Comparative Evaluation of Alkalinity Assessment Methods and Long-Term Phase Evolution in Electrochemical Realkalization of Historic Concrete: A Four-Year Field Study

Concrete buildings from the twentieth century are a vital part of our cultural landscape, yet they face a silent enemy that threatens their structural safety: the slow, natural process of carbonation. As air enters the tiny pores of concrete, carbon dioxide reacts with the material's internal chemistry, gradually lowering its pH level. This chemical shift is critical because the steel bars hidden inside the concrete rely on a highly alkaline, or basic, environment to stay protected. When the pH drops too low, that protective layer dissolves, and the steel begins to rust. To save these historic structures, engineers use a technique called electrochemical realkalization. This process sends an electric current through the concrete to push fresh alkaline materials deep into the pores, effectively resetting the chemical environment and stopping the rust before it starts. However, a major question has lingered for decades: does this treatment truly work over the long term, and how can we tell if it is still protecting the steel years after the repair is finished?

For years, the standard way to check if a concrete repair was successful has been a simple visual test. Technicians spray a liquid called phenolphthalein onto a fresh slice of the concrete. If the liquid turns a bright pink, it means the area is still alkaline and safe. If it stays clear, the assumption has been that the concrete has become acidic and the steel is at risk. This method is quick and easy, but a new four-year field study conducted on a historic school building in Ningbo, China, suggests that this simple spray test may be lying to us. The researchers found that while the spray test often showed clear, "failed" concrete, the actual chemical environment inside the material remained protective. The study reveals that the traditional test is blind to a specific type of chemical balance that keeps the concrete safe, leading to a potential misdiagnosis of healthy, repaired buildings.

The research team focused on the columns of the former County Rural Normal School, a building over eighty years old that had undergone the electrochemical repair. They monitored these columns for four years, taking samples at regular intervals to see what was happening inside. When they used the standard spray test on the concrete cores, the results were discouraging. The liquid remained clear, even on the surface layers, suggesting that the alkalinity had vanished and the repair had failed. The team even tried soaking the samples in water to ensure the liquid could penetrate, but the spray still failed to turn pink. Based on this test alone, one would conclude that the steel bars were exposed to dangerous conditions.

However, when the researchers looked deeper using a different method, the story changed completely. Instead of just spraying the surface, they ground the concrete into a fine powder, mixed it with water, and measured the pH of the resulting liquid. This approach, which allows the water to fully dissolve the solid particles, told a very different story. The liquid extracted from the concrete consistently showed a pH level between 9.20 and 9.53. This is a crucial finding because it is well above the threshold needed to keep steel from rusting. The concrete was not dead or acidic; it was alive with a stable, protective alkalinity that the spray test simply could not detect.

The reason for this discrepancy lies in the specific chemistry of the repaired concrete. Over the four years of exposure, the concrete developed a unique internal structure dominated by calcium carbonate, a common mineral also found in limestone and chalk. While the spray test relies on water quickly dissolving chemicals on the surface to create a color change, the calcium carbonate in these repaired columns dissolves very slowly. The spray test is like a quick snapshot that misses the slow, steady release of protection happening inside. The powder method, by breaking the concrete apart and letting it soak, forces that slow-release chemistry to happen all at once, revealing the true, stable alkaline environment that exists within the solid matrix.

To understand exactly what was holding this environment together, the team analyzed the microscopic composition of the concrete using advanced imaging and thermal analysis. They discovered that the protective environment was maintained by a solid-state balance. The concrete contained a significant amount of calcium carbonate, about 20 percent of its weight, along with small amounts of calcium hydroxide and trace amounts of sodium hydroxide. These materials work together in a delicate equilibrium. Even though the amount of the highly reactive calcium hydroxide was low, the abundance of calcium carbonate acted as a buffer, slowly releasing the necessary chemicals to keep the pH high enough to protect the steel. This internal reservoir of alkalinity is what the spray test failed to see because it could not break the solid bonds to release the chemicals in the short time it was applied.

The researchers also connected these chemical findings to the actual condition of the steel bars inside the building. Before the repair, the steel was in a state of active corrosion, with electrical measurements showing a high risk of rust. After the electrochemical treatment, the electrical potential of the steel shifted dramatically, moving into a safe zone where the metal is protected. This real-world data confirmed that the treatment was working. The steel was not corroding, even though the surface spray test suggested the concrete was no longer alkaline. The study proves that the steel is safe because of the deep, buffered chemical environment, not the surface condition that the spray test measures.

This four-year investigation highlights a critical flaw in how we evaluate the health of historic concrete. The traditional spray test, while useful for quick checks, is insufficient for assessing long-term repairs because it cannot detect the slow-dissolving minerals that provide lasting protection. Relying on it could lead engineers to believe a building is in danger when it is actually safe, or worse, to perform unnecessary and invasive treatments on structures that are already stable. The study suggests that a more reliable approach involves analyzing the concrete's internal chemistry through powder dissolution or using non-destructive sensors that can monitor the actual electrical state of the steel. By shifting focus from a simple surface color change to the complex, balanced chemistry inside the material, we can better preserve the concrete heritage that supports our modern world.