Civil Engineering Reference
In-Depth Information
16.2.6 Air entrainment admixtures
Almost one-third of the concrete volume cast in the nordic countries must
possess adequate freeze-thaw durability. Since the damage of freezing and
thawing involves expansion of water on freezing, air-entraining admixtures
are used to introduce and stabilize microscopic air bubbles in concrete.
These bubbles should be distinguished from accidentally entrapped air,
which is in the form of larger bubbles left behind during the compaction of
fresh concrete. The use air-entraining admixtures improve the durability of
concrete exposed to cycles of freezing and thawing.
Air-entraining agents belong to a class of chemicals called 'surfactants'. A
surfactant is a material whose molecules are strongly adsorbed at air-water
or solid-water interfaces. Such substances have a dual nature, one portion of
their molecules being polar and the other one being markedly non-polar.
For more than 50 years, neutralized Vinsol natural resin has been effectively
used for air entrainment. other synthetically manufactured surfactant agents
have been introduced in recent years, based on blends of alkylbenzene
sulfonates or salts of sulfonated hydrocarbons. Their use increased due to
the high cost and limited availability of Vinsol resin.
nagi et al. (2007) compared the properties of concrete containing a
variety of air-entraining admixtures: two based on synthetic surfactants
(sodium olefin sulfonate and alpha olefin sulfonate, indicated with ES and
eA, respectively) and two based on Vinsol resin (indicated with en and
eV). Admixtures eS, eA and en are commercially available while eV is
an admixture accepted and used by most highway agencies. The air content
of the concretes prepared with the air-entraining admixtures was set at 6
±1.0%. The adequacy of an air-entraining admixture in a given concrete can
be estimated by a spacing factor. The latter is an index of the maximum
distance of any point in the cement paste from the periphery of nearby air
void. A maximum spacing factor of 0.2 mm is required for satisfactory frost
protection. The spacing factor for the four mixes was within this reference
limit. Compressive, flexural and split tensile strength values of concrete
containing eV and eA admixtures, at both 7 and 28 days, were relatively
high compared to a non-air- entraining concrete. The compressive strengths
of the concretes containing the other two admixtures, en and eS, were lower
(i.e., less than 70% of the reference concrete).
The difficulty with the use of any air-entraining admixture is that the
spacing factor cannot be controlled in advance. This difficulty is avoided if
rigid-foam particles of suitable size are used, such as compressible hollow
plastic microspheres. They have a diameter of 10-60 mm which is a narrower
range of sizes than in the case of air-entraining admixtures.
