International Concrete Abstracts Portal

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.

Conventional approaches to concrete three-dimensional (3-D) printing relies on printing concrete in a straight (linear) print path, with layers overlaid on top of each other. This results in interlayer and interfilament joints being potential weak spots that compromise the mechanical performance. This paper evaluates simple alterations to the print geometry to mitigate some of these effects. A printable mixture with 30% of limestone powder replacing cement (by mass), with a 28-day compressive strength of approximately 70 MPa in the strongest direction is used. S- and 3-shaped print paths are evaluated as alternatives to the linear print path. Staggering of the layers ensures that the interfilament joints do not lie on the same plane along the depth. Flexural strength enhancement is observed when print geometries are changed and/or layers are staggered. The study shows that print geometry modifications mitigate mechanical property reductions attributed to interfilament defects in 3-D concrete printing.

The rapid growth of the construction industry in Asia and the consequent updating of design specifications put forward higher performance requirements for structural components, which results in a large number of existing shear walls that are not compliant with the current seismic standards. A prospective retrofitting method, which is based on replacing the existing boundary concrete or attaching external boundary columns to nonconforming shear walls, is experimentally studied. Four shear-wall specimens were designed according to the current Chinese design code: one using plain concrete boundary columns and three using ultra-high-toughness boundary columns (UHTBCs), adopting three different strengthening strategies relevant to the boundary size and the connection form. Cyclic performance, damage patterns due to UHTBCs, and connection form are discussed based on the experimental results, from which it was ascertained that shear walls with UHTBCs show improved seismic performance, compatible with the requirements of the current seismic design code, even for the reduced-boundary UHTBCs and non-connection specimens. The predictive equation for the sectional moment capacity of shear walls with UHTBCs was discussed as a practical tool for retrofitting applications. This study highlights the most important features of a rapid retrofitting measure to improve the resilience of existing nonconforming shearwall structures, while also proving to be an effective measure for newly constructed structures.