Questions from the Field

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 on precast.org.

If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Ron writes:

I have always complied with the vacuum testing requirements of ASTM C1227 and am looking at the water testing procedures for the first time. When section 9.2.2 of ASTM C1227 states that we must “refill the tank” after 24 hours, does that mean we have to pump all water out of the tank and restart?

NPCA Technical Services engineers answered:

Not at all. ASTM C1227 Section 9.2.2 standardizes the water pressure test with very succinct steps.

1. Seal the tank – This step refers to any seams in the tank, not the installation of a cover. If you are testing a monolithically poured septic tank, you would not have a seam to seal.

2. Fill with water and let stand for 24 hours – You should fill the tank to the operational level and then not disturb it for a full 24-hour cycle. During this time, the concrete surface may absorb water and slightly lower the water level.

3. Refill the tank – Any change in the water level during the first 24 hours of the test should not be interpreted as leakage or a lack of watertightness. In fact, a determination of watertightness at this stage of the test is premature and contrary to the specification itself. The slight change in water level during the first 24 hours is normal. Simply add water to the tank to return the water level to its previous level.

4. Let stand for 1 hour – After the 24-hour mark, the tank must hold the refilled water level for one hour. If the water level remains unchanged after 1 hour, then the tank passes the ASTM C1227 water pressure performance test.

Chris writes:

How are 3D-printed concrete structures reinforced?

NPCA Technical Services engineers answered:

3D printing of concrete is still a very new technology and research is ongoing on how to build these structures to perform as needed. There are several methods of reinforcement under consideration including fibers, hand or mechanically placed mesh or insertions of rebar during 3D layering. Another consideration is simply printing the concrete around existing, pre-assembled reinforcing. NPCA will continue to monitor this evolving technology to see if and how it rises to meet the stringent requirements of reinforced precast concrete structures.