The thickness of the concrete protective layer is insufficient, and the external harmful medium easily penetrates the protective layer to reach the surface of the steel bar, causing the steel bar to rust and swell, the concrete tending to crack, or even falling off, thereby accelerating the corrosion of the steel bar. In the coastal areas, the reinforced concrete structure caused by the insufficient thickness of the concrete protective layer is destroyed much faster than the inland areas. For example, there are quite a few reinforced concrete structures in the water conservancy projects in the coastal areas of our province. Due to the insufficient thickness of the protective layer, they often run for less than ten years. In the shortest period of only two or three years, the steel bars have been completely rusted, and the concrete protective layer is peeled off in a large area. Extremely threatened.

It is precisely because of the above reasons that the Ministry of Communications industry standard "Seaport Engineering Concrete Structure Anti-corrosion Technical Specifications" (JTJ275-2000) is lower than the national standard "Concrete Structural Engineering Construction Quality Acceptance Specification" (GB50204-2015) for the minimum thickness of concrete protective layer higher requirement.

The thicker the concrete protective layer, the more favorable the structural durability, but under the condition that the structural section size is constant, the effective section size of the component is reduced and the bearing capacity is reduced. Therefore, the steel cage must be made strictly according to the design drawings during the construction process. The concrete protection layer is too thin and too thick, which will cause major safety accidents. The Code for Acceptance of Construction Quality of Concrete Structures (GB50204-2015) has strict requirements on the allowable deviation of the thickness of the protective layer: the allowable deviation of the thickness of the protective layer of the plate is (+8 mm, -5 mm), and the beam type The allowable deviation value is (+10mm, -7 mm).

According to the statistics of many years of testing data, the thickness of the protective layer of the water conservancy project in our province is less than 30%, and the thickness of the bottom and the bottom of the protective layer is negatively deviated by more than 80%; the reinforced concrete structure with large section height, side It has a positive deviation from the top protective layer thickness of 80% or more. This leaves a hidden danger to the safe operation of the project.