Post-Fire Mechanical Properties of Steel Fiber Light Aggregate Steel Tube Concrete Short Columns

Literature Overview and Research Significance

The study by Wang Xintang, Yin Jie, and Zhou Ming, published in the Journal of Natural Disasters (2013, Vol. 22, No. 1, pp. 198–206), investigates the post-fire mechanical performance of steel fiber reinforced light aggregate concrete (LAC) filled steel tube short columns. Funded by the National Natural Science Foundation of China (51078187) and the Zhejiang Provincial Public Welfare Technology Application Research Project (20212C23036), this research was conducted at Ningbo University and Zhejiang Tianlu Engineering Design Co., Ltd.

Light aggregate concrete is widely used in applications requiring reduced structural weight, such as high-rise buildings, bridges, and offshore platforms. However, LAC typically exhibits lower strength and stiffness compared to normal-weight concrete, and its performance under fire conditions is of particular concern. The addition of steel fibers is known to improve crack resistance and post-peak ductility, but the combined effect of steel fibers, light aggregate, and fire exposure on steel tube composite columns has not been adequately studied.

Experimental Program and Test Configuration

The experimental program included 15 fire-exposed specimens and 3 unexposed control specimens, all of which were steel fiber reinforced LAC filled circular steel tube short columns. The specimens were subjected to different fire conditions, with exposure temperatures reaching up to 700°C, which corresponds to the peak temperature of a standard fire curve (ISO 834).

Specimen Parameters

The steel fiber dosage was varied at four levels: 0 kg/m³ (control without fibers), 39 kg/m³, 78 kg/m³, and 117 kg/m³. These dosages represent different fiber volume fractions, with 39 kg/m³ corresponding to approximately 0.5% by volume for typical steel fibers with density around 7800 kg/m³.

Key Findings and Performance Analysis

Effect of Steel Fiber Dosage on Post-Fire Bearing Capacity

The most significant finding is that the addition of steel fibers improves the post-fire bearing capacity of the steel tube LAC columns, but the improvement is not monotonically increasing with fiber dosage. The optimal dosage was found to be 39 kg/m³, which provided an 11% improvement in post-fire bearing capacity compared to the fiber-free specimen when exposed to 700°C. Beyond this dosage, increasing the fiber content led to a decrease in post-fire bearing capacity.

This non-monotonic behavior can be explained by the following mechanisms:

• At low to moderate dosages (0–39 kg/m³), steel fibers bridge microcracks in the LAC, maintaining structural integrity and load transfer capacity even after thermal degradation of the concrete matrix.
• At higher dosages (78–117 kg/m³), the fibers may create localized stress concentrations and reduce workability, leading to imperfect compaction and void formation. After fire exposure, these voids and the associated weakened zones become critical failure sites.
• The thermal expansion mismatch between steel fibers and the light aggregate matrix can induce additional microcracking at elevated temperatures, particularly when fiber volume fraction is high.

Effect of Steel Fiber Content on Room Temperature Performance

At room temperature (unexposed specimens), steel fibers also improve the bearing capacity of the steel tube LAC columns. The improvement is attributed to the crack-bridging effect of fibers, which enhances the post-cracking tensile strength and ductility of the LAC core. This improved ductility allows the concrete to sustain higher compressive stresses under confinement by the steel tube.

Failure Modes

The failure modes of the specimens varied with steel fiber dosage and fire exposure:

• Fiber-free specimens exhibited brittle failure with sudden concrete crushing and steel tube local buckling.
• Specimens with 39 kg/m³ fibers showed improved ductility with gradual concrete crushing and progressive steel tube deformation.
• Specimens with higher fiber dosages (78–117 kg/m³) showed mixed behavior, with some specimens exhibiting reduced ductility due to workability-related defects.

Engineering Practice Implications

From a pipe welding and structural integrity perspective, the post-fire performance of CSTC columns is critical for fire safety design. The steel tube provides passive fire protection to the concrete core, but the tube itself also degrades at high temperatures. The yield strength of structural steel typically retains about 50% of its room temperature value at 600°C, and the modulus of elasticity degrades even more rapidly.

The finding that 39 kg/m³ is the optimal steel fiber dosage has direct implications for mix design specifications. Engineers should avoid the temptation to use higher fiber dosages assuming "more is better," as the experimental evidence clearly shows diminishing and then negative returns. The optimal dosage should be verified through trial mixes and workability tests specific to the aggregate type and cementitious system used.

For welding quality control, the steel tube material must maintain adequate mechanical properties after fire exposure. Post-fire inspection should include:

• Visual examination for excessive oxidation, scaling, or discoloration.
• Ultrasonic testing for cracks or internal defects induced by thermal cycling.
• Hardness testing to assess tempering effects on the steel tube material.
• Dimensional inspection for distortion or warping.

Study Insights and Independent Reflection

The research addresses a genuinely important gap in the knowledge of post-fire behavior of lightweight composite columns. The identification of an optimal steel fiber dosage that is not the maximum tested is a valuable finding that challenges common assumptions in fiber-reinforced concrete design. The practical implication is that mix design optimization for fire-resistant applications requires experimental validation rather than simple extrapolation from room temperature performance.

However, several limitations should be acknowledged. The study focuses on short columns under axial compression, which is the most favorable loading condition. Real structures may experience eccentric loading, shear, or combined loading after fire, which could alter the relative effectiveness of steel fibers. Additionally, the study does not address the residual deformation and deflection of columns after fire cooling, which is critical for assessing structural usability and repair feasibility.

The fire exposure conditions in the study should be carefully evaluated. A uniform 700°C exposure is a simplification compared to real fire scenarios where temperature gradients develop across the column cross-section. The thermal gradient between the outer steel tube surface and the inner concrete core can induce thermal stresses that affect the post-fire mechanical properties.

Reference Value and Outlook

This paper provides valuable experimental data on the post-fire behavior of steel fiber reinforced LAC filled steel tube columns, with the key finding that moderate fiber dosage (39 kg/m³) optimizes post-fire bearing capacity. The research supports the use of steel fiber LAC in fire-prone applications, provided that the fiber dosage is carefully optimized and verified through testing. Future research should extend to eccentric loading conditions, longer columns, and realistic fire curves with thermal gradients. Additionally, studies on the repair and rehabilitation of post-fire CSTC columns would be highly beneficial for practical engineering applications. For pipe manufacturers, the research underscores the importance of material consistency and the need for post-fire inspection protocols that account for the composite nature of the structure.