Long-Term and Seismic Performance of Concrete-Filled Steel Tube Columns with Conventional and High-Volume SCM Concrete

Long-Term and Seismic Performance of Concrete-Filled Steel Tube Columns with Conventional and High-Volume SCM Concrete

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Long-Term and Seismic Performance of Concrete-Filled Steel Tube Columns with Conventional and High-Volume SCM Concrete
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    Final technical report.
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    Production of Portland Cement for concrete is a major source of CO2 emission. Concrete can be made more sustainable by replacing a large volume of the cement with Supplementary Cementitous Materials (SCMs) such as fly ash and slag. The amount of cement that can be replaced with SCMs in conventional concrete structures is limited due to slow strength gain, of such concretes. Concrete-Filled Tubes (CFT) are composite structural elements that consist of a steel tube with concrete infill. In CFT sections, neither formwork nor rebar is needed since the steel tube serves as both, which can reduce construction time and cost. In bridge construction, the steel tube itself has strength to support some dead weight of the superstructure prior to casting of the concrete. This can significantly reduce delays for concrete curing and eliminates the need for high early strength of the concrete, making slow-curing concrete, such as SCM concrete a realistic alternative. In this research program, two 20-inch diameter CFT columns were tested for creep and shrinkage behavior under sustained axial loading. Subsequently, the specimens were tested until failure under combined loading consisting of constant axial load and increasing cyclic lateral load to evaluate seismic performance. One tube was filled with conventional self-consolidating concrete (SCC), while self-consolidating SCM concrete was used in the other specimen. The objective was to determine if the performance of CFTs filled with SCM concrete is comparable to the performance of CFTs with conventional concrete. In the SCM concrete, 80% of the cement was replaced with fly ash and slag, which reduces the carbon footprint of the mixture greatly. Sealed and unsealed cylinders were made from both concrete mixtures and tested for creep and shrinkage behavior as a comparison. The results indicate that the long-term performance of CFTs filled with low early-strength concrete, such as SCM concrete, is fully comparable to performance CFTs filled with conventional concrete. Slightly more load was shed to the steel in the SCM specimen, but the concrete core of the SCM specimen was shown to creep considerably less. The results from the long-term tests were compared to four existing models for creep and shrinkage, to determine if these models provided acceptable predictions for the long-term behavior of structural CFT components. Some models captured the behavior quite well in some cases, while other models provided inferior predictions. Results from the seismic performance tests were compared to results from two specimens previously tested for seismic performance. One was identical to the SCC specimen tested in this research program, apart from the embedment depth of the tube, which was slightly smaller. The other specimen was 30 inches in diameter but the steel tube was of the same thickness. The results showed that the seismic performance of the CFT SCM specimen was almost identical to the seismic performance of CFTs filled with conventional concrete. Observed damage, Force-Displacement response, and Moment-Drift ratio were all very similar for all 20 in. specimens compared.
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