Self-leveling concrete is developed with low-calcium alkali-activated fly ash (AAF) binder paste. The rheological behavior of AAF pastes with different compositions is evaluated. AAF pastes are proportioned with alkali-silicate activating solutions to ensure specific reactive oxide ratios for comparable geopolymer strength. The yield stress and the viscosity of the AAF binder paste vary with the silica content and the silica modulus (SiO2/Na2O mass ratio) in the alkali-silicate activating solution. The slump and flow behaviors of concrete mixtures made with AAF paste are evaluated. The requirements of the AAF binder characteristics, paste content, and aggregate packing for achieving self-leveling flow characteristics under gravity-induced flow are assessed. The transition from a frictional to a flow-type behavior in concrete mixtures depends on the AAF binder paste content. Self-leveling is achieved without the use of admixtures with an AAF binder paste of low yield stress and at a paste content of 45%. Improving the aggregate packing using the Fuller-Thompson curve and reducing the yield stress of the AAF binder paste increase the flow achieved in concrete mixtures. The specifications for cement-based self-consolidating concrete (SCC) are closely applicable for self-leveling AAF-based concrete.

With aging, concrete structures exhibit deterioration due to multiple reasons. Consequently, repair processes become overwhelmingly essential to extend the service life of structures. This experimental study investigated nano-modified concrete cast and cured under cyclic freezing/low temperatures, including its applicability to partial-depth repair. Seven mixtures, incorporating general-use cement, fly ash (0 to 25%), and nanosilica (0 to 4%) with a cold weather admixture system (antifreeze/accelerator) were tested. The mixtures were evaluated based on fresh, hardened, and durability properties as well as their compatibility with parent/substrate concrete. In addition, mercury intrusion porosimetry and thermogravimetric analysis were conducted to assess the evolution of microstructure under cold temperatures. The incorporation of 4% nanosilica in the cementitious binder, even with the presence of 15% fly ash, markedly enhanced the performance of concrete cast and cured under low temperatures without protection; thus, it may present a viable option for cold weather applications including repair.

It is well known that the workability of concrete will decrease when doped with secondary fly ash (FA). The authors reported a new FA composite with surface modification which can improve the fluidity of cement and the workability of concrete. A polycarboxylate (PC) high-range water-reducing admixture (HRWRA), which contained poly ethylene glycol (PEG) side chain, carboxylic groups, and hydroxysilane groups, was synthesized by free radical copolymerization. It was subsequently grafted onto fly ash (FA) beads. The Si-OH groups on the surface of alkali-activated FA beads interacted with the PC molecules through covalent hydroxysilane linkage. In the PC-modified FA beads, new infrared (IR) peaks appeared at 2900 and 1100 cm−1 that were assigned to the vibration of C-H and C-O-C groups, respectively. A peak shift in 29Si NMR from −80 to −86 ppm also confirmed the successful grafting of the PC molecules onto the FA beads. Thermal analyses indicated that each of the PC moieties accounted for 2.1 wt. % of the modified FA beads. Compared with the crude FA and the alkali-activated one, the PC-modified FA significantly improved the workability of the cement paste and enhanced the mechanical properties of the cement after hydration for 7 days. Thus, the PC-modified FA composite could serve as a promising additive for cementitious materials.

This research studies the effect of retarder, accelerator, stabilizer, air-entraining agent, and shrinkage-reducing admixture (SRA) on plastic shrinkage cracking in self-consolidating concrete (SCC). The main objective is to identify the dominant cracking cause—that is, plastic settlement or plastic shrinkage—in an SCC containing a particular admixture. During experimentation, crack-free concretes were achieved by adding air-entraining agent and SRA, while accelerator and retarder increased the crack area. The impact of admixtures on the cracking mechanism was identified by comparing the respective vertical and horizontal deformations. It was observed that the crack-free concretes had moderate settlement and horizontal shrinkage, while the cracked specimens exhibited significant deformation either vertically or horizontally.

Freezing-and-thawing (F-T) resistance is a key parameter in evaluating the durability of concrete. The response of concrete under F-T environment varies depending on the mixture proportion and materials used. This paper focuses on the F-T behavior and damage resistance of normal-strength (NC), high-strength (HSC), high-performance (HPC), and ultra-high-performance (UHPC) concrete. The mechanisms causing F-T damage are discussed, specifically based on expansion of freezable water under negative temperature and thermal stress arising from differences in the coefficient of thermal expansion of cement and aggregates. To quantify damage, two parameters—namely, mass loss ratio (MLR) and relative dynamic elastic modulus (RDEM)—are compiled for different classes of concrete. Results show that UHPC exhibited much lower increase in MLR and reduction in RDEM than NC and HPC, respectively. The effects of F-T loading on other mechanical properties of concrete such as compressive strength, flexural strength, tensile strength and stress-strain relationship are also investigated in this paper as possible parameters to help characterize F-T resistance. It is found that F-T will decrease the peak stress but increase the peak strain, and the flexural strength has the fastest loss rate for NC, HPC, HSC and UHPC, respectively. As concrete under F-T environment is often exposed to chloride, the significance of sodium chloride (NaCl) concentration and chloride diffusion coefficient (CDC) on HSC and UHPC under NaCl solution are studied. UHPC exhibits better resistance on chloride diffusion after F-T action due to denser internal pore structure. To improve the F-T resistance of concrete, the performance of two supplementary cementitious admixtures, fly ash and silica fume, to partially replace cement are studied. Results show that the appropriate fly ash replacement of 10 to 30% or silica fume replacement of 5 to 10% is found to enhance the F-T resistance. In addition, introducing fibers such as PVA or PP can improve the F-T resistance significantly, although using the wrong proportion may have a negative effect. Using combined admixture of polyvinyl alcohol and polyethylene fiber with 1.5% volume in cement-based composites reduces strength degradation caused by F-T loadings.