A concrete system is identified as highly printable if it can offer minimal resistance to handling while sustaining high load resistance and structural stability. One of the major complexities of three-dimensional (3D) concrete printing lies in its sensitivity to materials and equipment that varies the time among layers, hydration time, and shear history. While nanoclays are effective additives for enhancing structural buildup, methylcellulose is introduced as a secondary additive to significantly amplify the nanoclays’ effect on the static yield stress while prolonging the open time between layers and increasing filament cohesiveness. The compatibility of these two systems at different contents is studied by characterizing rheological properties such as static yield stress, steady-state viscosity, and storage modulus, as well as the heat of hydration through isothermal calorimetry. The hybrid system is found to increase the static yield stress by up to 900% compared to the reference paste at only 3.0 wt.% total content by mass.

A factorial experimental design plan was used to compare admixtures (high-range water-reducing admixture, viscosity-modifying agent, accelerator, calcium silicate hydrate [CSH] seeds, and nanoclay) in a cement and silica fume blend. Two methods were tested to measure the structuration rate: the constant velocity method and the creep recovery method. The measurements were performed with a rotational rheometer with a double-helical spiral geometry to reduce slippage. The evolution of yield stress and thixotropy of the mixtures at four resting times was evaluated, providing insight into the stress that the recently printed structure can withstand. The creep recovery method generally provides a higher static yield stress than the constant velocity method, except for the stronger mixtures, raising additional questions on the effect of the paste history on microstructural buildup mechanisms. When the extruder begins, shear is applied and the microstructure is broken, causing the dynamic yield stress to be lower than the static yield stress. The effect of the admixtures on thixotropy is discussed.

Deicing chemical solutions can profoundly affect concrete’s physical and chemical properties. It is a known fact that salt solutions are highly conductive in comparison with pure water and are expected to alter concrete’s electrical resistivity as well as other transport properties. In this study, the influence of NaCl, CaCl2, and MgCl2 on transport properties of cementitious materials was investigated. The first part of the project evaluated the continuous exposure for 1 year, while the second part evaluated the wetting-drying cyclic exposure for 6 months (27 cycles). This paper presents the results of the cyclic exposure. Results obtained with standard testing methodologies can be misleading and should be interpreted with caution because transport properties were influenced by different factors, especially the exposure history. In addition, each salt affected each individual transport property differently. Cyclic exposed samples presented similar results as those subjected to 1 year of continuous exposure.

A pilot-scale field investigation was conducted through which: 1) a refined ultra-high-performance concrete (UHPC) mixture was prepared in a ready mixed concrete plant; 2) a large reinforced UHPC block was constructed through placement, consolidation, and finishing of UHPC; and 3) a commonly available concrete curing (insulating) blanket was applied for field thermal curing of the UHPC block using the exothermic heat of hydration of the cementitious binder in UHPC. Monitoring of the reinforced UHPC block temperature over time confirmed the development of a reasonably uniform temperature and a viable temperature time history, which suited thermal curing of UHPC without any heat input. In-place nondestructive inspection of the reinforced UHPC structure pointed at timely setting and strength development, leading to achievement of ultra-high-performance status. Specimens were cored from the large reinforced concrete block and subjected to laboratory testing. The experimental results indicated that the field thermal curing was more effective than the laboratory thermal curing considered in the project, and that the pilot-scale production of the UHPC mixture produced compressive strengths approaching 170 MPa (24.7 ksi).

The stresses, strains, and curvatures due to the shrinkage restraint of new concrete bridge deck overlays by the underlying older substrate are investigated. A time history analysis method is derived that, for each time increment, computes free-shrinkage and creep strains, enforces compatibility and equilibrium using a time-dependent stiffness matrix, and determines incremental mechanical and total strains. A new model for tensile creep strains has test-predicted ratios for experimental results reported by others that average 1.00, with a coefficient of variation of 11%. The resulting stresses and mechanical strains are nonlinear across the depth of the member, with large stress gradients in the top and bottom faces of the overlay at early ages of drying. Simplified analytical methods proposed by others are often not accurate: neglecting swelling of the substrate underestimates the mechanical strains, neglecting tensile creep markedly overestimates the mechanical strains, and assuming uniform free shrinkage through the overlay thickness initially overestimates the mechanical strains but subsequently underestimates them at older ages. The studies also found that application of a waterproofing membrane at the top of the overlay 3 days after the end of curing has very little effect on the maximum tensile stresses in the overlay. The age-adjusted equivalent modulus method accurately estimates the overlay tensile stress at early ages, but fails to predict the time of cracking.