Research in Progress (ACI Fall 2022, Dallas, TX) High early strength concrete (HESC) has numerous applications, where several methods are available to achieve its high early strength. Generally concrete mixtures are designed with high content of ASTM C1600 cement, low water to cement ratio, and a high dosage of superplasticizers. The strength properties of HESC have been widely studied, but limited documentation is made of its durability characteristics. With that objective, this study focuses on understanding its freeze-thaw resistance, surface scaling, autogenous & drying shrinkage; by examining the effect of using: (i) Low (LCCM), standard (SCCM), and high (HCCM) cement content of ASTM Type III and CSA/CSA blend cement; (ii) Different types and combinations of admixtures; (iii) Internal curing (IC) using light-weight aggregates (LWA). It has been observed that irrespective of cement content, both type III and CSA/CSA blend cement, achieved a minimum compressive strength of 1800 psi or flexural strength of 380 psi within 6 hours of mixing. Concrete mixtures designed with type III cement required using an accelerating admixture to achieve strength thresholds. HCCM exhibited concerns such as excessive shrinkage, higher heat of hydration, poor aggregate-past bond, and reduced permeability, while LCCM performed most satisfactorily. Comparing mixtures prepared with and without IC, LWA was very effective in mitigating excessive shrinkage. A well-entrained air-void distribution is known to be most effective in ensuring frost durability, but the complex chemistry between superplasticizers and higher heat of hydration associated with HESC makes it challenging to deliver a well-distributed air-void system.

Synergy of Nanoparticles with Supplementary Cementitious Materials in Concrete (ACI Fall 2022, Dallas, TX) This talk will cover a performance-based study on the effects of attapulgite nanoclay and carbon nanotubes (CNTs) on the fresh and hardened properties of mortars with partial cement replacement with fly ash and blast furnace slag. Shear rheology was applied to measure yield stress and viscosity, and the tack test was applied to measure static cohesion. Results show that the additives had differing effects on 100% cement mortars versus blended (50% cement, 25% fly ash, 25% slag). The nanoclays increased yield stress and static cohesion and decreased plastic viscosity in both systems, but these effects were more marked in the cement mortar than in the blended. On the other hand, CNTs increased all measured rheological parameters in the cement mortar and decreased them in the blended. The rheological results highlight the importance of considering the binder system when utilizing additives with exceptional surface. In the hardened state, electrical resistivity, compressive strength, and tensile strength were evaluated. Results indicate that although nanoclays are utilized primarily as a rheological modifier, they have the potential to enhance mechanical properties.