Frequently Asked Questions

How much does air entrainment with Fosroc Durox reduce the compressive strength of concrete and is air-entrained concrete suitable for high-strength structural concrete in India?

The relationship between air entrainment and concrete compressive strength follows a consistent empirical rule: each 1 per cent of entrained air content (total air content minus natural entrapped air) reduces the 28-day compressive strength of concrete by approximately 5 per cent relative to the non-air-entrained control mix of the same w/c ratio. For frost-resistant highway concrete with a target air content of 5 to 7 per cent (the standard range for concrete in severe freeze-thaw exposure environments), the strength reduction is 25 to 35 per cent. This means that to achieve the same design compressive strength in air-entrained concrete as in non-air-entrained concrete, the mix design must compensate for the air-entrainment strength penalty by reducing the water-cement ratio — typically by reducing the mixing water content rather than increasing the cement content, since air also improves workability and reduces the water needed for the target consistency. For example: if the concrete design requires 30 MPa at 28 days (M30 grade), the non-air-entrained mix at w/c 0.50 would achieve this target; the air-entrained version (5 per cent air, 25 per cent strength reduction) would achieve only 22.5 MPa at the same w/c 0.50; to restore 30 MPa with 5 per cent air, the w/c ratio must be reduced to approximately 0.42 to 0.43 — which requires either a reduction in water content (accepting the workability reduction and relying on the improved workability from the air entrainment itself) or the addition of a water-reducing admixture (Conplast SP) to achieve the lower w/c at the required workability. For high-strength structural concrete (M50 to M80), air entrainment is generally NOT specified because: the strength penalty from 5 to 6 per cent air is approximately 25 to 30 per cent — unacceptably large for high-strength concrete; high-strength concrete (low w/c ratio, dense microstructure) already has very low capillary porosity and adequate freeze-thaw resistance through strength and density alone; and the mixing and quality control of high-strength concrete with accurate air content requires special care that is difficult on typical Indian ready-mix plants. Air entrainment with Durox is appropriate for M20 to M35 grade frost-resistant concrete for roads, runways, and bridge decks in northern India — not for structural high-strength concrete.

What is the target total air content for frost-resistant concrete in the Delhi NCR region and how is the air content measured and controlled on site?

The target total air content for frost-resistant concrete in the Delhi NCR region depends on the severity of the freeze-thaw exposure expected at the structure location, the maximum aggregate size in the concrete mix, and the required minimum strength grade. Indian winter climate in Delhi NCR is at the mild to moderate frost exposure boundary: Delhi minimum recorded temperatures are typically minus 1 to minus 4 degrees Celsius in December-January, with frost conditions occurring in some years in the surrounding plains; the Yamuna Expressway, NH-48 (Delhi-Jaipur), and other elevated highway structures in the NCR see bridge deck temperature extremes in winter that can reach minus 5 to minus 8 degrees; frost exposure in Delhi NCR is classified as Exposure Condition F1 (moderate freeze-thaw in IS 456 terms) rather than the severe F2 category that applies to Himalayan and high-altitude concrete. For Delhi NCR Exposure F1 conditions: target total air content 3.5 to 5.5 per cent for 20 mm maximum aggregate size concrete; target total air content 4 to 6 per cent for 14 mm maximum aggregate size concrete; these targets are appropriate for bridge decks, elevated roads, and exposed slabs in Delhi NCR and western UP for the current climate; higher altitudes and areas north of Delhi (towards Haridwar, Chandigarh, Ambala) may require the higher end of the range. Air content measurement on site: the standard test is the pressure method (ASTM C231 or IS 1199 Part 6) using a pressure air meter — this is the fastest, most reliable on-site test; a weighed concrete sample is placed in the air meter bowl, the lid sealed, and water injected to fill the bowl completely; the pressure is applied, the gauge read, and the air content calculated from the pressure drop; calibration of the air meter with the specific aggregate type used in the mix is essential (porous aggregates can absorb the pressurised water, giving falsely high air content readings); air content testing should be done at the discharge point from the transit mixer truck at the site, not at the batching plant — significant air can be lost during truck discharge and pump delivery, and the target air content must be maintained at the point of placement, not just at the batching plant.

Can Fosroc Durox be used in concrete pumping applications and does the air-entraining admixture cause any problems with concrete pump operation?

Fosroc Durox and air-entrained concrete in general can be pumped through concrete pumps, but there are specific considerations and precautions that must be understood to avoid pump problems and air content loss during pumping. The relationship between air entrainment and concrete pumpability is nuanced: at moderate air contents (3 to 5 per cent), air-entrained concrete can be somewhat more pumpable than non-air-entrained concrete of the same w/c ratio and aggregate grading, because the entrained air bubbles act as a lubricant in the concrete mix and improve the cohesion and workability that allow the mix to slide through the pump and pipeline without segregation; at higher air contents (greater than 6 to 7 per cent), the air bubbles begin to reduce the cohesion of the mix and can cause pump surging and poor pump delivery — the highly aerated mix does not maintain the uniform hydraulic pressure distribution in the pipeline that good pumpability requires. The more significant concern with pumping air-entrained concrete is air loss during pumping: as the concrete passes through the pump cylinder and enters the delivery pipeline under the pump pressure (typically 30 to 100 bar depending on pump capacity and delivery distance), the pressure compresses the entrained air bubbles and reduces the measured air content in the concrete at the pipe outlet compared to the air content at the pump intake; this is a reversible effect — the air expands again as the concrete is discharged from the pipeline at low pressure, restoring the design air content in the placed concrete. However, the pressure in the pump pipeline can destroy some of the weakly bonded air bubbles (particularly the larger bubbles that were marginally stable in the fresh concrete) by forcing the air out of solution into the surrounding mix water; this is a permanent air loss that is not recovered after discharge. In practice, the air loss during pumping of air-entrained concrete is approximately 0.5 to 2.5 per cent of total air, depending on the pump pressure, pipeline length, aggregate gradation, and concrete consistency. The target fresh air content at the pump intake must be adjusted upward by the expected air loss during pumping to ensure the target air content is achieved in the concrete at the point of placement.