While being an essential component of concrete, water in excessive quantities will provoke damages that can be very hard to repair. Exposure to moisture can also compromise the life span of concrete structures and the resistance of the materials over the years. Some of the circumstances that are likely to affect the durability of the structure are:
- Water standing in the concrete for too long
- Leaks of water
- Groundwater with poor drainage
- Scarce protection for the structure
- High humidity levels
If underestimated, the destructive power of water can cause a number of dangers:
Steel reinforcement corrosion
Corrosion takes place when water and oxygen stay in touch with steel for a prolonged amount of time. Without one of these components, the damages of corrosion will not occur: dry steel, for example, won’t corrode on its own if not when in contact with moisture. When water damages happen, prompt intervention with the proper drying procedures is essential to stop the corrosion from taking place. Additionally, impermeability of concrete can be a crucial factor in preventing corrosion of the reinforcement steel lying underneath.
Another element that protects steel from corrosion is the reinforcing bars, as they develop a passive layer functioning as a barrier for the steel thanks to the high alkalinity of the concrete. In these circumstances, the reinforcing steel is granted strong protection from corrosion.
However, the durability of this layer can decrease fast in time with unfavourable climate conditions. Carbon dioxide, in particular, along with bicarbonate ions, causes carbonation and lowers the pH of the concrete which can slowly break the passive layer. The calcium carbonate operates within the aggregates dissolving and decreasing the strength of the concrete, until it crumbles down to pieces. The progression of the damage and the acidification of the concrete will eventually get to the steel reinforcement, starting a process of corrosion that will make it expand, too. This is often caused by salty elements as well as acid rains. The process of carbonisation is slow, and is affected by the concrete’s density and the humidity to which it is exposed.
Humidity levels also directly affect the pH in concrete: the higher the humidity, the higher the pH and the temperature of the concrete. When the pH of the concrete raises to higher levels, so will the chances that the bonds between parts will fail.
There are several substances that can interact in different ways with concrete. Low quality concrete, especially, can have small holes all over that will allow access to more water with its corrosive substances. The deeper these substances go, the more materials they can damage.
Chlorides, for instance, will destroy the layer of protection formed around the rebars over the steel reinforcement. In absence of an iron oxide protection film around the steel, the corrosion process will start to take place.
Another corrosive substance is the Sulfate contained in dissolved forms in the water that makes it through the concrete.
The sources of Sulfate attacks can be:
- High-sulfate lands
Sulfate attacks can be very dangerous, changing the concrete’s composition and microstructure. This will cause cracking of the concrete, which can sometimes be very extended; expansion of the mass can also be another dangerous consequence, as well as the bond failure between aggregate and cement paste.
Along with corrosive substances, pores in concrete also make way for microbial growth. Bacteria, microbe and mold grow best in environments with high humidity and temperatures. Despite the fact that concrete does not provide organic materials in sufficient quantities for the mold to be able to feed itself, its pores can trap lots of organisms that can be food sources for mold to grow, such as dust, pollen, microorganisms and salts. The growth of mold also involves the release of its acids which can cause serious structural damages to the strength and the durability of the materials.
Deterioration of the concrete
Cracking and expansion of deteriorating concrete constitute another serious damage caused by water. This slow process is caused by alkali hydroxides reactions in aggregates: cracks will facilitate the intrusion of water, which will penetrate through the concrete and make it to the reinforcement bars, initiating corrosion process even in high-grade buildings.
There can be two different forms of alkali reactions in aggregates depending on the element present in the concrete aggregates:
Alkali-Silica reaction (ASR)
Alkali hydroxide present in concrete reacts with silica, which can be alarming as aggregates often contain silica elements.
The reaction results in the creation of a gel that absorbs the water in its surrounding environment, whether that is stuck in the concrete or in the concrete paste; the gel then swells up, increasing in volume and causes expansive pressure that will eventually make the concrete crack.
Cracking of the concrete is one of the most common indicators that an alkali reaction with silica is in course.
Alkali-Carbonate reaction (ACR)
This type of reactions is more rare: carbonate materials tend to be less commonly used in concrete and aggregates, as they are normally unsuitable for construction purposes. The reaction with carbonate causes similar damages as the alkali reaction with silica, involving expansion pressure with subsequent cracking.
Water that is stuck in the aggregates can represent a high-risk to the life span of non-air entrained concrete. In fact, if temperatures decrease enough that the water freezes to ice, its mass will expand by 9% its original size and will start occupying more room. This unpredictable increase in volume cannot be sustained by the structure as there is no space available for the expanding water and will lead to distress for the concrete, the most obvious consequence being cracks.
Furthermore, the formation of thaws will make the ice defrost some bits into water, which will penetrate further down the concrete cracks, freeze again and expend again, increasing the amount and the sizes of the original cracks. This cycle will keep going causing greater damage to the concrete. Spalling, scaling and exposed aggregates are evident sign of a freeze and thaw cycle that is in course of action within the concrete.
Keeping water, humidity and wetness away from the concrete, or properly drying the materials as soon as the water damage took place, can prevent any damage to the structure in order to keep the building’s strength and preserve its original life span.