At present, there are three main theories for evaluating the advantages and disadvantages of particle grading as follows.
The first one is the maximum density theory. The maximum density theory believes that the best gradation should be the one with the minimum porosity and maximum density. The second one is the surface area theory. This theory believes that the optimal gradation should be the one with small surface area of the aggregate and small amount of cement paste used to wrap the surface, while the third one is the particle interference theory. Particle interference theory believes that if the upper level of aggregate produces a void exactly equal to the next level of aggregate particle size, then the next level of aggregate filling the void does not “interfere” with the grade as the most optimal grade. All three theories have their own rationality, but in the end, which gradation is suitable for modern concrete needs further study. All three theories have their own rationality, but which gradation is suitable for modern concrete needs further study.
Chen XiYun in the article “coarse aggregate on the concrete performance” pointed out that in actual production, you can use two or three kinds of coarse aggregates in different proportions to match, so that the coarse aggregate can achieve the minimum void ratio. With 16 ~ 31.5 mm single-grain crushed stone and 5 ~ 16 mm continuous grade crushed stone with, when the ratio is 6:4, or with 20 ~ 40 mm single-grain crushed stone and 10 ~ 20 mm single-grain crushed stone with, the ratio of 3:7, the concrete and strength are better. The text points out that 16 to 31.5mm and 5 to 16mm ratio of 6:4, to obtain the minimum void ratio, but the range of continuous grading is too large, if you change the ratio of each particle size in 16 to 31.5mm and 5 to 16mm, then the results are not necessarily so, about 20 to 40mm single-graded crushed stone and 10 to 20mm single-graded crushed stone with the ratio of 3:7, the void ratio may lower, but the surface area is relatively large, the general situation is not used.
Jiandong Zheng also pointed out that the ratio of small aggregates (5-20 mm) to large aggregates (20-40 mm), with 4:6 as the excellent gradation, but this experimental study is plastic concrete, the study is not for modern large fluid concrete.
Fusheng Zhu established a fine numerical model of concrete with nine different grouping ratios and simulated the damage process of concrete under uniaxial compression using the finite element analysis system MFPA. The results show that the concrete compressive strength is directly affected by the different batching ratios. It is proposed that when the ratio of 5-20 mm and 20-40 mm is 4:6, the best concrete performance is formulated. The ratio of the two is best in 3:7 to 4.5:5.5, if more than this range requires secondary screening or adjustment to close to the optimal aggregate ratio. Research points out that the following 20mm stones should not exceed 40% of the concrete aggregate, this is because the smaller the stone, the surface area increases, the water demand increases, 20 to 40mm stones should not exceed 60% to 70%, because too many large stones, the void ratio increases, if there are no small stones to fill, then the need for cement mortar to fill, which will affect the performance of concrete.
The study yielded two aggregate proportions, but the difference between the two grain sizes is large, and for modern concrete, where the aggregates used are 5 to 25 mm, the resulting aggregate proportions are not applicable.
Lixia Xie selected three types of limestone quality aggregates from 3 to 8 mm, 8 to 15 mm and 15 to 25 mm mixed in different proportions and investigated the effect of coarse aggregate gradation on the performance of normal and high strength grades of machined sand concrete. The slump of C30 concrete (3-8 mm: 8-15 mm: 15-25 mm = 20:40:40) prepared with tertiary-mix coarse aggregate was the largest, and its resistance to segregation was also better. In contrast, the slump of C60 concrete prepared with coarse aggregates of tertiary mix was the smallest, while the slump of C60-2 concrete (20% of 3-8mm aggregates and 80% of 15-25mm aggregates) was the largest, and the anti-separation properties of C60 concrete were all better than those of C30 concrete. The results show that for normal strength grade concrete, the concrete prepared with three-stage coarse aggregate has high strength and the specific surface area of coarse aggregate has a greater influence on the concrete properties; while for high-strength grade concrete, the concrete prepared with two-stage coarse aggregate has high strength and the coarse aggregate pile void ratio has a greater influence on its concrete properties.
Geng Xiuchun also pointed out that the compressive strength of concrete depends on both its water-cement ratio and the gradation of its coarse aggregate. For the low strength grade of concrete, the coarse aggregate with larger particle size should be used to reduce its total surface area, while for the high strength grade of concrete, the coarse aggregate with smaller particle size should be used to reduce the tendency of water secretion around the coarse aggregate, thus improving the quality of the interface transition zone and increasing the strength of concrete.
