exposing flatwork

How to Expose Aggregate in Flatwork

by: Art Sampson

begin by placing your concrete flatwork as normal, screeding or striking off the surface to desired grades, be sure to float edges smooth and closing any holes.

A trick I use is to spray a small amount of water over the surface to help assist in bull floating or Darby finishing, this will help speed the process of filling all the holes and getting a nice wet grout at the surface (this is only because the top layer of paste is sacrificial and will be removed, outside of exposed aggregate never use water to finish air entrained concrete ever! ) be sure not to over work the surface. and avoid cold joints at all costs, if you get a cold joint you should cut it out with a shovel and float the fresh concrete into the joint. take your time not to get retarder into cold joints or unfinished areas to be floated.

After the surface water starts to dry, or as directed by manufacturer, start spraying the surface with Surface Retarder usually after about 20 mins. (weather permitting). Be sure to spray all the areas to be exposed being sure not to miss any spots, the retarder will usually be colored to help you see where its applied. A low pressure spray can is the best way to apply retarder, within reason you cannot put too much retarder on the surface but keep in mind you get about 100-200 sq.ft./gal.

After surface water is dry or about 45 min. Its a good idea to cover the area with plastic. this will prevent surface dehydration this is very important in hot weather. ACP International produce Surface Etch it has a built in membrane that seals the water in helping to cure and ultimately giving you a cleaner wash without leaving paste on the aggregate, then covering is optional.

Now here's the hard part, once the concrete reaches 600 psi, usually 6 - 12 or 12 - 24 hours after placement (temp. effects this dramatically keep an eye or know your concrete) begin hosing off the top layer of paste that has been retarded. the use of a broom will help move the fine particles along as you wash. caution if stones are coming out or you see that the concrete is still soft stop! and wait. A nice uniform layer of paste should wash away leaving a beautiful exposed aggregate finish.

The best way to prevent unsightly cracking is to put control joints in your concrete. The basic rule of thumb is 1 cut every 2 to 3 times in feet the thickness in inches (ie. 4" slab should have joints every 8 to 12 feet). Frost is a major contributor to cracking, and where applicable, extra precaution should be taken. The proper curing of concrete is essential in the reduction of cracks in freshly poured concrete.

Choosing a Sealer

The 'Wet Look' vs the 'Dry Look'

Wet Look - Film Formers	Dry Look - Penetrating Sealers
Advantages
tend to be less costly better stain protection (i.e., oil, grease, etc.) usually compatible with curing method used glossy to medium gloss look.	should not change the concrete's appearance less frequent application needed
Disadvantages
can darken the concrete may appear blotchy if not evenly applied will wear away, requiring more frequent applications may create a slippery surface	usually more costly not as good of a stain protector cannot be applied over a film former

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