Ultra-high-performance concrete (UHPC) is a new class of cementitious material with unique characteristics, including self-consolidation, and excellent mechanical and durability properties. To achieve the desired properties, a very dense internal structure and a very low water-binder ratio (w/b) are necessary. Due to the very different mixture design compared to conventional concrete, it is critical to incorporate different types of chemical admixtures to achieve appropriate fresh concrete behavior of UHPC. To ensure the successful placement of UHPC, it is important to have a good understanding of the workability and rheological characteristics of UHPC with different types and dosages of chemical admixtures. This paper presents a detailed study of the impact of high-range water reducer, workability-retaining admixture, and anti-foaming admixture on the workability and rheological characteristics over different mixture elapsed times. Besides the flowability, both Bingham and modified Bingham models were used to obtain key rheological parameters, including yield stress, viscosity, and thixotropy. Furthermore, the authors developed stability indexes to assess the fiber stability of UHPC in both fresh and hardened states. Based on the experimental results, the paper presented suggested criteria to ensure appropriate flowability and fiber stability for UHPC placement.

To ensure a successful outcome when using cementitious materials during three-dimensional (3D) printing operations, the effects of chemical admixtures on rheological and setting behavior must be carefully adjusted to accommodate the needs for pumping, extrusion, deposition, and self-sustainability without the support of formwork. This paper highlights potential solutions offered by chemical admixtures, while discussing various testing methods and important influencing parameters. The impact of commercial polymers on viscosity, initial yield stress, thixotropy, and their variations over time are reported. Influencing factors, such as mixing energy and material interactions, are discussed. Accelerating and strength-enhancing admixtures are used to illustrate the adjustment of setting and early strength development of concrete. Understanding the possibilities of modifying fresh concrete properties will help to improve the robotic construction process as well as the design or adaptation of the printing equipment.

Durability and long-term performance of concrete exposed to deleterious ions and environmental conditions are major concerns. The rapid chloride permeability (RCP) test is commonly used in specifications in the United States to evaluate the permeability of concrete. To evaluate the critical factors that control the service life of structures, the investigation of various concrete mixtures is required. In this paper, the performance of 54 concrete mixtures containing three types of water-reducing admixtures, two types of aggregates, and two levels of cement contents are evaluated in the RCP and freezing-and-thawing tests and the air void structure of selected mixtures are analyzed. It was found that the use of supplementary cementitious materials (SCMs) significantly enhances the performance of concrete mixtures in the RCP test. In addition, mixtures containing up to 30% of Class C fly ash and 50% slag content achieved exceptional durability performance in both RCP and freezing-and-thawing (F-T) tests. The “very-low” RCP values were found for mixtures containing Class F fly ash and polycarboxylate ether (PCE) admixture.

Concerns over the availability and quality of conventional fly ash for use in concrete has become widespread in recent years. Fluidized bed combustion (FBC) ash, with its annual U.S. production exceeding 14 million tons, could serve as a reasonable alternative. In this study, two compositionally different fly ashes from circulating FBC (CFBC) power plants were evaluated for their compliance with the ASTM C618 standard and their impact on the fresh and hardened properties of concrete. The pozzolanic reactivity of the fly ashes was also quantified based on the emerging RILEM test method. These fly ashes met the chemical and physical requirements of ASTM, except for elevated LOI (in both fly ashes) and elevated SO3 (in one fly ash). Despite this, concrete with proper slump, air content, and strength development could be produced by proper dosing of chemical admixtures. The elevated SO3 was found not to produce deleterious expansions.

With the potential decline in supplies of today’s most widely used supplementary cementitious materials (SCMs) such as fly ash and slag, there is growing interest in the use of natural pozzolans (for example, pumice, volcanic ash), processed pozzolans (for example, calcined clays and shales), and manufactured pozzolans (for example, ground glass). Establishing the pozzolanic reactivity of these materials is an essential part of the evaluation process. Currently, pozzolans are assessed using ASTM C618 and C311, with the strength activity test being the only real performance indicator. Unfortunately, it is not possible to accurately determine the contribution of the pozzolanic reactivity to the strength in this test. This paper presents the development of a new pozzolanic reactivity test method based on the previous strength-activity-with-lime test but modified to increase the rate of the pozzolanic reaction. In this test, the solution-to-binder ratio is kept constant with workability adjusted using chemical admixtures. A range of mixing solutions containing combinations of KOH, NaOH, and K2SO4 and various curing regimes were investigated. The outcome of the test is compared with results from the current ASTM C311 and CSA A3004-E1 test methods for a wide range of pozzolanic (and inert) materials.