How to solve the quality problem of cement silo in cement clinker production line
1、Project Overview
The cement silo of a cement clinker production line was completed and put into operation in 2007. The cement silo is a reinforced concrete circular silo structure with a total height of 61.81m, an inner diameter of 22.5m, and a silo wall of 380mm. Its reinforcement is all non-prestressed steel bars, and the concrete design strength grade is c30.
After the project was completed and put into use, the owner company’s personnel found during routine inspections that there were multiple vertical and circumferential cracks on the outer surface of the silo from +14.166 to +50m elevation, and some cracks were wider than 0.2mm, exceeding the specification requirements. At the circumferential cracks, it was also found that the concrete surface layer of the silo wall had different degrees of hollowing and the concrete protective layer fell off. For this reason, the owner company required technical reinforcement of cement silo.
2. Analysis of the cause of the accident
After discovering the cracking problem in the cement silo, the owner company commissioned a testing agency to conduct on-site testing of the silo wall concrete strength, annular reinforcement, cracks, etc. According to the analysis of the attached test results, the concrete strength of the silo wall basically meets the original design requirements; the area of the annular reinforcement in the silo wall is from +14.166 to +50.000m, which mostly cannot meet the original design requirements; the crack width is between +23.5 and +34.2m, which mostly does not meet the specification requirements.
After on-site inspection, the vertical and circumferential cracks on the silo wall were analyzed as follows: vertical cracks were caused by the combined effects of silo wall stress and temperature; circumferential cracks were caused by insufficient circumferential reinforcement area and unreasonable circumferential reinforcement joint connection structure. Since the raw material homogenization silo was constructed using the slipform process, the horizontal circumferential reinforcement joints were tied and overlapped, rather than welded or mechanically connected; and at the locations where concrete was hollow and detached, the reinforcement protective layer was too thin, and the bonding force and anchoring strength between the reinforcement and concrete were insufficient;
3. Reinforcement treatment（Scheme selection）
Based on the analysis of the current situation and causes of the accident in the raw material homogenization silo, two technical treatment schemes are initially considered: reinforcement of the outer wall of the cylinder with reinforced concrete and external prestressing reinforcement. The reinforcement effect and economic efficiency of the two methods are compared and analyzed as follows.
3.1.1 Use reinforced concrete to increase the cross-section of the cylinder wall
This method is to use 200mm thick reinforced concrete to increase the cross-section of the cylinder wall from the foundation ring beam to the +50m elevation on the outside of the cylinder. According to calculations, this method requires reverse mold construction, and the construction period is about 6 months. Considering that the cement silo is in operation during the reinforcement, the construction period is long. At the same time, there is a stress lag problem between the newly added part of the structure and the steel bars in the original silo wall, and the material efficiency cannot be fully exerted. In addition, it is estimated that the cost of the project is about 2 million yuan. Therefore, from the perspectives of reinforcement effect and economy, this method is not very good.
3.1.2 External prestressing reinforcement
Since the vertical and transverse cracks on the silo wall are mainly caused by insufficient area of circumferential reinforcement and temperature stress, a certain number of steel strands can be arranged along the circumferential direction on the outer surface of the silo wall and external prestressing can be applied to make the internal silo wall circumferential reinforcement and the external prestressed steel strands work together to achieve the reinforcement effect. At present, the silo is in use and operation, and the components are under stress. The use of external prestressed steel strands can largely ensure that the circumferential reinforcement and steel strands work together, which has a relatively obvious effect on structural reinforcement and may even close some cracks. It is estimated that the cost of the reinforcement project is about 1 million yuan.
Combining the above two schemes, considering the reinforcement effect, economy and actual situation on site, it is decided that the external prestressed reinforcement method for this cylinder is the most reasonable and economical.
