Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and plant evaluations.
MOLLY ASKS: Is it acceptable to have a crack in a 36-inch pipe? We just received a load of pipe, and there are several with cracks.
NPCA EXPERT ANSWER: Like any good engineering answer, it depends.
It is recommended to evaluate cracks in precast drainage products before installation using AASHTO R-73. The document outlines acceptance and provides pictorial representation of what acceptable precast drainage products should look like.
For reinforced concrete pipe, per AASHTO R-73 paragraph 4.3.2: Any continuous crack having a surface width less than 0.01 in. wide, provided it does not pass through the wall, shall be acceptable.
For reinforced concrete pipe, per AASHTO R-73 paragraph 4.3.3, any continuous crack having a surface width of 0.01 in. or greater and less than 12 inches long, provided it does not pass through the wall, shall be acceptable. There is additional guidance for end cracks.
DAN ASKS: This project requires a design life of at least 75 years. What do I need to do differently to meet this requirement?
NPCA EXPERT ANSWER: Design life is an AASHTO LRFD term defined in the Bridge Design Specifications (10th Edition, Section 1, Article 1.2 ‘Definitions,’ p. 1-2) as the period of time on which the statistical derivation of transient loads is based — 75 years in this case. It’s a load-calibration concept, not a durability guarantee. Since ASTM C1577 requires designs to be done per AASHTO LRFD, a standard C1577 box already carries a minimum 75-year design-life basis without any changes to your mix, cover or reinforcement. The actual lifespan could, in fact, be even longer.
GIL ASKS: In ASTM C990, “Standard Specification for Joints for Concrete Pipe, Manholes, and Precast Box Sections Using Preformed Flexible Joint Sealants,” the stated intent of the standard is to “prevent the flow of solids through the joint,” yet there are performance requirements in Section 10 that specify a 10-psi hydrostatic test under straight alignment. Section 10 mentions this is only a quality control test, but there appears to be a wide gap between the intended function of preventing the movement of solids through the joint and the hydrostatic test requirement. Why is this?
NPCA EXPERT ANSWER: While we cannot offer any interpretations of ASTM standards, the following reflects our best understanding of the correct use of ASTM C990 and, by extension, AASHTO M198.
The choice of wording used to describe testing requirements for joint sealing materials was apparently intended to discourage the specification of a hydrostatic test for precast concrete products with mortar joints installed in horizontal alignments, where more suitable products with leak-resistant joints should be specified, such as ASTM C443 or C1628 for pipe joints and C1677 for box culvert joints. The reference to hydrostatic testing in Section 10 specifically notes that tests are to be conducted “in vertical alignment,” which matches the typical joint orientation of manholes, wet wells and lift stations that may not be able to be equipped with the aforementioned leak-resistant joints but may still be tested and proven to prevent the passage of water. ASTM C990 clearly states that the hydrostatic test described in the standard is intended only for use as a quality control test and not as a simulated service test, so any field performance testing and proper procedures for those tests should instead be included in the project specifications as required.