Self-Compacting Concrete for Steel Tube Arches - Mix Design and Application

Literature Overview

Published in the Journal of Building Materials in 2010 by Jiang Zhengwu, Li Xiangtao, Sun Zhenping, and Wang Peiming from the Ministry of Education Key Laboratory of Advanced Civil Engineering Materials at Tongji University, this paper addresses the formulation and application of self-compacting concrete (SCC) specifically tailored for steel tube arch bridge construction. Supported by the Eleventh Five-Year National Science and Technology Support Program (2006BAJ05B07-05), the research focuses on the unique requirements imposed by the continuous pumping and jacking construction method used in steel tube arch bridges. The authors propose performance requirements and evaluation methods for steel tube arch SCC, develop mix design techniques through optimization of basic parameters, admixture combinations, and mineral admixture selection, and successfully produce and apply a high-performance SCC with compressive strength exceeding C60 grade.

Core Technical Points and Interpretation

Construction Method Requirements

The continuous pumping and jacking construction method for steel tube arch bridges imposes stringent requirements on the concrete:

• Continuous pumping: The concrete must maintain flowability over extended pumping distances and durations without segregation or bleeding.
• Jacking process: The concrete must be cast into the steel tube while it is being jacked into position, requiring immediate workability without vibration.
• No vibration: Since the steel tube geometry prevents the use of internal vibrators, the concrete must self-compact under its own weight.
• Rapid strength development: The jacking process requires the concrete to gain sufficient strength quickly to maintain the arch shape during subsequent construction stages.

Performance Requirements and Evaluation Methods

The paper proposes a comprehensive set of performance indicators for steel tube arch SCC:

The requirement of zero slump loss over 4 hours is particularly demanding, as it ensures that the concrete maintains its workability throughout the entire pumping and jacking cycle, which can take several hours depending on the bridge span and construction logistics.

Mix Design Technology

The mix design approach involves three key optimization strategies:

1. Basic parameter optimization: The water-binder ratio, aggregate gradation, and binder composition are systematically varied to achieve the target rheological properties. The use of a well-graded aggregate skeleton with a high solid fraction reduces the paste volume while maintaining flowability.
2. Admixture combination: A carefully designed polycarboxylate ether (PCE) superplasticizer is combined with viscosity-modifying agents (VMA) such as microsilica, bentonite, or hydroxypropyl methylcellulose (HPMC) to achieve the required flowability without segregation. The VMA increases the paste viscosity, stabilizing the aggregate suspension and preventing bleeding.
3. Mineral admixture selection: Supplementary cementitious materials (SCMs) such as fly ash, silica fume, and ground granulated blast furnace slag (GGBFS) are used to optimize the particle size distribution, reduce heat of hydration, and enhance long-term strength development. The inclusion of expansive agents (such as calcium sulfate or magnesium oxide) provides shrinkage compensation to counteract the high shrinkage tendency of high-strength concrete.

Key Mix Design Parameters

The achieved mix design demonstrates the following characteristics:

• Compressive strength: Exceeding C60 grade (≥ 60 MPa at 28 days), providing adequate structural capacity for the arch ribs.
• Autogenous shrinkage compensation: The expansive component compensates for both autogenous shrinkage (which occurs in low water-binder ratio concretes due to internal drying) and early-age drying shrinkage, ensuring full confinement of the steel tube.
• Segregation resistance: The VMA-enhanced paste provides sufficient yield stress to prevent aggregate settlement during the extended pumping and jacking process.
• Bleeding resistance: Zero open-air bleeding ensures that no water separation occurs at the steel tube-concrete interface, which would create a weak zone detrimental to bond strength.

Engineering Practice Implications

Steel Tube Surface Preparation

From a steel pipe manufacturing and construction perspective, the application of SCC in steel tube arches imposes specific requirements on the steel tube:

• Internal cleanliness: The internal surface of the steel tube must be free of rust, scale, oil, and moisture to ensure proper bond development between the concrete and the steel tube. Shot blasting to Sa 2.5 or higher is recommended.
• Internal coating compatibility: If an internal epoxy coating is applied for corrosion protection, it must be compatible with the SCC and should not impair the bond. Unbonded coatings may reduce the interfacial shear strength, which is critical for composite action.
• Geometric accuracy: The steel tube should have consistent inner diameter to ensure uniform concrete thickness. Variations in wall thickness can lead to uneven confinement pressure on the concrete.

Welding Considerations

Steel tube arch bridges typically involve welding of steel tube segments and the connection of arch ribs to the deck and other structural elements. The use of C60-grade SCC does not directly affect the welding of steel components, but the following considerations are relevant:

1. Thermal effects on concrete: Welding operations near or on the steel tube can cause local heating of the adjacent concrete, potentially leading to microcracking or strength loss in the heat-affected zone of the concrete. Preheating and controlled heat input are essential.
2. Residual stress interaction: The residual stresses from welding can interact with the confinement stresses from the concrete, potentially affecting the buckling behavior of the steel tube wall.
3. Corrosion protection: The SCC provides some cathodic protection to the internal steel surface, but external corrosion protection (coating, galvanizing) remains essential for the service life of the bridge.

Construction Quality Control

The successful application of SCC in steel tube arches requires rigorous quality control at multiple stages:

• Batch consistency: The SCC mix must be produced with tight control over water content, admixture dosage, and aggregate gradation. Automated batching systems with real-time feedback are recommended.
• Pumping performance: The pumping pressure and flow rate must be monitored continuously to detect any signs of segregation or workability loss during the pumping process.
• In-situ strength verification: Since the concrete is enclosed within the steel tube, traditional core sampling is impractical. Non-destructive testing methods such as ultrasonic pulse velocity (UPV) and rebound hammer tests should be used for in-situ strength verification.
• Curing management: The high binder content and low water-binder ratio of C60 SCC make it susceptible to early-age cracking due to thermal gradients and autogenous shrinkage. Internal curing agents or external curing methods must be employed.

Study Insights and Implications

This paper represents a significant advancement in the application of advanced concrete technology to steel tube arch bridge construction. The achievement of C60-grade SCC with zero slump loss over 4 hours and complete shrinkage compensation is a remarkable feat of materials engineering. For steel pipe manufacturers, the key insight is that the selection of steel grade and surface treatment for SRC applications must be coordinated with the concrete mix design to ensure optimal composite performance. The shrinkage compensation feature of the SCC is particularly important because it addresses one of the primary causes of debonding identified in Topic 1 — concrete shrinkage-induced interfacial stress. By compensating for shrinkage, the SCC maintains a tight bond with the steel tube throughout the service life of the bridge. The successful engineering application reported in the paper provides confidence that these advanced materials can be reliably used in large-scale infrastructure projects. The integration of SCC technology with steel tube arch construction represents a paradigm shift from traditional vibrated concrete methods, enabling faster construction, higher quality, and improved long-term performance.