Quality defects and preventive measures of cast-in-place concrete engineering

In the whole project management, the management of on-site pouring concrete is a small sub-project, but it is related to the overall quality of the unit project. There are many reasons for the quality defects of cast-in-place concrete engineering. The author has been engaged in the management of cast-in-place concrete in construction site for many years, so this paper only discusses the quality defects, causes and preventive measures of cast-in-place concrete in construction.

1. Quality defects of formwork engineering

Formwork has a very important influence on concrete structure engineering. Although it is not a part of the structure itself, the "traces" left on the concrete structure can be seen everywhere, which has a great influence on the quality of concrete structure engineering, whether it is structural performance or appearance quality. All concrete structure engineering quality must first close the template. Common template defects are as follows:

(1) The rigidity and stability of formwork installation can't meet the specified requirements. In the process of concrete pouring, the formwork is deformed, displaced and sometimes even collapsed.

(2) The joint of formwork is lax and the reinforcement method is improper, which leads to the ultra-thin deviation of concrete size and honeycomb pits, which directly affects the quality of concrete.

(3) sundries left in the formwork or uncleaned sundries will cause defects such as slag inclusion in concrete.

(4) The coating of the isolation agent is uneven, which pollutes the steel bar when coating the isolation agent, resulting in the destruction of the bonding force between the steel bar and the concrete.

2. The main reason of template construction quality.

2. 1 Formwork and its support were not designed before construction.

Before installing the formwork, the formwork and its support must be designed. The design should ensure the bearing capacity, rigidity and stability of the formwork and its support, so that it can bear the weight, lateral pressure and construction load of concrete, thus ensuring the construction safety and concrete forming quality.

2.2 There is no technical disclosure or the disclosure content is incomplete and non-targeted.

Qualified technical disclosure must at least achieve the following two points: First, it must meet the standards. The second is to be targeted. At work, some technical personnel who do disclosure are not familiar with the corresponding standards at least, indicating that they have not carefully consulted or studied them. The so-called disclosure is copied from the model text moved between technicians, and the content principle explains the construction practices and operating procedures. At the same time, the so-called audit of the technical department of the unit only signed a name, and did not read and study its content carefully. Appropriate technical disclosure has become a form, lacking operability, and the inspection is just a dead letter. In addition, the technical disclosure is not targeted. Although there is a certain * * * in the process of target construction, the construction of each project has its own personality. It is necessary to fully understand and integrate theory with practice, so as to make the disclosure instructive and operable.

2.3 The quality inspector relaxes the quality inspection.

For some people, the quality inspection of formwork engineering is a simple matter, but in fact, formwork engineering has the duality of quality and safety. If the quality inspector fails to check in place, the existing quality hidden dangers cannot be discharged, which will lead to concrete slag inclusion, honeycomb, pits and even template collapse. Therefore, the template project of concrete acceptance specification is listed as a sub-project separately, and the regulations must be seriously implemented.

2.4 premature removal of formwork

When the formwork is removed too early, the strength of concrete is easy to be insufficient, resulting in settlement and deformation, lack of edges and corners, cracking and so on. In construction, the requirement of concrete strength during formwork removal is usually judged by the strength of concrete specimens cured under the same conditions. In construction practice, due to the need to catch up with the progress, it is often judged by "experience", and some construction sites often dismantle the formwork in advance on the grounds of formwork turnover, which is extremely harmful to the quality of concrete projects and safe production, and must be strictly managed.

2.5 template besmear brushs isolation agent

Special tools must be used when painting, and suitable isolation agent should be selected at the same time. Be careful not to use oily isolation agent that affects the structure or hinders the construction of decoration engineering. At the same time, because the isolation agent pollutes the joint between steel bar and concrete, it may have obvious adverse effects on the stress of concrete structure, so the isolation agent should be brushed to avoid polluting steel bar.

3. Quality defects in concrete engineering construction

The concrete sub-engineer carries out a series of technical work, and completes the physical work from the entrance inspection of raw materials such as cement, sand, stone, admixture and admixture, concrete mix design, weighing, mixing, transportation, pouring, curing, formwork removal and specimen testing until the concrete reaches the predetermined strength. In the process of construction, each technological process is interrelated and affects each other, and improper handling of any process in construction will affect the final quality of concrete engineering.

3. 1 Appearance quality defect

Appearance quality is mainly manifested as honeycomb, pits, holes, lack of edges and corners, delamination, cracks, inconsistent color difference and other defects. In addition to the above reasons, these defects are also related to water cement ratio of concrete, aggregate gradation and vibration.

3.2 Strength defects

The main factors causing concrete strength defects are cement grade, quality of raw materials such as sand, stone and water, concrete construction mixture ratio, water cement ratio, raw material measurement, mixing method and time, additives and admixtures, poor construction management and inadequate maintenance in winter.

4. Preventive measures for quality defects in concrete engineering

4. 1 Strictly implement the concrete mixture ratio.

During construction, the mixture ratio commonly used in the construction area or provided by the laboratory should be selected according to the concrete strength grade required by the structural design, and whether the actual moisture content of the on-site spare sand and gravel is considered in the adopted mixture ratio. If it is not considered, it should be adjusted according to the moisture content of specific materials. For the convenience of construction, two bags of cement are generally used as the blanking unit to convert into the construction mixture ratio.

In the process of construction, the adjusted construction mixture ratio must be strictly controlled, and the construction mixture ratio should be adjusted in time according to the actual situation of sand and gravel materials or when the moisture content changes greatly in rainy days. The mix proportion is not converted according to the actual moisture content of sand and gravel, and the material dosage is not accurately weighed according to the requirements of mix proportion, which is often an important factor affecting the quality of concrete construction.

4.2 Strengthen the management of water cement ratio and raw material measurement.

During the construction process, although the mixing ratio of concrete was written next to the mixing table and a scale was set in front of the mixer, the operators did not carefully control the water consumption and the sand and gravel were not weighed in strict accordance with the regulations. As we all know, the amount of concrete added directly affects the strength of concrete, but in many construction sites, the personnel in charge of concrete mixing, foreman, quality inspector or technical director of the project, and even supervision engineer rarely check the accuracy of the amount of concrete added, and the measurement of various components of concrete is also a mere formality, turning a blind eye and pretending to cope with the inspection only when the inspectors arrive at the scene.

This phenomenon is caused by poor management. In actual construction, various components must be weighed on each board according to the requirements of concrete mixture ratio. At the same time, it is still necessary to identify the mix proportion of each concrete used for the first time, and at least keep a group of 28d standard curing specimens to verify the consistency between the actual quality of concrete and the design requirements.

4.3 Strengthen the management of incoming inspection of raw materials

Cement is one of the most important components of concrete. When cement comes into the site, check its variety, grade, packaging, production date, etc. , and whether it meets the requirements of engineering design standards. At the same time, it is necessary to re-examine the strength, safety and other important performance indicators of cement, and its quality must meet the current national standards.

Carry out witness sampling inspection on the sand and stone entering the site. The indexes of inspection must meet the design requirements of concrete strength, and the inspection lot and inspection method should comply with the provisions of the standard.

4.4 Strengthen the management of mixing, vibrating and curing of concrete.

Concrete mixing time should be determined according to mixer type, drum size, aggregate variety and concrete slump.

Vibrators should be selected according to the design thickness of concrete. For concrete with large plane area and thickness of 20-30cm, flat vibrator should be selected. For members with small vibrating area and certain vertical depth or large-scale structural concrete with large thickness, plug-in vibrator should be selected. During operation, the concrete should be inserted quickly and pulled slowly, and the vibrating time should be determined based on the fact that the concrete is no longer obviously sinking and deceiving each other, and the concrete surface is horizontal and cement slurry appears. The insertion points should be staggered evenly, and the spacing between the insertion points should not exceed 1.5 times the action radius of the vibrator. Only by following these procedures can the concrete be vibrated and compacted.

After vibrating compaction, concrete gradually solidifies and hardens. This process is mainly realized by the hydration of cement, which must be completed under suitable temperature and humidity conditions. It is necessary to maintain concrete to ensure proper hardening conditions and increase its strength. In the process of construction, effective measures should be taken for maintenance according to the specific conditions such as raw materials, mixture ratio, pouring position and season.

5. Conclusion

To sum up, any construction site should be "three haves", namely "standards, systems and regulations". Standard is the most basic condition for improving project quality, quality management system is the prerequisite for improving project quality, and a sound system is the guarantee for improving project quality. The quality control of cast-in-place concrete engineering is to manage the "real object" and "behavior" in the construction process to ensure the engineering quality.