Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs 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.
I’m working on a six-story mid-rise building in Annapolis, Md. It was built in the late 1960s. The walls are 12-inch concrete masonry units and the floors are flanged precast slabs that start at 2 inches thick and increase up to 18 inches thick at the ribs. I want to verify if this system has a one-hour fire rating.
NPCA Technical Services engineers answered:
ACI/TMS 216.1-14, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies,” states, “Concrete bearing or nonbearing walls, floor slabs, and roof slabs required to provide fire-resistance ratings of 1 to 4 hours shall comply with the minimum equivalent thickness values in Table 4.2.”
The equivalent thickness of slabs, walls or other barrier elements with surfaces that are not flat are determined in accordance with sections 4.2.2 through 4.2.4.
Section 4.2.3 states, “For flanged walls, and floor and roof panels where the flanges taper, the equivalent thickness shall be determined at the location of the lesser distance of two times the minimum thickness or 6 inches from the point of the minimum thickness of the flange.”
In your case, if the minimum thickness of the slab is 2 inches, you would need to come over 4 inches (2t) and measure the thickness at this location. This would constitute your equivalent thickness. Assuming the worst case in Table 4.2 and using siliceous aggregates, this thickness would have to be 3.5 inches to provide a one-hour rating.
A sample could also be tested per ASTM E119 – 16a, “Standard Test Methods for Fire Tests of Building Construction and Materials.”
The full ACI/TMS 216.1-14 is available at https://www.concrete.org/store/productdetail.aspx?ItemID=216114.
I just had my first experience testing a manhole via a 10-inch vacuum, and it did not go so well. We hired a local contractor who does this daily. My first structure was a 16-foot-tall double drop manhole with 10 feet of backfill, which passed with flying colors. However, we were not so lucky on the second structure, which was 12 feet tall with 12 feet of backfill. On the outlet side of the manhole, I had a 2-foot, 4-inch piece of pipe that connected into the boot placed into the manhole. The balloon plug was placed in this piece but not past the joint connecting it to the next full length of pipe. Testing began. I was told at 6 inches that it failed. The lid was removed to look inside, and the 2-foot, 4-inch piece had been sucked out of the hub and pulled completely into the manhole. Having pulled these joints apart in the past, I know it takes a good amount of force to make this happen. I am now digging this up and will reinforce this piece so this does not happen a second time. What is the formula for figuring the necessary vacuum to be pulled to equal a true 10-inch vacuum? Better yet, can you give me that number along with the math it takes to get it? The water table is not an issue. Therefore, there is no hydrostatic pressure to factor.
NPCA Technical Services engineers answered:
We hope this experience will not keep you from further work requiring vacuum testing. The reason for conducting this test is similar to air testing for pipelines. The owners are looking to obtain the most leak-resistant sewer system and a means to verify that.
The standard you are likely being tested to is ASTM C1244, “Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill.” The original intent of this standard was to test structures prior to backfill. For certain installed conditions, the test criteria can exceed the range of the resilient booted connection. However, that is little comfort to sewer owners who want to test a completed system to assure the backfilling process didn’t create some type of defect. Currently, ASTM C13 is developing a new standard to address vacuum testing of manholes after installation. You can read more about the new standard on page 6.
The problem you encountered with a pipe “implosion” is strictly a power of pressure issue. Whether compressed for positive pressure or sucked down to create a vacuum for negative pressure, air will exert a force against an object adjacent to it with a different pressure zone. In your case, the manhole interior at the known failure was at 6 inches of mercury (Hg), which equates to roughly 3 psi of reduced differential pressure to the outside of the manhole (1 inch of Hg = 0.49 psi, or approximately 0.50 psi). Therefore, when the 30-inch-diameter pipe was plugged, it had a differential force of 3 psi trying to push it into the manhole. That is measured to the pipe’s outside diameter. If this 30-inch-diameter pipe is a B-wall RCP, the minimum force is 3,225 pounds using the equation: Outside Area of Pipe (square inches) x Force (pounds/square inch).
If this is SDR35 PVC, that force is reduced to 2,412 pounds, but this pipe has less weight and resisting friction than the concrete pipe. Regardless, if either pipe is not adequately blocked off with adequate struts, it will likely move, as you have proven. A similar situation can occur with a pipeline tested with positive pressure. In this case, the pipes are sealed at both at ends and pressurized, pushing the pipes apart and into the manhole structure if it is not adequately blocked.
To learn more about vacuum testing, visit precast.org/vtmanholes.