Effect of Aggregate Grading on Concrete Performance (II)

Liu Jiahui pointed out that the aggregates were firstly classified, re-matched to form new aggregates, repeatedly tested, and then the corresponding aggregate gradation was designed according to the ideal gradation curve, based on which orthogonal tests were conducted to determine the final mix ratio.

Rui Jie combined seven gradations from fine to coarse after sieving the mechanism sand for the situation that the mechanism sand is usually coarse and the gradation exceeds the upper limit of zone II, and studied the effect of mechanism sand gradation on the performance of high-strength grade concrete. The test results show that: the mechanism sand grading curve between the median and lower limit (coarse), 0.3mm sieve passage rate of 20%, the best concrete performance; grading curve between the median and upper limit (fine), the highest compressive strength of concrete when the fineness modulus is about 3.0. The same fineness modulus grading is not necessarily the same, the middle is small and large, the fineness modulus may also be 3.0, so in addition to the fineness modulus, but also need to point out the range of mechanical sand grading.

 

 

By formulating workability tests for concrete mixes of different strength grades, Yue Haijun determined a standard for full grading of mechanism sand, which mixes zone I and II from the perspective of satisfying the workability of concrete. The mechanism sand grading significantly affects the workability of concrete, and particles below 0.3mm are important for workability, smearing and water secretion of concrete; therefore, it is necessary to ensure a certain amount of particles below 0.3 mm. It is proposed in the paper that for conventional concrete, the content of particles below 0.3mm should be at least 15%; for high performance concrete, the best sand requires a cumulative sieve allowance of more than 70% for 0.6mm sieve and 85% to 95% for 0.3mm sieve because the amount of cementitious materials used becomes larger and the mineral admixture in concrete can ensure its viscosity. The study mainly studied the cumulative sieve margin range of fine particles, but the sieve margin of particles larger than 0.6 mm was not given.

In order to reveal the influence of the gradation of mechanism sand on the performance of concrete and the mechanism of action, Ai Changfa divided the mechanism sand into Ⅰ (particle size ≥1.18mm) and Ⅱ (particle size <1.18mm) components with 1.18mm as the dividing point, and prepared C50 concrete with six different gradations of mechanism sand and conducted performance tests on them. The test results show that the fineness modulus is only a macroscopic index to characterize the coarseness and fineness of the mechanism sand, which cannot truly reflect the particle gradation, and the decisive factor to determine the quality of the mechanism sand is the particle gradation, and it is found that the Ⅰ component of the mechanism sand mainly affects the water retention of the concrete, and the Ⅱ component mainly affects the water retention and cohesion of the concrete. In order to ensure the good working performance of concrete, it is recommended that the content ratio of Ⅰ and Ⅱ components should be kept around 1:2, and the particle composition ratio of Ⅰ component should be controlled effectively; in order to give full play to the role of filling density and secondary skeleton structure of mechanism sand, it is recommended that the accumulated sieve residual percentage of 1.18mm sieve grade should be controlled near the median value of the grade, and the sieve residual percentage of 4.75, 2.36 and 1.18mm sieve grades should be controlled by 2:3:1. Concrete performance optimization provides strong support, but only for C50 concrete, if for other strength classes of concrete whether there are different results need to be further.

Li Beixing pointed out that the actual production of mechanism sand stone powder content is generally high (about 10% to 20%), 0.075mm sieve residual part is also more, China’s specification for the part below 0.15mm sieve is within 10%, he proposed to appropriately relax the range of particles below 0.15mm sieve, both in line with the actual production of mechanism sand, but also to reduce the production cost of mechanism sand, while reducing the pollution. However, if the stone powder has high methylene blue value, it will affect the various properties of concrete, so if the range below 0.15mm is to be relaxed, the methylene blue value must be controlled. If the stone powder is blended as mineral admixture and tested with relevant standards, this can ensure the quality of the mechanism sand and also make full use of the stone powder.

Yang Wenlie pointed out that the medium coarse state of the mechanism of the sand is easier to formulate excellent performance of concrete. In the case of ensuring good aggregate grading in concrete, generally low strength grade concrete needs to use higher sand rate, small fineness modulus of the mechanism sand; high strength grade concrete needs to use lower sand rate, fineness modulus of the mechanism sand; mechanism sand when the stone powder methylene blue value is high, it will have a greater impact on the admixture content, concrete strength and compatibility.

Wang Jiliang pointed out that the moderate amount (7%-10.5%) of stone powder in the mechanism sand does not deteriorate the working performance of concrete, but can improve the working performance of concrete, and the compressive strength of concrete gradually increases with the increase of stone powder mass fraction; when the stone powder mass fraction is less than 7%, the elastic modulus of concrete made of mechanism sand is close to that of concrete made of natural sand, but when the stone powder mass fraction is higher than 10%, the elastic modulus of concrete made of mechanism sand decreases. When the mass fraction of stone dust is less than 7%, the modulus of elasticity of concrete prepared by mechanism sand is close to that of concrete prepared by natural sand, but when the mass fraction of stone dust is higher than 10%, the modulus of elasticity of concrete prepared by mechanism sand decreases; the content of stone dust has a greater influence on the early dry shrinkage value of high-strength concrete prepared by mechanism sand, but has little influence on the dry shrinkage value at the later age, meanwhile, the mixing of fly ash can reduce the dry shrinkage value of concrete prepared by mechanism sand at each age; the creepiness and creep strength of concrete prepared by mechanism sand with the mass fraction of stone dust of 7% increase gradually. The creep degree and creep coefficient of concrete prepared with 7% of stone dust mass fraction are closer to those of concrete prepared with natural sand.

Wang Yuli pointed out that when stone powder was added to river sand, the frost resistance of concrete could be improved; when the stone powder content was between 5% and 10%, the frost resistance of concrete was the best; when the stone powder content exceeded 15%, the frost resistance of concrete decreased. This may be due to the microaggregate effect produced by the right amount of stone powder can improve the concrete compactness and reduce the concrete porosity, thus improving the frost resistance of concrete.

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