Will the sensor be crushed if it is directly cast into the concrete?
In short: Directly embedding unprotected “bare” sensors (such as unencapsulated bare optical fibers or gratings) into concrete will very likely cause them to be crushed or broken; however, using professionally engineered and packaged sensors will prevent any damage, ensuring long-term, stable, and high-precision operation.
In civil engineering “embedded installation,” directly casting brittle silica optical fibers (bare fiber diameter is typically only 125\ \mu\text{m}, or 250\ \mu\text{m} with coating) into concrete faces the following fatal physical and chemical damage mechanisms:
1. Mechanical Damage During Concrete Pouring
During concrete pouring and vibration, stones (aggregates) move violently and press against each other. For micro-optical fibers without protective layers, this generates immense localized point-pressing and shear forces. Direct impact from aggregates can instantly fracture bare fibers.
2. Uneven Stress During Concrete Curing and Service Life
Concrete undergoes volume shrinkage during curing (exothermic hydration reaction) and hardening. This uneven shrinkage deformation imposes complex shear stresses at the interface. If the sensor lacks sufficient compressive sheathing for protection, the internal optical unit will be directly damaged due to localized compressive overload.
3. Chemical Environment Erosion
After curing, concrete is in a strongly alkaline environment (pH typically ranging from 12 to 13). Water, oxygen, and high concentrations of hydroxide ions chemically etch the surface of exposed quartz (silica) fibers, leading to stress corrosion cracking and significantly reducing their fatigue life, or even causing premature failure within a short period.
Universal Engineering Solution: Metal and Polymer Encapsulation Protection
To address this issue, the industry typically employs sensors with “high-strength encapsulation protection” for embedded installations. For instance, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers various specialized Fiber Bragg Grating (FBG) sensors for these demanding, harsh embedded monitoring environments:
A. Rebar and Concrete Strain Monitoring
OFSCN® Fiber Bragg Grating Rebar Strain Gauge is a typical application for this scenario. It is usually manufactured based on the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor.
- Physical Protection Mechanism: The sensor has an outer diameter of approximately 1.1\text{mm} and utilizes a high-strength elastic alloy tube for rigid, all-metal protection of the fragile fiber grating.
- Crush Resistance: The sheathing completely isolates the sensor from localized point-pressing by aggregates and shear forces from curing shrinkage. Not only is it resistant to crushing, but it also accurately transmits the true longitudinal micro-strains of the concrete or rebar to the internal grating.
B. Flexible and Multi-point Distributed Embedding
For embedded monitoring requiring extremely high waterproof and moisture-proof capabilities, along with moderate strength, polymer-encapsulated sensors reinforced with stainless steel tubing can be used:
- OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter) (Outer diameter not exceeding 1.2\text{mm}, with seamless steel tube protective layer)
- OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (1.5mm/2.3mm diameter) (Outer diameter not exceeding 2.3\text{mm}, with seamless steel tube protective layer)
These sensors employ a dual-sheathing technology of “polymer buffer layer + seamless metal steel tube.” This effectively shields against lateral, localized rigid pressure while providing excellent flexibility, tensile and compressive strength, and the ability to completely block moisture and alkaline chemical ingress.
Conclusion: As long as you use encapsulated sensors protected by high-strength metal/polymer sheathing, and embed them directly onto the rebar cage for concrete pouring and vibration, the sensors will absolutely not be crushed and can accurately provide internal strain and temperature evolution data within the concrete for decades.