Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Frank writes:
What is concrete bleed water?
NPCA Technical Services engineers answer:
Bleeding in fresh concrete refers to the process where free water in the concrete mix is pushed upward to the concrete surface because of the settlement of heavier solid particles in the mix, such as cement. Some bleeding is normal, but excessive bleeding can be problematic. Fortunately, bleed water can be controlled.
The water-to-cement ratio (w/c) of a concrete mix has the greatest impact on the amount of bleed water a fresh concrete mix will exhibit. A higher w/c can lead to more bleeding. Additionally, the mix proportions, raw materials and the form height also affect bleeding.
The goal is not necessarily to eliminate bleed water, because it is an inherent characteristic and result of properly curing concrete. Instead, it is important to manage bleed water and prevent excessive bleeding. An appropriate amount of bleed water is beneficial and can help ensure moist curing conditions for the unformed concrete surface.
According to the commentary in Section 4.4.4 of the NPCA QC Manual, “Excess bleed water on the surface should be removed using a squeegee or a rubber hose pulled across the surface before finishing the concrete. The surface should then be floated using a magnesium float, followed by troweling, if required. For hard, dense surfaces, repeated troweling may be needed.”
Diane writes:
What is the ASTM standard for watertightness testing by vacuum (negative pressure) of a septic tank?
NPCA Technical Services engineers answer:
There are two ASTM standards that cover vacuum testing for precast concrete septic tanks.
The first is ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks.” This is the standard for precast concrete septic tanks that covers concrete materials, tank design, etc., and also includes hydrostatic watertightness testing as well as the vacuum testing method for watertightness. This vacuum test can be conducted at the precast manufacturing plant or on the jobsite prior to installation. This is the one that is most often used and specified.
The other ASTM standard that covers vacuum testing for precast concrete tanks is ASTM C1719, “Standard Test Method for Installed Precast Concrete Tanks and Accessories by the Negative Air Pressure (Vacuum) Test Prior to Backfill.” This document only covers the test method. It does not address concrete materials, tank design or anything else. An important note for this test method is that it is to be conducted on the jobsite when the tank has been installed but not yet backfilled. This standard also can be used for precast grease interceptors and some other structures.
Vacuum testing not only demonstrates that the structure is watertight – a characteristic critical to the performance of the septic tank and the entire treatment system – but it also can be used to demonstrate structural integrity.
Note that it is generally not advised to vacuum test septic tanks made of alternative materials as they could implode.
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