We make precast septic tanks and often get water trails on the tank surfaces, even though we use admixtures. Is there a way to avoid these, or will they always have some trails?

The condition you describe is called sand streaking, and it’s caused by excessive bleeding. As bleedwater moves upward along the form in long channels, it washes away some of the paste and exposes sand. Sand streaks can be avoided by reducing bleeding. You can doe this by taking one of the following steps.

  • Reduce water content.
  • Add an air-entraining mixture.
  • Add a water-reducing admixture.
  • Increase cement content.
  • Add fly ash to the concrete as a supplementary cementing material, or use a blended cement.
  • Blend a fine sand (blow sand) with the concrete sand to increase the amount of material passing the No. 50 and No. 100 screens.