# Influence of Aggregate Gradation on the Engineering Properties of Lightweight Aggregate Concrete
## 3 Results and Discussion
### Influence of Aggregates' Gradation on the Properties of Concrete Strength  
As shown in Table 5, the compressive strength of LWAC with various mix proportions was approximately 22 MPa to 26 MPa when the w/c is 0.7. For NWC with the same w/c, the compressive strength was about 21 MPa to 24 MPa, which was close to that of LWAC. However, the compressive strength of LWAC with various mix proportions was approximately 39 MPa to 58 MPa when the w/c is 0.4, while that of NWC was about 51 MPa to 56 MPa. The discrepancy between them was considerably evident. Figure 6 shows the relationship between the concrete compressive strength with uniform size aggregates and that of concrete with standard-grade aggregates (fit in with the ASTM specification). As shown in the figure, the compressive strength of LWAC which uses 7 mm uniform-sized aggregates is 11% higher than that of LWAC with standard gradation aggregates when the w/c is 0.7. Furthermore, the difference approaches 29% when the w/c is 0.4. The compressive strength of NWC which uses uniform size aggregates was mostly lower than that of NWC with standard-grade aggregates with high or low w/c. The values were in the range of 89% to 102%. This indicates that NWC with standard gradation aggregates possesses comparatively higher compressive strength. However, LWAC with standard gradation aggregates does not exhibit better compressive strength. There is a tendency for the compressive strength of LWAC increases as the aggregate grain size decreases, this appeared regardless of the w/c value. The larger SSA the aggregates possess, the higher compressive strength of the concrete. This is especially conspicuous under the low w/c. Like composite material, lightweight aggregate is the weak-phased material in LWAC (especially at the low w/c). Under the same total volume of aggregates, the number of aggregate particles increase as the grain size of the aggregates decreases. The distribution of weak-phased material in concrete  
Figure 5: Slump of normal-weight concrete (NWC) and lightweight aggregate concrete (LWAC). tends towards uniform. The uniformity of concrete is consequently improved, so that the concrete represents preferable strength. Furthermore, at the low water-cement ratio (w/c = 0.4), the difference of compressive strength with varied gradation is obvious. This is because the particle strength and elastic modulus of lightweight aggregates are both lower than those of mortar, so that the strength behavior of LWAC is mainly controlled by aggregates [14; 15; 16; 17; 18; 19; 20; 21; 22]. Hence the influence of aggregate gradation on the strength of concrete is extremely obvious. The strength and elastic modulus of natural aggregate are higher than that of mortar, and the achievable strength of NWC is mainly predominated by mortar. The aggregates merely present the reinforced particles in concrete. Therefore, the effect of the variation of aggregates' gradation on the concrete strength is not evident (shown in Figure 6b). This phenomenon occurs in LWAC with a high w/c, as well (shown in Figure 6a). The strength of mortar reduces as the w/c is high. The strength of lightweight aggregates could be higher than that of mortar at this time, and the lightweight aggregates possess the strength properties like natural aggregates. The lightweight aggregates perform the reinforced particles in LWAC, and the strength of concrete tends to be decided by mortar. For this reason, the strength of LWAC and NWC is almost identical (as w/c = 0.7) under the same mix condition. Furthermore, the influence of aggregate gradation has a similar tendency. As a result, the standard grade of aggregates that follows the specification requirement should still be adopted in NWC and high w/c ratio LWAC for improving the characteristics of strength and workability. In LWAC proportioning (low w/c), if the major concern is concrete strength, the standard gradation advised by ASTM could not be followed.