Mix Design of C60 Machine-Made Sand Self-Compacting Steel Tube Concrete

Literature Overview and Engineering Context

The study by Li Beixing and colleagues, published in the Concrete journal in 2010, addresses the mix design of C60 self-compacting steel tube concrete (SC-SCC) using machine-made sand as the fine aggregate. The research was conducted as part of the Western Transportation Construction Science and Technology Program, with the Xiaohe Grand Bridge as the engineering backdrop. The use of self-compacting concrete (SCC) in steel tube concrete (STC) applications is advantageous because it eliminates the need for vibration, which is difficult to achieve inside a steel tube, and it ensures full compaction and bonding between the concrete and the steel tube.

The use of machine-made sand, which contains a relatively high proportion of stone powder (7% in this study), presents both challenges and opportunities. The stone powder acts as a filler and provides additional viscosity and lubrication, which can be beneficial for the self-compacting properties of the concrete. However, excessive stone powder can reduce the workability and increase the water demand, which must be carefully controlled.

Mix Design Parameters and Optimization

The study employed a three-component admixture technology combining polycarboxylate superplasticizer, mineral powder (fly ash or slag), and expansive agent. The key design parameters and their optimized values are summarized below:

The achievement of a 28-day compressive strength exceeding 75 MPa with a binder content of only 535 kg/m3 is a significant accomplishment. This is made possible by the synergistic effects of the three-component admixture system and the beneficial properties of the stone powder in the machine-made sand.

The polycarboxylate superplasticizer provides high water reduction and enhances the workability of the fresh concrete. The mineral powder contributes to the long-term strength development through secondary hydration reactions and reduces the heat of hydration, which is important for mass concrete applications. The expansive agent compensates for drying shrinkage and improves the bond between the concrete and the steel tube.

Technical Analysis of Key Design Decisions

The decision to use machine-made sand with a high stone powder content (7%) is a critical design choice. In conventional concrete design, stone powder is often considered a deleterious component because it increases the water demand and reduces the strength. However, in self-compacting concrete, the stone powder serves multiple beneficial functions:

1. Viscosity enhancement: The fine particles increase the viscosity of the paste, which improves the resistance to segregation and bleeding.
2. Lubrication: The stone powder particles act as ball bearings between the larger aggregate particles, reducing the internal friction and improving the flowability.
3. Filling effect: The fine particles fill the voids between the larger particles, leading to a denser microstructure and improved strength.

The balance between these beneficial effects and the potential drawbacks (increased water demand, reduced strength) is achieved through the careful selection of the polycarboxylate superplasticizer dosage and the mineral powder content.

The use of a relatively low binder content (535 kg/m3) is advantageous from both an economic and environmental perspective. A lower binder content reduces the cost of materials and the carbon footprint of the concrete. It also reduces the heat of hydration, which is important for preventing thermal cracking in mass concrete applications such as bridge piers.

Quality Control and Testing Requirements

The production of C60 self-compacting steel tube concrete requires rigorous quality control at every stage of the process. The following quality control measures are recommended:

The slump spread and T50 time are particularly important for self-compacting concrete because they directly relate to the ability of the concrete to flow into the steel tube and fill all voids without vibration. A slump spread greater than 650 mm ensures that the concrete can flow around any internal reinforcement or obstructions, while a T50 time less than 15 seconds ensures that the concrete does not lose its flowability before it has filled the entire tube.

Engineering Practice Implications

For engineers specifying C60 self-compacting steel tube concrete for bridge piers or similar applications, the following practical considerations should be noted:

1. Steel tube preparation: The interior of the steel tube should be cleaned and free of rust, oil, and debris before concrete placement. A thin layer of epoxy primer may be applied to enhance the bond between the concrete and the steel tube.
2. Placement method: The concrete should be placed from the bottom of the tube upward to avoid the formation of air voids. A tremie pipe or a flexible discharge pipe should be used to control the placement rate and prevent segregation.
3. Curing: The concrete should be cured for a minimum of 7 days under moist conditions to ensure adequate strength development. For high-performance concrete, extended curing for 14 to 28 days is recommended.
4. Temperature control: The ambient temperature during placement should be between 5 and 35 degrees Celsius. Extreme temperatures can affect the setting time and the fresh properties of the self-compacting concrete.

Key Questions and Reflections

One question that arises is how the long-term durability of the C60 self-compacting concrete performs in the aggressive environment of a bridge pier. The low binder content and the use of mineral powder may affect the permeability and the resistance to chloride ingress and sulfate attack. Long-term durability testing, including accelerated chloride diffusion and sulfate resistance tests, should be conducted to verify the long-term performance.

Another consideration is the effect of the stone powder content on the shrinkage and creep of the concrete. A higher stone powder content may increase the shrinkage due to the higher paste volume, which can affect the long-term dimensional stability of the structure. The expansive agent is intended to compensate for shrinkage, but its effectiveness over the long term should be verified.

Study Insights and Outlook

This study demonstrates that the use of machine-made sand with a high stone powder content is a viable and beneficial approach for the production of high-strength self-compacting concrete for steel tube applications. The achievement of a 28-day compressive strength exceeding 75 MPa with a relatively low binder content is a significant contribution to sustainable concrete technology. The three-component admixture system provides a robust framework for mix optimization, and the careful control of fresh and hardened properties ensures reliable performance in the field. Future research should focus on the long-term durability, the effect of different stone powder contents, and the applicability of this approach to other high-strength concrete grades. The integration of these findings into design codes and specifications for self-compacting steel tube concrete would facilitate wider adoption of this technology in bridge and infrastructure engineering.