Detection of the thickness of the protective layer of reinforced concrete

1.0.1 In order to strengthen the construction quality of the concrete structure project, unify the province's concrete internal reinforcement position and reinforcement protection layer thickness detection method, improve the detection accuracy of each detection unit, formulate this detection procedure, the concrete internal reinforcement protection layer thickness detection standard is "concrete structure Code for Construction Quality Acceptance (GB50204-2002).

1.0.2 This regulation is applicable to the detection of the position of the steel bars and the thickness of the steel bar protective layer of the concrete structure of construction projects.

1.0.3 In addition to the provisions of this regulation, the thickness of the protective layer of reinforced concrete inside the concrete structure shall be tested in accordance with the relevant national mandatory standards.

2 Terminology

2.1 Terminology

2.1.1 Electromagnetic induction method steel bar detector detection method

A probe composed of a single or multiple coils generates an electromagnetic field. When steel bars or other metal objects are located in the electromagnetic field, the magnetic field lines will be deformed. The interference caused by the metal changes the distribution of the electromagnetic field intensity, which is detected by the probe and displayed by the instrument. If the size and material of the detected steel bar are properly calibrated, it can be used to detect the location, diameter and thickness of the concrete protective layer of the steel bar.

2.1.2 Radar detection method

The electromagnetic wave is emitted by the radar antenna, reflected back from the interface of the substance with different electrical properties such as steel bars in the concrete, and received by the antenna on the concrete surface again, and the condition of the reflector is detected based on the received electromagnetic wave.

2.1.3 Actual protective layer thickness of steel bar

For smooth round steel bars, the minimum distance between the concrete surface and the steel bar surface. For ribbed steel bars, the value is shown in Figure 2.1.3.

Figure 2.1.3 Thickness of protective layer of ribbed steel bar C ≈C

2.1.4 Indicate the thickness of the protective layer of reinforcement

The thickness of the steel protective layer displayed by the instrument during the inspection .

2.1.5 Indication diameter of rebar

The steel bar diameter indicated by the instrument during testing.

2.1.6 Test deviation of rebar position

The minimum distance between the rebar axis indicated by the instrument and the actual axis of the rebar.

2.2 Symbol

3 Reinforcement position and protective layer thickness detection

3.1 General

3.1.1 Appropriate instruments should be selected according to the specifications, depth and spacing of the measured steel bars, and operated according to the instrument manual.

3.1.2 For the battery-powered instrument, the power supply should be sufficient during the test, and the instrument and battery should be maintained after the test. For instruments that can be powered by batteries or external power supplies, the instrument should be calibrated separately under two power supply conditions.

3.1.3 The instrument should be preheated or zeroed before testing. When zeroing, the probe must be kept away from metal objects. During the testing process, you should always check whether the instrument deviates from the initial state and perform zero adjustment in time.

3.1.4 The following information should be available before testing:

1 Project name and name of construction, design, construction and supervision unit;

2 The name of the structure or component and the corresponding steel design drawings;

3 Whether the concrete is prepared with ferromagnetic raw materials;

4 The variety, brand, design specifications, thickness of the design protective layer, whether there are reserved pipes and metal embedded parts in the structural members of the detection site;

5 Necessary construction records and other relevant materials;

6 Reason for detection.

3.1.5 According to the reinforcement design data, determine the possible distribution of reinforcement in the inspection area and select the appropriate inspection surface. The detection surface should be a concrete surface, which should be clean and smooth, and avoid metal embedded parts.

3.1.6 For components with a decorative layer, the decorative layer should be clean, flat, and well combined with the base concrete. Neither the main material of the facing layer nor the interlayer should contain metals. The veneer layer containing metal materials should be removed. For the finish layer with a thickness of more than 50mm, it should be cleared for inspection or bored verification. No testing shall be carried out on the overhead facing.

3.1.7 For concrete containing ferromagnetic raw materials, sufficient laboratory verification should be conducted before testing.

3.1.8 For the detection of the thickness of the protective layer of steel bars, a non-damaged or partially damaged method may be used, or a non-damaged method and a partial damage method may be used for correction.

3.1.9 The non-destructive testing method is suitable for a large number of structural components and large-area testing because it does not damage the structure to be tested. However, its detection accuracy is greatly affected by the accuracy of the instrument and the experience of the testing personnel.

3.1.10 The local damage detection method is suitable for sampling inspection of a small number of structural test points due to damage to the structure under test. The detection accuracy is high, and it can be used in combination with the non-destructive detection method to modify the detection result of the non-destructive method.

3.1.11 The structural parts and the number of components of the steel bar thickness inspection shall meet the following requirements:

1 The structural parts of the steel bar protective layer thickness inspection shall be jointly selected by the supervision (construction), construction and other parties according to the importance of structural members;

2 For beams and plates, 2% of the number of components should be extracted and no less than 5 components should be tested; when there are cantilevered components, the proportion of the extracted components is that of beams and plates. It should not be less than 50%.

3.1.12 For the selected beam members, the thickness of the protective layer of all longitudinally stressed steel bars shall be inspected; for the selected plate members, the thickness of the protective layer of not less than 6 longitudinally stressed steel bars shall be selected for inspection. For each steel bar, one point should be measured at a representative location.

3.2 Instrument performance requirements

3.2.1 The instrument should have a product certificate and a unique identification on the obvious position of the instrument, including name, model, and factory number.

3.2.2 The instrument should be calibrated regularly. Under normal circumstances, the instrument calibration is valid for one year.

3.2.3 When one of the following conditions occurs, the instrument should be calibrated:

1 Before the new instrument is put into use;

2 The calibration validity period is exceeded;

3 The detection data is abnormal and cannot be adjusted;

4 After repair or replacement of main parts (such as probes, antennas, etc.).

3.3 Electromagnetic induction method steel bar detector detection technology

3.3.1 Before testing, the steel bar detector of the electromagnetic induction method should be calibrated according to the concrete used for testing structural members. For the calibration method, see Appendix A.

3.3.2 When the ratio of the thickness of the reinforced concrete protective layer to the steel bar diameter is less than 2.5 and the thickness of the concrete protective layer is less than 50mm, the test error should not be greater than ± 1mm, and should not be greater than ± 5% in other cases.

3.3.3 The initial positioning of the steel bars to be tested should be performed before testing.

3.3.4 When testing the position of the steel bar, the probe moves regularly on the detection surface until the instrument shows the strongest received signal or the minimum thickness of the protective layer, and the design data is used to determine the position of the steel bar. At this time, the center line of the probe and the axis of the steel bar are basically Overlap, make a mark in the corresponding position. Mark the other adjacent steel bars one by one according to the above steps.

3.3.5 The thickness of the protective layer can be tested after the reinforcement is positioned:

1 Set the range of the instrument and the diameter of the steel bar, select the position of the adjacent steel bar along the axis of the measured steel bar, and avoid the steel bar joint, and read the thickness value C of the indicated protective layer . Repeat the test twice at the same position of each steel bar, taking 1 reading each time.

2 When the difference between the thickness values ​​of the two protective layers read at the same place is greater than 1mm, check whether the instrument deviates from the standard state and adjust in time (such as re-zeroing). Regardless of whether the instrument is adjusted or not, the previous test data is discarded, and the test is repeated 2 times and compared again. If the difference between the thickness values ​​of the two protective layers is still greater than 1 mm, the test instrument should be replaced or drilled or chiseled. Method verification.

Note: Most instruments require that the diameter of the steel bar is known to detect the thickness of the protective layer. In this case, the instrument must be set according to the actual diameter of the steel bar.

3.3.6 When the actual protective layer thickness value is less than the minimum indication of the instrument, the method of additional pads can be used for detection. The pads attached to the instrument should be preferred. The self-made pads should not cause electromagnetic interference to the instrument. The surface is smooth and smooth, and the thickness deviation in all directions is not more than 0.2mm. Thickness of added block C It should be deducted in the calculation.

3.3.7 When testing the distance between the steel bars, the adjacent steel bars to be tested should be marked out one by one without any omission, and should not be less than 7 bars, then measure the distance between the axis of the first bar and the last bar, and Calculate the number of intervals.

3.3.8 When encountering one of the following situations, at least 30% of steel bars should be selected and no less than 6 (when the actual number of inspections is less than 6 should be all extracted), using drilling, chiseling and other methods to verify:

1 The instrument requires that the diameter of the steel bar is known to determine the thickness of the protective layer, and the actual diameter of the steel bar is unknown or objectionable;

2 The actual number of steel bars, their position and design are quite different;

3 Concrete made with ferromagnetic raw materials;

4 Detect the thickness of the protective layer of steel bar without removing the decorative layer of the component;

5 Reinforcement and concrete materials are significantly different from the calibration test piece.

3.3.9 The steel bars shall not be damaged when drilling or picking. Vernier calipers are used for the actual measurement, and the measurement accuracy is 0.1mm.

3.4 Radar detection technology

3.4.1 The radar method is suitable for large-area scanning inspection of structures and components. Before testing, the dielectric constant of the radar should be calibrated according to the concrete used for testing structural members. For the calibration method, see Appendix B.

3.4.2 The detection error of the thickness of the steel bar protective layer should be less than ± 2mm, and should not be greater than ± 5% in any case; the test deviation of the steel bar spacing should be less than ± 3mm, and should not be greater than ± 5% in any case.

3.4.3 According to the engineering data, determine the testing conditions, select the instruments that meet the requirements for testing accuracy, and perform laboratory calibration if necessary.

3.4.4 According to the arrangement direction of the steel bars in the structure or component under test, the radar probe or antenna is scanned perpendicular to the axis of the steel bar under test. The instrument collects and records the reflected signal of the tested part. After proper processing, the instrument can display According to the cross-sectional image of the measured part, the depth and spacing of the steel bar can be estimated according to the position of the reflected wave of the steel bar displayed.

3.4.5 When testing the spacing between bars, the number of bars to be tested should not be less than 7 (6 intervals).

3.4.6 When encountering one of the following situations, at least 30% of the steel bars shall be selected and no less than 6 (when the actual number of inspections is less than 6, all shall be extracted), and verification shall be carried out by drilling and chiseling.

1 When the actual number of steel bars, position and design have a large deviation or there is no data to refer to;

2 Using concrete with ferromagnetic raw materials;

3 The moisture content of the concrete is high, or the material of the concrete is quite different from the calibration test piece;

4 The electromagnetic properties of the finish layer are quite different from the concrete;

5 Reinforcement and concrete materials are significantly different from the calibration test piece.

3.4.7 The steel bars shall not be damaged when drilling or picking. Vernier calipers are used for the actual measurement, and the measurement accuracy is 0.1mm.

3.5 Test data processing

3.5.1 Calculate the average value of the thickness of the reinforced concrete protective layer as follows:

C = ( + －2C ) / 2 (3.5.1)

Where C ——The average thickness of the reinforced concrete protective layer at the i-th measuring point, accurate to 0.5mm;

, ——Indicated protective layer thickness value of the 1st and 2nd test, accurate to 1mm ;.

C ——The thickness of the probe pad is accurate to 0.1mm.

3.5.2 When using the drilling and chiseling method for verification, the correction factor should be determined as follows:

(3.5.2)

In the formula ——Correction factor, accurate to 0.01;

Ci——The actual protective layer thickness of the steel bar at the i-th measuring point, accurate to 0.5mm;

Then multiply the correction factor by the average value of the indicated protective layer thickness to obtain the concrete protective layer thickness value.

3.5.3 When detecting the steel bar spacing, the results can be given in drawing mode according to actual needs, the maximum spacing and minimum spacing of the tested steel bars can be analyzed, and the average steel bar spacing S can be calculated as follows:

(3.5.3)

In the formula S——the average spacing of the steel bars, accurate to 1mm;

l——The total length of n steel bar spacing, accurate to 1mm.

4 Determination of test results

4.0.1 During the inspection of the thickness of the steel bar protective layer, the allowable deviation of the thickness of the longitudinal steel bar protective layer is + 10mm and -7mm for beam members and + 8mm and -5mm for plate members.

4.0.2 The thickness of the protective layer of longitudinally stressed steel bars of beams and plates shall be separately inspected and accepted.

4.0.3 The acceptance of the thickness of the protective layer of structural solid reinforcement shall meet the following requirements:

1 When the pass rate of the inspection of the thickness of all protective layers of steel bars is 90% or more, the inspection result of the thickness of the protective layers of steel bars shall be judged as qualified;

2 When the pass rate of the inspection of the thickness of all the protective layers of steel bars is less than 90% but not less than 80%, the same number of components can be selected for inspection; when the pass point rate calculated based on the sum of two samples is 90% or more, the steel bars The inspection result of the thickness of the protective layer shall still be judged as qualified;

3 The maximum deviation of unqualified points in each sampling inspection result shall not be greater than 1.5 times the allowable deviation specified in Article 4.0.1.

Appendix A Calibration Method of Electromagnetic Induction Steel Bar Detector

A.1 Production of calibration test piece

A.1.1 According to the sensitivity of the instrument to isolation materials, one of the following methods can be arbitrarily selected to make calibration test pieces:

1 Use concrete, wood, plastic, epoxy resin and other materials that do not cause electromagnetic interference to the instrument to make a rectangular parallelepiped specimen. Embed a steel bar with a certain diameter in it. When the steel bar is buried, the two ends should be exposed. It should be above 50mm. The surface of the test piece should be flat, the axis of the rebar should be parallel to the surface of the test piece, the embedding depth of the rebar should be different from the four sides of the test piece, and the vertical distance difference between the two exposed ends of the rebar and the four parallel surfaces of the test piece should be within Within 0.5mm. The size of the test piece and the depth of reinforcement can be set according to the range of the instrument. Rebar with a diameter of 16mm to 25mm should be selected, and the variation range of its embedding depth should be between 10mm to 60mm. Refer to Figure A.1 for sample size.

Figure A.1 Schematic diagram of the test piece for calibration

1－16mm diameter steel bar 2－Calibration test piece

2 Use flat plates with a certain thickness that do not cause electromagnetic interference to the instrument, and the thickness difference on all four sides should not exceed 0.2mm, as the insulation material on the reinforcement.

3 Use concrete, wood, plastic, epoxy resin and other materials that do not cause electromagnetic interference to the instrument to make a rectangular parallelepiped test piece. Reserve a number of holes in the test piece that are parallel to the surface of the test piece. The distance between each hole and the surface of the test piece is different. The distance should be at least 10mm ~ 60mm, and the minimum distance deviation between the two ends of the hole and the surface of the test piece should not be greater than 0.5mm. The diameter of the hole is slightly larger than the selected rebar for calibration, generally 16mm ~ 25mm.

A.1.2 When the test data of the instrument is different for different insulation materials, the calibration test piece must be made of concrete, and the test pieces of different strength levels should be made according to the local commonly used raw materials, and the instrument should be calibrated separately.

A.1.3 For the test piece made of concrete, any raw material shall not contain ferromagnetic, and should be used after the concrete age reaches 28d.

A.2 Calibration items and index requirements

A.2.1 The detection error of rebar position should be less than ± 3mm, and should not be greater than ± 5% in any case.

A.2.2 The detection error of the protective layer of reinforcing steel shall be less than ± 1mm ​​within the range of the thickness of the protective layer of 10 ~ 60mm.

A.2.3 For digital display instruments with the function of detecting the diameter of steel bars, the diameter detection error should be less than ± 2mm.

A.3 Calibration procedure

A.3.1 During the calibration process, always ensure that the power supply voltage of the instrument is stable, the power supply is sufficient, and the external electromagnetic interference is minimized.

A.3.2 Mark the actual axis position of the steel bar on each test surface of the test piece, and use a Vernier Caliper to measure the actual protective layer thickness value of the two exposed end surfaces of the steel bar on each test surface, take the average value, accurate to 0.1mm, and measure Reinforcement diameter, accurate to 0.1mm.

A.3.3 Operate the instrument correctly, scan on the test piece, mark the rebar axis specified by the instrument, and measure the maximum deviation between the rebar axis measured by the instrument on the surface of the test piece and the actual rebar axis with a steel tape measure. Record the instrument to indicate the protective layer thickness value. For instruments with a diameter detection function, diameter detection should be performed.

A.3.4 Compare the measured value of the instrument with the actual measured value. When all the items of the instrument meet the requirements of A.2, the instrument is judged to be qualified. When part of the project index and range range meet the requirements of A.2, it can be judged as partially qualified, but the scope of use of the instrument should be limited, and the items and range range that it meets and the items and range range that do not meet should be indicated.

A.3.5 The certified or partially qualified instrument shall indicate the steel grade, specifications and concrete material of the calibration test piece used.

Appendix B Radar Calibration Method

B.1 Manufacture of calibration test pieces

B.1.1 Select local commonly used raw materials and strength grades to make concrete slabs, and use the same mixture to make concrete test blocks for correcting the dielectric permittivity of concrete. For the size, refer to the requirements of the instrument manual. When there are many test pieces, the calibration concrete slab should correspond to the test block for correcting the dielectric constant one by one.

B.1.2 The test block for correcting the dielectric constant of concrete is a plain concrete test block.

B.1.3 The concrete slab should be made of single-layer reinforced steel mesh, and should be made of round steel of 8mm ～ 12mm, the spacing should be 100mm ～ 150mm, and the depth of the protective layer of the steel reinforcement should cover four sections of 15mm, 40mm, 65mm, 90mm The reinforcement mesh of the thickness of the protective layer should have at least 8 intervals. The two ends of the steel bar should be exposed, and the thickness difference of the protective layer at both ends should not be greater than 0.5mm, otherwise the test piece should be re-made.

B.1.4 The raw materials used to make concrete shall not contain ferromagnetism. The test piece shall be watered and covered for curing within 7 days after pouring. The natural curing shall be used after 7 days. The test piece shall be used after the concrete age reaches 28 days.

B.2 Calibration items and index requirements

B.2.1 The test deviation of the thickness of the steel bar protective layer should be less than ± 2mm, and should not be greater than ± 5% in any case.

B.2.2 The test deviation of the steel bar spacing should be less than ± 3mm, and should not be greater than ± 5% in any case.

B.3 Calibration procedure

B.3.1 During the calibration process, always ensure that the power supply voltage of the instrument is stable, the power supply is sufficient, and the external electromagnetic interference is minimized.

B.3.2 Use the test block to correct the dielectric constant to calibrate the radar.

B.3.3 At both ends of the exposed steel bars, measure the total length of the 6-segment steel bar with a steel tape measure, take the average value, and calculate the actual average distance of the steel bar, accurate to 1mm. At the same time, use the vernier caliper to measure the actual thickness of the protective layer on the two exposed ends of the steel bar.

B.3.4 Operate the instrument correctly, scan on the test piece, mark the rebar axis specified by the instrument, and calculate the average rebar distance according to the scan results. Record the instrument to indicate the protective layer thickness value.

B.3.5 Compare the measured value of the instrument with the actual measured value. When all the indicators of the instrument meet the requirements of B.2, the instrument is judged to be qualified. When part of the project index and measuring range meet the requirements of B.2, it can be judged that the instrument is partially qualified, but the scope of use of the instrument should be limited, and the items and measuring range that it meets and the items and measuring range that do not meet should be indicated.