Development prospect of concrete

Concrete is the most widely used building material in civil engineering. Design and manufacture concrete according to the predetermined performance, and develop new varieties of lightweight, high-strength and multifunctional concrete. Using modern new technology, vigorously develop new technology and new equipment; The extensive utilization of industrial waste residue as raw material is a subject that needs to be solved continuously in the future. Concrete talents are relatively scarce, so we cooperate with recruitment service providers such as Concrete Yingcai.com to fill the vacancy.

The Development Direction of Modern Concrete —— Commercial Concrete

Commercial concrete is provided with cast-in-place concrete to the construction site through centralized pre-mixing and long-distance transportation; It is a high-tech building material product combining modern concrete and modern construction technology, including: large fluidity concrete, flowing concrete, pumping concrete, self-compacting concrete, impervious and crack-resistant mass concrete, high-strength concrete and high-performance concrete.

Commercial concrete refers to the supply of concrete with certain requirements to the construction site through centralized mixing and long-distance transportation. It includes the process of mixing, transporting, pumping and pouring the mixture. Strictly speaking, commercial concrete refers to the process and product of concrete, not the variety of concrete, and should include large fluidity concrete, flowing concrete, pumping concrete, high strength concrete, mass concrete, impermeability and crack resistance concrete or high performance concrete. Therefore, commercial concrete is a combination of modern concrete and modern construction technology, and its popularity can represent the concrete construction level and modernization of a country or region. Centralized mixing of commercial concrete is mainly used in cast-in-place concrete engineering. It takes 1 ~ 2 hours from mixing, transportation to pouring of concrete, sometimes more than 2 hours. Therefore, the reasonable supply radius of commercial concrete mixing station should be within l0km. With the popularization and development of commercial concrete, cast-in-place concrete has become the future development direction. In many big cities in China, such as Beijing, Shanghai, Tianjin, Guangzhou and Shenzhen. There are more than 100 commercial concrete mixing stations, and their scale and technical level are no less than those of developed countries. Many small and medium-sized cities are also promoting the application of commercial concrete.

abstract

When fluid concrete is used as commercial concrete, the fluidity and fluidity loss of fresh concrete should be strictly controlled. Due to the long transportation distance, traffic jam and other factors, the slump loss is required to be small, the concrete should be kept flowing within 2 hours (sometimes more than 2 hours), and pumping is required when pouring. Although the post-mixing method can solve the problems of slump loss and pumping, it increases the mixing time or times, affects the output of commercial concrete and complicates the mixing operation. Even if the superplasticizer is mixed before pumping and stirred in the mixer for 3 minutes, the dispersion effect of superplasticizer can not be fully exerted and the uniformity of the mixture is poor. Therefore, at least in China, the post-mixing method is not easy to popularize, and it is better to adopt the same mixing method. Therefore, it is necessary to study a new superplasticizer to ensure that the fluidity of fresh concrete remains above 2 hours without affecting the strength of hardened concrete, especially the early strength.

About 70% of commercial concrete in China is labeled C25~C40, and C50~C60 is used in some important projects, and C70~C80 is used in some special cases. In order to reduce the dosage of cement, improve the workability of fresh concrete and improve the performance of hardened concrete, especially the durability, fly ash should be added. In this way, when 10% ~ 25% fly ash is added, the unit cement consumption can be reduced by 10% ~ 20%. The calculation shows that adding 20% fly ash to benchmark concrete can save energy 10%. Adding superplasticizer to benchmark concrete (reducing cement consumption 15%) can save energy 15%. When fly ash and superplasticizer are mixed at the same time, it can save energy by 25.5%. Therefore, it is the most energy-saving to mix fly ash and superplasticizer at the same time to prepare fluid concrete, and satisfactory results can be achieved in performance and energy saving.

Adding superplasticizer improves the rheological properties of fluid concrete, that is, the yield value, plastic viscosity and hysteresis loop are reduced, so it is almost close to Newtonian fluid. This increases the pumpability of fluid concrete. When 0.4% ~ 0.8% (preferably 0.75%) of superplasticizer is added to the benchmark concrete, the pumping pressure of fluid concrete is reduced by 25% ~ 35%.

Slump loss, segregation and pump blockage will occur in pumped concrete under the action of pump pressure. The key is to adjust the uniformity, stability, fluidity and dryness of the mixture through the mixture ratio of concrete and the composition of water reducer. When pumping concrete, fine powder (

Fluidized concrete is mainly used for cast-in-place concrete of foundations, beams, columns, frames, bridges and other high-rise buildings, as well as integral pouring of T-joints. In particular, steel bars are dense, so it is difficult or unnecessary to vibrate ("self-collapse" or "self-leveling").

Selection of concrete

Commercial concrete is concrete that provides certain requirements to the construction site through centralized mixing. It includes the process of mixing, transporting, pumping and pouring the mixture. The only requirement of commercial concrete in market competition is the lowest cost and price under the premise of ensuring workability, strength and durability. The technical way to reduce the cost is to correctly select raw materials and mixture ratio.

Characteristics of commercial concrete

1. Because it is centralized mixing, the quality and mixture ratio of raw materials can be strictly controlled online to ensure the quality requirements of concrete;

2. The mixture is required to have good workability, that is, high fluidity, small slump loss, no bleeding, no segregation and good pumpability;

3. Economical, requiring low cost and high cost performance.

Selection and requirements of raw materials

Selection of cement: Portland cement, ordinary silicon cement or slag cement are usually used, and the basic requirements for cement are:

When the label 1. is the same, choose cement with large abundance coefficient, because cement is the "foundation" for concrete to obtain strength;

2. Under the same strength, choose cement with small water demand. The water requirement of standard consistency of cement is 265,438+0% ~ 27%, and the cement with low water requirement can reduce the cement consumption when preparing concrete.

3. Choose C3 high and C3A low (

4. Rational use of different grades of cement. When preparing fluid concrete below C40, 32.5Mpa ordinary silicon cement should be used; Preparation of C40 and above high performance concrete, should use 42.5Mpa Portland cement or ordinary silicon cement;

5. Choose cement varieties correctly for concrete with different purposes, such as early strength or winter construction, try to use R-type portland cement, and use slag cement or ordinary silicon cement for mass concrete.

Selection of fine mineral admixture

Commonly used mineral fine admixtures include fly ash, ground slag, zeolite powder, silicon powder and so on. When preparing commercial concrete, the basic requirements for fine mineral admixture are:

1. has low price and certain hydration activity, and can replace part of cement. Under the condition of ensuring strength and other properties, more fine mineral admixtures should be added to reduce the cost of concrete;

2. The water demand ratio is small (

3. Reasonable use of different kinds of fine admixtures. When preparing fluid concrete below C60, Grade II fly ash is used, Grade I fly ash or ground slag is used for C60~C80, and silica powder is added to high-performance concrete above 100Mpa.

Selection of aggregate

Both coarse aggregate and fine aggregate should meet the requirements of relevant standards. Correct selection of aggregate can ensure the workability, strength and economy of concrete.

1. Fine aggregate: the sand has a reasonable particle gradation and low silt content, which is beneficial to the improvement of strength and workability. Artificial sand and weathered mountain sand have large water demand, unreasonable particle shape and gradation, which reduces the fluidity of the mixture. River sand is an ideal fine aggregate, and the fineness modulus should be selected correctly when it is used. Coarse sand is used when preparing high-strength concrete, and medium sand is used for ordinary fluid concrete. The fineness modulus of sand affects the sand ratio and water consumption of concrete. High sand ratio, large water consumption and fast slump loss. Low sand ratio is easy to cause bleeding and segregation.

2. Coarse aggregate: the maximum particle size and gradation of stone affect the water consumption, sand ratio and workability of concrete. When preparing high-strength concrete and high-performance concrete, high-strength macadam should be used, and its maximum particle size should be 19mm or 25mm, because the strength of high-strength concrete is nearly half that of stone. Ordinary fluid concrete uses gravel with the maximum particle size of 25mm or 31.5mm. When pumping technology is adopted, the maximum particle size of gravel should be less than one third of the pump outlet pipe diameter, otherwise the pump will be blocked. There are few continuously graded gravels in the market, and most of them are single graded. At this time, secondary gravel will be used. If a single size stone is used, the sand ratio should be increased.

The sand ratio of concrete is related to the maximum particle size of stones, and the sand ratio of large stones is small, while the sand ratio of small stones is large. Among them, there is the problem of reasonable cooperation. When preparing fluid concrete, if gravel with larger particle size (such as 3 1.5mm) is mixed with medium and fine sand (Mx=2.50), the sand ratio and water consumption can be reduced, thus reducing the concrete cost.

Selection of additives

Admixtures used in commercial concrete should include air-entraining water reducer, high-efficiency retarding air-entraining water reducer, retarding water reducer, high-efficiency retarding water reducer, pumping agent and high-efficiency pumping agent. Principles for selecting additives:

1. According to the type of concrete, choose the corresponding admixture varieties;

2. According to the raw materials, mix proportion and label of concrete, determine the requirements for water reduction rate and admixture content;

3. Determine the requirements for slump loss, setting time and early strength according to the engineering type, climatic conditions, transportation distance, pumping height and other factors;

4. Other special requirements (such as impermeability, frost resistance, corrosion resistance and wear resistance, etc.). ).

Finally, the admixture can only be applied after concrete trial and economic evaluation.

Mix proportion optimization

The technology of commercial concrete is different from that of on-site mixing concrete. Due to the existence of transportation distance and time, slump loss must be controlled. Therefore, when designing the concrete mixture ratio, the following factors should be considered:

1. Determine the initial slump according to the transportation distance and time: short distance (10 km) or 2h, 20 cm ~ 22 cm.

2. Control slump loss, that is, control the slump before entering the pump should be greater than 15cm. Because of the plunge; 20cm, after pouring, the concrete keeps a large fluidity for a long time, with poor stability, easy segregation and slow setting.

3. Control of initial setting time: when pouring beams and columns, the initial setting time is 8 h ~ 12h, and the mass concrete is12 h ~15 h..

4. As a building material product, commercial concrete must consider economy when participating in market competition, and its price is the lowest on the premise of ensuring technical performance. The general requirements for commercial concrete are: stability, reliability, applicability and economy.

Traditional concrete mix design methods (that is, assumed volume density method and absolute volume method) are based on strength, that is, mix design is based on "water cement ratio rule". Based on workability, strength and durability, the formulas of water consumption and sand ratio are derived through the concrete volume model, and the formula is combined with the water cement ratio rule to realize the full calculation of FLC and high performance concrete composition and mix ratio. Compared with the traditional design method, the full calculation method makes the concrete mix design move from semi-quantitative to full quantitative and from experience to science. Compared with the traditional mix proportion design, the full calculation method is more convenient and quick to get the optimized concrete mix proportion.