Factors Affecting Corrosion Rate in Reinforced Concrete Manholes
Authors: Kasahara Hisatoshi , Fujimoto Norihiro , Okamura Yousuke 
Published: December 2020

There are approximately 600,000 reinforced concrete manholes for communications owned by NTT in Japan. Reinforced concrete sometimes degrades and the reinforcing bars (rebar) are exposed on the surface. When rebar is exposed, corrosion progresses and the structural strength of the manhole degrades. Basically, there is a high level of humidity inside the manhole chamber, and this facilitates the corrosion of the rebar. Since the rebar in the ceiling plays an important role in maintaining the strength of the entire manhole, NTT conducts inspection and repair especially for the exposed rebar in the ceiling. In this study, the corrosion rate of the rebar exposed in manholes is investigated based on exposure tests and experiments using atmospheric corrosion monitoring (ACM) sensors. The environmental factors contributing to the corrosion rate are also investigated.

In order to simulate exposed rebar, we conducted an exposure test in which rebar samples were installed in the manhole ceiling. The rebar was installed in multiple manholes for 1 year, and the corrosion rate of the reinforcing bar was calculated from the weight change due to corrosion.

The results show that the rebar corrosion rate in the manhole in which dew condensation was generated on the ceiling tended to be high. The product of corrosion, a crystalline compound, was identified by X-ray diffraction, and Magnetite was mainly detected. Since Magnetite easily develops over long exposure to a wet environment, the existence of corrosion due to dew condensation was confirmed. Although Goethite and Lepidocrocite were identified through X-ray diffraction, Akaganeite that forms in the presence of chloride was not observed.

As shown in Fig. 1, the temperature and humidity inside the manhole chamber, the temperature of the ceiling, and the temperature of the soil immediately above the manhole were measured. From the temperature and humidity in the manhole chamber, the dew point temperature, which is the threshold for dew condensation on an object, i.e., the ceiling in this case, was calculated.

As the results in Fig. 2 show, in winter, the cold ground temperature was transmitted through the soil and concrete lowering the ceiling temperature of the manhole. Meanwhile, the temperature in the manhole chamber was relatively warm. This resulted in the ceiling temperature becoming lower than the dew point temperature.

To clarify the relationship among the dew point temperature, amount of dew condensation, and corrosion rate, experiments were conducted using an ACM sensor. The ACM sensor comprises Fe, Ag, and an insulator separating the two metals. When water droplets are formed on the sensor surface, Fe is eluted through corrosion. The emitted electrons undergo the following reaction at the surface of Ag: O2 + 2H2O + 4e- → 4OH-. The e- in the reaction equation is measured as the current. The measured current corresponds to the amount of eluted Fe. Therefore, the corrosiveness can be evaluated using the current value. An acrylic box, in which the temperature can be controlled using flowing cooling water, was prepared inside the thermostat. We attached the ACM sensor to the box to generate a temperature difference between the ACM sensor temperature and the air temperature. Based on the temperature difference between the ACM sensor temperature and the dew point temperature calculated from the air temperature (approximately 15℃) and humidity (approximately 98%), several patterns of results were obtained.

When the temperature difference (dew point temperature – ACM sensor temperature) is less than 0.5 ℃, dew condensation is not visually apparent as represented in Fig. 3 (left), but a weak current is observed. This current is not caused by dew condensation but by a water film formed in a high humidity environment. In the high humidity manhole, it can be said that the environment facilitates corrosion of iron through the water film even if there is no visible dew condensation. When the temperature difference is greater than approximately 0.8℃, the corrosion current rapidly increases as shown in Fig. 4. As the corrosion current increases rapidly, the surface of the ACM sensor becomes covered in mist and dew condenses on the surface of the ACM sensor as shown in Fig. 3 (Right). Due to this, dew condensation is generated when the temperature difference exceeds approximately 0.8℃, and corrosion seems to be promoted. Since dew condensation repeatedly forms, the iron ion concentration in the vicinity of the rebar is kept low, so the corrosion rate increases due to the dew condensation.

Figure 1