The compatibility of fresh concrete has a very great influence on the pumpability, construction performance, mechanical properties and durability of the concrete after hardening, and the compatibility of fresh concrete depends to a large extent on the air content of the concrete. In the actual construction, the best range of air content of concrete should be determined in the trial preparation process according to the project site and the different functions of concrete, and controlled effectively to help ensure the construction quality of concrete.
Types of air bubbles in concrete concrete vibrating before its internal contains a large number of bubbles, mainly including air bubbles introduced during concrete mixing, transportation and discharging, air bubbles introduced by water reducing agents and tiny bubbles introduced by air-entraining agents.
(1) concrete mixing, transportation and the introduction of air bubbles in the process of discharging. This type of air bubble diameter is large, unevenly distributed and extremely unstable, very easy to polymerize into a larger bubble diameter bubbles, easy to break, so called unstable bubbles. This unstable bubble introduced by the mechanical mixing of both the fluidity of the concrete, or the mechanical properties and durability of concrete after hardening will have a negative impact.
(2) Water reducing agent introduced bubbles. Water reducing agent can introduce a certain amount of bubbles, due to the same electrical repulsion, these bubbles are between the cement particles as ball bearings to disperse the cement particles, thus increasing the sliding effect between the cement particles. However, these bubbles are not uniform in size, irregular in shape and unstable, with the transportation and pounding, often gather and converge with each other to become large bubbles, and eventually overflow outward to the concrete surface to form the apparent bubbles, resulting in honeycomb pockmarked defects.
(3) Air bubbles introduced by air-entraining agents. Air-entraining agent can make the concrete internal formation of many sizes between (20-200) um and uniform distribution of micro-fine bubbles. This type of bubble surface of the liquid film is relatively strong, from the perspective of thermodynamics, that is, the high potential of the liquid film, can prevent bubble agglomeration, bubbles are relatively stable, not easy to break. Its bubbles introduced with water-reducing agents are fundamentally different, and are beneficial to the impermeability and other durability of concrete. Defoamer is beneficial to the exclusion of large air bubbles in concrete. Adding defoamer can, on the one hand, eliminate the air bubbles between concrete and formwork to a certain extent, effectively prevent or eliminate the generation of honeycomb and pockmarked concrete surface, so that the surface of concrete has a high degree of flatness and gloss. On the other hand, defoamer can eliminate a large number of air bubbles inside the concrete, reduce the air content and internal porosity of concrete, and improve the mechanical properties and durability of concrete. Defoamer in concrete mainly eliminates the air bubbles introduced by the water reducing agent, therefore, the project is often compounded with defoamer in the polycarboxylic acid water reducing agent to solve the problem of large air-entraining polycarboxylic acid water reducing agent.
Defoamer and polycarboxylic acid water reducing agent compounding because of the polycarboxylic acid water reducing agent master batch itself contains a large amount of gas, high surface activity, good foam retention, when used directly in concrete, will cause high gas content of concrete, the apparent bubbles and low strength and other adverse effects, so the need to compound the appropriate amount of defoamer to eliminate the large air bubbles in concrete. The basic performance testing of defoamer and polycarboxylic acid water reducing agent compounding generally includes the compatibility of defoamer and water reducing agent and the effect of defoamer on concrete performance.
(1) defoamer and water reducing agent compatibility defoamer and polycarboxylic acid water reducing agent compounding the use of the difficulty is the compatibility with the water reducing agent. By testing the dissolution state of the defoamer in the polycarboxylic acid water reducing agent can evaluate the compatibility of defoamer and water reducing agent, defoamer in the polycarboxylic acid water reducing agent in the solubility of good, and a long time without delamination, then good compatibility, and water reducing agent can be compounded; and poor compatibility of defoamer can not be compounded with water reducing agent, can only be added to the concrete alone. The defoamer and polycarboxylic acid water reducing agent are added into the net cement slurry, and the initial flow rate and flow rate time loss of the net cement slurry can be used to evaluate the compatibility of defoamer and polycarboxylic acid water reducing agent by testing. The defoamer with good compatibility with polycarboxylic acid water reducing agent should be the defoamer that has no obvious adverse effect on the initial flow rate and flow rate time loss of net cement slurry.
(2) The effect of defoamer on concrete properties The effect of defoamer on concrete properties is manifested in two aspects: the working properties of concrete and the mechanical properties after hardening. Generally, the impact of defoamer on concrete performance is evaluated by testing the slump and slump loss, air content and strength of concrete. Can significantly reduce the air content of concrete, concrete slump and slump loss of less impact and the strength of concrete has a significant increase in the effect of defoamer good.
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