Analysis of causes of accidents using concrete admixtures and preventive measures

Mixing accidents of different brands of admixtures

Due to mechanical failure during the construction of zero-layer panels in a certain engineering factory building, the mixing station changed the production line during the supply process, but high-performance water-reducing materials from two different manufacturers were used. Agent, the next day the constructor reported that the concrete setting time and construction sequence were reversed. The part constructed first had not yet initially set, and the part constructed later could not be closed, and the part constructed later had not set partially and had a white slurry.

　1. Reason analysis

　(1) Due to the replacement of the production line during the supply process, the raw materials, especially the admixtures, were not unified, resulting in a large difference in the concrete setting time, which could not be correct Determine the accurate closing time to affect the appearance of the project.

(2) During the construction process, the moving of the pump caused the phenomenon of car pressing, and the slump of some cars could not meet the construction requirements. The on-site personnel used pumping agents for secondary fluidization adjustment, and the dosage could not be strictly controlled. And the mixture was not stirred evenly, resulting in partial non-condensation in the later construction parts.

2. Preventive measures:

(1) Different admixture manufacturers have different slump-preserving and retarding components, and the difference in setting time is larger than that of naphthalene series. This is also the case with polycarboxylic acid additives. One of the characteristics that distinguishes the agent from the naphthalene series. The supply process should ensure the stability and consistency of raw materials. When there are multiple admixture manufacturers supplying, it is best to achieve unified performance indicators;

(2) The state of the concrete cannot meet the construction requirements and requires secondary When adjusting fluidization, it is advisable to use a simple high-efficiency water-reducing agent to avoid introducing excessive retardant ingredients and affecting the normal setting time. The material must be stirred fully and evenly before loading and construction.

Measuring scale out-of-control accident

Metering was out of control and the amount of water-reducing agent was too large, resulting in abnormal concrete conditions. During the supply of C40 concrete for a certain housing construction project, the construction unit reported that the concrete ran out badly after pouring and could not even be vibrated. After the on-site construction workers observed the subsequent loading of materials, they all had the same problem and returned it to the station for processing. According to the on-site reaction, the quality inspector’s station discharged materials in the same state, and it was inferred that the admixture was over-adulterated. A few months later, the mixing building was undergoing fire-proof renovations. During production at night, a similar situation occurred again. The quality inspector reduced the dosage from 1.5% to 0.9% in normal use, but still could not guarantee the out-of-machine condition.

　1. Reason analysis:

　(1) For the first time, due to the aging of the sensor of the admixture metering scale, the sensitivity became dull, resulting in an increase in the metering of admixtures, thus affecting the state of the concrete. exception.

　(2) The second accident was due to the lack of wind protection during the renovation of the external wall panels of the mixing building. When the wind picked up during production at night, the admixture scale swayed slightly, causing the metering to go out of control.

The difference between polycarboxylic acid-based and naphthalene-based admixtures in the concrete mixing process is that when the naphthalene-based admixture is over-mixed, the ammeter of the mixer displays a significantly smaller value, and the outgoing concrete appears to be close to segregation; When the polycarboxylic acid admixture is over-mixed, the performance of the mixer ammeter is not obvious, and the state of the concrete mixture is normal. However, after 1-2 minutes of observation, obvious slurry running and bottom-grabbing conditions will appear.

2. Preventive measures:

(1) Due to the high water reduction rate of polycarboxylic admixtures, measurement accuracy must be ensured, so it is necessary to increase the self-test frequency of the weighing scale. According to Regularly replace the weighing scale sensor for output.

(2) Strengthen the technical reserves and targeted learning of polycarboxylic acid admixtures, accumulate production experience, master the knowledge of admixtures, and achieve flexible application.

Examples of high-performance concrete supply

(1) Avoid mixing two types of admixtures

In 2009, when a company was supplying a municipal project, due to simultaneous The high-performance concrete supplied in other parts (casting piles, caps, tie beams, etc.) uses naphthalene-based admixtures, and the box beams supplied are produced with a polycarboxylic acid admixture mix. The mixers and tankers should be thoroughly cleaned. Otherwise, after mixing with residual naphthalene-based water-reducing agent, the working performance of the concrete will be extremely poor. When polycarboxylic acid was initially used to supply the concrete, the slump of the out-of-machine concrete was around 210mm, but the workability and fluidity were poor. It was only 100mm to 120mm after arriving at the construction site, and there was almost no fluidity. After the vibrator was pulled out during vibration The hole is not easy to close.

1. Cause analysis:

(1) The mixer and tanker were not cleaned thoroughly;

(2) Polycarboxylic acid and naphthalene series are two completely different types By comparing their performance, it is easy to find that the pH value of naphthalene series is around 9.5, which is alkaline, while the pH value of polycarboxylic acid additives is around 5.5, which is weakly acidic and easy to react.

2. Preventive measures:

(1) Downgrade concrete in abnormal condition for temporary construction use;

(2) When producing again, two kinds of admixtures Strictly use them separately, fix the production line, fix the mixing truck and pump truck during production, and clean them;

(3) Due to the weak acidity of the polycarboxylic acid admixture, long-term contact with iron containers will cause reactions. Affecting its quality, it must be packed in a separate polyethylene plastic tank, and a separate PVC pipe and metering device are required;

(4) If the production task is relatively heavy, in order to improve production efficiency, all naphthalene series Admixtures.

　(2) Slump control

Precast box beams have higher requirements for appearance. Therefore, for the stability of concrete quality, especially the stability and uniformity of slump, It plays an important role in reducing defects such as water streaks, cloud spots, color difference, and even holes and excessive laitance.

1. Reason analysis:

(1) The polycarboxylic acid admixture dosage is small, the water reduction rate is high, and the productionis extremely sensitive to water consumption. When the water consumption fluctuates, increasing or decreasing by 3 to 5kg, the slump changes significantly, with slight bleeding or no expansion.

(2) There are errors in measurement, overmixing occurs, and the slump of concrete out of the machine is too large. After mixing and transportation, it is slightly segregated when it arrives at the construction site. When testing its basic performance, there is leakage. Water and hardening and other phenomena. As the concrete used for testing, it is difficult to vibrate during pouring and the upper part is seriously hardened. When removing the formwork, there was mold sticking. There were obvious water marks on the bottom of the test box beams, and there was honeycombing on the inner sides, and the appearance was poor.

2. Preventive measures:

(1) Increase the frequency of testing of aggregate moisture content to ensure accurate production water consumption. It is recommended that the sand moisture content be measured during each working shift or during continuous supply. Measure once every two hours and increase the frequency to ensure the accuracy of water consumption; for stones, it is best to dry screen directly. In the early stage of supply, in order to ensure the quality of the stones, we washed the stones 2 to 3 times with water and stored them for a week to dry. However, during the actual production process, the outside of the large pile of stones was dry but the inside was still wet, and the water content in different layers was uneven. The stability of slump has a greater impact. Later, it was changed to direct dry screening, and the effect was better;

(2) Machine operators were not given the authority to adjust the water consumption;

(3) The mixing time was extended to ensure the uniformity of mixing;

(4) Communicate and coordinate with the admixture manufacturers to increase the admixture dosage from the designed 1.2% to 2.0% to reduce the relative error and reduce the measurement sensitivity;

( 5) Upgrade the management software of the production control system and replace sensors to ensure measurement accuracy.

　(3) Pouring construction control

The average summer temperature in North China exceeds 25℃ and the maximum temperature exceeds 38℃. The slump loss of concrete supplied during the day is large, resulting in changes in construction operation time. Short, affecting the construction effect and poor appearance.

1. Cause analysis:

(1) The temperature is high, and aggregates such as sand and gravel have a higher temperature after being exposed to the sun. The temperature of the concrete after the machine reaches 30℃~35℃, which accelerates Hydration reaction;

(2) High-temperature work of workers affects construction efficiency. Occasionally at the end of concrete pouring for box beams, the upper concrete loses water too quickly and cannot be vibrated, resulting in honeycombs, pitted surfaces and surface. Quality problems such as layer cracking.

2. Preventive measures:

(1) In view of the characteristics of concrete construction in summer and many unfavorable factors, after communicating with the construction party, the concrete pouring time of precast box beams in summer will be adjusted every morning. Construction is carried out before 6 o’clock and after 8 o’clock in the afternoon to control the temperature of the concrete and ensure the quality;

(2) Improvement of the construction technology, an attached vibrator is installed on the side of the outer mold of the box girder, and a plug-in vibrator is used The vibration method combines the vibrator and the attached vibrator to improve work efficiency and vibration effect;

　(3) Increase the frequency of debugging of admixtures and consider main cementitious materials such as admixtures and cement Matching factors such as temperature changes and other factors, the formula of admixtures should be adjusted in a timely manner to meet construction performance standards.

Application summary

The concrete admixture polycarboxylate water-reducing agent has outstanding advantages and strong vitality, but any new thing also has its two sides, and its advantages are also relatively Yes, only by being good at giving full play to its advantages, improving its shortcomings, and correctly understanding and applying it can we achieve the greatest results. Domestic polycarboxylic acid-based water-reducing agents have not been used for a long time. Most people have not fully grasped its mechanism of action. Understanding it also requires a process. Moreover, the knowledge about concrete does not come from theory but from experiments. But these The experiment was conducted under the guidance of the most basic theory. I believe that the technology for rational use of polycarboxylic acid-based water-reducing agents in the future is mainly the concrete pre-wetted aggregate technology introduced in this book. This technology proposes a new set of ideas from theoretical basis to practical application, which is especially suitable for the polycarboxylic acid-based water-reducing agent in our country. Practical application of water reducing agent.

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