International Concrete Abstracts Portal

The movement of ultra-high-performance concrete (UHPC) toward wide scale acceptance within the concrete industry has generated interest in developing improved test methods to provide quality assurance for this material. Most test methods currently used to measure the tensile behavior of ultra-high-performance concrete require specialized testing equipment that is not typically owned by precast or ready-mix production facilities. These test methods provide reliable data for quality assurance of newly developed concrete mixes, but they are impractical as quality-control tests, which would need to be performed for every UHPC placement. This paper presents the development of a simple and inexpensive test to measure tensile strength and ductility for UHPC and serve as a quality-control test. This method was developed from the double-punch test, commonly referred to as the “Barcelona test,” but has been revised to incorporate substantial changes to the loading and data collection requirements to eliminate the need for expensive, specialized equipment. It was determined that the modified test method could produce reliable results using a load-controlled testing procedure with manually recorded data points taken every 0.635 mm (0.025 inches) of vertical displacement for ductile concrete specimens. It was also determined that specimen surface grinding, loading rate, and punch alignment did not significantly influence the test results. However, the fabrication of the specimens, specifically the rate and method at which the molds were filled, had a significant effect on the results. Accordingly, any recommended standardized test method based off of this procedure should have requirements on specimen fabrication.

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is a high-strength cementitious material whose composition can be designed to achieve given properties in the fresh and hardened state. Thanks to the low water/binder ratio (around 0.2) and to the presence of mineral additions, it is characterized by a low porosity which is often taken as a guarantee of intrinsic – almost unlimited – durability. However, the number of researches on the long-term behavior of this type of material is relatively limited, in particular with respect to the corrosion behavior of steel and to the role of steel fibers on concrete durability. This note presents the results of research aimed at studying the influence of fibers on durability properties. Besides tests for the characterization at fresh and hardened states, durability-related parameters such as electrical resistivity, resistance to chloride penetration, absorption rate, and water absorption are investigated. The results show that a higher amount of fibers increased the porosity of fiber/concrete interfacial zone and therefore large fibers content negatively influences the parameters related to durability. However, the investigated parameters indicate higher performances with respect to ordinary concretes even in UHPFRC with large fiber content, except for electrical resistivity.

Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) is a type of concrete with superior mechanical and durability properties, which might be improved even further with the addition of nano-materials. This work studies the influence of adding nano-additions to two UHPFRCs with compressive strength around 150MPa (21755 psi), with and without crystalline admixtures. Two nano-materials were considered: cellulose nano-crystals (4-5 nm diameter, 50–500 nm length, 0.157-0.197 μin diameter, 1.97-19.7 μin length); in a dosage up to 0.15% by the cement weight; and aluminum oxide nanofibers (diameter 4-11nm, length 100-900nm, 0.157-0.433 μin diameter, 3.94-35.4 μin length) in a dosage of 0.25% by the cement weight. Water content of the mixes with nanomaterials was modified to maintain workability in a similar range aiming to maintain the self-compacting behavior. The following properties were analyzed: workability, compressive strength, modulus of elasticity and tensile properties calculated through a simplified inverse analysis after performing four-point bending tests. The study considered the effect of using three levels of mixing energy to ensure a proper dispersion of all the components, and its effect in the aforementioned properties. The results show a potential effect of these nanomaterials as nanoreinforcement, with slightly better ultimate strength and strain values for the higher energy level.

In this contribution, the experience of the application of the SIA 2052 guidelines for testing of Ultra-high-performance fiber-reinforced concrete (UHPFRC) at Empa is summarized. From the point of view of testing, UHPFRC is a challenging material that sets high requirements with regard to laboratory equipment, specimens’ preparation and evaluation of results. The most important tests for UHPFRC classification, namely the evaluation of the uniaxial tensile behavior and of the flexural behavior, are discussed. In particular, a practical example illustrates to which extent probe preparation can influence the measurements of the material properties in tension.

Several studies have indicated that the shear capacity of fiber-reinforced ultra-high-performance concrete (UHPC) girders outperforms that of conventionally reinforced high-strength concrete girders. However, the extremely high material and production cost of fiber-reinforced UHPC girders limits its applications. This paper presents the experimental and analytical investigations performed to evaluate the shear capacity and economics of using welded wire reinforcement (WWR) in place of random steel fibers in UHPC precast/prestressed I-girders. Two economical, practical, and nonproprietary UHPC mixtures that eliminate the use of steel fibers were developed and tested for their mechanical properties. Two full-scale precast/prestressed concrete girders were designed and fabricated using the developed mixtures and reinforced using orthogonal WWR. The shear testing of the two girders indicated that their average shear capacity exceeds that of comparable fiber-reinforced UHP girders while being 62% less in total material cost. In addition, the production of welded wire-reinforced UHPC girders complies with current industry practices, and eliminates handling, mixing, and consolidation challenges associated with the production of fiber-reinforced UHPC girders.