Time has shown that concrete is not as resistant against ageing as first expected. There are different reasons to this occurrence, and we will here give a brief summary of the main reasons.
Chloride (salt) is often spread on roads and bridges in order to prevent ice from being created on road surfaces. On structures close to water, the salt is brought by wind or sucked in through cracks directly from the ocean. Once inside, the chloride causes corrosion at the rebars. The rebars expand when they corrode and, consequently, crack the surrounding concrete. These damages lead to an increased penetration of water and chloride, which speed up the aging procedure. Ultimately, large pieces of cracked concrete start to fall out. The higher concentration of chloride, the bigger the pieces.
A similar cracking procedure of the concrete is occurring when water and moist inside the concrete freezes and thereby expands. The concrete can only withstand a specific amount of freezing cycles. This means that a concrete structure e.g. a bridge in an area where the temperature fluctuates around the freezing point will be age a lot faster than one a less seasonal climate. Temperature variation around the freezing point is alone enough to completely disintigrate the concrete.
Since concrete is a complex material the penetration of salt and water, and thereby the damages, differ throughout the construction. Some parts can be almost intact while others are seriously and deeply injured. When repairing a damaged construction it is of utmost importance to remove all of the damaged concrete to avoid encapsulation of these parts. Otherwise, hidden weakness in the construction are left out. A proper removal of the damaged concrete will therefore give a surface with various depths.
In certain locations, such as Scandinavia, the temperature fluctuations descirbed above can occur more than hundred times
each year. This has forced countries with these conditions to focus on research regarding this problem, and not only concerning damages, but also development of methods for practical and efficient repair of the damaged parts. This research and development has led to the sophisticated process called Hydrodemolition.
Concrete is naturally basic but in contact with the air a chemical reaction caused by the carbon dioxide (Co2) is decreasing the pH value. When decreasing too low (pH < 9) the corrosion of the reinforcement will accelerate (compared to the chloride penetration mentioned above). When this phenomenon occurs the carbonated concrete must be replaced.