Concrete looks workable when it leaves the mixer, but once the formwork comes off the surface is dense with air voids, and the cube cured under the same conditions falls a notch below the design strength. The batching plant's first reaction is to add cement to make up the strength; the cost goes up and the voids are still there. Most of the high air content does not come from the cement: the polycarboxylate water-reducing agent itself carries entraining components, and the higher the dosage the more pronounced the entrainment; a high mud content in the sand and gravel stabilises the bubbles; and too long a mixing time or a wrong charging order also rolls air into the mix. Bring the air content back into the design range first, and then talk about strength.
Whether the Air Voids Are Even or Clustered at the Surface Tells the Causes Apart
Air voids distributed evenly through the section, small and dense, mean the whole mix is over-entrained, which is tied to the water-reducing agent and the sand and gravel. Voids clustered on the top surface and at the formwork contact face mostly mean the vibration is inadequate or bleeding has carried the bubbles up. Voids that are large and isolated, more than five millimetres across, are generally air rolled in during vibration and not released. When the air content measured in the laboratory differs greatly from the on-site measurement, the problem often lies in the transport and placing stage.
Order of Handling: Check the Admixture First, Then the Aggregate, and Use the Defoamer Last
First check the dosage and the entraining components of the water-reducing agent; if the dosage is high, cut it or switch to a low-entraining grade. Next check the mud content and grading of the sand and gravel — high mud content stabilises the bubbles, and switching to clean aggregate usually brings the air content back down. Then look at mixing and transport: lengthen the mixing time a little or adjust the charging order so the rolled-in air has a chance to escape. Only when none of this works is the defoamer used, added to the mixing water or the water-reducing agent, starting from a low dosage and settling it with comparison tests on air content and strength.
Selection Points: Read Compatibility, Workability and Strength Together
A concrete defoamer must first be compatible with the water-reducing agent system; one that pairs poorly with a polycarboxylate water-reducing agent will stratify and settle, and once dosed it drags even the water-reducing ratio. Next look at the effect on workability — defoam too hard and the mix turns harsh, bleeding increases and pumping resistance grows with it. Third, look at defoaming speed and persistence: where the interval from discharge to placing is long, choose a persistent type, or the air content climbs back on the way.
Chemi Tech Solutions
For concrete, mortar and cement-based materials, Chemi Tech offers powder-type and emulsion-type defoamers, covering ready-mixed concrete, precast elements and dry-mixed mortar.
Compared with the emulsion type, the powder type suits dry-mixed mortar and situations needing powder pre-mixing, dispersing evenly and not readily stratifying; the emulsion type is convenient to add in a ready-mixed concrete plant and has a wider compatibility window with the polycarboxylate water-reducing agent. The two differ in their strength of air control and the processes they fit.
When the aggregate mud content is high or the water-reducing agent carries too many entraining components, raising the defoamer dosage still cannot hold the swing in air content; the problems in the material source and the mix proportion have to be controlled at the source. The specific dosing plan must be judged by the system conditions.
State the cementitious materials, the admixture system and the air-content requirement together, and the technical team will give a direction by operating condition and arrange a trial mix. The cement mortar defoaming powder solution and the building-materials defoaming solution can be looked at first.
Summary
When concrete has many air voids and loses strength, it is mostly that the air content was not held; the foundation lies in the material and the mix proportion, not in how much defoamer was added. Sort out the entraining components of the water-reducing agent, the mud content of the sand and gravel, and the mixing and transport, and the air content will hold steady. The defoamer is an auxiliary — when choosing it, watch the compatibility with the water-reducing agent and the effect on workability, and do not defoam too hard. Chemi Tech has both powder and emulsion products, matched to the material system. If you have the mix design and the aggregate test report, we can look at them together.