by Prefabricated components are the main components of prefabricated concrete buildings.
Common types of prefabricated components include exterior wall panels (“sandwich” structure), interior wall panels, composite panels, balcony panels, air-conditioning panels, stairs, partition panels, prefabricated beams, prefabricated columns, etc.
In the production of prefabricated components, due to various reasons, such as concrete mix proportion, cement quality, sand and gravel specifications, construction technology, steam curing process, process control, transportation mode and other factors, the quality of prefabricated components is less controllable, which will lead to various common quality defects, such as honeycomb, pockmarked surface, bubbles, missing edges and corners.
These common quality defects usually have little impact on structures and buildings, and are secondary quality defects, However, in projects with high appearance requirements (such as fair faced concrete projects), such problems will become the main problem; Quality problems such as out of tolerance flatness and geometric dimension deviation of components may not cause structural defects, but may affect building functions and construction efficiency; Common quality defects, such as cracks, insufficient strength and reinforcement cover problems, may affect the structural safety and are important quality defects.
In order to reduce the frequency of common quality problems of prefabricated components in production, inspection and other links, improve product quality and ensure project quality.
The common quality problems, cause analysis and pre control measures of prefabricated components are introduced below.
01 Honeycomb refers to a honeycomb like hole formed between bubbles or stones due to local looseness, less mortar and more stones in the concrete structure (see Figure 1).
Fig.
1 Honeycomb defect of prefabricated components is caused by improper concrete mix proportion or inaccurate measurement of sand, stone, cement and water, resulting in less mortar and more stones.
The sand and stone are poorly graded, with less sand and more stones.
The concrete mixing time is not enough, the mixing is uneven, and the workability is poor.
The mold gap is not blocked tightly, causing mortar leakage in the gap during pouring and vibration.
One time concrete pouring or unclear layering.
The concrete vibration time is short and the concrete is not dense.
Pre control measures: (1) Strictly control the concrete mix proportion to ensure accurate measurement, uniform concrete mixing and proper slump.
(2) Control the mixing time of concrete, which shall not be less than the specification.
(3) The die seam is tight.
(4) The concrete shall be poured in layers (when the height of the end face of the prefabricated component is greater than 300mm, the concrete shall be poured in layers, and the pouring height of each layer of concrete shall not exceed 300mm), and the concrete shall be vibrated in layers until the bubbles are removed.
(5) During concrete pouring, the mold shall be checked for mortar leakage and deformation at any time.
If there is mortar leakage and deformation, remedial measures shall be taken in time.
(6) The vibrating equipment shall be determined according to different concrete varieties, working performance, specifications and shapes of prefabricated components and other factors.
Before vibration, reasonable operation procedures for vibration molding shall be formulated.
02 Pitted surface refers to a rough surface formed by partial lack of grout or many small pits, pits, etc.
on the component surface (Fig.
2).
Fig.
2 The reasons for the pitted surface defects of prefabricated components are: the mold surface is rough or the impurities such as cement slurry residue are not cleaned up, and the concrete surface is damaged when the mold is removed; the mold cleaning and mold release agent painting process is improper, causing the water in the concrete to be absorbed by the mold, resulting in excessive water loss of the concrete and pitted surface; The joint of the mould is not tight, and the grout leaks locally; The mold isolating agent is applied unevenly, or is partially omitted or invalid, and the concrete surface is bonded with the formwork, causing pockmarks; The concrete is not vibrated compactly, and bubbles are not discharged, which stop on the surface of the formwork to form pits.
Pre control measures: (1) Before the production of components, the mold surface shall be cleaned to make the surface flat and smooth and ensure no rust.
(2) The contact surface between the mold and the concrete shall be coated with the isolating agent, which must be uniform in the process of painting, without missing or accumulating.
(3) The concrete shall be vibrated and compacted evenly in layers until bubbles are eliminated.
(4) Before pouring concrete, carefully check the firmness of the mold and whether the gap is blocked.
(5) Corresponding quality assurance measures shall be taken during outdoor production.
03 Hole means that the depth and length of the hole in the concrete exceed the thickness of the protective layer (Figure 3).
Fig.
3 The reasons for the hole defects of prefabricated components are: at the places with dense reinforcement or reserved holes and embedded parts, the concrete blanking is placed, and the upper layer of concrete is poured without vibration; Segregation on the concrete, mortar separation, stones in piles, serious mortar bleeding, and no vibration; The concrete is too much and too thick at one time, and the vibrator cannot vibrate, forming loose holes; Mud lump and other sundries fall into the concrete, and the concrete is stuck.
Pre control measures: (1) Fine aggregate concrete shall be used for pouring at places with dense reinforcement and complex parts.
(2) Carefully vibrate in layers to prevent missing vibration.
(3) The sand and gravel mixed with clay blocks, mold tools and other debris falling into the concrete shall be cleaned in time.
04 Bubble refers to that after the prefabricated component is demoulded, except for individual atmospheric bubbles, there are many small bubbles on the component surface, which are dense in sheet shape (Figure 4).
Fig.
4 The causes of bubble defects in prefabricated components are: unreasonable sand and gravel grading, too much coarse aggregate and too little fine aggregate; Improper aggregate size and excessive content of needle shaped particles; Concrete with large water consumption and high water cement ratio; Poor quality and effect of release agent or unsuitable release agent selected; It is related to insufficient and uneven vibration during concrete pouring; The pouring thickness often exceeds the requirements of the technical specifications.
Because the bubble travel is too long, even if the vibration time meets the requirements, the bubble cannot be completely discharged.
Pre control measures: (1) Strictly control the materials, control the aggregate size and the content of needle and flake particles, carefully screen the materials when preparing, and remove unqualified materials.
(2) Optimize the concrete mix proportion.
(3) The formwork shall be cleaned, and the release agent with good effect shall be selected and applied evenly.
(4) The pouring shall be carried out in layers, and the height of one feeding shall not exceed 300mm.
For the precast beams of long components, the crown block shall be commanded to move back and forth, and the materials shall be uniformly distributed..
