Steel corrosion and concrete freeze-thaw cycle

Corrosion of steel bars and freeze-thaw cycles of concrete can cause damage to the structure of concrete. When the steel bars are corroded, the rust spreads, causing the concrete to crack and the bond between the steel bars and the concrete to be lost. When water penetrates the concrete surface and enters the interior, the volume of the frozen and condensed water expands. After repeated freeze-thaw cycles, the concrete is cracked and deepened at the microscopic level, so that the concrete is crushed and the concrete is permanently irreversible. damage.

In wet and cold weather conditions, reinforced concrete pavements, bridges, parking lots and other building structures that may use deicing salt should be made of epoxy resin or hot dip plating, stainless steel reinforcement and other materials. Epoxy rebar can be easily identified by a light green coating on the surface. A cheaper method is to use zinc phosphate as a rust preventive coating for steel bars. Zinc phosphate reacts with calcium ions and hydroxide ions to form stable hydroxyapatite. Waterproof materials are also used to protect reinforced concrete, such as non-woven geotextiles with interlayers filled with bentonite. Calcium nitrite Ca(NO2)2 is used as a corrosion inhibitor and is added in a ratio of 1-2% relative to the weight of the cement to protect the steel from corrosion. Because nitrite ion is a mild oxidant, it is combined with ferrous ions (Fe) on the surface of steel to precipitate insoluble iron hydroxide (Fe(OH)3). Kunshan concrete

Carbonation

To be correct, it should be called carbonation, and it is customary to call it carbonization. The pore water in concrete is usually alkaline. According to the Pourbaix diagram, the steel bars are inert at pH values greater than 11, and no corrosion occurs. The reaction of carbon dioxide in the air with the alkali in the cement makes the pore water more acidic, thereby lowering the pH. From the time the component is made, carbon dioxide will carbonate the concrete on the surface of the component and will continue to deepen. If the component cracks, the carbon dioxide in the air will more easily enter the interior of the concrete. Usually during the structural design process, the minimum thickness of the protective layer of the steel is determined according to the building code. If the carbonization of the concrete weakens this value, it may cause structural damage caused by the corrosion of the steel.


The method of testing the degree of carbonization of the surface of the member is to drill a hole in the surface thereof and drop it with phenolphthalein. If there is no carbonized portion, it will become pink. By measuring the depth of the crucible without discoloration, the depth of the carbonized layer can be known.


Chlorination corrosion


Chlorides, including sodium chloride, corrode steel bars in concrete. Therefore, only water can be used when mixing concrete. It is also prohibited to use salt to de-ice the concrete pavement.


Alkali aggregate reaction


Alkali Aggregate Reaction (AAR, or Alkali Silica Reaction, ASR for short) means that when the alkali of the cement is too strong, the amorphous silicon component (SiO2) in the aggregate dissolves and dissociates. In high pH (12.5 - 13.5) water, it reacts with hydroxide ions in cement to form silicate, which reacts with calcium hydroxide in cement to form hydrated calcium silicate, causing uneven expansion of concrete, leading to cracking damage. . It occurs under the conditions that (1) the aggregate contains relevant active ingredients - amorphous silica; (2) there are enough hydroxide ions in the environment; (3 concrete has sufficient humidity, relative humidity is greater than 75 %. This reaction is called cancer of concrete, and it is reflected in concrete whether or not reinforcing steel is strengthened. For example, concrete dam. Suzhou concrete


Crystal transformation of high alumina cement


High alumina cement is resistant to weak acids, especially sulfates, while early strength increases rapidly, with high strength and durability. It was widely used after the Second World War. However, due to the transformation of internal hydrate crystals, its strength will decrease with time and become more serious in hot and humid environments. In the United Kingdom, with the collapse of three roofs using high-aluminum prestressed concrete beams, the cement was banned in the local area in 1976, although it was later proved to have manufacturing defects, but the ban was retained.


Sulfate corrosion


Sulfate in groundwater reacts with Portland cement to form expansive by-products such as ettringite or thaumasitein leading to early failure of concrete.


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