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Wednesday, June 20, 2012

Building a Freeway: Portland Cement Concrete Pavement



Placing Portland Cement Concrete Pavement (PCCP) is one of the final steps involved when building a road, but it’s far from an afterthought…

Type of pavement is a consideration very early in the planning phase when designers look into various factors, including pavement design life, traffic volume and type, soil conditions and maintainability. Based on these factors the designers determine everything from the pavement's thickness to how strong it needs to be. 
 
Let’s take a look at how it’s made
Portland Cement Concrete Pavement (PCCP) consists of cement, sand, aggregate (rocks) and water. There are also admixtures – materials often added to the concrete mix to alter its properties. These admixtures can do a number of things to the concrete mix. Depending on the need, they may be added to reduce water, slow the setting rate, accelerate the setting time or add color to concrete, among other things.

A few facts about PCCP
  • When it’s this hot outside (100 degrees and up), crews have to mix and place the PCCP at night when temperatures are cooler. There are a few reasons for this, one having to do with water evaporation. In higher temperatures, the water in the concrete evaporates faster (more water could be added to maintain the consistency, but that has the unwanted effect of reducing concrete's strength). Conversely, if the weather gets too cold in the winter months, crews can’t place PCCP because the water will freeze and inhibit the PCCP from curing properly.
  • Portland Cement Concrete can be purchased or mixed on site.
  • PCCP has a long life-span. In Arizona it’s used mostly on freeways within the Valley and Tucson metro area. Because of its durability, PCCP is great for these high-traffic areas.
  • A curing compound is sprayed onto the PCCP after it is placed. It creates a kind of membrane that prevents moisture from evaporating from concrete (again, water is a key component to PCCP’s strength).
  • Crews aren’t done once the PCCP is placed. The PCCP is not ready for traffic unless it meets certain criteria. ADOT tests for strength, thickness and smoothness before vehicles are allowed on the road.
Posted by Angela DeWelles   |  Labels:  
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