The study examines the mechanical and microstructural performance of eco-friendly mortar mixes that incorporate Recycled Steel Fibers (RSF) derived from waste tires. Four mortar formulations with varying RSF content (0%, 0.5%, 1%, and 1.5% by volume) were evaluated for compressive strength, flexural strength, and electrical conductivity. Experimental results revealed that a 1.5% RSF mixture exhibited remarkable improvements in flexural strength, achieving a 67% increase compared to the control formulation while delivering a 12.6% enhancement in compressive strength. However, the 0.5% RSF mix showed reduced performance due to poor fiber dispersion, underscoring the importance of proper fiber distribution. Specific resistance decreased with RSF addition, indicating enhanced electrical conductivity, with the lowest specific resistance observed at 0.5% RSF on day 28. An empirical model using a fiber reinforcing index (ξ) was developed to predict strength behavior. A quadratic relationship was found to best describe compressive strength gains, while a linear model effectively captured the flexural strength trend. The models were calibrated using both experimental data and literature values, achieving high predictive accuracy. Electrical conductivity increased with RSF addition, and the slope of the specific resistance during loading correlated strongly with mechanical strength, highlighting its potential as a non-destructive structural health monitoring (SHM) indicator. SEM analysis confirmed improved matrix integrity and fiber–matrix interaction at the optimal 1% RSF content, which balanced strength gains and sensing capability. The study establishes RSF as a viable sustainable alternative to virgin steel fibers, providing both mechanical enhancements and self-sensing properties. This novel integration of electrical monitoring with mechanical testing and modeling provides new insights into recycled-fiber composites by enabling simultaneous enhancement of structural performance and real-time damage monitoring.

Nonlinear ultrasonics (NLU) has grown in importance over the past years. The viability of applying the newly developed NLU technique called the Sideband Peak Count-Index (SPC-I) to estimate the 28-day unconfined compressive strength (UCS) of cylindrical concrete specimens is investigated. In addition, the SPC-I technique is combined with the conventional ultrasonic pulse velocity (UPV) technique to achieve more precise UCS estimations than would be possible with a single estimation parameter. Sixteen conventional general-purpose concrete specimens are cast utilizing water-to-cement (W/C) ratios of 0.4, 0.5, 0.55, and 0.6. To ensure the reproducibility of results, experimental operations are performed utilizing consistent testing protocols and instruments. The SPC-I is determined to be a viable parameter that can be used to estimate the UCS value of concrete specimens with high accuracy. In addition, when the SPC-I is combined with the UPV, the resulting estimates become even more accurate and reliable than when utilizing only UPV values.

Carbon fiber-reinforced thermoplastic (CFRTP) composites represent a promising advanced coating solution for steel pipelines, offering superior chemical resistance, mechanical reinforcement, and durability compared to conventional polymer coatings. This study compares the performance of single-layer and multi-layer coatings made of carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites bonded to low-carbon steel substrates. Four composite configurations were investigated: 1-layer unidirectional (UD), 2-layer biaxial, 7-layer UD, and 7-layer biaxial samples. Both UD and biaxial ([0/90]) layups were bonded to low-carbon steel substrates using epoxy adhesive. The ultrasonic nondestructive evaluation was employed to characterize coating quality and detect subsurface defects. Then the mechanical integrity was assessed via three-point bending tests and the microstructural features were examined using optical microscopy. The experimental results demonstrated that while 1- layer coatings provided negligible structural reinforcement (flexural strength: 5.1-5.3 MPa), they induced detectable alterations in ultrasonic response characteristics, confirming the sensitivity of hybrid ultrasonic techniques. The 7-layer coatings significantly enhanced mechanical behavior, with the UD layup achieving a flexural strength of 908 MPa (approaching the 900 MPa design target) compared to 638 MPa for the 7-layer biaxial configuration. The hybrid ultrasonic indices (Sideband Peak Count Index - SPC-I, and Spectral Dissipation Index - SDI) increased significantly with ply count and were significantly higher for the 7-layer biaxial configuration (SPCI = 2.91 ± 0.51, SDI = 2.78 ± 0.33) compared to the 7-layer UD (SPC-I = 1.70 ± 0.21, SDI = 1.94 ± 0.22), reflecting greater microstructural heterogeneity and interfacial defects. The findings demonstrate the superior sensitivity of hybrid ultrasonic indices for detecting microstructural variations (interfaces, porosity) that influence the mechanical integrity of multi-layer composite coatings. This highlights their potential for nondestructive evaluation and quality control of composite pipeline coating systems. Prototype steel pipe sections coated with CFRTP composites demonstrated the feasibility of the coating system for real-world pipeline applications.

This study investigates the optimization of 3D mortar printing mixtures and processes to enhance sustainability and efficiency in construction. Utilizing a novel 3D mortar printer equipped with a 14-bar pumping system and interchangeable nozzles of 1.5, 2, and 3 cm diameters, the research incorporates sustainable materials such as fly ash, silica fume, and polypropylene fibers. These materials are selected to improve the mechanical properties of the mortar while reducing the environmental impact of construction practices. The study analyzes the effects of key parameters, including binder type (Fast-Setting Cement (FSC) vs. Ordinary Portland Cement (OPC)), water-to-powder (W/P) ratio, and nozzle diameter, on the fresh properties and mechanical performance of printed elements. The research evaluates the printability, rheological properties, and hardened properties, such as flow table tests and flexural strengths, of various mixtures. Early-age performance is monitored by both traditional mechanical tests and advanced ultrasonic techniques. Flexural strength tests at 4h, 6h, and 24h are complemented by ultrasonic pulse velocity (UPV) measurements, hybrid ultrasonic indices (Sideband Peak Count Index (SPC-I), and Spectral Dissipation Index (SDI)). The optimized FSC mixtures (W/P ratios 0.34 and 0.37) exhibited excellent workability (flow 142–150%) and rapid strength gain, reaching flexural strengths of 3.06 and 1.83 MPa at 4h and about 4.3 MPa at 24h. Ultrasonic monitoring revealed that UPV values increased to 3.1 km/s by 24h, while hybrid ultrasonic indices peaked at 6h (SPC-I 1.1–1.6 and SDI 1.1–1.4 relative to the initial values) before dropping by more than 50% at 24h, correlating with transient microstructural adjustments observed via scanning electron microscopy (SEM). By establishing correlations between printing parameters, mix designs, and material properties, the study aims to develop a nonproprietary optimized 3D mortar (printing) mixtures and processes. The results demonstrate that carefully selected mix designs and printing parameters can significantly enhance the sustainability, efficiency, and performance of 3D concrete printing (3DCP), potentially leading to its wider adoption in the construction industry. Ultrasonic evaluation provided novel insights into early-age material behavior, strengthening the understanding of printability versus buildability. Specifically, this work is one of the first to integrate hybrid ultrasonic indices into 3DCP monitoring, a novel approach that reveals early microstructural evolution beyond standard tests. This comprehensive approach offers valuable guidance for improving construction practices, reducing environmental impact, and increasing the viability of additive manufacturing in the built environment. This research contributes to the advancement of 3D mortar printing technology, promoting sustainable and efficient construction practices. Graphical abstract

The cement industry is a major contributor to global CO₂ emissions, creating a need for monitoring techniques that support carbon capture strategies while assessing material performance. This study investigates the accelerated carbonation curing of cement mortar using linear and nonlinear ultrasonic sensing methods, alongside mechanical and gravimetric measurements. Mortar specimens were carbonated for 1–28 days and evaluated using ultrasonic pulse velocity (UPV), the Sideband Peak Count Index (SPC-I) for nonlinear ultrasonic response, compressive strength testing, and mass-based CO₂ uptake analysis. UPV showed sensitivity primarily to bulk material changes, with comparatively less distinction among the observed responses during carbonation curing. In contrast, the SPC-I captured distinct nonlinear responses associated with matrix evolution. Early-age carbonation (<7 days) produced increased nonlinearity, attributed to shrinkage-induced microcracking, whereas extended curing led to reduced SPC-I values, consistent with carbonation curing age. These trends exhibited an inverse correlation with compressive strength, which increased by up to 38.9% on the 28th day compared to the control specimens. Gravimetric analysis confirmed effective CO₂ sequestration, with average specimen mass gains reaching 2.62%. The findings demonstrate that nonlinear ultrasonic sensing provides a sensitive, nondestructive approach for monitoring carbonation curing and linking acoustic signatures to mechanical performance and carbon uptake in cement-based materials.