The strength grade of concrete means the compressive strength of concrete. The regulations of our country stipulates that the strength grades of concrete structure are C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, 80, totally 14 levels. Concrete of strength grade higher than C50 is called high-strength concrete. The concrete strength grade of reinforced concrete structure should not be lower than C15. In our country, grades of commodity concrete strength are mainly C30 and C40. If the strength of the concrete cannot meet the standards and requirements, not only will affect the construction quality, also will leave the building safe potential risks, threatening the usage of the building. There are many reasons leading to low strength of structure concrete,
1. Reasons leading to low strength of structure concrete
1.1 Maintenance of structural concrete cannot meet the requirements
Cast-in-place floor is covered by plastic film in the pre-process in case of crack in the early stage. When it is used in practice, constructor will uncover film and water for maintenance before setting out construction. However, insufficient maintenance personnel and unreasonable ways in the water maintenance lead to low strength of cast-in-place floor.
1.2 Low accuracy of “technical specification for compressive strength of concrete for rebound detection”
In the process of structural concrete pouring construction, construction units choose technical specification for compressive strength of concrete for rebound detection as a standard. But the accuracy of this standard is lower than that in nowadays concrete construction technology.
Non-standard drill core testing operation leads to inaccurate result
Drill core testing operation is the main way to test the strength of concrete. But when used in practice, it has to be operated under strict requirements.
2 Countermeasures to the low strength of structure concrete
2.1 Make a preparation of early maintenance in the process of concrete construction
It has several requirements in the following ways:
First, the better maintenance in the early stage contributes to a nearly standard maintenance condition;
Second, with more sufficient maintenance in the early stage, the stability and crack resistance of structural concrete can be further improved, which will prevent structural concrete from cracking when it is put in use.
Third, early maintenance also has a significant impact on the carbonation of structural concrete, which is to say that better maintenance means slower carbonation. In this case, the service life of structural concrete can be greatly extended.
2.2 New technology for early maintenance of structural concrete
（1）Early maintenance of concrete slab
A new method of using plate and thatch covered by plastic film will be used in the early maintenance of concrete slab. The basic requirements of this method are as follows: After the concrete pouring, we should use a vibrator to help mix concrete, then put a plate on the flat concrete, and roll the plastic film on the thatch from front to back. It should be mentioned that the beginning and end of the film should be pressed by the plate in the process of flattening, which can make sure they will be glued together with concrete. This method can prevent insufficient maintenance resulted by natural causes.
（2）As for the maintenance of concrete columns and shear wall, it should be noted that once the protection film is removed, the plastic film must be immediately wrap on them. But before wrapping, construction personnel must water the surface of columns. When wrapping they need to ensure that it’s tight so that the water will not evaporate and the maintenance can be improved. As for the maintenance of shear walls, once the protection film is removed, construction personnel should immediately paint curing liquid on the wall, especially in winter. If in a high temperature, we can use water for maintenance which has to be done every hour with two people responsible for it. The maintenance time of shear walls should be more than 14 days, and the continuous maintenance should not be less than 7 days. We should pay attention when painting protective liquid so as to ensure that a waterproof film will be formed on the surface of shear walls. If it does not meet this requirement, we can paint it again.
In a word, with more sufficient maintenance in the early stage, the stability and crack resistance of structural concrete can be further improved, which will prevent structural concrete from cracking when it is put in use. The strength of the concrete can be ensured. Disputes between the supply and demand shall be reduced.