Features of reinforced concrete corrosion processes in aggressive environments and methods of corrosion protection of building structures

Abstract

The article deals with the production of concrete with high corrosion resistance in the most common environments and in sulfate, chloride, acid, etc. It is shown that the corrosion resistance of concrete is determined by two main indicators - a slight permeability to an aggressive environment and positive reactivity with the components of an aggressive environment. Quantitative values of these indicators are given. The use of concrete with high corrosion resistance makes it possible to create concrete and reinforced concrete structures with high durability in aggressive environments with out the use of additional (secondary) protection against corrosion. Respectively, on average it is 1300, 1850 and 2700 mg/dm³. In addition, these values may vary depending on the diffusion coefficient of chlorine ions. It is shown that with a change in the pH of the environment (from 3.5 to 1.0), which is in contact with the concretere inforcement, the rate of concrete corrosion changes from 0.3 to 20 mm/year, respectively. The essence of corrosion processesin a sulfate environment lies in the structure of concrete, gypsum and hydrosulfoaluminates with an increased volume of solid phases, which causes the appearance of internal stresses exceeding the strength of concrete, which inevitably leads to the destruction of concrete. Sulfate corrosion is observed under the action of various sulfate solutions or when using aggregates that contain gypsumim purities (sulfide-containing aggregate in concrete). Classic measures to prevent sulfate corrosion are the use of sulfate-resistantce ments that contain a small amount of aluminates and tricalcium salt, the use of mineralad mixtures that bind the calcium hydroxide of the cement stone into low-base calcium silicates. Reducing the permeability of concreteto SO₄²-ions coming from an aggressive environment is an effective means of protection against sulfate corrosion. This is achieved by introducing admixture complexes into the composition of concrete, including water-reducing and mineral iones. An example can be the use of concrete modifiers of the MB series. Such similar complexes make it possible to obtain highly sulfate-resistant concreteon or dinary medium-aluminate portlandcements, which eliminates the need to use deficient sulfate-resistant portlandcement, which ensures high corrosion resistance of concrete. Increasing the water proofing grade of concreteto W8 and above prevents corrosion damage to concrete. However, this applies to highly diluted solutions of acids. Notice able corrosion was observe din ordinary concretes of W4 water proofing grades at pH 6.5 and below, in particularly low-permeability concretes of W10 - W12 grades at pH 3.5 and below. Inrecent decades, two destructive processes have attracted the attention of researchers – the late formation of ettringite and taumansite. Late formation of ettringite (in hardened concrete) in the absence of exposure to an aggressive sulfate environment was observed in cement concrete with an unbalanced content of aluminates and sulfates in the cement. In the case when the concrete hardens at an elevated temperature, the monosulfate form of hydrosulfoaluminate is mainly formed. At the next moment, the monosulfate form can be transformed into the trisulfate form with the addition of an additional amount of water and an increase in volume.