by In the construction of concrete components, due to improper construction technology or poor construction management, defects such as sand line, sand spot, pockmarked surface (exposed stone, bubble and sticky skin), lack of edge, corner and loose top often appear on the surface of components, which affects the durability and appearance quality of the project to a certain extent.
Combined with work practice, this section puts forward some prevention and control suggestions on how to eliminate and reduce this common quality problem to ensure project durability and apparent quality.
（1） Sand lines, sand spots, surface sand pattern defects caused by bleeding or slight slurry leakage on the concrete surface.
If the sand is not fully cemented by the cement slurry and is exposed, it can fall off by scraping with a wooden board.
Flakes (width greater than 10mm) are called sand spots, and linear ones are called sand lines.
1.
Causes: the bleeding rate of the selected cement is high, and the bleeding water is not discharged in time, so that the water accumulated on the surface flows down along the gap between the formwork and concrete to form sand lines and sand spots; Excessive sand ratio in mix proportion design or construction; The formwork is not spliced tightly, the grout stop is not solid, or the vibrator touches the formwork and leaks grout during vibration.
2.
For prevention and control measures, cement varieties with low bleeding rate shall be selected as far as possible; The sand ratio should not be too large during concrete trial mixing.
During construction, the material shall be cut in strict accordance with the mix proportion to control the sand content; Strictly control the content of stone powder in coarse aggregate (gravel); In case of bleeding, it shall be removed in time (sponge can be used to dry), especially to ensure that there is no ponding at the edge of the formwork; The mortar stop of formwork joint shall be dense, and there shall be no mortar leakage during concrete vibration, so as to avoid the vibrator from touching the formwork.
（2） The surface of pockmarked concrete is partially lack of slurry and many small pits and pits to form a rough surface, but there is no exposed reinforcement, mainly including commonly known defects such as “exposed stone”, “sticking skin”, “bubble” and so on.
1.
Causes: the surface of the formwork is rough and unclean, with dry and hard cement slurry and other sundries, uneven application of release agent or improper selection of release agent, and pitted surface is caused by bonding the formwork on the concrete surface during formwork removal; The formwork joint is not tight, the grout stop strip is not replaced in time, and the grout stop is not solid, resulting in local grout leakage during concrete pouring; The concrete vibration is not dense, and the air bubbles cannot be discharged due to missing vibration, and some air bubbles remain on the formwork surface to form pits; During concrete pouring, the layer thickness is not well controlled, and the blanking height of each layer is too high, which makes it impossible to discharge bubbles to the greatest extent during vibration, especially when encountering the inclined plane.
During blanking, the concrete plane is often much higher than the top of the inclined plane.
Under the action of vibration force, the residual gas in the material is squeezed and rose, dissociated to the inclined plane of the formwork, blocked and then collected into a pile, resulting in the formation of a large number of bubbles.
2.
Prevention and control measures: the formwork surface shall be cleaned, and the release agent shall be painted evenly without omission; The mortar stop of formwork joint shall be tight without mortar leakage; During concrete construction, blanking shall be carried out in layers.
The layer thickness shall not be too large (generally not more than 40cm) and shall be vibrated and compacted layer by layer to prevent missing vibration, and holes shall be opened at appropriate parts to fully eliminate bubbles.
（3） Honeycomb, cavity and exposed reinforcement concrete surface have no slurry, and the exposed stone depth is greater than 5mm but not greater than the protective layer or 50mm, which is called honeycomb; Caves and serious honeycombs with a depth greater than the protective layer or 50mm are called cavities; The reinforcement inside the concrete is not wrapped by the concrete and exposed on the surface, which is called exposed reinforcement.
1.
The main causes of exposed reinforcement on the concrete surface are unreasonable setting of reinforcement cushion block and unstable binding and fixation of cushion block, resulting in displacement of cushion block during concrete vibration, reinforcement clinging to the formwork, and exposed reinforcement after formwork removal; Or because the reinforcement of the concrete section is too dense, the large aggregate cannot be wrapped by mortar, stuck on the reinforcement, and the cement slurry cannot fill around the reinforcement, resulting in exposed reinforcement at the dense part of the reinforcement.
For the exposed reinforcement, first clean up the concrete slag and rust on the exposed reinforcement, then rinse with water, use 1:2 ~ 2.5 cement mortar and add an appropriate amount of 108 glue for plastering and leveling; If the exposed reinforcement is deep, all the weak concrete shall be chiseled off, washed clean and wetted, tamped with fine aggregate concrete with a higher strength grade, and covered with film for curing 2 Pitted surface refers to the irregular small pits on the concrete surface the size of countless mung beans.
The diameter is usually not greater than 5mm.
The main reason is the poor workability of concrete.
After concrete pouring, there are less mortar and more stones in some places, forming honeycombs; Or honeycomb and pitted surface are caused by poor vibration quality or missing vibration after concrete is put into the formwork, and bubbles are not completely discharged.
Only by controlling the concrete mixing quality and vibrating according to the specification requirements can the generation of pitted surface be effectively controlled.
The pockmarks on the concrete surface have no great impact on the structure and are usually not treated.
If treatment is required, 1:2 ~ 2.5 cement mortar can be used.
If necessary, a certain proportion of white cement can be mixed to color or 108 glue can be added to enhance the bonding force; Then press the mortar into the pitting with a scraper, and then scrape it flat; After the repair, cover it with gunny bags or plastic cloth for moisturizing and curing.
Honeycomb refers to that there is no slurry on the concrete surface and there are voids between aggregates, forming more or less 3 Honeycomb hole, the size is like honeycomb, the shape is irregular, the exposed stone depth is greater than 5mm, the depth does not expose the main reinforcement, and the stirrup may be exposed.
The main cause is the serious slurry leakage of the formwork; The slump of concrete is too small, which is formed by under vibration or missing vibration; The concrete mixing and vibration are insufficient, resulting in uneven and non dense concrete.
The mixing time of concrete can be extended, and the layered thickness of concrete shall not exceed 30cm.
The vibration workers must carefully vibrate according to the vibration requirements, especially strengthen the vibration of formwork corners and joint parts, which can effectively control the generation of honeycombs.
The repair method can refer to surface treatment method 4 Missing mausoleum and angle 1 Causes: edges and corners are knocked off due to improper operation and impact during formwork removal, or due to improper operation during hoisting; Premature disassembly; The edges and corners are not vibrated tightly, or there are more mortar and less stones, resulting in corner falling due to low strength..
