A method and system for detecting the strength of concrete

Abstract

The application relates to the technical field of building engineering detection, in particular to a concrete strength detection method and system. The method comprises the following steps: placing a thermal reference block with known thermal physical properties in the center of a target area on the surface of concrete to be detected; applying synchronous thermal pulses with the same energy to the concrete surfaces on the two sides of the thermal reference block in a symmetrical position, recording the formed thermal response by using an infrared thermal imager, and generating thermal response data, wherein two thermal spots are formed on the surface of the concrete; monitoring the environmental temperature fluctuation by using the thermal reference block, subtracting the amount of the environmental temperature fluctuation from the thermal response data in real time, and obtaining the thermal decay curve of the thermal spot; analyzing the isotherm profile distortion degree of the thermal field corresponding to the thermal decay curve, and quantifying the thermal field structure polarization index. The application solves the problems of environmental interference and internal structure quantification in the existing concrete strength detection by using the building engineering detection technology.

Description

Technical Field

[0001] This invention relates to the field of building engineering testing technology, and in particular to a method and system for testing concrete strength. Background Technology

[0002] Traditional methods for testing concrete strength include physical testing techniques such as rebound hammer testing and ultrasonic testing. However, these methods still face significant challenges in the testing of low-strength concrete. Traditional rebound hammer and ultrasonic methods exhibit insufficient sensitivity in testing low-grade concrete (C20 and below). The measurement curves tend to flatten in the low-strength range, making it difficult to accurately distinguish between similar strength grades such as C15 and C10, resulting in low testing accuracy and susceptibility to the influence of surface hardening layers. Meanwhile, while existing thermal testing methods show potential, they are severely limited by external factors such as ambient temperature fluctuations, uneven surface emissivity, and air convection disturbances. The lack of effective environmental noise cancellation mechanisms leads to poor repeatability of measurement results, making it difficult to obtain reliable and stable data in complex field environments. More importantly, existing thermal methods mainly focus on defect imaging and internal structure visualization, and have not yet established a stable quantitative relationship between thermal response characteristics and the concrete's intrinsic strength. There is a lack of effective models and theoretical support for converting thermal parameters into practical engineering strength values.

[0003] In summary, there is an urgent need for a new non-destructive testing method that can accurately detect low-strength concrete in complex environments and establish a reliable correlation between thermal properties and strength. Summary of the Invention

[0004] Therefore, it is necessary to provide a concrete strength testing method and system to solve at least one of the above-mentioned technical problems.

[0005] To achieve the above objectives, a method for testing concrete strength includes the following steps:

[0006] Step S1: Place a thermophysical reference block with known thermophysical properties at the center of the target area on the concrete surface to be tested;

[0007] Step S2: Using the thermal reference block as the center, apply synchronous thermal pulses of the same energy to the concrete surfaces at symmetrical positions on both sides of it, and record the resulting thermal response with an infrared thermal imager to generate thermal response data, in which two hot spots are formed on the concrete surface.

[0008] Step S3: Monitor ambient temperature fluctuations using a thermal reference block, and subtract the amount of ambient temperature fluctuations from the thermal response data in real time to obtain the thermal decay curve of the hot spot;

[0009] Step S4: Analyze the distortion of the isotherm profile of the thermal field corresponding to the thermal decay curve and quantify it as the thermal field structure polarization index;

[0010] Step S5: Calculate the time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature to obtain the initial effective thermal relaxation time; use the thermal decay curve to compensate the time constant of the initial effective thermal relaxation time to obtain the effective thermal relaxation time.

[0011] Step S6: Substitute the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation to calculate the estimated equivalent strength of the concrete.

[0012] This invention uses a thermal reference block introduced in step S1 as a thermal benchmark, providing a stable reference point for subsequent measurements. Step S2 employs a double-sided symmetrical thermal pulse method to enhance signal strength and provide redundant verification. Step S3 utilizes the thermal reference block to monitor and compensate for ambient temperature fluctuations in real time, effectively eliminating environmental interference factors and significantly improving the measurement's anti-interference capability and repeatability. Step S4 innovatively analyzes the distortion degree of the isothermal contour of the thermal field, transforming the non-uniformity of the concrete's internal structure into a quantifiable thermal field structure polarization index, breaking through the limitations of traditional thermal methods that only focus on the temperature decay rate. Step S5 accurately captures the thermal diffusion characteristics of concrete through multiple time feature analyses and exponential fitting compensation, improving the detection sensitivity in the low-strength range. Step S6 employs a dual-parameter strength mapping model that comprehensively considers two key factors: structural non-uniformity and density, and introduces age and moisture content corrections, achieving accurate non-destructive testing of low-strength concrete (especially C20 and below) in complex on-site environments, providing rapid and reliable technical support for safety assessments of historical building renovations, temporary structures, and rural self-built houses.

[0013] Therefore, this invention provides a method for testing concrete strength. By introducing a thermal reference block as a dynamic environmental monitor, real-time cancellation of environmental noise is achieved. Simultaneously, the morphological distortion of heat diffusion is analyzed, using the "shape" of heat diffusion as a quantitative indicator of the non-uniformity of the internal structure of the concrete. Furthermore, a two-parameter strength mapping model is established by combining precisely calculated thermal relaxation time. This method cleverly solves the problems of environmental interference and internal structure quantification, realizing a non-destructive testing technology for low-strength concrete that is interference-resistant, high-precision, and easy to operate. It provides reliable technical support for safety assessments of historical building renovations, temporary structures, and rural self-built houses.

[0014] Preferably, the present invention also provides a concrete strength testing system for performing the concrete strength testing method described above, the concrete strength testing system comprising:

[0015] The thermal reference block, made of silicon nitride ceramic sheet with a high emissivity black coating, is placed at the center of the concrete surface to be tested.

[0016] The dual-head pulse heat source includes two power-adjustable focused infrared lamps and a synchronization controller, which are used to apply synchronous heat pulses of the same energy to the concrete surface, forming two hot spots on the concrete surface.

[0017] Infrared thermal imagers, with a thermal sensitivity of not less than 0.05℃, are used to continuously record the thermal response process and generate thermal response data;

[0018] The data processing unit is communicatively connected to the infrared thermal imager to receive thermal response data and communicatively connected to the synchronization controller of the dual-head pulse heat source to control the application of thermal pulses. This data processing unit is used to perform the following operations:

[0019] By using a thermal reference block to monitor ambient temperature fluctuations, the amount of ambient temperature fluctuations is subtracted from the thermal response data in real time to obtain the thermal decay curve of the hot spot.

[0020] The distortion of the isotherm profile of the thermal field corresponding to the thermal decay curve is analyzed and quantified as the thermal field structure polarization index.

[0021] The time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature is calculated to obtain the initial effective thermal relaxation time. The time constant is then compensated for the initial effective thermal relaxation time to obtain the effective thermal relaxation time.

[0022] By substituting the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation, the estimated equivalent strength of concrete is calculated.

[0023] The display unit, which is connected in communication with the data processing unit, is used to display thermal response data, thermal decay curves, thermal field structure polarization index, effective thermal relaxation time, and estimated equivalent intensity value.

[0024] The concrete strength testing system provided by this invention utilizes a thermal reference block made of silicon nitride ceramic sheets, which possesses precisely known thermophysical properties and stable radiation characteristics, providing a reliable benchmark for eliminating environmental interference. The dual-head pulse heat source design ensures precise control and spatial symmetry of energy input, enhancing measurement reliability. A high-sensitivity infrared thermal imager enables accurate capture of minute temperature differences, improving the resolution of thermal field analysis. The data processing unit integrates core algorithms such as environmental fluctuation compensation, thermal field morphology distortion analysis, thermal relaxation time calculation, and strength mapping, achieving automatic conversion from thermal response data to concrete strength. The display unit intuitively presents the testing process and results, facilitating on-site evaluation and decision-making. The entire system achieves portable, automated, and intelligent non-destructive testing of concrete strength, particularly suitable for rapid on-site testing of low-strength concrete. It significantly reduces testing time and labor costs, improves testing accuracy and reliability, and provides an efficient and practical technical means for building structure safety assessment, with broad engineering application prospects. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the steps involved in a concrete strength testing method.

[0026] Figure 2 This is a schematic diagram of the concrete strength testing system of the present invention.

[0027] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0028] The technical method of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0029] Furthermore, the accompanying drawings are merely illustrative of the invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor methods and / or microcontroller methods.

[0030] It should be understood that although the terms "first," "second," etc., may be used herein to describe various units, these units should not be limited by these terms. These terms are used merely to distinguish one unit from another. For example, without departing from the scope of the exemplary embodiments, a first unit may be referred to as a second unit, and similarly, a second unit may be referred to as a first unit. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0031] To achieve the above objectives, please refer to Figures 1 to 2 This invention provides a method for testing the strength of concrete, comprising the following steps:

[0032] Step S1: Place a thermophysical reference block with known thermophysical properties at the center of the target area on the concrete surface to be tested;

[0033] In this embodiment of the invention, a 1m × 1m test area is marked on the concrete surface to be tested. A thermal reference block made of a 10mm × 10mm × 1mm silicon nitride ceramic sheet is placed at the center of the area. This reference block has known thermophysical properties (specific heat capacity 700J / kg·K, thermal conductivity 30W / m·K), and its surface is coated with a high emissivity black coating with a thickness of 25±5μm and an emissivity ε=0.95±0.02, giving it stable radiation characteristics in the infrared band. The reference block is fixed to the concrete surface using silicone adhesive with a thermal conductivity of less than 0.2W / m·K, with the adhesive layer thickness controlled within 0.1mm to ensure good contact. Ambient temperature, humidity, and other parameters are measured and recorded. The temperature of the reference block is collected for 30 seconds, and the temperature standard deviation is calculated to ensure it is less than 0.05℃, indicating that the reference block has reached thermal equilibrium. Finally, the actual heat capacity value of the reference block at the current ambient temperature is calculated as the calibration heat capacity benchmark.

[0034] Step S2: Using the thermal reference block as the center, apply synchronous thermal pulses of the same energy to the concrete surfaces at symmetrical positions on both sides of it, and record the resulting thermal response with an infrared thermal imager to generate thermal response data, in which two hot spots are formed on the concrete surface.

[0035] In this embodiment of the invention, two heating points are marked 10 cm to each side of a thermal reference block. Two 800W focusing infrared lamps are adjusted so that their beam centers are precisely aligned with the marked points, and the spot diameter is controlled within 20±2 mm. The actual output energy of the two heat sources is measured using a heat flux density meter. The output energy difference between the two heat sources is kept within ±2% by fine-tuning the power controller, with a typical heat flux of 3.5 kW / m². An infrared thermal imager with a thermal sensitivity of not less than 0.05℃ is set up, covering the entire test area, and the sampling frequency is set to 25 Hz. The pulse duration of the electronic synchronization controller is set to 0.5 seconds to ensure that the two heat sources start and stop strictly synchronously. The infrared thermal imager is started to record, and then the synchronization controller is triggered to apply a 0.5-second thermal pulse to both heating points simultaneously. The thermal response process is recorded for another 60 seconds to obtain a complete bimodal thermal response spectrum.

[0036] Step S3: Monitor ambient temperature fluctuations using a thermal reference block, and subtract the amount of ambient temperature fluctuations from the thermal response data in real time to obtain the thermal decay curve of the hot spot;

[0037] In this embodiment of the invention, the average temperature value within a 5mm × 5mm area around the center point of the thermal reference block is extracted from the bimodal thermal response spectrum to form a time series. The average temperature of the reference block within the first 10 seconds of the thermal pulse is calculated as the environmental reference temperature. Then, the difference between the reference block temperature series and the reference temperature is calculated to obtain the temperature fluctuation series caused by environmental factors. A Butterworth low-pass filter with a cutoff frequency of 0.5Hz is applied to the temperature fluctuation series to obtain a smoothed environmental fluctuation series. The time delay (approximately 3.5 seconds) and attenuation coefficient (approximately 0.85) are calculated based on the ratio of the thermal diffusivity coefficient of the thermal reference block to that of concrete (approximately 30). The smoothed environmental fluctuation series is time-shifted and amplitude-adjusted according to the conduction correction parameters to generate an environmental fluctuation compensation matrix. The value at the corresponding position in the environmental fluctuation compensation matrix is ​​subtracted from the original hot spot temperature curve extracted from the thermal response data to obtain a pure thermal attenuation curve that eliminates environmental influences.

[0038] Step S4: Analyze the distortion of the isotherm profile of the thermal field corresponding to the thermal decay curve and quantify it as the thermal field structure polarization index;

[0039] In this embodiment of the invention, the pure thermal decay curve is analyzed to determine three characteristic time points where the hot spot temperature decays from its peak to 75%, 50%, and 25%. At these three time points, the isothermal contours of the hot spot are extracted from the corrected thermal field diagram. The geometric center coordinates of each contour are calculated, and the contour is translated to the origin and converted to polar coordinates. The radial distances in 72 directions are calculated at 5° intervals, and the average radius value is taken as the radius of the ideal circular contour. The difference in radial distance between the actual contour and the ideal circular contour is calculated to form a radial deviation sequence. The radial deviation sequence is subjected to Fourier transform to extract the amplitudes of the first and second order components, which respectively characterize the ellipticity and biaxial asymmetry of the contour. The ratio of the standard deviation of the radial deviation sequence to the average radius is calculated to obtain the dimensionless distortion coefficient. The distortion coefficients at the three time points are weighted and averaged to obtain the comprehensive distortion index. The average value and difference of the comprehensive distortion index of the two hot spots are calculated, and finally, the polarization index of the thermal field structure is calculated.

[0040] Step S5: Calculate the time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature to obtain the initial effective thermal relaxation time; use the thermal decay curve to compensate the time constant of the initial effective thermal relaxation time to obtain the effective thermal relaxation time.

[0041] In this embodiment of the invention, the highest temperature value and its corresponding time point are identified from the pure thermal decay curve, and determined as the peak temperature and peak time. 37% (i.e., 1 / e) of the peak temperature is calculated as the characteristic threshold for thermal decay. A three-point moving average filter is applied to the thermal decay curve to eliminate minor data fluctuations. Starting from the peak time, a linear interpolation method is used to accurately determine the time point when the temperature first drops to the threshold temperature. The time interval from the peak time to the threshold time point is calculated to obtain the characteristic decay time. The arithmetic mean of the characteristic decay times of the two hotspots is calculated, and a correction formula is applied based on the ambient temperature to obtain the initial effective thermal relaxation time. The thermal decay curve is fitted with an exponential function, and the time constant τ_fit in the fitting parameters is extracted. The deviation rate between the initial effective thermal relaxation time and the fitted time constant is calculated, and different strategies are used to adjust according to the magnitude of the deviation rate, ultimately obtaining the effective thermal relaxation time τ.

[0042] Step S6: Substitute the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation to calculate the estimated equivalent strength of the concrete.

[0043] In this embodiment of the invention, the thermal field structure polarization index PI and the effective thermal relaxation time τ are divided by a preset standard reference value to obtain normalized characteristic values. A binary linear regression formula is used to calculate the basic strength value. The product of the normalized characteristic values ​​is calculated and multiplied by the interaction coefficient to serve as an adjustment amount reflecting the mutual influence of the parameters. Based on the estimated age of the concrete and the surface moisture content, age correction coefficients and humidity correction coefficients are calculated. The basic strength value is added to the adjustment amount to obtain the corrected strength value. Then, based on the corrected strength value (F_corr), age correction coefficient (K_age), and humidity correction coefficient (K_moisture), a multi-factor correction value is calculated. The standard deviation and upper and lower limits of the 95% confidence interval of the multi-factor correction value are calculated. The multi-factor correction value is compared with the standard concrete strength grades (C10, C15, C20, etc.), and the concrete strength grade is determined based on the overlap between the confidence interval and the standard strength grade. The final output includes the estimated equivalent strength, including the numerical strength estimate (accurate to 0.1 MPa), the 95% confidence interval, and the estimated concrete strength grade, thus completing the entire concrete strength testing process.

[0044] Preferably, the monitoring process for ambient temperature fluctuations in step S3 includes:

[0045] Calculate the average temperature of the thermal reference block before the thermal pulse, and use it as the temperature reference point under steady-state environmental conditions;

[0046] Extract the temperature sequence of the thermal reference block region over time, subtract the temperature reference point, and obtain the temperature fluctuation sequence caused by environmental factors;

[0047] A low-pass filter is applied to the temperature fluctuation sequence to remove random noise while retaining the true trend of environmental temperature change, resulting in a smoothed environmental fluctuation sequence.

[0048] Based on the difference in thermophysical properties between the thermal reference block and concrete, the attenuation coefficient and time delay of environmental fluctuations transmitted from the thermal reference block to the hot spot area are calculated to obtain the transmission correction parameters.

[0049] Based on the transmission correction parameters, the smoothed environmental fluctuation sequence is time-shifted and amplitude-adjusted to generate an environmental fluctuation compensation matrix.

[0050] The corresponding environmental fluctuation compensation matrix is ​​subtracted from the original hot spot temperature curve extracted from the thermal response data to obtain the thermal attenuation curve that eliminates the environmental impact.

[0051] In this embodiment of the invention, when monitoring ambient temperature fluctuations, the temperature values ​​of the thermal reference block area are first sampled within 10 seconds before the heat pulse, with a sampling frequency of 10Hz, resulting in 100 temperature data points. These 100 temperature data points are summed and divided by 100 to calculate the average temperature value T_base, which serves as the temperature baseline under steady-state conditions. Next, the average temperature value within a 5mm × 5mm area surrounding the center point of the thermal reference block is extracted from the bimodal thermal response spectrum, forming a time series T_ref(t), where t represents the time from the start of the heat pulse, the total recording time is 60 seconds, and the sampling interval is 0.04 seconds. The environmental fluctuation sequence ΔT_env(t) is obtained by calculating T_ref(t) - T_base, which reflects temperature fluctuations caused by purely environmental factors (such as airflow, radiation changes, etc.). A Butterworth low-pass filter with a cutoff frequency of 0.5 Hz and a filter order of 3 is applied to the environmental fluctuation sequence ΔT_env(t). The filtering formula is ΔT_smooth(t) = LPF[ΔT_env(t)], where LPF represents the low-pass filtering operation. The resulting smoothed environmental fluctuation sequence ΔT_smooth(t) preserves the true trend of environmental temperature changes while removing high-frequency random noise. Based on the difference in thermophysical properties between the thermal reference block and concrete, conduction correction parameters are calculated. Specifically, the thermal diffusivity α_ref of the thermal reference block is 1.5 × 10⁻⁶. m² / s, the thermal diffusivity α_conc of concrete is 5.0 × m² / s, the ratio of the two is 30. According to the thermal diffusion theory, the time delay Δt for environmental fluctuations to be conducted from the thermal reference block to the hot spot region is the square of the distance between the two points divided by the difference in thermal diffusion coefficients. For a hot spot region with a distance of 10cm, the calculated time delay Δt = 3.5 seconds. The attenuation coefficient K is calculated based on the thermal resistance ratio of the two materials: K = (λ_ref × ρ_ref × c_ref) / (λ_conc × ρ_conc × c_conc), where λ_ref is the thermal conductivity of the thermal reference block (silicon nitride ceramic), ρ_ref is the density of the thermal reference block, c_ref is the specific heat capacity of the thermal reference block, λ_conc is the thermal conductivity of the concrete to be tested, ρ_conc is the density of the concrete to be tested, and c_conc is the specific heat capacity of the concrete to be tested. Substituting the specific parameter values, K = 0.85 is calculated. The environmental fluctuation sequence is smoothed according to the conduction correction parameters to generate the environmental fluctuation compensation matrix ΔT_comp(t,x,y). The specific operation is as follows: For each pixel (x, y) in the thermal response map, the time delay correction Δt_d = Δt × (d / 10cm)² is calculated based on its distance d from the center of the reference block. Then, the smoothed environmental fluctuation sequence is time-shifted and amplitude-adjusted: ΔT_comp(t, x, y) = K × ΔT_smooth(t - Δt_d) × exp(-d / d_0), where d_0 is the characteristic attenuation distance, which is 15cm. The value of the corresponding position in the corresponding environmental fluctuation compensation matrix is ​​subtracted from the original hot spot temperature curve T_orig(t) extracted from the thermal response data to obtain the corrected thermal attenuation curve T_corr(t) = T_orig(t) - ΔT_comp(t, x_0, y_0), where (x_0, y_0) are the coordinates of the hot spot center point. The corrected thermal attenuation curve T_corr(t) only reflects the thermal conductivity characteristics of the concrete itself, eliminating the influence of external factors such as environmental temperature fluctuations and airflow disturbances.

[0052] Preferably, the analysis process of the isothermal contour distortion degree of the thermal field in step S4 includes:

[0053] Determine the time point at which the hot spot decays from its peak temperature to 50%;

[0054] Extract the closed isotherm profile of the hot spot at the decay time point;

[0055] Calculate the geometric center coordinates of the closed isotherm profile, translate the closed isotherm profile to the origin of the coordinate system, and obtain the standardized profile.

[0056] Calculate the average radius value of the standardized profile;

[0057] Using the average radius value as the ideal circular profile, the radial distance difference between the standardized profile and the ideal circular profile is calculated to obtain the radial deviation sequence;

[0058] Perform a Fourier transform on the radial deviation sequence to extract the amplitudes of the first and second order components;

[0059] The ratio of the standard deviation of the radial deviation sequence to the mean radius is calculated to obtain the dimensionless distortion coefficient that characterizes the degree of profile irregularity.

[0060] The comprehensive distortion index is obtained by weighted averaging of thermal field morphology distortion of hot spots at multiple time points based on the dimensionless distortion coefficient.

[0061] The polarization index of the thermal field structure is calculated based on the comprehensive distortion index.

[0062] In this embodiment of the invention, the precise time point t_50% when the hot spot temperature drops from its highest temperature value Tmax to 50% (i.e., 0.5 × Tmax) is determined by analyzing the pure thermal decay curve T_corr(t). Specifically, starting from the peak time tmax, the search proceeds backward along the time axis, using linear interpolation to find the moment when the temperature is exactly 0.5 × Tmax. For typical C15 strength concrete, this time point usually occurs 8 to 12 seconds after the end of the thermal pulse. At the determined t_50% time, the isothermal contour of the hot spot is extracted from the corrected thermal field map. The extraction method involves marking pixels with a temperature value set to 0.5 × Tmax ± 0.1℃ as contour points, and then connecting these contour points using an edge tracking algorithm to form a closed isothermal contour C(x,y). For each hot spot, typically 200 to 300 contour point coordinates are obtained, forming a complete closed curve.

[0063] Calculate the profile of a closed isotherm Geometric center coordinates The calculation formula is:

[0064] , ;

[0065] in For the outline of the first The coordinates of the points This represents the total number of contour points.

[0066] outline Translate to the origin, i.e. To obtain a standardized profile Standardize the profile Convert to polar coordinates ,in This represents the distance from the origin to the contour point. This represents the polar angle, ranging from 0 to 2π. In the polar coordinate system, radial distances in 72 directions are calculated at 5° intervals. ,in = ×5°×π / 180, =0,1,2,...,71. Calculate the average of these 72 radial distances. ,Will The radius is taken as the ideal circular profile. For each angle... Calculate the radial distance difference between the actual profile and the ideal circular profile. , forming a radial deviation sequence { }, =0,1,2,...,71.

[0067] For radial deviation sequence { The discrete Fourier transform is calculated using the following formula:

[0068] ;

[0069] in =0,1,2,...,71 The imaginary unit commonly used in engineering (i.e. ), to distinguish it from the circular index value Extracting first-order components from the Fourier transform results. and second-order components amplitude and The first-order component characterizes the degree of ellipticization of the profile, while the second-order component characterizes the degree of "dumbbell-shaped" distortion of the profile. The radial deviation sequence is calculated. Standard deviation Then calculate the dimensionless distortion coefficient. This coefficient characterizes the degree of irregularity in the profile; a larger value indicates a more significant non-uniformity in the internal structure of the concrete. Repeat the above steps to calculate the dimensionless distortion coefficients D_25%, D_50%, and D_75% for the hot spot at 25% (t_25%) and 75% (t_75%) of the peak temperature decay.

[0070] The comprehensive distortion index DI is calculated according to the preset weights as 0.2×D_25%+0.5×D_50%+0.3×D_75%. The average comprehensive distortion index of the two hot spots is calculated as DI_avg=(DI_1+DI_2) / 2 and the difference in comprehensive distortion index is calculated as DI_diff=|DI_1-DI_2| / DI_avg, where DI_1 is the comprehensive distortion index of the first hot spot and DI_2 is the comprehensive distortion index of the second hot spot.

[0071] Finally, the polarization index of the thermal field structure was calculated:

[0072] PI=0.6×DI_avg+0.2×DI_diff+0.2×(Z_1+Z_2) / 4;

[0073] Z_1=|F_1(1)| / r_avg+|F_1(2)| / r_avg;

[0074] Z_2=|F_2(1)| / r_avg+|F_2(2)| / r_avg;

[0075] Wherein, PI is the thermal field structure polarization index, Z_1 is the normalized harmonic distortion coefficient of the first hot spot, Z_2 is the normalized harmonic distortion coefficient of the second hot spot, F_1(1) and F_1(2) are the first and second order Fourier transform components of the radial deviation of the first hot spot at time t_50%, F_2(1) and F_2(2) are the first and second order Fourier transform components of the radial deviation of the second hot spot at time t_50%, DI_avg is the average value of the comprehensive distortion index, DI_diff is the degree of difference of the comprehensive distortion index, and r_avg is the average radius of the isotherm profile. The thermal field structure polarization index PI is a dimensionless parameter, and its value is usually between 0.05 and 0.5. The PI value of low-strength concrete (such as C10) is usually above 0.3, while the PI value of high-strength concrete (such as C20) is usually below 0.2.

[0076] Preferably, the calculation process of the initial effective thermal relaxation time in step S5 includes:

[0077] Find the highest temperature value of the thermal decay curve and its corresponding time point, and determine it as the peak temperature and peak time.

[0078] Calculate 37% of the peak temperature as the characteristic threshold for heat decay;

[0079] A three-point moving average filter is applied to the thermal decay curve to eliminate minor data fluctuations and obtain a smooth decay curve.

[0080] Starting from the peak moment, the thermal decay curve is searched backward to determine the time point when the temperature first drops to the characteristic threshold, which is recorded as the characteristic threshold time point;

[0081] Calculate the time interval from the peak time to the characteristic threshold time point to obtain the characteristic decay time;

[0082] Calculate the arithmetic mean of the characteristic decay times of the two hot spots to obtain the hot spot decay time;

[0083] The hot spot decay time is calibrated by applying a temperature correction formula based on the ambient temperature during the test to obtain the initial effective thermal relaxation time.

[0084] In this embodiment of the invention, the pure thermal decay curve T_corr(t) is analyzed. By traversing the entire time series, the highest temperature value Tmax and its corresponding peak time tmax are found, which are then determined as the peak temperature and peak time. For typical C15 strength concrete, the peak temperature is reached within 0.5 to 1.5 seconds after the thermal pulse ends, and the peak temperature is usually 3 to 5°C higher than the ambient temperature. The characteristic threshold temperature T_threshold is calculated as 0.37 × Tmax + T_ambient, where T_ambient is the ambient reference temperature. The characteristic threshold temperature corresponds to the heat decaying to 37% of the peak value (i.e., 1 / e, where e is the base of the natural logarithm). This threshold is chosen based on the characteristic decay ratio in the thermal diffusion theory, which can accurately reflect the thermal conductivity characteristics of the material. A three-point moving average filter is applied to the pure thermal decay curve T_corr(t), i.e., T_smooth(i) = [T_corr(i-1) + T_corr(i) + T_corr(i+1)] / 3, where T_smooth(i) is the temperature value of the i-th point in the discrete data point sequence after the three-point moving average filter, and T_corr(i-1), T_corr(i), and T_corr(i+1) are the temperature values ​​of the corrected thermal decay curve at the (i-1), i, and (i+1) sampling times, respectively, where i ranges from 1 to n-1, and n is the total number of data points. This filtering operation eliminates small data fluctuations in the curve, ensuring the accuracy of subsequent time point determination. Starting from the peak time tmax, the smooth decay curve T_smooth(t) is searched backward along the time axis to find the time point t_threshold when the temperature first drops to the characteristic threshold T_threshold. Since temperature data is sampled discretely, linear interpolation is typically used to determine the precise threshold crossover time between two adjacent sampling points.

[0085] t_threshold = t_i + (t_i+1 - t_i) × (T_threshold - T_smooth(i)) / (T_smooth(i+1) - T_smooth(i)), where t_i and t_i+1 are two adjacent sampling times, and T_smooth(i) > T_threshold > T_smooth(i+1). The characteristic decay time τ = t_threshold - tmax is calculated in seconds. The characteristic decay time reflects the rate of heat diffusion in concrete and is directly related to the concrete's density and internal structure. The characteristic decay times τ1 and τ2 are calculated for two hot spots, and then their arithmetic mean τ_avg = (τ1 + τ2) / 2 is calculated to obtain the hot spot decay time. The average value of the two hot spots eliminates the influence of local inhomogeneities and improves the representativeness of the measurement. The hot spot decay time is calibrated based on the ambient temperature T_test during testing. The initial effective thermal relaxation time τ_0 is calculated as follows: τ_avg × [1 + α × (T_test - T_ref)], where T_ref is the standard reference temperature of 20℃, and α is a temperature correction factor with a value of 0.015 / ℃. This calibration process considers the influence of ambient temperature on the thermal diffusivity of concrete, ensuring the comparability of thermal relaxation times measured under different environmental conditions. For low-strength concrete (such as C10-C15), the initial effective thermal relaxation time τ_0 is typically in the range of 15 to 25 seconds, while for higher-strength concrete (such as C20-C25), τ_0 is typically in the range of 8 to 15 seconds. The initial effective thermal relaxation time is an important indicator of concrete density and provides a key parameter for subsequent strength assessment.

[0086] Preferably, the calculation process for estimating the equivalent strength in step S6 includes:

[0087] The normalized characteristic values ​​are obtained by dividing the thermal field structure polarization index and the effective thermal relaxation time by their respective preset standard reference values.

[0088] Based on the polarization index of the thermal field structure and the effective thermal relaxation time, the basic strength value is calculated using a binary linear regression formula.

[0089] Calculate the product term of the normalized eigenvalues ​​as an adjustment measure to reflect the mutual influence of the parameters;

[0090] Obtain and calculate the age correction factor and humidity correction factor based on the estimated age of the concrete and the surface moisture content;

[0091] The corrected strength value is obtained by adding the basic strength value to the adjustment amount, and then the multi-factor correction value is calculated based on the corrected strength value, the age correction factor, and the humidity correction factor.

[0092] Calculate the standard deviation and 95% confidence interval of the multi-factor corrected values;

[0093] The multi-factor correction values ​​are compared with the standard concrete strength grade based on the standard deviation and 95% confidence interval to determine the concrete strength grade, which is then used as the estimated equivalent strength.

[0094] In this embodiment of the invention, the polarization index PI of the thermal field structure and the effective thermal relaxation time τ are divided by preset standard reference values ​​to obtain normalized characteristic values. The standard reference values ​​are PI_ref = 0.25 and τ_ref = 15 seconds, which represent characteristic parameters of typical C15 strength concrete. The normalized characteristic values ​​are calculated as PI_norm = PI / PI_ref and τ_norm = τ / τ_ref. Normalization makes the two parameters with different dimensions comparable, facilitating subsequent multi-parameter comprehensive analysis. Based on the polarization index of the thermal field structure and the effective thermal relaxation time, the strength base value F_base is calculated using a binary linear regression formula. + ×PI_norm+ ×τ_norm, where , , The coefficient is determined experimentally for the low-strength concrete range (C10-C20). =15.0, =-8.5, =-4.2. This regression formula reflects the basic relationship between concrete strength and thermal property parameters, where PI_norm is negatively correlated with strength (the larger the PI value, the more heterogeneous the internal structure, and the lower the strength), and τ_norm is also negatively correlated with strength (the longer the thermal relaxation time, the slower the thermal diffusion, the lower the density, and the lower the strength). The product term PI_norm × τ_norm of the normalized eigenvalues ​​is calculated, multiplied by the interaction coefficient. =2.1, thus obtaining the parameter interaction adjustment amount Δ_interact= ×PI_norm×τ_norm. This interaction term reflects the nonlinear interaction between the two parameters, especially in very low-strength concrete, where the combined effects of internal structural inhomogeneity and low density are more pronounced. The estimated age (in days) and surface moisture content (W) of the concrete are obtained, and the age correction factor K_age and the moisture correction factor K_moisture are calculated.

[0095] The formula for calculating the age correction factor is: K_age = 1.0 - 0.2 × exp(-Age / 7);

[0096] For mature concrete with an age of more than 28 days, K_age is close to 1.0; for early-stage concrete with an age of only 7 days, K_age is about 0.85.

[0097] The formula for calculating the humidity correction factor is:

[0098] K_moisture = 1.0 - 0.05 × (W - 3);

[0099] When the surface moisture content W is 3%, K_moisture = 1.0; when the moisture content increases to 8%, K_moisture decreases to 0.75. The corrected strength value F_corr = F_base + Δ_interact is obtained by adding the base strength value and the interaction adjustment. Then, based on the corrected strength value, the age correction factor, and the humidity correction factor, the multi-factor correction value F_multi = F_corr × K_age × K_moisture is calculated, in MPa. This multi-factor correction value comprehensively considers the influence of factors such as the thermal properties of concrete, age, and moisture content. The standard deviation of the multi-factor correction value is calculated as σ_F = 0.5 + 0.1 × F_multi, in MPa. The standard deviation increases with increasing strength value, reflecting the relatively large measurement uncertainty in the high-strength range. Calculate the upper and lower limits of the 95% confidence interval: F_upper = F_multi + 1.96 × σ_F and F_lower = F_multi - 1.96 × σ_F, where F_upper is the upper limit of the 95% confidence interval and F_lower is the lower limit. The confidence interval reflects the reliability range of the strength estimate. Compare the multi-factor correction value F_multi with the standard concrete strength grade to determine the concrete strength grade. The specific comparison rules are: when F_lower ≤ 10.0 and F_upper ≥ 10.0, it is determined to be C10; when F_lower ≤ 15.0 and F_upper ≥ 15.0, it is determined to be C15; when F_lower ≤ 20.0 and F_upper ≥ 20.0, it is determined to be C20. If F_lower < 10.0 and F_multi < 10.0, it is determined to be below C10; if F_lower > 20.0, it is determined to be above C20. The final output is the estimated equivalent strength result, including the numerical strength estimate F_multi (accurate to 0.1MPa), the 95% confidence interval [F_lower, F_upper], and the estimated concrete strength grade, such as "Estimated equivalent strength: 13.5MPa, 95% confidence interval [11.8, 15.2]MPa, estimated strength grade C15".

[0100] Preferably, step S4 further includes analyzing the thermal field morphological distortion at multiple time points:

[0101] In addition to the time point when the hot spot decays from the peak temperature to 50%, isotherm profiles are also extracted at the time points when the hot spot decays from the peak temperature to 25% and 75%.

[0102] Calculate the distortion coefficient values ​​at the three time points based on the isotherm profiles;

[0103] The distortion coefficients at the three time points were weighted and averaged to obtain the comprehensive distortion index, with the weights corresponding to the 50%, 25%, and 75% time points being 0.2, 0.5, and 0.3, respectively.

[0104] In this embodiment of the invention, by analyzing the pure thermal decay curve T_corr(t), three characteristic time points t_25%, t_50%, and t_75% are determined when the hot spot decays from its highest temperature value Tmax to 25%, 50%, and 75%, respectively. Specifically, the determination method starts from the peak time tmax and searches backward along the time axis, using linear interpolation to find three moments when the temperature is exactly 0.75×Tmax, 0.5×Tmax, and 0.25×Tmax. For typical C15 strength concrete, t_75% typically occurs 4 to 6 seconds after the end of the thermal pulse, t_50% occurs 8 to 12 seconds, and t_25% occurs 18 to 25 seconds. These three time points represent the early, middle, and late stages of heat diffusion, respectively, capturing the complete dynamic characteristics of the heat diffusion process. At the determined t_25%, t_50%, and t_75% time points, the isothermal contours of the hot spot are extracted from the corrected thermal field diagram. The extraction method involves marking pixels with temperature values ​​of 0.75×Tmax±0.1℃, 0.5×Tmax±0.1℃, and 0.25×Tmax±0.1℃ as contour points. An edge tracking algorithm is then used to connect these contour points, forming three closed isothermal contours: C_75%(x,y), C_50%(x,y), and C_25%(x,y). For each time point of each hot spot, typically 200 to 300 contour point coordinates are obtained to form a complete closed curve. Standardization is performed on the isothermal contours at the three time points: the geometric center coordinates are calculated, the contour is translated to the origin, converted to polar coordinates, the average radius is calculated, an ideal circular reference contour is generated, the radial deviation sequence is calculated, and finally, the dimensionless distortion coefficients D_75%, D_50%, and D_25% are calculated. The specific calculation process is the same as the distortion coefficient calculation method for a single time point, but it is applied to the contour data at three different time points. The distortion coefficients D_75%, D_50%, and D_25% at the three time points were weighted and averaged to calculate the comprehensive distortion index DI = 0.2 × D_25% + 0.5 × D_50% + 0.3 × D_75%. In this weighting scheme, the distortion coefficient D_50% at the mid-stage (50% decay point) was assigned the highest weight of 0.5, because the thermal field morphology best reflects the internal structural characteristics of concrete at this point; the distortion coefficient D_75% at the early stage (75% decay point) was assigned a weight of 0.3, as the thermal field morphology mainly reflects surface characteristics at this point; and the distortion coefficient D_25% at the late stage (25% decay point) was assigned a weight of 0.2, as the thermal field has spread further, and the influence of boundary effects and deep structures is enhanced. This multi-time-point comprehensive analysis overcomes the limitations of single-time-point assessment, comprehensively capturing the structural characteristics of concrete at different depths and scales.For the two hot spots, calculate the comprehensive distortion indices DI_1 and DI_2 respectively, and then calculate the average value DI_avg=(DI_1+DI_2) / 2 and the difference DI_diff=|DI_1-DI_2| / DI_avg.

[0105] Finally, using the average value of the comprehensive distortion index DI_avg and the difference in the comprehensive distortion index DI_diff obtained from the multi-time point analysis, the polarization index of the thermal field structure under the multi-time point analysis is calculated again.

[0106] This multi-time-point analysis method significantly improves the stability and representativeness of thermal field morphological distortion assessment, providing a more reliable indicator for the accurate quantification of the non-uniformity of the internal structure of concrete.

[0107] Preferably, the time constant compensation for the initial effective thermal relaxation time using the thermal decay curve in step S5 includes:

[0108] The thermal decay curve is fitted with an exponential function to obtain the fitting parameters;

[0109] Extract the time constant from the fitted parameters;

[0110] The initial effective thermal relaxation time was verified and adjusted using a time constant to obtain the effective thermal relaxation time.

[0111] In this embodiment of the invention, a set of temperature data points {t_i, T_i} after the peak time tmax is extracted from the pure thermal decay curve T_corr(t), where i = 1, 2, ..., n, t_i represents the time point, T_i represents the corresponding temperature value, and n is the total number of data points, typically within 60 seconds. The extracted temperature data point set is fitted using the exponential function model T(t) = T_ambient + ΔT × exp(-(t - tmax) / τ_fit), where T_ambient is the ambient reference temperature, ΔT is the temperature increment parameter, and τ_fit is the time constant to be determined. The optimal fitting parameters are determined using the least squares method, i.e., solving for the residual sum of squares S = ∑[T_i - T(t_i)]. The minimum parameter value is determined. An iterative optimization algorithm is used, with initial values ​​set as T_ambient = end temperature, ΔT = peak temperature - T_ambient, and τ_fit = initial effective thermal relaxation time τ_0. Iterative calculations are performed until the parameter change rate is less than 0.1% or the maximum number of iterations (50) is reached. The time constant τ_fit is extracted from the fitting results. This parameter characterizes the characteristic timescale of heat decay in concrete, reflecting the overall thermal diffusion performance of the concrete. Fittings are performed on two hot spots to obtain the time constants τ_fit1 for the first hot spot and τ_fit2 for the second hot spot. The average value τ_fit_avg = (τ_fit1 + τ_fit2) / 2 is calculated. The goodness-of-fit R-value is then calculated. =1-∑[T_i-T(t_i)] / ∑[T_i-T_mean] Where T_mean is the average of all temperature data. The goodness-of-fit R² is used to evaluate the accuracy of the exponential model in describing the actual thermal decay process. A value closer to 1 indicates a more accurate fit. For a typical concrete heat decay curve, R0 Typically, it is above 0.95. Calculate the deviation rate δ = |τ_0 - τ_fit_avg| / τ_0 between the initial effective thermal relaxation time τ_0 and the fitted time constant τ_fit_avg. If the deviation rate δ is less than 10%, the verification is successful, and the initial effective thermal relaxation time τ_0 is directly used as the final effective thermal relaxation time τ. If the deviation rate δ is between 10% and 30%, a weighted adjustment is performed, and τ = (2 × τ_0 + τ_fit_avg) / 3 is calculated. If the deviation rate δ is greater than 30%, it indicates that the thermal decay process has obvious non-exponential characteristics or measurement anomalies. In this case, a more complex double-exponential model T(t) = T_ambient + ΔT1 × exp(-(t - tmax) / τ1) + ΔT2 × exp(-(t - tmax) / τ2) is used for refitting, and then the equivalent time constant τ_eq = (ΔT1 × ... The effective thermal relaxation time τ = (τ_0 + τ_eq) / 2 is ultimately determined by the formula τ1 + ΔT2 × τ2) / (ΔT1 + ΔT2). Here, ΔT1 is the initial temperature increment of the rapid decay component (in °C), representing the temperature rise caused by heat absorbed by the concrete surface; τ1 is the time constant of the rapid decay component (in seconds), representing the characteristic time of surface heat diffusion, with a relatively small value (usually 3-8 seconds); ΔT2 is the initial temperature increment of the slow decay component (in °C), representing the temperature rise caused by heat penetrating deep into the concrete; and τ2 is the time constant of the slow decay component (in seconds), representing the characteristic time of deep heat diffusion, with a relatively large value (usually 15-30 seconds). The equivalent time constant τ_eq is calculated by weighted averaging of the temperature increments of each component to comprehensively characterize the overall thermal relaxation characteristics. The initial effective thermal relaxation time is verified and adjusted using exponential function fitting, enhancing the robustness of the thermal relaxation time measurement, especially for low-strength concrete with complex internal structures, whose thermal decay process often exhibits multi-timescale characteristics. The final effective thermal relaxation time τ combines the advantages of both direct measurement and model fitting methods, providing a more accurate thermal characteristic parameter for concrete strength assessment.

[0112] Preferably, the thermal reference block in step S1 is made of silicon nitride ceramic sheet, and its surface is coated with a uniform high emissivity black coating, so that it has stable radiation characteristics in the infrared band.

[0113] In this embodiment of the invention, the thermal reference block is made of a silicon nitride ceramic sheet with dimensions of 10mm × 10mm × 1mm. This material has high thermal conductivity (30W / m·K), low heat capacity (700J / kg·K), and high temperature stability (temperature range -50℃ to 500℃). The silicon nitride ceramic is selected from high-purity (purity ≥99.5%) and high-density (density ≥3.1g / cm³) materials to ensure the consistency and stability of physical properties. The surface of the thermal reference block is coated with a high-emissivity black coating with a thickness of 25±5μm. The coating material is carbon black-filled polyacrylate with an emissivity ε=0.95±0.02 (within the 8-14μm infrared band), ensuring stable and near-blackbody radiation characteristics within the working band of the infrared thermal imager. The coating is achieved through a precision spraying process, with the coating thickness variation coefficient controlled within 5% and the surface roughness Ra<1.0μm to ensure spatial uniformity of emissivity. Before use, the thermal reference block undergoes rigorous thermophysical property calibration, including measuring its thermal conductivity, specific heat capacity, and thermal diffusivity at three temperature points: 20℃, 30℃, and 40℃, to ensure parameter accuracy. The thermal reference block is installed at the center of the target area on the concrete surface to be tested, and fixed using silicone adhesive with a thermal conductivity of less than 0.2 W / m·K. The adhesive layer thickness is controlled within 0.1 mm to ensure full contact between the reference block and the concrete surface while minimizing thermal coupling effects. After fixing, it is left to stand for 15 minutes to allow the reference block to reach thermal equilibrium with the environment. During this period, the surface temperature of the reference block is monitored, and temperature fluctuations are controlled within ±0.1℃ before subsequent testing can proceed. In the testing system, the thermal reference block serves as a thermal reference standard and an environmental fluctuation monitor. Its high thermal conductivity and low heat capacity enable it to respond quickly to changes in ambient temperature, while its stable emissivity ensures the accuracy of infrared thermometry, providing a reliable benchmark for subsequent environmental noise cancellation.

[0114] Preferably, the synchronous thermal pulse in step S2 is achieved by two side-by-side focused infrared lamps, the pulse duration is 0.5 seconds, and the output energy difference between the two focused infrared lamps does not exceed ±2%.

[0115] In this embodiment of the invention, the synchronized thermal pulse is achieved through two side-by-side focused infrared lamps. The infrared lamps are halogen quartz bulbs with a rated power of 800W, an operating voltage of 220V, and an emission spectrum concentrated in the 1.0-3.0μm band, exhibiting a thermal radiation efficiency ≥85%. Each infrared lamp is equipped with an elliptical reflector cup with a focal length of 150mm. The inner surface of the reflector cup is aluminum-plated and polished, achieving a reflectivity ≥95%. The two infrared lamps are mounted on a precision adjustment bracket, which allows for ±10mm fine-tuning in three directions to ensure accurate spot positioning. The distance between the infrared lamps is fixed at 200mm, and the vertical distance from the concrete surface is 300mm, with the illumination angle perpendicular to the concrete surface. The center of the light spots from the two infrared lamps is located 100mm to each side of the thermal reference block, forming a symmetrical layout. The spot diameter of the infrared lamps is controlled within the range of 20±2mm, achieved by adjusting the focusing device. The two infrared lamps are connected to the same electronic synchronization controller, which uses a solid-state relay drive circuit with a switching time <1ms, ensuring synchronous start-up and shutdown of the two lamps. The pulse duration is precisely set to 0.5 seconds, with a time error of <±5ms, controlled by a high-precision timing circuit. Before applying the thermal pulse, a heat flux meter is used to measure the heat flux of the two infrared lamps at the target location. By fine-tuning the power input of the infrared lamps, the difference in heat flux between the two light spots is controlled within ±2%, with a typical value of 3.5kW / m². Before pulse triggering, the system performs a self-check to confirm that the infrared thermal imager has started recording. Then, the synchronous controller sends a trigger signal, simultaneously activating both infrared lamps, which are simultaneously turned off after a precise 0.5 seconds. This precisely controlled synchronous thermal pulse method ensures that the two hot spots receive exactly the same heat input, providing reliable initial conditions for subsequent thermal field analysis. At the same time, the short pulse characteristic avoids overheating of the concrete surface, maintaining the non-destructive nature of the measurement.

[0116] Preferably, the present invention also provides a concrete strength testing system for performing the concrete strength testing method described above, the concrete strength testing system comprising:

[0117] The thermal reference block, made of silicon nitride ceramic sheet with a high emissivity black coating, is placed at the center of the concrete surface to be tested.

[0118] The dual-head pulse heat source includes two power-adjustable focused infrared lamps and a synchronization controller, which are used to apply synchronous heat pulses of the same energy to the concrete surface, forming two hot spots on the concrete surface.

[0119] Infrared thermal imagers, with a thermal sensitivity of not less than 0.05℃, are used to continuously record the thermal response process and generate thermal response data;

[0120] The data processing unit is communicatively connected to the infrared thermal imager to receive thermal response data and communicatively connected to the synchronization controller of the dual-head pulse heat source to control the application of thermal pulses. This data processing unit is used to perform the following operations:

[0121] By using a thermal reference block to monitor ambient temperature fluctuations, the amount of ambient temperature fluctuations is subtracted from the thermal response data in real time to obtain the thermal decay curve of the hot spot.

[0122] The distortion of the isotherm profile of the thermal field corresponding to the thermal decay curve is analyzed and quantified as the thermal field structure polarization index.

[0123] The time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature is calculated to obtain the initial effective thermal relaxation time. The time constant is then compensated for the initial effective thermal relaxation time to obtain the effective thermal relaxation time.

[0124] By substituting the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation, the estimated equivalent strength of concrete is calculated.

[0125] The display unit, which is connected in communication with the data processing unit, is used to display thermal response data, thermal decay curves, thermal field structure polarization index, effective thermal relaxation time, and estimated equivalent intensity value.

[0126] In the embodiments of this invention:

[0127] 1. System Initialization Phase: First, the data processing unit is started, and the system performs a self-test to confirm the connection status of each component. The data processing unit establishes a data communication link with the infrared thermal imager and a control communication link with the synchronous controller of the dual-head pulse heat source. The system loads the pre-calibrated intensity correlation equations and reference database.

[0128] 2. Test Preparation Stage: The operator cleans the concrete surface to be tested, ensuring it is dry and free of obvious contaminants. A 1m x 1m square area is marked in the center of the test area. The thermal reference block (a 10mm x 10mm x 1mm silicon nitride ceramic sheet coated with a high-emissivity black coating) is fixed in the center of the marked area. The position of the dual-head pulse heat source is adjusted so that the two focusing infrared lamps are aligned with positions 10cm to either side of the thermal reference block, and the power output is adjusted to ensure that the difference in output energy between the two infrared lamps does not exceed ±2%.

[0129] 3. Environmental Parameter Acquisition Phase: The system measures and records environmental parameters such as temperature and humidity. The infrared thermal imager begins continuous monitoring of the test area, acquiring the initial temperature distribution of the thermal reference block and the surrounding concrete surface, and establishing a temperature baseline for 10 seconds.

[0130] 4. Thermal Pulse Excitation Stage: The data processing unit sends a trigger signal to the synchronization controller. The synchronization controller precisely controls the two focusing infrared lamps to turn on simultaneously for 0.5 seconds and then turn off, applying a thermal pulse of equal energy to the concrete surface, forming two hot spots on the surface. The infrared thermal imager continuously records the temperature changes of the entire area at a frequency of not less than 25Hz, including the complete thermal response process for 60 seconds before, during, and after the application of the thermal pulse.

[0131] 5. Data Processing Stage: The data processing unit receives the thermal response data transmitted by the infrared thermal imager and performs the following processing:

[0132] The temperature sequence of the thermal reference block region is extracted, the ambient temperature fluctuation is calculated, and the ambient temperature fluctuation compensation matrix is ​​generated.

[0133] The corrected thermal decay curve is obtained by subtracting the environmental fluctuation compensation from the original thermal response data.

[0134] Hot spot isotherm profiles were extracted at multiple time points (peak temperature decayed to 25%, 50%, and 75%).

[0135] Analyze the morphological distortion of the isotherm profile and calculate the polarization index of the thermal field structure;

[0136] Determine the time required for the hot spot temperature to decay from its peak value to 37%, and calculate the initial effective thermal relaxation time;

[0137] The thermal decay curve is fitted with an exponential function to extract the time constant and compensate for the initial effective thermal relaxation time.

[0138] The estimated equivalent strength of concrete is calculated by substituting the thermal field structure polarization index and effective thermal relaxation time into a pre-calibrated correlation equation.

[0139] 6. Result Output Stage: The data processing unit transmits the processing results to the display unit. The display unit presents key data in graphical and numerical form, including:

[0140] Original thermal response image and corrected thermal field distribution map;

[0141] Thermal decay curve of the hot spot center point;

[0142] Isotherm profiles and their distortion analysis results;

[0143] The polarization index of the thermal field structure (dimensionless).

[0144] Effective thermal relaxation time (seconds);

[0145] Estimated equivalent strength value (MPa) and its 95% confidence interval;

[0146] Estimated concrete strength grade (e.g., C10, C15, C20, etc.).

[0147] 7. Data Storage and Report Generation Stage: The system stores test results, raw data, and processing procedures in the database, generating a test report that includes test point location information, environmental parameters, measurement results, and intensity assessment conclusions. The report can be viewed through the display unit or exported as a standard format file.

[0148] Please see Figure 2 The diagram shown is a structural schematic of the concrete strength testing system of this invention. It adopts a symmetrical design, with a thermal reference block as the central axis, and the components are rationally distributed, forming a complete testing chain from excitation to detection and result output. Figure 2 The specific explanation is as follows:

[0149] Detection target area: The concrete to be tested is located at the bottom of the figure and serves as the target object to be tested; the thermal reference block is a small black block located in the center of the concrete surface, serving as a temperature benchmark and environmental monitoring reference point; the hot spot is a two symmetrical red elliptical area generated by a dual-headed pulse heat source, which is the temperature response zone formed by the heat pulse on the concrete surface.

[0150] Excitation Components: The dual-head pulse heat source consists of two focused infrared lamps, which are orange oval in shape and symmetrically distributed on both sides of the thermal reference block. Each focused infrared lamp contains a focusing lens system that emits synchronous thermal pulses onto the concrete surface. The synchronization controller is located between the two infrared lamps to ensure the precise synchronous operation of the dual-head heat source.

[0151] Detection component: The infrared thermal imager is the blue device located at the top center of the image. It has a large lens system that continuously captures the temperature distribution changes throughout the entire test area.

[0152] Data processing and display components: The data processing unit is a green rectangular module located in the upper left of the figure, serving as the core of the control and analysis of the entire system; the display unit is an orange rectangular module located in the upper right of the figure, containing a white display screen to display the detection results in real time (for example, the intensity level "C25" is displayed in the figure).

[0153] Data flow direction (the data flow direction in the diagram is clearly indicated by connecting lines of different colors):

[0154] 1. Control process:

[0155] The data processing unit sends "control signals" to the synchronization controller via the thick green line;

[0156] The synchronization controller sends synchronization control commands to the two focusing infrared lamps via the purple line;

[0157] 2. Data Acquisition Process:

[0158] The infrared thermal imager transmits "thermal response data" to the data processing unit via a thick blue line;

[0159] The data processing unit transmits the "processing results" to the display unit via a thick green line;

[0160] 3. Physical excitation process:

[0161] Two focused infrared lamps emit thermal pulses onto the concrete surface through orange radiation.

[0162] The infrared thermal imager observes the entire test area through the blue dashed triangular region.

[0163] Figure 2 Working principle: Based on thermal pulse excitation-infrared response analysis technology, a standardized thermal excitation is generated on the concrete surface through a symmetrical dual-head heat source. The thermal response process is monitored in real time using an infrared thermal imager. The data processing unit analyzes the thermal field characteristics and time response characteristics, and finally outputs the concrete strength grade assessment result.

[0164] Therefore, the embodiments should be considered as exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of the equivalents of the application are intended to be included within the invention.

[0165] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

Claims

1. A method for testing the strength of concrete, characterized in that, Includes the following steps: Step S1: Place a thermophysical reference block with known thermophysical properties at the center of the target area on the concrete surface to be tested; Step S2: Using the thermal reference block as the center, apply synchronous thermal pulses of the same energy to the concrete surfaces at symmetrical positions on both sides of it, and record the resulting thermal response with an infrared thermal imager to generate thermal response data, in which two hot spots are formed on the concrete surface. Step S3: Monitor ambient temperature fluctuations using a thermal reference block, and subtract the amount of ambient temperature fluctuations from the thermal response data in real time to obtain the thermal decay curve of the hot spot; Step S4: Analyze the isotherm profile distortion degree of the thermal field corresponding to the thermal decay curve, and quantify it as the thermal field structure polarization index. The analysis process of the isotherm profile distortion degree of the thermal field includes: The decay time point from peak temperature to 50% of the hot spot is determined; the closed isotherm profile of the hot spot is extracted at the decay time point; the geometric center coordinates of the closed isotherm profile are calculated, and the closed isotherm profile is translated to the origin to obtain the standardized profile; the average radius value of the standardized profile is calculated; the average radius value is used as the ideal circular profile, and the radial distance difference between the standardized profile and the ideal circular profile is calculated to obtain the radial deviation sequence; the Fourier transform of the radial deviation sequence is performed to extract the amplitudes of the first and second order components; the ratio of the standard deviation of the radial deviation sequence to the average radius is calculated to obtain the dimensionless distortion coefficient characterizing the irregularity of the profile; the thermal field morphology distortion degree of the hot spot is weighted and averaged at multiple time points based on the dimensionless distortion coefficient to obtain the comprehensive distortion index; the thermal field structure polarization index is calculated based on the comprehensive distortion index. Step S5: Calculate the time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature to obtain the initial effective thermal relaxation time; use the thermal decay curve to compensate the time constant of the initial effective thermal relaxation time to obtain the effective thermal relaxation time. Step S6: Substitute the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation to calculate the estimated equivalent strength of the concrete. The calculation process for the estimated equivalent strength includes: Normalized characteristic values ​​are obtained by dividing the thermal field structure polarization index and effective thermal relaxation time by their respective preset standard reference values. Based on the thermal field structure polarization index and effective thermal relaxation time, the basic strength value is calculated using a binary linear regression formula. The product term of the normalized characteristic values ​​is calculated as an adjustment amount reflecting the mutual influence of parameters. Age correction coefficients and humidity correction coefficients are obtained and calculated based on the estimated age of the concrete and the surface moisture content. The corrected strength value is obtained by adding the basic strength value to the adjustment amount. Then, multi-factor correction values ​​are calculated based on the corrected strength value, age correction coefficient, and humidity correction coefficient. The standard deviation and 95% confidence interval of the multi-factor correction values ​​are calculated. Based on the standard deviation and 95% confidence interval, the multi-factor correction values ​​are compared with the standard concrete strength grade to determine the concrete strength grade, which serves as the estimated equivalent strength.

2. The concrete strength testing method according to claim 1, characterized in that, The monitoring process for ambient temperature fluctuations in step S3 includes: Calculate the average temperature of the thermal reference block 10 seconds before the thermal pulse, and use it as the temperature reference point under steady-state environmental conditions; Extract the temperature sequence of the thermal reference block region over time, subtract the temperature reference point, and obtain the temperature fluctuation sequence caused by environmental factors; A low-pass filter is applied to the temperature fluctuation sequence to remove random noise while retaining the true trend of environmental temperature change, resulting in a smoothed environmental fluctuation sequence. Based on the difference in thermophysical properties between the thermal reference block and concrete, the attenuation coefficient and time delay of environmental fluctuations transmitted from the thermal reference block to the hot spot area are calculated to obtain the transmission correction parameters. Based on the transmission correction parameters, the smoothed environmental fluctuation sequence is time-shifted and amplitude-adjusted to generate an environmental fluctuation compensation matrix. The corresponding environmental fluctuation compensation matrix is ​​subtracted from the original hot spot temperature curve extracted from the thermal response data to obtain the thermal attenuation curve that eliminates the environmental impact.

3. The concrete strength testing method according to claim 1, characterized in that, The calculation process for the initial effective thermal relaxation time in step S5 includes: Find the highest temperature value of the thermal decay curve and its corresponding time point, and determine it as the peak temperature and peak time. Calculate 37% of the peak temperature as the characteristic threshold for heat decay; A three-point moving average filter is applied to the thermal decay curve to eliminate minor data fluctuations and obtain a smooth decay curve. Starting from the peak moment, the thermal decay curve is searched backward to determine the time point when the temperature first drops to the characteristic threshold, which is recorded as the characteristic threshold time point; Calculate the time interval from the peak time to the characteristic threshold time point to obtain the characteristic decay time; Calculate the arithmetic mean of the characteristic decay times of the two hot spots to obtain the hot spot decay time; The hot spot decay time is calibrated by applying a temperature correction formula based on the ambient temperature during the test to obtain the initial effective thermal relaxation time.

4. The concrete strength testing method according to claim 1, characterized in that, Step S4 also includes analyzing the thermal field morphological distortion at multiple time points: In addition to the time point when the hot spot decays from the peak temperature to 50%, isotherm profiles are also extracted at the time points when the hot spot decays from the peak temperature to 25% and 75%. Calculate the distortion coefficient values ​​at the three time points based on the isotherm profiles; The distortion coefficients at the three time points were weighted and averaged to obtain the comprehensive distortion index, with the weights corresponding to the 50%, 25%, and 75% time points being 0.2, 0.5, and 0.3, respectively.

5. The method for testing concrete strength according to claim 1, characterized in that, Step S5, which uses the thermal decay curve to compensate for the time constant of the initial effective thermal relaxation time, includes: The thermal decay curve is fitted with an exponential function to obtain the fitting parameters; Extract the time constant from the fitted parameters; The initial effective thermal relaxation time was verified and adjusted using a time constant to obtain the effective thermal relaxation time.

6. The method for testing concrete strength according to claim 1, characterized in that, The thermal reference block in step S1 is made of silicon nitride ceramic sheet, and its surface is coated with a uniform high emissivity black coating, which gives it stable radiation characteristics in the infrared band.

7. The method for testing concrete strength according to claim 1, characterized in that, The synchronous thermal pulse in step S2 is achieved by two side-by-side focused infrared lamps, with a pulse duration of 0.5 seconds and the output energy difference between the two focused infrared lamps not exceeding ±2%.

8. A concrete strength testing system, characterized in that, For performing the concrete strength testing method as described in claim 1, the concrete strength testing system comprises: The thermal reference block, made of silicon nitride ceramic sheet with a high emissivity black coating, is placed at the center of the concrete surface to be tested. The dual-head pulse heat source includes two power-adjustable focused infrared lamps and a synchronization controller, which are used to apply synchronous heat pulses of the same energy to the concrete surface, forming two hot spots on the concrete surface. Infrared thermal imagers, with a thermal sensitivity of not less than 0.05℃, are used to continuously record the thermal response process and generate thermal response data; The data processing unit is communicatively connected to the infrared thermal imager to receive thermal response data and communicatively connected to the synchronization controller of the dual-head pulse heat source to control the application of thermal pulses. This data processing unit is used to perform the following operations: By using a thermal reference block to monitor ambient temperature fluctuations, the amount of ambient temperature fluctuations is subtracted from the thermal response data in real time to obtain the thermal decay curve of the hot spot. The distortion of the isotherm profile of the thermal field corresponding to the thermal decay curve is analyzed and quantified as the thermal field structure polarization index. The time required for the hot spot temperature to decay from the peak value to 37% of the peak temperature is calculated to obtain the initial effective thermal relaxation time. The time constant is then compensated for the initial effective thermal relaxation time to obtain the effective thermal relaxation time. By substituting the thermal field structure polarization index and effective thermal relaxation time into the pre-calibrated correlation equation, the estimated equivalent strength of concrete is calculated. The display unit, which is connected in communication with the data processing unit, is used to display thermal response data, thermal decay curves, thermal field structure polarization index, effective thermal relaxation time, and estimated equivalent intensity value.

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