Sand + crushed coarse aggregate ( b ) Sand + gravel coarse aggregate
Self-consolidating concrete (SCC) mixtures designated for precast, prestressed applications should be highly workable to flow easily through restricted spacing and completely encapsulate reinforcements without any mechanical vibration. Key workability characteristics of SCC can be described in terms of filling ability, passing ability, and resistance to segregation. These properties are typically characterized by data that relate to specific testing methods. In general, these methods include the components required for evaluating simultaneously filling ability, passing ability, and resistance to segregation, since these properties are rather interrelated. In this investigation, 33 SCC mixtures made with various mixture proportioning parameters, including maximum size and type of aggregate, type and content of binder, and w/cm were evaluated. The mixtures were prepared using crushed aggregate and gravel of three different nominal sizes, w/cm of 0.33 and 0.38, and three binder compositions: Type MS cement, Type HE cement with 30% of slag replacement, and Type HE cement with 20% of Class F fly ash. Comparisons and correlations among various test results used in evaluating the workability responses obtained for these mixtures are established. This is done to highlight advantages and limitations of the various test methods that can be used to assess workability of SCC designated for prestressed applications. Appropriate combinations of test methods that can be used to assess workability of SCC at the precast plant are recommended, and ranges of acceptance of the various test methods are established.
WORKABILITY, TESTING, AND PERFORMANCE OF SELF-CONSOLIDATING CONCRETE
Self-consolidating concrete (SCC) is a new category of high-performance concrete that exhibits a low resistance to flow to ensure high flowability and a moderate viscosity to maintain a homogeneous deformation through restricted sections, such as closely spaced reinforcement. This paper reviews the benefits of using SCC to facilitate the casting of densely reinforced sections and improve productivity and onsite working conditions. Workability requirements necessary to secure self-consolidation and the principles involved in proportioning such highly flowable concrete are discussed. Field-oriented tests useful in evaluating the deformability, filling capacity, and stability of SCC are presented. The performance of concrete mixes proportioned according to two main approaches needed to ensure high deformability, low risk of blockage during flow, and proper stability are compared. Such approaches involved the proportioning of concrete with a moderate water-to-cementitious material ratio (w/cm) of 0.41 and using a viscosity-enhancing admixture to increase stability, as well as mixes without any viscosity-enhancing admixture, but with lower w/cm of 0.35-0.38 to reduce free water content and provide stability. Mixes with both moderate and high contents of ternary cementitious materials were evaluated. The performance of each concrete was compared to that of a flowable concrete with 250-mm slump.
Growth and Characterization of β-RDX Single-Crystal Particles
1,3,5-Trinitrohexahydro-s-triazine (RDX) single-crystal particles crystallized in the β phase have been successfully grown by sublimation–recrystallization. Synchrotron X-ray diffraction results confirm that the recrystallized products are β-RDX single crystals. Compared to the traditional solution deposition methods, the sublimation method used in this work breaks through the mass limitations during the growth of β-RDX. The distribution of α- and β-RDX on different substrates reveals that hydrophilic substrates are prone to grow β-RDX single crystals and that hydrophobic substrates prefer α-RDX single crystals. The sublimation rate dominates the growth of pure β-RDX. The mechanisms of the phase transition from β-RDX to α-RDX during the process of sublimation–recrystallization have been studied. The current results can promote the potential application of β-RDX in the propellant, explosives, and military fields.