Questions from the Field is a selection of questions NPCA Technical Service engineers received from calls, emails and comments on blog posts or magazine articles posted to Precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/plant-resources/technical-services.
Steve asks: We have been asked by a local utility to provide a precast concrete fire wall using refractory materials. The precast concrete panels must be 2½ inches thick and rated at a minimum of a four-hour fire rating. Can you assist with some suggestions?
NPCA technical experts answer: To get the refractory properties you need to achieve the desired fire rating, you could investigate using calcium aluminate cement (CAC). Contact your local cement supplier about availability and its ability to perform the mix design.
Also, if needed, ACI has a guide available that covers calcium aluminate cement. The guide is PRC-225-19: “Guide to the Selection and Use of Hydraulic Cements.”
If you cannot get to a four-hour fire rating by simply using calcium aluminate cement, you may have to look at an intumescent or cementitious coating for the precast concrete panels. We suggest searching for a coating’s supplier at “find precast products and supplies” button on precast.org. A coatings supplier will be able to discuss the specifics and options available.
Mike asks: A project engineer is asking for the sound transmission coefficient for a 14-foot-tall, 4-inch-thick precast concrete noise barrier wall, standard weight concrete.
NPCA technical experts answer: To get an accurate number for your specific wall, measure the sound transmission class (STC) by using ASTM E413, “Classification for Rating Sound Insulation.”
For a general number, the Precast/Prestressed Concrete Institute Design Handbook has a section on precast acoustics that shows a 4-inch-thick precast concrete wall with a density of approximately 164 lbs/ft3 has an STC of 49.
A document published by the U.S. Department of Housing and Urban Development called the “Noise Notebook,” Chapter 4, Appendix A has a 4-inch-thick slab of concrete with the same density having an STC of 44. PI