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Cracking the Code: Detecting concrete cover with Precision
March 20,2023

Definition of Reinforcement Cover:

The concrete protective layer is part of the concrete that protects and avoids the direct exposure of the steel bars in the concrete member. The thickness of the protective layer is from the concrete surface to the outer edge of the outermost steel bar (including stirrups, structural bars, distribution bars, etc.) the minimum distance between.

Reinforced concrete is equipped with a protective layer of steel bars, as the name suggests, to protect the steel bars from corrosion.


The role of concrete cover:

The concrete cover directly determines the structure, durability and functionality of the building, so it is a very important construction measure. Its role is mainly manifested in the following aspects:


Maintain structural durability

If the protective layer in concrete is too thin, there is an intuitive hazard, and it is easy to infiltrate moist gas and water, which may cause the steel bar to corrode and expand, thereby destroying the concrete and affecting its use and structural safety. In the specification, the requirement of "minimum value of reinforcement cover" is also specified.


Withstand external force

The protective layer has an anchoring force on the steel bar, using the anchoring force between the concrete and the steel bar, the two are closely combined and work together.

When the protective layer is too thin or missing, its anchoring force is reduced, thereby reducing the structure's ability to resist axial force and bending moment. If the protective layer is too thick, cracks will easily occur on the concrete surface, which will affect the stress of the steel bars in the concrete.


Affect the bearing capacity of the structure

From the formula for calculating the flexural capacity of the normal section, it can be seen that,

When the gluten protective layer is too thick and the effective height h0 of the section is reduced, the bending capacity of the section will be reduced. Concrete is prone to cracking, and air and moisture will come into contact with the steel bars through the cracks, which will lead to a series of problems such as corrosion, expansion, and loss of the protective layer of the steel bars. It can be seen that the thickness of the protective layer is an important factor affecting the bearing capacity of the structure.


Thermal insulation

The concrete protective layer has a certain heat insulation effect and can protect the steel bars that are easy to soften in case of fire. If the thickness of the protective layer is too thin, it is easy to crack under a high-temperature environment, causing the strength of the steel bars to decrease due to overheating, thus destroying the overall structure.

All in all, the thickness of the protective layer should neither be too small nor too large and should be within the allowable range of the specification. Therefore, in addition to checking the size deviation during the construction process, the thickness of the reinforcement cover in the structural entity should also be inspected.

Reasons for exceeding the standard of steel bar protection

(1) The processing and production of steel bars are not in place;

(2) The installation is not in place, and the steel skeleton is placed in an offset position;

(3) The steel pads are not installed properly;

(4) The formwork is not installed properly.


Main Factors Affecting Detection ·

1. During the on-site inspection, the number of components to be inspected is not selected according to the relevant specifications and standards, but the number of components to be inspected is determined by the size of the building area, so that the number of selected components does not meet the requirements of the specifications.

For some unit projects with a small area, only 1~2 components are selected, which does not meet the minimum requirement of 2% of the total number of components to be inspected, and not less than 5 components; Among them , the proportion of cantilevered components is less than 50%; some unit projects do not extract components according to the importance of structural components, but randomly select components on the same floor or the same unit, which is not representative at all.

2. During the on-site inspection, the flatness of the datum surface of the inspected components is too poor, especially the top surface of the cantilevered balcony and the top surface of the indoor floor is not carefully treated, and the inspection is carried out directly on them, which makes the inspection data fluctuate. Low, the discrete value is too large and becomes invalid data.

3. During the on-site inspection, the operation of the inspection instrument is not standard or standardized. For example, the scanning speed of the instrument is too fast or too slow, which cannot ensure that the instrument sensor is directly above the steel bar. The obtained data has a large error and cannot truly reflect the measured value. The thickness of the protective layer of the component.


4. During the on-site inspection, the stirrups were not pre-scanned and the stirrups will affect the measurement results. For the case where the stirrup spacing is less than 120mm, the stirrups should be scanned in advance and the actual scanned stirrup spacing value should be entered in the instrument system settings.

5. When testing the beam components of the frame structure, before the bottom filling wall is built, the detection is not carried out in time, and finally, it cannot be detected in fact, or only the exposed components without masonry at the bottom can be detected, which makes the detection unrepresentative.

6. During the on-site inspection, the detection position of the component is not selected properly. For example, when detecting beam components of frame structures, lack of in-depth and detailed understanding of the location of steel bars, joints, and stirrup dense areas in advance, and do not avoid beam-column joints and other steel-intensive areas, often the number of detected steel bars is greater than the actual number. The number of root causes of misjudgment or becomes invalid data.

7. During the on-site inspection, for intersecting beams of equal cross-section, equal span, and equal reinforcement, it is not carefully analyzed and correctly distinguished which beam's steel bars are on the top and which beam's steel bars are on the bottom, sometimes causing the protective layer of a certain beam to exceed The thickness does not match the reality.

8. During on-site testing, when the qualified rate reaches more than 80% and less than 90%, the number of samples is not increased in time for re-evaluation according to relevant standards. When the test is unqualified, there is no corresponding treatment plan and measure, which makes the on-site test fundamentally meaningless.

On-site inspection steps·

The inspection of the thickness of concrete cover of steel bars can be carried out by non-destructive or partial damage methods, or by non-destructive methods and local damage methods for calibration. When the non-destructive method is used for inspection, the testing instruments used shall be subject to metrological inspection, and the testing operation shall comply with the provisions of the corresponding regulations.

On-site detection technology is very important, and is related to the accuracy of detection data. The main detection steps are as follows:

1. The surface of the component needs to be clean, smooth, and free of dirt and plaster.

2. According to the drawings, design documents, and the introduction of on-site supervisors, there are no embedded metal parts or other objects that affect the detection data at the measuring point.

3. Select the range of the instrument, which is divided into two ranges. The first range is used when the thickness of the measured protective layer is small; the second range is used when the thickness of the measured protective layer is large.


4. Instrument parameter setting (take Langry LR-G200 integrated ferro scanner as an example):

(1) Component name setting;

(2) Set the diameter of the steel bar to be tested;

(3) Set the design protective layer thickness of the steel bar to be tested;

(4) Stirrup correction, the pre-scanning stirrup spacing can be set to [100], [80], [60], or [40] according to the scanning result, and the instrument will perform corresponding compensation correction. If the stirrup spacing is greater than 120mm, you need to set the parameter to [>120]. This detection is set to [>120].

5. Signal reset calibration

When the detection environment changes or there is a large deviation between the measured steel bar cover thickness and the design value, it is necessary to reset the signal for calibration of the instrument. In any measurement mode, you can press【…】 key to activate the signal calibration function.

Note: When the instrument i

s calibrated, it should be operated on air and away from ferromagnetic substances. Press the [OK] key to start the calibration process according to the interface prompts, and wait for the instrument to exit after self-calibration; if the calibration signal is abnormal, it will prompt that the calibration has failed, and needs to be repeated. calibration.

6. Determine the location, direction, and thickness of the protective layer of the reinforcement.

To determine the direction of the steel bar, it is necessary to avoid the interference of unrelated steel bars. Place the instrument on the surface of the concrete to be tested, parallel to the steel bar to be tested, and move the instrument slowly to the right at a uniform speed to start measurement. When the instrument is close to the steel bar, a green aiming frame appears. The aiming frame slowly approaches the center line. When the aiming frame and the center line coincide, the aiming frame turns red and the red indicator light turns on. There is a beep to prompt, and the laser light in front of the instrument will make a red vertical line to indicate that the instrument has detected Rebar, located directly below the red line. If it is set to the automatic storage mode, it will automatically save the determined protective layer thickness value. If it is the manual storage mode, you need to press the [FN] key to save the thickness value, and the thickness value will be displayed at the bottom of the screen.

During the detection process, the signal reset calibration operation is performed every 10 minutes or so, which can improve the test accuracy.

7. Arrangement of test results: users can upload data through a USB interface or Bluetooth interface, among which Bluetooth transmission is an extended function that needs to be uploaded with special mobile phone software, and users can choose according to their needs.



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