Source: Luban alliance of construction engineering this paper is selected from the construction of protective layer of reinforced concrete components, selection and use of cushion blocks, by Fang bin of Fenghua Transportation Design Institute.
Therefore, the larger the cushion block size is, the weaker the bad section will be.
The reinforcement workers must work on duty.
However, the thickness of the marble cushion block may not meet the design requirements.
Simple connection with reinforcement, self-locking and no falling off.
Good technical disclosure.
At present, this problem has attracted enough attention.
3.
For example, the service life of many bridges is about 15 ~ 20 years, far from reaching the design life.
At the same time, the protective layer can prevent the infiltration of humid gas and water, avoid the corrosion and expansion of reinforcement and damage the concrete, so as to maintain the durability of the structure and ensure its normal safe use function and service life.
At the same time, from the design and calculation model, it will also reduce the effective height of the section and reduce the flexural capacity of the section.
The author expounds how to ensure the thickness of reinforced concrete protective layer from two aspects of protective layer construction and cushion block selection, which is worth learning.
On the premise of adopting mass production and strengthening supervision to ensure construction quality, I personally think it is the best choice at present.
In case of damage and displacement of reinforcement, they shall correct and repair in time to ensure the correct position of reinforcement in the concrete.
4) Fine aggregate concrete fine aggregate concrete is an ideal cushion block material and is recommended.
3) The steel bar head brushed with antirust paint has the advantages of good combination, small volume and little impact on the strength of components.
Problems needing attention in construction.
If it is not well combined with the concrete, or the expansion and contraction performance is too different, it is easy to produce cracks or even fall off around the cushion block, so that the cushion block becomes a bad section with weak strength and a breakthrough point of water vapor erosion.
The fabrication shall be standardized and the size shall be accurate.
If the bonding performance is not good, cracks around the pad will easily cause corrosion of internal reinforcement.
2.
Due to the limited service life, there is no relevant investigation record for reference.
Correct the bad habits during pouring.
Cement mortar has defects in strength, and it is generally better not to use it.
To solve the problem of concrete protective layer, it is now expounded from two aspects: 1.
Therefore, concrete cover is an important structure in reinforced concrete members, but it is easy to ignore.
2) It is well combined with concrete and has close expansion and contraction performance.
After the removal of some formwork, the reinforcement can be found to be exposed, which must be corrected.
Due to the smooth surface of the cushion block and different bonding strength with the surrounding concrete, small cracks are easy to appear around the cushion block; There is a certain contrast between the color and concrete, and the appearance is not very good.
In particular, the transportation vehicles are strictly prohibited to enter.
The size deviation of formwork will also lead to insufficient or excessive protective layer.
2) The number of cushion blocks shall meet the requirements, and the vertical and horizontal spacing of cushion blocks shall be controlled at about 100cm, which shall not be too large or too small.
However, individuals have doubts about the bonding properties and anti-aging properties of plastic blocks and concrete.
3) When the reinforcement diameter is small, the cushion block shall be properly densified.
During reinforcement processing, the setting out size shall be correct, especially for some drawings with dense and complex reinforcement layout, the setting out of reinforcement shall be carried out according to the actual situation after calculation, so as to avoid the inability to install due to the dense reinforcement at the junction point and the size deviation of the protective layer.
Therefore, the construction unit shall carefully carry out the joint review of drawings and technical disclosure, so as to make the construction personnel understand that different construction parts have different requirements for the thickness of protective layer, rather than uniform.
This will cause the displacement of steel reinforcement and the deviation of concrete cover thickness.
These two problems are common in the construction process.
In the previous inspection, it is often found that the reinforcement is offset and sagging, resulting in the thin or even missing protective layer.
At present, there are no specific specifications and requirements for cushion block.
Its disadvantage is that the stiffness is not enough, which is easy to make the reinforcement mesh sag.
At present, the problems of reinforcement corrosion and concrete cracking caused by improper setting of protective layer are common, which seriously reduces the service life of the structure.
It is recommended to be used in non corrosive concealed works.
A movable operation platform shall be set up.
From the author’s practical experience, the following points are summarized for reference: 1) it should have sufficient strength, not easy to break and corrosion resistance.
The concrete protective layer enables the reinforcement to have anchoring force on the reinforcement under the grip of the concrete, so that the reinforcement and concrete can bear the force together, and the steel-concrete structure can play a normal function and role; The concrete surface with too thick protective layer is prone to cracks.
II.
Use of cushion blocks.
The dimensions meet the requirements.
2 type and selection of cushion block generally speaking, concrete, cement mortar, marble, granite, plastic cushion block and steel bar head brushed with anti rust paint can be used as cushion blocks, which are also more used materials.
1.
The thickness of protective layer refers to the distance from the outside of reinforcement to the concrete surface.
1) Cement mortar is ideal in combination with concrete and color, but the cushion block shall generally not be lower than the strength grade of concrete.
Pay attention not to use bamboo and wood materials such as formwork leftovers, and pay attention to the selection of size according to the requirements of different structures.
2) The construction personnel shall not walk on the beam and slab reinforcement at will and trample on the reinforcement mesh at will.
However, the effect on the safe service life of buildings and the waste of social resources is huge and shapeless.
1) during construction, especially in the parts with dense reinforcement, due to the difficulty of vibration, the operators sometimes pry the reinforcement with a vibrating rod or use the vibrating reinforcement to transmit the vibration force.
There shall be self-locking devices such as grooves to facilitate the fixing of reinforcement and prevent sliding.
However, the commonly used square cushion block shape should be improved and the self-locking function should be added, as shown in the left figure.
Therefore, attention shall be paid to the fabrication and installation of formwork.
Selection and use of cushion block 1.
3) The color shall be consistent with or close to the concrete, and the volume shall be as small as possible.
Personally, I think this is easy to be ignored.
Personally, I believe that firmness, durability, beauty and applicability are the most basic requirements of all buildings, and durability needs a long test, which is easier to be ignored and the most difficult to trace.
In addition, the anti rust effect directly affects the service life of the members, which is difficult to supervise and is not recommended, especially on the flexural and tensile members.
When the height of the structure is greater than 1.5m, the specification of concrete protective layer cushion block should be 10cm × 10cm × Thickness of concrete cover.
Under the joint action of overloaded vehicles and other reasons, some even need to be rebuilt.
The thickness of reinforced concrete protective layer depends on many factors, such as the environmental category of concrete structure, the strength grade of concrete, the specification of reinforcement and the type of concrete components.
4) When fine aggregate concrete is used, the cushion block is generally one grade higher than the concrete strength grade, and the specification can be 7cm × 7cm × Thickness of concrete cover.
Concrete cover is one of the most important factors to maintain structural durability in reinforced concrete components, especially in construction…
Attention shall be paid to the use of cushion blocks: 1) cushion blocks of different sizes shall be used for different construction parts.
2) Marble and granite leftovers have high strength and easy to obtain.
The combination of cushion block and concrete is difficult to achieve the same effect as the overall pouring.
5) Compared with the traditional cushion block, the plastic cushion block has the following advantages: it has the characteristics of standardization, finalization and universality, and the quality is stable; The thickness of the protective layer can be adjusted by adjusting different placement angles, which reduces the uneven thickness caused by human factors and is convenient to use; The cushion block has the advantages of light weight, small volume, high strength and corrosion resistance; The support is stable and not easy to fall off.
If there is a certain contrast between the color and concrete, the appearance is not very good.
