Air entrainment is a process whereby many small air bubbles are incorporated into concrete and become part of the matrix that binds aggregate together in hardened concrete. The air bubbles are dispersed throughout the cement paste but are not, by definition, part of the paste (Dolch, 1984). Air entrainment has been used to protect
hardened concrete from damage due to repeated freezing and thawing cycles. The air bubbles with typical diameters from 0.02 to 1.0 mm are generated by addition of air- entraining admixtures (AEAs) (Hewlett, 1998). The entrained air bubbles are produced through agitation of the concrete during mixing process and stabilized due to reduction of surface tension of water through adsorption of surfactants from the AEAs onto the bubbles’ surfaces. Most AEAs are polymeric hydrocarbons that terminate in a polar group, typically carboxylic acid or sulfonic acid, both of which are anionic surfactants, similar to those of the traditional melamine and naphthalene based superplasticizers although their molecular weight is smaller. In using these AEAs, the charged sheath of surfactants surrounding each bubble leads to mutual repulsion, thus preventing coalescence in the form of larger bubbles and further stabilizing the entrained air bubbles.
Air entrainment is known to alter the properties of fresh concrete. This includes the improvement of workability by increasing slump. It is estimated that the slump increases by about 10 mm per 1% entrained air (ACI 211, 1991).
Comparatively, the air entrained concrete is well known to be more cohesive than non-air entrained concreteand has a lower tendency to segregate, whether by bleeding of water or separation of aggregate from the mortar matrix (Dolch, 1984; Hewlett, 1998; Neville, 1995; Page, 1981).
The orientation of the anionic surfactants from the AEA around the air bubble causes its surface to possess negative charges, whereas the cement particles become positively charged during hydration due to adsorption of calcium ions (Ramachandran and Feldman, 1984). This results in adhesion of the air bubbles with the cement particles and also with oppositely charged zones on aggregate particles. According to Kreijger (1980), the compounds of AEA will adsorb onto the cement surfaces through
the negatively charged head, which is hydrophilic. The non-polar chain is hydrophobic and repels water. This will cause a slight dispersion as shown in Fig.2.6.
He also proposed that the air bubbles are able to form bridges between the cement particles, giving an increased yield value. Once flow occurs, the spherical bubbles move easily past each other and the plastic viscosity decreases. This model is consistent with results obtained on air-entrained cement pastes (Bruere, 1958). The net effect is an aggregate-air-cement-air-aggregate type of bridge, improving the cohesion and further stabilizing the air void system (Hewlett, 1998). Such a system would permit relatively free motion of fresh concrete in shear with the stabilized air bubbles acting like compressible ball bearings. However, observed effect of air- entrainment by Tattersall and Banfill (1983) suggests that the yield value decreases rather than increase as proposed by Kreijger (1980). They suggest that steric separation of cement particles by air bubbles is the dominant effect and the bridging effect may not build up because of air bubbles interfering with inter-particle contacts.
A recent study by Struble and Jiang (2004) indicated that the yield stress of cement paste increases with increasing air entrainment. This is consistent with what Kreijger (1980) suggested. The same study shows that the plastic viscosity increases with air entrainment in cement paste with superplasticizer, but decreases with air entrainment in cement paste without superplasticizer.
Fig.2.6 – Structure of air-entrained cement paste (Kreijger, 1980)
It is also believed that the mass of the cement particles attracted around each air bubble helps to disperse the air bubbles in the mixture and reduces the tendency of air bubbles floating to the surfaces while, in turn, the floatation force of the air bubbles decreases the possibility and rate of settlement of cement particles (Du and Folliard, 2005) and aggregate in normal-weight aggregate concrete. In lightweight aggregate concrete (LWAC), however, the floatation forces of the air bubbles and lightweight aggregates are in the same direction, thus air entrainment may have significant effect on the stability of fresh LWAC.
From the literature, it appears that although there are some exceptions, it may be reasonable to acknowledge that the major effect of adding a superplasticizer is on the yield value, with little or no effect on the plastic viscosity (Gjứrv, 1998). This means that a series of mixtures of increasing superplasticizer content, with other factors remaining constant, will produce a series of parallel flow lines as shown in Fig.2.7 (a). On the other hand, air entrainment will affect the plastic viscosity more than the yield value (Gjứrv, 1998). Therefore, a series of flow curves with different air content in concrete should be fan-shaped as shown in Fig. 2.7 (b).
Fig.2.7 – Effect of increasing superplasticizer dosage (a) and air content (b).
τ
γ&
increasing superplasticizer
τ
increasing air content
γ&
a b