Safeguarding structures from high levels of salt
Building materials are at risk from sea salt and high levels of salt in the underground
Concrete is a highly versatile building material, but it has two arch enemies: chlorides and carbon dioxide. For bridges, bridge piers and tunnels on the Arabian Peninsula made of reinforced concrete, these dangers double up, since the materials are at risk from sea salt and high levels of salt in the underground.
All year long, the salty sea air from the Gulf blows inland. Through atmospheric humidity and precipitation, the ocean salts penetrate into the concrete’s porous structure and cause immense damage. In 2011, Saudi Arabia spent more than $14bn for corrosion protection and repair of infrastructure projects, more than any other country in the world. Consequently, bridges and infrastructure projects in the region need special protection in order to increase their longevity and decrease the cost of repair.
Salt water can cause considerable damage to reinforced concrete. Once the salt has entered the concrete, the continuing capillary absorption of water slowly transports the chlorides to the steel reinforcing rods which, as a result, start to rust. Since corrosion products require more space than the metal, the reinforcing steel expands causing the concrete to spall.
New moisture draws the salts further into the structure’s core. “Water molecules give the deposited chlorides a change to penetrate deeper into the concrete,” explains Anoop D’souza, Regional Sales Manager at Wacker Chemicals Middle East. “The process is gradual and in some cases only becomes visible after several years.”
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.
The benefits of Silane
The most efficient way of protecting concrete is to drastically reduce water uptake. Without water, steel will no longer corrode even in carbonated concrete. 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 other substance classes in their resistance to UV radiation, thermal stress, aggressive substances and microbiological influences. While they efficiently penetrate into the concrete, they protect the material against external influences without sealing it. The building material can continue to release water vapour 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 D’souza. “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 specialised in this area, for example, a civil engineering consultant,” says D’souza.
A detailed examination with magnets and ultrasonic and radar testing is able 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 D’souza.
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.
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. 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.
Preparing the site
Prior to the actual application via the airless technique, WACKER 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 atomise and lead to wastage, while too little pressure would result in uneven application. As a result, lumps would 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, as the silanes cannot penetrate sufficiently.
Further problems can arise, if the building-component temperature is 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, 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.
Deep impregnation with water-repellent chemicals is able to protect intact concrete against water and salts for many years and is much cheaper than carrying out full repair work at a later stage. Scientific studies show that by using this method, bridge piers need to be repaired far less frequently and in some cases not at all. There are several projects in southern Germany, Japan, China, and in the GCC where the use of water-repellent chemicals have proved their benefits.
“Many projects in the Gulf region are built close to the sea which exposes them to high chloride ingress,” says D’Souza, “Therefore, it is important to protect structures, bridges and tunnels from the penetration of chloride. Inland structures need an equal level of protection in order to ensure long term durability and construction sustainability.”
To cater to the special protection needs in the region, WACKER tests special silane formulations at its Technical Centre in Dubai. This way it is possible to adapt them to the specific construction materials and needs of the region.
“Although the application conditions vary greatly with location and climate, hydrophobic impregnation is in demand around the world,” says D’Souza. “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 and WACKER’s silane treatment protects structures even under extreme loads.”
Thus, reinforced concrete structures in the Arabian Peninsula are often exposed to an aggressive mixture of salt and water and high levels of salt underground. Without protection, these salts can penetrate deep into the concrete and cause damage, to the reinforcement as well as the fabric of bridges, bridge piers and highways. A water-repellent treatment, with special hydrophobic silanes, can offer effective and long-lasting protection for many years.
