Mechanical Properties and Design Methodology of Steel Pipe Recycled Concrete Members

Literature Overview

This paper by Yang Youfu from Fuzhou University, published in Industrial Construction in 2006, Volume 36, Issue 11, explores the application of recycled concrete within steel pipe concrete (SRC) members. The research addresses a growing environmental and resource sustainability concern by investigating whether recycled concrete aggregate (RCA) can be successfully used in SRC columns, beams, and combined bending-compression members. The paper presents preliminary research results on axial compression short columns, pure bending members, and bending-compression members, while discussing several fundamental issues related to mechanical properties and design methods.

Background and Motivation

Recycled concrete technology involves the mechanical processing of demolished concrete waste to produce aggregate that can replace natural aggregate in new concrete mixes. The primary motivation is threefold:

• Reducing the volume of construction and demolition waste in landfills
• Decreasing the extraction of natural aggregate resources
• Lowering the carbon footprint of concrete production through reduced transportation and processing

However, recycled concrete aggregate typically exhibits higher water absorption, lower density, and higher porosity compared to natural aggregate. These properties translate to reduced concrete strength, increased permeability, and potentially different creep and shrinkage behavior. The question of whether these property differences significantly affect the performance of SRC members—where the steel pipe provides confinement and the concrete provides compressive strength—is the central concern addressed by this paper.

Key Technical Parameters and Design Considerations

The paper discusses several critical design parameters for steel pipe recycled concrete (SPRC) members:

Analysis of Confinement Effect and Composite Action

The confinement effect in SRC members is the mechanism by which the steel pipe restrains the lateral expansion of concrete under axial compression, thereby increasing the concrete's compressive strength and ductility. In conventional SRC members using natural aggregate concrete, the confinement effect coefficient can be reliably predicted using established models. However, the paper raises important questions about whether these models remain valid when RCA is used.

The key differences in confinement behavior with RCA concrete include:

• RCA concrete typically has a higher initial cracking strain due to the weaker interface between the recycled aggregate and the cement paste matrix.
• The lateral expansion of RCA concrete under compression may be greater than that of natural aggregate concrete at equivalent stress levels.
• The bond between the steel pipe and RCA concrete may be affected by the higher water absorption of RCA, which can lead to differential shrinkage and debonding.

The composite action factor, which represents the efficiency of load transfer between the steel pipe and the concrete core, may also be affected by RCA properties. If the RCA concrete has a lower modulus of elasticity, the strain compatibility between steel and concrete may be reduced, leading to a lower effective composite action.

Design Methodology Discussion

The paper proposes that the design of SPRC members should consider the following modifications to conventional SRC design methods:

• Reduction factors for the design strength of RCA concrete based on replacement rate
• Adjustments to the confinement effect coefficient to account for the different lateral expansion behavior of RCA concrete
• Separate consideration of the steel pipe and concrete contributions to bearing capacity, with appropriate interaction factors
• Verification of bond strength between steel pipe and RCA concrete through pull-out tests

The paper also discusses the importance of considering long-term behavior, including creep and shrinkage, in the design of SPRC members. The higher porosity of RCA can lead to greater long-term deformations, which may affect the serviceability of structures and the stress distribution between steel and concrete over time.

Engineering Practice and Quality Control

From a quality control perspective, the production of SPRC members requires additional attention to several factors:

• The quality and consistency of recycled aggregate, including cleanliness, grading, and water absorption
• The concrete mix design, particularly the water-cement ratio and admixture selection
• The concrete placement and compaction within the steel pipe, which may be more challenging with RCA concrete due to different workability characteristics
• The curing regime, as RCA concrete may require longer curing times to achieve full strength development

For welding operations on steel pipes that will be filled with RCA concrete, standard welding procedures for the pipe material remain applicable. However, the engineer should ensure that the welding quality meets requirements before concrete placement, as the steel pipe serves as both a structural component and a permanent formwork.

Study Insights and Implications

This paper makes an important contribution to the field of sustainable construction by demonstrating that recycled concrete can be used in SRC members with appropriate design modifications. The preliminary nature of the research means that further investigation is needed, particularly regarding long-term performance, seismic behavior, and fire resistance of SPRC members. The environmental benefits of using RCA in SRC members are significant, and if the design methods can be reliably established, this technology has the potential to make a substantial contribution to reducing the environmental impact of the construction industry. The paper's discussion of multiple performance aspects—bearing capacity, hysteretic behavior, creep, fire resistance, and post-fire performance—provides a comprehensive framework for future research and practical application.