Q: Where can I find information on vacuum testing?
Q: How deep can a circular manhole be buried?
Q: Can I use rebar as a lifting loop?
Q: I am considering starting up my own precast concrete plant. What should I do?
Q: What is proper curing?
Q: How do I properly size a precast concrete grease interceptor?
Q: What are the different types of cements? How are they different?
Q: What is sustainability and LEED?
Q: Can I save money and become more sustainable?
Q: What paperwork do I need to get the Contractor for LEED?
Q: What LEED Credits does my precast product contribute toward?
Q: Where can I find information on vacuum testing?
A: ASTM C 1244, Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to backfill, is the only vacuum testing specification specific to precast concrete manholes. ASTM C 1227, Specification for Precast Concrete Septic Tanks, contains a section on testing which discusses vacuum testing procedures for precast concrete septic tanks.
NPCA publishes the Septic Tank Best Practices Manual, which details best practices for production of precast concrete septic tanks.
Q: How deep can a circular manhole be buried?
A: You may have heard someone say that manholes can be buried to an infinite depth. However, studies have shown that circular manholes can be buried to any depth up to approximately 500 feet. With this flexibility many say that burial depth is infinite, but studies do not necessarily support such a claim.
Q: Can I use rebar as a lifting loop?
A: Specifications that address the proper lifting of precast concrete products are available through The NPCA Shop and include:
- NPCA QC Manual, Section 2.3.1 (comments) states that reinforcing bars should not be used as lifting devices, but smooth bars as per ASTM A36 can be used.
- ANSI Standard A10.0-1983, Concrete and Masonry Work – Safety Requirements
- 704, Requirements for Precast Concrete
- ASTM C 857, Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures
- ASTM C 890, Standard Practice for Minimum Structural Design Loading for Monolithic or Sectional Precast Concrete Water and Wastewater Structures
- ASTM C 913, Standard Specification for Precast Concrete Water and Wastewater Structures
These specifications define safety factors for all lifting devices and loops. In general, the use of rebar as a lifting loop is not covered by any specification. The precaster is in a precarious situation when using rebar as a lifting loop because many believe they can use them since specifications do not specifically disapprove them. However, in the event of a failure that causes injury, the precaster can be liable since specifications also do not specifically approve them.
If you are investigating using rebar as a lifting loop, you should educate yourself as much as possible on the parameters of the bar that will affect the likelihood of failure. The Precaster’s Notebook, available through The NPCA Shop, devotes an entire chapter (chapter 4) to the art of lifting precast concrete products and is a good start.
Q: I am considering starting up my own precast concrete plant. What should I do?
A: The answer to this question is largely dependent on which products you wish to produce, what type of production process you choose and whether or not (and how soon, if so) you will likely expand to other product lines. It is highly recommended that you hire an independent consultant to help with plant layout, equipment selection and other issues. Your consultant will likely need to know:
- How much you are willing to invest
- Whether you wish you buy new or used equipment
- You preference for wet-cast or dry-cast production
- The various dimensions of any properties you are interested in purchasing or leasing
- What products you wish to produce (both now and in the future), etc.
You should plan on attending the next Production & Quality School to begin learning as much about precast concrete production as possible. You should also begin plans to attend the next The Precast Show to make contact with various equipment manufacturers and other suppliers who will become an invaluable resource to you as you build your company. Good Luck!
A: Curing is a very delicate part of producing precast concrete. However, the ability to cure properly and consistently at the plant offers a distinct advantage over cast-in-place concrete products, which must be cured at whatever ambient conditions exist on the day of pour.
Proper curing ensures that water is not allowed to escape the product. The amount of water in any given batch must be completely consumed by the chemical reaction between water and cement. If water is allowed to escape the product, it leaves unused cement in the mix that cannot hydrate and contribute to the concrete strength.
Leave the forms on as long as practical, since the formwork helps to maintain moisture. Once you strip the forms, however, it is important to maintain a warm, humid environment around the product until your specified handling strength is obtained. Ideally, it is beneficial to maintain a positive curing environment until the specified concrete compressive strength is reached. The use of a curing compound can help prevent moisture loss, but you can also mist the product. Always keep the product away from direct sunlight.
Q: How do I properly size a precast concrete grease interceptor?
A: This question is asked frequently because sizing criteria for grease interceptors, regardless of construction material, is inconsistent from one municipality to another. Many specifiers require sizing according to the Uniform Plumbing Code (Appendix H) sizing formula, but others do not. Contact the NPCA Technical Services Department for more information.
Q: What are the different types of cements? How are they different?
A: Portland cement is produced by pulverizing clinker and contains the following basic components:
- C3S (Tricalcium silicate)
- C2S (Dicalcium silicate)
- C3A (Tricalcium Aluminate)
- C4AF (Tetracalcium aluminoferrite)
Varying quantities of these cement ingredients are used to produce different types of Portland cements. ASTM C 150 defines five basic cement types with the following properties:
- Type I Normal
- Type II Moderate Sulfate Resistance
- Type III High Early Strength
- Type IV Low Heat of Hydration
- Type V High Sulfate Resistance
Cement types I, II and III can contain air-entrainment (Type IA, IIA and IIIA) but air-entrained cement is not available everywhere. As a result, air entrainment is typically achieved with use of an admixture rather that with the use of air-entrained cement.
When some (or all) of the cement content in a concrete mix is substituted with pozzolans or slags, the result is a blended cement. ASTM C 595 recognizes the following five classes of blended cements:
- Type IS Portland blast-furnace slag cement
- Type IP (or Type P) Portland-pozzolan cement
- Type I(PM) Pozzolan-modified Portland cement
- Type S Slag Cement
- Type I(SM) Slag-modified Portland cement
Due to their special nature, blended cements require some study. You should visit the NPCA Online Store for the most updated version of ASTM C 595 for further information.
Q: What is sustainability and LEED?
A: Sustainability is generally accepted as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” (Brundtland Commission 1987) Precasters can become more sustainable by making both their products and manufacturing plants more environmentally friendly.
According to the USGBC, Leadership in Energy and Environmental Design is a third-party certification program and the nationally accepted benchmark for the design, construction and operation of high-performance green buildings and neighborhoods. Developed by the U.S. Green Building Council in 2000 through a consensus-based process, LEED serves as a tool for buildings of all types and sizes. LEED certification offers third-party validation of a project’s green features and verifies that the building is operating exactly the way it was designed to.
For more information, please visit NPCA’s sustainability page.
Q: Can I save money and become more sustainable?
A: There are three common categories when money is involved in sustainability. Some sustainable initiatives can easily be achieved and will save you money. Some will cost you money up front but it will pay for itself over the span of a few years. Lastly, there are sustainable decisions which will never make you money but should be done simply because it’s the right thing to do.
For more information, please visit NPCA’s sustainability page.
Q: What paperwork do I need to get the Contractor for LEED?
A: Typically the paperwork involved will be linking your concrete’s materials components proximity to the job site and the quantity of recycled content of your product. NPCA has a LEED Reference Guide which outlines some of the LEED credits and precast’s contribution toward those credits. In addition, NPCA has a LEED Calculator online to help guide you through the process of completing the necessary paperwork.
For more information, please visit NPCA’s sustainability page.
Q: What LEED Credits does my precast product contribute toward?
A: Each product will contribute to a different set of credits. NPCA will soon have a free downloadable set of product related LEED Reference guides (coming soon). If you have additional questions please contact Claude Goguen.
