However, due to the influence of equipment level, technology and management, system and many other factors, there are still some problems in the commercial concrete industry. Based on my own working experience, the author analyzes the present situation of commercial concrete and puts forward some suggestions for reference. The author doesn't mean to offend anyone or unit, but only wants to cause discussion and discuss the quality problems that have a bearing on a century-old plan.
First, there are various problems in the use of commercial concrete at present. Generally speaking, it can be divided into management factors, mixing station's own factors, imperfect management in the construction process and other factors:
(1) Institutional factors: First of all, the construction market is not standardized, and some projects are not standardized during the contracting process. Before the contract is awarded, the project owner often determines subcontractors (including suppliers of commercial concrete) in advance, and the contractor often meets the owner's requirements in order to get the project. Under this premise, if the supplier of commercial concrete is clearly defined, a lot of troubles will be avoided in the construction process; If the supplier of commercial concrete is not well defined, it will increase the difficulty of construction and management in the construction process and have a certain impact on the quality of the project.
Secondly, there is a lack of necessary supervision and management of supply enterprises. At present, the management of commercial concrete still stays in the mode of all suppliers, that is, for commercial concrete, as long as the supplier provides the factory certificate of concrete and ensures that it passes the completion acceptance; However, little attention has been paid to the mix design, raw material quality, production technology, production and testing equipment, and the perfection of measurement system of commercial concrete.
(B) mixing station supply enterprise's own factors:
1. Technically, the specification cannot be strictly implemented.
(1), the strength configuration of concrete is low. Some mixing stations blindly pursue low cost, do not calculate the value strictly according to the specification requirements, and adopt low proportion, which leads to low concrete preparation strength.
(2) the changed concrete raw materials are not inspected according to the specification requirements.
As we all know, when the sources of coarse and fine aggregates and cement additives change, it is necessary not only to test the indexes of the changed raw materials, but also to redesign the ratio. But in fact, when the source channel of raw materials changes, some mixing stations do not test the changed raw materials; Some mixing stations mix aggregates from different sources, and some even mix cement from different manufacturers, because the materials are widely available and there is a lack of management on site. This will seriously affect the quality of commercial concrete and bring quality risks to structural engineering.
③ Some mixing stations ignored the influence of coarse aggregate strength on concrete strength, and did not measure the strength of coarse aggregate.
The quality standard and inspection method of ordinary concrete crushed stones or pebbles (JGJ53-92) stipulates; The strength of crushed stone can be expressed by the compressive strength and crushing index value of rock. When the strength grade of concrete is C60 or above, the compressive strength of rock should be tested. In other cases, if in doubt or considered necessary, the compressive strength of rock can also be tested. The ratio of rock compressive strength to concrete strength grade should not be less than 1.5. For concrete with low strength grade, the strength of coarse aggregate can generally meet the requirements without inspection. But for high-strength concrete, not all the coarse aggregate strength can meet the requirements, so it must be tested. In practical work, most mixing stations have stone inspection reports. Stone crushing index values are not listed, some concrete strength grades are above C50, and no single inspection has been carried out. This kind of behavior, no matter what the quality, can't help worrying.
(4) The water content of sand and gravel can't be measured in time in rainy and snowy seasons, and the water cement ratio can't be adjusted in time. According to the regulations, the mixing station should take corresponding measures to keep the water content of sand and gravel aggregate stable, and measure the water content at least once per shift. In rainy and snowy weather, the number of measurements should be increased and the water consumption should be adjusted in time to meet the requirements of concrete strength grade and construction workability.
At present, many mixing stations do not have instruments for automatically measuring the water content of sand and gravel. In most cases, the water content is judged by experience and the water consumption is adjusted. Some of them are not estimated or adjusted at all. In this way, the water-cement ratio is too high in the rainy and snowy season, and the workability of the mixture cannot be guaranteed, let alone the strength of concrete, which not only increases the construction difficulty, but also affects the strength of the engineering structure.
⑤ Improper use of admixtures According to the Technical Specification for Application of Concrete Admixtures (GBJ 1 19-88), the choice of admixtures should be based on the performance requirements of concrete, construction conditions and climatic conditions, combined with factors such as raw materials and mixture ratio, and the variety and dosage of admixtures should be finally determined through experiments.
It is difficult to meet the above requirements when additives are actually used. There are objective factors, and more are human factors. Some mixing station leaders and technicians are not very clear about the use of additives. They don't ask about the type and applicability, and they don't determine the dose through experiments, so they use it. What's more, adding retarder or early strength agent into the compound admixture with retarding, water reducing or early strength agent leads to the superposition of admixtures, which not only wastes the admixture, but also fails to improve the performance of concrete.
2. The equipment and testing tools of the mixing station cannot meet the requirements;
(1) Some mixing stations have used mixing equipment for a long time, and the equipment has not been maintained and tested in time. Once the metering system is out of alignment, the concrete configuration cannot be carried out accurately according to the proportion table; When such concrete is used in engineering, the consequences are unimaginable.
(2) For the measurement of additives, many mixing stations do not have metering devices, and they do not intend to install metering devices. Therefore, the concrete mixture ratio in the laboratory is correct. Because there is no special admixture metering device in the mixing station, the actual operation is in the hands of migrant workers. It is difficult to achieve a measurement error of 2% by adding additives to non-measuring instruments such as shovels, spoons and bowls.
(3) At present, many mixing stations do not have instruments for automatically measuring the moisture content of sand and gravel.
3. There is a lack of contact between the mixing station and the site. Most mixing stations still have the phenomenon of behind closed doors, that is, managers do not go deep into the field to conduct actual investigations, but design concrete mixture ratio according to experience and information and requirements provided by construction units; As for whether the concrete it produces meets the needs of the project and the quality in the later period, there is often no follow-up observation.
4. Insufficient attention is paid to the durability of commercial concrete.
Many projects (such as gift-giving projects and time-limited projects). In order to speed up the construction progress, the mixing station is required to provide early strength concrete. Therefore, in order to meet the requirements of customers, the mixing station has adopted measures such as using early strength cement, early strength agent and increasing the dosage of cement to achieve the purpose of early strength. In fact, this kind of behavior is irresponsible short-term behavior.
As we all know, all the factors that can improve the early strength of concrete will affect the later strength of concrete, so there will be a large strength surplus when preparing concrete. In order to make up for this loss of later strength, this will undoubtedly cause great waste. In fact, the problem is often much more serious. The consequence of early high strength is the early deterioration of concrete structures. Therefore, it is suggested that "the late strength of concrete should be used as much as possible to extend the design strength from 28 days to 45 days or 56 days".
The production of concrete must follow certain laws and take a certain time, and we can't just emphasize the construction speed. In any case, the most important goal should be to ensure the long-term durability of concrete.
5. The mixing station failed to implement the latest standards in time. Article 5.2. 1 of the Code for Indoor Environmental Pollution Control of Civil Building Projects (GB50325-200 1) implemented on June12002 stipulates that "inorganic nonmetallic building materials and decoration materials used in civil building projects must have radioactive indicators and test reports". Article 3. 1. 1 stipulates that "inorganic nonmetallic materials used in civil construction projects, including sand, stone, brick and cement, shall comply with the provisions in Table 3. 1. 1", that is, the internal irradiation index and external irradiation index shall not be less than 1.0.
(C) the construction process management is not perfect
1. After the concrete is transported to the construction site, the construction unit will add water at will for the convenience of construction, so as to change the slump of the concrete and make the water-cement ratio too large, resulting in the concrete strength failing to meet the design requirements.
As we all know, under the same material and process conditions, the strength of concrete mainly depends on the water-cement ratio, that is, the strength of concrete decreases with the increase of water-cement ratio, which is the law of water-cement ratio. However, some technicians and operators do not know enough about the durability and workability of concrete damaged by adding water on site, thinking that diluting the mixture with water will increase the slump and facilitate mixing, pumping and pouring. According to relevant data, the strength of concrete may decrease 10 MPa for every 0.05 increase of reasonable water-cement ratio. In addition, mixed concrete is a mixed structure filled with water and cement, cement slurry and sand, mortar and stones. Adding water into it will aggravate the segregation and bleeding of concrete, affect the strength and durability of hardened concrete, and at the same time, it will cause the phenomenon of formwork expansion and slurry leakage during concrete pouring, resulting in honeycomb, pits and stones.
2. When the construction unit finds that the commercial concrete delivered to the site does not meet the requirements (mainly manifested in the segregation, slump, workability and initial setting time of the concrete delivered to the site, etc.), it will not be ordered to leave, but will continue to be used in the project; For individual projects, after concrete arrived at the site, there was no vehicle-by-vehicle inspection at all.
3. Due to imperfect management, commercial concrete has been misused in some projects.
4. For individual projects, it is found that the quality of reserved commercial concrete cannot meet the actual needs after concrete pouring. If you place an order again, either time is not allowed or the mixing station does not produce a small amount of concrete. In this regard, the construction unit only simply mixes concrete on site instead of commercial concrete, which is used in engineering. It is worrying whether its strength is qualified and whether there are adverse reactions and shrinkage cracks between different concretes.
(4) All contractors (mainly supervision units, construction units and construction units, etc.). ) did not give full play to its due role, and there was a lack of contact and disconnection between various units, mainly because the supervision unit did not play its due role in supervision and management.
Of course, there are other quality problems in the production and use of commercial concrete, so I won't describe them here.
Second, some suggestions:
In order to ensure the engineering quality and the sustainable development of the commercial concrete specialty and improve the competitiveness of the mixing station, the author thinks it is necessary to strengthen the industrial management and technical research of the commercial concrete, and the measures are as follows:
1. Standardize the construction market, strengthen legal supervision and management, and put an end to illegal subcontracting and unfair low-price bidding.
2. When selecting the supplier of commercial concrete, the contractor should choose a mixing station with high qualification level and good reputation, and at the same time, it should also consider whether the geographical location is suitable and whether the production capacity can meet the needs of on-site construction.
3. When signing a contract with the commercial concrete mixing station, the contractor must specify and fill in the concrete technical parameters, construction site conditions and site technical director in detail, and report to the supervision unit for review. If necessary, visit the mixing station to see whether the production technology is advanced, whether the laboratory instruments and equipment are complete, and whether the production scale can meet the needs of on-site construction.
4. To strengthen the supervision and management of the mixing station, it is mainly to give play to the supervision and management role of the supervision unit. The author thinks that the mixing station belongs to the subcontracting unit of the project, and the supervision unit should not only review the qualification grade and inspect its production capacity, but also supervise and manage it during the operation to see whether its system is implemented, whether its management is perfect and whether its behavior is standardized, so as to ensure the quality of the project. At the same time, the government departments also strengthen the macro control and guidance of the mixing station.
5. The mixing station should constantly improve and develop itself, especially after the concept of "high-strength and high-performance concrete" is introduced into the concrete industry at present. In addition to constantly improving management, standardizing behavior and improving equipment, it should also pay attention to the research of "high-strength and high-performance concrete" technology to meet the requirements of modern concrete mixing.
① Strengthen the technical transformation of the mixing station. The key to the transformation of mixing station is to adopt advanced mixing equipment and metering device. The mixing station should gradually realize mechanical feeding, computer measurement control and management; Choose forced or dump mixer; Storage and addition device using bulk cement, additives and superfine active admixture; There should be a laboratory suitable for the qualification of the enterprise to meet the requirements of preparation and mixing of various performance concrete; There are sewage treatment and reuse devices to strictly control dust, noise and water pollution. Take effective measures from the aspects of technology, materials and equipment to improve the durability of concrete and reduce the cost of concrete.
② Establish a database of raw materials such as cement, aggregate and admixture. Based on the continuous research and application of commercial concrete, the mixing station should establish a database of raw materials such as cement, aggregate and admixture, and form its own internal data, so that it can't make up its mind and analyze it. I hope this situation can be improved as soon as possible.
(3) In order to meet the increasing requirements of users for the performance and strength of concrete, the mixing station should strengthen the research on the technology of "high strength and high performance concrete", which mainly includes:
A, use local materials, prepare high-strength concrete with various performance requirements, and continue to increase the application proportion of C50, C55 and C60 high-strength concrete, so as to effectively meet the requirements of uniformity, impermeability, low shrinkage and pumpability in engineering application, and correspondingly improve its durability.
B. development and application of superplasticizer. Superplasticizer, also known as high-performance admixture, must have high water reducing rate and good plastic retention. The water reduction rate is not less than 25%, and the slump loss of 120min is not more than 20 mm. At the same time, the chloride ion content, total alkali content and the requirements for cement adaptability should be strictly controlled. Under the condition that the dosage of cement, sand, stone and admixture remains unchanged, and the slump and fluidity of fresh concrete remain unchanged, replacing ordinary superplasticizer with superplasticizer can further reduce the water consumption, increase the 28d strength by more than 65,438+00%, and correspondingly improve the durability of concrete.
Development and application of superfine active admixture. Ultra-fine active admixture can not only improve the submicroscopic structure in concrete, improve the interfacial strength between coarse aggregate and mortar, but also fill the capillary inside concrete, which plays a role in strengthening and compacting. Ultra-fine active admixture is made of industrial waste (silica fume, ultra-fine slag, fly ash, zeolite powder or other industrial waste residue) by grinding, and the general specific surface area should not be less than 400 cm2/g. The application of ultra-fine active admixture can not only improve the performance of concrete, but also save cement.
D development and application of high performance concrete. According to the needs of engineering, the performance index is formulated, and various high-performance concrete are prepared by using high-efficiency water reducer and superfine active binder. At present, the self-compacting concrete shrinkage compensating concrete used in engineering belongs to high performance concrete and should be popularized and applied in engineering.
6. In the application process of commercial concrete, the construction unit should not only check the commercial concrete car by car, but also regulate its own behavior. In this regard, the supervision unit should give full play to its supervision and management role to ensure the quality of the project.
7. There should be a durability clause in the commercial concrete engineering contract. At present, China is in the transition period from planned economy to market economy. If the economic lever is not introduced into the durability of the project, it is impossible to motivate the engineers and technicians to pay strict attention to the durability. The current reality is that whether it can be built quickly and completed as soon as possible is directly related to personal economic benefits. However, if a project has problems after 5 ~ 10 years, even if it needs to be overhauled or rebuilt, no unit or individual will bear the economic responsibility, and sometimes the project is even rated as an excellent project. On the contrary, some projects show good persistence, but they are not rewarded. If this situation does not change, contractors and mixing stations are unwilling to spend energy to do durability prevention research or take measures, and there are many examples in China. It is very necessary to set aside a small amount of funds from the design and construction funds for durability prevention research and set up a long-term durability incentive fund.
8. Establish a database of major concrete projects and concrete raw materials. All kinds of construction projects in China, especially large-scale projects, may have their own historical files, but they are scattered and lack of information exchange, which leads to repeated lessons. 1982 The United States established the "AEPI (Building and Engineering Performance Information Center)" supported by the National Science Foundation, and has collected 40,000 cases so far. Establishing such a center and standardizing data files will provide valuable information for the development of China's economic construction.
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