Shear performance of reinforced concrete beams incorporating recycled concrete aggregate and high-volume fly ash
Introduction
Concrete is the most widely used construction material. Efforts aimed at producing environmentally friendly concrete can play a major role in securing environmentally friendly construction. Candidate technologies for sustainable concrete materials include the incorporation of supplementary cementitious materials (SCMs), such as fly ash as a partial replacement for portland cement, the incorporation of recycled materials in concrete production, and, in particular, recycled concrete aggregate (RCA), as well as the use of highly durable concrete to increase service life.
Portland cement production accounts for a significant portion of the total greenhouse gas emission. The production of Portland cement is responsible for about 7% of total worldwide CO2 emissions (Nuaklong et al., 2015). Therefore, replacing portland cement with an alternative cementitious material and/or industrial by-products can decrease the carbon footprint of the concrete.
Over 900 million tons of construction and demolition waste is produced annually in Europe, the U.S., and Japan (WBCSD, 2012). Although the use of RCA does not significantly reduce CO2 emissions, it can significantly contribute to reducing the depletion of natural resources (virgin aggregate) and decreasing the need for landfills.
When using a high volume of RCA and SCMs, it is necessary to understand the structural performance of this new class of concrete materials. Limited studies regarding the structural performance of high-volume fly ash concrete (HVFAC) are available. Arezoumandi et al. (2013) replaced 70% of cement with Class C fly ash in full-scale beams with longitudinal reinforcement ratios of 1.27%, 2.03%, and 2.71%. The concrete had compressive strengths at 28 days of 31 MPa. Results of the studies showed that the HVFAC beams can develop a shear strength that is around 12% higher than the reference beam made without any fly ash, which had a 28-day strength of 29 MPa. Rao et al. (2011) replaced 50% of cement with Class F fly ash in reinforced concrete (RC) beams made with longitudinal reinforcement ratios of 0.6%–2.9% and reported a slightly lower shear strength than the reference specimens without any fly ash.
A number of studies were conducted to evaluate the shear behavior of concrete made with RCA. Sogo et al. (2004) used RCA from foundation concrete of 10–40 years of age. Concrete mixtures were proportioned with a water-to-cementitious materials ratio (w/cm) of 0.3–0.6 and a RCA replacement of 100%. Beam specimens with a longitudinal reinforcement ratio varying between 2.39% and 4.22% with different shear reinforcement ratios of 0, 0.26%, and 0.53% were studied. The authors found that the shear strengths of beams made with RCA were up to 20% lower than those of the control beams in which RCA and virgin fine were used. A decrease in shear capacity of up to 30% was observed for concrete made with both fine and coarse RCA (Sogo et al., 2004). Yagishita et al. (1994) used three different types of RCA of relatively low, medium, and high quality as a full replacement of virgin coarse aggregate. The authors reported an 8% lower shear strength for the beams made with high-grade RCA compared to concrete made with virgin aggregate. Gonzalez-Fonteboa and Martinez-Abella (2007) tested beams made with a 50% RCA replacement. Beams were cast with 2.39% of longitudinal reinforcement and four shear reinforcement ratios ranging from 0 to 0.22%. Concrete mixtures with slump values of 50–100 mm, proportioned with w/cm of 0.55 and containing 8% silica fume were prepared. The average compressive strength at 115 days was 40 MPa. No significant difference in the shear strength was observed between the RCA-made and control beams. Table 1 offers a summary of various studies of the shear strength of concrete made with RCA.
The study reported in this paper investigates the shear capacity of 24 full-scale RC beams fabricated with high volume fly ash and RCA. Four concrete mixtures were employed for casting the beams: conventional concrete (CC) without any fly ash or RCA as the reference; fly ash concrete with 50% of Class C fly ash replacement (FA50 beams); RCA concrete with 50% of coarse RCA replacement (RCA50 beams); and sustainable concrete (SC) proportioned with 50% Class C fly ash and 50% RCA. The beams were fabricated with three different longitudinal reinforcement ratios of 1.27%, 2.03%, and 2.71%. The test results were compared with theoretical models provided by six different design codes as well as a shear database for CC. The experimental results were also compared to analytical approaches based on fracture mechanics as well as the modified compression field theory approach. Parametric and non-parametric statistical data analyses were conducted to compare the shear strength characteristics of the various beam elements.
Section snippets
Research significance
With the increasing use of sustainable concrete made with RCA and fly ash, more information regarding the structural behavior of such sustainable materials is required for safe implementation. The research presented here contributes to the evaluation of the structural performance of concrete containing high-volume fly ash and RCA in shear. The results presented here should be of interest to owner agencies and engineers considering the design and use of sustainable concrete for structural
Experimental program
As stated earlier, the experimental program involved the testing of four concrete mixtures and three longitudinal reinforcement ratios with two replicate beams per mixture. The constituent materials and mixture proportioning of the four mixtures are elaborated below.
Test results, analysis, and discussion
The performances of the various beams are discussed in this section. The shear strength and steel strain were compared to values that can be deducted from various standards as well as a database proposed by Reineck et al. (2003). A statistical data analysis was conducted to evaluate the shear strength results that were normalized to the square and cube root of compressive strength.
Conclusions
This paper compares the shear strength of concrete proportioned with 50% fly ash and 50% RCA, which is referred to as sustainable concrete (SC), to conventional concrete, concrete with 50% fly ash, and concrete with 50% RCA. The results presented in this study are providing a better understanding of the performance of such sustainable concrete mixtures for potential structural applications. The data obtained through testing 24 full-scale beams were analyzed. The beams had a longitudinal
Acknowledgment
The authors gratefully acknowledge the financial support provided by the RE-CAST University Transportation Center at Missouri S&T and O.H. Ammann Research Fellowship. The assistance of the technical staff at Missouri S&T for preparing and testing the beam samples is highly appreciated. The conclusions and opinions expressed in this paper are those of the authors and do not necessarily reflect the official views or policies of the funding institutions.
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