The earth’s natural resources are being consumed at a very high rate for many years. The potential depletion of resources, CO2 emissions and high energy consumption rates in the process of production, increase the necessity of recycling. All sectors of society are responsible for these concerns, especially the construction industry.
As a major construction material, concrete is increasingly judged by its environmental impacts and reusing the readily available concrete is becoming very important. Considering that much of the US infrastructures and urban buildings now require renovation and replacement, the concrete left behind can be a valuable source of aggregate for new concrete. Such concrete is usually called Recycled Aggregate Concrete. Through cost analyses, it is shown that using recycling concrete as aggregate for new concrete production can be a cost-effective method for construction.
(1) Raise public awareness of recycled concrete including its impact on reducing landfills and construction waste;
(2) Compare parameters of recycled aggregate concrete to natural aggregate concrete and evaluate the differences;
(3) Describe how recycled concrete can save natural resources and ecological environment leading to sustainable and greener world;
(4) Explain the effect of adding different construction waste materials to concrete and the impact of this on recycled concrete properties.
Petrographic Characterization of Recycled Concrete Aggregates (RCAs)
Presented By: Mengesha Beyene
Affiliation: SES Group & Associates, LLC
Description: One of the challenges of using RCAs in concrete or other construction applications is the absence of a well-defined method to assess the quality of the aggregate. RCA stockpiles may contain aggregates from different sources. To characterize the quality these mixed RCAs or RCA derived from demolition of a known concrete pavement or structure, statistically representative aggregate particles need to be examined using appropriate techniques. Petrographic methods in conjunction with quantitative image analyses is a useful tool to characterize the quality of RCAs. We considered 4 RCAs sources in this study. Representative aggregate particles were randomly selected from each sieve size of the coarse RCAs and examined closely by techniques adopted from ASTM C 295 which is used to characterize natural aggregates. Semi-quantitative tabulated data on the type and a preliminary idea on the quality of the RCAs were obtained from this level of examination. Representative aggregate particles from each group of RCAs were selected to make oversized fluorescent thin sections to perform fluorescent microscopy to study the quality of the paste density/quality of paste-aggregate bond in the interfacial transition zones. Quantitative image analysis was performed on representative particles by using high quality fluorescent thin section images to determine the total area fraction of aggregates, relatively denser paste, relatively porous paste, and larger voids/cracks. A colored composite map was created, combining all phases in the concrete. This provided clarity about the overall quality of the ITZs/cement/cementitious paste. These methods will provide information about any prior deterioration mechanisms that may affect quality – such as alkali aggregate reactions, freezing and thawing, and sulfate attack. Petrographic characterization of RCAs may be a helpful tool to decide whether a given RCA source can be recommended to be used as a concrete aggregate or for other applications.
Characterizations of Recycled Concrete Aggregate (RCA) for Concrete Applications
Presented By: Jiong Hu
Affiliation: University of Nebraska-Lincoln
Description: As sources of quality natural aggregates become challenging or economically prohibitive to obtain in some markets, and the economic and sustainability benefits of concrete recycling become increasingly enticing to stakeholders, use of Recycled Concrete Aggregate (RCA) in new concrete is receiving renewed interest in recent years. Major obstacles that hinder the use of RCA in concrete construction cited by practitioners are the lack of specifications/procedures for qualifying an RCA source and the unclear impact of RCA on concrete performance. Many of these problems can be resolved by an accurate characterization of RCA, which includes geometrical properties, physical and mechanical characteristics, and chemical characteristics/compositions. The purpose of this research is to identify the most effective and practical RCA characterization methods and procedures through experimental study. The presentation includes a summary of different methods used for RCA characterization and their effectiveness, as well as the impacts of critical RCA characteristics on fresh and hardened concrete performance. Results of characteristics of RCA collected from different sources will also be presented.
Evaluation of Reclaimed Asphalt Pavement based Cementitious Materials
Presented By: Xijun Shi
Affiliation: Texas A&M University
Description: Potential issues associated with the depletion of good aggregate sources and management of excess reclaimed asphalt pavement (RAP) stockpiles increasingly motivate the use of RAP in cementitious materials as aggregate replacements. A systematic study on using RAP in cementitious materials was conducted by the speaker in the past several years. The study indicates that, if designed properly, RAP based cementitious materials could have satisfactory performance for different civil applications. Based on the results, the following conclusions are drawn: (1) asphalt is the major weakness zone, (2) controlling asphalt content is the key for designing RAP based cementitious materials with adequate strengths; limiting agglomerated RAP particles and using dense gradation mix design help improve the strengths, and (3) RAP based cementitious materials could have improved fracture properties and crack resistance under restrained drying relative to conventional concrete.
Degree of Hydration and Remaining Unhydrated Cement in Concrete Waste
Presented By: Daniele Kulisch
Affiliation: Technion - Israel Institute of Technology
Description: The reuse of material recycled from concrete waste in new concrete requires a beneficiation process, which usually separates the natural aggregate from the attached cement paste. Old cement paste is considered detrimental to the quality of the recycled aggregates and therefore, its removal is necessary. The majority of research on recycling of concrete focus on recycled aggregate, leaving the separated cement paste out of scope. This work focuses on the old cement paste, which contains remaining unhydrated cement and therefore, presents a recovery potential for further use in new concrete not as inert filler but rather as an active additive with cementing properties. Unhydrated cement remains in all concrete types, that is complete cement hydration is not achieved. This may be due to several factors: particle size of the cement, water to cement (w/c) ratio of the paste, curing duration and conditions, lack of water or space for hydration, etc. A cement paste is prepared in a laboratory with w/c ratio of 0.4. At the age of 7 days, the hydration is stopped by the solvent exchange in isopropyl alcohol and the degree of hydration is estimated by different methods, including X-ray diffraction (XRD), thermogravimetric analysis, and isothermal calorimetry. Results show that the degree of hydration is indeed below 100% varying from 0.68 to 0.76 depending on the analytical method, which indicates that cement hydration is not complete and considerable part of cement remains unhydrated. XRD results also provide quantification of the remaining unhydrated cement, which represents approximately 20% by weight. These results indicate that the material has a recovery potential and it could be further used in new concrete as active addition with active cementing properties.