In-Depth Analysis of Circular Cracking in Concrete Utility Poles

In the production and application of concrete poles, circular cracking (commonly referred to as "ring cracking") is one of the most closely watched quality indicators in the industry. According to the national standard Circular Concrete Poles (GB4623-2025), for reinforced concrete poles, circular cracks with a width not exceeding 0.20 mm under the cracking test load are considered permissible, and a residual width of less than 0.05 mm after unloading is deemed acceptable. Nevertheless, even when the crack width falls within the standard tolerance, end users often raise concerns during actual acceptance inspections. A thorough investigation into the causes of circular cracking is not only a technical subject but also a manifestation of our commitment to quality.

Core Mechanism of Circular Cracking

The compressive strength of concrete far exceeds its tensile strength, with the latter typically being only about one-tenth of the former. When the circumferential tensile stress in a local area of the pole exceeds the concrete's own tensile limit, circular cracks develop. Combined with finite element analysis results, the circumferential tensile stress is relatively concentrated near the 2.4?meter height mark, making this zone particularly susceptible to circular cracking. This explains why such cracks predominantly appear in specific segments of the lower?middle portion of the pole.

From the perspective of the manufacturing process, the triggers for circular cracking are mainly concentrated in the following aspects:

Improper Prestressing Application: If the prestressing tension does not meet the process requirements, or if the prestress loss during application is excessive, the concrete will not receive sufficient precompressive stress to effectively counteract the tensile stresses induced by subsequent loads. Generally, the concrete precompressive stress should be controlled within 20% of the concrete grade. Conversely, if the prestress value is too high, the transverse tensile stress increases and can also exceed the concrete's tensile resistance, leading to cracks.

Substandard Demolding Strength: Inadequate control over concrete batching accuracy, centrifugal compaction density, and steam curing regimes (heating and holding times) can all result in insufficient demolding strength. If the pole is subjected to stress before reaching adequate demolding strength, irreversible structural circular cracking is highly likely to occur.

Non?standard Handling and Stacking: Collisions, rolling, swinging during demolding, lifting, and transportation, as well as incorrect positioning of lifting points and stacking supports, subject the pole to impact loads or localized stresses beyond its design values, thereby inducing circular cracks.