Preventing sand streaking: controlling entrained air content

Preventing sand streaking: controlling entrained air content

Q. 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?

A. The condition you describe is called sand streaking, and it’s caused by excessive bleeding. As bleed-water 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 do this by taking one of the following steps.

* Reduce water content.

* Add an air-entraining admixture.

* 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.

Q. We have started having problems with variations in air content of our air-entrained mix designs. Air content stays uniform for a while, then goes out of the specification range. Usually, measured air content is too low, but sometimes we get exceptionally high values. I know that many factors affect air content. Is there a systematic method for tracking down the problem?

A. Assuming that your air content measuring methods are accurate, fluctuations in air content can be caused by changes in the materials used to make concrete, changes in mix proportions, and changes in production methods. When the dosage of air-entraining agent remains constant, expect changes in the following variables to have the effects noted.

Materials

Cement

* Air content decreases as cement fineness increases.

* Air content increases as cement alkali content increases.

Fly Ash

* Air content decreases with increasing carbon content of the fly ash. Loss on ignition is a good indicator of fly ash carbon content.

Aggregate

* Air content decreases with increases in the amount of minus 200 mesh fines in the aggregate.

* Air content increases with increasing amounts of intermediate sand sizes.

Mixing water

* Air content decreases when truck mixer washwater is used as mixing water.

Other admixtures

* Air content increases when lingo-sulfonate water reducers and retarders are used.

* Air content decreases when some coloring pigments are used.

* Melamine-based superplasticizers may decrease air or have little effect. Napthalene and lignosulfonate superplasticizers increase air content.

* Calcium chloride increases air content.

Mix proportions

Cement content

* Air content decreases with an increase in cement content.

Sand content

* Air content increases with increasing sand content.

Slump (Water content)

* Air content increases with increasing water content and slump up to a slump of about 6 or 7 inches. Then, higher slumps decrease air content.

Production procedures

Batching sequence

* Air content decreases when air-entraining agent is simultaneously batched with cement or with other admixtures.

* Air content increases when air-entraining agent is added late in the batching sequence.

Mixer capacity

* Air content increases if the mixer is loaded to less than rated capacity and decreases if the mixer is overloaded.

Mixer speed

* Air content increases up to about 20 rpm and decreases at higher speeds.

Mixer condition

* Air content decreases if mixer blades are badly worn or if hardened mortar accumulates on the drum and mixer blades.

Temperature

* Air content decreases with an increase in concrete temperature.

Haul time

* Air content decreases during long hauls, especially during hot weather.

Don’t overlook the possibility of errors in measuring air content. When the pressure method is used, an erroneously high air content will be read if the concrete is incompletely consolidated in the air meter bowl.

When the volumetric method is used with extremely sticky concrete, it’s hard to wash out all of the air. Thus, an erroneously low air content may be read. Other variations in testing methods can affect the accuracy of measured air content.

COPYRIGHT 2005 Hanley-Wood, Inc.

COPYRIGHT 2005 Gale Group