International Concrete Abstracts Portal

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.

The reduction in the shear capacity using recycled coarse aggregate made from crushed concrete is evaluated in terms of tensile cracking and fracture surface characteristics. An experimental investigation is presented into the fracture and flexure-shear behaviors of recycled aggregate concrete (RAC). Replacing natural aggregate in concrete proportioned for 30 MPa compressive strength with recycled coarse aggregate results in lower compressive and tensile strengths. The tensile fracture surface characteristics vary between RAC and natural aggregate concrete (NAC). While the surface area created in the tensile fracture of RAC is larger than that of NAC, the fracture surface profile in RAC has a smaller roughness than that of NAC. In the flexure-shear response of reinforced concrete beams, the dilatancy determined from the slip and crack opening displacements measured across the shear crack is smaller in RAC than NAC. The failure in the reinforced beam is due to the frictional stress transfer loss across the primary shear crack. There is a larger decrease in the shear capacity with the use of RAC than indicated by the reduction in compressive strength. The reduced shear capacity of reinforced RAC is due to the combined influences of reduced tensile strength and crack surface roughness. The design provisions require calibration for crack surface roughness when using RAC in structural applications.

Prompt-gamma activation analysis (PGAA) is an elemental analysis method based on radiative neutron capture that has a high sensitivity to chlorine (Cl). To evaluate the feasibility of replacing the conventional wet chemistry method, ASTM C1152, with PGAA, four mixtures of concrete were prepared with Cl added, ranging from a 0.004 to 0.067% mass fraction of Cl in concrete. The PGAA method detected levels of 100 μg/g Cl in concrete. While both the PGAA and C1152 methods gave results systematically below the nominal values of added Cl, the PGAA data showed excellent correlation (R2 of 0.999) with the C1152 results measured on the same samples. Given that PGAA can measure Cl in concrete as well as C1152 and is faster and less labor-intensive, it can be a candidate for development as a standard method for an alternative to the latter.

Supplementary cementitious materials (SCMs) have been used to replace portland cement and are used in conjunction with advanced mixture design approaches in reinforced concrete for the purpose of creating low-carbon concrete (LCC). In this research, functionally graded concrete (FGC) was used with LCC to provide strength and serviceability for reinforced concrete one-way slab strips by placing a higher-strength/stiffness concrete in the flexural compression region and LCC in all other locations. The behavior of FGC slab strips with varied connection types, placement methods, reinforcement ratios, and ages were compared to uniform specimens with different types of LCC and conventional concrete. Behavior was evaluated through load deflection, cracking, and strains during four-point bending, which were measured using distributed sensing, including distributed fiber-optic sensing and digital image correlation. Limited differences in behavior existed among specimens with the same reinforcement ratios. However, some FGC specimens had higher stiffness and ultimate capacity. Implications of FGC, including cracking behavior at the interface, are also discussed.

This paper investigates the efficacy of ultra-high-performance concrete (UHPC) jacketing as an option for seismic retrofit (repair or strengthening) of structural components that have been damaged by reinforcement corrosion. Previous work has illustrated that UHPC cover fully mitigates corrosion in the absence of service cracks and significantly reduces the corrosion rate in the case of preexisting cracks. In the present experimental study, cover replacement by UHPC is used to repair and strengthen corroded columns. Six lap-spliced columns designed based on pre-1970s design standards were constructed and subjected to artificial corrosion. Parameters of the investigation were: a) the aspect ratio of the specimens; b) the bar size (to account for the effect of bar diameter loss on bond); and c) the condition of the specimen (repair or strengthening after damage due to application of simulated seismic load to assess the effectiveness of retrofitting corroded components, even after having endured earthquake damage). The results show that thin UHPC jackets replacing conventional concrete cover suffice to impart a significant increase in strength and ductility of the columns. The jackets also endow the corroded and unconfined lap splices with significant force and deformation development capacity, thus alleviating a source of excessive column flexibility in existing construction.