Protection for Steel Reinforcement - Wacker Chemie AG


Protection for Steel Reinforcement

Road salt, rainwater, atmospheric humidity and carbon dioxide can seriously damage reinforced concrete structures. That’s why, especially for large infrastructure projects such as bridges, highways and road tunnels, engineers rely on hydrophobic impregnation with silanes. This is also the policy of Kajima, the biggest construction company in Japan.

For test purposes, a silane creme is applied to a concrete cube using a brush at a Burghausen applications technology laboratory.

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.

“The most efficient way of protecting concrete is to drastically reduce water uptake.”

Hiroshi Kanzawa, Wacker Asahikasei Silicone, Japan
The 346.8-kilometer-long Tomei Expressway is part of the Asian Highway 1, which spans 20,557 kilometers from Tokyo through South and North Korea, China, Southeastern Asia, India, Iran and Turkey and over the Bosporus.

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.

Built by Kajima, the Toyota Arrows bridge on the Shin-Tomei Expressway between Tokyo and Nagoya was hydrophobically impregnated with silanes in 2004.

Pressure and Dosage Decisive

Liquid products, on the other hand, are applied by “flooding.” This means that the product is applied to the wall under very low pressure. It runs down the wall, soaking into it as it goes. For liquid products, several work steps (two to three) are often required in order to apply sufficient material.

Prior to the actual application via the airless technique, WACKER building-protection expert Auer recommends carrying out spray tests to find the right application pressure for the relevant product, environmental and substrate conditions. Too much pressure would cause the product to atomize and lead to wastage, while too little pressure would result in uneven application – lumps form on the surface.

Beside material dosage and active-ingredient concentration in the water repellent, there are a number of other factors that determine the silanes’ penetration depth, including weather conditions and the porosity and moisture content of the concrete to be treated. If the moisture content is greater than four percent, for example, treatment must not proceed – according to the German technical contractual conditions and guidelines for civil engineering structures – as the silanes cannot penetrate sufficiently. Further problems can arise if the building-component temperature is too low – below five degrees Celsius. At such low temperatures, there is a risk of water condensation on the component, preventing the effective penetration of the water repellent.

For quality control purposes, in Germany, reference surfaces must be prepared prior to the actual application to test how the hydrophobic impregnation works on the concrete to be treated. Among other things, this test area is used to determine the amount of water-repellent agent needed. Normally, two or three test surfaces with different dosage are prepared; exactly 28 days later, service engineers determine the quality of the water-repellent treatment. Only then do they decide what dosage to apply to the concrete structure.

Best Protection even under Extreme Conditions

Although the application conditions vary greatly with location and climate, hydrophobic impregnation with silanes is in demand around the world. “In Japan alone, 125 million square meters of concrete surfaces have been treated since the market launch,” says Wacker Asahikasei manager Kanzawa. Especially in the case of calls for bids for expensive infrastructure projects, contracting authorities increasingly demand that the results should be durable – renovation cycles that are too short could ruin a bidder’s chances. The operator didn’t give the construction of the new highway beside the Tomei Expressway the motto “100-year road” without good reason.

Whether roads and highway bridges in Japan, harbor piers in Dubai, tunnels in Europe or dams in China – WACKER’s silane treatment protects structures even under extreme loads. All around the world.