Commercial concrete supplies concrete with certain requirements to construction sites in a concentrated manner. It includes processes such as mixing, transporting, pumping and pouring of the mixture. The only requirement for commercial concrete in the market competition is to ensure the lowest cost and price under the premise of ensuring workability, strength and durability. The technical way to reduce costs is to choose the right raw materials and mix ratios.
Characteristics of commercial concrete
1. Because it is concentrated mixing, it can strictly control the quality and proportion of raw materials online, and can guarantee the quality requirements of concrete;
2. It is required that the mixture has good workability, that is, high fluidity, small slump loss, no bleeding, good pumpability, and concrete price.
3. Economical, low cost, high performance and price ratio.
Selection and requirements of raw materials
Cement selection: Portland cement, plain silica cement or slag cement is usually used. The basic requirements for cement are:
1. When the same label is used, choose a cement with a large rich coefficient because cement is the "foundation" for the strength of concrete;
2. For cements with the same strength, choose a cement with a small amount of water. The standard consistency of cement requires 21%~27% of water. When using concrete, the cement with small water requirement can reduce the amount of cement;
3. Choose C3 S high, C3A low (<8%), low alkali content (<1%), moderate surface area (3400cm2/g ~ 3600cm2 / g), good graded cement;
4. Reasonably use different grades of cement. 32.5Mpa silicon cement is used for the preparation of fluid concrete below C40; 42.5Mpa Portland cement or ordinary silicon cement is used for high performance concrete above C40;
5. For the concrete of different purposes, the cement type should be correctly selected. For example, R-type Portland cement should be used as much as possible for early strength or winter construction, and slag cement or ordinary silicon cement should be used for mass concrete.
Selection of mineral fine admixture
Commonly used mineral fine admixtures are fly ash, ground slag, zeolite powder, silicon powder, and the like. The basic requirements for mineral fine admixtures when formulating commercial concrete are:
1. Low price, certain hydration activity, can replace part of cement, in the case of ensuring strength and other properties, should be mixed with mineral fine admixture, so that the cost of concrete is reduced;
2. The water demand ratio is small (<100%), and the particle gradation is reasonable to improve the fluidity of the mixture;
3. Rational use of different types of fine admixtures, the use of class II fly ash when formulating fluid concrete below C60, C60 ~ C80 using grade I fly ash or ground slag, high performance concrete doped silicon powder above 100Mpa.
Both coarse and fine aggregates should meet the requirements of the relevant standards. Proper selection of aggregates ensures concrete workability, strength and economy.
1. Fine aggregate: The grain size of the sand is reasonable, and the low mud content is beneficial to the improvement of strength and workability. The artificial sand and weathered sand have large water demand, unreasonable particle shape and gradation, which reduces the fluidity of the mixture. River sand is the ideal fine aggregate, and the fineness modulus should be selected correctly. Coarse sand is used for the preparation of high-strength concrete, and medium sand for ordinary fluid concrete. The fineness modulus of the sand affects the sand rate and water consumption of the concrete. The sand rate is high and the water consumption is large, and the slump loss is fast. Low sand rate is prone to bleeding and segregation.
2. Coarse aggregate: The maximum particle size and gradation of the stone affects the water consumption, sanding rate and workability of the concrete. High-strength concrete and high-performance concrete should be prepared with high-strength gravel. The maximum particle size should be 19mm or 25mm, because the strength of high-strength concrete is nearly one-half of the strength of the stone. Ordinary fluid concrete adopts the maximum particle size of 25mm or 31.5mm gravel. When using the pumping process, the maximum particle size of the stone should be less than one third of the diameter of the pump outlet pipe, otherwise the pumping phenomenon will occur. There are fewer gravel in the market for continuous grading, most of which are single-grain grades. At this time, secondary stones should be used. If a single-grain stone is used, the sanding rate should be increased. Kunshan Jianguo Concrete
The sand rate of concrete is related to the maximum particle size of the stone. The sandstone rate of small stone is small and the sandstone rate of small stone is large. Among them, there is a problem of reasonable cooperation. In the preparation of fluid concrete, the use of larger particle size (such as 31.5mm) crushed stone and medium fine sand (Mx = 2.50) can reduce the sand rate and water consumption, thus reducing the cost of concrete.
The admixture used for commercial concrete shall include: air-entraining water reducing agent, high-efficiency retarding air-entraining water reducing agent, retarding water reducing agent, high-efficiency retarding water reducing agent, pumping agent, high-efficiency pumping agent, and the like. The principle of choosing an admixture:
1. Select the appropriate admixture type according to the type of concrete being formulated;
2. Determine the water reduction rate and dosage requirements of the admixture according to the raw materials, mix ratio and label of the concrete;
3. According to the type of project, climatic conditions, transportation distance, pumping height and other factors, determine the requirements for the degree of slump loss, condensation time and early strength;
4. Other special requirements (such as impermeability, frost resistance, corrosion resistance, wear resistance, etc.).
Finally, through the concrete test, the admixture can be applied after the economic evaluation.