George EL DAOUD
Importance of Hydration Process as Protection Against Corrosion
Fix Concrete Technologies experts are aware of importance of concrete quality on every stage of the work, which leads to long-lasting concrete. We call it durability engineering. Corrosion mechanism of steel exposed to the atmosphere is not the same as in reinforced concrete. Cover makes the difference. Chemical environment of the steel changes significantly in the concrete.
Concrete cover performs several functions. With respect to corrosion, the primary function is to serve as protection against environment exposure. Its effectiveness depends on chemical composition and concrete structure, specially on:
· total porosity,
· pores size,
· pores distribution,
· degree of pores interconnection.
Let me recall a few basic principles of how water, cement and sand become concrete. Concrete is a hardened composite material composed of fine and coarse aggregates (impermeable and mostly chemically inactive) embedded in a matrix of hydrated Portland cement (pore solutions are chemically active). As explained in previous blog, corrosion is electrochemical process. So naturally effectiveness of concrete cover against steel corrosion depends on the cement paste, its pore structure and composition of pores solution.
Few words about cement paste. Its structure develops as a result of chemical reaction between Portland cement and water in hydration process. When concrete sets, residual water is enclosed in the pores of new material and some of the water is used to form hydration products. As curing proceeds, it can be replaced by the curing water. Volume of hydrated products is greater than a volume of cement grains. As a consequence, free spaces are filled with hydration products.
Degree of cement paste space filling depends on water/cement (W/C) ratio. In general, the lower w/c, the lower porosity. While hydration proceeds, two different types of pores are developed: capillary pores and gel pores. Capillary pores are larger and not filled up with hydration products. Gel pores are relatively smaller and developed within particles of hydration products. As indicated on the drawings, in low w/c ratio concretes, the paste structure is much denser and more impermeable, comparing to high w/c ratio concretes. Naturally low w/c ratio concretes are stronger and less permeable.
The water in the pores after hydration process is rather pore solution which contains dissolved alkali and hydroxide and other ions derived from the cement. Reinforcement embedded in the concrete is in contact with pore solution. Stable passivating film, which protects against corrosion, is formed and maintained on the steel surface immediately during hydration process.
Corrosion will start once passivating film is destabilised. Sometimes ions other than expected may be found in the concrete. The ion of the greatest concern is the chloride Cl- ion. There are different sources of chloride ions, which will be discussed subsequently in dedicated post.
Concrete cover is a barrier for chloride ions to migrate inside, however its effectiveness depends mainly on low permeability, high density and thickness. Additionally, it is essential to maintain good quality of workmanship on the site. Fix Concrete Technologies supervisors are aware of the importance of proper quality in this process, so our products are by definition more durable. It is very important to remember, improper curing (cracks) and inadequate compaction (honeycombing) can totally destroy expected positive impact on corrosion protection from properly designed concrete mix.