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.

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.

Due to global warming, the temperature of earth surface increased by 0.95 to 1.20℃ in the past 4 decades. The increase in temperature has significant effects on the concrete industry, causing alterations in concrete curing conditions and degradation in strength and durability properties. The understanding of changes in concrete properties due to variations in curing conditions from climate change is an imminent task that has to be resolved. Among the durability properties of concrete, freeze-thaw (FT) resistance is most directly affected by climate change. However, in all of the studies conducted on the FT behavior of concrete, the dramatic changes in environmental conditions due to climate change were not considered. Therefore, the focus of this study is to understand the FT performance of concrete from extreme temperature and relative humidity (RH) changes in curing conditions. To find the relationship between the curing condition change and FT resistance levels as a function of time, a 3-D satisfaction surface graph was developed using the Bayesian probabilistic method. Then, an example of drawing the 3-D satisfaction surface diagrams for FT resistance based on the weather conditions in New York City between 2001 and 2100 was shown. Furthermore, considering the reduction rate of the average annual FT cycle due to climate change, this study confirmed that FT resistance performance increased. This approach contributes to a performance-based evaluation (PBE) strategy for concrete exposed to FT cycles under various environmental conditions. The study details and results are discussed in the paper.

This study employs advanced nonlinear finite element (FE) modeling to investigate interface shear transfer (IST) behavior in reinforced concrete connections, a crucial factor for bridge durability and safety. The research examines shear-transfer mechanisms at the interface between precast girders and cast-in-place deck segments through three experimental methods: beam, pushoff, and Iosipescu four-point bending tests. FE simulations evaluated stress distributions, IST capacity, and failure mechanisms. Validation against experimental data shows that the Iosipescu test provides the most accurate representation of IST behavior, exhibiting a stress distribution error margin of only 1%, closely aligned with observed failure patterns. In contrast, the pushoff test showed a 30% deviation from empirical data, indicating reduced accuracy in predicting real-world IST behavior. These findings highlight the importance of incorporating the Iosipescu test into IST evaluation protocols, as its greater precision enhances design methodologies for concrete bridges, reduces structural failure risks, and informs future updates to IST-related codes.

The durability of manufactured sand concrete is substantially influenced by variations in parent rock lithology, fineness modulus, and stone powder content of the manufactured sand. This study develops a predictive model for the relative dynamic elastic modulus of manufactured sand concrete using six machine learning algorithms. The results demonstrate that the CPO (crested porcupine optimizer)-optimized XGBoost model exhibits superior prediction accuracy and stability. The algorithm-based optimization reveals that manufactured sand produced from limestone, iron ore tailings, and quartzite demonstrates improved frost resistance in concrete. The optimal fineness modulus range was found to be 2.6 to 2.86; stone powder content should be maintained between 3 and 12% for optimal performance. The study further proposes a mixture ratio optimization scheme that takes into account frost resistance, material cost, and carbon emissions, so that the cost and carbon emissions of single concrete are reduced, and the frost resistance is further improved.