by Precast concrete components need to be lifted, stacked and transported for many times, and cracks, corners and other defects are easy to occur in these processes; In addition, in the production process, due to unqualified raw materials, improper maintenance, insufficient vibration time and other reasons, some minor surface bubbles, looseness and other defects will also occur.
In order to ensure the appearance quality of prefabricated components, it is necessary to find out the causes of defects, prepare repair plans, and repair the defects before the components leave the factory.
Taking the three common defects of cracks, bubbles and missing corners and edges as examples, the main causes of defects, prevention measures and repair methods are analyzed.
1 main cause of defect formation crack crack shall not be non penetrating crack, and the crack width shall not exceed 0.2mm.
Cause of formation: raw materials such as cement, crushed stone and admixture are unqualified; Improper mix proportion leads to excessive water cement ratio; During demoulding or lifting of components, the concrete strength does not reach the specified value or is impacted and squeezed by large external forces; During the curing process, the temperature difference between the inside and outside of the component is too large, and the temperature rises and falls too fast.
Reasons for bubble formation: inaccurate measurement of coarse and fine aggregates and water during mixing results in less mortar and more stones; The concrete mixing time is not enough, the concrete is not mixed evenly, and the workability is poor; The concrete vibration is not dense, or the vibration is missed, and the vibration time is insufficient; The spacing of reinforcement is not uniform, and the local reinforcement is too dense, resulting in the separation of local concrete stones and mortar.
Cause analysis of missing corners and edges: the formwork is removed in advance before the strength is reached, and the strength of edges and corners is not enough; During demoulding or lifting, it is impacted and squeezed by large external force.
2 defect prevention and control measures crack prevention and control measures: do a good job in raw material mobilization inspection; Strictly control concrete water cement ratio and slump; In the process of demoulding and lifting of components, avoid collision and extrusion by large external forces; Check the temperature measuring probe and steam pipe every half an hour during the curing process to ensure that the temperature rise and fall speed is not too fast.
Bubble prevention measures: the measurement is accurate and the error is within the allowable range; Ensure sufficient concrete mixing time and uniform mixing; Ensure sufficient vibration time, and the vibration table operator shall be controlled by special personnel; Strengthen the on-site process control and inspection of reinforcement binding, and construct in strict accordance with the drawing.
Preventive measures: the formwork can be removed only after the formwork removal strength of prefabricated components meets the specification requirements; The release agent shall be sprayed evenly, and there shall be no missing or non spraying; In the process of formwork removal and lifting, prefabricated components shall be protected from collision and extrusion by large external force.
3.
Defect repair method and evaluation standard repair procedure 1.
First, the quality inspector shall inspect the components after the mold, identify the components to be repaired, and judge the defect type.
2.
Clean up the concrete residue at the base and use special repair materials (strength grade is the same as that of concrete).
3.
After the repair materials are solidified, water them regularly for curing.
If cracking and falling off are found, smash it again and repair it again.
4.
The quality inspection personnel shall supervise the whole process and evaluate and record the repair quality according to the evaluation standard.
Repair tools and materials repair tools mainly include spatula, ash bucket bucket, sponge, fine sand paper, water drill, angle grinder, etc.
For cracks and bubbles, the prefabricated component repair material composed of cement (sand) slurry bonded by butadiene styrene copolymer lotion and cement (sand) is used as the repair material.
The prefabricated component repair material has high bonding strength, fast setting time, good durability, non-toxic and harmless, and has no rust effect on steel bars.
It is a green environmental protection product.
For missing edges and corners, the repair materials shall be fine aggregate concrete or precast component concrete of the same grade.
Repair method 1.
Crack repair method: first, the concrete surface near the crack shall be pretreated to remove laitance, ash, oil stains and other objects on the base surface, remove loose parts such as bumps and layers on the surface and loose stones such as honeycombs and pockmarks, wash them with water, and then fully soak the base concrete with water.
The repair mortar shall be uniformly applied to the repaired surface by circular brushing method with nylon brush, and the thickness of the coating shall be controlled.
It shall be cured for no less than 2D after 2-4h brushing, and covered with moisture.
2.
Bubble repair method: first clean the base concrete near the bubble with a brush, fully saturate the pores with water, then repair with cement mortar.
After drying, grind the repair surface with fine sand paper to make the repaired part and the nearby prefabricated components smooth and integrated, and reduce the repair color difference.
3.
Repair method of missing edges and corners: clean the parts to be repaired, chisel if necessary, and then soak the base with water.
Then, apply a layer of cement mortar on the concrete surface, erect formwork for repair, and use fine aggregate concrete or precast component concrete of the same grade for repair.
After repair, moisturizing curing shall not be less than 7d.
After drying, grind the repair surface with fine sand paper to make the repaired part and the prefabricated component become a whole and reduce the color difference between the repaired part and the prefabricated component body.
4 Conclusion 1.
There are many reasons for appearance defects, such as air bubbles caused by uncompacted concrete vibration, and holes in serious cases.
The prevention and control measures for defects shall be controlled from the quality of raw materials, the production process and the whole process of later stacking, hoisting and transportation.
2.
The quality inspector shall inspect the components that have been molded on the same day, judge the type of the components after finding the defects, and timely notify the repair workers to repair them.
The quality inspectors must supervise the whole process of the repair process, and evaluate and record the repair quality according to the evaluation standards.
3.
Defects shall not be removed from the site without treatment.
Once serious defects such as large cracks and holes are found, the prefabricated components shall be scrapped, and the unqualified report shall be filled in for filing, so as to be well documented..
