*2.1. Raw Materials*

Ordinary Portland cement, with a strength grade of 52.5, fly ash of class II, and ground granulated blast furnace slag (GGBS) of class S95, was used as a binder material. The detailed physical and mechanical properties of this cement are listed in Table 1, where it can be seen that it met the specification of China code GB 175 [27]. The detailed physical and mechanical properties of the fly ash and GGBS are listed in Table 2, where it can be seen that they met the specification of China code GB/T 51003 [28]. Pictures of the binder materials are presented in Figure 1.

**Table 1.** Physical and mechanical properties of cement.




**Figure 1.** Pictures of binder materials: (**a**) cement; (**b**) fly ash; (**c**) GGBS.

Manufactured sand crushed from limestone and river sand were used as fine aggregates. Their physical and mechanical properties are listed in Table 3. Crushed limestones with maximum particle sizes of 20 mm, 16 mm, and 10 mm from 1 company were used as coarse aggregates. Their physical and mechanical properties are listed in Table 4. Their particle gradations are presented in Figure 2 and are adjusted according to the principle of maximum close-packing density. The gradations of fine and coarse aggregates met the specifications of China codes GB/T 14684 and GB/T 14685 [29,30]. Pictures of the fine and coarse aggregates are presented in Figure 3.

**Table 3.** Physical and mechanical properties of fine aggregates.



**Figure 2.** Particle size distribution of the coarse aggregate used.

**Table 4.** Physical and mechanical properties of coarse aggregates.

**Figure 3.** Pictures of aggregates: (**a**) manufactured sand; (**b**) river sand; (**c**) 5~10 mm aggregate; (**d**) 10~16 mm aggregate.

A polycarboxylate-based superplasticizer was used as a water reducer with a waterreducing rate of 29% and solid content of 23%. The mixing water was tap water.

#### *2.2. Mix Proportion Design*

In this study, the absolute volume method is used to design the mix proportion of the SCC [31,32]. The target compressive strength of SCC was 69 MPa, which is standard for concrete of strength grade C60 with a guarantee rate of 95%. The target workability of the fresh SCC was a slump larger than 260 mm, a slump flow of 700 mm, and a flow time *T*<sup>500</sup> less than 5 s. The target pouring quality of the fresh SCC was a density greater than 2450 kg/m3.

A total of 11 groups of mixtures for SCC were designed, with the influencing factors detailed in Table 5. The influencing factors included the ratio *R*<sup>r</sup> of manufactured sand substituted by river sand, the ratios *R*<sup>G</sup> and *R*<sup>F</sup> of GGBS and fly ash in the total mass of the binders, and the maximum particle size of coarse aggregates (MPS). The water-to-binder ratio *w/b* was kept at 0.31, and the sand ratio *β*<sup>s</sup> was kept constant at 50%.


**Table 5.** The influencing factors designed in the mix proportion of the SCCs.

The detailed mix proportions of the SCCs are presented in Table 6. The water content was 180 kg/m3. The water reducer was kept constant at 1.6% of the total mass of the binders. This is to eliminate the influence of the water reducer content on the fresh performance of the SCC.


**Table 6.** The detailed mix proportions of the SCCs (kg/m3).

#### **3. Experimental Verification**

#### *3.1. Fresh Performance*

A horizontal shaft forced mixer with a maximum capacity of 100 L was used to mix the SCC. The slump, slump flow and slump flow time *T*<sup>500</sup> of the fresh SCC were tested immediately after mixing according to the Chinese code JGJ/T 283 [1], which is identical to ASTM C1611/C1611M [33]. The slump flow time *T*<sup>500</sup> is the time from the lifting of the slump cone to the diameter of the slump expansion surface reaching 500 mm [34]. Results with comparisons to target indices and specified filling ability are presented in Figure 4.

The reasonable mixing of machine-made sand and river sand can optimize the particle grading of sand and bring about good workability for fresh SCC [14,15]. In this test, the workability of SCC first decreased with the increasing ratio of river sand up to 27.5%, then increased with the ratio of river sand from 27.5% to 45%. This is similar to the study of pumped concrete with manufactured sand and river sand [17]. Compared with R0, R45 and R100, R27 has the worst fresh performance, with a smaller slump of 230 mm, a lower slump flow of 600 mm and a longer flow time *T*<sup>500</sup> of 11 s, while R35 has good fresh performance, with a suitable slump of 245 mm, a higher slump flow of 700 mm and a shorter flow time *T*<sup>500</sup> of 3.5 s.

Fly ash and GGBS are conducive to the slump and slump flow of fresh SCC R35-F30 and R35-G30 compared to R35. However, the addition of GGBS prolongs the *T*500, while the addition of fly ash has no obvious influence on the *T*500. The *T*<sup>500</sup> of R35-G30 is 1.43 times that of R35, while the *T*<sup>500</sup> of R35-G30 was equal to that of R35. Compared with R35-G30, R35-F30 had the same slump, a higher slump flow and a shorter *T*500. This indicates that fly ash improved the working performance of the SCC. The reason is mainly due to the special morphological effects of fly ash, with microspheres featuring smooth surfaces, fine particle size and dense texture. Then, the fresh SCC with fly ash would adsorb a small amount of water in the mixing process [35], reducing the internal friction resistance and improving the flowability of the fresh concrete [8,36].

Compared with R35, R35-G10F20 and R35-G20F10 presented with a higher slump and slump flow. This indicated that the hybrid fly ash and GGBS contributed to the improvement of flowability of SCC, although no superposition effect of fly ash and GGBS was observed in these indexes. However, the negative superposition effect of hybrid fly ash and GGBS appeared on the *T*500. The *T*<sup>500</sup> of R35-G10F20 and R35-G20F10 were longer than both R35-F30 and R35-G30.

Compared with R35, R35-C16 had a better flowability, with a higher slump of 265 mm, a higher slump flow of 720 mm and a flow time *T*<sup>500</sup> of 8.8 s, while R35-C10 had a similar slump, a slightly lower slump flow and the longest *T*<sup>500</sup> of 11.3 s. This indicates that the gradation of the coarse aggregates had effects on the slump and slump flow, and especially on the flow time of the fresh SCC. The *T*<sup>500</sup> of the SCC increased with the decreasing maximum particle size of the coarse aggregate.

**Figure 4.** Workability of fresh SCC: (**a**) slump; (**b**) slump flow; (**c**) slump flow time *T*500.
