# Influence of Aggregate Gradation on the Engineering Properties of Lightweight Aggregate Concrete
## 3 Results and Discussion
### Influence of Aggregates' Gradation on the Properties of Fresh Concrete  
The mixing duration was two minutes. The slump and air content tests proceeded after mixing for eight minutes. The slump test results are shown in Table 5 and Figure 5. From Table 5, it can be seen that the LWAC slumps were in the range of 130 mm to 220 mm. The NWC slumps were in the range of 70 mm to 200 mm when the w/c = 0.7. As for the w/c = 0.4, the LWAC slumps were within 45 mm to 65 mm, while the NWC slumps were within 25 mm to 60 mm. Figure 5a shows a LWAC slump in which 7 mm uniform size aggregates were 130 mm when the w/c is 0.7. The slump increased to 220 mm as the size of the aggregate changed to 16 mm. The increment was about 70%. NWC exhibited the same trend. The workability of the concrete was obviously affected by the aggregate grade. Figure 2 shows that the SSA of the aggregates and the slump are inversely proportional to grain size. The concrete slump becomes larger as well. The same tendency appeared at the low water-cement ratio (w/c = 0.4, shown in Figure 5b). The main reason is that the amount of paste adhering to the aggregate surface becomes reduced as the SSA decreases. When the total volume of paste is fixed, the paste provided for flowing increases relatively. Therefore, the workability of the concrete becomes better. This is not so obvious at lower water-cement ratios because of the high paste consistency in the concrete. The differences between LWAC and NWC were further compared. Under the same mix proportions, the LWAC slump was larger than that of NWC slump. The possible reason is that the coarse aggregates of LWAC were oven-dried. Thirty minutes of lightweight aggregate absorption was used while mixing. The redundant water, which could not be completely absorbed by the lightweight aggregates at the beginning of the mixing process, transformed into mixing water. Therefore, the workability of LWAC was enhanced. Moreover, the shape of lightweight aggregates was much rounder than that of normal weight aggregates. Therefore, the resistance was relatively low when flowing. Combining the characteristics of lightweight aggregates mentioned above, all cause a slightly better LWAC workability than that for NWC under the same mix proportions. According to the air content test result in Table 5, the air contents of LWAC were in the range of 1.5% to 2.5%. Compared to the air content in NWC with the same aggregates grade, the air content of LWAC was about 0.4-1.1% higher than that of NWC. The major cause is the great quantity of pores existing on the shell of lightweight aggregates. The air content measurement for concrete is the pressure method, so that the value might be highly estimated because of the influence from such surface pores. Therefore, the air content of LWAC is higher than that of NWC. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Fresh Concrete**} & \multicolumn{2}{c}{**Hardened Concrete**} & \multicolumn{2}{c}{**Durability**} \\ \cline{2-7}
**Item** & **Slump** & **Air Content** & **Unit Weight** & **Compressive** & **Perment** & **Electrical** \\  & **(mm)** & **(\%)** & **(kg/m\({}^{3}\))** & **Strength (MPa)** & **Volume (cm\({}^{3}\))** & **Resistance (k\(\Omega\)-cm)** \\ \hline L0.7-S & 180 & 2.1 & 1857 & 23.8 & 16 & 15.9 \\ L0.7-16 & 220 & 1.5 & 1874 & 21.9 & 17 & 15.9 \\ L0.7-11 & 180 & 1.9 & 1852 & 24.5 & 19 & 15.9 \\  
The specific gravity of normal-weight aggregates was about 2.65 for each grain size. The specific gravity of lightweight aggregates was about 1.46 for each grain size. By virtue of the similar aggregate specific gravity for each size of aggregate, the influence on the unit weight of aggregates' grade was not obvious for both LWAC and NWC. Comparing the unit weight of LWAC and NWC, LWAC, which uses the expanded shale lightweight aggregates, is about 20% lower than NWC on average. The unit weight of LWAC depends on the specific gravity and amount of lightweight aggregate.