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Making Concrete Slip Resistant

Broom Finish

Concrete finishers have been broom finishing their surfaces for about as long as there has been concrete. Typically decorative concrete surfaces are not broom finished, although dyes and stains can be applied very successfully to broomed finishes. Even stamped finishes can be broomed, although that's a bit difficult—impossible if you are using a powdered release agent. There are better ways to make stamped surfaces slip resistant, which we will get into later.

The typical process for a broom finish is:

  • Pour the slab
  • Strike off with a screed
  • Bull float
  • Wait for the bleed water to evaporate—although with low water-cement ratio exterior concrete with the proper amount of air, there might not be much bleed water. Bleed water is a result of the wet concrete settling and with entrained air, it doesn't settle much and therefore little water comes to the surface. The proper amount of air is always critical in any exterior concrete that will be exposed to free-thaw action. For concrete with ¾ or 1-inch aggregate, order the concrete with 6% entrained air (plus or minus 1%)—and make sure you are getting it, otherwise the surface will spall. For smaller aggregate you need more air—7% for ½ inch and 7.5% for 3/8 inch.
  • Trowel—there's some disagreement here. In many cases, today's finishers won't trowel a slab that's getting a broom-finished surface, just bull float and broom. One veteran finisher, however, told me "I like to use a fresno to get the bull float lines out." Bob Simonelli, with Structural Services Inc., says that some troweling is OK, "but be careful not to over-finish the surface and work some of the air out." Advice in a 1996 edition of Concrete Construction's Problem Clinic, however, says you can trowel twice before brooming, but be sure to keep the trowel flat during the second troweling and begin brooming "immediately after the second troweling." If you get the surface troweled hard, it will be difficult to get much texture. PCA's Cement Mason's Guide says to use a damp broom after troweling.
  • Broom the surface by running a concrete broom perpendicular to the slope, if there is one. On concrete that's intended to drain, though, broom marks should be run towards the drain. One thing to note is that a broom-finished exterior surface is just as durable as a smooth finish.
  • Cure the concrete—You can (and must) cure broom-finished concrete with sheets of polyethylene or by spraying on curing compound. For plain gray concrete, a curing agent with some color (typically white) in it helps you to see where it's been applied. The color dissipates after a few weeks. For decorative concrete, use a cure & seal. Don't forget the curing!

A good broom finish is something of an art. You can even create decorative effects by running the broom texture in various directions. Typically the broom should be run from side to side of the concrete without stopping. With a standard broom, you should pull the broom towards you, then lift it and set it back on the far side to pull it across again. Marion Brush makes a brush (the Auto Glide) where the head automatically tilts to the correct angle, so you can get a good broom finish whether you are pushing or pulling the broom.

Brooms are available from a variety of sources. They come in various widths and the block that holds the bristles can be made from wood, aluminum, or plastic. Brooms tend to be wet a lot and the plastic blocks (high-density polyethylene) won't rot or warp. Bristle materials can be horsehair, polypropylene, or nylon and come in various stiffnesses and sizes to produce different textures. For extreme textures, wire combs are available to produce tined finishes.

Marion Brush Co. specializes in concrete brooms. Their president, Gary Bolden, says that "there are a lot of variables in getting a good broom finish: the slump of the concrete, the weather (sun, wind), and the timing. Every contractor has a tendency to do a slab in their own way. Some might broom sooner than others. Some may pour and broom finish and use a softer brush so they can get on the slab sooner." Marion's Chameleon concrete broom allows you to change out an insert to get different textures or to adjust to conditions. The color-coded inserts come in five textures from supersoft to rough.

Marion's concrete brushes are made from 612 nylon, which Bolden admits is one of the most expensive materials used for brooms, but which he says will outwear other materials by 3 or 4 times. The other advantages of this nylon are that it stays cleaner during use and that it has a memory. No, it can't remember your phone number, but if the bristles get bent, you can pour boiling water on them and they will return to their original shape.

A couple of other interesting kinds of brooms are handleless and brooms attached to bull floats or fresnos. The handleless brooms are pulled back and forth across the surface with ropes—Marion Brush and Cleform make these. This invention is especially handy on very wide pours where it would be difficult to push a broom all the way across and where the bull float handle begins to get so heavy that it pushes the broom too deeply into the concrete. Also available from Marion are brooms attached to bull floats or fresnos. This allows the float to support the weight of the handle rather than the broom. Bolden says that this positions the broom so that the finish is made by the sides of the bristles rather than the tips, which provides a more uniform finish.

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