International Concrete Abstracts Portal

Ultra-high-performance geopolymer concrete (UHP-GPC) can exhibit high to exceptional strength. Given the importance of UHP-GPC’s mechanical properties, the prediction of its 28d compressive strength (f’_c) remains insufficiently explored. This study predicts UHP-GPC’s f’_c based on alkali-activated materials, sand, fiber volume, water-to-geopolymer binder, and alkali activator ratios. Advanced statistical modeling and a spectrum of ensemble machine learning (ML) algorithms, including random forest (RF), gradient boosting (GB), extreme gradient boosting (XGB), and stacking, are utilized to predict UHP-GPC’s strength. The derived models reveal the significance of fiber, slag, and sand as the most significant factors influencing the 28d f’_c of UHP-GPC. All the ML models demonstrate higher precision in forecasting f’_c of UHP-GPC compared to statistical modeling, with R²s peaking at 0.85. Equations are derived to predict the strength of UHP-GPC. This article reveals that UHP-GPC with superior mechanical properties can be designed for further sustainability.

This study introduces a new anchorage strategy using ultra-high-performance concrete (UHPC) to attach unbonded post-tensioning (PT) strands to existing foundations. This solution complements a seismic retrofit scheme investigated by the authors, which transforms non-ductile cast-in-place reinforced concrete (RC) shear walls into unbonded post-tensioned rocking shear walls, following concepts of selective weakening and self-centering. In the proposed PT anchorage scheme, mild steel reinforcements are inserted through the shear wall thickness and into the foundation. Subsequently, UHPC is cast around the wall base, forming a vertical extension connected to the foundation, which is used to anchor the unbonded PT strands. The feasibility and performance of the anchorage scheme were investigated through a combination of laboratory testing and numerical simulations. Pull-out testing on four scaled-down anchorage specimens was conducted in the laboratory. Hairline cracks were observed in the UHPC during testing. Additionally, 3D finite element (FE) models were created, validated, and used to study the performance of the proposed anchorage scheme under lateral loading. The simulation results support the effectiveness of the proposed anchorage strategy.

Ultra-high performance concrete (UHPC) is considered a material with high strength and good ductility. However, it was found in the experiments that the ductility of slender UHPC walls at high axial- load ratios (ALRs) was not as good as expected. The improvement on the ALR limit of the walls by using UHPC is limited. Thus, this study theoretically investigated the ALR limit of slender UHPC wall-type piers. Equivalent UHPC stress block and equivalent steel strip methods were used to calculate the bearing capacity of UHPC wall-type piers. The calculation results were in good agreement with the summarized experimental and numerical results. Based on the experimental observations and the proposed calculation method, the failure mechanism of the UHPC wall-type piers was theoretically analyzed. Equations for determining the ALR limit of UHPC wall-type piers and suggestions for designing UHPC wall-type piers were proposed. It was suggested that high-strength steel bars should be used with caution in T-section UHPC wall-type piers, especially when the reinforcement ratio is higher than 3%. This study provided references for the compilation of the Chinese Code, “Technical Specification for Ultra-High Performance Concrete Structures.”

Ultra-high-performance concrete (UHPC) enables thinner, longer-span elements with fewer or no reinforcing bars. This study investigates the shear behavior of reinforcing bar-free UHPC panels with a thickness of 4 in. (101.6 mm) and 2.0% volumetric content of straight steel fibers. The panels were tested under combined shear and axial loads using the universal panel tester (UPT) facility. The UPT experiments were complemented with small-scale direct tension tests (DTTs) and large-scale tension strip tests (TSTs) to investigate the effect of UHPC tensile characteristics on shear. The panels exhibited ductile responses with post-peak residual shear capacities higher than 20% of the maximum shear stress, with the TSTs providing an improved correlation to UHPC shear than the DTTs. Test results showed that the relative effect of axial loads on UHPC shear can be greater than the relative effect on conventional concrete per ACI 318. It was also found that a correlation exists between fiber alignment and UHPC’s tensile behavior, which can alter the localization stress by as much as 39%.

This paper presents the behavior of unreinforced cylindrical concrete elements confined with a hybrid system, consisting of an ultra-high-performance concrete (UHPC) jacket and basalt fiber-reinforced polymer (BFRP) grids. For exploring the feasibility of the proposed strengthening scheme, a series of tests are conducted to evaluate material properties and to obtain results related to interfacial bond, load-bearing capacity, axial responses, and failure modes. To understand the function of the individual components, a total of 57 cylinders are loaded under augmented confining conditions, including plain cores with ordinary concrete (CONT), plain cores with UHPC jackets (Type A), and plain cores with UHPC jackets plus BFRP grids (Type B). By preloading the cores at up to 60% of the control capacity (60%fc′) before applying the confinement system, the repercussions of inherent damage that can take place in vertical members on site are simulated. The compressive strength of UHPC rapidly develops within 7 days, whereas its splitting strength noticeably ascends after 14 days. The adhesion between the ordinary concrete and UHPC increases over time. While the Type B specimens outperform their Type A counterparts in terms of axial capacity by more than 18%, reliance on the BFRP grids is reduced with the growth of UHPC’s strength and adhesion because of the interaction between the hardened UHPC and the core concrete. The adverse effects of the preloading are noteworthy for both types, especially when exceeding a level of 30%fc′. The BFRP grid-wrapping alleviates the occurrence of a catastrophic collapse in the jacketed cylinders, resulting from a combination of the axial distress and lateral expansion of the core. Analytical models explain the load-carrying mechanism of the strengthened concrete, including confinement pressure and BFRP stress. Through parametric investigations, the significance of the constituents is clarified, and design recommendations are suggested.