Fly ashes that do not meet the ASTM C618 specifications areconsidered “off-spec” and are not used as supplementary cementitious materials (SCMs). In this research, four off-spec fly ashes (OFAs) were sourced from different parts of the United States and the characteristics of these OFA concretes were measured to compare their performance with that of the mixtures containing 0% OFA. The first objective of this study is to assess the influence of OFA reactivity and replacement levels on concrete characteristics. The second objective is to assess the influence of constituent material characteristics such as shape and size of coarse aggregate, fineness modulus of fine aggregate, and cementitious content of the concrete mixture on the fresh and hardened characteristics of concretes containing OFAs. Results indicate that at sufficient degrees of reactivity and replacement levels, OFAs can provide characteristics comparable to that of conventional ordinary portlandcement (OPC) concrete while improving the consistency of theconcrete. Findings from sensitivity analysis reveal that the degree of reactivity (DoR) of the OFA has a high influence on the hardened characteristics of concrete. Finally, the life cycle assessment of concrete mixtures containing OFAs indicate that greenhouse gas emissions can be reduced up to 45% when compared to conventional mixtures.

Thermal-cured alkali-activated binders (AABs) are a potential replacement for traditional portland cement (PC) in concrete, primarily for precast applications. To avoid this energy-intensive regime and encourage wider application, this study investigates the development of ambient-cured AABs by adding graphene oxide (GO) nanoparticles. The mechanical strength and durability characteristics are determined for alkali-activated slag (AAS) mortar specimens prepared using 4, 6, and 8 molar (4, 6, and 8 M) concentrations of sodium hydroxide in the alkaline activator. The different percentages of GO by weight of slag are 0.0, 0.03, 0.06, and 0.09%. The mechanical parameters considered are compressive, flexural, and splitting tensile strengths. The durability parameters investigated are the rapid chloride permeability test (RCPT), sorptivity, and acid resistance. The performance of ambient-cured AAS mortar specimens containing GO is compared with thermalcured AAS mortar specimens (without any GO inclusions) and the control cement mortar (PC) to evaluate the effect of GO on the mortar characteristics. The strength of AAS mortar is observed to be higher both with and without GO inclusions for the molarity of sodium hydroxide greater than 4 M. The mixture containing 0.06% GO with a 4 M activator is found to exhibit optimal mechanical and durability characteristics. Mineralogical, chemical, and microstructural investigations confirm that the addition of GO to the ambient-cured AAS accelerates the rate of hydration, even at a lower concentration of the activator (4 M) due to its high specific surface area and consequent formation of a greater number of nucleation sites. Hence, ambient-cured AAS mortar prepared using 4 M sodium hydroxide and 0.06% GO is recommended for practical use.

This study investigated utilizing acoustic emission (AE) monitoring to assess the abrasion performance of fiber-reinforced self-consolidating concrete at cold temperatures (-20°C). In addition, the study targeted correlating the abrasion damage to AE data through AE intensity analysis parameters. Seven concrete mixtures were developed with variable water-binder (W/B) ratios (0.4 and 0.55), fiber types (steel and polypropylene synthetic fibers), fiber lengths (19 and 38 mm), and fiber volumes (0.2 and 1%). Tests on 100 mm cubic samples were conducted at -20°C and 25°C, for comparison, according to the rotating cutter technique in conjunction with AE monitoring. Characteristics of the AE signals, such as signal amplitudes, number of hits, and signal strength, were collected and underwent b-value and intensity analyses resulting in three subsidiary parameters: b-value, severity (Sr), and the historic index (H (t)). A clear correlation between abrasion damage progress and AE parameters was noticed. Analyzing AE parameters along with experimental measurements generally revealed a better abrasion resistance for all mixtures when tested at -20℃ compared to that at room temperature. The mixtures with steel fibers, lower W/B values, shorter fibers, and higher fiber volume showed improved abrasion resistance irrespective of temperature. Noticeably, the mixtures containing longer fibers, higher W/B values, or lower fiber dosage experienced a more pronounced enhancement ratio in the abrasion resistance when cooled down to sub-zero temperature. Two damage classification charts were developed to infer the mass loss percentage and wear depth due to abrasion using intensity analysis parameters: Sr and H (t).

An investigation was performed on the drying shrinkage and tensile drying creep characteristics of a nonproprietary ultra-high-performance concrete (UHPC) mixture. The mixture was formulated using metakaolin as the supplementary cementitious material (SCM) and limestone powder as the mineral filler. Cylindrical specimens with dimensions of 52 x 400 mm (2.05 x 16 in.) were fabricated and loaded at 7 and 11 days from casting to various stress levels for 90 days. Additional specimens were fabricated from a proprietary mixture with a silica fume-ground quartz formulation to study the effects of mixture composition. Simultaneous free drying shrinkage measurements were recorded in accompanying specimens placed in the same room environment. Attention was given to the effect of the casting orientation, age at loading, and mixture composition on the drying shrinkage and drying creep behavior of the samples. These tests show that the metakaolin-limestone powder mixture has significantly lower drying shrinkage and specific drying creep than the silica fume-ground quartz mixture. Additionally, the age at loading influences primary creep behavior while not affecting secondary creep at the same stress level. It seems that fiber orientation plays a significant role in the drying creep behavior of UHPC and that cracked UHPC under constant tensile stress undergoes a significant amount of fiber slip.

This paper presents a set of procedures and a recently developed direct tension test for determining the uniaxial tensile strength and full stress-strain behavior of ultra-high-performance concrete (UHPC). The proposed set of procedures aim to establish an upper and lower bound for the tensile strength based on preferential casting orientation. Results from this research show that an upper and lower bound of strength could be established when properly executed casting procedures are in place. On the other hand, the proposed direct tension test can capture the full stressstrain behavior of the material at pre- and post-cracking stages, for both strain-hardening and strain-softening samples. Results from the direct tension tests performed during this research favor the use of contactless extensometers to avoid stress concentrations that induce early localization at the regions close to the attachment points when using traditional measuring methods.