by The introduction is made of mechanical crushing, shaping and screening after soil removal.
The parent rock with particle size less than 4.75mm is granite, limestone rock and mine tailings particles, but it does not include soft and weathered particles, called machine-made sand.
Ready mixed cement concrete made of machine made sand as the main fine aggregate is called ready mixed machine made sand concrete.
The mixing, transportation, pouring, vibrating, curing, formwork removal and other construction requirements of machine-made sand concrete are basically the same as those of natural sand concrete, but due to the influence of the characteristics of machine-made sand, its raw material quality control, concrete mix design, production and construction process quality control measures are different from those of natural sand concrete.
1 Quality control measures for raw materials The quality of machine-made sand determines the quality of concrete and directly affects the quality of construction projects.
The user of machine-made sand shall reinspect the machine-made sand entering the site according to the requirements of the current national standards and specifications.
The machine-made sand that fails to pass the reinspection shall not be used.
The stone powder content, parent rock strength, crushing value index, water absorption, etc.
contained in machine-made sand will affect the relevant performance of machine-made sand concrete, which must be strictly controlled in use.
1.1 Stone powder content control The content of particles smaller than 0.075mm in machine-made sand is called stone powder.
As machine-made sand is made by mechanical crushing, its particles are sharp and angular, and its surface is rough, which is conducive to improving the binding force of aggregate and cement, but is unfavorable to the workability of concrete.
There is an appropriate amount of stone powder in the machine-made sand, which makes up for this defect.
The composition of stone powder is the same as that of machine-made sand, which mainly plays the role of micro aggregate in concrete and can supplement the fine particles missing in concrete.
The presence of an appropriate amount of stone powder in machine-made sand increases the proportion of solid surface area to water, enhances water retention, improves bleeding, improves the workability of concrete, and improves the compactness and durability of concrete.
However, when the stone powder content exceeds a certain limit, with the increase of the stone powder content, the water consumption of the concrete mixture will increase, and the mixture will become dry and thick, which will also lead to concrete cracking and strength reduction.
In order to study the influence of the content of machine-made sand and stone powder on the performance of concrete, during the preparation of the specification, the two participating units selected the machine-made sand with MB ＜ 1.4 and the content of stone powder of 0, 5%, 7%, 10% and 13% respectively to prepare C25, C35, C45 and C60 pumped concrete, and tested the influence of machine-made sand with different content of stone powder on the performance of concrete mixture, concrete mechanical performance and concrete durability.
The test results show that in terms of the workability of concrete mixture, when the content of stone powder is 5~10%, the workability of concrete with strength grade C35 and below is better, and when the content of stone powder is less than 7%, the workability of concrete with strength grade C35 and above is better; In terms of mechanical properties of concrete, the peak compressive strength of premixed machine-made sand concrete with strength grades of C25 and C35 appears at about 7% of the stone powder content, while the peak compressive strength of premixed machine-made sand concrete with high strength grade of C60 appears at about 5% of the stone powder content; In terms of concrete durability, the curing methods of coating and spraying curing agent can effectively reduce concrete cracking.
When the stone powder content is 7%, the concrete has good crack resistance; In terms of impermeability, the content of stone powder in a certain range can improve the chloride ion permeability and water permeability resistance of ready mixed machine-made sand concrete, but when the content of stone powder exceeds 10%, the impermeability will decline.
Therefore, dust removal shall be carried out during the production of machine-made sand to ensure that the content of stone powder can meet the requirements in Table 1.
1.2 Control of the parent rock strength and crushing value index of machine-made sand In view of the fact that the parent rock strength and crushing value index of machine-made sand will directly affect the mechanical properties and durability of concrete, the crushing value index of machine-made sand and the strength limit of the parent rock of machine-made sand shall comply with the provisions in Table 2.
1.3 Water absorption rate and water content control The water absorption rate of machine-made sand is one of the main measures to control the water binder ratio of concrete and the working performance of the mixture.
The water absorption rate of machine-made sand should not be greater than 3%.
When the water content is too high, the stacked machine-made sand is prone to serious water content stratification (that is, the water content at the bottom is significantly higher than that at the top), which has a great impact on the performance of concrete mixture, thus affecting the concrete quality.
The water content of the incoming machine-made sand should not be greater than 8%.
1.4 Appearance quality control of machine-made sand Under the condition of the same nominal particle size, the specific surface area of non reshaped slender particles is larger than that of round particles after reshaping treatment.
When reaching the same mortar consistency, more cement paste needs to be coated.
The machine-made sand produced by reshaping can save the cement consumption in the machine-made sand concrete, and can better improve the mixture performance and physical performance of the machine-made sand concrete.
In order to study the influence of the type of machine-made sand on the performance of concrete, the preparation team organized two participating units to conduct tests on the influencing factors respectively.
The test results show that the overall performance of the concrete with shaped machine-made sand is better than that of the concrete without shaped machine-made sand.
2 Mix proportion design control and adjustment When the premixed machine-made sand concrete meets the requirements of engineering design and construction, it should follow the principles of low cement consumption, low water consumption and low shrinkage performance to design the concrete mix proportion.
The fineness modulus of machine-made sand has a significant impact on the working performance and strength of concrete.
When using machine-made sand to mix concrete, if the fineness modulus of machine-made sand is too large, there will be too many coarse particles, unreasonable grading, and poor workability.
If it is too small, there will be too much fine powder, large water demand, increased cement consumption, and low concrete strength.
The appropriate fineness modulus range should be 2.3~3.2.
In order to ensure the workability of ready mixed machine-made sand concrete, its sand ratio should be appropriately increased..
