Types of bubbles in concrete
Before concrete is vibrated, it contains a large number of air bubbles. These mainly include air bubbles introduced during the mixing, transportation and pouring of the concrete, air bubbles introduced by the addition of water reducers, and tiny air bubbles introduced by the addition of air-entraining agents.
(1) Air bubbles introduced during the mixing, transportation and pouring of concrete. These air bubbles are relatively large in diameter, unevenly distributed and highly unstable. They are prone to coalesce into larger bubbles and are easily burst. Therefore, they are called unstable bubbles. These unstable bubbles introduced by mechanical stirring have adverse effects on the fluidity of concrete and the mechanical properties and durability of the hardened concrete.
(2) Air bubbles introduced by water-reducing agents. Water-reducing agents can introduce a certain amount of air bubbles. Due to the same electrostatic repulsion force, these bubbles are located between the cement particles, like ball bearings, dispersing the cement particles and increasing the sliding effect between the cement particles. However, these bubbles are uneven in size, irregular in shape and unstable. As transportation and vibration proceed, they tend to aggregate and merge into large bubbles, eventually overflowing to the concrete surface and forming apparent bubbles, resulting in honeycomb-like defects.
(3) Air bubbles introduced by air-entraining agents. Air-entraining agents can cause the concrete to form many fine bubbles with dimensions ranging from (20-200) um and evenly distributed. These bubbles have a relatively firm liquid film on their surfaces. From a thermodynamic perspective, the liquid film's electromotive potential is relatively high, which can prevent bubble coalescence and make the bubbles relatively stable and less prone to bursting. They are fundamentally different from the air bubbles introduced by water-reducing agents and are beneficial to the impermeability and other durability of the concrete.
Defoamers are beneficial for eliminating air bubbles in concrete. By adding defoamers, on one hand, it can to some extent eliminate the air bubbles between the concrete and the formwork, effectively preventing or eliminating the occurrence of honeycombing and rough surfaces on the concrete surface, making the surface of the concrete have a higher degree of flatness and glossiness. On the other hand, defoamers can significantly eliminate the air bubbles inside the concrete, reducing the air content and internal porosity of the concrete, and improving the mechanical properties and durability of the concrete. The defoamers in concrete mainly eliminate the air bubbles introduced by the water reducer. Therefore, in engineering, defoamers are often compounded with polymeric acid type water reducers to solve the problem of excessive air entrainment caused by polymeric acid type water reducers.
Formulation of defoamer and polycarboxylate superplasticizer
Due to the high gas content and high surface activity of the master solution of polycarboxylate superplasticizer, as well as its good bubble retention property, when directly used in concrete, it will cause adverse effects such as high gas content in the concrete, numerous apparent bubbles, and low strength. Therefore, an appropriate amount of defoamer needs to be compounded to eliminate the air bubbles in the concrete. The basic performance tests of the compound of defoamer and polycarboxylate superplasticizer generally include the compatibility between the defoamer and the superplasticizer, as well as the influence of the defoamer on the performance of the concrete.
Compatibility between defoamer and water reducer
The difficulty in combining defoamer with polycarboxylate-based water reducer lies in the compatibility issue between them. By testing the dissolution state of the defoamer in the polycarboxylate-based water reducer, the compatibility between the defoamer and the water reducer can be evaluated. If the defoamer has good dissolution in the polycarboxylate-based water reducer and does not separate for a long time, the compatibility is good and it can be combined with the water reducer; while a defoamer with poor compatibility cannot be combined with the water reducer and can only be added alone to the concrete. When the defoamer and the polycarboxylate-based water reducer are added to the cement paste, the initial flowability and the time loss of flowability of the cement paste can also be tested to evaluate the compatibility between the defoamer and the polycarboxylate-based water reducer. A defoamer with good compatibility with the polycarboxylate-based water reducer should be one that has no significant adverse effects on the initial flowability and time loss of flowability of the cement paste.
Effects of defoamer on concrete performance
The effects of defoamer on concrete performance are manifested in two aspects: the working performance of the concrete and the mechanical properties after hardening. Generally, the effects of the defoamer on the concrete performance are evaluated by testing the slump and slump loss, gas content, and strength of the concrete. Defoamers that can significantly reduce the gas content of the concrete, have little effect on the slump and slump loss of the concrete, and significantly improve the strength of the concrete have better effects.
