The early-age material parameters of three-dimensional (3-D)-printable concrete defined under the umbrella of printability, namely, pumpability, extrudability, buildability, and the “printability window/open time,” are subjective measures. The need to correlate and successively substitute these subjective measures with objective and accepted material properties, such as tensile strength, shear strength, and compressive strength, is paramount. This study validates new testing methodologies to quantify the tensile and shear strengths of printable fiber-reinforced concretes still in their fresh state. A tailored mixture with high sulfoaluminate cement and nonstructural basalt fibers has been assumed as a reference. The relation between the previously mentioned parameters and rheological parameters, such as yield strength obtained through International Center for Aggregates Research (ICAR) rheometer tests, is also explored. Furthermore, in an attempt to pave the way and contribute toward a better understanding of the mechanical properties of 3-D-printed concrete, to be further transferred into design procedures, a comparative study analyzing the work of fracture per unit crack width in three-point bending has been performed on printed and companion nominally identical monolithically cast specimens, investigating the effects of printing directions, position in the printed circuit, and specimen slenderness (length to depth) ratio.

In marine structures, concrete requires adequate resistance against chloride ion penetration. As a result, numerous studies have been conducted to enhance the mechanical properties and durability of concrete by incorporating various pozzolans. This research has investigated the curing conditions of samples including zeolite and metakaolite mixed with Micro nanobubble water in artificial seawater and standard conditions. The results indicated that incorporating zeolite and metakaolin mixed with Micro nanobubble water, which was cured in artificial seawater conditions, compared to similar samples that were cured in standard conditions, improved the mechanical properties and durability of concrete samples. The compressive strength of 28 days concrete samples containing 10% metakaolin mixed with 100% Micro nanobubble water and samples consisting of 10% zeolite blended with 100% Micro nanobubble water cured in seawater in comparison to the control sample cured in the standard condition indicated an increase of 25.06% and 20.9%, respectively. The most results were obtained with a compound of 10% metakaolin, and 10% zeolite with 100% Micro nanobubble cured in seawater (MK10Z10NB100CS) which rose significantly Compressive, Tensile and Flexural Strength by 11.13, 14, and 9.1%, respectively, in comparison with to the MK10Z10NB100 sample cured in the standard condition. Furthermore, it decreased considerably 24-hr water absorption and Chloride Penetration at 90 days by 27.70 and 82.89%, respectively, in comparison with the control sample cured in standard conditions.

The penetration of chloride ions causes degradation of reinforcing bars, which directly affects the service life of the element. In this study, four different alternatives for the construction of a reinforced concrete (RC) caisson parapet beam are investigated: conventional RC, the addition of a corrosion inhibitor to concrete, and the use of glass fiber-reinforced bars (GFRP) and galvanized steel instead of steel bars. The durability of the RC element under marine environment was studied based on measurements performed both in-place and in well-controlled laboratory conditions on specimens prepared in the laboratory, as well as specimens taken from the actual structural element. It was concluded that the exposure of fresh concrete to seawater splash has no effect on mechanical properties. In addition, galvanized rods were found to be a less effective protection strategy compared to the other alternatives studied. GFRP bars, however, provide better protection than the other tested alternatives, although chloride ion penetration in these bars was found to be more accelerated in an alkaline environment compared to a chloride environment. In contrast to the prevailing approach, which considers plain concrete and according to which the electrical resistance of the concrete decreases because of chloride penetration, this study found that electrical resistance in the reinforced element is increased due to an increase in the amount of corrosion products formed between steel and concrete if no cracks occur. Furthermore, it was found that the potential measured using the half-cell method in all the alternatives slowly increased with time, as well as the corrosion risk in the three alternatives with reinforcing steel. The remaining question is whether this change of potential is a direct characteristic of the corrosion risk. Therefore, more research in this direction is needed.

The use of blended cements enables the production of concretes with low embodied carbon and improved resistance to chloride penetration compared to general-purpose (GP) cement concrete. This paper reports the chloride diffusion characteristics in terms of the apparent diffusion coefficient (Da), surface chloride concentration (Cs), and corresponding aging factors (a and b) of low-carbon concrete (LCC) derived from up to 9-year long-term exposure of small reinforced concrete slabs in both laboratory-simulated and field marine tidal conditions. LCC with either 30% fly ash or 50% slag provides slightly to significantly lower 28-day compressive strength than GP cement concrete at the same water-binder ratio but significantly better resistance to chloride penetration. The long-term chloride profile necessary to determine the concrete cover where the chloride threshold is reached can be determined with the Da.t0, Cs.t0, and corresponding age factors a and b, where t0 is the 1-year time of exposure. The improved resistance to chloride penetration by the use of fly ash and slag as cement replacements was largely due to their intrinsic influence on the microstructure of the concrete. The results highlight that the difference in chloride penetration arises from the change in test methods, thus the importance of calibration when data obtained from laboratory concrete were used as inputs for service-life design.

Studies have revealed that kaolinite-containing marine clay (MC) exhibits pozzolanic reactivity after thermal activation, making it a sound supplementary cementitious material for concrete production. In this study, calcined MC with two different finenesses was used to substitute cement in the mortar at 0, 10, 20, and 30% replacement ratios. The pozzolanic activity of calcined MC was confirmed by the reduced portlandite content in hydration products (by quantitative X-ray diffraction [QXRD]) and the formation of the calcium-silicate-hydrate (C-S-H) and calcium-aluminumsilicate-hydrate (C-A-S-H) gels (by scanning electron microscopy/energy-dispersive X-ray spectroscopy [SEM-EDS]). The microstructural, mechanical, and transport properties of mortar were investigated. The pore volume increased with the incorporation of calcined MC, while obvious pore refinement was noted. Due to the cement dilution effect and higher pore volume in the mortar, the strength and modulus of elasticity generally decreased with higher MC content. Chloride transport in the mortar was greatly inhibited due to the reduced pore sizes and higher chloride-binding capacity of calcined MC. With the addition of 10% fine calcined MC, the low-carbon mortar exhibits an earlier and increased heat release, similar strength, 8% lower water absorption, 14% lower water sorptivity, 44% lower chloride migration coefficient, and 12% loss in modulus of elasticity.