International Concrete Abstracts Portal

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 one-inch 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 indices. 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 indicated that lower calcium content in supplementary cementitious material (SCM) replacement, either due to higher fly ash content with lower calcium compared to OPC or with silica fume with almost zero calcium content, with bacteria, may have a significant effect on the healing progress.

To improve the ductility of fiber-reinforced polymer reinforced concrete structures, the hybrid reinforcement with glass fiber-reinforced polymer (GFRP) and stainless steel (SS) is selected in this paper. Nine seawater sea sand concrete beams were designed and tested. The effects of concrete strength, effective reinforcement ratio ρ₂, and reinforcement type in the tensile zone on the flexural behavior of the beams were analyzed. The test results show that with the same concrete strength and the same effective reinforcement ratio ρ₂, the ductility of hybrid reinforced beams is higher than GFRP reinforced beams; the comparison of mid-span deflection of the GFRP bars and hybrid reinforced beams are not only depend on the reinforcement type, but also depend on the total stiffness of reinforcement before SS bars yield in the tensile zone and whether the SS bars are yielding in the tensile zone. Meanwhile, theoretical analysis was conducted for cracking moment, ultimate flexural load-carrying capacity, and mid-span deflections. A new ultimate flexural load-carrying calculation equation was proposed, which predicted the experimental values in good agreement.

To promote the application of fiber-reinforced polymer (FRP) bars reinforced ultra-high-performance seawater sea-sand concrete (FRP-UHPSSC) structures in marine construction, four-point static bending tests were carried out on 16 FRP-UHPSSC beams with different reinforcement ratios, height of cross-section, and type of FRP bars to investigate the ultimate load-carrying capacity, the midspan deflection, and the failure modes of the beams. The experimental results show that all the test beams are brittle failures, and the failure mode of the beams is shear failure when the ratio of the actual reinforcement ratio to the balanced one is higher than 2.73. Increasing the reinforcement ratio and the beam section height both improve the bending moment at ultimate load and the flexural stiffness at the service limit state. The Steel-FRP composite bars (SFCB) reinforced UHPSSC beams have the maximal bending moment at ultimate load, and the basalt fiber reinforced polymer (BFRP) bar reinforced UHPSSC beams have the optimal ductility. The deviation of ultimate bending moment and midspan deflection obtained by the proposed calculation method is reduced from 7.5 to 2.8%, and from 15 to 3%, respectively, compared with current specifications for FRP-reinforced concrete structures.

The composition of OPC changed in North America with the addition of ground limestone in 2004 (since the adoption of ASTM C150-04a), which reacts to form carboaluminate hydration products. This paper discusses the potential influence of limestone addition on porosity, pore connectivity, formation factor, and electrical properties of cementitious systems. The carboaluminate reaction products can result in a system with limestone that has an equivalent water-to-powder ratio (w/p) that is approximately 0.07 lower than the system without limestone (occurring at the minimum porosity). When reactive alumina is added to the system, a greater amount of limestone reacts, and a reduction in porosity occurs. The carboaluminate phases impact the transport properties of mixtures to a greater extent for mixtures with moderately low w/p and aluminous SCMs. This has implications on standards and specifications, which are based on historic research and testing using cements not containing limestone, and therefore would have a higher porosity and lower formation factor than cements manufactured in the US after approximately 2004 at the same w/p.

Due to the excellent deformation coordination ability and permeability, bentonite has been widely introduced to modify concrete in underground geotechnical engineering. However, the underlying mechanism for bentonite modification remains unexplored. A series of experiments was performed to clarify the modification mechanism of bentonite. The results showed that all strengths decreased upon bentonite addition, while high toughness was achieved. The micro-test results revealed that bentonite promotes the dissolution of calcium hydroxide (CH) and the nucleation of calcium silicate hydrate (C-S-H) in the interfacial transition zone (ITZ). The hydration products produced by the reactive ions and ultrafine bentonite particles continuously reduced the porosity and Ca/Si ratio in ITZ, strengthened the interface bonding, and controlled the coalescence of microcracks. Inversely, bentonite particles tend to adsorb large amounts of water and hinder the available water from accessing cement grains, which results in an increased porosity and slower hydration progress of cement grains. The loose microstructure cannot be compensated for by reinforced interfacial bonding and inevitably results in the deterioration of mechanical performance in composites.