International Concrete Abstracts Portal

This paper presents research focused on the development of a test method that can be used to gauge the susceptibility of a post-tensioning (PT) grout to form soft grout. Depending on the grout formulation, soft grout may have a lower pH, retain excessive moisture, and be corrosive to the tendon. While relatively rare, it has been documented in bridge construction in the U.S. and abroad and in some cases has prompted the replacement of PT tendons.

One of the causes of the soft grout is thought to be the result of the use of low reactivity fillers such as ground limestone. When tendons are deviated significantly, these fillers can segregate and then accumulate into a mass of material that does not harden. The modified inclined tube test (MITT) was developed based on the Euronorm inclined tube test. None of the commercially available PT grouts produced soft grout when the grout was mixed and injected in accordance with the manufacturer’s recommendations and tested well before their expiration date. Additional mix water or residual water in the tube, however, produced soft grout consistently in one of the PT grouts.

This study employs machine learning (ML) to predict ultrasonic pulse velocity (UPV) based on the mix composition and curing conditions of concrete. A dataset was compiled using 1495 experimental tests. Extreme Gradient Boosting (XGBoost) and Support Vector Regression (SVR) were applied to predict UPV in both direct and surface transmissions. The Monte Carlo approach was used to assess model performance under input fluctuations. Feature importance analyses, including the Shapley Additive Explanation (SHAP), were conducted to evaluate the influence of input variables on wave propagation velocity in concrete. Based on the results, XGBoost outperformed SVR in predicting both direct and surface UPV. The accuracy of the XGBoost model was reflected in average R² values of 0.8724 and 0.9088 for direct and surface UPV, respectively. For the SVR algorithm, R² values were 0.8362 and 0.8465 for direct and surface UPV, respectively. In contrast, linear regression exhibited poor performance, with average R² values of 0.6856 and 0.6801 for direct and surface UPV. Among the input features, curing pressure had the greatest impact on UPV, followed by cement content. Water content and concrete age also demonstrated high importance. In contrast, sulfite in fine aggregates and the type of coarse aggregates were the least influential variables. Overall, the findings indicate that ML approaches can reliably predict UPV in healthy concrete, offering a useful step toward more precise health monitoring through the detection of UPV deviations caused by potential damage.

Adding fibers, especially steel fibers, to cementitious composites is one of the most commonly used methods to improve the mechanical properties of cementitious composites. While the high price becomes the most concerning factor in the use of steel fibers. This study aims to investigate the influence of the content of multiscale fibers, including nanocellulose, sisal fibers, and steel fibers, on the fracture properties of cementitious composites. The fracture properties will be evaluated using the initial fracture toughness, unstable fracture toughness, and fracture energy through the notched beam bending tests. The results demonstrate that replacing steel fiber with an appropriate amount of sisal fiber effectively improves fracture properties, indicating a balancing point between fracture-impeding property and price/ environment. Specifically, under total macro fiber volume fractions of 1% and 1.5%, the 0.2 % sisal fiber replacement to the steel fibers exhibits the best fracture impeding properties. Additionally, the incorporation of nanocellulose (2% optimal in the research) enables the formation of a multi-scale crack resistance system at the nano-micro level, further enhancing the fracture-impeding properties of cementitious composites. Moreover, the research found that adding the fibers collaboratively can cultivate a better enhancement in fracture-impeding properties than adding them separately.

This study investigates the feasibility of utilizing a hybrid combination of recycled steel fibers (RSF) obtained from scrap tires and manufactured steel fibers (MSF) in concrete developed for pavement overlay applications. A total of five concrete mixtures with different combinations of MSF and RSF, along with a reference concrete mixture, were studied to evaluate fresh and mechanical properties. The experimental findings demonstrate that the concretes incorporating a hybrid combination of RSF with hooked-end MSF exhibit comparable or higher splitting tensile strength, flexural strength, and residual flexural strength to that of concretes containing only hooked-end MSF, straight MSF, and RSF. This enhanced mechanical performance can be ascribed to the multiscale fiber reinforcement effect that controls different scales (micro to macro) of cracking, thereby providing higher resistance to crack propagation. The concretes containing only RSF show lower splitting tensile strength, flexural strength, and residual flexural strength compared to concrete solely reinforced with straight MSF or other steel fiber-reinforced concrete (SFRC) mixtures due to the presence of various impurities in the RSF, such as thick steel wires, residual rubber, and tire textiles. Interestingly, blending RSF with hooked-end MSF overcomes these limitations, enhancing tensile strength, flexural strength, and residual flexural strength, while significantly reducing costs and promoting sustainability. Lastly, the findings from the pavement overlay design suggest that utilizing a hybrid combination of RSF with hooked-end MSF can reduce the design thickness of bonded concrete overlays by 50% compared to plain concrete without fiber reinforcement, making it a practical and efficient solution.

This research investigates the impact of using washed waste fines (WWF), a byproduct from ready-mixed concrete (RMC) plants, as a partial replacement for natural sand in concrete. Cylindrical (100mm x 200mm) and cubic mortar specimens (50mm x 50mm x 50mm) were created with 20% WWF substitution. Hardened properties such as compressive strength, tensile strength, UPV, and durability parameters such as chloride migration coefficient and carbonation coefficient were evaluated. The study also examined the microstructure of concrete using a Scanning Electron Microscope (SEM). Results showed that incorporating WWF enhanced both the hardened and durability properties of concrete, increasing compressive strength by 25% compared to the control case. Additionally, WWF decreased the non-steady-state chloride migration and carbonation coefficients, indicating improved durability. SEM analysis revealed a denser microstructure, and WWF incorporation reduced the permeable porosity and absorption capacity of concrete.