International Concrete Abstracts Portal

The concept of multi-generational concrete recycling is increasingly relevant as many existing recycled concrete structures near the end of their service lives. This study examines the performance variation and recyclability of multi-generational concrete subjected to chloride salt dry-wet cycling. After 30 dry-wet cycles, natural aggregate concrete, designed with three different strength grades, was crushed to produce the first generation of recycled fine aggregate, which was then used to prepare the second generation of concrete. This second generation was subjected to the same dry-wet cycling and subsequently crushed to yield a second generation of recycled fine aggregate. The results demonstrate a significant decline in the performance of the second generation of concrete, with an average compressive strength reaching only 89.52% of the first generation. Notably, the performance deterioration was more pronounced in lower-strength mixes, which exhibited increased porosity, greater mass loss, and deeper chloride penetration. Both generations of recycled fine aggregate met the standards for Class III aggregate; however, some properties of the recycled fine aggregate derived from higher-strength concrete qualified for Class II aggregate status. Additionally, a regression analysis model was developed to predict the attenuation coefficients for the third generation of concrete with design strengths of 30, 45, and 60 MPa, yielding coefficients of 56.84, 67.75, and 71.72%, respectively. This study underscores the potential for multi-generational use of recycled fine aggregates and highlights the importance of selecting appropriate design strengths to enhance durability and recyclability in chloride-rich environments.

The determination of the static coefficient of friction between steel and concrete is essential for the design and safety of structures, particularly in systems operating under low axial stresses, such as foundation slabs supporting waste storage casks. In such applications, sliding resistance and shear transfer at the steel–concrete interface play a critical role in ensuring stability and overall structural performance. Inadequate friction at this interface can lead to sliding, reducing the structure’s capacity to resist lateral forces and potentially resulting in serviceability or safety concerns. This study presents an innovative approach to evaluate the static coefficient of friction between steel, prepared to a specific steel surface roughness level (SSPC-SP 6), and concrete with varying surface roughness profiles, including light sandblast, light-to-medium sandblast, medium bush hammer, and heavy sandblast finishes. Tests were performed under low normal stresses (18, 33, and 50 kPa) and shear displacement rates (3, 5, 7, and 9 mm/s). A custom test setup was developed to apply controlled displacement to a concrete block while measuring the horizontal force required to initiate sliding against the steel plate. The results indicate that the static coefficient of friction across all concrete surface roughness levels ranges from 0.68 to 0.75, with a mean value of 0.72. Statistical analysis at a 95% confidence level reveals that variations in concrete surface roughness, shear displacement rates, and applied normal stresses do not produce significant differences in the static coefficient of friction. Consequently, utilizing concrete with light sandblast surface preparation in the field is sufficient to achieve a static coefficient of friction comparable to aggressive surface roughness profiles. These findings simplify construction practices while ensuring reliable shear transfer and sliding resistance at steel-concrete interfaces in low axial stress applications.

Pumping concrete is widely reported to modify the air volume of fresh concrete. The study compares changes in the air volume and air void distribution in both fresh and hardened concrete before pumping and after the concrete is discharged from the pump hose. This comparison is made for 62 different concrete mixtures from 20 field projects using 18 different concrete pumps. These results show that after pumping, the air volume and SAM Number are sometimes significantly changed, but when checking the hardened concrete, there is minimal change in the air volume and air void spacing. Further, evidence is given for the air to restabilize within the fresh concrete before the concrete hardens.

The calculation of the nominal flexural strength of concrete members prestressed with hybrid (i.e., a combination of bonded and unbonded (steel and/or carbon fiber reinforced polymer (CFRP)) tendons is dependent on determining the stress in the unbonded prestressed reinforcement. Current provisions in the ACI CODE-318-25 are only applicable to members with either unbonded or bonded steel tendons. Additionally, while ACI PRC-440.4R-04 is adopted for unbonded CFRP tendons, neither ACI provisions address the use of hybrid tendons. This paper presents a closed-form analytical solution for the stress at ultimate derived based on the Modified Deformation-Based Approach (MDBA) that is applicable to beams prestressed with unbonded, hybrid (steel or FRP), external with deviators or internal tendons, with and without non-prestressed reinforcement. An assessment of its accuracy and applicability in calculating the nominal flexural strength is examined using a large database of 330 beams and slabs (prestressed with steel and/or CFRP tendons) compiled from test results by the authors as well as those available in the literature. Results predicted by the proposed approach exhibited excellent accuracy when compared to those predicted using ACI CODE-318 or ACI PRC-440 stress equations. They also show that the approach is universally applicable to any combination of bonded and/or unbonded (steel and/or CFRP) tendons, span-to-depth ratio, as well as loading applications.

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.