Prompt gamma activation analysis (PGAA) is an elemental analysis method based on radiative neutron capture that has a high sensitivity to chlorine (Cl). To evaluate the feasibility of replacing the conventional wet chemistry method, ASTM C1152 (acid-soluble chloride in mortar and concrete), with PGAA, 4 mixtures of concrete were prepared with Cl added ranging from a 0.004 to 0.067% mass fraction of Cl in concrete. The PGAA method detected levels of 100 µg/g Cl in concrete. While both PGAA and C1152 methods gave results systematically below the nominal values of added Cl, the PGAA data showed excellent correlation (R2 of 0.999) with the C1152 results measured on the same samples. Given that PGAA can measure Cl in concrete and the C1152 and is faster and less labor-intensive, it can be a candidate for development as a standard method for an alternative to the latter.

Determining the in-place properties of mass concrete placements is elusive, and currently there are minimal to no test methods available that are both predictive and a direct measurement of mechanical properties. This paper presents a three-stage testing framework that uses common laboratory equipment and laboratory scale specimens to quantify thermal and mechanical properties of mass high-strength concrete placements. To evaluate this framework, four mass placements of varying sizes and insulations were cast, and temperature histories were measured at several locations within each placement, where maximum temperatures of 107 to 119°C (225 to 246°F) were recorded. The laboratory curing protocols were then developed using this mass placement temperature data and the three-stage testing framework to cure laboratory specimens to represent each mass placement. Laboratory curing protocols developed for center and intermediate regions of the mass placements reasonably replicated thermal histories of the mass placements, while the first stage of the three-stage framework reasonably replicated temperatures near the edge of the mass placements. Additionally, there were statistically significant relationships detected between calibration variables used to develop laboratory curing protocols and measured compressive strength. Overall, the proposed three-stage testing framework is a measurable step toward creating a predictive laboratory curing protocol by accounting for the mixture characteristics of thermomechanical properties of high-strength concretes.

High cement content is often found in concrete mix designs to achieve the unique fresh-state behavior requirements of 3D Printable Concrete (3DPC), i.e., to ensure rapid stiffening of an extruded layer without collapsing under the stress applied by the following layers. Some materials with high water absorption, such as recycled concrete aggregates, have been incorporated in concrete mix designs to minimize environmental impact, nevertheless, the fine powder fraction that remains from the recycled aggregate processing still poses a challenge. In the case of 3DCP, few studies are available regarding mix designs using Recycled Concrete Powder (RCP) for 3D printing. In this context, this study presents the use of RCP as a filler to produce a printable mixture with low cement content. An RCP with 50 μm average particle size was obtained as a by-product from Recycled Concrete Aggregate production. Portland cement pastes were produced with 0%, 10%, 20%, 30%, 40% and 50% of cement mass replacement by RCP to evaluate its effects on the hydration reaction, rheology, and compressive strength. It was found that the studied RCP replacement was not detrimental for the hydration reaction of Portland cement during the initial hours, and at the same time it was capable of modifying the rheological parameters of the paste proportionally to the packing density of its solid fraction. The obtained results indicated the viability of 3DCP with up to 50% cement replacement by RCP. It was concluded that RCP presents good potential for decreasing the cement consumption of 3DPC, which in turn could decrease its associated environmental impact while providing a destination for a by-product from recycled concrete aggregate production.

Conventional approaches to concrete three-dimensional (3-D) printing relies on printing concrete in a straight (linear) print path, with layers overlaid on top of each other. This results in interlayer and interfilament joints being potential weak spots that compromise the mechanical performance. This paper evaluates simple alterations to the print geometry to mitigate some of these effects. A printable mixture with 30% of limestone powder replacing cement (by mass), with a 28-day compressive strength of approximately 70 MPa in the strongest direction is used. S- and 3-shaped print paths are evaluated as alternatives to the linear print path. Staggering of the layers ensures that the interfilament joints do not lie on the same plane along the depth. Flexural strength enhancement is observed when print geometries are changed and/or layers are staggered. The study shows that print geometry modifications mitigate mechanical property reductions attributed to interfilament defects in 3-D concrete printing.

Exposure to high temperature is well known to cause concrete degradation and lead to compressive strength loss. However, most research focuses on concrete exposed to high temperatures for more than 1 hour, and the available predictive equations for concrete strength loss due to heat exposure do not consider the effects of concrete thermal mass or account for variation in concrete thermal properties. This work proposes a methodology to create a predictive equation for the compressive strength loss in concrete exposed to heat. The proposed method leverages concrete temperature data from transient thermal analyses of concrete specimens correlated to results from experimental testing. The resulting equation from the analyzed data set predicted compressive strength loss with a root-mean-square error (RMSE) of 1.35% absolute error of the measured strength loss, and the maximum absolute underprediction in strength loss was 12.4% across all 26 cases examined.