polished concrete

EHH week 14: concrete floor topping

The finished floor upstairs will be a polished concrete topping.  At just 2 inches thick, the concrete has pea gravel aggregate and is reinforced with steel mesh.  It was pumped into the house, then screeded and troweled to a smooth, flat surface.  Flat is crucial for the floor polishing to look good.


The subfloor is red!  The color comes from a waterproof coating sprayed over the entire plywood subfloor.  Since concrete is wet at first, we did not want all that water to soak the plywood that Model Remodel so carefully protected from rain all summer.  The coating also acts as a bond breaker, allowing the concrete to shrink and move independently of the plywood.


Since the finished floor will be “cast in concrete” it was crucial to lay out all the floor penetrations before the concrete is poured.  Items like electrical outlets, heating grilles, and some plumbing pipes were blocked out to the precise size of each recessed item.


The pour was done in two halves on two different days with a “cold” joint between.  After the concrete had hardened, control joints were saw cut into the surface using a wet saw and a vacuum to limit dust.  These joints are intended to control the surface cracks as the concrete dries so they fall in nice straight rectangular lines, instead of unsightly jagged cracks.  Both these joints will be filled with caulk to match the color of the concrete once it is polished.


A week after pour day, still no cracks!

EHH week 19: concrete floor polishing

The interior concrete floors have cured long enough that they are ready for polishing.  A heavy machine fitted with diamond tipped blades (reminds me of an electric shaving razor) makes several passes over the concrete surface.  Its job is to remove the “cream,” the uppermost 1/8 of an inch or so of cement that forms the skin of the cured concrete. 


Removing the cream exposes the pea gravel aggregate which gives the finished surface a lovely “salt and pepper” appearance.  The trick is to remove just enough to expose the aggregate without overdoing it so that the salt and pepper look gets too chunky.


Once the removal step is complete, polishing begins with successively finer grit, just like sanding a piece of wood.  The final grit for this project is 400, which we feel is not too shiny but still lets you see into the depth of the concrete, so to speak.  Before polishing the main floor, the contractor did a test patch to the satisfaction of architect and homeowner so everyone agreed on the look we were targeting.


As part of the polishing, a densifier is added to essentially fill in the pores of the concrete so it is resistant to water and stains.  The polished floor was then covered with protection boards for the duration of construction.  Just before turning the finished house over to the homeowners, the protection will be removed and one last densifier coat will be applied.  They will look gorgeous!