Q. Does self-consolidating concrete exert much higher pressure on forms than more traditional concrete?

A. Yes. Although research has shown that SCC can set up soon after placement in forms, there is also evidence that a bump of the forms or a little unintended vibration can cause the concrete to suddenly become fluid again. The safest approach is to design formwork for SCC so that it can support full liquid head. Based on that, if you assume that the unit weight of the concrete (actually its density) is 150 pounds/cubic foot, and you are pouring a 5-foot wall in one lift, you need to design the forms for 750 pounds/square foot of pressure. That's not so difficult to handle and most decent formwork will be fine. If you get to higher lifts, though, keep pressure in mind. But perhaps even more important on many jobs is that since SCC is so fluid, even small holes in the forms will leak. Check your forms over carefully prior to using SCC and you'll get the best surface finishes you've ever seen.

Q. I have a project where I need to place then stamp a ¼-inch-thick overlay. How can I choose the right overlay material for stamping?

A. Each manufacturer formulates its overlay products to have certain advantages when it comes to stamping patterns. The wish list of attributes and properties can get confusing, but here are some to look for when selecting a product:

  • An overlay material should spread easily—some are self-leveling.
  • It should establish a good bond with the existing concrete.
  • It should have a wide range for thickness of application, typically 1/16 to ½ inch.
  • It should be resistant to surface shrinkage and micro-cracking.
  • It should be easy to work with in its wet form.
  • It should be resistant to thermal shock (have a good temperature working range).
  • It should be plastic enough to record detailed impressions.
  • It should have setting characteristics that provide the maximum window of opportunity for stamping impressions.
  • Many integral colors should be available, and the overlay should tolerate colored powdered release agents and small amounts of color hardener for adding color variation.
  • The hardened overlay should be stainable with chemical stains.
  • The finished surface should have a natural concrete appearance, as opposed to a shiny “resin” look.