PSI - Issue 64

Jian-Neng Wang et al. / Procedia Structural Integrity 64 (2024) 1605–1612 / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Several types of optical wave-guide sensors or optical fiber-based refractometer have been proposed, including long-period fiber gratings, Fabry-Perot interferometers, Michelson interferometers, and Mach-Zehnder interferometers (MZI). The MZIs made with a pair of lateral offset zones separated by a few centimeters are suitable for sensing applications that they offer many advantages such as low cost, robustness, low insertion losses, relatively simple fabrication, compact, ease of use, immunity to electro-magnetic interference, and sensitivity either to the chloride ion concentration or the external refractive index (RI) [Tang et al. (2007), Wang (2011), and Wang et al. (2012)] or pH [Al-Hayali et al. (2022) and Nguyen et al. (2014)]. Excessive chloride ions possess the ability to corrode the steels of reinforced concrete structures that affect the safety and health of concrete structures. In addition, the pH of concrete structures decreases to a value close to 9.5 that externally exhibits carbonation and durability problems [Liu et al. (2017)]. In this paper, we present the development and assessment of two individual optical fiber sensing platforms either for monitoring chloride concentration or pH range in structural health monitoring of concrete structures. The optical fiber MZI sensing platform setup for chloride ion concentrations and the spectral responses of the MZI for the aqueous samples of fresh concrete supernatant (W/C = 0.35 and W/C = 0.65) with salt water solutions in different concentration range from 0.015% to 12.5% were measured. Behnood et al. focused on pH monitoring in concrete either using non electrochemical methods (e.g., OH titration) or using electrochemical methods (e.g., electrode method) for different materials such as concrete, paste, and mortar. Lin published recent development and applications of optical and fiber-optic pH sensors in 2000, including pH sensors based on conductive polymers, imaging fibers, microparticles, and nanospheres, as well as micron and submicron fiber-optic pH sensors, distributed fiber optic pH sensors, pH sensors for high acidity and alkalinity, etc. However, our proposed optical fiber pH sensing system with single mode fiber and broadband light source (200-1100 nm) is proposed to implement structural health monitoring of concrete especially in pH measurement without any chemical coating or compound. The optical fiber pH sensing system is composed of a fiber optical sensing system with a Visible-NIR bifurcated fibers, a CUV-UV cuvette holder, a broadband light source, an UV-NIR spectrometer, and a personal computer for data acquisition. We measured different pHs of the standard solutions with different pHs of 1, 4.08, 4.95, 5.99, 7.09, 8.07, 9.75. For optical fiber Mach-Zehnder interferometer, the relationship between the sensing length L and the wavelength difference between the two adjacent minimum interference signals (or fringe period) as Eq. 1 [Villatoro et al. (2007)], where λ is the center wavelength of the source, β 1 and β 2 is the propagation constants of the interference mode, L is the length of the interferometer, and Λ is the interference fringe spacing. Our home-made Mach-Zehnder interferometers were fabricated and confirmed to meet the requirement of Eq. 1: the fringe period of the interferometer is almost inversely proportional to the interferometer length [Wang et al. (2012)].

Λ = β 2πλ 1 −β 2 ( ) · λ : the center wavelength of the source β 1 and β 2 : the propagation constants of the interference mode

(1)

L : the length of the interferometer Λ : the interference fringe spacing

2.

Materials and methods

2.1. Materials and specimen preparation The proportions of fresh concrete are based on weight, for water /cement ratio is equal to 0.35, the concrete is composed of 910 g cement, 2150 g fine aggregate, and 4100 g coarse aggregate. In addition, for water /cement ratio is equal to 0.65, the concrete is composed of 910 g cement, 2060 g fine aggregate, and 4100 g coarse aggregate. The used cement meets the requirement of ASTM C150/C150M-22: Standard Specification for Portland Cement . The specific gravities and other properties of used fine and coarse aggregates were conducted and meet the source property requirements, such as ASTM standard methods such as ASTM C127-15 ： Standard Test Method for Relative