Concrete testing method

Abstract

The invention relates to the technical field of concrete testing, in particular to a concrete testing method which comprises the following steps: manufacturing a plurality of groups of standard-size concrete test pieces according to a preset concrete mix proportion, and respectively placing the standard-size concrete test pieces under a simulated curing condition and a standard curing condition for curing in a preset curing period to form a first test group and a first control group; based on a plurality of detection points, carrying out a rebound test and an ultrasonic test on the concrete test pieces in the first test group, and determining a high-frequency ultrasonic section to determine an ultrasonic attenuation coefficient of a single concrete test piece, according to the ultrasonic propagation speed, the ultrasonic attenuation coefficient and the detection rebound value, constructing a concrete strength-based correction model to obtain the comprehensive compressive strength of the concrete test pieces in the first test group, and determining the standard compressive strength corresponding to the concrete test pieces in the first control group, and determining an initial influence coefficient of the simulated maintenance condition according to the comprehensive compressive strength and the standard compressive strength. The compressive strength of the concrete structure is effectively tested.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete testing, and particularly to a testing method for concrete. Background Art

[0002] Building underground concrete structures are usually affected by multiple factors such as groundwater erosion, temperature and humidity changes, and ground stress. Sulfates in groundwater can cause concrete cracking and spalling, reducing the strength of concrete, and chlorides can trigger steel bar corrosion. Temperature changes in the underground environment can cause thermal expansion and contraction of concrete, resulting in concrete cracks and reducing the overall compressive strength of concrete.

[0003] In the prior art, the rebound method or the core drilling method is usually used for testing the compressive strength of concrete. The rebound method is relatively simple to operate, but the measured value of compressive strength is often low and it is difficult to accurately reflect the actual compressive strength of concrete. Although the core drilling method can provide accurate test results, coring the exterior wall of the underground structure will cause local structural damage and may cause potential problems such as water seepage in the future, and its application scenario is greatly limited. To balance accuracy and structural integrity, the ultrasonic-rebound method has become a feasible alternative, but when the ultrasonic-rebound method is applied, ultrasonic attenuation will occur during ultrasonic transmission, affecting the measurement accuracy.

[0004] Therefore, there is an urgent need for a testing method for concrete that takes into account the ultrasonic attenuation problem while realizing the testing of the compressive strength of the concrete structure, improving the accuracy of the measured value of the compressive strength and verifying the long-term stability of the concrete structure in a complex environment. Summary of the Invention

[0005] For this reason, the present invention provides a testing method for concrete to overcome the problem that when the ultrasonic-rebound method is used to measure the compressive strength of concrete in the prior art, the influence of ultrasonic attenuation on the compressive strength value is not considered, resulting in a large gap between the measured compressive strength and the actual compressive strength.

[0006] To achieve the above object, the present invention provides a testing method for concrete, including: Step S1, making a number of standard-size concrete specimens according to a preset concrete mix ratio, and placing them under simulated curing conditions and standard curing conditions respectively for curing with a preset curing period to obtain a first test group and a first control group; Step S2, performing a rebound test on the concrete specimens in the first test group based on a number of detection points to obtain detected rebound values, and performing an ultrasonic test on the concrete specimens based on the detection points to obtain the ultrasonic propagation velocity and the cross-measured ultrasonic spectrum, where the cross-measured ultrasonic spectrum includes an emitted ultrasonic spectrum and a received ultrasonic spectrum; Step S3: For a single concrete specimen within the first test group, determine the high-frequency ultrasonic segment based on the pair-measured ultrasonic spectrogram to determine the ultrasonic attenuation coefficient of the single concrete specimen. Step S4: Construct a concrete strength correction model based on ultrasonic attenuation according to the ultrasonic propagation velocity, the ultrasonic attenuation coefficient, and the detected rebound value to obtain the comprehensive compressive strength of the concrete specimens within the first test group. Step S5: Determine the standard compressive strength corresponding to the concrete specimens within the first control group, and determine the initial influence coefficient of the simulated curing conditions on the concrete compressive strength according to the comprehensive compressive strength and the standard compressive strength.

[0007] Further, in Step S2, determine the relative detection points based on several detection points for pair-measured ultrasonic testing, and determine the corresponding ultrasonic propagation velocity according to the relative distance between the relative detection points and the ultrasonic propagation time.

[0008] Further, in Step S3, for a single group of relative detection points, divide the frequency ranges of the transmitted ultrasonic spectrogram and the received ultrasonic spectrogram into several frequency intervals respectively to determine the energy values within each frequency interval, construct an energy distribution curve based on the energy values, and determine the high-frequency ultrasonic segment according to the energy distribution curve.

[0009] Further, in Step S3, determine the initial ultrasonic energy of the transmitted ultrasonic spectrogram in the high-frequency ultrasonic segment and the attenuated ultrasonic energy of the received ultrasonic spectrogram in the high-frequency ultrasonic segment, and determine the single ultrasonic attenuation coefficient of the relative detection points according to the initial ultrasonic energy, the relative distance, and the attenuated ultrasonic energy.

[0010] Further, in Step S3, determine the relative weights of each group of relative detection points based on the detection positions of several groups of relative detection points, and determine the ultrasonic attenuation coefficient of the single concrete specimen according to the relative weights and the single ultrasonic attenuation coefficient.

[0011] Further, in Step S4, determine the comprehensive rebound value according to the rebound values at several detection points of each concrete specimen within the first test group, determine the comprehensive ultrasonic propagation velocity according to the ultrasonic propagation velocities at several groups of relative detection points of each concrete specimen within the first test group, and construct a concrete strength correction model according to the comprehensive rebound value, the comprehensive ultrasonic propagation velocity, and the ultrasonic attenuation coefficient.

[0012] Further, in Step S4, determine the individual compressive strength of each concrete specimen according to the concrete strength correction model, and determine the comprehensive compressive strength of the concrete specimens within the first test group according to the individual compressive strength.

[0013] Further, it also includes: placing concrete specimens under simulated curing conditions and standard curing conditions respectively for curing for a number of curing cycles to obtain a second test group and a second control group, respectively performing rebound tests and ultrasonic tests on the second test group and the second control group to obtain the comprehensive compressive strength and the standard compressive strength at each curing cycle, and correcting the initial influence coefficient according to the curing cycle and the corresponding comprehensive compressive strength and standard compressive strength to obtain a corrected influence coefficient.

[0014] Further, determine the strength difference according to the comprehensive compressive strength and the standard compressive strength at each curing cycle, determine the correction factor according to the strength difference, and determine the corrected influence coefficient according to the correction factor and the initial influence coefficient.

[0015] Further, determine the relationship function between the compressive strength of the concrete specimen and the curing cycle under the simulated curing conditions according to the corrected influence coefficient, the comprehensive compressive strength and a number of curing cycles, and determine the service maintenance time of the concrete specimen under the simulated curing conditions according to the relationship function and the preset compressive strength threshold.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows. By setting up a comparison of concrete specimen groups under simulated curing and standard curing conditions, the present invention accurately evaluates the influence of the underground environment on the performance of concrete structures. Not only determines the compressive strength of concrete through ultrasonic-rebound testing, but also considers the attenuation coefficient of ultrasonic waves during propagation. Based on these data, a concrete strength correction model based on ultrasonic attenuation is constructed to obtain the comprehensive compressive strength of the concrete specimen under simulated curing conditions, and the initial influence coefficient is determined by comparing with the standard compressive strength under standard curing conditions, which helps to more accurately evaluate the compressive strength of concrete under simulated curing conditions, helps to accurately master the change law of the compressive strength of concrete under simulated curing conditions, improves the accuracy and reliability of the compressive strength test results, and thus provides a scientific and reliable reference for the design, construction and quality control of building underground concrete structures.

[0017] Further, the present invention combines the ultrasonic-rebound method and the ultrasonic attenuation coefficient to reflect the internal state of concrete from more dimensions, considers the attenuation of ultrasonic wave energy when propagating inside the concrete, and at the same time combines the rebound value and the comprehensive ultrasonic propagation speed to jointly reflect the compressive strength of concrete, making the evaluation of the compressive strength more accurate, and helping to further master the change law of the compressive strength of concrete under simulated curing conditions, and providing a scientific and reliable reference for the design, construction and quality control of building underground concrete structures.

[0018] Furthermore, by comparing the compressive strengths (comprehensive compressive strength and standard compressive strength) under different curing periods, the present invention helps to further evaluate the influence of different curing conditions on the internal structure of concrete, and also helps to deeply study the variation law of the compressive strength of concrete specimens under simulated curing conditions at different curing periods, further providing a scientific basis for the design, construction and quality control of underground concrete structures in buildings.

[0019] Furthermore, the present invention determines the strength difference according to the comprehensive compressive strength and the standard compressive strength under each curing period to determine the correction factor, and determines the corrected influence coefficient according to the correction factor and the initial influence coefficient, further accurately simulating the influence parameters of curing conditions on the compressive strength of concrete specimens, which helps to improve the accuracy and reliability of the compressive strength test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a step diagram of the test method for the concrete in the embodiment of the present invention; Figure 2 is a step diagram of determining the high-frequency ultrasonic section in the embodiment of the present invention; Figure 3 is a step diagram of constructing a concrete strength correction model in the embodiment of the present invention; Figure 4 is a step diagram of correcting the initial influence coefficient in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] In order to make the objectives and advantages of the present invention clearer, the present invention will be further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0022] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention and do not limit the protection scope of the present invention.

[0023] It should be noted that in the description of the present invention, the terms indicating directions or positional relationships such as "upper", "lower", "left", "right", "inner", "outer", etc. are based on the directions or positional relationships shown in the drawings. This is only for convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention.

[0024] Furthermore, it should be noted that, in the description of the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0025] See also Figure 1 , which is a step diagram of a concrete testing method according to an embodiment of the present invention. Specifically, the present invention provides a concrete testing method, comprising: Step S1, preparing several groups of standard-sized concrete specimens according to a preset concrete mix ratio, placing them under simulated curing conditions and standard curing conditions for a preset curing cycle, respectively, to obtain a first test group and a first control group; Step S2: performing a rebound test on the concrete specimens in the first test group based on a plurality of detection points to obtain a detection rebound value, and performing an ultrasonic test on the concrete specimens based on the detection points to obtain an ultrasonic propagation velocity and a measured ultrasonic spectrum, wherein the measured ultrasonic spectrum includes a transmitted ultrasonic spectrum and a received ultrasonic spectrum; Step S3, for a single concrete specimen in the first test group, determining a high-frequency ultrasonic segment based on the measured ultrasonic spectrum to determine the ultrasonic attenuation coefficient of the single concrete specimen; Step S4, constructing a concrete strength correction model based on ultrasonic attenuation according to the ultrasonic propagation velocity, the ultrasonic attenuation coefficient, and the detection rebound value to obtain the comprehensive compressive strength of the concrete specimens in the first test group; Step S5, determining the standard compressive strength corresponding to the concrete specimens in the first control group, and determining the initial influence coefficient of the simulated curing conditions on the concrete compressive strength based on the comprehensive compressive strength and the standard compressive strength.

[0026] It is understood that the ultrasonic-rebound method uses ultrasonic transducers and a rebound hammer placed on the surface of a concrete structure to obtain data. The data is then substituted into established regional or project-specific compressive strength estimation formulas to estimate the compressive strength of concrete. However, because defects such as pores and cracks within concrete attenuate ultrasonic waves of varying frequencies to varying degrees, and high-frequency ultrasonic waves are more sensitive to tiny defects, ignoring ultrasonic attenuation can lead to inaccurate compressive strength values. Therefore, the present invention combines the ultrasonic propagation velocity, ultrasonic attenuation coefficient, and rebound value to construct a concrete strength correction model based on ultrasonic attenuation to comprehensively assess the compressive strength of concrete.

[0027] It can be understood that the concrete specimens and the raw materials of the actually poured underground concrete structure of the building are of the same batch. The simulated curing condition is to simulate the underground environment of the building, and the standard curing condition is an environment that enables the concrete to undergo a hydration reaction in a relatively stable environment. By curing the concrete specimens under the simulated curing condition to obtain the first test group, and curing the concrete specimens under the standard curing condition to obtain the first control group, the compressive strength of the concrete under two different curing conditions is obtained, and then the influence degree parameter of the simulated curing condition on the compressive strength of the concrete, that is, the initial influence parameter, is obtained to characterize the influence on the compressive strength of the concrete under the simulated curing condition.

[0028] In a specific embodiment, preferably, the concrete specimen is a cube specimen, and the standard size of the concrete specimen is taken as 150 mm × 150 mm × 150 mm. The preset concrete mix ratio of the concrete specimen is cement: sand: gravel = 1:1:2, and the water-cement ratio is 0.42. The number of groups ranges from 8 groups to 12 groups, preferably, the number of groups is 12 groups; the value of the preset curing period is 28 days. In practice, the shape, standard size, preset concrete mix ratio, number of concrete specimen groups, and preset curing period of the concrete specimen can all be determined according to the actual situation, and no specific limitations are made here and will not be elaborated further.

[0029] It can be understood that the initial influence coefficient is a characterization parameter of the influence degree of the simulated curing condition on the compressive strength of the concrete.

[0030] In a specific embodiment, the standard curing condition is that the standard curing temperature ranges from 18 to 22 °C, preferably, the standard curing temperature is taken as 20 °C; the standard relative humidity is above 95%. The simulated curing condition is that the simulated curing temperature ranges from 10 °C to 25 °C, preferably, the simulated curing temperature is 15 to 22 °C, and the simulated relative humidity ranges from 85% to 95%, preferably, the simulated relative humidity is 88% to 92%. Initial influence coefficient = (standard compressive strength - comprehensive compressive strength) / standard compressive strength. In practice, the relevant environmental parameters of the standard curing condition and the simulated curing condition can be determined according to the actual situation, and no specific limitations are made here and will not be elaborated further.

[0031] The present invention accurately evaluates the influence of the underground environment on the performance of concrete structures by setting up a comparison of concrete specimen groups under simulated curing and standard curing conditions. It not only determines the compressive strength of concrete through ultrasonic-rebound testing but also considers the attenuation coefficient of ultrasonic waves during propagation. Based on these data, a concrete strength correction model based on ultrasonic attenuation is constructed to obtain the comprehensive compressive strength of concrete specimens under simulated curing conditions, and the initial influence coefficient is determined by comparing it with the standard compressive strength under standard curing conditions. This helps to more accurately evaluate the compressive strength of concrete under simulated curing conditions, accurately grasp the change law of the compressive strength of concrete under simulated curing conditions, improve the accuracy and reliability of the compressive strength test results, and thus provide a scientific and reliable reference for the design, construction, and quality control of concrete projects.

[0032] Specifically, in step S2, relative detection points are determined based on several detection points for cross-hole ultrasonic testing, and the corresponding ultrasonic propagation speed is determined according to the relative distance between the relative detection points and the ultrasonic propagation time.

[0033] It can be understood that the ultrasonic transducer emits and receives ultrasonic waves at the relative detection points. The ultrasonic propagation speed is the ultrasonic propagation speed between the relative detection points among several detection points corresponding to each concrete specimen. The relative detection points are a pair of points used for cross-hole ultrasonic testing, and setting several detection points ensures that detection information can be obtained comprehensively and accurately.

[0034] In a specific embodiment, the positions of the relative detection points are determined based on the shape and size of the concrete specimen. For an object with a regular shape such as a cube specimen, a group of relative detection points is set every 20 mm to 30 mm along the height direction of the cube, so that the internal conditions at different positions can be comprehensively detected. Preferably, the distance between the relative detection points is 25 mm. The ultrasonic propagation time is the time experienced by the ultrasonic signal from the emission point to the reception point of the relative detection points, the relative distance is the distance between a pair of relative detection points, and the ultrasonic propagation speed corresponding to the relative detection points is the ratio of the relative distance to the ultrasonic propagation time.

[0035] Please participate Figure 2 As shown, it is a step diagram for determining the high-frequency ultrasonic section in an embodiment of the present invention. Specifically, in step S3, for a single group of relative detection points, the frequency ranges of the transmitted ultrasonic spectrum and the received ultrasonic spectrum are respectively divided into several frequency intervals to determine the energy values within each frequency interval. An energy distribution curve is constructed based on the energy values, and the high-frequency ultrasonic section is determined according to the energy distribution curve.

[0036] It can be understood that concrete is a heterogeneous material with various microscopic defects such as pores and microcracks inside. During the propagation of high-frequency ultrasonic waves, they are more vulnerable to the influence of these microscopic structures, and the energy attenuation, scattering, etc. can more intuitively reflect the compactness, pore size and distribution, etc. inside the concrete. By determining the high-frequency ultrasonic segments corresponding to the respective detection points, the microscopic characteristics inside the concrete can be analyzed more meticulously, thereby more accurately evaluating the compressive strength of the concrete.

[0037] It can be understood that in ultrasonic testing, for each group of relative detection points (transmitting point and receiving point), there is a corresponding spectrogram. Each detection point has its unique spectrogram characteristics for the received ultrasonic signal. By comparing the transmitted ultrasonic spectrogram and the received ultrasonic spectrogram, the attenuation of certain frequency components in the received ultrasonic spectrogram can be obtained.

[0038] In a specific embodiment, the entire spectrogram frequency range of the transmitted ultrasonic spectrogram and the received ultrasonic spectrogram is respectively divided into several smaller frequency intervals. The energy value within each corresponding frequency interval can be obtained by integrating the square of the amplitude of each frequency point within that interval. Based on the respective energy values of the transmitted ultrasonic spectrogram and the received ultrasonic spectrogram, an energy distribution curve is constructed respectively, and the frequency point where the energy starts to rapidly decline is found as the starting point of the high-frequency ultrasonic segment. The frequency band above the starting point of the high-frequency ultrasonic segment is used as the high-frequency ultrasonic segment. In implementation, the high-frequency ultrasonic segment can also be determined according to a preset segmentation frequency. The preset segmentation frequency is 200 kHz. When the ultrasonic wave frequency of a normal concrete structure is higher than 200 kHz, the sensitivity to tiny defects inside the concrete is significantly enhanced. Therefore, the frequency band above 200 kHz can be set as the high-frequency ultrasonic segment. In implementation, the determination method of the high-frequency ultrasonic segment is not unique, and no specific limitation is made here and will not be elaborated further.

[0039] Specifically, in step S3, the initial ultrasonic energy of the transmitted ultrasonic spectrogram in the high-frequency ultrasonic segment and the attenuated ultrasonic energy of the received ultrasonic spectrogram in the high-frequency ultrasonic segment are determined, and the single ultrasonic attenuation coefficient of the relative detection points is determined according to the initial ultrasonic energy, relative distance, and attenuated ultrasonic energy.

[0040] It can be understood that in the high-frequency ultrasonic segment of the transmitted ultrasonic spectrogram, the energy magnitude of the ultrasonic wave at the time of transmission, that is, the initial ultrasonic energy, can be read, which reflects the output energy level of the ultrasonic wave generator. When the ultrasonic wave passes through the concrete, its energy will attenuate, and the magnitude of this attenuated energy can be observed through the received ultrasonic spectrogram in the high-frequency ultrasonic segment, that is, the attenuated ultrasonic energy. The single ultrasonic attenuation coefficient between the relative detection points is determined based on the energy attenuation principle of the ultrasonic wave during the propagation process.

[0041] In a specific embodiment, the initial ultrasonic energy is determined by integrating the energy spectrum of the transmitted ultrasonic spectrum in the high-frequency ultrasonic band or taking its peak value, etc., and the attenuated ultrasonic energy is obtained by integrating the energy spectrum of the received ultrasonic spectrum in the high-frequency ultrasonic band or taking its peak value, etc. According to the attenuation formula of ultrasonic waves propagating in a medium , is the attenuated ultrasonic energy, is the initial ultrasonic energy, is the relative distance, and the single ultrasonic attenuation coefficient relative to the detection point can be obtained .

[0042] Specifically, in step S3, based on the detection positions of several groups of relative detection points, the relative weights of each group of relative detection points are determined, and the ultrasonic attenuation coefficient of a single concrete specimen is determined according to the relative weights and the single ultrasonic attenuation coefficient.

[0043] It can be understood that the area of the concrete specimen near the center of the specimen is less affected by external factors and can better reflect the true performance inside the concrete. However, the performance of the edge area may be different from that inside due to factors such as contact with the mold and curing conditions. Therefore, the detection points in the central area are more representative of reflecting the overall ultrasonic attenuation coefficient and should have a higher weight.

[0044] In a specific embodiment, taking the geometric center of the concrete specimen as a reference, the distances from each relative detection point to the geometric center are calculated. The closer the detection point is to the geometric center, the higher the relative weight. For every certain proportion reduction in the distance, the relative weight increases by a certain value. The ultrasonic attenuation coefficient of the single concrete specimen is the weighted average of the relative weight and the single ultrasonic attenuation coefficient. In practice, the relative weight can be determined according to the actual situation, which is not specifically limited here and will not be elaborated further.

[0045] Please refer to Figure 3 shown, which is the step diagram for constructing the concrete strength correction model in the embodiment of the present invention. Specifically, in step S4, the comprehensive rebound value is determined according to the rebound values at several detection points of each concrete specimen in the first test group, the comprehensive ultrasonic propagation velocity is determined according to the ultrasonic propagation velocities at several groups of relative detection points of each concrete specimen in the first test group, and the concrete strength correction model is constructed according to the comprehensive rebound value, the comprehensive ultrasonic propagation velocity, and the ultrasonic attenuation coefficient.

[0046] It can be understood that the rebound method is sensitive to the surface, and the ultrasonic method is sensitive to the internal structure. The combination of the two can evaluate the performance of concrete from different angles. Considering the energy loss during the propagation of ultrasonic waves, a concrete strength correction model can be further constructed to accurately evaluate the compressive strength of the concrete specimen.

[0047] In a specific embodiment, the comprehensive rebound value is the sum mean of the rebound values at several detection points of a single concrete specimen within the first test group, and the comprehensive ultrasonic propagation velocity is the sum mean of the ultrasonic propagation velocities at several groups of relative detection points of a single concrete specimen within the first test group. To more accurately evaluate the compressive strength of concrete, a neural network model can be used, taking the comprehensive rebound value, the comprehensive ultrasonic propagation velocity, and the ultrasonic attenuation coefficient as input features, and the concrete compressive strength as the output feature to construct a concrete strength correction model. By training data such as the comprehensive rebound value, the comprehensive ultrasonic propagation velocity, and the ultrasonic attenuation coefficient, the mapping relationship between the input features and the compressive strength of the concrete specimen can be obtained. In implementation, the construction method of the concrete strength correction model can be determined according to the actual situation, which is not specifically limited here and will not be elaborated further.

[0048] In implementation, the influence of factors such as the mix ratio, age, and moisture content of the concrete specimen on ultrasonic attenuation and strength can also be considered, and methods such as multiple regression analysis can be used to determine the quantitative relationships between these factors and ultrasonic attenuation, rebound value, and concrete strength, so as to improve the accuracy of strength prediction, etc.

[0049] The present invention combines the ultrasonic rebound method and the ultrasonic attenuation coefficient to reflect the internal state of concrete from more dimensions, considers the attenuation of energy when ultrasonic waves propagate inside the concrete, and simultaneously combines the rebound value and the comprehensive ultrasonic propagation velocity to jointly reflect the compressive strength of the concrete, making the evaluation of the compressive strength more accurate, and helping to further master the variation law of the compressive strength of concrete under simulated curing conditions, providing a scientific and reliable reference for the design, construction, and quality control of concrete projects.

[0050] Specifically, in step S4, the individual compressive strength of each concrete specimen is determined according to the concrete strength correction model, and the comprehensive compressive strength of the concrete specimens within the first test group is determined according to the individual compressive strength.

[0051] It can be understood that since there may be some slight differences in the production and curing processes of each concrete specimen, such as local non-uniformity of raw materials and different compaction degrees during the pouring process, etc., the corresponding individual compressive strength is determined according to the respective concrete strength correction model, and the individual compressive strengths of the concrete specimens within the first test group are synthesized to overall evaluate the compressive strength level of the concrete specimens within the first test group during the preset curing period, which is convenient for further evaluating the safety, durability, and other properties of the concrete structure.

[0052] In a specific embodiment, the individual compressive strength can be obtained according to the concrete strength correction model, and the comprehensive compressive strength of the concrete specimens within the first test group is the sum mean of the individual compressive strengths of the concrete specimens within the first test group.

[0053] Please refer to Figure 4 as shown, which is the step diagram for correcting the initial influence coefficient in the embodiment of the present invention. Specifically, it further includes: placing concrete specimens under simulated curing conditions and standard curing conditions for curing for several curing cycles to obtain a second test group and a second control group, respectively performing rebound tests and ultrasonic tests on the second test group and the second control group to obtain the comprehensive compressive strength and standard compressive strength at each curing cycle, and correcting the initial influence coefficient according to the curing cycle and the corresponding comprehensive compressive strength and standard compressive strength to obtain the corrected influence coefficient.

[0054] It can be understood that the second test group is cured under simulated curing conditions, and the second control group is cured under standard curing conditions. By performing rebound tests and ultrasonic tests on the second test group and the second control group in the above manner, the comprehensive compressive strength and standard compressive strength of the corresponding concrete specimens at each curing cycle are obtained. The initial influence coefficient is the influence degree parameter of the simulated curing conditions on the concrete specimens in a single curing cycle.

[0055] In a specific embodiment, the values of several curing cycles are 28 days, 56 days, and 90 days. In practice, the values of the curing cycle can be determined according to the actual research purpose and research conditions, and no specific limitation is made here and will not be elaborated further.

[0056] The present invention helps to further evaluate the influence of different curing conditions on the internal structure of concrete by comparing the compressive strengths (comprehensive compressive strength and standard compressive strength) at different curing cycles, and also helps to deeply study the change law of the compressive strength of concrete specimens under simulated curing conditions at different curing cycles, and further provides a scientific and reliable reference for the design, construction, and quality control of underground concrete projects.

[0057] Specifically, determine the strength difference according to the comprehensive compressive strength and standard compressive strength at each curing cycle, determine the correction factor according to the strength difference, and determine the corrected influence coefficient according to the correction factor and the initial influence coefficient.

[0058] It can be understood that the strength difference is the difference in the compressive strength of concrete specimens under simulated curing conditions and standard curing conditions at each curing cycle. Taking the standard compressive strength as the reference, determine the correction factor according to the strength difference at different curing cycles, and correct the initial influence coefficient to make it more accurately represent the influence degree of the simulated curing conditions on the concrete specimens. The strength difference between the comprehensive compressive strength and the standard compressive strength at each curing cycle is positively correlated with the correction factor. The larger the strength difference, the greater the adjustment required for the initial influence factor, and thus the larger the correction factor.

[0059] In a specific embodiment, a relationship function between the strength difference and the correction factor can be established through a linear relationship or a non-linear relationship, and regression analysis is performed to make it satisfy that the strength difference and the correction factor are positively correlated, so as to obtain the final correction factor. The correction influence coefficient is the product of the correction factor and the initial influence coefficient. In practice, the correction factor can be determined according to the actual situation, which is not specifically limited here and will not be elaborated further.

[0060] The present invention determines the strength difference according to the comprehensive compressive strength and the standard compressive strength under each curing period to determine the correction factor, and determines the correction influence coefficient according to the correction factor and the initial influence coefficient, further accurately simulating the influence parameters of the curing conditions on the compressive strength of concrete specimens, which helps to improve the accuracy and reliability of the compressive strength test results.

[0061] Specifically, according to the correction influence coefficient, the comprehensive compressive strength and several curing periods, a relationship function between the compressive strength of the concrete specimen and the curing period under the simulated curing conditions is determined, and according to the relationship function and the preset compressive strength threshold, the service maintenance time of the concrete specimen under the simulated curing conditions is determined.

[0062] It can be understood that in the relationship function between the compressive strength of the concrete specimen and the curing period under the simulated curing conditions, the comprehensive compressive strength is the dependent variable and the curing period is the independent variable. As the curing period changes, the comprehensive compressive strength of the concrete specimen also changes. Therefore, by combining the correction influence parameters of the concrete under the simulated curing conditions, the comprehensive compressive strength, and the curing period, the relationship function between the curing period and the compressive strength is determined to describe the relationship between time and compressive strength. Based on this, the compressive strength maintenance time of the concrete is determined according to the preset compressive strength threshold.

[0063] In a specific embodiment, the relationship function is in the form of an exponential function, the comprehensive compressive strength is the dependent variable, the curing period is the base, and the correction influence coefficient is the exponent. In practice, the relationship function can also be determined in the form of a logarithmic function or a power function, which is not specifically limited here and will not be elaborated further.

[0064] In a specific embodiment, substituting the preset compressive strength threshold into the relationship function, the service maintenance time of the concrete specimen under the corresponding simulated curing conditions can be obtained. The value range of the preset compressive strength threshold is 75% - 85% of the designed compressive strength. Preferably, the value of the preset compressive strength threshold is 83% of the designed compressive strength. The designed compressive strength is the compressive strength of the building underground concrete structure during actual design, such as the compressive strength of the basement. In practice, the preset compressive strength threshold can be determined according to the actual situation, which is not specifically limited here and will not be elaborated further.

[0065] Example 1: A. Specimen production and curing According to the preset concrete mix ratio corresponding to the underground structure of the building, 10 groups of cubic concrete specimens with dimensions of 150 mm × 150 mm × 150 mm were made. Five of them were placed under simulated curing conditions where the simulated curing temperature fluctuated between 15 and 22 °C and the simulated relative humidity fluctuated between 88% and 92%. The other five were placed under standard curing conditions with a temperature of 20 °C ± 2 °C and a relative humidity above 95%. The two groups were cured for 28 days respectively to obtain the first test group and the control group.

[0066] B. Rebound and ultrasonic testing On each face of each cubic concrete specimen, a number of detection points were evenly arranged, and the distance between opposite detection points was 25 mm. Using a calibrated rebound hammer, perpendicular to the concrete test surface, slowly apply pressure, accurately read and record the rebound value of each detection point. And a high-precision ultrasonic detector was selected. Apply an appropriate amount of coupling agent (such as vaseline, butter, etc.) at the detection point to ensure that the ultrasonic transducer is tightly coupled with the concrete surface. Adopt the opposite-side measurement method to measure and record the ultrasonic propagation time of the ultrasonic wave at the position of the opposite detection points to determine the ultrasonic propagation speed.

[0067] C. Ultrasonic attenuation Using an ultrasonic detector with spectrum analysis function, collect the opposite-side ultrasonic spectra for each group of opposite detection points. The opposite-side ultrasonic spectra include the transmitted ultrasonic spectrum and the received ultrasonic spectrum. By analyzing the attenuation differences of ultrasonic waves with different frequencies, determine the ultrasonic attenuation coefficient of the high-frequency ultrasonic segment of the ultrasonic wave inside the concrete to quantify the attenuation degree of the ultrasonic energy propagation between opposite detection points.

[0068] D. Establish a strength correction model Sort out the rebound values, ultrasonic propagation speeds, and ultrasonic attenuation coefficients obtained in the above steps to form a complete data set. Use mathematical methods such as artificial neural networks to establish a concrete strength correction model based on ultrasonic attenuation based on this data set to obtain the compressive strength of each concrete specimen.

[0069] E. Initial influence coefficient evaluation and correction Compare the compressive strengths of concrete specimens under simulated curing conditions and standard curing conditions to quantify the influence degree of simulated curing conditions on the compressive strength of concrete to obtain the initial influence coefficient. Based on the above curing conditions, increase the curing period, cure the concrete specimens at different curing periods, analyze the change trend of the compressive strength of concrete with time, and the difference from the standard curing conditions.

[0070] Use the conventional ultrasonic-rebound method, the pressure method and the present invention to test the compressive strength of 28-day-old concrete specimens. Some of the results are shown in Table 1: Table 1 Comparison results of the compressive strength of concrete specimens

[0071] It can be seen that the pressure method directly conducts pressure tests on concrete specimens, which can intuitively reflect the actual compressive capacity of concrete and most accurately reflect the compressive strength of concrete specimens. The ultrasonic rebound method is based on the mutual relationship between the ultrasonic propagation velocity, the rebound value and the compressive strength of concrete, and comprehensively reflects the compressive strength of concrete with the sound velocity and the rebound value. However, if there are laitance, oil stains, loose layers, etc. on the surface of the concrete specimen, it will cause deviation in the rebound value result, and this method is affected by materials and mix ratios, which will change the relationship between the ultrasonic propagation velocity, the rebound value and the compressive strength, thus affecting the accuracy of the test result. And the ultrasonic-rebound method considering ultrasonic attenuation in the present invention has higher accuracy of the test result.

[0072] The present invention accurately evaluates the influence of the underground environment on the concrete performance by setting up a comparison of concrete specimen groups under simulated curing and standard curing conditions. It not only determines the compressive strength of concrete through ultrasonic-rebound tests, but also considers the attenuation coefficient of ultrasonic waves during propagation. Based on these data, a concrete strength correction model based on ultrasonic attenuation is constructed to obtain the comprehensive compressive strength of the concrete specimens under simulated curing conditions, and the initial influence coefficient is determined by comparing with the standard compressive strength under standard curing conditions, which helps to more accurately evaluate the compressive strength of concrete under simulated curing conditions, helps to accurately master the change law of the compressive strength of concrete under simulated curing conditions, improves the accuracy and reliability of the compressive strength test result, and thus provides a scientific and reliable reference for the design, construction and quality control of underground concrete projects.

[0073] So far, the technical solution of the present invention has been described in conjunction with the preferred embodiments shown in the drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A testing method for concrete, characterized in that, Including: Step S1: Fabricate several groups of concrete specimens with standard dimensions according to a preset concrete mix ratio, and place them under simulated curing conditions and standard curing conditions respectively for curing with a preset curing period to obtain a first test group and a first control group; Step S2: Conduct rebound tests on the concrete specimens in the first test group based on a number of detection points to obtain detected rebound values, and conduct ultrasonic tests on the concrete specimens based on the detection points to obtain ultrasonic propagation velocities and cross - side ultrasonic spectrograms. The cross - side ultrasonic spectrograms include transmitted ultrasonic spectrograms and received ultrasonic spectrograms; Step S3: For a single concrete specimen in the first test group, determine the high - frequency ultrasonic segment based on the cross - side ultrasonic spectrogram to determine the ultrasonic attenuation coefficient of the single concrete specimen; Step S4: Construct a concrete strength correction model based on ultrasonic attenuation according to the ultrasonic propagation velocity, the ultrasonic attenuation coefficient, and the detected rebound value to obtain the comprehensive compressive strength of the concrete specimens in the first test group; Step S5: Determine the standard compressive strength corresponding to the concrete specimens in the first control group, and determine the initial influence coefficient of the simulated curing conditions on the concrete compressive strength according to the comprehensive compressive strength and the standard compressive strength.

2. The testing method of concrete according to claim 1, wherein In step S2, determine relative detection points based on a number of detection points for cross - side ultrasonic testing, and determine the corresponding ultrasonic propagation velocity according to the relative distance between the relative detection points and the ultrasonic propagation time.

3. The testing method of concrete according to claim 2, wherein In step S3, for a single group of relative detection points, divide the frequency ranges of the transmitted ultrasonic spectrogram and the received ultrasonic spectrogram into several frequency intervals respectively to determine the energy values within each frequency interval, construct an energy distribution curve based on the energy values, and determine the high - frequency ultrasonic segment according to the energy distribution curve.

4. The testing method of concrete according to claim 3, wherein In step S3, determine the initial ultrasonic energy of the transmitted ultrasonic spectrogram in the high - frequency ultrasonic segment and the attenuated ultrasonic energy of the received ultrasonic spectrogram in the high - frequency ultrasonic segment, and determine the single ultrasonic attenuation coefficient of the relative detection points according to the initial ultrasonic energy, the relative distance, and the attenuated ultrasonic energy.

5. The testing method of concrete according to claim 4, wherein In step S3, determine the relative weights of each group of relative detection points based on the detection positions of several groups of relative detection points, and determine the ultrasonic attenuation coefficient of a single concrete specimen according to the relative weights and the single ultrasonic attenuation coefficient.

6. The testing method of concrete according to claim 1, characterized in that, In step S4, determine the comprehensive rebound value according to the rebound values at a number of detection points of each concrete specimen in the first test group, determine the comprehensive ultrasonic propagation velocity according to the ultrasonic propagation velocities at several groups of relative detection points of each concrete specimen in the first test group, and construct a concrete strength correction model according to the comprehensive rebound value, the comprehensive ultrasonic propagation velocity, and the ultrasonic attenuation coefficient.

7. The testing method for concrete according to claim 6, characterized in that, In step S4, determine the individual compressive strength of each concrete specimen according to the concrete strength correction model, and determine the comprehensive compressive strength of the concrete specimens in the first test group according to the individual compressive strength.

8. The testing method of concrete according to claim 1, characterized in that, Also including: The concrete specimens are respectively placed under simulated curing conditions and standard curing conditions for curing at several curing cycles to obtain a second test group and a second control group. The rebound test and ultrasonic test are respectively carried out on the second test group and the second control group to obtain the comprehensive compressive strength and standard compressive strength at each curing cycle. The initial influence coefficient is corrected according to the curing cycle and the corresponding comprehensive compressive strength and standard compressive strength to obtain the corrected influence coefficient.

9. The testing method of concrete according to claim 8, wherein, The strength difference is determined according to the comprehensive compressive strength and standard compressive strength at each curing cycle. The correction factor is determined according to the strength difference. The corrected influence coefficient is determined according to the correction factor and the initial influence coefficient.

10. The testing method of concrete according to claim 9, characterized in that, According to the corrected influence coefficient, comprehensive compressive strength and several curing cycles, the relationship function between the compressive strength and curing cycle of the concrete specimens under simulated curing conditions is determined. The service maintenance time of the concrete specimens under simulated curing conditions is determined according to the relationship function and the preset compressive strength threshold.

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