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Effect of Acid Rain on Concrete

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Abstract

Hangzhou, a rapidly developing city in Southeast China, was evaluated for damages caused by acid rain, an emerging issue among coal-consuming nations. The status of acid rain in Hangzhou between 1983 and 2015 was analyzed using both ex-situ and in-situ analysis methods such as dry-wet cycling, for example, which simulates the erosion of concrete over time. Experiments investigating sulfate attacks on concrete under different conditions allowed researchers Yuanzhu Zhang, Luoyi Gu, Weiguang Li, and Qianlu Zhang to estimate the economic loss of residential concrete structures in Hangzhou by acid rain. Research demonstrated a progressive decrease in cube strength resulting in loose and powdery concrete, which categorized Hangzhou’s acid rain problem as moderate. Researchers estimate an economic loss of over 2 million USD every year for the city.

Introduction

Acid rain, which is mainly caused by SO2 and NO2 emissions, is a result of the rapid economic growth in China over the last 30 years. The average annual pH value from 1983 to 2015 varied from 4.0 to 5.2 pH units. Investigating statistics obtained by the State Environmental Protection Agency (SEPA) showed that SO2 emissions in 1995 were 1.6 million tonnes higher than in 1990. The conditions of concrete within Chinese urban environments are very poor (and are getting worse) because acid rain has reached more than 30% of China. Conveniently, Hangzhou is placed between and blocked by three mountain faces, which add additional strain on the environment and the concrete found within the city. Long-term acid rain has been proven to produce hydration products, which in turn expand concrete by 1.5 to 2.2 times its original size. This results in the gradual production of cracks and spalls, which in turn negatively affect concrete strength and durability. The gradual degradation of concrete structures in Hangzhou results in an annual economic loss of approximately 2 million USD. While the effects of pH change on wildlife and forestry have been widely studied, the effects of pH change on concrete structures are lacking research. Further research could help the public, as well as decision-makers, realize the importance of strong environmental policies. This experiment clearly demonstrated that society has a long way to go in the field of acid rain control.

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Methods and Materials

Concrete was prepared using Portland cement (grade 32.5), 25 mm gravel particles, and river sand (2.53 g/cm3), then formed into 10 cm3 cubes. The concrete was cured at 20°C in 95% humidity for 28 days, then dried at 80°C for an additional 48 hours. Forty-five of the most well-made cubic specimens were chosen for dry-wet cycling. The compressive strength of the specimens was determined to be 2304g and 34.6 MPa and was calculated using automatic sulfate dry-wet cycling test equipment. Specimens were slowly submerged in an 8% solution of Na2SO4 with a pH of 4.8 to replicate rain in Hangzhou. After 15 hours the cubic specimens were air-dried for 30 minutes, oven-dried at 80°C for six hours, then cooled at room temperature for five hours. After every 15 cycles, three blocks were removed and analyzed. Blocks that did not fit in with the group (lacking surface damage) were removed from testing and disposed of. The compressive strength, weight, and appearance of each block were recorded using a Universal Testing Machine (UTM). The pH was maintained using anhydrous sulfuric acid and was measured using an audiometer before every test.

Results

After 15 cycles, concrete that was previously gray and without holes became rust-colored and pitted with corrosion holes. The scale of corrosion holes and discoloration increased over time. Compressive strength also dropped after 15 cycles, which can be attributed to the calcium hydroxide reacting with a surplus of H+ molecules. The mass of the concrete increased substantially over the first 15 days, which was followed by a slow decrease in mass over the following 45 days. This change in mass can be attributed to concrete’s capillary pores getting filled with products of the acid rain, which subsequently decompose and progressively reduce the mass of the concrete. After 75 cycles, specimen surface layers formed honeycomb voids and became loose and powdery. The compressive strength of the cement gradually decreased until cycle number 75, when the compressive strength increased slightly due to extra strength provided by filled capillary pores. After 90 days though, the concrete specimens began forming clear cracks and spalls.

Discussion

This study revealed that although most of the effects of pH change on concrete are invisible to the naked eye, a distinct frosty aggregate will form on the surface of sulfate-attacked concrete after just ten years of service.

This experiment, which investigates only residential buildings, required several complicated calculations to estimate the surface area of exposed concrete in Hangzhou. The calculation takes into consideration the cost and frequency of maintenance and repair of buildings, population statistics, region size, climate, and more. Residential buildings are the focus of this study because many large-scale buildings and industrial sites are composed of a wide array of materials such as glass, metal, coated in paint, etc. An increased number of variables hurt the accuracy and credibility of the researcher’s estimate, which is why residential buildings are the main focus of this study. Literature review investigating population statistics of Hangzhou between 2002 and 2014 revealed the population of rural areas to be slowly decreasing, while the urban population showed rapid growth. Furthermore, researchers learned that over the last 30 years, acid rain in Hangzhou (which has a pH value of between 4.0 and 5.2) has reached approximately 93% of the city. With all of these factors in mind, researchers were able to produce a rough estimate of the annual economic loss of concrete due to acid rain in Hangzhou at about 2 million USD. Visible surface damage and a prominent decline in compressive strength proves acid rain to be 

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