6.4 C OMPARISON OF STABILITY OF NON - AIR AND AIR ENTRAINED LWAC WITH
6.4.2 Effect of yield stress on fluidisation of fresh concrete under vibration
The results above showed that the non-air entrained concrete had better stability under vibration than the air entrained concrete at higher yield stresses of about 650 Pa, and vice versa at lower yield stresses of about 200 and 350 Pa. In a study by Banfill et al (1999), it was found that the fluidity of fresh concrete under vibration is influenced by the peak vibrational velocity and that the rheology of the unvibrated concrete (i.e. original yield stress and plastic viscosity) governs its response to vibration. The peak vibrational velocity is a function of the frequency and amplitude of vibration. He suggests that fluidisation of fresh concrete under vibration occurs when the energy input by the vibration is sufficient to reduce the yield stress as the force of attraction between the cement particles and the internal friction between the aggregate particles have been overcome. In another study on the effect of vibration on workability of concrete, Tattersall (1989) suggests that for a given concrete there is a minimum amplitude below which vibration has no effect on the concrete because the yield stress is not reduced sufficiently for flow to occur. In addition, there is also a maximum limiting frequency above which the vibration is ineffective. The minimum amplitude is dependant on the fresh concrete mixture, the applied frequency, and the maximum limiting frequency. The above indicates that this minimum amplitude necessary for the fluidisation of fresh concrete may be related to
the yield stress of the concrete. The higher the yield stress of the fresh concrete, the higher the minimum amplitude may be required to achieve fluidisation. The requirement of a minimum amplitude for vibration and consolidation is also mentioned in a report by ACI 309 (1993).
The lower MI shown by the Series I non-air entrained concrete in Fig.6.5 may be due to the higher yield stress of the concrete so that the energy input from the vibration was insufficient to reduce the yield stress to a level low enough to bring about fluidisation even though the vibration time for Series I concrete was longer than that for Series II concrete. This implied that the applied vibration amplitude of 0.21 mm was probably below the minimum amplitude required to reduce the yield stress sufficiently to bring about fluidisation at the vibratory frequency of 50 Hz. This also implied that fluidisation of the Series I non-air entrained concrete in Fig.6.5 might not have occurred during vibration due to the higher yield stress of the concrete, resulting in lower MI values. Figure 6.7 shows the effect of the yield stress on the MI for the Series I non-air entrained concrete. These concrete had similar plastic viscosity. The MI remained rather constant at about 3 – 6% when the yield stress was above approximately 400 Pa. It is likely that the applied amplitude of 0.21 mm was insufficient to overcome the yield stress to bring about fluidisation of the Series I non- air entrained concrete when the yield stress was above approximately 400 Pa. As the yield stress decreased from about 400 to 40 Pa, the MI increased significantly to about 40%. From these, it appears that for given amplitude and frequency of vibration, there is a critical yield stress of fresh concrete above which the fluidisation may not occur, thus segregation of the concrete may be limited.
0 10 20 30 40 50
0 200 400 600 800 1000 1200 1400 1600 1800 Yield stress (Pa)
MI (%)
F6.5
Fig.6.7 – Effect of yield stress on the Mass Deviation Index (MI) of Series I non-air entrained concrete
The origin of the yield stress may be contributed by three primary sources as discussed earlier in Section 4.4.1 (page 93). These include the mechanical interlocking between aggregates, the attractive colloidal forces between the cement and other submicron particles, and a colloidal gel of hydrated calcium silicate that forms around the cement particles as a result of cement hydration. However, the mechanism by which vibration reduces the yield stress is not fully understood. Petrou et al (2000) speculated that vibration causes the larger aggregates to jiggle, thus unlocking the initially interlocked structure and reducing the internal friction. The vibration may also deflocculate the cement particles, and break the initially weak chemical bonds resulting from the early hydration products in fresh cement paste.
Therefore, the reduction in the yield stress of concrete under vibration may be related to the weakening of the mechanical, electrostatic, and chemical bonds among its ingredients.
Although the air and non-air entrained concrete in Fig.6.5 had similar yield stress, it is believed that the yield stress in each type of the concrete may be dominated by different sources. The air entrained concrete may have a greater electrostatic attraction due to the attraction between the air bubbles and the cement particles. This explains why the air entrained concrete is more cohesive. On the other hand, the non-air entrained concrete may have a greater internal friction due to a higher volume of solid particles and coarse aggregates. In view of these, it was likely that for given amplitude and frequency of vibration, the critical yield stress for the air and non-air entrained concrete was different.
In this case, the critical yield stress of the air entrained concrete was likely to be higher than that of the non-air entrained concrete. The reason is that at similar yield stress, the air entrained concrete had lower plastic viscosity. Thus, at the yield stress of about 650 Pa, the same vibratory energy might have caused fluidisation in the air entrained concrete but not necessary for the non-air entrained ones (Fig.6.8). This may explain why the non-air entrained concrete of Series I in Fig.6.5 had lower MI than the air-entrained ones. For Series II concrete, it was likely that fluidisation occurred in both air entrained and non-air entrained concrete during vibration (Fig.6.6
& Fig.6.8) as the yield stress of the concrete was lower.
In summary, the air entrained concrete had better stability than the non-air entrained one at similar yield stress so long as the concrete was fluidised during vibration. However, it was noted that the air entrained concrete became less stable during vibration as entrained air content was increased.
Fig.6.8 – Schematic presentation of critical yield stress of air and non-air entrained concrete: Air entrained concrete might have higher critical yield stress. Series I non-air entrained concrete (yield stress about 650 Pa) might not have fluidised while the air entrained concrete and Series II non-air entrained concrete were fluidised under vibration.