Vacuum-Assisted Concrete Filling Process Test for CFST Arch Segments

Literature Overview

This study by Zheng Jialian, Wang Jianjun, Feng Zhi, Han Yu, and Qin Dayan, published in the "China Journal of Highway and Transport" (2014, Vol. 27, No. 6), presents a process test for vacuum-assisted concrete filling of concrete-filled steel tube (CFST) arch segments. The research was conducted in collaboration with the Guangxi Zhuang Autonomous Region Department of Transportation and the Guangxi Zhuang Autonomous Region Highway Bridge Engineering General Corporation, funded by the Guangxi Transportation Science and Technology Project (2011-17). The test was conducted near the Hejiang Yangtze River Bridge site, using two arch segments with 50 m span and 660 mm tube diameter.

Core Technical Approach

Vacuum-Assisted Filling Process

The vacuum-assisted concrete filling method involves:

Sealing both ends of the steel tube segment
Creating a vacuum inside the tube through a vacuum pump connected to a dedicated port
Pouring concrete through a filling port while maintaining vacuum pressure
The negative pressure draws concrete into the tube, reducing the risk of air entrapment
Releasing the vacuum after complete filling

Test Configuration

Parameter	Specification
Number of test segments	2
Segment span	50 m
Steel tube diameter	660 mm
Test location	Near Hejiang Yangtze River Bridge site
Environmental conditions	Natural (outdoor)
Inspection ages	6, 17, 21, 28, 56 days
Inspection methods	Ultrasonic testing + core drilling
Inspection locations	Key cross-sections + arch crown area

Quality Assessment Results

Ultrasonic Testing Results

Inspection Age	Wave Velocity (m/s)	Void Ratio	Assessment
6 days	Higher than conventional	Smaller	Good initial quality
17 days	Higher than conventional	Smaller	Stable improvement
21 days	Higher than conventional	Smaller	Consistent results
28 days	Higher than conventional	Smaller	Standard age verification
56 days	Higher than conventional	Smaller	Long-term stability confirmed

Core Drilling Investigation

Core samples taken from the arch crown area confirmed:

High concrete density with minimal voids
Uniform concrete quality throughout the section
Good bond between concrete and steel tube inner surface
No evidence of segregation or honeycombing

Technical Interpretation for Steel Pipe and Welding Engineers

Relevance to Steel Tube Quality

The vacuum-assisted filling process is particularly sensitive to steel tube quality:

Internal surface condition: Surface irregularities, weld beads, or coatings inside the tube can trap air and create voids even with vacuum assistance. Internal surface preparation is therefore critical.
Weld seam protrusion: For longitudinally welded tubes, the internal weld bead creates a geometric discontinuity that can serve as a void nucleation site. The following welding quality requirements are essential:

Welding Parameter	Requirement	Rationale
Internal weld bead height	≤1.0 mm	Minimizes air entrapment
Weld surface smoothness	Ra ≤ 12.5 μm	Ensures uniform concrete flow
Weld fusion quality	100% full penetration	Prevents voids behind weld
Post-weld grinding	Internal flush grinding	Eliminates geometric discontinuities

Tube roundness: Out-of-roundness creates areas of varying cross-section that affect vacuum distribution and concrete filling uniformity. The maximum allowable out-of-roundness should be limited to 0.3% of nominal diameter for vacuum-assisted filling applications.
End sealability: The tube ends must be capable of forming a reliable seal for vacuum application. Welded end plates or mechanical seals require careful design to prevent air leakage.

Comparison with Conventional Filling Methods

Aspect	Vacuum-Assisted	Conventional (Gravity/Vibration)
Void content	Significantly lower	Higher, especially in horizontal sections
Wave velocity	Higher	Lower
Filling uniformity	More uniform	Less uniform, segregation possible
Labor intensity	Moderate (vacuum equipment needed)	Higher (vibration, monitoring)
Equipment cost	Higher initial investment	Lower
Quality consistency	Higher	Lower
Applicability	Horizontal and inclined tubes	Best for vertical tubes

Process Optimization Recommendations

Based on the study findings and engineering experience, the following optimization measures are recommended:

Vacuum level: Maintain vacuum pressure of -80 to -95 kPa during filling for optimal air removal
Concrete mix design: Use low-viscosity, high-flowability concrete with slump flow ≥650 mm to facilitate filling under vacuum
Filling rate: Control concrete delivery rate to match vacuum pump capacity, preventing pressure fluctuations
Temperature control: Maintain concrete temperature between 15–25°C to optimize flow properties and prevent premature setting
Sequential filling: For long segments, use multiple filling ports with sequential activation to ensure complete filling from both ends

Quality Control Framework

Pre-Filling Inspection Checklist

Verify steel tube dimensional accuracy (diameter, wall thickness, roundness)
Inspect internal weld quality and surface condition
Test vacuum system integrity (leak rate <1 kPa/min)
Confirm concrete mix properties (slump flow, air content, temperature)
Check end seals and filling port connections

Post-Filling Verification Protocol

Immediate: Visual inspection of filling ports for overflow confirmation
24 hours: External tapping test to identify obvious voids
6 days: First ultrasonic testing for early quality assessment
28 days: Comprehensive ultrasonic testing at multiple cross-sections
56 days: Core drilling at critical locations for definitive quality verification

Engineering Practice Implications

The study was conducted in the context of large-span CFST arch bridge construction, where the quality of concrete filling directly affects:

Structural load-bearing capacity
Long-term durability
Service life prediction
Maintenance requirements

The Hejiang Yangtze River Bridge project demonstrated that vacuum-assisted filling provides:

Superior quality compared to conventional methods
Improved quality consistency across multiple segments
Reduced risk of hidden defects that may not be detected until loading
Better economic performance despite higher equipment costs

Steel Pipe Supplier Requirements

For steel tube suppliers providing tubes for vacuum-assisted filling applications, the following additional requirements should be incorporated into specifications:

Internal surface roughness: Ra ≤ 25 μm (mechanically polished or naturally smooth from seamless manufacturing)
Maximum internal weld bead: ≤0.5 mm after grinding (for welded tubes)
Roundness tolerance: ≤0.3% of nominal diameter
Length tolerance: ±2 mm per segment
Surface treatment: Clean, dry, free from oil, rust, and contaminants
Packaging: End-protected to prevent deformation during transport and storage

Study Insights and Outlook

The vacuum-assisted concrete filling process represents a significant advancement in CFST construction technology, particularly for large-span arch bridges where quality consistency is paramount. The study's multi-age inspection protocol (6, 17, 21, 28, 56 days) provides valuable data on the time-dependent quality development, confirming that the quality advantages of vacuum-assisted filling are maintained throughout the concrete curing period. The combination of ultrasonic testing and core drilling provides a comprehensive quality verification approach that should be adopted as standard practice. For the steel pipe manufacturing industry, this research highlights the importance of internal surface quality and dimensional precision, suggesting that future specifications for CFST tubes should include more stringent internal quality requirements to fully realize the benefits of advanced filling technologies.