Acid also Impacts Concrete
Another danger threatens from the air: so-called carbonation. Here, atmospheric humidity and rainwater carry carbon dioxide from the atmosphere into the concrete pores. Carbonic acid forms, which then turns into calcium carbonate. The otherwise alkaline concrete thus becomes increasingly acidic from the outside in. If this process reaches the steel, it loses its corrosion resistance and, in the presence of moisture and oxygen, begins to rust.
“Although water is important in preparing concrete, it can also be destructive by acting as a carrier medium,” says Hiroshi Kanzawa of Wacker Asahikasei Silicones (AWS). In all examples, the building material usually absorbs the harmful substances when it comes into contact with water. “Thus, the most efficient way of protecting concrete is to drastically reduce water uptake,” emphasizes Kanzawa. Without water, steel would no longer corrode even in carbonated concrete.
Silanes Have Proven Themselves
The biggest construction company in Japan, Kajima Corporation is also involved in the new highway project. “Here, we rely on the principle of hydrophobic impregnation,” says Dr. Daisuke Hayashi, research engineer of Kajima Technical Research Institute. The water-repellent protective zone created by this process significantly reduces the uptake of harmful substances. Silanes with long alkyl chains, such as isooctylsilanes, have proven to be the ideal product group here. Silanes outperform rival substance classes in their resistance to UV radiation, thermal stress, aggressive substances and microbiological influences. While they efficiently penetrate into the concrete, they do not fully seal it, but only protect it against external influences. The building material can continue to release water vapor from the inside and so dry out.
To unfold its effect, the process draws on the concrete’s material properties: “Silanes form extremely stable bonds with the silicate matrix of the pores and capillary walls,” says WACKER building-protection expert Auer. The protective molecules resemble conventional quartz molecules with an additional organic group. This makes the protection particularly durable and the hydrophobic effect lasts for decades.
However, to achieve optimum results, users must observe a number of conditions. “Before any restoration work is carried out, we recommend that an accurate analysis of the structure’s condition be conducted by professionals specializing in this area, e.g. a civil engineering consultant,” says Auer. The structure is given a detailed examination with magnets and ultrasonic and radar testing to determine the temperature and moisture content of the concrete and the ambient air, the depth of carbonation and compressive strength, as well as the depth of the reinforcing steel. For older concrete structures, the chloride content deep down in the material can also be determined. “In special cases, an extracted drill core can be examined in the lab,” says Auer. Once the structure’s condition has been ascertained exactly, a civil engineering consultant plans the appropriate repair measure and, normally, puts out an invitation to tender.
Spray or Flood
There are liquid as well as cream-like products available for the hydrophobic impregnation of concrete. Creams can be applied with so-called airless spray guns. These instruments have a suction tube that can be immersed directly in the product container and make it possible to meter the active ingredient exactly. However, the main advantage of spraying cream-like products is that the application can be carried out in a single step.