Ultra-high performance concrete (UHPC) is a state-of-the-art cementitious composite. Since the concept of this novel concrete mixture emerged in the 1990s, significant advancements have been made with numerous benefits such as high strength, flowability, high post-cracking tensile resistance, improved durability, reduced maintenance, and extended longevity. Currently, UHPC is employed around the globe alongside recently published practice guidelines. Although numerous research projects were undertaken to examine the behavior of UHPC-incorporated structures, there still are many gaps to be explored. Of interest are the development of robust and reliable mixtures and their application to primary load-bearing members for bridges and buildings, including various site demonstration projects that would promote the use of this leading-edge construction material. This Special Publication (SP) contains nine papers selected from three technical sessions held in the ACI Spring Convention in March 2022. All manuscripts were reviewed by at least two experts in accordance with the ACI publication policy. The Editors wish to thank all contributing authors and anonymous reviewers for their rigorous efforts. The Editors also gratefully acknowledge Ms. Barbara Coleman at ACI for her knowledgeable guidance. Yail J. Kim, Steven Nolan, and Antonio Nanni Editors University of Colorado Denver Florida Department of Transportation University of Miami

Embedding magnetic particles into cement paste produces a smart material in which the rheological properties of the resultant paste can be actively controlled through the use of magnetorheological principles. This research investigates the rheological behavior of cement-based MR pastes with and without air entrainment to gain a better understanding of the effects of air-entrained bubbles on MR cement pastes. Such information would be critical for the use of such MR Pastes in 3D concrete printing applications. It is revealed that the incorporation of entrained air results in increasing the MR response and this effect is related to the bubble bridge effect.

This study examines the impact of deicers on the compressive strength and microstructure of concrete at ambient temperature in sub-zero areas. In this study, after seven days of curing in plain water, concrete specimens were exposed to four deicer chemical solutions: sodium chloride, sodium acetate, calcium nitrate, and urea at 3%, 6%, and 9% concentrations, respectively. The specimens were tested for compressive strength after 14 days, 28 days, and 90 days of exposure. All tested deicers, except calcium nitrate, have a propensity to decrease the compressive strength of concrete. Exposure to sodium acetate, which appears to have the most detrimental effect, decreased the compressive strength of concrete by a maximum of 30.79% at a concentration of 9%, whereas exposure to calcium nitrate increased the compressive strength of concrete by 17% at a concentration of 3%. Deicers changed the microstructure of concrete, which was investigated using Field Emission Scanning Electron Microscopy (FESEM). This was followed by X-ray diffraction (XRD) for qualitative analysis of phases present in deicer-treated concrete specimens. The desirability function was used to determine the optimal exposure period and calcium nitrate concentration for concrete in subzero environments, which were respectively 10 to 11 days and 8.8 to 9%.

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 (acid-soluble chloride in mortar and concrete), with PGAA, 4 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 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 and the 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.

The applications of alkali-activated slag (AAS) face challenges such as poor workability, rapid setting, and high autogenous shrinkage, which require chemical admixtures (CAs) to adjust the performance of AAS. Unfortunately, there are limited specific CAs available to tune AAS properties. To address this gap, this study proposes using a ubiquitous, naturally occurring compound, L-ascorbic acid (LAA), as a multi-functional performance-enhancing additive for AAS to overcome the major challenges of AAS. The findings showed that LAA can function as a retarder, plasticizer, strength enhancer, and autogenous shrinkage reducer for AAS. When 0.5% LAA was added, the compressive strengths of AAS mortars at 3d and 28ds increased by 28.9% and 19.6%, respectively, and the 28d autogenous shrinkage decreased by 43.1%. Both surface adsorption and ion complexation have been confirmed as the working mechanisms of LAA in hydrated AAS.