The current paper investigates the effects of partial cement replacement with Nano-TiO₂ (NT) in varying weight proportions in concrete. In the C20/25 grade of concrete, Nano-TiO₂ (NT) was added by weight of cement with partial replacement of 0%, 0.5%, 1.5%, 2.0%, 2.5%, and 3.0% using Portland Pozzolana Cement. The physical and mechanical properties of the resulting concrete were assessed, as well as aspects of durability such as sorptivity and non-destructive tests (NDT) such as Ultrasonic Pulse Velocity (UPV). Compared with the control mixture, the fresh concrete produced showed a drastic reduction in slump with increasing percentage replacement, with a 54% reduction at 3.0% replacement. Furthermore, for 1.5% Nano-TiO₂ (NT), the compressive, flexural, and split tensile strengths peaked at 7, 28, 56, and 90 days, after which the values decreased. The addition of Nano-TiO₂ (NT) improved the homogeneity and integrity of the resulting concrete based on the UPV values. As the percentage of Nano-TiO₂ (NT) increased, chloride penetration decreased. From microstructural studies, it can be concluded that Nano-TiO₂ (NT) acts as a filler material and can be used as a partial replacement for cement in concrete up to 2% by weight.

The use of performance-based specifications (PBS) may increase quality and sustainability while lowering project costs through innovations in concrete material selection and proportioning. A preliminary survey was conducted showing that barriers to implementation for PBS still exist, the main barrier being the enforcement of the specification, followed by cost and time. This study aims to develop guidelines to overcome the identified barriers by presenting a laboratory-scale case study of six concrete mixtures that both conform (one) and do not conform (five) to Georgia Department of Transportation specifications. This case study includes experimental results on mechanical (flexural and compressive strength) and resistivity performance properties, as well as three additional parameters: time, cost, and CO₂ emissions associated with each mixture design. This study showed that innovation in material use and mixture design can increase durability and performance while reducing overall project cost and environmental impact.

This research focuses on the evaluation of the sustainability of recycled ultra-high-performance concrete (R-UHPC), from a life cycle analysis perspective and with reference to a case study example dealing with structures exposed to extremely aggressive environments. This involves the assessment of the self-healing capacity of R-UHPC, as guaranteed by the recycled UHPC aggregates themselves. Recycled aggregates (RA) were created by crushing four-month-old UHPC specimens with an average compressive strength of 150 MPa. Different fractions of recycled aggregates (0 to 2 mm) and two different percentages (50 and 100%) were used as a substitute for natural aggregates in the production of R-UHPC. Notched beam specimens were pre-cracked to 150 µm using a three-point flexural test. The autogenous self-healing potential of R-UHPC, stimulated also by the addition of a crystalline admixture, was explored using water absorption tests and microscopic crack healing at a pre-determined time (0 days, 1 month, 3 months, and 6 months) following pre-cracking. Continuous wet/dry healing conditions were maintained throughout the experimental campaign. The specimens using recycled UHPC aggregates demonstrated improved self-healing properties to those containing natural aggregates, especially from the 2nd month to the 6th month. To address the potential environmental benefits of this novel material in comparison to the conventional ones, a Life Cycle Assessment (LCA) analysis was conducted adopting the 10 CML-IA baseline impact categories, together with a Life Cycle Cost (LCC) analysis to determine the related economic viability. Both LCA and LCC methodologies are here integrated into a holistic design approach to address not only the sustainability concerns but also to promote the spread of innovative solutions for the concrete construction industry. As a case study unit, a basin for the collection and cooling of geothermal waters has been selected. This is meant as representative of both the possibility offered, in terms of structural design optimization and reduction of resource consumption, and of reduced maintenance guaranteed by the retained mechanical performance and durability realized by the self-healing capacity of the R-UHPC.

Mixed recycled aggregate (MRA) is considered a sustainable construction material, and its use in precast concrete is currently banned due to its poor engineering performance. This paper aims to evaluate the feasibility of partial replacement of natural coarse aggregate with MRA in self-compacting concrete (SCC) for manufacturing architectural precast concrete sandwich wall panels. To this end, five MRAs from recycling plants were characterized, out of which two were selected to develop SCC. SCC mixtures with three replacement levels and three water compensation degrees were produced, and their physical, mechanical, durability and aesthetic properties were examined. The results showed that the incorporation of MRA dominated the mechanical properties of SCC, while the water compensation degree primarily affected the flowability and carbonation resistance. The presence of MRA had no considerable effect on the aesthetic characteristics. Up to 10% MRA in weight of total aggregates could be used in precast SCC.

The built environment is facing an increasing challenge of emission reduction regarding both embodied and operational carbon. As an ultra-durable concrete, engineered cementitious composites (ECC) reduce the need for repair, thus resulting in a prominent reduction of lifecycle footprints. Here, a new version of low-carbon ECC was developed for cast-in-place applications by sequestering CO₂ via mineralization. Two waste streams were pre-carbonated and incorporated into ECC as fine aggregate and supplementary cementitious material, respectively. At 28 d, the CO₂-sequestered ECC exhibited a compressive strength of 32.2 MPa (4670 psi), a tensile strength of 3.5 MPa (508 psi), and a strain capacity of 2.9%. Multiple fine cracks were distinctly identified, with a residual crack width of 38 µm (0.0015 in.) and a self-healing behavior comparable to that of conventional ECC. The new ECC sequestered 97.7 kg/m³ (164.7 lb/yd³) CO₂ (equivalent to 4.7 wt% of final mixture) and demonstrated a 42% reduction in cradle-to-gate emissions compared to conventional concrete at the same strength level. This study demonstrates the viability of turning waste CO₂ gas into durable construction materials and proposes a potential path toward carbon neutrality.