Questions from the Field

Questions from the Field is a selection of questions the NPCA Technical Services Department receives from calls, emails and comments on blogs or magazine articles on precast.org.  If you have a technical question that needs an answer, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Zoel writes:

How does air velocity, humidity and temperature affect bleeding of concrete?

Technical Services Department answers:

While bleeding is primarily attributable to water content and poorly graded aggregates, air velocity, temperature and moisture can have an effect. This effect varies based on what kind of concrete structure you are pouring. Air movement and humidity will influence bleed water rates to a higher degree on a large slab than it would with a deep structure with a smaller exposed area. Bleed rates will be dictated, among other things, by setting time. Cold temperatures will slow setting and as a result can extend the bleeding period. High humidity and lack of air circulation can also contribute to prolonged bleeding. The longer ambient conditions allow bleed water to remain on the concrete surface and not evaporate, the longer the bleeding period may extend.

ACI 302.1R-04, “Guide for Concrete Floor and Slab Construction” recommends using “concrete approaching the highest as-placed temperature permitted by the contract documents.”

It is important to note that while this may reduce bleeding, it may lead to other issues, such as reduced long-term strengths. There are also recommendations to use heaters or fans to move the air and evaporate excess surface moisture while concrete is in its plastic stage. This must also be done cautiously as accelerated evaporation may also lead to plastic shrinkage cracking. If using heaters, avoid using non-vented heaters as they may accelerate carbonation, which can create a soft, dusty, chalky surface.

Jeffery writes:

What is the standard for achieving SSD aggregates?

Technical Services Department answers:

Achieving saturated-surface-dry aggregates can be done using methods described in ASTM C127, “Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate” and ASTM C128, “Standard Test Method for Relative Density (Specific Gravity) and Absorption of Fine Aggregate.”

Usually, SSD weights are given in the aggregate mill certificates through the absorption capacity. Take the alternating current and multiply it by the oven-dry weight and you get your SSD weight. 

Ann writes:

We had a sanitary manhole that was installed and backfilled, and the engineer on site performed a vacuum test and subsequently failed the manhole based on the results. The engineer stopped the timer as soon as the pressure dropped. Could you please explain the acceptance procedure for ASTM C1244?

NPCA Technical Services Department answers:

As the title suggests, ASTM C1244, “Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill” would not apply in this situation. If in fact the engineer was trying to use this method, not only is it not applicable, but he was performing it incorrectly. ASTM C1244 does allow for a drop in pressure from 10 to 9 inches of mercury, which is the determining factor in passing or failing the structure. For example, if your manhole has a 4-foot diameter and is 12 feet deep, ASTM C1244 allows 30 seconds for that pressure drop. 

Regardless, ASTM C1244 is not applicable when the manhole is backfilled.  See two clear references within the title and scope on the standard’s first page.

If the authority having jurisdiction wants to perform a test after backfill, there are precautions and adjustments to be taken. Also, I would urge you to contact your precast manhole supplier for additional support and advice. 

David writes:

Can the neutralite solution for cement burns be applied after the damage has been done?

Technical Services Department answers:

Neutralite is formulated to control both chronic and allergic dermatitis by acting as a topical skin buffer and neutralizer. Test results show that it neutralizes the skin’s alkalinity after contact with wet concrete and reduces hexavalent chromium to undetectable limits. The user fills up the bottle with tap water, shakes it to dissolve the crystals and sprays the resulting solution onto the affected skin immediately after contact with wet concrete. The solution should be applied until all visible cement is removed from the skin. Then, the affected area should be rinsed with clean water. The manufacturer states that the product serves only as a topical buffer and neutralizer and won’t prevent acute irritant contact dermatitis caused by long-term skin exposure to wet concrete.