The core mold vibration process involves the internal and external molds being vertically positioned on the bottom tray within the pit. The dry, hardened concrete of the pipe mold is subjected to the strong vibrational force generated by the high-frequency vibrator of the internal mold, liquefying the concrete mixture, filling the pipe mold, expelling air, and becoming increasingly compact. The top of the core mold vibrated pipe is fitted with a shaping ring, which gently rubs and adheres tightly together under liquid pressure. The core mold vibration process creates pipes by tightly forming the concrete under strong impact forces.

In core mold vibrated concrete pipes, annular cracks easily appear on both the inner and outer walls within 200-300 mm of the socket end. Additionally, annular or longitudinal cracks may appear in the straight sections of the pipe. Annular cracking at the pipe ends significantly reduces the allowable normal force of the pipe in pipe jacking projects, easily causing engineering accidents and posing a hidden danger. If the concrete settles during the ground-level process, the settlement ring will crack, with even spacing similar to the reinforcement spacing, and severe internal and external cracks will appear.

Main Causes of Cracks

A. Steel Reinforcement Cage Does Not Meet Specifications:

The overall stiffness of the steel reinforcement cage is poor, with brazing or debonding issues. The connection between the double layers of reinforcement is not secure, and the ends of the cage are not tight. When the socket is pressed during shaping, the pressure transmits force through the aggregate to form the steel cage. After the pressure is released, the circumferential tendons partially rebound, causing annular cracks near the insertion point.

The vertical tendons have varying lengths, and the ends of the cage are uneven. When the shaping socket is pressed, the force on the longitudinal reinforcement is uneven. After the pressure is released, the elastic action of the steel reinforcement causes it to reset, resulting in annular cracks of varying lengths in the socket due to the uneven distribution of stress.

The bearing surface of the cage is uneven and not perpendicular to the axis of the cage, resulting in deviations in the concrete cover on the upper and lower parts of the pipe, making the pipe prone to bending or cracking.

The longitudinal reinforcement diameter is too small. Under the required pressing pressure, the entire steel reinforcement cage cannot withstand the pressure, causing the cage to deform and settle, resulting in damage or even collapse of the pipe.

The dimensions of the steel reinforcement cage are inaccurate, with ovality and uneven cover thickness. In some areas, the cover is too thin. When the socket is pressed, the pressure is transmitted through the aggregate to the reinforcement. After the pressure is released, the circumferential area partially rebounds, causing annular cracks or partial cracking near the socket. Currently, the diameter of the steel reinforcement cage is adjusted to an appropriate size relative to the thickness of the concrete cover of the pipe.

B. When removing the internal or external molds, the lifting centerline does not match the centerline of the internal and external molds, leading to tilting and collision.

C. Uneven ground in the stripping area and a non-horizontal base result in uneven spacing around the pipe body, causing the pipe to be subjected to frictional and horizontal forces. The formwork pulls on the pipe body, creating cracks.

When removing the formwork, if the lifting process is not done at a consistent speed and there are pauses, the increased frictional resistance can easily lead to cracks in the stationary areas.

D. Using a single hoist for demolding, the arrangement of the steel wire rope causes displacement of the center when demolding starts from the formwork and moves upwards towards the socket, resulting in cracks or damage to the nozzle at the formwork entrance.

E. If the roller drum sides are not smooth and the inner and outer formwork taper is not controlled within 3%, the frictional resistance during pulling is high, easily leading to jamming or cracking.

F. Local bending of the steel mold often leads to cracks in the affected areas.

G. Large horizontal deviations in the forming machine frame and vertical deviations in the installation of the core mold and external mold prevent the maintenance of verticality.

H. Short steam curing time and rapid heating speed lead to a large temperature difference between the surface of the socket or the outer surface of the pipe and the inner wall of the pipe.

I. Unreasonable concrete mix ratio. Excessive water content or water-cement ratio in the concrete mixture, insufficient bonding strength, and large aggregate diameter.

In summary, to eliminate and reduce cracks, core mold vibration achieves this by controlling the entire production process of the pipe.