In situ monitoring on prestress losses in the reinforced structure with fiber-optic sensors
Introduction
Prestress technique is frequently used in the modern building activity to reduce the deadweight of structures and improve their durability and reliability. Typical application example can be seen in the construction of bridges and nuclear reactor containments. Effective determination of prestress losses is important in the design of prestressed concrete structures. Over-predicting prestress losses results in an overly conservative design for service load stresses, and under-predicting prestress losses, can result in cracking at service loads. For example, according to the study of Mo and Hwang [1], the seismic response of prestressed concrete frames is significantly affected by the value of service prestress losses. Moreover, the knowledge of prestress loss is essential for correct prediction of creep effects on the deformation and strength of the reinforced-prestressed concrete members [2]. Therefore, measurement and prediction of the value of prestress losses is an imperative task in the design of reinforced structures and healthy evaluation.
Mechanisms like relaxation in the steel strand and creep and shrinkage in the concrete will lead to prestress loss and then change the performance of the prestressing system gradually. These mechanisms depend on several different environmental and material factors that make the loss of prestress difficult to predict. The value of prestress loss is therefore expected to be measured and monitored experimentally to ensure that the prestress level is sufficient. Many approaches have been developed to implement the measuring task in the reinforced structures [3]. For the real time monitoring of the nuclear reactor containment strength with grouted tendons, Sun et al. [4] suggested that it can be realized through overall deformation measurements by installing permanent strain transducers on the containment concrete surface. The main difficulty of Sun’s method is that an immovable and unchangeable reference point should be pre-selected. By using the vibrating wire strain gauges, Onyemelukwe et al. [5] determined the time-dependent prestress loss variation and distribution in the pretensioned concrete girders of a bridge. However, the application of strain gauges for prestress loss measurement is restricted due to their limited durability and unsuitable for being embedded into the concrete.
Unlike conventional electrically-based sensors, optical-fiber sensors fabricated with high-strength silica, for advantages of small bulk, strong corrosion-resistance, high precision, accurate locating and anti-electromagnetic-wave interference etc., have been used for measuring and monitoring of the stress, strain, temperature, vibration and the crack initiation and growth in advanced materials and structures [6], [7]. A growing number of researchers are now embedding optical-fiber sensors into concrete civil structures to monitor various physical perturbations in the structures [8]. Ansari et al. [9] developed a fiber-optic refractive index sensor for the in situ determination of air content in fresh concrete. Nanni et al. [10] implemented the optical-fiber sensor to monitor stress concentrations in small concrete specimens. Wolff and Miesseler [11] employed the optical time-domain reflectometry technique for strain measurements on composite prestressing strands used to reinforce concrete bridges. Marten et al. [12] reported the implementation of an optical fiber sensor for the non-destructive quantitative evaluation of advanced concrete-based civil structures. Nevertheless, Mendez and Morse [13] indicated that, for accurate and reliable measurements, it is essential to ensure that there is adequate and stable strain transfer between the host structure and the fiber sensor elements.
In this report, an attempt is performed on in situ monitoring of the prestress losses and the implementation of fiber-optic sensor in the large structures outside the laboratory. A novel prestressed concrete structure, that is a large-scale sewage-treating tank, is selected as the testing model. The measuring procedure for prestress loss with fiber-optic sensor is also discussed.
Section snippets
Principle of in situ monitoring of prestress loss
Illustration of the principle of in situ monitoring of prestress loss with optical-fiber sensor is shown in Fig. 1. The light produced by a laser irradiates to the steel-strand through a sending optical fiber, and will be reflected by a reflector that is fixed on the steel-strand. The reflected light is collected through a receiving optical fiber and finally transmitted to a photo-electronic checker. Because a linear relationship exists between the received light-intensity and the displacement
Results and discussion
A novel reinforced-prestressed technique was employed in a sewage-treating tank, as shown in Fig. 4. To provide the first-hand data about the new structure, according to the requirement of design, the calibrated in-situ monitoring system with optical fiber sensor was applied on the real-scale sewage-treating tank to measure the prestress loss. The radius of the sewage-treating tank is of 27,000 mm. Four pieces of steel-strands were arranged in the tank of interest. Different styles were adopted
Conclusion remarks
The following conclusions summarized the findings presented in this paper:
- 1.
An optical-fiber-sensor-based in situ monitoring system for loss of prestress was developed and applied in a real-scale sewage treating tank outside the laboratory. Results indicate that the developed system is feasible for on-line monitoring of the prestress loss in large scale structures.
- 2.
Values of instantaneous loss of prestress are apparently affected by the arrangement of steel-strand in concrete. For the measured
Acknowledgements
The authors are grateful for the supports provided by China Natural Science Foundation (50505012) and CFKSTIP, Ministry of Education of China (704020). The first author also extends thanks to the supports of Shanghai Rising-Star Program (05QMX1416).
References (17)
- et al.
The effect of prestress losses on the seismic response of prestressed concrete frames
Computers & Structures
(1996) Determining aging coefficient and time-dependent lossess and deformations of prestresses members with due consideration of percentage of steel
Nuclear Engineering and Design
(1995)Thirty years of measured prestress at Swedish nuclear reactor containments
Nuclear Engineering and Design
(2005)- et al.
Strength monitoring of a prestressed concrete containment with grounted tendons
Nuclear Engineering and Design
(2002) - et al.
Optical fiber sensors for monitoring of welding residual stresses
Journal of Materials Processing Technology
(2003) - et al.
Field measured prestress concrete losses versus design code estimates
Experimental Mechanics
(2003) - et al.
Optical Fiber Sensors: Principles and Components
(1988) - D.R. Huston, P.L. Fuhr, P.J. Kajenski, T.P. Ambrose, W.B. Spillman. Installation and preliminary results from...
Cited by (30)
Measurement of cable forces for automated monitoring of engineering structures using fiber optic sensors: A review
2021, Automation in ConstructionCitation Excerpt :Besides, in the case of bonded tendons, the presence of the packaged FBG sensors deployed using the third method may compromise the bond between the cable and the concrete. The prestressed concrete beams instrumented with FBG sensors have been tested under static loads [72], dynamic loads [45], and fatigue loads [73]. The FBG sensors survived after 200 million fatigue cycles.
A cost-effective approach for assessment of pre-stressing force in bridges using piezoelectric transducers
2021, Measurement: Journal of the International Measurement ConfederationFriction characteristics of post-tensioning tendons in full-scale structures
2019, Engineering StructuresCitation Excerpt :Various number of strands were inserted into the ducts in turn as shown in Table 2 to investigate the effect of the filling ratio on friction coefficients, where Smart Strands occupied a certain portion and regular strands occupied the remaining portion of the total number of strands in a duct. Although an attempt has been made to apply FBG sensors to estimate the distribution of prestressing force and prestress losses in previous studies [11,15–17,44], friction coefficients were not derived in these studies. However, FBG sensing technology was extended to a direct evaluation of the friction coefficients in this study.
Measurement of existing prestressing force in concrete structures through an embedded vibrating beam strain gauge
2016, Measurement: Journal of the International Measurement ConfederationCitation Excerpt :Measurement of existing stresses using induced magnetic field is given in [11]. A more elaborate study on the effect of grouting on the techniques mentioned in the above studies [4–6,8–11] is required. A sensitivity based finite element model updating technique is developed in [12] for the inverse analysis of the existing prestress force in each element of prestressed girders.
Structural monitoring using fiber optic sensors of a pre-stressed concrete viaduct during construction phases
2014, Case Studies in Nondestructive Testing and EvaluationCitation Excerpt :Over the past 20 years, a large number of applications of optic fiber sensors have been developed for the monitoring of great infrastructures, both new and existing [17]. With regard to new prestressed RC structures, these monitoring systems allow – for example – to appraise the expected stress loss during the construction phases and the service life [18–20]. In the phase of concrete casting, they are also very useful in order to evaluate the effect of the deformations related to the shrinkage, and afterwards, to monitor the response of the new structure under growing loads [21].
Configuration optimization of magnetostrictive transducers for longitudinal guided wave inspection in seven-wire steel strands
2010, NDT and E InternationalCitation Excerpt :With the growing number of applications of the use of prestressing technique in civil engineering, a significant amount of research attention has been paid to the monitoring of the condition of seven-wire steel strands. Various monitoring methods have been applied to these spiral structures [2,3]. The ultrasonic guided wave technique is an emerging nondestructive testing method which has great potential in the inspection of many waveguides, such as pipes, plates, rods and rails [4–8].