by This article is selected from “Construction of Reinforced Concrete Component Protection Layer, Selection and Use of Cushion Blocks” by Fang Bin from Fenghua Traffic Design Institute. The author elaborates on how to ensure the thickness of the reinforced concrete protective layer from two aspects: the construction of the protective layer and the selection of cushion blocks, which is worth learning..
The concrete protective layer enables the steel bars to be wrapped in concrete and has anchoring force on the steel bars, allowing them to bear the force together with the concrete and allowing the steel-concrete structure to function normally; However, if the protective layer is too thick on the concrete surface, it is easy to produce cracks. At the same time, from the design calculation model, it can also lead to a decrease in the effective height of the section, reducing the bending bearing capacity of the section. At the same time, the protective layer can prevent the infiltration of humid gases and water, prevent the corrosion and expansion of steel bars and damage the concrete, thereby maintaining the durability of the structure, ensuring its normal safe use function and service life..
Therefore, the concrete protective layer is an important structure in reinforced concrete components, but it is also an easily overlooked part. Currently, it is not uncommon for steel corrosion and concrete cracking to occur due to improper protective layer settings, which seriously reduces the service life of the structure. For example, many bridges have a service life of around 15-20 years, far from reaching the design life. Under the combined action of overloaded vehicles and other reasons, some even need to be rebuilt..
To solve the problem of concrete protective layer, we will now elaborate on two aspects: construction of protective layer and selection of cushion blocks:.
The thickness of the protective layer refers to the distance from the outside of the steel reinforcement to the surface of the concrete. The thickness of the reinforced concrete protective layer depends on many factors such as the environmental category in which the concrete structure is located, the strength grade of the concrete, the specifications of the steel bars, and the type of concrete components..
Therefore, the construction unit should carefully conduct drawing review and technical disclosure work, so that the construction personnel understand that different construction parts have different requirements for the thickness of the protective layer, rather than necessarily being uniform. When processing steel bars, the layout dimensions must be correct, especially for some densely arranged and complex drawings. The steel bars must be calculated and laid out according to the actual situation to avoid installation difficulties and deviation in the size of the protective layer due to the dense distribution of steel bars at the junction. The deviation of template size can also lead to insufficient or excessive protective layer, so attention should be paid to template production and installation, with standardized production and precise dimensions..
1) During construction, especially in areas with dense steel bars, due to the difficulty of vibration, operators sometimes use vibrating rods to pry the steel bars or use vibrating steel bars to transmit vibration force. This will cause displacement of the steel reinforcement and deviation in the thickness of the concrete protective layer..
2) Construction personnel should not walk on the steel bars of beams and slabs at will, and should not step on the steel mesh at will. Especially, transportation vehicles are strictly prohibited from entering. A movable operating platform should be set up, and steel workers must follow the shift. If any damage or displacement of the steel bars is found, they should be corrected and repaired in a timely manner to ensure the correct position of the steel bars in the concrete..
These two issues are quite common in the construction process, and in previous inspections, it was often found that the steel bars were misaligned and sagging, resulting in a thin or even missing protective layer. Some templates can be removed and the steel bars are exposed, which is a problem that must be corrected..
At present, specific specifications and requirements for cushion blocks cannot be found. Based on the author’s practical experience, the following points have been summarized for reference:.
1) Should have sufficient strength and not be easily damaged.
