A requirement for achieving sustainable concrete structures is to develop a quantitative method for designing concrete mixtures that yields the target rheological properties and compressive strength. Toward this objective, this paper proposes a mathematical model approach to improve the sustainability of the concrete industry. A postulation that packing density, a function of the concrete mixture, provides the link between concrete mixture, rheological properties, and compressive strength was investigated. Rheological models for yield stress and plastic viscosity and a compressive strength model were adopted with packing density as a central variable. The rheological models employ a cell description that is representative of fresh concrete. The compressive strength model is based on excess paste theory to account for the concrete mixture proportions, gradation of the aggregate particles, and porosity. An experimental program was developed to calibrate and test these models. Results revealed that packing density provides a consistent and reliable link and that the concrete mixture composition can be designed to achieve the target rheological properties and hardened properties and ensure quality control. Consequently, a new mixture proportioning methodology was developed and proposed as an improvement to the ACI 211.1 mixture design method. Furthermore, a case study was conducted to test the applicability and adequacy of this proposed method. This research outcome, which provides a quantitative approach to designing concrete mixtures to meet specific strength requirements and rheology, can also be used to ensure quality control before concrete is cast.

This research focuses on the evaluation of the sustainability of recycled ultra-high-performance concrete (R-UHPC) in a life cycle analysis (LCA) 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 R-UHPC aggregates themselves. Recycled aggregates (RA) were created by crushing 4-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 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 R-UHPC aggregates demonstrated improved self-healing properties to those containing natural aggregates, especially from the second to the sixth month. To address the potential environmental benefits of this novel material in comparison to the conventional ones, an 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 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 collection and cooling of geothermal waters was selected. This is 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 R-UHPC.

Major cities worldwide are densely built with concrete structures. Moreover, urban infrastructures partly cause, and are in return adversely impacted by, the urban heat island (UHI) effect that elevates localized temperature, further to the rise caused by the climate change-induced (CCI) impact of CO2 emissions. While the influence of temperature on carbonation is generally well established based on experimental research, there are hardly any analytical engineering methods specifically for evaluating temperature effect on natural carbonation. In the present study, an equation referred to as the temperature correction factor (TCF) submodel, capable of accounting for temperature effect on natural carbonation, was nested into the natural carbonation prediction (NCP) model, then used to conduct the evaluation.

The second aspect of the present study was employment of the TCF submodel for evaluation of CCI temperature rise on natural carbonation. The scope of evaluation covered 130 selected major cities, geographically located globally and strategically representative of the diverse climate regions worldwide. It was found that tropical climate regions exhibit a more significant increase in CCI carbonation progression compared to that of temperate regions. For structural concretes of normal to moderate strengths, CCI carbonation increases from 34 to 46% in tropical regions and by as low as 9.43/0% in cold/subpolar temperate regions. The silver bullet solution to CCI adverse effects is the use of high-strength concretes, which is a conundrum as this measure undermines or negates sustainability principles. Evidently, the projected CCI global temperature rise significantly increases concrete carbonation in major climate zones. Research is needed into the development of counter measures and design provisions for climate resilience of concrete structures.

The overdesign of concrete mixtures and substandard concrete acceptance testing practices significantly impact the concrete industry’s role in sustainable construction. This study evaluates the impact of overdesign on the sustainability of concrete and embodied carbon emissions at the national and project scales. In addition, this paper reviews quality results from a concrete producer survey; established industry standards and their role in acceptance testing in the building codes; the reliance on proper acceptance testing by the licensed design professional, building code official, and the project owner; and the carbon footprints that result from overdesign of concrete mixtures. In 2020, a field survey conducted on over 100 projects documented Pennsylvania’s quality of field testing. Of those surveyed, only 15% of the projects met the testing criteria within the ASTM and building code requirements. As a result, the total overdesign-induced cement consumption is as large as 6.7% of the estimated cement used in the United States.

The built environment is facing an increasing challenge of reducing emissions 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 life-cycle footprints. Herein, a new version of low-carbon ECC was developed for cast-in-place applications by sequestering CO2 through mineralization. Two waste streams were pre-carbonated and incorporated into ECC as fine aggregate and supplementary cementitious material, respectively. At 28 days, the CO2-sequestered ECC exhibited a compressive strength of 32.2 MPa (4670 psi), tensile strength of 3.5 MPa (508 psi), and strain capacity of 2.9%. Multiple fine cracks were distinctly identified, with a residual crack width of 38 μm (0.0015 in.) and a selfhealing 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 towards carbon neutrality.