International Concrete Abstracts Portal

The concept of multi-generational concrete recycling is increasingly relevant as many existing recycled concrete structures near the end of their service lives. This study examines the performance variation and recyclability of multi-generational concrete subjected to chloride salt dry-wet cycling. After 30 dry-wet cycles, natural aggregate concrete, designed with three different strength grades, was crushed to produce the first generation of recycled fine aggregate, which was then used to prepare the second generation of concrete. This second generation was subjected to the same dry-wet cycling and subsequently crushed to yield a second generation of recycled fine aggregate. The results demonstrate a significant decline in the performance of the second generation of concrete, with an average compressive strength reaching only 89.52% of the first generation. Notably, the performance deterioration was more pronounced in lower-strength mixes, which exhibited increased porosity, greater mass loss, and deeper chloride penetration. Both generations of recycled fine aggregate met the standards for Class III aggregate; however, some properties of the recycled fine aggregate derived from higher-strength concrete qualified for Class II aggregate status. Additionally, a regression analysis model was developed to predict the attenuation coefficients for the third generation of concrete with design strengths of 30, 45, and 60 MPa, yielding coefficients of 56.84, 67.75, and 71.72%, respectively. This study underscores the potential for multi-generational use of recycled fine aggregates and highlights the importance of selecting appropriate design strengths to enhance durability and recyclability in chloride-rich environments.

Ultra-high-performance concrete (UHPC) is composed of a high volume fraction of binder and steel fibers, and a very low water content, resulting in enhanced strength and ductility along with higher cost and environmental impacts. This study develops a UHPC mixture amenable for three-dimensional (3-D) printing, with 30% of cement (by mass) replaced with a combination of replacement materials. The proportioned UHPC mixture with 1.5% fiber volume fraction demonstrates 28-day compressive strengths of >120 MPa (17.4 kip), and limited anisotropy when tested in the three orthogonal directions. Furthermore, 3-D-printed layered composites are developed where UHPC (with and without fiber reinforcement) and conventional concrete layers are synergistically used in appropriate locations of the beam to achieve mechanical performance that is comparable to 3-D-printed UHPC sections. Such manufacturing flexibility offered by 3-D printing allows conserving resources and attaining desirable economic and environmental outcomes, as is shown using life cycle and techno-economic analyses (LCA/TEA). Experimental and theoretical analyses of load-carrying capacity and preliminary LCA/TEA show that >50% of the fiber-reinforced UHPC beam volume (in the compression zone) can be replaced with conventional concrete, resulting in only a <20% reduction in peak load-carrying capacity, but >35% reduction in cost and >20% reduction in CO2 emissions. These findings show that targeted layering of different materials through 3-D printing enables the development and construction of 3-D-printed performance-equivalent structural members with lower cost and environmental impacts.

Reinforced concrete members derive flexural strength from reinforcing steel, which acts together with the concrete to mobilize the required capacity. Design standards stipulate mandatory norms that must be complied with during the design process. Non-compliance with these provisions can increase the risk of corrosion, compromising the safety and integrity of the structure. Concrete protects the reinforcing steel against corrosion, but it can also become a contributing factor when its microstructure is poor due to non-compliance with these norms. Assessing the residual flexural capacity is essential for making informed decisions regarding repair or demolition. The proposed model in this paper enables computation of the reduction in flexural strength based either on gravimetric mass-loss percentage or on measured corrosion current density. A design chart is also proposed to facilitate practical application, enabling engineers to assess residual capacity and decide on repair or demolition.

The term “mass concrete” characterizes a specific concrete condition that typically requires unique considerations to mitigate extreme temperature effects on a structure. Mass concrete has historically been defined by the physical dimensions of a massive concrete element with the intent of identifying when differential temperatures may induce early-onset cracking, leading to reduced service life. More recently, in addition to differential temperature considerations, extreme upper temperature limits have been imposed by the American Concrete Institute to prevent long-term concrete degradation. Studies dating back to 2007 show that shafts as small as 48 in. (1.2 m) in diameter can exceed both differential and peak temperature limits; in 2020, augered cast-in-place piles as small as 30 in. (0.76 m) in diameter exceeded one or both limits. This suggests the term “mass concrete” is misleading when considering today’s high-early-strength or high-performance mixture designs. This study applies numerical modeling coupled with field measurements to investigate the effects of concrete mixture design, drilled shaft diameter, and environmental conditions on heat energy production and temperature. Further, the outcome of this study focuses on developing criteria that combine the effects of both size and cementitious material content to determine whether unsafe temperature conditions may arise for a given drilled shaft design.

This study comprehensively investigates the development of ambient-cured self-consolidating geopolymer concrete (SCGC) based on the chemical composition of binders and alkaline activators. Five factors of the chemical composition of binders and alkaline activators, each with four levels, are used to evaluate and optimize the workability and compressive strength of the high-strength SCGC. The designed SCGC mixtures provided sufficient workability properties and compressive strength between 28 and 70.3 MPa (4061 and 10,196 psi). It was found that the SCGC mixture with a binder content of 600 kg/m3 (37.4 lb/ft3), a CaO/(SiO2 + Al2O3) mass ratio of 0.55, an Na2O/binder mass ratio of 0.11, an SiO2/Na2O mass ratio of 1.2, and an Na2O/H2O mass ratio of 0.35 was the optimum mixture, which achieved a slump flow of 770 mm (30.3 in.), 28-day compressive strength of 70.3 MPa (10,196 psi), and final setting time of 80 minutes. The CaO/(SiO2 + Al2O3) ratio in binders, binder content, and Na2O/binder mass ratio have been found to be the most influential factors on the workability and compressive strength of ambient-cured SCGC. Microstructural analysis of SCGC mixtures showed that the increase in the CaO/(SiO2 + Al2O3) ratio promoted the formation of calcium- aluminate-silicate-hydrate (C-A-S-H) gels and enhanced the compressive strength by filling voids and creating a compact and dense microstructure.