Chemical Characterization of Recycled Concrete Aggregates Using a Handheld X-Ray Fluorescence Device
Abstract
Recycled concrete aggregates (RCA) are an important source of material that can be used in construction to mitigate the problems associated with use of virgin aggregates. The volume of waste generated by the construction industry is increasing at a rapid pace with demolished concrete forming a major part of the total waste. Aggregates are the largest component of concrete, and conventional sources of natural aggregates are diminishing due to environmental and cost considerations. Increased use of RCA in lieu of virgin aggregates slows the depletion of non-renewable natural resources and produces a more environmentally friendly infrastructure as our nations strive to progress towards a more sustainable future.Globally, a large amount of infrastructure construction is planned and underway. One potential way of reducing the consumption of natural aggregates is to replace natural aggregates with RCA extracted from construction and demolition (C&D) waste. However, the use of RCA in concrete applications comes with its own set of challenges due to RCA’s non-homogenous composition. Produced by crushing concrete, RCA includes both the virgin aggregate from the original concrete and the adhered mortar. Many of the properties of RCA are influenced by the presence of the mortar adhered to its surface. Because of this, use of RCA has historically been mostly limited to non-structural concrete applications like construction of gutters, pavements and small retaining or barrier walls. RCA is also often used in applications such as in pavement foundations (unbound and bound bases and subbases). Primary barriers to increased use of RCA include its variability in composition, including the residual mortar content, and the potential to contain contaminants which can negatively affect the properties of concrete (such as chlorides or sulfates) or present environmental concerns (such as heavy metals) (Snyder et al. 2018). Chemical characterization of RCA and determination of residual mortar content prior to use in new concrete or other infrastructure applications provides critical information to users, providing confidence in its usability and potential impacts on the performance of new concrete or base materials. To date, no method of rapid chemical characterization of RCA has been developed and accepted. Some of the existing methods implemented in the industry for elemental analysis of samples include atomic absorption spectrometry (AAS), Neutron Activation Analysis (NAA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) which are non-destructive testing techniques that each require a high investment cost and are time consuming. To overcome the short comings of the aforementioned methods, this research study proposes the use of a portable handheld XRF (PHXRF) for chemically characterizing recycled concrete aggregates and determining the residual mortar content of the samples. In this study the PHXRF was used for collecting quantitative data from the elemental analysis of RCA samples acquired from four different sources (highway, airfield pavement, and C&D waste) across the state of North Carolina. The results of this quantitative PHXRF data were compared against XRF "whole rock analysis" results for the validation of PHXRF. First, the actual mortar content of the RCA samples was determined using the thermal shock method and then a stepwise regression was performed on the PHXRF results based on size (No.4, No.12 & No.50) to determine a regression model to help compute the predicted values of the mortar content. The predicted mortar content value was compared against the mortar content determined through laboratory testing to determine the accuracy of the model in predicting the mortar. The second objective of the research was to determine the accuracy of the PHXRF device and choose the best representative size for PHXRF analysis. This objective was achieved by comparing the PHXRF results of the RCA samples with the whole rock analysis test results and using simple linear regression to observe the R2 values for each element. Based on the R2 value, the most appropriate size of the aggregate used for the test was determined, and the regression equation of this size RCA was used to compute the predicted weight % of major and trace elements.