# Acoustic Emission Analysis of the Cracking Behavior in ECC-LWSCC Composites
## 1 Introduction  
In recent years, there has been growing interest in the application of lightweight concrete (LWC) within the construction industry due to its cost-effectiveness and structural advantages. LWC is designed by fully or partially replacing normal weight aggregates with lightweight aggregates, which have low density. This reduction in self-weight results in smaller concrete cross-sections and decreased dead loads [1, 2, 3]. Additionally, lightweight aggregates in concrete offer benefits such as reduced thermal conductivity and improved insulation properties due to air voids [4]. The incorporation of lightweight aggregates into self-consolidating concrete (SCC) creates lightweight self-consolidating concrete (LWSCC), which flows easily into congested reinforcements and complex formwork [4]. Concrete structures face degradation from aging and environmental factors, leading to cracks, spalling, deflections, and steel reinforcement corrosion [5]. These problems can also stem from construction defects and increased structural loads [6]. Therefore, aging structures often require repairs to maintain design integrity, resist higher loads, ensure serviceability, and prevent sudden failures [5, 7, 6]. Typically, repair involvesremoving damaged concrete and applying a new repair layer [8], making the selection of high-performance, economical repair materials critical for effective rehabilitation. Rehabilitating concrete structures demands materials with superior properties such as high ductility, strength, energy absorption, and durability [9]. Fiber-reinforced cement-based composites, like engineering cementitious composites (ECCs), offer these attributes. ECCs exhibit enhanced tensile strength, high ductility, and minimal crack widths [10]. Compared to regular concrete, ECC can achieve 600 times the tensile ductility [11; 12], with a 2% fiber content enhancing its tensile capacity by 3-5% and showing pseudo strain hardening [12; 13]. Polyvinyl alcohol (PVA) and steel fibers (SFs) notably improve ECC's cracking behavior, with SF contributing to superior ductility, crack load, ultimate load, and energy absorption [14]. ECCs typically exhibit tensile strengths of 4-6 MPa, compressive strengths of 30-80 MPa, and strain capacities of 0.4% to 0.65% [15], allowing repaired structures to perform well under fatigue and impact loads [9]. Combining ECC and SCC has enhanced structural performance in various studies. For example, [PERSON] et al. [16] analyzed hybrid composite beams of SCC and ECC, noting improved ductility and energy absorption without shear reinforcement. [PERSON] et al. [17] tested lightweight ECC composites with PVA or SF under impact loading, demonstrating superior impact resistance. [PERSON] et al. [18] studied ECC in beam-column joints, finding SF ECC provided the highest ductility and load capacities. Acoustic emission (AE) monitoring is valuable for the health monitoring of materials and structures, detecting transient waves emitted by damage events [19]. AE's high sensitivity enables the detection of micro-cracking and other low-energy sources in concrete structures [19]. AE has proven effective in characterizing fracture processes in fiber-reinforced concrete [20], assessing bridge integrity [21], monitoring crack propagation in shear walls [22], and evaluating pre-stressed concrete beams [23]. AE analysis in composite beams includes damage detection, classification, and quantification. For example, studies have used parameters like average frequency versus rise time and AE hits versus test time to analyze fracture mechanisms in lightweight composite beams [24]. Rise time/amplitude (RA) analysis has differentiated flexural and shear failures in multi-layered beams [25].