ACI Online Learning & Presentations

Concrete plays a vital part in construction industry. However, the production of concrete, notably cement, pose sustainability issues that need to be managed. To improve the sustainability of concrete, innovative admixtures, which contain Calcium Silicate Hydrate (C-S-H) nanoparticles, were developed at Master Builders Solutions. The patented admixtures can significantly enhance concrete strength at early and late ages. The use of this concrete admixtures enhances a reduction of considerable amounts of cementitious materials, typically in range from 50 lb/yd3 (30 kg/m3) to 100 lb/yd3 (60 kg/m3) to achieve similar strengths of those control concrete mixtures without the admixtures. In other hand, these innovative admixtures also enable a higher replacement of portland cement with supplementary materials such as fly ash, slag cement and milled limestone, reduction of heat-curing and/or curing time. The information presented will show how the use of the nanoparticle-based admixtures can provide options to modify, improve properties, increase the speeds of concrete curing and construction process, and reduce CO2-footprint of the concrete mixtures.

The presentation demonstrates the acceleration of carbonation kinetics rate in nanostructured interfaces of cementitious systems, crucial for CO2 mineralization in concrete, using 2D carbon-based nanomaterials. Highly exfoliated/nearly monolayered graphene nanoplatelets (GNPs) with 3-5x higher specific surface area than the surface area of cement grains were used. NanoIR mapping on sub-10 nm nanomaterial/C-S-H interfaces of carbonated specimens shown the formation of active sites that generate large quantities of Ca(OH)2 and CaO essential for CO2 mineralization, i.e., CaCO3 formation. A 50-80% higher nanoscale modulus of elasticity and elastic strain energy absorption capability of the carbonated nanostructured interfaces over the material without the GNP reinforcement was demonstrated through quantitative nanomechanical property mapping. Such enhancements in the carbonation kinetics and nanomechanical properties indicate improved CO2 mitigation potential and increased strengthening and toughening/resiliency of carbonated engineered concrete, important performance factors for the material’s serviceability.

Presented By: Surendra Shah, The University of Texas at Arlington Carbon Nano Fibers used to produce Ultra High-Performance Concrete (UHPC) is revo-lutionizing the concrete industry. This technology yields safe, simple, and sustainable UHPC for use in the infrastructure, building construction and other design applications where its low shrinkage and creep, superior bond strength and tensile performance are an asset. It has the unique, sustainability benefit of using CO2 captured from waste streams as the feedstock for producing the required nano fibers used in the mix. This presentation will describe the nano-poro-mechanical structure of the matrix, fundamen-tal characteristics of the nano technology and give concrete examples of applications where this material is being used today. It will explain why UHPC is an ideal material for use in resilient/sustainable design and how engineers and concrete professionals can leverage this material for longer lasting projects with smaller environmental footprints than traditional applications.

As the knowledge, testing and experience of using synthetic macrofiber reinforced concrete continues to grow for use in infrastructure projects, their successful use and benefits are now being realized through full scale and long term demonstration projects. Over the past several years, there has been a renewed interest in the use of fiber reinforcement in concrete pavements for parking lots, white toppings, bridge decks and roadways. Various technical organizations such as the American Concrete Institute, American Concrete Pavement Association and the National Concrete Pavement Technology Center have developed new guidance and recommendations on how to properly select and use fiber types in concrete. However, many prospective engineers, architectural firms and clients are now requesting additional information as to the environmental impacts of using fibers in replacement of traditional reinforcement or as an added material in concrete to improve durability and useful service life.

With low-carbon concrete requirements gaining momentum worldwide, the construction industry needs to take sustainability education to the next level. In view of the fact that concrete's climate change impact and its solutions vary across regions, education should be provided uniformly and in a large-scale manner based on local conditions. The ACI 130 Committee has developed and organized the Eco Concrete Competition since 2017. This competition aims to promote the idea of environmental performance in concrete mix designs and to think critically about low-impact solutions while considering their performance. This presentation will discuss the structure and experiences of organizing three versions of the competition. The importance of life cycle accounting, the context-specificity of solutions, and concrete performance accounting in educating the upcoming generation of civil engineers will be elaborated. The streamlined life cycle assessment (LCA) tool developed for this competition will be showcased as a possible educational tool for civil engineering students. Finally, lessons learned by and from student participants will be provided to clarify the points that should be emphasized in the LCA education of concrete.

Carbon Nano Fibers used to produce Ultra High-Performance Concrete (UHPC) is revolutionizing the concrete industry. This technology yields safe, simple, and sustainable UHPC for use in the infrastructure, building construction and other design applications where its low shrinkage and creep, superior bond strength and tensile performance are an asset. It has the unique, sustainability benefit of using CO2 captured from waste streams as the feedstock for producing the required nano fibers used in the mix. This presentation will describe the nanoporomechanical structure of the matrix, fundamental characteristics of the nano technology and give concrete examples of applications where this material is being used today. It will explain why UHPC is an ideal material for use in resilient/sustainable design and how engineers and concrete professionals can leverage this material for longer lasting projects with smaller environmental footprints than traditional applications.

To reduce the carbon footprint of the Portland cement (PC) industry, the prevailing practice is to partially replace the PC in concrete with supplementary cementitious materials (SCMs). Each SCM, owing to its distinctive chemical composition and molecular structure, affects hydration kinetics and microstructural evolution of cementitious binders uniquely. Current computational models cannot produce reliable predictions of hydration kinetics of complex [PC + SCM] binders. In the past two decades, the combination of Big data and machine learning (ML) has emerged as a promising tool to produce predictions for material properties. This study employs ML models to produce predictions of hydration kinetics of PC replaced by various SCMs at different replacement levels. However, ML cannot produce highly reliable predictions of hydration kinetics of [PC + SCM] binders because it is hard for ML to completely learn highly nonlinear correlations from a small database. To enhance prediction accuracy, we introduce two methods – Fourier transformation and phase boundary nucleation model – to reduce the degree of complexity of the database, which allows ML to produce highly reliable predictions. Furthermore, thermodynamic constraints derived from thermodynamic criteria are applied to inform and guide the ML model.

Calcium sulfoaluminate cements (CSACs)—for which CO2 emissions are ~50% lower compared to Portland cement (PC)—present a tremendous opportunity to develop sustainable binders for construction infrastructure. To further reduce the energy-intensity and carbon footprint of CSAC, supplementary cementitious materials (SCMs: e.g., a mixture of limestone and fly ash) can be used—at least in theory to replace up to 50% of the CSAC in the binder. That said, owing to the substantial diversity in SCMs’ compositions—plus the massive combinatorial spaces, and complex SCM-CSAC interactions—current computational models are unable to produce a priori predictions of properties of [CSAC + SCM] binders. This study presents a deep learning (DL) model capable of producing a priori, high-fidelity predictions of composition- and time-dependent hydration kinetics, phase assemblage development, and compressive strength development in [CSAC + SCM] pastes. The DL is coupled with a thermodynamic model that constrains and guides the DL, thus ensuring that predictions do not violate fundamental materials laws. The training and outcomes of the DL are ultimately leveraged to develop a high-fidelity prediction tool to determine optimum precursor chemistry and mixture designs of [CSAC + SCM] binders that exhibit superior compliance-relevant properties compared to PC concretes, while restricting the CSAC content to ~50%.

Multiple recent initiatives advocate for reducing embodied carbon, i.e., greenhouse gas emissions, associated with concrete production. Even though readily implementable strategies exist, practitioners are concerned that embodied carbon reductions may adversely affect concrete performance and durability, resulting in detrimental effects from a life cycle perspective. To address such concerns, this study investigates the correlations between embodied carbon of concrete mixtures, mixture design parameters, and experimentally measured mechanical and durability properties. The analyzed dataset included 145 mixtures, featuring a variety of mixture designs from laboratory and field studies. The results indicate that the cement content is the most significant predictor of GWP and that considerable savings can be achieved with cement reduction and replacement without compromising performance. Clear correlations of GWP with compressive strength were not identified, while the results demonstrated that reduced GWP and increased surface electrical resistivity (the utilized durability indicator) often occur in synergy. The study also provides the tie between Performance Engineered Mixtures and embodied carbon and thereby provides practical insights into mixture design optimization that includes both sustainability and durability.

An important part of improving the embodied carbon of the built environment is reducing the carbon emissions associated with concrete. The beneficial use of carbon dioxide (CO2) in ready mixed concrete production has been developed and installed as a retrofit technology with industrial users. An optimum dose of CO2 added to concrete as an admixture leads to the in-situ formation of mineralized calcium carbonate (CaCO3) and can increase the concrete compressive strength. The improved performance can be leveraged to design concrete mixture proportions for a more efficient use of portland cement along with the use of CO2 to reduce the carbon footprint of concrete. The physicochemical aspects of CO2 mineralization, the fresh and hardened concrete performance, durability performance and life cycle impacts will be discussed.

To reach the crucial milestone of generating efficient decarbonization pathways in sustainable concrete constructions, new technological developments to increase carbon sequestration in concrete are needed, well beyond the current carbon mitigation approaches limited to cement manufacturing. The objective of this presentation is to design sustainable and eco-friendly concrete by replacing cement with carbon-based agricultural byproducts and reinforcing it with graphene-based nanomaterials. This is a promising way for developing eco-efficient and highly resilient concrete, with at least 2x increased tensile load carrying and strain energy absorption capability; and 2-3x higher CO2 sequestration ability. The output of the proposed study will help promoting low-cost pathways for nullifying the greenhouse gas emission in the concrete and construction industry, while optimizing concrete’s carbon uptake and mineralization capacity, by engineering the nanostructured interfaces to accelerate carbonation kinetics.

The large volume o Rui He f the using of Portland cement makes the industry a large emitter of CO2 despite it has a relatively low carbon footprint compared to most other construction materials. The CO2 curing has been found to be an effective method to achieve carbon neutrality of cementitious materials. In this work, cementitious materials with the addition of nano-silica were cured by CO2. The carbon footprint of cementitious materials with and without the addition of nano-silica was calculated. The mechanical performance of cementitious materials cured by CO2 is tested. The chemical composition of cementitious materials cured by CO2 incorporated with various dosage of nano-silica is characterized by thermogravimetric analysis (TGA) and X-ray powder diffraction (XRD). The pH value change of cementitious materials after CO2 curing is measured. The pore structure of cementitious materials is characterized by mercury intrusion porosimetry (MIP) and 3D X-ray microscopy (Micro-CT) techniques. The morphology of CO2 curing products is also characterized by scanning electron microscope (SEM) analysis. The results indicates that the CO2 curing method can significantly reduce the car-bon footprint of cementitious materials. The addition of nano-silica can improve the mechanical performance and result in a dense microstructure of cementitious materials.