International Concrete Abstracts Portal

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.

The concrete industry is increasingly adopting the production of environmentally sustainable green concrete. Using recycled coarse aggregate (RCA) produced from demolished concrete infrastructures and waste tire-derived crumb rubber (CR) in the concrete mixture provides a sustainable construction practice and can enhance structural performance. This study investigates the uniaxial compressive behavior of concrete columns composed of RCA, CR, and polypropylene (PP) fiber. A total of 26 columns 150 x 150 x 950 mm in size were tested under uniaxial compression loading. Test parameters included longitudinal reinforcement ratio (0, 1.4, and 2.0%), tie spacing (75 and 150 mm), CR content (0, 5, 10, and 15% of the volume of natural fine aggregate), and percentage of PP fiber (0 and 0.5% of the volume of the total mixture) with 30% RCA replacement (by weight of natural coarse aggregate). The compressive behavior, failure mechanism, influence of longitudinal and transverse reinforcement, dilation, ductility, and toughness were examined. This study demonstrated that incorporating fiber into the concrete made with RCA and CR waste materials improved the axial capacity, resulting in fiber-reinforced rubberized recycled concrete (FRRC) columns with enhanced ductility and toughness. These findings support the development of sustainable concrete for structural columns, justifying their applicability to existing design codes.

A high cement content is often found in concrete mixture designs to achieve the unique fresh-state behavior requirements of three dimensional (3-D) printable concrete (3DPC) to ensure rapid stiffening of an extruded layer without collapsing under the stress applied by the following layers. Some materials with high water absorption, such as recycled concrete aggregates, have been incorporated in concrete mixture designs to minimize environmental impact; nevertheless, the fine powder fraction that remains from the recycled aggregate processing still poses a challenge. In the case of 3DCP, few studies are available regarding mixture designs using recycled concrete powder (RCP) for 3-D printing. In this context, this study presents the use of RCP as a filler to produce a printable mixture with low cement content. An RCP with 50 μm average particle size was obtained as a by-product from recycled concrete aggregate production. Portland cement pastes were produced with 0, 10, 20, 30, 40, and 50% of cement mass replacement by RCP to evaluate its effects on the hydration reaction, rheology, and compressive strength. It was found that the studied RCP replacement was not detrimental for the hydration reaction of portland cement during the initial hours, and at the same time, it was capable of modifying the rheological parameters of the paste proportionally to the packing density of its solid fraction. The obtained results indicated the viability of 3DCP with up to 50% cement replacement by RCP. It was concluded that RCP presents good potential for decreasing the cement consumption of 3DPC, which in turn could decrease its associated environmental impact while providing a destination for a by-product from recycled concrete aggregate production.

This paper investigates the structural performance of lightweight self-consolidating concrete (LWSCC) and lightweight vibrated concrete (LWVC) beam-column joints (BCJs) reinforced with monofilament polyvinyl alcohol (PVA) fibers under quasistatic reversed cyclic loading. A total of eight exterior BCJs with different lightweight aggregate types (coarse and fine expanded slate aggregates), different PVA fiber lengths (8 and 12 mm [0.315 and 0.472 in.]), and different percentages of fiber (0.3 and 1%) were cast and tested. The structural performance of the tested joints was assessed in terms of failure mode, hysteretic response, stiffness degradation, ductility, brittleness index, and energy dissipation capacity. The results revealed that LWSCC specimens made with expanded slate lightweight fine aggregates (LF) appeared to have better structural performance under reversed cyclic loading than specimens containing expanded slate lightweight coarse aggregates (LC). Shortening the length of PVA fibers enhanced the structural performance of LWSCC BCJs in terms of initial stiffness, load-carrying capacity, ductility, cracking activity, and energy dissipation capacity compared to longer fibers. The results also indicated that using an optimized LWVC mixture with 1% PVA8 fibers and a high LC/LF aggregate ratio helped to develop joints with significantly enhanced load-carrying capacity, ductility, and energy dissipation while maintaining reduced self-weight of 28% lower than normalweight concrete (NWC).

The early-age material parameters of three-dimensional (3-D)-printable concrete defined under the umbrella of printability, namely, pumpability, extrudability, buildability, and the “printability window/open time,” are subjective measures. The need to correlate and successively substitute these subjective measures with objective and accepted material properties, such as tensile strength, shear strength, and compressive strength, is paramount. This study validates new testing methodologies to quantify the tensile and shear strengths of printable fiber-reinforced concretes still in their fresh state. A tailored mixture with high sulfoaluminate cement and nonstructural basalt fibers has been assumed as a reference. The relation between the previously mentioned parameters and rheological parameters, such as yield strength obtained through International Center for Aggregates Research (ICAR) rheometer tests, is also explored. Furthermore, in an attempt to pave the way and contribute toward a better understanding of the mechanical properties of 3-D-printed concrete, to be further transferred into design procedures, a comparative study analyzing the work of fracture per unit crack width in three-point bending has been performed on printed and companion nominally identical monolithically cast specimens, investigating the effects of printing directions, position in the printed circuit, and specimen slenderness (length to depth) ratio.