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.

The atmospheric purification capacity of concrete has not beenadequately investigated. This study examines the feasibility ofusing sustainable foam-concrete granules as a porous materialfor reducing air pollutants in concrete. To enable the removal of nitrogen oxide (NOx) and sulfur oxide (SOx) using titanium dioxide (TiO2) nanoparticles, foam concrete was crushed into granules with porosity exceeding 30%. Ordinary portland cement (OPC), fly ash (FA), and slag cement were used as source cementitious materials. OPC was replaced with 0 to 40% FA and 0 or 40% slag cement by weight. Test results indicate that 30% FA and unit cementitious materials content exceeding 500 kg/m3 (31.2 lb/ft3) are optimal for replacing cement and foam-concrete granules, respectively. Considering the particle-size distribution and specific surface area, 6 to 13 mm (0.24 to 0.51 in.) and 6 to 9 mm (0.24 to 0.35 in.), were selected as optimal granule sizes. The coating procedures yielded improved SOx and NOx removal, with the removal rates reaching 83.8 and 45% using granules of 6 to 9 mm (0.24 to 0.35 in.), respectively. Consequently, the foam-concrete granules coated with TiO2 nanoparticles are promising in developing porous concrete with the reduction capability of air pollutants.

This study tested the effect of using taconite as an aggregate replacement in concrete. Taconite is a by-product from iron ore mining that has the potential to be used in concrete production as a coarse and/or fine aggregate. Replacing the aggregate in a concrete pavement with taconite could decrease the demand for increasingly scarce high-quality aggregates. The mechanical properties of concrete made with only fine, only coarse, or both fine and coarse taconite aggregates were tested. Properties tested include compressive, flexural, and tensile strength; elastic modulus; and coefficient of thermal expansion. All concretes made with taconite coarse and fine aggregate, either alone or in combination, produced concrete with acceptable mechanical properties for use in paving. The use of taconite coarse aggregate increased all mechanical properties tested, while the use of taconite fine aggregate had mixed effects on mechanical properties, but values of all properties tested remained within normal ranges. Fresh concrete properties were also tested, and taconite was found to decrease workability. This work shows that both coarse and fine taconite aggregates have the potential to be used as viable aggregates for concrete.

The use of calcined clays in cement (kaolinitic and common calcined clays) introduces challenges due to the high alumina content, large specific surface area, and high-alkali content. This paper examines the performance against sulfate attacks, alkali-silica reactions, and chloride penetration in mixtures with calcined illitic shale (CIS). Replacement of high and moderate alkali content cement with 25% CIS significantly reduces the expansion in the alkali-silica reaction (ASR) test, as the pozzolanic reaction contributes to the combination of alkalis. Replacing 20% white portland cement with CIS allows the formulation of sulfate-resistant cement by limiting the formation of gypsum and ettringite. The incorporation of 25% CIS in concrete does not significantly increase chloride ingress. Therefore, blended cements with replacement levels of 20 to 25% of portland cement by CIS reduce or maintain the performance against ASR, sulfate attack, and chloride penetration.

Digital fabrication and automated manufacturing technologies have been explored for civil engineering applications in the recent past and have rapidly gained momentum. Research and industrial development activities have been primarily focused on three-dimensional (3D) printing of concrete using the basic principle of extrusion along a predefined, automatically guided path. While the automated placement and shaping of concrete has advanced and has been refined significantly, the installation of reinforcement in the concrete is still largely done using traditional methods by manual placement of conventional steel reinforcing bar in a cavity between 3D-printed walls of formwork, which is subsequently filled by conventional cast-in-place concrete or grout. The concept for the construction of a structure in an entirely automated, digitally controlled process using alternative methods of structural reinforcement is currently still to be developed. Structural reinforcement is a key requirement in any efficient and economical concrete structure, and it is a challenge to invent a process for placing this reinforcement using an automated process in line with the printing process of concrete.