What are the five inspection items for the main structure?

1 Testing of concrete compressive strength: The compressive strength of concrete is a key indicator for evaluating the reliability of concrete structures, as it directly affects the structure’s load-bearing capacity and seismic performance. The main testing methods include the rebound method, core drilling method, and ultrasonic-rebound combined method. In engineering practice, the rebound method is primarily used. The rebound method involves striking the concrete surface with a rebound hammer, measuring the distance the hammer bounces back, and using this rebound value to estimate the strength of the concrete. This is a surface non-destructive testing method that relies on the correlation between the hardness and strength of the concrete surface to determine the compressive strength of the concrete. According to the Technical Specification for Testing the Compressive Strength of Concrete by the Rebound Method, the number of components in each test batch should not exceed 30; if the number is less than 30, it can still be considered as one batch. Each component should have no fewer than 10 measurement areas; 16 impacts are made on each area. The 3 highest values and the 3 lowest values are excluded, and angle correction as well as surface correction must be applied. Additionally, the carbonation depth has an impact on these measurements (the carbonation depth is measured using a phenolphthalein alcohol solution, and correction is required when the depth exceeds 1.0 mm).

2 Inspection of the thickness of the rebar cover layer: The thickness of the rebar cover layer has a direct impact on the durability and fire resistance of reinforced concrete structures. An overly thin protective layer can cause the rebar to rust prematurely, while an overly thick one may lead to cracks. The thickness of the rebar cover is generally tested non-destructively using electromagnetic principle detection methods. By emitting electromagnetic signals from the probe, a secondary induced magnetic field is generated in the rebar surrounding the protective layer; this field is detected by the probe. After processing by the instrument, a measurement of the thickness of the rebar’s protective layer is obtained. In accordance with GB50204-2023 \"Code for Acceptance of Construction Quality of Concrete Structural Engineering\" and JGJ/T152-2019 \"Technical Specification for Inspection of Steel Bars in Concrete\", the sampling principle for the steel bar cover thickness is as follows: for beam and slab elements, 2% of the total number of such elements, with a minimum of 5 elements, should be sampled for inspection. For cantilever beams, the number of components to be sampled shall be no less than 5% of the total, and at least 10 pieces. For cantilever slabs, the number of components to be sampled shall be no less than 10% of the total, and at least 20 pieces. Allowable deviation: (1) For beam elements, the allowable deviation in the thickness of the protective layer is +10mm and -7mm. (2) For slab elements, the allowable deviation in the thickness of the protective layer is +8mm and -5mm. A pass rate of 90% or higher for all thickness inspections of the protective layers is considered acceptable ； If it is between 80% and 90%, double sampling can be conducted for re-evaluation.

Inspection of floor slab thickness: The thickness of floor slabs is related to the stiffness and load-bearing capacity of the structure. Insufficient thickness can lead to vibration of the floors, poor sound insulation, and even safety hazards. Non-destructive testing is carried out using the electromagnetic principle detection method. The transmitting probe and the receiving probe are placed on the surface and bottom of the floor slab to be tested, respectively. The transmitting probe emits electromagnetic signals, while the receiving probe captures the attenuated magnetic field signals; after processing by the instrument, the thickness of the floor slab is determined. High-rise buildings should use reinforced concrete structures with cast-in-place floor slabs, whose standard thickness is 10 cm; cast-in-place floor slabs in ordinary buildings that are less than 8 cm thick are considered substandard. The thickness of floor slabs in general floors should not be less than 80 mm; when concealed pipes are embedded in the slab, it should not be less than 100 mm ； The thickness of the top floor slab should not be less than 120 mm. In accordance with GB50204-2023 \"Code for Acceptance of Construction Quality of Concrete Structural Engineering\", the sampling principle for floor slab thickness is: 1% of the representative rooms shall be sampled, with a minimum of 3 rooms ； For large-scale spatial structures, the inspection surfaces can be divided along the longitudinal and transverse axes; 10% of them should be inspected randomly, with a minimum of 3 surfaces ； For 6 floors or less, no fewer than 3 floor slabs shall be sampled; for 6 floors or more, no fewer than 6 floor slabs shall be sampled. No less than 5 testing points should be sampled from each floor slab for inspection. When the designed thickness of a cast-in-place floor slab is ≤150mm, the allowable deviation is +8mm/-5mm. Over 90% of the measurement points should fall within the allowable deviation range.

4 Inspection of steel bar distribution (quantity, spacing): The quantity and spacing of steel bars directly affect the structure’s bending and shear resistance, and are key inspection items to ensure consistency between design and construction. Non-destructive testing is generally used for the distribution of rebar, with rebar scanners employed to determine the location and quantity of the rebar as well as the thickness of the protective layer. Inspection items: 1. Spacing of main reinforcement bars ； 2 stirrup spacing ； 3 Steel bar connection ； 4 Setting of positioning ribs.

5. Pull-out test of post-installed rebar: The pull-out test of post-installed rebar is a key method for evaluating the quality of anchoring, and it directly affects the reliability of structural connection joints. Reinforcement bar pull-out testing is a field test to determine the anchoring strength of anchors; generally, 48–72 hours after the reinforcement bars are installed, a tensiometer (jack) can be used to carry out pull-out tests on these bars. In accordance with Article C.2.3 of JGJ145-2013 \"Code for Post-anchoring Technology of Concrete Structures\", the sampling quantity for post-installed rebar is as follows: (1) For important structural elements: 3% of the total number of rebars in each inspection lot, with a minimum of 5 pieces; (2) For ordinary structural elements: 1% of the total number of rebars in each inspection lot, with a minimum of 3 pieces; (3) For non-critical projects: 0.1% of the total number of anchorage elements in each inspection lot, with a minimum of 3 pieces. Testing time: The age of the rebar bonded with adhesive must meet the required standards, and for bolts bonded chemically, the bonding material must have reached the required level of curing at the time of testing. The above are the main aspects of the five tests for the main structure, with non-destructive testing being the primary method and destructive testing used as a supplement. Different regions may have varying requirements for the five inspection items related to the main structure; in some areas, inspections for structural dimension deviations and structural deformation are added. In practical engineering applications, it is necessary to follow the requirements set by the local housing and construction authorities.

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