Identifying an Overlooked Key Parameter on the Sustainability of RCA Usage in Cementitious Composites
Presented By: Rui Bai
Affiliation: Purdue University - West Lafayette
Description: The use of recycled concrete aggregate (RCA) in the production of new cementitious composites has significant sustainability benefits in terms of reduction of net waste generation and abiotic depletion of natural resources. However, the use of RCA instead of natural aggregate may reduce the strength of the composite for a given water-to-cement ratio (w/c). This reduction of the strength affects the sustainability of the composite since achieving the same strength as the reference mortar (with no RCA) will require more cement per volume of composite, increasing the carbon footprint of the material. However, the effect of RCA on the composite’s strength, and therefore on its carbon footprint, highly depends on many factors. In this study, we illustrate the role of a critical parameter in the sustainability of RCA usage in cementitious composites: the target strength of the new composite. A total of 12 mortar mixtures were studied. 3 mixtures without RCA (100% natural aggregate) and with different w/c were used as reference mixtures. For each reference mixture, mortars with 3 RCA replacement ratio were studied. A total of 3 cubic samples per mixture were cast, cured under standard conditions, and tested at 28 days to determine their compressive strength. A comparative Life Cycle Assessment (LCA) considering a functional unit of a cubic meter of mortar with 28-day compressive strength equal to the strength of the corresponding reference (no RCA) was carried out to assess the effect of the RCA on the mortar’s environmental impact as a function of the corresponding target strength. Results show that the same aggregate will have a different effect on the environmental footprint of cementitious composites as a function of the target strength. In the same way, depending on the quality of the recycled concrete aggregate, the maximum replacement will be considered sustainable in terms of carbon footprint for the same target strength.
Trends in Concrete Properties when Recycled Concrete Aggregates Are Used: Statistical Analysis of a Large Database
Presented By: Matthew Adams
Affiliation: New Jersey Institute of Technology
Description: This presentation focuses on the development and analysis of a database of mechanical properties of concrete containing coarse RCA generated from 115 peer-reviewed journal articles. A sensitivity study of RCA systems was conducted through a full-factorial analysis to explore how the mixture design proportions influence the RCA concrete hardened properties. Impacts of mixture design properties on particular mechanical properties are presented along with bivariate contour plots that can be used by concrete mixture designers to better estimate RCA concrete properties. This work supports an increased adoption of RCA by providing broad conclusions supported by statistical analysis that show typical performance of RCA concrete across a variety of design inputs.
Carbonation of Recycled Concrete Aggregates for Improved Properties of Recycled Concrete
Presented By: Qingli Dai
Affiliation: Michigan Technological University
Description: Most of RCAs were applied as coarse aggregates in recycled concrete production or unbounded base materials. The porous microstructure of coarse RCA with mortar can cause low interface bonding with hydrated cement, thus reducing the strength of concrete. Both dry- or web-carbonated RCAs are used to produce the recycled concrete. The mechanical and durability performance of recycled concrete is investigated through lab testing and data analysis. The performance data of recycled concrete will be compared with that of control concrete, and the concrete with as received RCAs. The leachate pH value changes of carbonated RCAs and as-received RCAs are also compared. This study will provide some useful suggestions for using carbonated RCAs for recycled concrete and unbounded applications.
Preventing and Assessing Alkali-Aggregate Reaction (AAR) in Recycled Concrete Aggregates: Challenges and Research Needs
Presented By: Leandro Sanchez
Affiliation: University of Ottawa,
Description: Pressure is mounting to decrease the carbon footprint of concrete construction. In this context, the use of recycled concrete aggregates (RCA) is amongst the most promising alternatives. Over the last decades, an important amount of information and data have been gathered particularly on the short and mid-term performance of RCA-made concrete. However, RCA is currently not as much used as it could be due to the lack of guidelines and quality control, especially regarding its durability and long-term performance. This work discusses the state of the art and the current practices related to the prevention and assessment of alkali-aggregate reaction (AAR) in RCA-made concrete. It states the current challenges and presents the potential for further studies in the field.
Assessing the Impact of TiO2 Nanoparticles on the Environmental Performance of Mortars with RCA
Presented By: Carlos Moro
Affiliation: Texas State University
Description: The World Meteorological Organization has recently reported that greenhouse gases have reached their highest level since 3–5 million years ago. A continuing rise would cause serious consequences e.g., rising temperatures, death of living beings, or water pollution. Cement is associated with those levels since its production encompasses around 8% of global CO2 emissions. To increase the environmental performance of cementitious materials, different approaches could be followed, for instance, the reuse of waste materials such as recycled aggregate (RA) or the addition of TiO2 nanoparticles due to its proactive effect during service life. However, no research has been found that examined the effect of nano-TiO2 addition on recycled mortars in terms of environmental impact. Consequently, the main objective of this research is to evaluate the sustainability of TiO2 nanoparticles in mortars made with either natural or recycled aggregate. Twelve mixtures with different percentages of nano-TiO2 substitution (0%, 0.5%, 1%, 2% by the weight of cement) and RA replacement (0%, 50%, 100%) were studied. A life cycle assessment focused on material production (cradle-to-gate) was performed. The functional unit (FU) used was 1 m3 of mortar with a given compressive strength. The mix design was modified to meet the fixed strength defined in the FU. In terms of global warming potential, mortars with RA reduced the environmental impact when 0.5% of nano-TiO2 was added. Considering waste generation and depletion of natural resources play a crucial role in the sustainability assessment of mortars with RA. Furthermore, when the compressive strength factor was considered in the FU, RA increased the total CO2 emissions due to the higher amount of cement needed. Finally, despite the apparent harmful effect of nano-TiO2 using a cradle-to-gate approach, these nanoparticles could highly enhance environmental performance due to their effects during service life.
Enhancing Performance and Reducing Environmental Impact of Concrete with Replacement of Recycled Concrete Aggregate (RCA) Treated with Various CO2 Pressures
Presented By: Jiong Hu
Affiliation: University of Nebraska-Lincoln
Description: Recent studies demonstrated that the carbonation treatment of recycled concrete aggregate (RCA) could enhance its properties due to the conversion of adhesive paste to stronger and denser products. In addition, the use of RCA and sequestration of CO2 during the CO2-treatment process can help to reduce the carbon footprint of concrete. This study assesses the performance of recycled aggregate concrete (RAC) developed from CO2-treated RCA. RCAs obtained from 20-plus old highway and airfield pavements were treated under different pressures (5, 10, 20, 40, and 60 psi) of CO2, and then the physical and mechanical properties of RCA were examined. The complete substitution of natural coarse aggregate was carried out using both untreated and treated RCA, followed by an assessment of the resulting RAC's fresh, mechanical, and durability properties. Furthermore, the environmental performance of concrete incorporating untreated and treated RCA was evaluated. The experimental findings revealed that the CO2 treatment pressure had a significant influence on RCA characteristics, leading to notable improvements in the mechanical and durability properties of RAC. Results demonstrated that by employing CO2 treatment at pressures of 20, 40, and 60 psi, concrete incorporating complete RCA replacement can achieve properties comparable to concrete with natural aggregate. Moreover, the RAC produced using CO2-treated RCA demonstrated a reduced CO2 equivalent when compared to concrete incorporating natural aggregate or untreated RCA. These findings underscore the potential of CO2-treated RCA as a viable and environmentally friendly alternative to natural aggregate for sustainable concrete production.
Recycled Concrete Aggregate in Concrete Pavement: Pavement Design and Life Cycle Assessment
Presented By: Xijun Shi
Affiliation: Texas State University
Description: Pavement recycling has become a widespread practice in numerous states across the USA. Despite extensive laboratory studies on the material properties of concrete containing recycled concrete aggregate, there has been minimal focus on addressing concrete pavement design considerations and evaluating the potential sustainability advantages of this application. This study aims to fill this gap by developing pavement design recommendations that account for the changes in concrete properties resulting from the inclusion of recycled concrete aggregate. Additionally, a life cycle assessment case study has been conducted to comprehensively analyze the environmental impact of such practices. The findings of this study unequivocally support the conclusion that integrating recycled concrete aggregate into concrete pavement is not only a technically sound strategy but also holds the potential for substantial sustainability benefits.