Snow-capped Mount Fuji is a majestic landmark in central Japan. From the sacred mountain’s summit, you can see a vista of tea plantations and rice paddies stretching as far as the Pacific coast. Through this landscape, the Tomei Expressway winds like a gray dragon. This main traffic link between Tokyo and Nagoya will, over the coming years, be relieved by the construction of a parallel highway. The motto chosen by the operator, NEXCO (Nippon Expressway Company), for this project is “100-year road.” The use of modern technologies and innovative protective measures should allow future generations to profit from these essential routes, too. In this earthquake-prone country, the safety and durability of a structure play a key role. That’s why, to be fit for the future, this billion-dollar construction project requires innovative concrete technologies on top of sophisticated tunnel work and complicated bridge building.
Damage due to Salt and Water
“Concrete is a highly versatile building material, but it has two arch enemies: chlorides and carbon dioxide,” explains Dr. Noboru Sakata, General Manager of Kajima Corporation. For the Tomei Expressway, these dangers double up, since the materials are at risk from road salt in melt water and road spray, as well as sea salt. All year long, the salty sea air from the Pacific blows inland. Through atmospheric humidity and precipitation, the ocean salts penetrate into concrete’s porous structure. Once the salt has entered the concrete, the continuing capillary absorption of water slowly transports the chlorides into the material’s interior. By the time the sodium chloride finally reaches the steel reinforcing rods, it’s usually already too late. The steel has started to rust. Since the corrosion products require more space than the original metal, the reinforcing steel expands and, in time, the concrete starts to spall.
In winter, on the other hand, large amounts of water mixed with road salts splash from the road onto the bridge and its piers, penetrating into the concrete. Only a few millimeters deep, at first. However, the next time it rains, new moisture will draw the salts further into the structure’s core. “Water molecules give the deposited chlorides a “piggyback” ride ever deeper into the concrete,” explains Dr. Dominik Auer, head of an applications laboratory for Construction Chemicals at WACKER in Burghausen. This is a gradual process, which only becomes visible after several years, but is all the more conspicuous then. In addition, there is freeze/thaw damage. Frozen water expands in the concrete capillaries, causing the surface to spall.