1 Characteristics of machine-made sand
According to GB/T 14684-2011 "Sand for Construction", artificial sand (artificial sand) does not include soft and weathered particles, but refers to the particle size not exceeding 4.75mm, which has undergone soil removal treatment, mechanical crushing and screening. Manufactured rock, mine tailings or industrial waste granules. The current fineness modulus of machine-made sand is generally between 2.6 and 3.6, which belongs to medium-coarse sand and contains a certain amount of stone powder. It not only has irregular particle gradation, but also has a rough surface and sharp edges and corners. However, the particle size and gradation of machine-made sand are not fixed, and there are many influencing factors, such as local production sources, processing equipment, processes, etc., but as long as it is machine-made sand that meets the requirements and the technical indicators are passed, it can be used in concrete. Determined by the characteristics of the machine-made sand itself, even if it is compared with the natural sand with the same gradation and fineness modulus, even the proportioning design and the forming and curing conditions of other materials are the same, the characteristics of concrete prepared with machine-made sand are: Keeping the slump unchanged, the water demand increases; the slump decreases, and the 28d standard strength increases. If the proportion of concrete is designed according to the law of natural sand, the water demand of machine-made sand is large, and the workability is relatively poor, and it is easy to produce bleeding, the most obvious is in the low-strength grade concrete with less cement.
2 The role of stone powder
For a long time, since stone powder and mud are particles in sand with a particle size of less than 0.075mm, there was no basis for identification before the methylene blue test method was introduced. It is generally believed that stone powder is harmful to concrete. Because of the lack of scientific understanding, people think that stone powder cannot be used. In some places, they even waste time, energy and money, trying their best to remove the stone powder in the sand. In fact, many domestic and foreign experts believe that it is beneficial to add appropriate stone powder to concrete. There are three functions of stone powder in machine-made sand concrete, namely:
① Hydration: Studies have shown that the ettringite formed in the early stage of hydration will be converted to monosulfur calcium sulfoaluminate in the later stage, which will reduce the strength of the cement stone, but adding stone powder containing calcium carbonate can effectively solve the problem. This problem; in addition, the stone powder is mainly calcium carbonate, and calcium carbonate can undergo hydration reaction with C3A to form hydrated calcium carbonate aluminate, thereby improving the strength of concrete.
②Filling function: Stone powder can fill the voids in the concrete and act as a filler for the concrete, thereby increasing the compactness of the concrete, thus playing the role of an inert admixture. For the characteristics of less cementitious material and poor mixture performance, as long as the machine-made sand concrete of medium and low strength grades is used, it can be effectively compensated. In "Technical Regulations for Mountain Sand Concrete" DBJ52-016-2010, for mountain sand concrete of C50~C55 strength grade (sand rate 37%~45%), it is proposed that the stone powder should not exceed 7%, this is because the stone powder content and the cementitious material The total amount (fine powder content of concrete) cannot exceed 550kg/m3. (Note: Mountain sand is a kind of machine-made sand. The "Technical Regulations for Mountain Sand Concrete" DBJ52-016-2010 specifically refers to the nominal particle size of less than 5.00mm. formed rock particles.)
③ Water retention and thickening effect: There is stone powder in the machine-made concrete, which can reduce the risk of segregation and bleeding of the concrete mixture. Because the stone powder can absorb the water in the concrete, which virtually increases the unilateral water consumption of the concrete, the higher the stone powder content, the greater the viscosity of the concrete; in addition, adding stone powder can also reduce the shrinkage of the concrete and compensate for the water used for hydration in the later stage of the concrete. Because, even if the concrete hardens, the moisture previously absorbed by the stone powder will gradually be released.
The study found that the content of stone powder should be moderate. The main component of stone powder in machine-made sand is calcium carbonate, but the hydration is not unlimited, and it is also limited by the composition of cement. If the stone powder content is too high, it is not conducive to the bonding between aggregate and cement stone, because free stone powder will appear in the cement stone or in the interface transition area, thereby reducing the performance of concrete. In addition, after the stone powder content exceeds a certain limit, it is not conducive to the durability of concrete, because the water retention capacity of the single stone powder is significantly reduced, and the drying shrinkage is significantly larger. Based on various studies, generally the content of stone powder in concrete below C50 should be controlled at 10% to 15%, while the content of stone powder in concrete above C50 should not exceed 10%.
3 Mix ratio design of pumping machine-made sand concrete
(1) Unilateral water consumption
Due to its own characteristics, river sand has relatively smooth particles, and its own internal friction force is small. The surface of machine-made sand is rough and has many edges and corners, resulting in a large stranding force between particles in the concrete. Because the viscosity of the machine-made sand pumped concrete is higher than that of the river sand concrete, the construction slump is generally maintained at 180-220mm, and the unilateral water consumption is generally around 175-185kg/m3, which is completely different from the lower water consumption of the river sand concrete. of. At the same slump, if a lower water consumption is used, it will inevitably lead to over-mixing of admixtures, resulting in high fluidity of machine-made sand concrete, but high viscosity and poor pumpability. Water is often added on the construction site to reduce viscosity. As a result, the quality of ready-mixed concrete cannot be guaranteed. Therefore, when designing the mix ratio of machine-made sand, the use of admixtures should not be excessively pursued to reduce the unilateral water consumption of machine-made sand concrete, and the unilateral water consumption of machine-made sand concrete should be designed to be slightly larger than that of river sand concrete.
(2) Sand rate
Generally speaking, in order to increase the production capacity, the manufactured sand produced is mostly medium and coarse sand, the fineness modulus of the sand is generally 2.6 to 3.6, the particles above 1.18mm are more, and the particles above 1.18mm of the inferior manufactured sand account for When it reaches 50% to 70%, the composition of 0.315mm in the sand fluctuates at 8% to 13%. Most engineering practices in my country show that medium sand is suitable for pumping, and the number of 0.315mm sieve holes in the sand has a great influence on the pumpability of concrete. The Japanese pumping concrete regulations stipulates 10% to 30%, and the fine aggregate gradation curve recommended by the American Concrete Association is recommended to be 20%; domestic engineering practice has also proved that if this value is too low, the conveying pipe is easy to be blocked. Shanghai, Beijing, Guangzhou, etc. The construction experience of ground pumping concrete shows that this value is above 15%. JGJ/T10-95 "Technical Regulations for Concrete Pumping" stipulates that the fine aggregate should comply with the "Quality Standards and Inspection Methods of Sand for Ordinary Concrete". particle size. However, the machine-made sand produced by the general sand field is slightly insufficient at 0.315mm, and at the same time more than 1.18mm, which brings certain difficulties to the preparation of pumped machine-made sand concrete.
The usual method is to appropriately increase the sand rate and increase the number of particles of 0.315mm to ensure the pumpability of machine-made sand concrete. However, there is also a misunderstanding in the preparation of machine-made sand for pumping concrete. The appropriate sand rate should be selected. When designing the mix ratio of machine-made sand concrete, if the sand rate is too large, although the workability is maintained, the early shrinkage of the fresh concrete will increase, and problems such as plastic cracking are prone to occur. It is more obvious when it is higher. Usually, in order to ensure the pumpability of the machine-made sand concrete and ensure the economy of the concrete preparation, the sand rate of the machine-made sand pumping concrete is generally 5% to 10% higher than that of the river sand concrete.
