Measuring height of poured concrete via fiber-optic temperature sensing

Currently, there is no automatic method available to measure the height of poured concrete during concreting of underground diaphragm walls (UDWs), and the conventional manual method interrupts the operation. To address this issue, a distributed fiber-optic sensing-based method for measuring the height of poured concrete during the concreting of UDWs was developed and successfully applied to the real-time measurement of the height of poured concrete. The proposed method was verified by performing a scale mode test, wherein an acrylic circular tube was used to simulate a UDW steel cage and a temperature-sensing fiber-optic cable was laid spirally to increase the spatial resolution of measurement to 0.1 m. The temperature distribution in the tube and real-time measurements of the height of the poured concrete during concreting were obtained. The proposed method was applied to the construction of a real UDW. A total of 3360 data points representing the spatiotemporal temperature distribution in the UDW steel cage were obtained. The results indicated that the temperature of the mud in the steel cage was approximately 25 °C–26 °C when no concreting operation was performed and that the temperature of the concrete layer increased to approximately 28 °C–31 °C during concreting. The height of the poured concrete was the boundary representing the transition from the temperature of the mud to that of the concrete, and the measurement precision reached ±0.5 m. The measurement results obtained via the proposed method were consistent with those obtained manually using a plumb bob, thus confirming the effectiveness of the proposed method for high-precision, real-time measurement of the height of poured concrete during the concreting of UDWs.

temperature measurement, height of poured concrete (Some figures may appear in colour only in the online journal) * Author to whom any correspondence should be addressed.
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Introduction
Underground diaphragm walls (UDWs) are major maintenance structures for the anchorage foundations of suspension bridges. They have high rigidity and impermeability, they are cost-effective and can be constructed rapidly, and occupy little space. Japan introduced UDW construction technology from Italy in 1959 and applied it to the construction of bridges, including the Hakucho Bridge and the Akashi Kaikyo Bridge. The Humen Pearl River Bridge was the first bridge in China that adopted the UDW technology. Since then, the design theory and framework of UDW have advanced significantly, and the applications of UDW have broadened. During the construction of a UDW, accurate measurement of the height of the poured concrete can improve the construction efficiency and quality. Currently, the primary method for performing this measurement involves the manual use of a plumb bob. However, this method has a low efficiency and large error, and it interrupts operation.
Distributed fiber-optic sensing (DFOS) is a new monitoring technique that integrates sensing and signal transmission using light as the carrier and optical fibers as the medium [1]. This technology uses ordinary single-mode optical fibers as sensors and demodulates stress and temperature information in optical signals transmitted through the optical fibers. With a sensing distance of approximately 100 km [2] and a spatial resolution of several centimeters [3] (both the performance indicators are typically not reached simultaneously; they are usually inversely proportional), it has been widely used in underground and geotechnical engineering. Soga et al used DFOS for real-time measurement of the strains and temperatures of UDWs, geothermal energy piles, subway shafts, and tunnel structures, thus providing valuable field data for the health assessment of geotechnical structures [4][5][6][7][8]. Lienhardt et al [9] developed a DFOS-based system for monitoring side slopes, which had a precision of 0.01 mm m −1 . Zhu et al [10] investigated the performance monitoring of soil-nailed side slopes. Damiano et al [11] investigated the application of DFOS for early detection of shallow granular soil slopes in a laboratory environment. Gao et al [12] applied Brillouin optical timedomain reflectometry (OTDR) to deformation monitoring of bored piles and obtained useful pile deformation data. Cheng et al [13] applied DFOS to deformation monitoring of overlying strata during mining, and the results confirmed the effectiveness of the technology.
Raman distributed temperature sensing (RDTS)-a type of DFOS technology-detects the temperature of the surrounding media using longitudinally laid fiber-optic cables and is capable of localizing points with abnormal temperatures for real-time warning. Khan et al conducted long-term research on dike leak detection and flood warning [14][15][16] and developed an RDTS-based automatic monitoring system capable of providing real-time warning of anomalies. RDTS has been used in pipeline inspection as a real-time, distributed diagnostic technology for measuring the internal temperatures of pipelines. It causes no interference to normal pipeline operation and is capable of all-weather spatiotemporal highfrequency monitoring. Mirzaei et al [17], Madabhushi et al [18], Zhou et al [19], deHaan et al [20], Großwig et al [21], and Niklès et al [22] made significant progress in this regard, and their results confirmed the feasibility of this technology for temperature sensing.
In summary, DFOS has been mainly used for the stress/strain and health monitoring of structures and ambient temperature monitoring. No application of RDTS for measuring the height of poured concrete during the concreting of UDWs has been reported. Additionally, in field applications, the measurement precision was limited by the spatiotemporal resolution of the demodulator, and RDTS temperature signals were mixed with heavy spatiotemporal noise. Thus, to apply RDTS systems to measurement of the height of poured concrete during the concreting of UDWs, it is important to improve the spatiotemporal resolution and achieve rapid, accurate extraction of poured concrete level height information from heavy noise.
To address these issues, in the present study, the use of RDTS for measuring the height of poured concrete during the concreting of UDWs was investigated experimentally. The temperatures of the contents of a steel cage at different spatial and temporal points during concreting were measured. The main objective of this study was to evaluate the applicability of RDTS technology for measuring the height of poured concrete.

RDTS technology
The RDTS system consisted of a long temperature-sensing fiber-optic cable and a demodulator terminal comprising a laser and an optical receiver, as shown in figure 1. During measurement, the cable was laid along the inner wall of the UDW steel cage from the top to the bottom and then from the bottom to the top, forming a U-shaped loop. To minimize the optical signal loss and distance measurement error, excessive bending of the cable was avoided. The cable was connected to a terminal located in a control room.
When Raman scattering occurs in a fiber-optic cable, the intensity of the backscattered light satisfies equations (1) and (2). In RDTS, the temperature information along the cable is demodulated through intensity modulation Here, η is the coupling coefficient between the laser and fiber-optic cable; K as and K s are the scattering coefficients of the anti-Stokes light and Stokes light, respectively; S b is the backscattering factor; α 0 , α as , and α s are the loss coefficients of the incident light, anti-Stokes light, and Stokes  light, respectively; k is the Boltzmann constant; h is Planck's constant; and ∆v represents the frequency shift of Raman scattering.
OTDR is the theoretical basis of RDTS for distributed temperature sensing, as shown in figure 2. Because the propagation rate of an optical wave in a fiber-optic cable is constant, the time differences between the incident light and scattered light are different at different positions along the cable. The time difference is determined by the number of times the laser emits and receives a laser pulse. The time difference between the incident light and scattered light at a position along the fiber-optic cable is related to the distance between the laser and that position: where l represents the distance to a point on the temperaturesensing fiber-optic cable, t represents the time difference between the incident light and scattered light at that point, and v represents the propagation rate of light in the temperaturesensing fiber-optic cable. A laser pulse signal can reveal the temperature of the contents of the UDW steel cage at different spatial points at a specific time point. The temperatures of the contents of the UDW steel cage at different spatial and temporal points can be determined by emitting laser pulse signals periodically into the temperature-sensing fiber-optic cable.

Measurement of height of poured concrete during concreting of UDW
The temperature-sensing fiber-optic cable was laid inside the UDW steel cage. The measured temperature is the temperature of the contents of the cage, which are mud before concreting. During concreting, because the density of concrete is higher than that of mud, the mud in the steel cage is gradually squeezed out of the cage by concrete, and the contents of the cage gradually change from mud to concrete from the bottom to the top. Because the mud and concrete have different temperatures, there is an abrupt temperature change in the temperature profile of the contents of the cage. According to this characteristic, the concrete-mud interface can be identified, as shown in figure 3 (where the vertical coordinate of the point with an abrupt temperature change corresponds to the height of the poured concrete).

Experimental scheme
To verify the reliability of RDTS-based method for measuring the height of poured concrete during the concreting of UDWs, a scale model test was performed. An acrylic circular tube was used to simulate a UDW steel cage, which was filled with dry concrete, as shown in figure 4. The temperature measured by the fiber-optic cable was the air temperature in the tube before concreting and was the air temperature or the temperature of the concrete during concreting. The height of the point with an abrupt temperature change was the height of the poured concrete.
A 3 mm-diameter armored loose stepped fiber-optic cable was used, which had adequate durability for the test and field applications of the system. The diameter of the optical fiber core, optical fiber cladding, and sheath are 62.5 µm, 125 µm, and 0.6 mm, respectively. The outer sheath is made of antistatic flame-retardant low-smoke zero-halogen materials, and the internal is composed of metal spiral armored sheath. The demodulator used had a maximum spatial resolution of 1 m and a temperature measurement precision of ±0.5 • C.
To improve the spatial resolution of the system, the fiberoptic cable was laid spirally and attached to the steel cage inside the acrylic tube using cable ties. A 0.1 m increase in the height of the cable corresponded to a 1 m increase in the length of the cable (or the scale of the cable). Thus, the spatial resolution of the RDTS system for measuring the height of poured concrete was improved from 1 to 0.1 m. During concreting, a ruler was used to measure the height of poured concrete to calibrate the measurements obtained with the RDTS system.

Test results
Concrete was poured at time intervals of 2 min to concrete the acrylic tube to heights of 0.1, 0.3, 0.5, and 0.8 m. Figure 5 shows the test results. The 2 m point on the scale of the cable corresponded to a height of 0 m in the acrylic tube. Before concreting, the temperature in the tube sensed by the fiber-optic cable reduced to 31 • C-31.5 • C. After the first pouring, the actual height of poured concrete in the tube was 0.1 m, and the measurements obtained with the fiber-optic cable indicated an abrupt temperature change in the 2-3 m interval on the scale of the cable. The temperature at the 3 m point on the scale of the cable decreased abruptly to 29 • C. This is because the temperature of the concrete was lower than the air temperature in the tube. The height of poured concrete measured with the cable was consistent with that measured with the ruler. After the second and third pourings, the actual heights of poured concrete in the tube had increased to 0.3 and 0.5 m, respectively, and an abrupt temperature change occurred at the 5 and 7 m points on the scale of the cable, respectively. The liquidlevel height measured with the cable was consistent with that measured with the ruler. After the fourth pouring, the actual liquid-level height had increased to 0.8 m, and the temperature along the entire length of the cable in the tube had decreased to 29 • C, indicating that the section of the cable in the tube was immersed in concrete. The height of poured concrete measured with the cable was consistent with that measured with the ruler.

Target project
The RDTS system was applied to the construction of a UDW of the Zhangjiagang-Jingjiang-Rugao Yangtze River Bridge, which had a main span measuring 2300 m and a UDW anchorage foundation. The height of poured concrete during the concreting of a straight UDW was measured using the RDTS system, as shown in figure 6. The UDW used a threesection steel cage with a height of 27 m × 3 = 81 m. During the fabrication of the steel cage, a temperature-sensing fiberoptic cable was laid symmetrically in a U-shaped loop in the second section, as shown in figure 7. The field test was performed from 15:00 September 3-0:00 4 September 2022, spanning 9 h. During the test, concreting and non-concreting operations were performed.

Collection of raw data
A 3 mm-diameter armored loose fiber-optic cable with an operating temperature range of −25 • C-120 • C was used. The demodulator used had a spatial resolution of 1 m. The temperature was sampled at 1-m intervals along the length of the cable (i.e., the distance between adjacent light scattering points in the cable was 1 m) every 10 s (i.e., the laser pulse emission frequency was 10 s). The temperature measurement precision was ±0.5 • C.
The temperature-sensing fiber-optic cable was approximately 70 m long. The lowest point sampled was 2 m above the bottom of the steel cage, and this point corresponded to the 0 m point on the scale of the cable. The data sampled from the 25 m-long section on the two sides of the U-shaped loop were deemed effective, and the data sampled from other sections of the cable were deemed ineffective and were not considered. A total of 3360 measurements were obtained. Figure 8 shows the spatiotemporal distribution of the raw data.

Measurement of height of poured concrete
As indicated by the spatiotemporal distribution of the temperature of the contents (mud or concrete) in the UDW steel cage (figure 8), the temperature in the steel cage was approximately 25 • C from 15:00 to 19:10, when no concreting operation was performed. The temperature variation in the steel cage was the natural temperature variation of the mud and reflected the temperature variation range of the mud in the steel cage. At 19:10,  the temperature at the 0-m point of the temperature-sensing fiber-optic cable increased from 25 • C to approximately 28 • C, and the temperature on the two sides of the steel cage increased symmetrically from the bottom to the top (i.e., the temperature increased along the height direction of the steel cage from the bottom to the top), indicating that concrete (whose temperature was higher than that of mud) was poured to the bottom of the steel cage and that the height of the poured concrete increased as the concreting operation proceeded.
The vertical coordinate of the point with an abrupt temperature change in the spatiotemporal distribution of the temperature measurements indicated the height of poured concrete. As shown in figure 8, the height of poured concrete was approximately 5 m at 19:30, approximately 20 m at 20:10, and approximately 25 m at 21:00. Owing to the interruption of construction caused by the plumb bob method, only four tests were conducted during the pouring process. At 19:24, 20:00, 20:30, and 21:00 Beijing time, AA and BB conducted a total of four comparative tests, as shown in figure 9. The measurement results of the two were 2 m and 2.45 m, 13 m and 13.1 m, 20 m and 19.8 m, and 27 m and 27 m, respectively. The measurements obtained with the RDTS system were calibrated against the measurements obtained manually using a plumb bob. The results indicated that the measurements obtained with the RDTS system were correct, and the precision reached ±0.5 m.

Conclusion
(1) A scale model test (where a UDW was simulated using a circular acrylic tube) was performed to measure the height of poured concrete, confirming the effectiveness of the proposed RDTS system for high-precision measurement of the height of poured concrete. Additionally, the results indicated that spiral laying of the temperature-sensing fiber-optic cable improved the spatial resolution of the system for measuring the height of poured concrete, and the degree of improvement depended on the actual laying of the cable. (2) The proposed system was applied to the construction of a UDW, and high-precision temperature measurements representing the spatiotemporal temperature distribution in the steel cage were obtained. The vertical coordinate of the point representing the transition between the temperatures of mud and concrete corresponded to the height of the poured concrete. The measurements obtained with the proposed RDTS system were consistent with those obtained manually using a plumb bob and had a high precision of ±0.5 m. (3) The precision of the RDTS system for measuring the height of poured concrete during the concreting of UDWs depends on the difference between the temperature of the original contents of the steel cage and that of the newly poured concrete. A larger difference in temperature corresponds to a higher measurement precision. The proposed method is generally applicable to situations, where the temperature difference between the original contents of the steel cage and the newly poured concrete is large.

Data availability statement
The data cannot be made publicly available upon publication because they contain commercially sensitive information. The data that support the findings of this study are available upon reasonable request from the authors.