Five common "physical examination" methods for concrete compressive strength

Release time:

2023-01-20

Five common "physical examination" methods for concrete compressive strength

Comparison of advantages and disadvantages

Qian Fei, Engineering Technology Department

1、 Test piece method

During construction, pour the mixed concrete into the specified cube test mold, vibrate and form it. After curing for 28d according to the specified temperature and humidity, conduct the pressure test strength test. Take the 150mm cube test piece as the standard piece, and convert the 100mm and 200mm cube test pieces according to the specified size conversion factor.

>>>>Benefits

1. The strength of concrete can be directly understood through the test.

2. During the construction, the equivalent curing pressure test results of the same curing test pieces made under the witness condition can be used as the basis for re-inspection of the strength grade of the structural entity after conversion.

>>>>Disadvantages

1. The specimen method can directly reflect the strength of the concrete itself, but it can not truly reflect the quality after construction.

2. Sometimes the test piece is qualified, but the concrete solid quality has a great relationship with the level, method and work attitude of the construction unit. It is difficult to determine the quality, which leads to certain quality and safety risks.

3. If the specimen is made carelessly and the curing is not standardized, it is easy to cause the specimen quality to be unqualified, but in fact the concrete quality and strength meet the requirements, thus causing unnecessary trouble.

2、 Rebound method

The surface hardness of concrete is measured by rebound instrument, and the compressive strength of concrete is deduced based on the carbonation depth of concrete. The rebound value measured by the rebound instrument is the hardness of the concrete surface, and the hardness of the material is related to the strength of the material, so the special strength curve of rebound value and strength is established to deduce the strength value.

>>>>Benefits

1. It is simple and flexible to use, with fast test speed and low inspection cost.

2. The inspector shall randomly select and inspect the concrete on site to timely grasp the true strength of concrete and the overall level of pouring.

>>>>Disadvantages

1. The accuracy is relatively poor, and a certain strength measurement curve is needed.

2. This method cannot be used when the concrete surface and internal quality have obvious differences, such as chemical corrosion or fire, frostbite during hardening, etc.

3、 Core drilling method

Use diamond drill bit to drill core samples (preferably 100mm diameter) on representative concrete structures, and after processing, sawing, grinding or patching at both ends, make them into cylinders for compressive strength measurement (height-diameter ratio should be 1). The strength of concrete with component age of not less than 28 days and strength of not less than 15MPa can be tested by core drilling method.

>>>>Benefits

1. The core drilling method is a local damage detection method that directly reflects the actual situation of the component concrete and can accurately measure its strength.

2. The actual concrete conditions inside the component can be directly observed from the extracted core sample, such as aggregate distribution, honeycomb pores, cracks, etc.

>>>>Disadvantages

1. High labor intensity, strict sampling process requirements, and high core sample processing requirements.

2. The flatness requirements of the two ends are very high. If uneven, the strength will be low and the structural members will be damaged locally.

3. The detection cost is high, and the component reinforcement is too dense to be extracted.

4、 Ultrasonic testing

Using the many characteristics of ultrasound (such as reflection and diffraction), we can determine whether there are defects in the interior and surface of concrete by observing the propagation changes of ultrasound on the ultrasonic detector, so as to evaluate the quality without damaging or damaging the tested materials and workpieces, and whether we feel that we are doing B-ultrasound.

>>>>Benefits

Ultrasonic testing can make relatively accurate judgments on the location and scope of cavities and non-dense areas in concrete, the depth of cracks, the thickness of surface damage layer, the combination quality of concrete poured at different times and the homogeneity of concrete.

>>>>Disadvantages

There are many factors that affect the sound speed, such as the type of cement, the amount of cement, the sand content, the type of coarse aggregate and the maximum particle size, water content, age, etc. When the materials used, water content and age are different, the relationship between the propagation speed and the strength of concrete will be very different, so it is difficult to accurately measure the strength of concrete using ultrasonic method.

5、 Rear loading and pulling method

Also known as pullout method, it is to drill holes on the hardened concrete surface, grind grooves, embed anchors and install a pullout instrument for pullout test, measure the ultimate pullout force, and test the concrete strength according to the relationship between the pre-established pullout force and the concrete strength.

>>>>Benefits

The Technical Specification for Testing Concrete Strength by Post-installation and Pull-out Method stipulates that when there is doubt about the concrete strength of the structure or component, or when the concrete strength of the old structure needs to be tested, the post-installation and pullout method can be used for testing, and the test results can be used as a main basis for evaluating the concrete quality.

>>>>Disadvantages

Due to the difference of various factors, the local strength curve should be established to further improve the accuracy of the test results when using the pull-out method to detect the concrete strength.

Summary

1. When testing the concrete strength, the method to be used should be determined comprehensively according to the specific conditions and testing conditions of the tested concrete structure.

2. When the strength of the concrete specimen fails to pass the evaluation, the concrete strength of the component can be estimated by using the non-damage or local damage detection method.

3. When it is necessary to accurately determine the strength grade of structural concrete and conditions are available, the core drilling method can be preferred.

4. When the concrete quality is relatively uniform, the rebound method or ultrasonic testing can be used. At present, the rebound method is widely used on the site. Considering the slow growth of concrete strength in the later stage, the author believes that when the rebound method is used to detect the concrete strength value, it is better to rebound after the concrete age reaches 60 days and estimate the final strength value of the test area.

5. Checking with core drilling method can improve the accuracy.

6. When the concrete strength is relatively low, the pull-out method should not be used because the measured concrete strength is on the high side.

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