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.
Lauren writes:
(Regarding precast concrete pavement) How much time does it take for precast concrete paving slabs to be created? How much time would it take for the slabs to be installed/replaced versus the traditional methods of construction? And, will the method that uses the engineered foam still have the quality and longevity as the leveled slab?
NPCA Technical Services engineers answered:
NPCA has recently published the Manual for Jointed Precast Concrete Pavement that addresses your questions in your inquiry. The manual can be found at precast.org/jprcp-manual.
Q1: How much time does it take for the pre-slabs to be created? This depends, but typically precasters who are manufacturing a jointed precast concrete paving slab with traditional reinforcement can cast 8-to-12 slabs a day. They are stripped from the form the following production day. When manufacturing pre-stress slabs, some precasters have been known to fabricate 30-plus panels in a production day.
Q2: How much time would it take for the slabs to be installed/replaced versus the traditional methods of construction? This also depends on if you are asking about a continuous roadway repair or intermittent roadway repair. Typically, precast pavement is installed in a 5-to-8-hour work window, which typically takes place overnight. Contractors have been able to install up to 50 precast slabs in that timeframe, resulting in 500-to-1000 feet of new pavement placed overnight with minimal disruptions.
Q3: Will the method that uses the engineered foam still have the quality and longevity as the leveled slab? This question is best answered by contacting a foam manufacturer, but by all indications, yes, the quality and longevity are comparable. There are a number of projects in New York and New Jersey that have used this method.
NPCA file photo
Dan writes:
When combining two rebar lengths together that have epoxy coating, how far back do you remove the coating before performing the weld?
NPCA Technical Services engineers answered:
According to the CRSI Manual of Standard Practice, reinforcing steel should be welded according to the American Welding Society, AWS D1.4/D1.4M. If the steel used for the coated bars meets ASTM A706, “Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement,” the bars are intended for welding without preheating and should be specified for applications that require an appreciable amount of welding. ASTM A615, “Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement,” reinforcing bars can be welded, but may require preheating the bars up to 500 degrees Fahrenheit. After completion of the welding on epoxy-coated bars, the damaged areas shall be repaired using patch materials meeting ASTM A775, “Standard Specification for Epoxy-Coated Steel Reinforcing Bars.”
NPCA file photo
As far as removing the coating, enough should be removed so no epoxy remains in the path of the weld. This depends on the size of the bar used and the required overlap, splice or development length.
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