International Concrete Abstracts Portal

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.

Numerous shear tests on high-strength high-performance fiber-reinforced cementitious composites (HS-HPFRCCs) and ultra-high-performance concrete (UHPC) over the last three decades have enriched the understanding of their shear strength. This study integrates these experiments, which focused on specific shear strength parameters, into a comprehensive analysis. The Initial Collection Database, containing 247 shear tests, was developed for this purpose. From this, the Evaluation Shear Database was derived using specific filtering criteria, resulting in 118 beams pertinent to HS-HPFRCC and UHPC materials. These databases are accessible to the engineering community to advance the evaluation and development of shear strength formulations in structural design codes. This study concludes with an analysis of a subset of the Evaluation Shear Database, consisting of beams with reported uniaxial tensile strength. This analysis demonstrates the Evaluation Shear Database’s applicability and highlights limitations in existing design equations. Notably, their reliance on a single predictor variable constrained predictive power.

With a well-thought-out packing theory for sand, fine aggregates, cement, a water-cement ratio lower than 0.2, and steel fibers, ultra-high-performance concrete (UHPC) achieves remarkable mechanical properties. Despite UHPC’s superior mechanical properties compared to conventional concrete, its use remains limited, especially in structural applications, due to factors such as high cost, lack of design standards and guidelines, and inadequate correlation between material properties and structural behavior. By compiling and synthesizing the behavior of 70 structural- or full-scale axial UHPC columns, this research provides a new set of generalized design and detailing guidelines for axial UHPC columns. The study first uses the assembled database to assess and revisit the current ACI 318 axial strength design factors for applicability for UHPC. Next, the behavior trends are carefully analyzed to provide detailed recommendations for proper transverse reinforcement (ρt volume), spacing-to-longitudinal reinforcing bar diameter ratio (s/db, where s represents the centerline-to-centerline spacing between transverse reinforcement), and UHPC steel fiber ratio for best use of confinement.