How to prevent eight common appearance quality defects of concrete structures. Summarize the reference!

The appearance quality defects of cast-in-place concrete structure will affect the perception, function, structural performance and service life of the project, and must be effectively prevented. This paper summarizes eight common appearance quality defects of cast-in-place concrete structures, analyzes the reasons, and gives preventive and treatment measures for reference!

1

stock

Honeycomb is mainly characterized by local concrete looseness, less mortar, more gravel, and gaps between gravel, forming honeycomb holes.

cause analysis

Small slump of concrete, improper mix proportion or inaccurate measurement of raw materials lead to less mortar, more stones, insufficient vibrating time or vibration leakage.

Insufficient concrete mixing and poor uniformity.

When the height of free fall exceeds the specified value, concrete segregates and gravel accumulates.

Template slurry leakage or excessive vibration.

The vibration time is insufficient, and the bubbles are not eliminated.

preventive measure

Strictly control the mixture ratio, strictly measure and check frequently.

Concrete mixing should be sufficient and uniform.

When the feeding height is more than 2m, a tandem drum or chute should be used.

Layered blanking, layered tamping, prevent vibration leakage.

Close the gap of formwork, and check and correct the slurry leakage at any time during pouring.

Treatment measures

For small honeycomb, after cleaning, use 1:2 cement mortar for leveling and compaction.

For larger honeycombs, the weak loose particles are chiseled away and cleaned, and then the formwork is formed and carefully filled with high-strength fine stone concrete.

The deep honeycomb can be buried with grouting pipe and exhaust pipe, and the surface can be plastered with mortar or poured with concrete, and then sealed and grouted with cement.

2

pitted skin

The pockmarked surface mainly shows that the air is not clean after concrete pouring, or there are pits on the concrete surface, and the surface is not smooth and uneven due to concrete leakage. There are countless small pits on the surface of the component, and no steel bars are exposed.

cause analysis

The template surface is rough or debris such as cement slurry residue is not cleaned up, which damages the concrete surface when the template is removed.

The water at the end of the formwork is wet or not wet enough, which absorbs the moisture of the concrete on the surface of the component, so that the concrete loses too much water and produces pits.

Uneven coating of template isolation agent, or partial brush leakage or failure, leads to pitting surface and template bonding on concrete surface.

Template flat-fell seam is not tight, local slurry leakage.

The concrete is vibrated falsely, and the bubbles are not discharged, so it stays on the surface of the formwork to form pits.

preventive measure

Template surface should be clean, can not have hard cement mortar and other sundry.

Before pouring concrete, formwork joints should be watered and fully wetted.

Template isolation agent should be evenly coated, and brush leakage is not allowed.

Template gap should be sealed with packing paper or putty.

The concrete shall be vibrated evenly and compactly in layers, and air bubbles shall be discharged by knocking the outside of the formwork with a wooden hammer.

Treatment measures

If the surface is painted, it should not be treated. If the surface is not painted, it should be partially watered and fully wetted, and the stone slag mortar should be removed with the original concrete mixture ratio, and the surface should be smoothed and calendered.

three

hole

Hole means that there is a big gap in the concrete structure, and there is no concrete or honeycomb, and the steel bar is partially or completely exposed.

cause analysis

In the dense parts of steel bars or reserved holes and embedded parts, the concrete is not cut smoothly, and the upper concrete will continue to be poured after vibrating.

Concrete segregation, mortar segregation, stone accumulation, serious slurry leakage and poor tamping.

Tools, wood blocks, mud blocks and other sundries fall into the concrete, and the concrete is stuck.

preventive measure

High-strength fine stone concrete should be used in the places with dense steel bars, and it should be carefully compacted by layers or manually compacted.

Where there are reserved holes, the materials shall be discharged from both sides at the same time and vibrated seriously.

Keep clear of sundries falling into concrete in time.

Treatment measures

Chisel away the loose concrete and weak cement slurry around the hole, rinse it with pressure water, support the formwork with a supporting box, sprinkle water to fully wet it, and then sprinkle water with 8% expansion agent mixed in high-strength fine stone concrete for tamping.

Remove pumice from the concrete surface treatment, and check whether the concrete surface after roughening is dense and whether there are obvious gaps. If there are any cracks, chisel them out and rinse them with water, and then brush a layer of cement slurry on the interface between new and old concrete to better strengthen the bonding between the new and old concrete surfaces.

Formwork support. Depending on the surface depth of reinforcement, when the surface depth of reinforcement is less than 5㎝, and the surface shape of reinforcement is V-shaped, or only the surface other than the main reinforcement has pits, formwork support is not needed. When the depth of reinforcement exceeds 5㎝ and the width inside and outside is the same, it is difficult to ensure its compactness without formwork support. The formwork must be supported. When supporting the formwork, a hole should be opened at the top, and the width of the hole should not be less than 5㎝, so that the concrete can flow smoothly and the vibrator can be inserted easily.

Pour concrete. Check the design strength of reinforced concrete. When strengthening, the concrete with one grade higher than the design strength of this part is reinforced with a certain amount of expansion agent, and the concrete will be vibrated and compacted. When the depth is less than 5㎝, the high-grade fine stone concrete is strengthened with 8% expansion agent; When the depth is greater than 5㎝, the high-grade concrete is strengthened with 8% expansion agent; When the surface is only pitted, and there is no reinforcement exposed (when the surface is partially pitted due to premature formwork removal), the cement mortar of 1:2.5 is mixed with 8. Because the depth of the above parts is not less than 5㎝, 8% expansion agent is added to the high-grade fine stone concrete for reinforcement, and some parts have pits on the surface due to premature formwork removal, and 8% expansion agent is added to the 1:2.5 cement mortar for reinforcement.

Maintenance. After the surface strength reaches 12 hours, it shall be cured by spraying water for not less than 5 days.

four

Expose tendon

In the process of concrete pouring, the vibration is not in place, the protective layer pad is not set or fixed firmly, the concrete slump is small, the formwork is removed too early, the external force is destroyed before hardening, and the steel bar is exposed after molding.

cause analysis

When pouring concrete, the protective cushion of steel bar is shifted, or the cushion is too little or leaked, so that the steel bar is exposed close to the template.

The cross section of structural members is small, the steel bars are too dense, stones stick to the steel bars, and cement mortar can't fill the steel bars, resulting in exposed steel bars.

Improper concrete mix proportion leads to segregation, slurry leakage near the formwork or slurry leakage of the formwork.

The concrete protective layer is too small or the protection place is mixed with leakage vibration or false vibration, or the vibrating rod knocks or tramples on the steel bar, so that the steel bar is exposed due to displacement.

The end of the wooden formwork is affected with damp, and it absorbs water, bonds or demoulds prematurely, and the edges and corners are missing when the formwork is removed, resulting in exposed tendons.

preventive measure

When pouring concrete, ensure that the position of reinforcement and the thickness of protective layer are correct.

Strengthen inspection; When the steel bars are dense, stones with appropriate particle size should be selected to ensure the accuracy and good workability of concrete mixture ratio; If the watering height exceeds 2 meters, a string of buckets or chutes should be used to prevent separation.

Template should be fully wet and carefully plug the gap; When vibrating concrete, it is forbidden to hit the steel bar. In places with dense steel bars, you can use blades or vibrators to vibrate.

When operating, avoid stepping on the steel bar, and straighten and correct in time if there is any bending or tripping; The protective layer concrete shall be vibrated and compacted; Grasp the demoulding time correctly to prevent premature demoulding and damage to edges and corners.

Treatment measures

After the exposed steel bars on the surface are cleaned, apply 1:2 cement mortar on the surface. If the exposed reinforcement is deep, the interface should be properly handled and compacted with high-grade fine stone concrete.

five

Rotting root

Mainly due to the template flat-fell seam is not tight, joint slurry leakage is not good, caused by vibrating concrete surface slurry leakage.

cause analysis

The gap at the root of the formwork is not tightly blocked and leaks slurry.

Before pouring, there is no stone mortar with the same mixture ratio and composition as concrete.

The workability of concrete is poor, the water-cement ratio is too large, and the stones sink to the bottom.

When the pouring height is too high, the concrete will be cut in one place, the concrete will separate or the stones will accumulate.

False vibration.

The mold is not cleaned and wetted well.

preventive measure

All vertical structure templates are paved with cement mortar with the same concrete mixture ratio.

Strict layered casting, template flat-fell seam tightly, reinforced protective layer pad layout evenly.

Clean the formwork before closing the mold, strictly control the slump of concrete and prevent segregation.

Suspension materials shall be dispersed and suspended.

Concrete shall be watered and maintained in time for not less than two weeks.

Treatment measures

Grouting method is used for repair, and concrete grouting material is used. The construction process is as follows: roughening of old concrete → washing with clear water for saturation → pouring, daubing, grouting → curing.

six

Staggered platform

Staggered abutment means that the joints of the upper and lower concrete layers are staggered by a certain distance to form steps.

cause analysis

Pay-off error is too big.

Template displacement and deformation, template support without alignment measures.

The top of the lower die is tilted or expanded, and the upper die is corrected and reset to form a staggered platform.

preventive measure

Formwork should have sufficient rigidity and smooth edges and corners, and used formwork should be corrected before installation.

When installing the formwork, it is necessary to ensure that the formwork is closely spliced, firmly supported and the overall stiffness is sufficient, especially to strengthen the fastening between the formwork and the old concrete.

If the pouring height is high, it is best to keep the last template when the last silo was demolished and splice it with the template of the new pouring silo.

Pay attention to the follow-up work of the concrete pouring process, monitor the deformation of the formwork after being stressed in real time, and adjust the deformed formwork in time.

When concrete is poured to the heights of 1/3 and 1/2, the formwork support shall be fastened once each, and then again after pouring.

Treatment measures

Try to deal with quality defects such as uneven countertop and appearance deformation by chiseling and polishing.

Chiseling into an inclined plane forms a gradual change. Generally, a flat chisel and a hand grinding wheel are used as tools. The slope of the inclined plane is generally greater than1:20-1:30, and the maximum should not be greater than 1: 10.

If the depression is serious and the chiseling and grinding method can't meet the requirements, chisel off the surface of the depression and fill it with first-class fine stone concrete or mortar. The treatment method is basically the same as that of pockmarked surface.

seven

crack

Concrete crack is the physical structural change of concrete structure caused by internal and external factors, and crack is the main reason for the decrease of bearing capacity, durability and waterproof of concrete structure.

cause analysis

Excessive water-cement ratio leads to pores and cracks on the surface.

Too much cement causes shrinkage and cracking.

Poor or untimely maintenance, surface dehydration and shrinkage cracks.

Slump is too large, pouring is too high and too thick, and laitance on the surface of slurry cracks.

Dismantle the formwork too early and pry the concrete improperly.

The concrete surface is not well plastered.

The protective layer of steel bar is too thin and cracks along the steel bar.

Lack of stirrups and temperature measuring steel bars leads to concrete cracking.

Mass concrete has no measures to reduce the temperature difference between inside and outside.

There is no reinforcement in the application center such as the corner of the hole.

preventive measure

After 6 hours of concrete pouring, the curing period is 7 days, once every 2 hours in the first 24 hours, once every 4 hours after 24 hours, and the top surface is covered with wet sacks to avoid exposure.

Vibration compaction does not segregate, and the board surface is plastered twice to reduce shrinkage.

Treatment measures

For micro-cracks, use pure cement slurry to fill and caulk, cover and maintain, clean up cracks, brush epoxy mortar twice after drying or stick epoxy glass cloth to seal the surface.

For deep or penetrating cracks, epoxy resin should be used for grouting and epoxy resin cement should be used to seal the surface.

eight

Lack of edges and corners

Phenomenon: concrete parts at right angles of beams, columns, slabs, walls and holes fall off, which are irregular and have defective edges and corners.

cause analysis

Excessive water-cement ratio leads to pores and cracks on the surface.

Too much cement causes shrinkage and cracking.

Poor or untimely maintenance, surface dehydration and shrinkage cracks.

Slump is too large, pouring is too high and too thick, and laitance on the surface of slurry cracks.

Dismantle the formwork too early and pry the concrete improperly.

The concrete surface is not well plastered.

The protective layer of steel bar is too thin and cracks along the steel bar.

Lack of stirrups and temperature measuring steel bars leads to concrete cracking.

Mass concrete has no measures to reduce the temperature difference between inside and outside.

There is no reinforcement in the application center such as the corner of the hole.

preventive measure

Before pouring concrete, the formwork should be fully wetted or coated with isolation agent.

Do a good job of concrete maintenance according to regulations.

Dismantle the form according to the specified time, and do a good job in protecting the finished product.

Treatment measures

When the missing edges and corners are small, brush them with a wire brush, rinse them with clear water and fully wet them, and then fill them with cement mortar of 1:2 or 1:2.5.

When the angle of missing edge is large, chisel away the false concrete and prominent aggregate particles, rinse them with water and moisten them, then repair the formwork with pisolite concrete higher than the original concrete, and maintain them carefully.

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