# What is "embedded" fiber optic sensing? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 22, 2026, 8:37am UTC](https://forum.ofscn.net/t/what-is-embedded-fiber-optic-sensing/3116 "2026-10-22T08:37:50Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 22, 2026, 8:37am UTC](https://forum.ofscn.net/t/what-is-embedded-fiber-optic-sensing/3116/1 "2026-10-22T08:37:50Z")
Why is fiber optics cast inside concrete dams or bridges? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [October 22, 2026, 8:38am UTC](https://forum.ofscn.net/t/what-is-embedded-fiber-optic-sensing/3116/2 "2026-10-22T08:38:32Z")
Embedding optical fibers or Fiber Bragg Grating (FBG) sensors within concrete dams, bridge piers, or box girders during the pouring phase is a critical “embedded/implantable” sensing technology in the field of Structural Health Monitoring (SHM) for civil engineering. The necessity of embedding fibers within the structure, rather than merely attaching them to the surface, is dictated by the material mechanics of massive concrete, the hardening process during construction, and the harsh service environment. * * * ### I. Core Engineering and Physical Monitoring Mechanisms #### 1. Construction Phase: Monitoring the “Hydration Heat” Temperature Field of Massive Concrete - **Hydration Heat Differential Effect** : The bodies of dams or the main piers of mega-bridges are massive concrete structures. The hydration reaction of cement is a vigorous exothermic process. Concrete has low thermal conductivity, making it difficult for heat to dissipate from the core. The core temperature can often exceed 60\ ^\circ\text{C} \sim 80\ ^\circ\text{C}, while the component surface is cooled by ambient temperature. - **Control of Through-Thickness Thermal Cracks** : A severe temperature gradient (exceeding 25\ ^\circ\text{C} difference) between the interior and exterior can generate tensile stress on the component’s surface. During the cooling and contraction phase, internal tensile stresses due to thermal contraction can form through-thickness cracks. Embedded fiber optic temperature measurement arrays can map the three-dimensional temperature field evolution within the component in real-time during pouring, providing first-hand data for water cooling, insulation, and temperature control. #### 2. Operational Phase: Measuring the True Stress/Strain Distribution in Core Cross-sections - **Overcoming Surface-Mount Limitations** : Surface-mounted strain gauges can only reflect superficial micro-strains and are easily disturbed by apparent anomalies caused by local solar radiation variations, wind erosion-induced drying shrinkage, or micro-cracks in the surface layer. - **Inferring Deep Stress State** : When a component is subjected to bending, torsion, or eccentric compression, the migration of the neutral axis position within the cross-section and the strain gradient distribution in the deep compressive and tensile zones must rely on strain sensors pre-embedded within the structure’s depth. The sensor forms a solid bond with the surrounding hardened concrete and deforms collaboratively, directly measuring internal strain values \varepsilon. Combined with the concrete’s elastic modulus, internal principal stresses \sigma can be calculated. #### 3. Early Detection of Internal Hidden Damage and Cracks - Under long-term fatigue loading or foundation settlement, the initiation and coalescence of micro-cracks in concrete often begin in areas of internal stress concentration (e.g., around the anchor zones of prestressed steel strands, at the interface between the dam body and its foundation). By the time cracks extend to the externally visible surface, the structure is usually in an irreversible stage of structural damage. Embedded sensors can capture localized strain abrupt changes or strain release signals in the early stages. #### 4. Ultimate Durability and Full Lifespan Service - **Vulnerability of External Surface Mounts** : Dams are subject to scour by water flow, erosion by high-humidity vapor, and damage from freeze-thaw cycles. Bridge surfaces endure impact wear from vehicle dynamic loads, and corrosion from rainwater and de-icing salts. Surface-mounted sensor cables are prone to aging, detachment, and mechanical damage. - **Natural Protective Barrier** : Once embedded within the concrete, the sensors are completely sealed by the alkaline concrete matrix, isolated from external mechanical impacts, ultraviolet radiation, and salt spray. This enables health monitoring for decades, potentially matching the lifespan of the dam itself. * * * ### II. Intrinsic Advantages of Optical Fibers Compared to Traditional Electrical Sensors 1. **Extremely Low Intrusion** : Optical fibers have a diameter of only 125\ \mu\text{m}. Encased in micro-metal tubes, the outer diameter typically ranges from 0.6\ \text{mm} \sim 2.0\ \text{mm}. Compared to the aggregate size of massive concrete structures, this minimally reduces the effective load-bearing area of the structural cross-section and does not create new stress concentration defects. 2. **Intrinsically Safe, Immune to Strong Electromagnetic Interference and Lightning Strikes** : Areas around hydropower dam sites often have generator sets and high-voltage switchyards. Extra-large bridges spanning seas or rivers are susceptible to lightning strikes. Optical fiber sensors use light modulation as a medium, are insulated, and carry no electrical signals, making them inherently immune to strong electromagnetic pulses (EMP) and lightning surges. 3. **Multiplexing on a Single Fiber (WDM) for Long-Distance Transmission**: Multiple Fiber Bragg Grating (FBG) measurement points can be cascaded on a single optical fiber using Wavelength Division Multiplexing (WDM). This eliminates the need for hundreds of heavy copper wire bundles required by traditional electrical strain gauges, significantly reducing structural damage caused by reserved openings during construction. * * * ### III. Specific Engineering Challenges and Product Encapsulation Technologies for Embedded Applications Bare quartz glass optical fibers are brittle. On-site concrete construction involves the impact of pouring coarse aggregates and intense shearing from high-frequency vibrators (vibration frequencies up to 100\ \text{Hz} \sim 200\ \text{Hz}). Bare fibers cannot be directly embedded and require specialized encapsulated structures that resist extrusion and shearing while ensuring good interfacial strain transfer. Within OFSCN®'s official product series, the core products for embedded monitoring in concrete structures are as follows: #### 1. Internal Strain Monitoring of Structures Suitable for embedding within concrete or tying to rebar frameworks to monitor anisotropic strain transfer inside massive concrete: - **[OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://www.ofscn.net/fbg-products/strain-sensor.html)**: Encapsulated in an elastic seamless alloy tube with a very small outer diameter (\le 1.1\ \text{mm}). It balances flexibility with compressive and impact resistance. Customizable gauge lengths are available for different structural spans. - **[OFSCN® Fiber Bragg Grating Rebar Strain Gauge](https://www.ofscn.net/fbg-products/rebar-meter.html)**: Specifically designed for measuring internal forces in reinforced concrete structures. It can be cascaded or welded onto main reinforcing bars to monitor the tensile and compressive deformation of internal reinforcement in components in real-time. ![OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://forum.ofscn.net/uploads/default/original/1X/90c11b90deb49f0cd88a54ecac0d956bcddf353c.jpeg) ![OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://forum.ofscn.net/uploads/default/original/1X/ae908bc1e181cfc4cdf263751c8670552a66522e.jpeg) ![OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://forum.ofscn.net/uploads/default/original/1X/4a28a6be888c7e0965d6a522716233e8b6403b54.jpeg) #### 2. Hydration Heat and Deep Temperature Field Monitoring of Massive Structures Used for pre-embedding within dam core blocks or large-section abutments to track hydration heat peaks and temperature recovery processes in real-time: - **[OFSCN® 300°C Fiber Bragg Grating Temperature Sensor](https://www.ofscn.net/fbg-products/temperature-sensor-300.html)**: Encapsulated using single-layer stainless steel seamless steel pipe armor technology, with a default outer diameter of 0.9\ \text{mm} (customizable down to 0.5\ \text{mm}). It is insensitive to mechanical strain internally, completely decoupling temperature response. It can operate stably for extended periods under harsh alkaline conditions (strong alkalinity of cement paste) and high-temperature impacts from hydration. ![OFSCN® 300°C Fiber Bragg Grating Temperature Sensor](https://forum.ofscn.net/uploads/default/original/1X/74932c0f3b71797adb12ca84a053059151051842.jpeg) ![OFSCN® 300°C Fiber Bragg Grating Temperature Sensor](https://forum.ofscn.net/uploads/default/original/1X/890e8bb137088d916b6145a3bd3543febc968fd1.jpeg)