# New Methods for Assessing External Sulfate Attack on Cement-Based Specimens
## 2 Materials and Methods
### Optical Fiber-Based and Extensometer Methods for Measurement of ESA-Induced Expansion  
The second method presented in this article aims to measure ESA-induced expansion using an optical fiber-based method. To perform this monitoring, three cement paste prismatic specimens (3 \(\times\) 4 \(\times\) 16 cm\({}^{3}\)) were cast with the CEM I 52.5 N CE CP2 NF at a water-to-cement ratio of 0.6. Figure 4a shows fresh cement paste poured into a mold common to three specimens. Each prism was crossed by a fiber embedded along its length. The location of the OF in each prism is detailed in Figure 4b. The outer parts of the OF  
Figure 3: The two stages of preparation of the cement paste specimen and the measurement of the penetration depth: (**a**) sample removed from bath; (**b**) specimen cut in half before being placed in a climatic chamber at 50% RH and T = 20 \({}^{\circ}\)C; (**c**) white precipitation measured with a stainless-steel ruler. were protected by a plastic sheath. A pair of stainless-steel pins was bonded to one side of each prism, allowing an extensometer to be used to measure expansion. The side of the prism equipped with the pair of pins was the side closest to the OF. The pins are bonded to the specimens with a strong chemical adhesive that is resistant to chemical attacks in solutions such as sulfate solution. There is a small hole at the top of the pins, which matches the extensometer pointers and the reference length bar (Figure 4c). A generating line was formed by aligning each pair of pins in the center axis of the prism at an initial length of 100 mm (see Figure 4d). Immediately after demolding, the prisms were placed in water to cure for 30 days. Then, the prisms were immersed in a 15 g/L Na\({}_{2}\)SO\({}_{4}\) solution to induce ESA. Expansions were then recorded, at different time intervals of the exposure, using a Rayleigh backscattering-based interrogation device [40, 41]. This device can give information about material expansion by measuring the changes in the frequency of the backscattered light caused by the stress-induced strain within the material. This technique, known as distributed fiber optic sensing, allows for the detection and monitoring of structural expansion. To apply this technique, a coherent light source, such as a laser, is launched into one end of an optical fiber. As the light travels through the fiber, a small fraction of it interacts with the material and is scattered back along the fiber in the opposite direction. Using optical fibers for the measurement of deformations offers several advantages over the use of invar wire. Optical fibers provide higher accuracy and sensitivity, allowing for precise measurements of small deformations. The precision of this technique enables measurements in the range of micrometers or even nanometers. Additionally, optical fibers are nonconductive and immune to electromagnetic interference, making them suitable for a wide range of environments. They also offer the capability for distributed sensing, enabling  
Figure 4: Experimental set-up: (**a**) cement paste just cast in the metallic mold; (**b**) geometry of cement paste prismatic specimen equipped with OF; (**c**) extensometer (left) and reference length bar (right); (**d**) cement paste prism equipped with a pair of pins. the measurement of multiple points along the fiber. Moreover, using fiber optic sensors provides a nonintrusive monitoring solution. When the material or structure undergoes deformation (expansion in the case of ESA), it induces strain on the optical fiber. This strain causes a change in the optical properties of the fiber, such as the wavelength or intensity of the light propagating through it. This change is then detected by the optical sensing system. The optical signal captured by the sensing system is processed to extract the relevant information related to the deformation.