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.
Is a vacuum test required for all manholes? What is the frequency requirement?
NPCA Technical Services engineers answered:
Frequency is dependent upon a few factors. If the vacuum testing occurs at the precast producer’s plant as part of its quality control/quality assurance program, the frequency of testing for manhole products used in sanitary sewer applications or other installations where installed measurable infiltration/exfiltration requirements exist would be established in the plant’s QC manual. There are no specific code requirements stating what that frequency shall be.
Similarly, per ASTM C1244, “Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill,” installed manhole structures do not have a testing frequency specified within the standard. Rather, the testing frequency is required to be stated in the project specification. Additionally, many states have included this testing in their specific sanitary sewer regulations along with the testing frequency expectation. These state regulations also permit the approval testing per an infiltration/exfiltration type of hydrostatic test, but many prefer the ease that a vacuum test provides versus the hydrostatic tests.
The NPCA article, “Vacuum Testing Precast Concrete Manholes,” was published in 2010. Since that time, the issue regarding whether it is preferred to test installed manholes before or after backfilling has still not been resolved. An ASTM Committee C13 task group is currently developing a specific installed and backfilled manhole vacuum test standard, but it has not yet been approved. If there is high groundwater on the outside of the installed manhole to be vacuum tested, analysis and care needs to be taken to ensure the combination of internal vacuum and external hydrostatic pressure does not exceed the recommended maximum forces on the resilient rubber pipe/manhole connector, which could lead to unintended leakage.
What is the effect of the variability of loading rate on the strength of a concrete cylinder? The majority of concrete compressive strength testing machines used in the industry are termed semi-automatic, but loading is in fact controlled manually by the operator, which relies on the operator’s ability to maintain a consistent load application rate. Would an automatic loading control improve the consistency of concrete cylinder strength test results?
NPCA Technical Services engineers answered:
As you know, a semi-automatic compressive strength testing machine relies on a hand-operated valve or lever to regulate the load rate on a cylinder. You mentioned the operator’s ability to keep the correct pace until failure, so let’s address that statement first. The intent in applying the load is to do so continuously and not intermittently to avoid shock. Section 7.5 of ASTM C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,” states the load rate should correspond to a stress rate on the specimen of 35 +/- 7 psi per second. Therefore, there is a tolerance of 14 psi during the application, and as long as the operator stays within that tolerance, their pace is acceptable.
Next, let’s address your question about how an automatic system would affect repeatability and consistency and, consequently, variability of test results. When we rely on manual adjustments during any testing procedure, there will inherently be some variability. In the case of operating a semi-automatic compression machine, each user will apply and adjust load rates differently and the associated variance will depend on the machine, the aptitude of the user, and the number of users in that plant. There’s no way to eliminate variability of application in this case, but there are measures that can be taken to minimize it. Multiple users should work together on how they will operate the machine and they need to be as consistent as possible in how they apply and adjust loading rates.
An automatic compression machine applies the load rate as required and does not involve manual adjustments of valves or levers. Using this apparatus removes the variability inherent with hand-operated load rates. There will still be variability based on the machine and its recommended and appropriate calibration. However, the automation improves consistency and reduces variability compared to a hand-operated system.