Reinforcement protective layer is a routine and required inspection project in reinforced concrete structure. In a broad sense, the protective layer of reinforcement is the part of concrete that protects the reinforcement and avoids the direct exposure of the reinforcement.

 

Different specifications differ in the definition of thickness, most of which is the distance from the outer edge of the reinforcement bar (excluding the stirrup) to the outer surface of the concrete.

 

In addition to the quality of raw materials, the protective layer of steel directly affects the durability of concrete structures.

 

As is known to all, concrete is directly exposed to the air and will be carbonized from the surface to the deep layer. When the carbonized layer comes into contact with the steel bar, it will lead to the corrosion of the steel bar.

 

In addition to the reduction of strength and performance, the corroded steel bar will also expand. The volume expansion coefficient of the deeply corroded steel bar can be more than 1.7. The expanded steel bar will lead to the cracking of surrounding concrete and the falling off of the protective layer of concrete, thus exposing the steel bar and causing further corrosion.

 

The carbonization rate of concrete is about 1mm every two years. A thick enough protective layer can effectively delay the process of carbonization depth reaching the position of steel reinforcement, which is also an important basis for determining the service life of concrete structure in the design.

 

The thickness of the protective layer is too small, which is directly harmful. In the code, the minimum value of the protective layer of reinforcement is also specified. However, in concrete members under stress, such as beams and columns, if the thickness of the protective layer is too large, the concrete is prone to crack, and the air and water contact with the steel bar through the crack, which will lead to a series of problems such as corrosion, expansion and loss of the protective layer mentioned above.

 

Therefore, the thickness of the protective layer should not be too small, nor too large, should be within the scope of the code. Therefore, in addition to the inspection of dimensional deviation during construction, the thickness of the protective layer of reinforcement in the structure should also be inspected.

 

The purpose of validating the thickness of the reinforced concrete protective layer is to strengthen the quality acceptance of the concrete structure, to truly reflect the position of the stressed steel reinforcement and other quality indicators, so as to ensure the safety of the structure.

 

The thickness of protective layer of reinforcement can be tested by damage method and non-damage method. Generally choose non-breakage method, in order to reduce the workload, when necessary with local breakage method for calibration.

 

The ZBL-R630 reinforced concrete rebar detector is used in a construction laboratory run by the director of a lock expansion project in a Chinese province.

 

Preparation work before testing

 

1. Read the drawings and design documents to understand the thickness of the protective layer, diameter of steel bars, spacing and distribution of steel bars and the location of embedded parts to be tested, and make records.

 

2. Determination of sampling quantity: in accordance with the relevant provisions on testing the thickness of reinforcement protection layer in appendix D of water transport engineering quality inspection standard (jts257-2008), at least 6 reinforcing bars shall be extracted from buttresses and gate walls for testing, and 2~3 points shall be measured in representative positions for each reinforcement.

 

3. Inspection matters before use of concrete and steel detector --

 

(1) check whether the concrete reinforcement detector is within the verification period, and conduct inspection on the standard parts to verify whether the equipment is in normal use.

 

(2) check the battery level. If the battery level is extremely low, the rebar detector will sound for 3 times, and then the icon of empty battery will flash in the bottom right corner of the screen. At this point, you should exit the detection interface immediately, and turn off the battery after returning to the main menu interface.

 

Field test procedure

 

Field testing technology is very important and related to the accuracy of test data. Taking the no.6 left lock wall of the lock chamber in the expansion and renovation project of a ship lock in a province of China as an example, the testing steps are as follows:

 

1. The surface of the component shall be clean, smooth, free from dirt and plastering layer.

 

2. According to the drawings, design documents and site supervisors, there are no embedded metal parts or other objects that affect the test data.

 

1. The measuring range of the rebar detector is selected and divided into two ranges. The first range is used for occasions where the thickness of the protective layer under test is small; The second range is used when the thickness of the protective layer under test is large.

 

4. Parameter setting of reinforcement detector :(1) setting of component number; (2) set the diameter of the steel bar under test, and the steel bar diameter of the component is 25mm; (3) set the design protective layer thickness of the steel bar under test, which is 50mm here;

 

(4) set the minimum thickness. If the value is not 0, if the thickness of the protective layer of the tested reinforcement is detected to be less than the set value, the rebar detector will give an alarm and prompt, and the thickness of the protective layer under test will not be displayed or stored. This time set the parameter to 0, so turn off the function;

 

(5) data correction. The effect of adjacent reinforcing bars on the test results cannot be completely eliminated by using this function, and no numerical correction will be carried out for this test.

 

5. Instrument reset: in the parameter setting interface, hold the sensor in the air and keep away from the ferromagnet. Press the "back" button to enter the test interface, and the rebar detector will automatically reset the sensor.

 

6. Determine the position, direction and thickness of the reinforcement. When determining the direction of the reinforcement bar, it is necessary to avoid interference from irrelevant reinforcement bars. When measuring the no. 6 longitudinal reinforcement bar in the left gate wall of the lock chamber, the transverse reinforcement bar shall be scanned first, and the survey line shall be arranged between the two adjacent transverse reinforcement bars.

 

Then, the probe is placed on the surface of the concrete under test, parallel to the steel bar under test, moving the sensor with uniform speed in a direction. As the probe gets closer and closer to the steel bar, the rolling bar becomes longer, the signal value becomes larger and larger, and the current thickness value decreases. When the probe passes the reinforcement bar, it will automatically lock the thickness of the protective layer of the reinforcement bar and press the "store" key to store the measured thickness of the protective layer.

 

In the detection process, a reset operation is carried out every 10min or so, which can improve the test accuracy.

 

7. Collation of test results. The measured data can be output by the software of the instrument or manually input into the computer. After finishing, 25 points of the component were measured, among which there were 3 unqualified points (all positive deviations), and the passing rate was 88%.

 

Evaluation of test results

 

According to the water transport engineering quality inspection standard (jts257-2008), the positive deviation of the actual thickness of the reinforcement protection layer of cast-in-place gate wall, retaining wall and other components should not exceed 15mm, and the negative deviation should not exceed 5mm. In each sampling test result, the maximum negative deviation value of the nonconforming point should not be greater than 7.5mm.

 

Qualification criteria for testing the thickness of the protective steel reinforcement of main members: when the percentage of qualified points for testing the thickness of all the protective steel reinforcement is 80% or more, the test result is qualified;

 

When the qualified point rate is less than 80% but not less than 70%, double sampling shall be conducted. When the qualified point rate calculated based on the sum of the two sampling quantities is 80% or more, the detection result of the steel bar protective layer thickness shall still be determined as qualified. To sum up, the thickness of the protective layer of no. 6 reinforcement on the left gate wall of the chamber is judged to be qualified.

 

Conclusion

 

The inspection of steel bar protective layer is only one of the items of steel bar working procedure acceptance and inspection. In order to ensure that it is within the scope of the specification, the corresponding control should be made on the steel bar cutting, installation, support and reinforcement, formwork assembly, concrete casting and other works, which can be said to be a miniature of the quality of each working procedure of members.

 

The rebar detector is simple to operate and easy to carry.

 

But in the previous inspection, can do the test qualified engineering is always few.

 

In such a background, the construction of all parties should improve the recognition of reinforced protective layer, intensify control, inspection, as the supervisor should be more strict control of reinforced protective layer, strengthen process acceptance check, the prepared concrete pouring process supervision and construction units to carry out the protection of protective measures, to ensure that the protective layer index meets the requirements.

 

While ensuring the quality of the project, improve the recognition level of leaders at all levels to the project quality and supervision work.