The thickness of the protective layer of reinforcing steel can ensure that the reinforcing steel and concrete will not be damaged, and it can block the corrosive medium contained in the surrounding environment outside the concrete structure, and another role is to increase the fitting force between the reinforcing steel and concrete, and the maximum effect of the compressive and tensile properties of reinforced concrete. The maximum effect of the compressive and tensile properties of reinforced concrete.
If the protective layer of reinforcing steel is too thin, corrosive substances are easy to erode the reinforcing steel; if the protective layer of reinforcing steel is too thick, it is a certain guarantee for the durability of the structure, but the original position of the structure will be changed, and the force will be affected; therefore, the thickness of the protective layer of reinforcing steel should be well controlled to improve its safety and durability.
1 Analysis of the importance of reinforcement protection layer
1.1 The combination of reinforcement and concrete mechanism of action
If the physical properties of the materials are analyzed, the tensile strength and compressive strength of reinforcing steel are relatively high, while the compressive strength of concrete is very strong, but its tensile strength is not high.
The common feature of these two materials is that they both have low temperature linear expansion coefficient and good bonding force, which combines these two materials perfectly, and then the whole structural members are made of steel to bear all the tensile strength and concrete to bear all the pressure, which brings out the force performance of both materials, and also allows these two to work together in coordination, and all the external loads are borne by these two materials. All external loads are borne by both materials.
1.2 Bond strength of reinforcement and concrete
There are many factors that can affect the bond strength of reinforcement and concrete, the thickness of the protective layer is the most influential, as well as the strength of the concrete itself, plus the location of the pouring, these are the main influencing factors, so it is very important to control the thickness of the protective layer of reinforcement.
2 Analysis of the significance of the thickness of reinforced concrete
The thickness of the protective layer of reinforcing steel can effectively prevent the reinforcing steel from rusting, and also make the bond strength of the reinforcing steel and concrete have a certain guarantee. This is mainly because the process of heat of hydration inside the concrete will have more calcium hydroxide, forming an alkaline environment, in which there will be a layer of passivation film generated, so that the phenomenon of corrosion of reinforcing steel will not occur. If the thickness of the protective layer of reinforced concrete is too small, then the surface of the concrete will have the phenomenon of carbonation, if this carbonation penetrates the protective layer is eroded to the surface of the reinforcing steel, then the passivation film of the reinforcing steel itself will be damaged, and then the phenomenon of corrosion will occur, and then the body of the reinforcing steel will be found to expand, and finally lead to the phenomenon of cracking of the concrete protective layer.
After the concrete is cracked, some harmful media such as carbon dioxide and oxygen will invade into the protective layer, which will make the reinforcing steel more corroded. This rusting phenomenon will make the section of the reinforcement reduced, and will also make its strength reduced, and then the reinforcement will not be able to work together with the concrete, and will also destroy the integrity of the concrete construction, if serious, the entire structure will be destroyed. Therefore, it is very important to control the thickness of the protective layer of reinforcing steel in reinforced concrete structures.
3 The hazards of irregular thickness of the protective layer of reinforcing steel
The protective layer is too thin and has the danger. Reinforcing steel and concrete to work well together is to have the right reinforcing steel protective layer, which will form the load-bearing capacity to ensure that the concrete in the process of carbonation will not allow the main reinforcement to be affected by carbonation and can last for decades, but also the corrosion of the main reinforcement inside the protective layer of steel to play a role in delaying the corrosion.
Some rebar protective layer is too thin, in the case of indoor or open air, will also be affected by the poor compactness of the concrete, reinforcing steel in the use of a few years will be a problem, the member of the force around the main reinforcement of the protective layer will have fallen off, cracking and other phenomena, the reinforcement will be exposed to the air, rust will be more serious, reinforcing steel members can not be used for a longer period of time.
4 Reasons for deviation of protective layer thickness of reinforcement
4.1 When supporting the mold or pouring concrete, the construction personnel walk and step on the reinforcement at will, and the vibrator vibrates the reinforcement, causing the reinforcement to deviate from the correct position.
4.2 The role of mat block is not understood, and the quality of mat block is not paid attention to. The number of mat blocks is small, and the mat blocks made on site have low strength and are easy to break. The mat blocks of vertical tendons are not firmly tied to the reinforcement and lose their proper function.
4.3 The part of steel reinforcement retaining layer deviation is mostly occurred at the negative moment reinforcement of the plate, this part of the reinforcement is mostly used in the design of fine diameter and small stiffness of Class I steel, which is easier to be stepped and bent.
4.4 Inaccurate elevation control during concrete construction, the thickness of concrete slab is not enough or super thick, resulting in deviation of reinforcement protective layer thickness.
4.5 The size of the reinforcement is not allowed or the position relationship between the reinforcement is not well considered, and the size of the processing hoop does not meet the requirements, which leads to the deviation of the protective layer thickness of the reinforcement in the nodes of columns, beams and slabs, causing the reinforcement to stick to the mold and shift.
5 measures to control the thickness of the protective layer of reinforcing steel
5.1 The control of beam class horizontal members and column, wall plate class vertical members.
To ensure that the protective layer mat has sufficient strength, not to be crushed in the process of tying the reinforcement and pouring concrete and lead to the displacement of reinforcement. In the traditional practice of the mat spacing size and then appropriately shorten. For general civil building components, the mat spacing of 800mm is more appropriate. Other types of buildings, structures, components, must be flexible, depending on the amount of reinforcement in the components.
5.2 plate members positive moment reinforcement.
According to the traditional practice mat spacing size to shorten to about 600mm is appropriate. In addition, the same to consider the strength of the mat, not due to the mat is crushed and the reinforcement moved down. 6-3 plate members negative moment reinforcement. When the drawings will be reviewed, the construction staff should negotiate with the juice unit and agreed, as far as possible, the negative moment reinforcement unified change for ribbed reinforcement, and as thick as possible diameter.
In this way, even after some trampling, because of its whole char 0 degree is larger, compared to the light round and fine reinforcement deformation value to be smaller, but also easy to deformation after the re-check adjustment. Seriously improve the handover inspection between professional teams, and early laying of the necessary channel, to prevent the reinforcement tying forming inspection after passing, and other professional teams stepping on the deformation caused by unqualified, but also for all kinds of managers in the supervision, supervision, sampling, re-inspection work to provide convenient access. Change the traditional point-like “iron horse” support tendons for 2 ~ 3m long “bench” hanging tendons, will be placed vertically above the negative moment tendons, with 12@500mm90. straight hook suspension of negative moment tendons, hook painted with the plate thickness mark. To be poured after the work surface concrete, then the horizontal to engage the hook, vertical lift, placed in the next concrete pouring surface turnover use.
The use of fine stone concrete “composite” mat. The top surface of this pad is cross concave, set the thickness of the plate mark and negative moment bars fixed in one, the spacing can be mastered in 1m left.
5.3 in the balcony, rain cover and other parts of the negative moment tendons more, can be used to add legs of the frame pipe, the negative tendons will be lifted to fix the position of the reinforcement. When the concrete surface is closed, the pipe will be taken out and the concrete surface will be smoothed.
The thickness of the protective layer of reinforcement should be paid attention to in the construction of building projects, and the importance of a reasonable protective layer of reinforcement to the engineering structure should be fully recognized under the premise of correct understanding of the stress mechanism of reinforcement and concrete.
In the construction of the control of the protective layer of reinforcing steel, we should fully consider the situation of the steel in the actual application, and then control its thickness, not too thin and not too thick, otherwise it will affect the application of the reinforcing steel structure, which is a difficult point in the construction, the construction personnel should pay special attention to.