International Concrete Abstracts Portal

The interference between steel fiber and coarse aggregate reduces the homogeneity of fiber distribution and orientation, which may compromise the expected reinforcing effectiveness of steel fibers in concrete. Traditional destructive testing techniques constrain the quality control of steel fiber distribution in prefabricated concrete segments; developing an inductance-based technique contributes to non-destructive characterization of steel fiber distribution. This work uses a Helmholtz coil to solve the magnetic field non-uniform distribution, thereby designing an inductor device to improve the accuracy of steel fiber distribution monitoring within concrete. On this basis, a multi-parameter experiment was designed to study the coupling effect of coarse aggregate and steel fiber, with key variables including water-to-binder ratio, coarse aggregate gradation, steel fiber mixing sequence, vibration duration, and casting flow distance. The C50 concrete mixture incorporates fly ash (75 kg/m³) as a supplementary cementitious material to improve workability and particle packing density. The primary findings are as follows: the induction-based method enables non-destructive evaluation of steel fiber content and orientation in steel fiber‑reinforced concrete containing coarse aggregate (SFRC‑CA), demonstrating high detection efficiency. The larger the aggregate size and water-binder ratio, the worse the steel fiber distribution uniformity. Improper vibration will lead to steel fiber thickness-related settlement, while the longer the flow distances, the more uneven the orientation of the fiber. These results offer important reference for material design and quality control of precast SFRC-CA components.

This study investigated the effects of sodium polyacrylate (PAAS) particle size and dosage on the workability, strength, and hydration of cementitious materials. The backscattered electron banding (BSE) was used to quantitatively analyze the degree of hydration of cement around PAAS voids under varying humidity levels. The results indicated that mortar fluidity, compressive strength, and flexural strength gradually decreased with increasing PAAS particle size and dosage. The incorporation of fine PAAS particles (45–50 μm) enhanced mechanical strength. While PAAS does not alter hydration product types, it promotes calcium carbonate formation, with calcium hydroxide, calcite, and C3S remaining dominant. Furthermore, it was found that higher humidity conditions and larger particle size PAAS particles can reduce the amount of unhydrated cement in the area around PAAS voids.

This research aims to prepare porous ceramsite with low thermal conductivity. The porous ceramsite was also used as fine aggregate to substitute river sand in pumice concrete. Its impact on improving the thermal insulation performance of pumice concrete was thoroughly investigated. The experimental method included high-temperature calcination; transient planar heat source analysis; as well as the use of X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) techniques. The investigation revealed that the best calcination parameters were a preheating temperature of 400°C (752°F), a preheating duration of 25 minutes, a calcination temperature of 1250°C (2282°F), and a calcination duration of 25 minutes. Under these conditions, the crushing index of the porous ceramsite was determined to be 29.1%, with a thermal conductivity of 0.138 W/(m·K) (0.08 Btu/h∙ft∙°F). It is worth noting that an increase in calcination temperature promotes the hole content in ceramsite, leading to a 52.19% increase in macropore volume and a corresponding decrease in thermal conductivity. Furthermore, as the replacement rate of ceramic aggregate increases, the thermal conductivity of pumice concrete gradually decreases, with values ranging from 18 to 34.8%. This reduction occurs because the replacement elevates the volume of coarse capillary pores and non-capillary pores in pumice concrete, increasing by 13.9% to 91.3% and 63.1% to 128.5%, respectively. Additionally, a prediction model for the thermal conductivity of pumice concrete has been established using the Mori-Tanaka homogenization method. The model’s verification accuracy falls within an error range of 5%, demonstrating its effectiveness in accurately predicting the thermal conductivity of pumice concrete.

This study evaluated the effect of Class F fly ash (5, 10, 15, and 20%) and silica fume (20%) as partial cement replacements on bacterial crack healing. Concrete cylinders were prepared, cracked into 25.4 mm disks, and submerged in fresh water. Healing progress was monitored over 18 weeks using microscopy and quantified through a healing index. Results showed that bacterial activity substantially improved healing compared to natural hydration in control specimens. Fly ash replacement did not prevent healing, and several disks across all percentages achieved complete crack closure. However, higher fly ash levels shortened the duration of bacterial activity, indicating sensitivity to calcium availability. At 20% fly ash, healing progressed more slowly but remained active at 18 weeks. In contrast, specimens containing 20% silica exhibited significantly lower healing efficiency, with few disks achieving full closure and overall lower healing indexes. These results confirm that bacteria-based self-healing concrete remains effective with fly ash but is constrained by high silica fume content due to very low to zero calcium content in silica fume. The findings suggest that lower calcium levels in supplementary cementitious materials (SCM) replacements, caused by higher fly ash content or the use of silica fume, may significantly influence bacterial healing.

The self-healing performance of zero cement-based one-part ambient cured alkali-activated engineered composites (AAECs) using 2% v/v polyvinyl alcohol (PVA) fibers and silica sand was evaluated. The variables in the study were: binary (fly ash class C ‘FA-C’ and ground granulated blast furnace slag ‘GGBFS’)/ternary (FA-C, fly ash class F ‘FA-F’ and (GGBFS) combination of precursors, two types of powder form alkaline reagents (type 1- calcium hydroxide: sodium meta-silicate = 1:2.5 and type 2 - calcium hydroxide; sodium sulfate = 2.5:1) and different pre-loading strain levels (0%, 0.5%, and 1%). The performance was based on the recovery of compressive/tensile strength, tensile strain hardening properties, crack-sealing, and microstructural characteristics after 365 days of water curing compared to conventional engineered cementitious composites (ECCs). All AAECs (binary/ternary or reagent type 1/2) exhibited enhanced/comparable self-healing performance compared to ECCs at both 0.5% and 1% pre-loading strain levels exhibiting maximum recovery of tensile strength, tensile strain capacity, stress index, tensile ductility and tensile elasticity up to 115%, 184%, 130%, 236% and 123%, respectively through preserving strain hardening and micro-cracking characteristics with up to 96% recovery of compressive strength. This was attributed to ongoing alkali activation and pozzolanic reactions forming C-S-H/C-A-S-H in binary with additional N-C-A-S-H in ternary and calcite binding phases leading to matrix densification, crack-sealing, and improved PVA fibre-matrix bonding as per SEM-EDS and XRD analyses. Generally, all the AAECs exhibited the ability of recovering properties and composites with reagent 2, demonstrating superior self-healing characteristics compared to their reagent 1 counterparts by achieving complete or higher recovery of tensile strength/strain capacities compared to their virgin counterparts. This study confirmed the viability of producing cement-free ambient-cured self-consolidating AAECs with powder form reagents having satisfactory strength, strain hardening, and self-healing characteristics for durable, sustainable construction.