by The prefabricated concrete members need to be lifted, stacked, transported and other links for many times, and cracks, corners and other defects are easily produced in these processes; In addition, in the production process, due to unqualified raw materials, improper maintenance, insufficient vibration time and other reasons, some small 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 a repair plan, and repair the defects before the components leave the factory.
Taking cracks, bubbles and missing corners and edges as examples, the main causes of defects, prevention measures and repair methods are analyzed.
1 The main cause of defect formation The crack shall not be non penetrating crack, and the crack width shall not exceed 0.2mm.
Cause: unqualified raw materials such as cement, crushed stone and admixture; Improper mix proportion leads to excessive water cement ratio; When the component is demoulded or lifted, 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 inside and outside the component is too large, and the temperature rises and falls too fast.
Reasons for bubble formation: the measurement of coarse and fine aggregates and water during mixing is inaccurate, resulting 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 omitted, and the vibration time is insufficient; The spacing of reinforcement is uneven, and the local reinforcement is too dense, resulting in the separation of local concrete stones and mortar.
Analysis of the causes of missing corners and edges: the formwork shall be removed in advance before the strength is reached, and the strength of edges and corners is not enough; During demoulding or hoisting, it is impacted and squeezed by large external forces.
2.
Defect prevention measures Crack prevention measures: do a good job in the incoming inspection of raw materials; Strictly control the water cement ratio of concrete and the slump; During the demoulding and hoisting of components, avoid being impacted and squeezed by large external forces; During the curing process, check the temperature measuring probe and steam pipe once every half an hour 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 even mixing; Ensure sufficient vibration time, and the vibration table operator shall be controlled by a specially assigned person; Strengthen the process control and inspection of rebar binding on site, and construct in strict accordance with the drawings.
Preventive measures: ensure that the formwork removal strength of prefabricated components meets the specification requirements before formwork removal; The release agent shall be sprayed evenly, and there shall be no missing spraying or non spraying; During formwork removal and hoisting, the prefabricated components shall not be impacted and squeezed by large external forces.
3 Defect repair method and assessment standard repair procedure 1.
First, the quality inspector shall check the components after the mold on site, identify the components to be repaired, and judge the defect type.
2.
Clean the concrete residue at the base and use special repair materials (with the same strength grade as the concrete) for repair.
3.
After the repair materials are solidified, they shall be watered regularly for curing.
In case of cracking and peeling, it shall be smashed again and repaired again.
4.
The quality inspection personnel shall supervise the whole process and assess and record the repair quality according to the assessment standards.
Repair tools and repair materials Repair tools mainly include spatula, ash bucket, sponge, fine sandpaper, water drill, angle grinder, etc.
For cracks and bubbles, the repair material is the prefabricated component repair material which is composed of the mixture of butadiene and styrene copolymer lotion and cement (sand) to bond cement (sand) slurry.
The prefabricated component repair material has high bonding strength, fast setting time, good durability, non-toxic and harmless, no rust effect on steel bars, and is a green environmental protection product.
For missing edges and corners, the repair materials shall be fine aggregate concrete or prefabricated component concrete of the same grade.
Repair method 1.
Crack repair method: first, pretreat the concrete surface near the crack, remove the laitance, floating ash, oil stain and other objects on the base surface, remove the loose parts such as pimples and layers on the surface, and remove the loose stones such as honeycombs and pitted surfaces, and wash them with water, then fully soak the base concrete with water.
Then use a nylon brush to evenly brush the repair mortar to the repaired surface with a round brushing method, control the thickness of the coating, and cure it for at least 2 days after 2~4 hours of brushing, and cover it with moisture.
2.
Bubble repair method: first, clean the base concrete near the bubble with a brush, fully saturate the pores with water, and then repair with cement mortar.
After drying, polish the repair surface with fine sandpaper, so that the repair area and the nearby prefabricated components are integrated as a whole, reducing the color difference of repair.
3.
Repair method for missing edges and corners: clean the parts to be repaired, roughen if necessary, and then soak the base course with water.
Then paint a layer of cement mortar on the concrete surface, and erect the formwork for repair.
The repair material is fine aggregate concrete or prefabricated component concrete of the same grade.
Moisturizing and curing shall not be less than 7d after repair.
After drying, polish the repair surface with fine sandpaper to make the repair part and the prefabricated component level as a whole, and reduce the color difference between the repair part and the prefabricated component body.
4 Conclusion 1.
There are many reasons for appearance defects, such as air bubbles caused by incomplete concrete vibration, and holes in serious cases.
The prevention measures for defects shall be taken from the quality of raw materials, production process and the whole process of stacking, hoisting and transportation.
2.
The quality inspector shall inspect the components after being molded on the same day, judge the type of components after finding defects, and timely notify the repair workers to repair.
During the repair process, the quality inspector must supervise the whole process, and evaluate and record the repair quality according to the evaluation standards.
3.
Do not leave the site without handling the defects.
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..
