A Study on the Carbonation Rate of Concrete Exposed in Different Climatic Conditions

Recently, the degradation of concrete has become a serious problem worldwide and one of the principle factors of degradation is the carbonation process. It is well established that environmental conditions affect the carbonation progress of concrete among the most important factors that can greatly affect the carbonation resistance of concrete are relative humidity (RH) and temperature. Carbonation has become a threat to concrete structures especially in urban and industrial areas. Thus, it is necessary to have a proper design to maintain the structure's stability against degradation caused by carbonation. Therefore, this study was conducted to evaluate the effects of different environmental and climatic conditions on the carbonation rate of concrete. The specimens were prepared using OPC and fly ash (FA). After 28 days of air curing, specimens were exposed in different climate conditions under sheltered and un-sheltered conditions. The carbonation tests were conducted at the ages of 6 and 12 months. It was found that the carbonation rates were significantly influenced by the climate and environmental conditions; the specimens exposed to a relatively dry environment and low annual precipitations have shown higher carbonation during one-year exposure. Moreover, in unsheltered conditions the annual precipitation significantly affects the carbonation rate of concrete. Furthermore, it was observed that a 20% replacement of FA does not enhance the carbonation resistance of concrete.

In saturated conditions, the carbonation progress is stopped and only a proceeds when concrete is enough dried to allow the diffusion of CO 2 . Moisture in concrete is essential for the chemical reactions of carbonation. The carbonation front reaches the maximum at a relative humidity between 50 and 70% (Wierig, 1984;Parrott, 1986). At higher relative humidity values above 70%, the carbonation hinders down due to the slower rate of diffusion of CO 2 through water-filled pores. At relative humidity values below 50% there is insufficient moisture to allow carbonation reactions to take place (Ghaforzai, 2021).
In addition to high CO 2 concentration, atmospheric temperature can also affect the carbonation of concrete because the rate of the oxidation reaction and CO 2 concentration is affected by the amount of heat energy available to drive the reaction. Besides carbonation resistance, the resistance of concrete against chloride penetration is also reduced at elevated temperatures, as ions become more mobile and salts become soluble. This situation can be possibly destructive, and it endangers the resistance of concrete against chloride diffusion because it leads to more portable chloride ions movement. Moreover, Relative humidity influences the permeation properties of concrete (Oberholster, 1986;Parrott, 1994Parrott, & 2006Basheer, Kropp & Cleland, 2001). Nagataki et al. have stated in research that air permeability is related to water evaporated from the concrete (Nagataki, Ujike and Konishi, 1986). While internal relative humidity significantly influences the air permeability. It has been observed that increase in RH reduces the air penetration of the concrete (Parrott, 1991;Saju et al., 2020).

Experimental Program Materials
In this study, two types of binders namely ordinary Portland cement (OPC), and fly ash are used. Table  1 includes the physical properties of materials. Fly ash is used as a replacement for cement. Crushed stone with a maximum size of 20mm was used as a coarse aggregate with density (SSD) of 2.88 g/cm 3 , fine modulus (FM) of 6.68, and water absorption of 0.71%. Washed sea sand was chosen as fine aggregates with density (SSD) of 2.53 g/cm 3 , fine modulus (FM) of 2.69, and water absorption: 1.61%. Moreover, tap water was mixed as mixing water for concrete.

Mixture
Two types of concrete mixtures (OPC and FA) were prepared with 60% W/B. FA content was maintained at 20% of cement mass. The mix proportion and fresh properties of concrete are shown in Table 2. In order to maintain appropriate fresh properties, both airentraining admixture; and air-entraining and waterreducing admixture were used based on the cement mass.

Specimen Design
Concrete prism specimens of 75x75x250 mm and cylindrical specimens with 100mm diameters were demolded at 24 hours after casting. Then, the specimens were moved to the temperature and humidity-controlled room. Concrete prisms and cylindrical specimens were stored in an air curing room for 28 days at a constant room temperature of 20 0 C and a relative humidity of 60%. After 28 days curing, 4 sides of the concrete prisms were coated by epoxy while two opposite sides were exposed to CO 2 diffusion and after 48 hours waiting for the specimens. The concrete prisms were exposed to CO 2 diffusion in accelerated carbonation chamber and natural exposure sites. For natural exposure, four distinct areas were selected; namely Japan, Afghanistan, Indonesia, and Malaysia. Furthermore, the specimens in the natural environment were exposed in sheltered and unsheltered conditions, while the specimens in the accelerated carbonation chamber were kept under constant temperature of 20±2°C, 60% relative humidity, and CO 2 concentration of (5±0.5%). Moreover, for decreasing the transportation cost the concrete prisms were divided into small cubic 75 mm along each side.

Testing Method
Compressive strength test was carried out according to JIS A 1152 at the ages of 28 and 91 days. In addition, the specimens were cured in controlled room air at a constant temperature of 20±2°C, 60% RH until the end of the test age. Accelerated carbonation depth was measured according to JIS A 1153:2003 at certain ages of 1, 4, 8, 13, and 26 weeks. While for natural exposure specimens, the carbonation test was per-formed at the age of 6 and 12 months. At the time of measurement, carbonation depths were measured in the laboratory, the concrete prisms were split, cleaned and 1% of phenolphthalein in the solution of 90% ethyl alcohol solution was sprayed on the fresh-cut surface. When the solution is sprayed on a broken concrete surface, the carbonated portion remains colorless (concrete color) and the noncarbonated portion turns to dark purple. Average carbonation depth was measured at 10 points from each side and taken the average carbonation depth at a specific age. Fig 1 shows the concrete strength development over a period of 28 and 91 days. The OPC and FA concrete specimens cured in air were tested. The FA was invoked as cement replacement in concrete and FA content was maintained 20% of cement mass in concrete. It was noticed that the strength increased up to 91 days for both normal OPC concrete and fly ash concrete, while for FA60 specimens the increment was not so significant. N60 (OPC) specimen has shown 17% development in strength from 28 days to 91 days.

Accelerated Carbonation
Accelerated carbonation depths of concrete with and without FA as partial cement replacement with a W/B ratio of 0.6 at the ages of 1, 4, 8, 13, and 26 weeks are shown in Fig 2. The carbonation progress was significant for both types of the mixtures due to high W/B ratio and air curing. It can be seen that the carbonation depth increases with the replacement of FA in the concrete compared with those without fly ash which partially replaces the cement. At the end of the testing, fly ash concrete has shown a 16 % increment compared to OPC. From Fig 2, it is clearly seen that OPC incorporation of its own demonstrated less increase in carbonation compared with FA concrete. b) Kabul is the capital of Afghanistan, located in the eastern part of Afghanistan. Kabul has a semiarid climate with cold winters and hot summers. The annual temperature at the exposure area (close to the specimen) was around 16.5 °C and the average humidity of 45%. It has four seasons the same as Fukuoka city in Japan. In winter, the temperature goes down below 0 °C and the precipitation is concentrated in January almost exclusively falling as snow, while the summer is hot and dry. The dry season is usually started in June and prolongs up to the end of August. The maximum temperature recorded in July was almost 38 °C.    The relative humidity ranges between 80-90 % (Fig 6).
Carbonation rate in natural exposure Carbonation depth was measured at the ages of 6 months and 12 months for all exposure sites. The specimens were kept in the sheltered and unsheltered conditions. The test results for both conditions are illustrated from Fig 7 up to Fig 10. Fig 7 represents the carbonation progress for the specimens exposed in Kabul, Afghanistan. The results are shown for 6 months and 12 months, both N60 and FA60 have shown significant increase in carbonation depth, and the similar trend to the accelerated carbonation depth shown in Fig 2. Furthermore, a strong relationship between carbonation development and exposure time for all concrete mixture was achieved. From the results, it can be seen that the carbonation depth is low in unsheltered conditions, which is influenced by rainfall and snowfall during the rainy season, but it is not so significant due to less amount of annual rainfall, which is around 316 mm. In the case of FA60 specimen, no significant difference has been seen for 6 months' exposure because the exposure started from Aug 2014, and from Aug-Jan the rainfall is less in amount. Therefore, fly ash concrete did not show the good resistance to CO 2 diffusion because fly ash affect sheat of hydration and the strength development becomes slow and resulting low carbonation resistance. In sheltered conditions, N60 and FA60 specimens have shown 40% and 47% increment in carbonation depth during 6 months respectively. While in unsheltered conditions, N60 and FA60 showed 46 and 39% the increment in carbonation depth, respectively.
Carbonation front progress in the Japanese environment is demonstrated in Fig 8, the trend of the carbonation progress under sheltered conditions is roughly the same as measured for the specimens exposed in Kabul. The increment was also same as mentioned above. More or less, for all exposure sites in normal conditions, the carbonation progress had a linear relationship with exposure time and increased with increasing exposure time. Furthermore, the concrete incorporating FA as cement replacement generally showed lower resistance to carbonation in natural exposure compared to OPC concrete, which has also been confirmed by the accelerated carbonation test presented in Fig 2, possibly due to the dominant effect of calcium hydroxide reduction on pore refinement. In addition, fly ash concrete has exhibited the larger carbonation depth under all exposure conditions (Khunthongkeaw, Tangterm-sirikul and Leelawat, 2006).   Moreover, the results represented in Fig 8 were measured for unsheltered conditions, which were remarkably different from the sheltered condition, the lower carbonation depths were obtained in the high rainfall environment. From Fig 7 and Fig 8, it can be concluded that environmental conditions have a great influence on the carbonation process of concrete. Concrete has a lower carbonation resistance in sheltered condition than unsheltered condition, where the concrete is no longer protected from rain. Fig 9 shows the carbonation front progress through one year in Makassar. The specimens show the same trend as it is illustrated in Fig 7 and  Fig 8. The climatic conditions of Indonesia are different from Afghanistan and Japan. High annual average temperature and humidity; the researches show, that carbonation occurs faster in warmer regions or sheltered conditions. Somewhat this statement is true for all exposure conditions at sheltered exposure sites, whereas it can be observed that despite the fact that Indonesia's annual temperature is higher than that of Afghanistan and Japan, but the carbonation depth is still lower than the other two regions, possibly due to the high relative humidity in Indonesia. Concrete face higher carbonation in high temperature unless the relative humidity is lower (Koichi et al., 2010). Fig  10 represents the carbonation progress of specimens exposed in the Malaysian environment. It is demonstrated that carbonation rates for the first six months of the exposure was significant, while the increment for all types of the mixture was negligible until 1 year. The increment in the first six months can be attributed to the dry weather conditions from January to March, and from March to February is usually the rainy season in Malaysia, which causes less increment in carbonation from six months.   This means that the drying and wetting processes have a great effect on the carbonation progress. The carbonation front after 1 year is also presented in the photo (1-4).

CONCLUSION:
The carbonation rate of all concrete mixtures apparent to natural exposure is significantly affected by the climatic conditions; Concrete exposed to a relatively dry environment has shown a higher rate of carbonation during the exposure time. It can be seen that the carbonation rate is higher when the RH is in the range of 45-70%. In addition, the carbonation rate is affected by the rainfall amount; the lower the rainfall the lower the carbonation rate. In Afghanistan, due to low annual rainfall and relative dry season, higher carbonation is observed.

ACKNOWLEDGEMENT:
The authors would like to thank Kyushu university concrete engineering Laboratory for providing research facilities and as well the Hasanuddin University, Indonesia and University Tun Hussein Onn Malaysia (UTHM) for assistance in conducting exposure test.

CONFLICTS OF INTEREST:
All authors declare that they have no conflicts of interest to disclose.