air temperature and relative humidity are shown in Figure 5.

The mean air temperature in Seoul from January 1983 to July 2023 was 12.73 °C, and the mean relative humidity was 63.13%RH. To account for seasonal variability, the dataset was further divided into warm (June–August) and cold (December–February) periods. During the warm period, the average, minimum, and maximum air temperatures were 24.63 °C, 21.23 °C, and 28.80 °C, respectively, whereas during the cold period, the corresponding temperatures were −0.57 °C, −4.26 °C, and 3.61 °C. In addition, the average CO₂ concentration in Seoul during 2019–2020 was 443.8 ppm, whereas the corresponding average CO₂ concentration at other regional monitoring stations during the same period was 418.5 ppm, indicating that Seoul was approximately 25 ppm higher.

3.2. Compressive Strength

The results of the core compressive strength tests and the rebound-hammer-based strength estimates for the investigated buildings are shown in Figure 6 and

Table 6. Measured compressive strength values.

Location		Core Specimens				Schmidt Hammer
		Compressive Strength (MPa)				Rebound Index (R)				Estimation Strength (MPa)
		#1	#2	#3	Ave.	#1	#2	#3	Ave.	A	B	C	Ave.
101	1F Northwest	25.4	25.4	24	24.9	50.2	49.4	48.3	49.3	28.2	23.7	28.4	26.8
	1F Southeast	24.5	23.4	17.7	21.9	46.5	43.7	44.3	44.8	24.6	20.9	26.4	24.0
	2F Northwest	25.7	24.4	26.7	25.6	49.4	47.6	51.9	49.6	28.5	23.9	28.6	27.0
	Partition wall	22.4	25.3	25.1	24.3	48.9	49.3	47.9	48.7	27.7	23.3	28.1	26.4
103	1F Northwest	22.7	22.5	20.3	21.8	50.8	51.8	49.7	50.8	29.4	24.6	29.1	27.7
	1F Southeast	31.0	35.0	30.3	32.1	55.0	54.3	52.7	54.0	32.0	26.6	30.5	29.7
	2F Southeast	18.1	14.8	15.1	16.0	40.7	42.2	38.7	40.5	21.2	18.2	24.5	21.3
	Partition wall	23.2	23.3	22.8	23.1	55.4	54.8	56.4	55.5	33.2	27.5	31.2	30.7

1. A: Japan Institute of Materials. 2. B: The Tokyo building materials inspection center. 3. C: Architectural Institute of Japan. 4. Value of age coefficient α_n: 0.63.

. The average core compressive strength was 23.7 MPa, and most locations showed no significant variation, with values generally ranging from 21 to 25 MPa. However, for the southeast-facing exterior wall of Building 103, the first-floor specimen showed the lowest strength (16 MPa), indicating the largest strength variation among all tested locations.

The average compressive strength estimated from the rebound-hammer measurements was 26.7 MPa. Except for the southeast-facing exterior wall on the first floor of Building 103, the rebound-hammer-based strength estimates were generally higher than the corresponding core compressive strengths. This trend is consistent with previous studies on carbonated concrete showing that surface carbonation increases near-surface hardness and rebound numbers, thereby leading to overestimation relative to core strengths [18,19]. Rebound-hammer readings primarily reflect the hardness of the near-surface zone, whereas core tests represent the bulk concrete. In particular, the near-surface region of aged concrete is often partially carbonated, and carbonation increases surface hardness and rebound numbers; consequently, applying generic conversion relationships may introduce an upward bias in rebound-based strength estimates compared with core test results [20].

3.3. Carbonation Depth

The carbonation depths of the investigated building are presented in Figure 7 and

Table 7. Carbonation depth and coefficient measurement results.

Location		Exposure	Finishing Material	Carbonation Depth (mm)	Age (Years)	Carbonation Coefficient (mm/√(Year))
101	1F Northwest	Exterior	water-based paint	42.9	39	6.9
	1F Southeast	Exterior	water-based paint	49.0	39	7.8
	2F Northwest	Exterior	water-based paint	49.0	39	7.8
	Partition wall	Interior	Wallpaper	47.8	39	7.7
	Kitchen	Interior	Wallpaper	27.0	39	4.3
	Staircase	Interior	Oil-based paint	0	39	0
103	1F Northwest	Exterior	water-based paint	43.9	39	7.0
	1F Southeast	Exterior	water-based paint	34.7	39	5.6
	2F Southeast	Exterior	water-based paint	67.0	39	10.7
	Partition wall	Interior	Wallpaper	53.5	39	8.6
	Kitchen	Interior	Tile	30.8	39	4.9
	Staircase	Interior	Oil-based paint	20.8	39	3.3

, and the calculated carbonation coefficients at each measurement location are shown in Figure 8. At each location, the carbonation coefficient was determined from the measured carbonation depths, and the average value was reported. For the exterior walls, an overall average value was calculated regardless of the exposure conditions (southeast- and northwest-facing).

The carbonation coefficient analysis results showed that the partition walls exhibited the greatest mean carbonation depth and the highest carbonation coefficient. In general, indoor CO₂ concentrations are reported to be higher than outdoor concentrations because of the respiration of occupants [21,22,23]. This is considered the primary reason for the relatively high carbonation depth and coefficient observed in the partition walls.

In contrast, the staircase exhibited the lowest carbonation coefficient. In Building 101, carbonation was not observed; the entire core specimen turned purple when sprayed with a 1% phenolphthalein solution. This is likely because the staircase surface was finished with synthetic paint over plastering mortar, which substantially reduced CO₂ diffusion into the concrete. This observation is consistent with previous findings reporting that applying synthetic (oil-based) paint, rather than emulsion (water-based) paint, improves the carbonation resistance of concrete [24].

The carbonation coefficients of the exterior wall concrete by orientation are presented in

Table 8. Comparative evaluation results of carbonation coefficient based on exposure environment.

Factors	Southeast Face	Northwest Face
Carbonation coefficient (mm/√year)	8.04	7.25
Carbonation coefficient ratio	1.11

. The carbonation coefficient of the southeast-facing exterior wall concrete was 8.04 mm/√year, whereas that of the northwest-facing exterior wall concrete was 7.25 mm/√year, indicating that the southeast-facing wall exhibited an approximately 10% higher value than the northwest-facing wall. Although the exterior wall specimens investigated in this study were not oriented exactly due south or due north, the comparison between the southeast- and northwest-facing walls suggests that the southeast-facing concrete may have been more strongly influenced by solar radiation, thereby providing relatively favorable conditions for CO₂ diffusion into the concrete. A detailed discussion of this issue is provided in the following sections.

4. Discussion

4.1. Effects of Exposure Condition and Orientation on Concrete Carbonation

Additional analyses were conducted to elucidate the relationship between carbonation depth and (i) solar radiation and (ii) exposure conditions, and to interpret the observed variability in both outdoor and indoor environments. For the exterior walls, the largest carbonation depth was observed on the second floor southeast-facing wall of Building 103, where the core compressive strength measured at the same location was the lowest (Figure 9). In Building 101, the northwest-facing wall exhibited no notable differences between the first and second floors in terms of core compressive strength or rebound-hammer-based estimated strength; however, the carbonation depth on the second floor was approximately 7 mm greater than that on the first floor. In addition, although the difference in compressive strength between the southeast-facing wall of Building 101 and the northwest-facing wall of Building 103 was negligible (≈0.1 MPa), the carbonation depth on the southeast-facing wall of Building 101 was approximately 6 mm greater. These results suggest that carbonation progress cannot be explained by strength alone, and that differences in orientation- and story-dependent solar/drying conditions and exposure conditions likely contributed to the observed carbonation.

Evidence supporting orientation-dependent solar conditions has been reported in previous studies. Hur [25] predicted potential sunshine duration by wall orientation and reported that solar radiation tends to increase as the orientation approaches south (see

Table 9. Prediction of sunshine availability based on direction [25].

Direction	West	45° West	South	45° East	East
Solar radiation time	3 h 8 m	6 h 7 m	8 h	6 h 51 m	3 h 53 m

). Tanuma et al. [26] analyzed carbonation depths from core specimens obtained from 1210 reinforced-concrete apartment buildings constructed between 1964 and 1996 and showed that, for outdoor concrete, carbonation progressed most rapidly on south-facing surfaces, followed by west-, east-, and north-facing surfaces. This trend was attributed to faster drying on south-facing surfaces after wetting events (e.g., rainfall) owing to higher solar radiation, which enhances CO₂ diffusion into concrete and thereby increases the carbonation coefficient. Ravahatra et al. [27] similarly measured carbonation depths from cores taken from an east–west-oriented reinforced-concrete wall constructed in 1979 after approximately 35 years of exposure and suggested that the south face may carbonate more rapidly because it dries faster. The reported mean carbonation depths were 19.6 mm for the north face and 24.2 mm for the south face; using the widely adopted square-root-of-time model, these values correspond to estimated carbonation coefficients of approximately 3.31 mm/√year (north) and 4.09 mm/√year (south), confirming faster carbonation on the south-facing surface.

This orientation dependency can be rationalized by the fact that solar exposure modifies the near-surface boundary conditions of concrete, mainly the temperature and moisture state, which govern both CO₂ transport and carbonation kinetics. In particular, atmospheric relative humidity controls the degree of saturation of near-surface pores and thus the effective CO₂ diffusivity and reaction rate; carbonation is often maximized at an intermediate RH of approximately 60–65%, whereas higher RH can suppress gas-phase CO₂ transport due to pore saturation, and very low RH can limit the reaction due to insufficient pore water availability [7,28]. Therefore, stronger insolation on south-facing walls is expected to increase the surface temperature and promote more frequent drying (and wetting–drying cycles), shifting near-surface moisture conditions toward ranges more favorable for carbonation, whereas north-facing surfaces tend to remain cooler and wetter for longer periods, potentially reducing carbonation progress. As shown in Figure 10, landscaping was present around the buildings investigated. Therefore, the first floor is likely to remain relatively moist owing to the influence of the surrounding landscaping, whereas the second floor is less affected and can dry more readily; accordingly, carbonation on the second floor is promoted under these more favorable drying conditions. In addition, carbonation tended to progress more rapidly on the southeast-facing wall than on the northwest-facing wall, which is consistent with solar-radiation-driven drying promoting conditions favorable for CO₂ diffusion. In particular, the southeast-facing wall on the second floor of Building 103 likely experienced both limited influence from landscaping and increased solar radiation, which plausibly contributed to the greatest measured carbonation depth.

Importantly, while outdoor exposure is often treated as the primary driver of carbonation, indoor carbonation can also exhibit pronounced spatial variability because indoor CO₂ concentration and moisture state are not spatially uniform but are strongly governed by occupancy patterns, room compartmentalization, and ventilation distribution. Field and experimental studies on residential units have shown that when internal doors are closed, CO₂ can increase rapidly in occupied rooms and maintaining low CO₂ becomes difficult even under a nominal whole-dwelling ventilation rate, indicating that room-to-room air exchange and supply/exhaust balance critically control the local CO₂ level [29]. In addition, quantitative interpretation of indoor CO₂ should consider occupant CO₂ generation, which varies with activity level and can be estimated using established generation-rate models [30]. In the present building, the highest carbonation rate coefficient observed at partition-wall locations can therefore be rationalized by the likelihood that these surfaces were adjacent to high-occupancy zones (e.g., rooms with extended residence time) where CO₂ accumulates under limited interzonal mixing (closed doors/partitions) and non-uniform ventilation supply, leading to a higher time-averaged CO₂ exposure at the concrete surface [29,30]. Moreover, carbonation kinetics are sensitive to the near-surface moisture condition; thus, microclimatic differences among indoor zones (temperature–humidity–airflow) can further amplify spatial contrasts.

Conversely, the lowest carbonation rate coefficient at the staircase is plausibly attributable to a combination of (i) surface finishing/coating and (ii) airflow characteristics of vertical shafts. Paint- or film-forming surface treatments can reduce CO₂ ingress and suppress carbonation depth by providing a diffusion barrier, and their effectiveness depends on coating type and integrity [24]. In addition, stairwells behave as vertical shafts where stack-effect-driven pressure differentials can promote substantial air movement through leakage paths and doors, potentially increasing air exchange and diluting locally generated CO₂ compared with enclosed rooms [31]. Finally, we acknowledge that direct measurements of room-resolved CO₂, temperature, and relative humidity were not available in this study; therefore, the above interpretation is presented as a mechanism-based explanation supported by indoor air quality literature.

4.2. Effects of Finishing Materials on Concrete Carbonation

To assess the effect of finishing materials on concrete carbonation, the carbonation coefficients of kitchens in Buildings 101 and 103 were compared. In Building 101, wallpaper was installed over plastering mortar, whereas in Building 103, tiles were installed over plastering mortar. The carbonation coefficients were 4.32 and 4.93 mm/√year for Buildings 101 and 103, respectively, indicating a slightly higher value at the tiled location. Although tile finishing is generally expected to reduce CO₂ diffusion and thus lower the carbonation rate, indoor carbonation can be strongly influenced by living conditions (e.g., ventilation, cooking frequency, and moisture conditions), which likely contributed to the observed results.

In addition, to examine the effect of paint type under outdoor conditions, the air-permeability coefficient of the painted surface was evaluated using the Torrent method, as shown in Figure 11. The test locations were selected on (i) the exterior walls coated with water-based paint and (ii) the staircase surfaces coated with oil-based paint. The measured air-permeability coefficients are presented in Figure 12. For the exterior wall coated with water-based paint, the results showed some variation depending on the measurement location, but the mean value was 54.54 × 10⁻¹⁶ cm/s. In contrast, the staircase surface coated with oil-based paint exhibited a much lower mean value of 11.73 × 10⁻¹⁶ cm/s, confirming that the air permeability of the oil-paint-coated concrete was markedly reduced. This observation is consistent with the carbonation results, in which carbonation in the exterior walls progressed to near the reinforcement cover depth, whereas carbonation in the staircase was negligible. The staircase investigated in this study was located inside the main entrance; therefore, its CO₂ concentration was expected to be similar to that of the outdoor environment. Although the exterior wall is directly exposed to rainfall, the staircase is sheltered, resulting in different exposure conditions. Nevertheless, the markedly slow carbonation in the staircase suggests that the effect of the oil-based paint was substantial, likely by significantly reducing gas (CO₂) transport through the near-surface region [7,24].

Overall, for exterior walls, carbonation was interpreted to be affected by orientation- and story-dependent solar/drying conditions and by exposure differences associated with the surrounding landscaping. In indoor environments, the influence of living conditions results in a large variability in the carbonation coefficient, making it difficult to isolate the effect of finishing materials. In contrast, under outdoor conditions, the staircase coated with oil-based paint exhibited a markedly reduced carbonation progression, indicating that the type of finishing material significantly influences concrete carbonation.

5. Conclusions

In this study, a durability assessment focusing on carbonation was conducted for an existing reinforced concrete structure scheduled for demolition in Seoul, South Korea, to examine the level of durability performance that has been generally assumed for building structures. The following conclusions were drawn:

• A comparison between the core compressive strength and the compressive strength estimated from the rebound-hammer measurements showed that, although the rebound-hammer-based estimates exhibited some scatter, they generally captured trends similar to those of the core strengths, indicating a meaningful correlation. However, the rebound-hammer-based strengths tended to be slightly higher than the corresponding core strengths, which can be attributed to the rebound hammer’s sensitivity to the near-surface zone and to carbonation-related surface densification in aged concrete. Accordingly, for structures with conditions comparable to those investigated, rebound-hammer testing can be used as a supplementary indicator for on-site durability assessment, particularly for rapid screening and relative comparisons among locations. Nevertheless, because rebound readings can be affected by age, surface condition, moisture state, and carbonation, strength estimation based solely on rebound measurements may be biased upward; therefore, core testing should be performed in parallel when higher accuracy or representative bulk strength is required.
• The measurement of carbonation depth at different locations indicated that indoor areas, where CO₂ concentrations are likely to be higher due to the respiration of occupants and daily activities, tended to exhibit greater carbonation depths. This suggests that carbonation progress can be strongly affected not only by outdoor exposure conditions but also by the indoor use environment, including CO₂ concentration, ventilation, and moisture.
• The influence of orientation-dependent solar radiation on the carbonation progress in exterior walls was examined. The measured carbonation depths were greater on the southwest-facing wall than on the northwest-facing wall, suggesting that increased solar radiation on the southwest wall may promote drying, thereby creating conditions more favorable for CO₂ diffusion into the concrete, accelerating carbonation. These findings imply that carbonation rates can vary by orientation, even within the same building; thus, orientation should be considered in durability assessment and maintenance planning.
• When the carbonation coefficients were compared under similar compressive strength conditions, the coefficient for the southeast-facing wall was estimated to be approximately 1.1 times that of the northwest-facing wall. This indicates that even when the material performance is comparable, carbonation can differ owing to exposure conditions (e.g., solar radiation, drying, rainfall, and shielding). Therefore, carbonation-based durability prediction is more appropriately performed by applying carbonation coefficients that reflect actual exposure conditions rather than relying solely on a single strength-based indicator.
• The effect of finishing materials on concrete carbonation was evaluated, and the results indicate that, under indoor conditions, the influence of finishing materials could not be clearly isolated because carbonation was strongly affected by living-related factors such as ventilation, cooking frequency, and moisture conditions. In contrast, under outdoor conditions, the staircase surface coated with oil-based paint exhibited a markedly lower air-permeability coefficient (Torrent method) than the exterior wall coated with water-based paint, and carbonation in the staircase was negligible compared with the pronounced carbonation observed in the exterior wall. These findings suggest that finishing materials, particularly paint type, can significantly influence carbonation resistance by controlling near-surface gas (CO₂) transport through concrete.
• This study was based on a snapshot field assessment of an existing reinforced-concrete building at a single time point; therefore, the effects of long-term seasonal temperature cycling (warm vs. cold periods) and other time-dependent environmental influences could not be quantitatively isolated from the measured carbonation depths. Further research is required to verify the roles of solar-driven drying and temperature on carbonation kinetics through accelerated laboratory carbonation tests under controlled boundary conditions that incorporate realistic warm–cold cycles and drying–wetting histories. In addition, complementary field investigations of comparable existing buildings in regions with colder climates (e.g., Gangwon Province) are needed to examine climate-dependent differences in carbonation progress under real exposure conditions.