China's ancient buildings are mainly wooden structures. The main load-bearing columns, beams, rafts, hoes, hoes, etc. are all wood. Wood is a biological material. During long-term use, it is susceptible to biological damage such as bacteria and insects, causing wood decay and insects, which threatens the safety of wood structures. In many cases, wood decay begins in the interior of the wood, but since the ancient construction wood structure cannot be easily dismantled, non-destructive testing techniques must be used to survey the material inside the wood. At present, there are many non-destructive testing or micro-damage detection methods developed in various countries, such as ultrasonic waves, stress waves, X-rays, γ-rays, Pilodyn, and Resistograph. Among them, ultrasonic waves, stress waves and resistance meters Detection is a more common method.
The resistance meter is a wood internal material testing instrument developed by Rinntech of Germany. It needs to penetrate a 1.5mm diameter probe into the interior of the wood for non-destructive testing. It is the current European, American, Japanese and Taiwanese wood structure. One of the commonly used equipment for material condition survey [5-9] . The instrument records the resistance encountered during the penetration of the wood during the test, and its size varies with the density of each tree species. According to the measured resistance curve, the decaying state inside the wood can only be qualitatively judged, and the degree of attenuation of the physical and mechanical properties of the wood caused by decay can not be quantitatively evaluated.
In this study, the local decaying old wood components replaced by the Forbidden City were used, and the specimens were visually classified according to the classification criteria of the decay grade, and the air dry density, bending strength and compressive strength were measured. The resistance test is carried out against the bending test piece. The purpose is to find out the relationship between the detected value and the physical and mechanical properties, and realize the quantitative analysis of the resistance test results, which provides a scientific basis for the quantitative evaluation of the wood structure material condition.
1 Materials and methods
1.1 Test materials The test materials were taken from the old decayed wood components of the single and double step beam melon column dismantled during the repair of the Forecourt Wuying Temple. The result of the identification of the wood species was Pinus sp. The test materials are summarized in Table 1. Wu Ying Dian was built in the Yongle Period of the Ming Dynasty and has a history of more than 600 years. In the 8th year of Tongzhi (AD 1869), Wu Yingdian was burned and burned down 37 buildings including the main hall, the apse, the temple door, the east hall and the bath hall. It was rebuilt in the same year [10] . In the twenty-sixth year of Guangxu (AD 1900), the front hall and the apse of Wuying Hall were once again burned. It is speculated that the materials used in this study were the wood used in the construction of 1869 or 1900, about 100 to 130 years ago.
The resistance meter is a wood internal material testing instrument developed by Rinntech of Germany. It needs to penetrate a 1.5mm diameter probe into the interior of the wood for non-destructive testing. It is the current European, American, Japanese and Taiwanese wood structure. One of the commonly used equipment for material condition survey [5-9] . The instrument records the resistance encountered during the penetration of the wood during the test, and its size varies with the density of each tree species. According to the measured resistance curve, the decaying state inside the wood can only be qualitatively judged, and the degree of attenuation of the physical and mechanical properties of the wood caused by decay can not be quantitatively evaluated.
In this study, the local decaying old wood components replaced by the Forbidden City were used, and the specimens were visually classified according to the classification criteria of the decay grade, and the air dry density, bending strength and compressive strength were measured. The resistance test is carried out against the bending test piece. The purpose is to find out the relationship between the detected value and the physical and mechanical properties, and realize the quantitative analysis of the resistance test results, which provides a scientific basis for the quantitative evaluation of the wood structure material condition.
1 Materials and methods
1.1 Test materials The test materials were taken from the old decayed wood components of the single and double step beam melon column dismantled during the repair of the Forecourt Wuying Temple. The result of the identification of the wood species was Pinus sp. The test materials are summarized in Table 1. Wu Ying Dian was built in the Yongle Period of the Ming Dynasty and has a history of more than 600 years. In the 8th year of Tongzhi (AD 1869), Wu Yingdian was burned and burned down 37 buildings including the main hall, the apse, the temple door, the east hall and the bath hall. It was rebuilt in the same year [10] . In the twenty-sixth year of Guangxu (AD 1900), the front hall and the apse of Wuying Hall were once again burned. It is speculated that the materials used in this study were the wood used in the construction of 1869 or 1900, about 100 to 130 years ago.
1.2 Sample sampling and test methods For each old wood component sample collected, in accordance with the national standard GB1929-91 "Wood physical and mechanical test material sawing and sample interception method" [12] in the health part and the different degrees of decay The samples for physical and mechanical testing were intercepted, including air dry density, flexural strength and compressive strength test samples, a total of 118 groups. Different degrees of decay of wood are classified into five grades according to the wood corrosion resistance classification standard (GB/T13942.2-92) [13] , see Table 2. “0†indicates uncorrupted materials, and “1â€, “2â€, “3â€, and “4†indicate four decay grades, respectively.
1.3 Resistance meter detection principle Wood resistance meter mainly includes probe and its protection device, microcomputer system and battery. The principle of wood resistance meter detection is that under the drive of the motor, a probe with a diameter of 1.5mm is pierced into the interior of the wood at a uniform speed, and the resistance data received during the penetration of the probe through the microcomputer system is recorded into the memory card. At the same time print out the output detection map. The abscissa of the map is equidistant from the depth of the probe penetration, and the density distribution inside the wood, the change in the density of the wood in the early and late, and the density change in the wood due to decay can be visually represented, as shown in Figure 1. . The test resistance meter model was Resistograph® 3450-P/S and the probe length was 45 cm.
1.4 Resistance test and analysis method Since the physical and mechanical properties of each sample are tested, the density test piece, the flexural strength test piece and the compressive strength test piece are taken from the same wooden strip, and 3 test pieces are ensured as much as possible during the interception. The degree of decay is the same, so the resistance test is performed only on the test piece that has been subjected to the bending strength test. The probe passes through the specimen in the radial direction during the test, and the position of the penetration is required to match the level of decay as much as possible. Apply the resistance image processing software (DECOM-Professional) to export the instrument storage information into the EXCEL file format containing the data information, draw the resistance change map according to the stored data (see Figure 1), and analyze the resistance of different decay grade samples. The instrument detects the change in value.
1.3 Resistance meter detection principle Wood resistance meter mainly includes probe and its protection device, microcomputer system and battery. The principle of wood resistance meter detection is that under the drive of the motor, a probe with a diameter of 1.5mm is pierced into the interior of the wood at a uniform speed, and the resistance data received during the penetration of the probe through the microcomputer system is recorded into the memory card. At the same time print out the output detection map. The abscissa of the map is equidistant from the depth of the probe penetration, and the density distribution inside the wood, the change in the density of the wood in the early and late, and the density change in the wood due to decay can be visually represented, as shown in Figure 1. . The test resistance meter model was Resistograph® 3450-P/S and the probe length was 45 cm.
1.4 Resistance test and analysis method Since the physical and mechanical properties of each sample are tested, the density test piece, the flexural strength test piece and the compressive strength test piece are taken from the same wooden strip, and 3 test pieces are ensured as much as possible during the interception. The degree of decay is the same, so the resistance test is performed only on the test piece that has been subjected to the bending strength test. The probe passes through the specimen in the radial direction during the test, and the position of the penetration is required to match the level of decay as much as possible. Apply the resistance image processing software (DECOM-Professional) to export the instrument storage information into the EXCEL file format containing the data information, draw the resistance change map according to the stored data (see Figure 1), and analyze the resistance of different decay grade samples. The instrument detects the change in value.
1.5 Data processing and analysis Cork pine is a gradual tree species in the early and late stages. It can be seen from Figure 1 that the resistance value from early wood to late wood in the same annual ring changes significantly. At the same time, because each bending strength test piece has a radial length of 2cm, it contains several annual rings, and there is one peak and one trough in each annual ring, and the resistance meter has high detection precision, and 100 values ​​are measured every 1mm. . Therefore, in order to eliminate the influence of the difference between the early and late material density on the detection results, and avoid the peak of the local micro-resin aggregation when the resistance meter is detected, or the sample measurement error caused by the trough value of the tiny decay, when performing data analysis The average and minimum intervals of each sample test result are equally divided into three equal parts, which are called peak area, average area and trough area. The average value of the peak region, the average value of the valley region, and the average value of the sample are calculated separately, which can reduce the influence of the maximum and minimum frequencies on the measurement results when the frequency is low.
Based on the air-dry density, flexural strength, compressive strength and measured value of the uncorrupted wood, the density and the residual rate of the flexural and compressive strength of each decay grade wood were calculated.
The correlation analysis method was used to analyze the relationship between the average value of the measured values ​​of the resistance meter, the average value of the peaks and the average value of the troughs and the physical and mechanical properties of the wood.
2 Results and discussion
2.1 Physical and mechanical properties of wood with different decay grades The results of physical and mechanical properties of wood with different decay grades are shown in Table 3. Since the "4" grade decayed wood cannot be processed into a sample, there is no physical and mechanical property test result of the "4" grade decayed wood. The undegraded portion of the old wood ("0" grade) has an average density of 0.439 g•cm -3 , a flexural strength of 61.04 MPa, and a compressive strength of 32.39 MPa, which is very close to the fresh and healthy Korean pine wood.
With the deepening of wood decay, wood density, flexural strength and compressive strength are significantly reduced, and the bending strength is most significantly reduced. The bending strength of "1" grade decays to 41.44MPa, to "3" level. When decaying, the flexural strength is only 7.71 MPa, and the bending strength of the wood is only 13% of the uncorroded wood (see Figure 2). With the deepening of the degree of decay, the compressive strength and air-dry density of the grain are also significantly reduced. When the "3" grade decays, the compressive strength of the grain is reduced to 18.41 MPa, which is about 57% of the uncorroded material; the air dry density Reduced to 0.353 g•cm-3, which is about 80% of the uncorroded material.
2.2 The resistance value of the resistance meter of different decay grade wood changes The resistance value of the resistance meter is expressed as Resi, and the variation range is from 0 to 500. Since the resistance meter varies greatly depending on the tree species and the moisture content of the wood, the resistance meter is not labeled at the factory. If the resistance meter is used directly, only qualitative analysis results can be obtained, and quantitative evaluation of physical and mechanical properties cannot be made. In order to achieve quantitative evaluation of the resistance test results, the resistance tester was tested on the samples whose physical and mechanical properties were measured.
During the investigation of the decay of the wooden structure of ancient buildings, it was found that most of the decay of wood began from the early wood, and in the three sections divided by the resistance detection value, the peak area was close to the late wood part, and the trough area was close to the early wood part. Due to the non-uniformity of the decay of the wood, this division takes into account the frequency of occurrence of each value and can more objectively reflect the actual material condition of each sample.
During the investigation of the decay of the wooden structure of ancient buildings, it was found that most of the decay of wood began from the early wood, and in the three sections divided by the resistance detection value, the peak area was close to the late wood part, and the trough area was close to the early wood part. Due to the non-uniformity of the decay of the wood, this division takes into account the frequency of occurrence of each value and can more objectively reflect the actual material condition of each sample.
2.3 Correlation analysis between the physical and mechanical properties and the wood resistance meter test results The correlation between the measured values ​​of the above resistance meter and the air dry density, flexural strength and the compressive strength of the grain is analyzed, and the resistance value and the gas are established respectively. A linear regression model of dry density, flexural strength, and compressive strength is shown in Figure 4 for a total of 15 models with n = 118 samples. There is a very significant linear correlation between the measured value of the resistance meter and the air dry density, flexural strength and compressive strength (P<0.01), and the correlation coefficient is between 0.29 and 0.52. Therefore, the resistance meter is used to detect the value and Linear regression models are predictive of wood density, flexural strength and compressive strength.
The value of the wood resistance meter is related to the distribution of the wood early and late wood and the degree of decay. The peak value appears in the undegraded late wood area, and the trough value appears in the early wood area or the decaying early and late wood areas. From the mean, peak value, trough value, maximum value and minimum value, the correlation between the average value and the air dry density and compressive strength is the largest, respectively 0.52 and 0.41, and the correlation coefficient between the mean and the bending strength. Smaller, it is 0.34. The correlation coefficient between wave peak and bending stress and compressive strength is small, 0.32 and 0.35, respectively, but the correlation coefficient with density is very large, 0.51. The correlation coefficients between trough value and density, flexural strength and compressive strength are relatively high, 0.47, 0.43 and 0.46, respectively. The correlation coefficient between the maximum value and the flexural strength and compressive strength is the smallest, 0.29 and 0.32, respectively, but the correlation coefficient with density is very large, 0.51. The correlation coefficients between minimum and density, flexural strength and compressive strength are relatively high, 0.40, 0.41 and 0.44, respectively. Therefore, when predicting wood density, the resistance meter average and density model, the resistance meter peak and density model, and the resistance meter maximum and density models are all accurate; when predicting the wood bending strength, the resistance meter trough value is used. The bending strength model or the resistance meter minimum and the bending strength model have good effects; it is best to use the compressive strength and the mean, trough or minimum model when predicting the compressive strength.
At present, China's national standards for the classification of wood decay grades only qualitatively describe the wood of each decay grade. There is no quantitative analysis of the mechanical properties of wood of different decay grades. Therefore, in the site investigation of the decay of wood structural materials, it is impossible to Judgment has been made about the physical and mechanical properties of decaying wood components. From the above test results, the physical and mechanical properties of each decaying grade wood are significantly attenuated, so it is feasible to quantitatively evaluate the wood of each decay grade of different tree species.
The above analysis of the physical and mechanical properties of different decay grades of cork pine wood, the change of the resistance meter and the correlation between the two shows that the air dry density, flexural strength, and compressive strength of each decay grade wood The measured values ​​of the resistance meter showed a trend of decreasing the characteristic value as the degree of decay increased, and the difference between the various decay levels was extremely significant. At the same time, there is a very significant correlation between the measured value of the resistance meter and the air dry density, bending strength and compressive strength. Therefore, when using a resistance meter to detect the internal decay of the old wood in the old building, for the cork pine wood, the established mathematical model and the measured value of the resistance meter of the tested material can quantitatively evaluate the material condition of the wood to be tested.
Conclusion after 3 <br> pine softwood structural material used in 100 to 130, non-wood decay ( "0" level) is the average air-dry density 0.439g • cm -3, flexural strength 61.04MPa, compressive strength parallel to grain It is 32.39 MPa. As the degree of wood decay increases, the bending strength decreases most significantly, followed by the compressive strength of the grain and the air dry density. There are significant differences in air dry density, flexural strength and compressive strength of different decay grade woods. When the wood reaches “3†grade decay, the flexural strength, the compressive strength of the grain and the air dry density are 13%, 57% and 80%, respectively, of the uncorroded wood.
The difference between the peak value and the trough value of the cork pine wood resistance meter of the same decay grade is between 35 and 55, and the difference between the maximum value and the minimum value is between 55 and 85, but the peak value, the trough value, and the maximum value. The difference between the residual rate of the minimum value and the average value is not significant. The average value of the uncorroded material detection value is 179.08. As the degree of decay increases, the detection value decreases. Especially when the “3†level is reached, the detection value decreases most obviously, and the average value decreases to 135.51, which is about 76 of the uncorroded material. %. There is a significant difference in the resistance values ​​of the various decay grade woods.
The correlation coefficient between the resistance meter detection value (except the minimum value) and the air dry density is the largest. The mean and peak value, trough value, maximum value, minimum value and air dry density, flexural strength and compressive strength of the grain showed a very significant linear correlation (P<0.01), and the correlation coefficient was 0.29~ Between 0.52. Therefore, for cork pine wood, the established material mathematical model and the measured value of the measured resistance meter can be used to quantitatively evaluate the material condition of the wood.
At present, China's national standards for the classification of wood decay grades only qualitatively describe the wood of each decay grade. There is no quantitative analysis of the mechanical properties of wood of different decay grades. Therefore, in the site investigation of the decay of wood structural materials, it is impossible to Judgment has been made about the physical and mechanical properties of decaying wood components. From the above test results, the physical and mechanical properties of each decaying grade wood are significantly attenuated, so it is feasible to quantitatively evaluate the wood of each decay grade of different tree species.
The above analysis of the physical and mechanical properties of different decay grades of cork pine wood, the change of the resistance meter and the correlation between the two shows that the air dry density, flexural strength, and compressive strength of each decay grade wood The measured values ​​of the resistance meter showed a trend of decreasing the characteristic value as the degree of decay increased, and the difference between the various decay levels was extremely significant. At the same time, there is a very significant correlation between the measured value of the resistance meter and the air dry density, bending strength and compressive strength. Therefore, when using a resistance meter to detect the internal decay of the old wood in the old building, for the cork pine wood, the established mathematical model and the measured value of the resistance meter of the tested material can quantitatively evaluate the material condition of the wood to be tested.
Conclusion after 3 <br> pine softwood structural material used in 100 to 130, non-wood decay ( "0" level) is the average air-dry density 0.439g • cm -3, flexural strength 61.04MPa, compressive strength parallel to grain It is 32.39 MPa. As the degree of wood decay increases, the bending strength decreases most significantly, followed by the compressive strength of the grain and the air dry density. There are significant differences in air dry density, flexural strength and compressive strength of different decay grade woods. When the wood reaches “3†grade decay, the flexural strength, the compressive strength of the grain and the air dry density are 13%, 57% and 80%, respectively, of the uncorroded wood.
The difference between the peak value and the trough value of the cork pine wood resistance meter of the same decay grade is between 35 and 55, and the difference between the maximum value and the minimum value is between 55 and 85, but the peak value, the trough value, and the maximum value. The difference between the residual rate of the minimum value and the average value is not significant. The average value of the uncorroded material detection value is 179.08. As the degree of decay increases, the detection value decreases. Especially when the “3†level is reached, the detection value decreases most obviously, and the average value decreases to 135.51, which is about 76 of the uncorroded material. %. There is a significant difference in the resistance values ​​of the various decay grade woods.
The correlation coefficient between the resistance meter detection value (except the minimum value) and the air dry density is the largest. The mean and peak value, trough value, maximum value, minimum value and air dry density, flexural strength and compressive strength of the grain showed a very significant linear correlation (P<0.01), and the correlation coefficient was 0.29~ Between 0.52. Therefore, for cork pine wood, the established material mathematical model and the measured value of the measured resistance meter can be used to quantitatively evaluate the material condition of the wood.
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