4.6 R ELATIONSHIP BETWEEN RHEOLOGICAL PARAMETERS AND SLUMP
4.6.2 Increase in slump of air entrained concrete at similar yield stress
Table 4.6 shows the properties of non-air and air entrained concrete in Series I with similar yield stress about 650 Pa. Figure 4.19 shows the relationship between the plastic viscosity and the slump of the concretes. The figure shows that the plastic viscosity decreased as the slump increased due to increasing air entrainment. This is not consistent with the absence of correlation between plastic viscosity and slump of the non-air entrained concrete as observed in Fig.4.18 in the previous section and by other researchers as well (Tattersall and Banfill 1983; Tattersall, 1976; Murata, 1984;
Mork, 1996). However, it is noted that the change in the plastic viscosity from these references was not achieved through air entrainment. In spite of this difference, it is known that air entrainment increases the slump of concrete (ACI 211, 1991).
Table 4.6 – Properties of non-air and air entrained concrete in Series I having similar yield stress of about 650 Pa
Density of fresh concrete
Plastic Mortar
density
Yield 28-day SP Air Slump
Mix MI
viscosity
stress strength content
No. (kg/m3) (%) (kg/m3) (kg/m3) (mm) (Pa) (Pa.s) (%) (MPa)
6-5 1850 70 616 60 5 50.4 6-6 1865 Non-air 110 723 56 6 --
entrained
6.26 4.5 2220
6-7 1855 75 697 58 5 51.2
6A-2 1835 6 2135 140 566 48 7 38.5 6A-3 1815 140 721 46 10 42.3
6A-4 8 2065 1775 165 710 38 17 46.4 6A-12 12 1935 1665 150 687 31 10 43.3
6A-13 1660 190 599 19 17 45.5 Air
entrained 5.22
14 1870
6A-14 1595 180 558 24 14 34.5
6A-16 1550 17 1775 180 612 19 19 33.3 6A-17 1535 190 594 19 13 29.0
0 10 20 30 40 50 60 70
50 75 100 125 150 175 200
Slump (mm)
Plastic viscosity (Pa.s)
non-air entrained air entrained 6%
8%
12% 14%
17% 17%
14%
4.5%
Fig.4.19 – Relationship between the plastic viscosity and slump of concrete in Series I at similar yield stress of about 650 Pa (Values besides corresponding data points are total air content).
In a report of Project EuroLightCon (1998), it is mentioned that the slump test tends to underestimate the workability of LWAC. In other words, the slump of LWAC is normally lower than the slump of normal-weight aggregate concrete (NWAC) at similar workability. This is due to a lower density of the concrete as lighter coarse aggregates are used. The driving force that leads to the slump in the concrete is derived from the self-weight of the concrete. Hence, the slump tends to be lower when the density of concrete decreases at similar yield stress.
In this study, apart from the increase in slump as entrained air content increased (Fig.4.19), the density of the concrete was also reduced at the same time (Fig.4.20). Hence, the slump was increased as the density of the air entrained concrete decreased (Fig.4.21). This is opposite to the finding stated in the Project EuroLightCon (1998) that the slump of LWAC is normally lower than the slump of NWAC at similar workability. The reason for this might be due to the decrease in the
plastic viscosity of the concrete that led to the increase in the slump. It has been shown that the plastic viscosity is a function of the final slump when the slump of the concrete is greater than 200 mm (Ferraris and de Larrard, 1998a). The reason for the trend shown in Fig.4.21 could be that when the concrete slumped under the influence of self-weight, the one with a lower plastic viscosity would flow with greater acceleration than another with a higher plastic viscosity. The concrete flowing with greater acceleration would require a larger internal opposing force to stop the flow.
1500 1600 1700 1800 1900
0 2 4 6 8 10 12 14 16 1
Air content (%) Density of fresh concrete (kg/m3 )
8 non-air entrained air entrained
Fig.4.20 – Effect of air entrainment on density of concrete in Series I at similar yield stress of about 650 Pa
50 75 100 125 150 175 200
1500 1600 1700 1800 1900
Density of fresh concrete (kg/m3)
Slump (mm)
non-air entrained air entrained
Fig.4.21 – Effect of change in density of concrete in Series I on slump due to air entrainment at similar yield stress of about 650 Pa
This is further explained by Wallevik J.E. (2003) who states that for a given yield stress, the concrete with a smaller spacing between the larger aggregates tends to form a self-bearing network due to interlocking of the aggregates during the slump test. This prevents further flow of the concrete leading to a lower slump. In the current study, as the entrained air content increased, the average spacing between the aggregates in the concrete was also increased, which was one of the reasons for the reduction of the plastic viscosity, as mentioned earlier in Section 4.4.2 (page 95). This explains the higher slump of the air entrained concrete when the plastic viscosity decreased.
In the study by Wallevik J.E. (2003), the concrete with the larger average spacing between the aggregates has a lower w/c, and thus, a higher average plastic viscosity due to lower water content. His results show that the concrete with higher average plastic viscosity has higher slump for a given yield stress. This is contrary to the results obtained for the air entrained concrete in the current study. The results for
the air entrained concrete with similar yield stress indicated that as the plastic viscosity decreased, the slump increased. However in both cases, the slump increased as the average spacing of the aggregates increased. This implied that the spacing of the aggregates might have significant effect on the slump of concrete with given yield stress. This will be further discussed in the next section. In summary, the increase in the entrained air content led to the decrease of both the plastic viscosity and the density of concrete, while the slump was increased.