# Properties of a Lightweight Fly Ash-Slag Alkali-Activated Concrete with Three Strength Grades
## 3 Experimental Results
### UPV and Splitting Tensile Strength  
The 28-day UPV of the NOPC and NAAC, using normal-weight aggregate (NWA) ranges between 3.66 and 3.92 km/s (Table 5), indicate that the concrete quality is \"good\". Since the 28-day UPV of the LAAC-3 and LOPC concrete ranged from 3.60 to 3.72 km/s, they can also be categorized as \"good\" quality concrete; while the 28-day UPV of the LAAC-1 and LAAC-2 concrete ranged between 3.23 and 3.39 km/s, which indicated that the quality of these two kinds of concrete was \"medium\" [30]. [PERSON] et al. [17] reported that the 28-day UPV of an NOPC, fly ash-based AAC and slag-based AAC with a compressive strength of 35 MPa was 3.52 km/s, 3.20 km/s and 3.31 km/s, respectively; while the 28-day UPV of the NOPC, fly ash-based AAC and slag-based AAC with a compressive strength of 65 MPa was 4.15 km/s, 3.93 km/s and 3.98 km/s, respectively. [PERSON] et al. [16] reported that the 28-day UPV of palm oil fuel ash-slag-based NAAC with NWA (the compressive strength is 37-42 MPa) ranged between 3.51 and 3.76 km/s; while the 28-day UPV of palm oil fuel ash-slag-based LAAC using the oil palm shell LWA (the compressive strength is about 29-33 MPa) was between 3.19 and 3.48 km/s. The measured values of UPV of all kinds of concrete in this study are shown in Figure 6a, which indicated that the UPV of the NAAC and LAAC increased as the compressive strength increased. For the NAAC in this investigation, the average UPV increased by 107 m/s for each 10 MPa increase in compressive strength, while for the LAAC, the average UPV increased by 182 m/s for each 10 MPa increase in compressive strength. The ultrasonic pulse velocity (UPV) of the NAAC and LAAC was lower than that of the NOPC and LOPC with a similar compressive strength, which indicated that more internal voids exist in the AAC compared to its cement counterpart. Figure 6b illustrates the measured values of the splitting tensile strength for all kinds of concrete used in this study and in previous studies. Table 5 and Figure 6b demonstrate that the splitting tensile strength of all the AACs increased with the compressive strength, and NAAC and LAAC achieved a higher splitting tensile strength than their cement counterparts with a similar compressive strength. ASTM C330/C330M-2014 specified that the splitting tensile strength of the structural grade lightweight aggregate concrete should be more than 2.0 MPa. All mixes in this study meet the minimum requirements of a structural lightweight aggregate concrete. In this study, the tensile/compressive strength  
ratio of the NAAC ranged from 9.47% to 11.80%, while that of the LAAC ranged from 8.42% to 10.46%, which was slightly lower than that of the NAAC. This may be due to the weaker stiffness of the spherical LWA particles used in LAAC than that of NWA used in the NAAC. [PERSON] et al. [13] reported that the tensile strength of an FA-based AAC increased significantly when 10% to 30% BFSP was added to the AAC matrix. [PERSON] et al. [39] found that their measured splitting tensile strengths of an FA-BFSP-based AAC were mostly lower than the predictions of ACI building code 318 [39]. [PERSON] et al. [12] found that their measured splitting tensile strengths of an FA-BFSP-based AAC were slightly lower than those obtained by [39]. [PERSON] et al. [47] reported the measured splitting tensile strengths of an FA-BFSP-based LAAC, using expanded clay LWAs with a 28-day compressive strength of 6.5-8.5 MPa, ranged from 1.02 MPa to 1.30 MPa, which were in good agreement with the predictions of ACI 318-08 code [39]. [PERSON] et al. [48] investigated the FA-based high early strength AAC, and found that the measured splitting tensile strengths were lower than the prediction of ACI building code 318 [39]. [PERSON] et al. [24] reported that the measured splitting tensile strengths of a slag-based LAAC (the compressive strengths were about 38.5-42 MPa), using expanded clay LWAs, varied in the range of 2.81-3.35 MPa, which were higher than the splitting strengths of LOPC with the same density and strength grade.