The problem is structural, not cosmetic
Damp on a basement wall is usually noticed as blistering paint and taken for a cosmetic issue. What water does inside reinforced concrete goes much deeper. Concrete protects reinforcing steel from corrosion through its alkaline nature. Persistent moisture and chloride ingress break down that protective layer, and the steel begins to rust.
The mechanics of corrosion
Rusting steel expands in volume. That expansion cracks the concrete cover, more water enters through the crack, and the process accelerates itself. Two things happen at once: the cross sectional area of the steel is reduced, and the bond between concrete and steel weakens. Both lower the building's capacity to carry horizontal loads.
What changes during an earthquake?
An earthquake applies repeated horizontal loads to a structure. What resists those loads is the tensile strength of the reinforcement and its bond with the concrete. A corroded column or shear wall performs below the capacity calculated in the design. The building stands on the assumption that it will withstand the design earthquake, while in reality it no longer has that capacity.
How the industry sees it
IZODER defines waterproofing as part of protecting buildings against earthquakes and draws attention to the importance of correct, complete application. Although Turkey's Building Waterproofing Regulation came into force in 2018, buildings waterproofed under the regulation account for only around 5 percent of the total building stock. Most of Turkey's building stock lacks this protection.
The situation along the Marmara and Black Sea line
Provinces such as Sakarya, Kocaeli, Yalova and Duzce sit in a first degree seismic zone and on alluvial soils with a high water table. The overlap of these two conditions takes waterproofing out of the realm of preference and makes it a requirement. Alluvial soil also carries liquefaction risk, which is another reason why controlling the water around the foundation matters.
What can be done in an existing building?
- If there is damp, salt efflorescence or blistering paint on basement walls, get an assessment without delay
- If you see rust staining and parallel cracking on the concrete surface, reinforcement corrosion may already have started
- If external excavation is possible, water ingress is stopped with drainage and membrane; if not, with injection
- If corrosion has progressed, waterproofing alone is not enough and a structural assessment is needed
Deciding to postpone waterproofing is in effect a decision to lower the building's seismic performance.
Moisture, reinforcement corrosion and load capacity
The load capacity of a reinforced concrete structure depends on the concrete and the steel within it working together. Concrete is normally an alkaline environment and forms a protective layer on the surface of the steel. That layer protects the steel from rusting.
Continuous moisture and contact with water accelerate carbonation of the concrete and destroy the alkaline environment. Once the protective layer is gone, the steel begins to rust. Rusting steel expands in volume, pushes against the surrounding concrete and cracks the cover. More water enters through the crack and the process accelerates by feeding itself.
The result is a reduction in the area of steel in the section and a weakening of the bond between concrete and steel. In other words, the structure can no longer carry the load it was designed for with the same margin of safety during an earthquake.
Why the process advances silently
Reinforcement corrosion develops over years and gives no visible sign for a long time. The first indications are usually these: brown staining on wall or column faces, blistering and spalling of the concrete surface, exposed and rusted reinforcement, and salt efflorescence on basement walls.
By the time these signs appear the process has already been running for some time. Waterproofing should therefore be treated as a preventive measure rather than something to consider once damage shows.
Soil liquefaction and its relationship with water
In saturated, loose, fine grained soils earthquake shaking raises the pressure of the water between the particles. As pressure rises the contact between particles is lost and the ground temporarily loses its bearing capacity. This is called soil liquefaction.
Drainage does not eliminate this phenomenon on its own, because liquefaction concerns a large mass of ground rather than a narrow band around the building. However, a permanently high water load around the structure does contribute to long term deterioration of foundation and basement elements. The two topics must be addressed separately, and one must not be treated as a substitute for the other.
Order of priority in buildings after an earthquake
In a damaged building waterproofing is not addressed on its own. The sequence is generally as follows.
Structural assessment. Whether cracks are structural or superficial, and whether there is any loss of section in load bearing elements, is assessed by a specialist. Waterproofing carried out while skipping this step does nothing but hide the problem.
Structural repair. Section repair and strengthening are carried out where required. Where there is reinforcement corrosion, rust removal and protective treatment take place at this stage.
Cutting off the water source. Active flow is stopped by injection, perimeter drainage is installed or surface water management is put right.
Waterproofing. Only after these three steps can the waterproofing layer work durably.
Material selection at moving cracks
In buildings in earthquake regions cracks are not static, they open and close with seasonal movement and aftershock effects. A rigid coating tears with the first movement in these conditions.
Elastic systems with high crack bridging capacity are therefore preferred in the region. Where injection is used, flexible polyurethane resins are chosen over rigid epoxy at moving cracks. At joints, what is needed is not the main material but a joint tape or bellows profile able to follow the movement.