Concrete deterioration behind the ghost town problem
In recent years, once-vibrant new towns and apartment buildings have become ghost towns, attracting attention as a social problem. This is largely because of changes in the industrial structure, such as an aging population combined with the diminishing number of children, and factory closures. However, the time-related deterioration of the buildings themselves, especially reinforced concrete buildings, is also a factor that cannot be overlooked.
Many houses built during the period of rapid economic growth from the mid-1950s to the early 1970s are now over 50 years old and deteriorating. In addition to concerns about their earthquake resistance, these buildings are experiencing an increasing number of visible problems, such as peeling tiles on exterior walls and collapsing corridors and stairs. Houses that used to be safe and familiar to people are gradually becoming something unacceptable socially and physically.
Concrete is a mixture of sand, gravel, water, and cement, and is a very familiar part of our daily lives. It is used in all sorts of places, including roads, sewers, utility poles, and houses. There was a time when it was believed to be an inexpensive, durable, and maintenance-free construction material. Because of its hardness, the image of concrete being unbreakable was prevalent.
However, while concrete is resistant to compressive forces, it is weak against tensile forces. For this reason, reinforced concrete, which is reinforced by reinforcing steels embedded in it, has been widely used. However, no matter how high the quality of reinforced concrete, deterioration will progress over time owing to various external factors
Deteriorated concrete carries the risk of collapse, peeling, and falling of walls and ceilings. Concrete, which could last more than 100 years under certain conditions, will have a much shorter life if not properly managed. In addition, there are many cases where buildings built relatively recently are demolished before the concrete has reached its true lifespan.
However, the idea of demolishing and building a new house as soon as it becomes old is being reconsidered in modern society. Compared to Europe, Japan is considered to have a stronger scrap-and-build mentality, but those days are coming to an end. Today, there is a growing demand for reuse and extending the lifespan of waste materials and building materials, in consideration of their environmental impact.
Meanwhile, there is an increasing number of cases where buildings that should be demolished owing to durability issues are forced to continue to be used because of complicated circumstances such as budgets, legal systems, and residents’ consent. In light of this situation, we need a new perspective on how buildings can be used for a long time in a sound condition.
This is where finishing materials have been drawing attention. These are materials used on the surface of buildings, and they play a role in preventing external influences and protecting the buildings. If we compare a building to a person, the structure is the body (bones and muscles), and the finishing materials are the clothing. Buildings also need “clothing” to protect them from the outside environment.
Good finishing materials protect buildings from deterioration factors
Many of you have seen cracks on the exterior walls and ceilings of old buildings. These cracks are caused by temperature differences, shrinkage due to drying, repeated loading, and other factors, but of particular note is a phenomenon called spalling, in which the reinforcing steel inside rusts and expands, pushing out the concrete from the inside.
By nature, reinforcing steel and concrete are chemically compatible materials. Concrete contains a large amount of sodium hydroxide, which makes it highly alkaline with a pH of around 13, forming a thin film called a passive film that protects steel. Steel is less likely to oxidize in an alkaline environment, and it lasts longer.
However, when carbon dioxide from the air penetrates concrete, a chemical reaction causes a phenomenon called carbonation, which weakens the alkalinity of the concrete. In this situation, steel is more likely to rust.
When carbonation promotes rusting of reinforcing steel, the volume of the area increases, destroying the concrete from the inside. Furthermore, deterioration factors such as water, carbon dioxide, chloride ions, will enter through the cracks, further shortening the lifespan of the concrete. To prevent this cycle, it is important to apply a finishing material to the surface to insulate concrete surfaces from outside influences.
Currently, there are many different types of finishing materials used to protect concrete surfaces, especially those called finish coatings, such as coating film types and penetrating water absorption inhibitors. Many studies have confirmed that each finishing material is effective in preventing the penetration of deterioration factors.
A good finishing material is truly highly functional “clothing” for a building, protecting it from the external environment. Even for old buildings, periodic repainting of the finish coating can be expected to extend the life of the building without total renovation or demolition.
However, finishing materials themselves gradually deteriorate owing to ultraviolet rays, heat, wind, and rain. In other words, finishing materials also have a lifespan. Currently, there is a great deal of research that qualitatively demonstrates the effectiveness of finishing materials, but there is still insufficient progress in quantitative evaluations, such as how long and to what extent protective performance lasts
If we can show numerically how many years the life of a building can be extended by using a certain finishing material, we can select the most suitable material for that building. Naturally, the materials that should be selected will be different for temporary buildings that only need to withstand a short period of time, and for buildings that are expected to be used for 100 years or longer.
If a method for evaluating the degree of deterioration of finishing materials can be established and its effectiveness successfully quantified, it will be possible to apply the most appropriate finishing materials on a case-by-case basis. This will not only reduce costs but will also contribute to environmentally friendly architecture by eliminating waste.
Establishing a method for evaluating the degree of deterioration of finishing materials for sustainable construction
Since 2016, I have been actively involved in research on the protection of concrete by finishing materials and have also started outdoor exposure experiments. Now, with the cooperation of various people, including joint researchers, we will be able to conduct experiments in areas with different climatic conditions, including Miyakojima. This will enable us to compare how deterioration progresses in different climates and provide clues for evaluating the long-term performance of finishing materials.
Results often differ between accelerated tests in the laboratory and exposures in the actual environment, and it is necessary to acquire data in exposure sites. It is especially important to confirm changes in real environments over a long-term span of 5 or 10 years.
In the future, sustainable development will become increasingly important in the building sector, considering carbon neutrality and SDGs. Though Japan is at a major turning point as it works to achieve the goal of zero greenhouse gas emissions by 2050, cement, the material used in concrete, is said to be a typical energy-intensive industry, and generates large amounts of carbon dioxide, especially when burning limestone.
Also, concrete production involves crushing rocks found in mountains to produce aggregate (crushed sand and stone), which inevitably affects the natural environment. Furthermore, when a building is rebuilt, there is also the problem of how to dispose of the rubble generated during demolition.
To reduce this environmental impact, the building and civil engineering industry has begun to use recycled aggregate and low-carbon concrete, but these environmentally friendly materials can still have issues with durability. That is why there is a demand for finishing materials to compensate for these weaknesses and extend the lifespan of buildings while reducing the environmental impact.
In the future, it would be ideal to be able to select the most suitable finishing materials from the building design stage according to the purpose of use and environmental conditions. If we can maintain the health of the building structure by renewing the finishing materials every appropriate number of years just like changing clothing, we can contribute to the creation of a sustainable society.
The work of architecture inevitably places a burden on nature. We need to make efforts to reduce that burden as much as possible and leave valuable buildings for future society. Supporting sustainable architecture from a perspective that has not received much attention until now – focusing on the surface of things. That is the future I am aiming for in my research.
* The information contained herein is current as of March 2025.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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