Analysis of the inspection contents and methods for concrete structural elements

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Analysis of the contents and methods for on-site inspection of concrete structures: In accordance with GB50204-2015 \"Code for Acceptance of Construction Quality of Concrete Structures\", for the acceptance of the construction quality of concrete structure sub-projects, in addition to the visual quality needing to be satisfactory, the on-site inspection of the structural elements must also be successful. The inspection of structural elements focuses on representative parts related to the safety of concrete structures, and includes three main aspects: ① concrete strength, ② thickness of the reinforcement cover, and ③ deviations in the structure’s position and dimensions. http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/59d44d83927ea1047df16ed44f1d015c.jpg The inspection of structural elements should be carried out by the construction unit under the supervision of the supervising agency, which shall also oversee the entire process. The construction unit shall develop a specific plan for the inspection of the structural elements, and it shall be implemented only after being reviewed and approved by the supervision unit. Inspection items for the structural entity, other than deviations in structural location and dimensions, shall be carried out by testing institutions with the appropriate qualifications. In the inspection of structural elements, when the test results for concrete strength or the thickness of the rebar cover do not meet the required standards, it is necessary to commission a qualified testing agency to conduct tests in accordance with the **current relevant standards. 1. Concrete strength testing: http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/dd52e5994f22fc98b696b1c3ee86bc28.jpg ① Concrete specimens cured under the same conditions: http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/a0286e94bc47b574df71ae6b9dce8818.jpg ② Rebound-core drilling method: The strength of concrete in structural elements should be tested separately for different strength grades, and the testing method should preferably involve specimens cured under the same conditions ； When the strength of specimens cured under identical conditions cannot be obtained, or when such strength does not meet the requirements, the rebound-core drilling method can be used for testing. The equivalent curing age for the inspection of concrete strength can be taken as the age at which the daily cumulative sum of the daily average temperature reaches 600°C·d; this age shall not be less than 14 days. Stages with a daily average temperature of 0°C or below are not included. During winter construction, when calculating the equivalent curing age, the temperature can be taken as the actual curing temperature of the structural components. Alternatively, based on the actual curing conditions of these components, it can be jointly determined by all parties involved—such as the supervisor and contractors—following the principle that the strength of specimens cured under the same conditions is equal to the strength of specimens cured under standard curing conditions after 28 days. 1. Inspection of the strength of specimens cured under identical conditions: (1) The structural elements or parts corresponding to the specimens cured under identical conditions shall be selected jointly by the construction party, the supervision party, and other relevant parties; furthermore, the sampling of such specimens should be carried out in a uniform manner throughout the construction period ； (2) Test specimens cured under the same conditions shall be sampled on-site at the location where the concrete is poured into the formwork ； (3) Test specimens cured under the same conditions should be placed in appropriate locations near the corresponding structural members, and the same curing methods should be employed ； (4) For specimens cured under the same conditions at the same strength grade, there should be no fewer than 10 groups, and at least 3 groups are required. For every two consecutive floors, no less than 1 set of samples should be taken ； Sampling shall be conducted at least once every 2,000 m3. 2. The strength values of the specimens under identical curing conditions in each group shall be determined based on the results of strength tests, in accordance with the provisions of the current **standard ‘Test Methods for Mechanical Properties of Ordinary Concrete’ GB/T50081. 3. For test specimens of the same strength grade that have been cured under the same conditions, their strength values shall be divided by 0.88 before evaluation in accordance with the relevant provisions of the current **standard \"Code for Inspection and Evaluation of Concrete Strength\" GB/T50107. If the evaluation results meet the requirements, the concrete strength of the actual structure can be deemed satisfactory. Strength testing by rebound-core drilling method: 1. The selection of components for rebound testing shall comply with the following provisions: (1) For columns, beams, walls, and slabs of the same concrete strength grade, the minimum number of components to be selected shall meet the requirements specified in the table below, and they shall be distributed evenly ； http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/24e5cdffe1692508be3931100818ed5c.png (2) Beams with a cross-sectional height of less than 300 mm and columns with side lengths of less than 300 mm should not be selected for sampling. 2. For each component, no fewer than 5 testing areas shall be selected for rebound testing and the calculation of rebound values; these requirements shall comply with the relevant provisions of the current industry standard \"Technical Specification for Determining the Compressive Strength of Concrete by Rebound Method\" JGJ/T23 regarding the testing of individual components. The rebound testing of floor members should be conducted at the bottom of the slab. 3. For concrete of the same strength grade, the average rebound values of the smallest test areas among the 5 test areas per component should be sorted, and a core sample should be taken from each of the 3 smallest test areas. Core samples should be drilled using a thin-walled hollow drill equipped with a water cooling system; their diameter should be 100 mm, and it should not be less than 3 times the maximum size of the concrete aggregates. 4. The ends of the core specimens should be filled and smoothed using epoxy mortar or polymer cement mortar; sulfur mortar can also be used for repair. The dimensional tolerances and visual quality of the processed core specimens shall comply with the following requirements: (1) The measured ratio of the height to the diameter of the core specimen shall not be less than 0.95, nor shall it be greater than 1.05 ； (2) The difference between any diameter along the core sample’s height and its average value should not exceed 2 mm ； (3) The unevenness of the end face of the core specimen shall not exceed 0.1 mm over a length of 100 mm ； (4) The deviation of the end face of the core sample specimen from the axis shall not be greater than 1° ； (5) The core sample shall have no cracks, defects, or other impurities such as rebar. 5. The measurement of the dimensions of the core sample specimen shall comply with the following provisions: (1) A vernier caliper shall be used to measure the diameter at two positions that are perpendicular to each other in the middle of the core sample specimen, and the arithmetic average of these values shall be taken as the diameter of the core sample specimen, accurate to 0.1 mm ； (2) The height of the core specimen shall be measured using a steel ruler, with an accuracy of 1 mm ； (3) The verticality shall be measured using a vernier protractor to determine the angle between the two end lines of the core sample and the axis, with an accuracy of 0.1° ； (4) For flatness, a steel rule or set square should be placed against the end face of the core sample specimen; while rotating the steel rule, a feeler gauge is used to measure the gap between the steel rule and the end face of the core sample specimen ； Other specialized equipment can also be used for measurement. 6. Core specimens shall undergo compressive strength tests in accordance with the provisions for cylindrical specimens in the current **standard, ‘Standard Test Methods for Mechanical Properties of Ordinary Concrete’ GB/T50081. 7. For components of the same strength grade, the concrete strength of the structural members may be deemed acceptable when the following conditions are met: (1) The arithmetic mean of the compressive strengths of three core samples is not less than 88% of the specified concrete strength grade value as required by the design ； (2) The minimum value of the compressive strengths of the three core samples shall be no less than 80% of the concrete strength grade required by the design. Inspection of the thickness of the steel bar protective layer: http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/be5a1efc6fca794dc31a093439e75e17.jpg
Inspection of the thickness of the steel bar protective layer
1. Selection of components for inspecting the thickness of the protective layer on structural elements
The selected components should be distributed evenly, and must comply with the following provisions:
(1) For non-cantilever beam and slab components, 2% of the total number of components should be sampled for inspection; this figure must not be less than 5 components. (2) For cantilever beams, 5% of the total number of components, with a minimum of 10 components, shall be selected for inspection ； When the number of cantilever beams is less than 10, all of them shall be inspected. (3) For cantilever slabs, 10% of the total number of components, with a minimum of 20 components, shall be selected for inspection ； When the number of cantilever slabs is less than 20, all of them shall be inspected. 2. For the selected beam members, the cover thickness of all longitudinal load-bearing rebars shall be inspected ； For the selected plate members, the cover thickness of no less than 6 longitudinal load-bearing rebars shall be sampled for inspection. For each rebar, three points at representative and different locations should be measured, and their average value should be taken. 3. For the inspection of the thickness of the reinforcement cover, non-destructive or partially destructive methods can be used, or a non-destructive method combined with a partially destructive method for calibration can be employed. When non-destructive testing methods are used, the testing equipment employed must have undergone calibration, and the testing procedures must comply with the relevant regulations. The testing error for the inspection of the rebar cover thickness should not be greater than 1 mm. 4. When checking the thickness of the reinforcement cover, the allowable deviation for the thickness of the cover around the longitudinal load-bearing rebars shall comply with the provisions in the table below. http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/bdba7325aaab886073c753aa3cac73fc.png5. The thickness of the protective layer around the longitudinal reinforcement in beam and slab elements shall be inspected separately, and it must comply with the following requirements: (1) If the pass rate for inspections of the protective layer thickness of all reinforcements is 90% or higher, then it can be considered satisfactory ； (2) When the pass rate for the inspection of the thickness of the protective cover around all rebar is less than 90% but not less than 80%, an equal number of additional components can be sampled for inspection ； When the pass rate calculated based on the sum of two samples is 90% or higher, it can still be considered qualified ； (3) The maximum deviation of defective points in each sampling inspection result shall not exceed 1.5 times the allowable deviation specified in Appendix F.0.4 of this specification. Inspection of structural position and dimension deviations http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/c39c64d0cabfefc0b1f2f43810c4ab30.jpg Inspection of structural position and dimension deviations 1. Inspection of the position and dimension deviations of structural elements: The elements to be selected should be distributed evenly, and the following requirements shall be met: (1) For beams and columns, 1% of the total number of elements should be sampled, with a minimum of 3 elements ； (2) 1% of the walls and panels shall be sampled from representative natural rooms, with a minimum of 3 rooms ； (3) The floor height shall be inspected by random sampling at 1% of the representative natural rooms, with a minimum of 3 rooms. 2. For the selected components, the inspection items and methods shall comply with the provisions in the table below; the allowable deviations and inspection methods shall conform to those specified in Tables 8.3.2 and 9.3.10 of GB50204-2015 \"Code for Acceptance of Construction Quality of Concrete Structures\", with accuracy up to 1 mm. http://img.civilcn.com/d/file/zhishi/aqwm/2019-09-23/8dfa6996699e13a750a7e9c87a6b13d7.jpg3. The inspection of wall thickness, slab thickness, and floor height can be carried out using non-destructive methods or partially destructive methods; it is also possible to use non-destructive methods along with partially destructive methods for calibration. When non-destructive testing methods are used, the testing equipment employed must have undergone calibration, and the testing procedures must comply with the provisions of **current relevant standards. 4. The positional and dimensional deviations of structural elements shall be inspected separately and must comply with the following requirements: (1) If the pass rate for the inspection items is 80% or higher, it can be deemed acceptable ； (2) When the pass rate of the inspection items is less than 80% but not less than 70%, an equal number of components can be sampled for further inspection ； When the pass rate calculated based on the sum of two samples is 80% or higher, it can still be considered qualified.