Surface Textures for Concrete

There are many standard surface textures for concrete and X there are a lot more you can "invent". To get an idea of why such possibilities exist it would be helpful to take a look at a cross section of a concrete slab. Correctly poured, spread and finished, the cross section would reveal an almost uniform pattern. The larger stones in the mix would be surrounded by smaller stones and spaces between would be filled with the finest aggregates (sand). Each piece of aggregate would be encased in cement which bonds to itself and bonds aggregate to aggregate.

Choose a surface—plain, striated, travertine, exposed aggregate—or invent one that meets your requirements

Travertine-like texture, also called "keystone finish," is unusually decorative, is often used around a pool, on large patio slabs or for a garden walk.

The more you work the top surface of poured concrete, the more action you use to spread and tamp it with shovel and rake (and even the strike board), the finer the top surface becomes. This is because working settles the larger stones to the bottom and, since they can't do anything else, the finer aggregates rise to the top. To a degree, this condition is necessary for any concrete finish which is relatively smooth. You don't want large stones too near the surface where they can break through the finish.

Working with a wood float is a finish in itself or it can be just a step in preparing the surface for a slick finish accomplished with a steel trowel. The wood float finish leaves a rough, slip-free surface which is practical for many purposes, especially on walks and paths where traction is an important consideration. The steel trowel finish can be made almost as slick as linoleum and this is good for interior slabs that will be overlaid with tiles or which will be used as is—for a recreation room that will be used for dancing, for example.

It may be arbitrary to say so, but it is our feeling that a good many of the deco­rator-type finishes are easier for the ama­teur to do than the float or trowel finishes. It takes a lot of practice to master the art of smoothing a concrete surface with float or trowel so that the resulting floor is level, uniformly textured and free of "arcs". On the other hand, anyone can use a garden hose to wash down a slab, remove a slim top layer of fine materials and re­veal the larger stones. This is a quickie method of getting an exposed aggregate texture, granted, but it's still effective so long as the timing is right and the stream of water directed uniformly over the en­tire surface.

When you have an idea for a particular texture, construct a small test slab to see if the results match your expectations. The test slab does not have to be large; step­ping-stone size will do. The important thing is that you carefully note the con­tents (proportions) of the mix, amount of spreading and tamping, time lapse between striking (if any) and the finishing touches. Go through this procedure even if you have to make three or four test slabs. They can always be used as stepping stones.

Striated Surfaces

Striations can take the form of straight, zigzag or wavy lines and are usually used to add some interest to a nonskid textured surface. A long handled broom can be used for the purpose or a small hand brush. Stiff bristles will produce a coarse texture while soft bristles result in a fine texture. How much time you wait after the con­crete has been fully prepared—poured, spread, struck, floated—will have some bearing on the results achieved. The softer the surface, the coarser the results if only because the bristles of the broom sink deeper. The more you let the concrete set the harder the surface becomes, and the shallower the lines and the finer the finish.

You can finish with a broom or brush over a floated surface or a troweled one. The float-broom finish will always be coarser than the trowel-broom finish. Straight lines are done by resting the broom on one edge of the pour and then pulling toward you. Each time you do as much as the width of the broom. You can jiggle the broom as you pull it or move it in such a manner that you create a wavy effect.

Swirl effect with steel trowel produces a finer finish than wood float. For a slick finish, allow the concrete to set somewhat before troweling it.

Travertine Effect

This is a very intriguing texture (see the photo on page 15) which involves an extra operation after the surface of the concrete has been broomed. The brooming is to in­sure a good bond when for an additional mortar coat

Swirl design with float is done with semicircu­lar or fan-like motions. The more pressure, the grittier the surface. Moist curing is last step.

To make the mortar coat, use a sack of white Portland cement with 2 cu. ft. of sand. Add about ¼ 1b. of color pigment and mix thoroughly. Any mineral oxide color can be used, but yellow is extremely effective. Add enough water so that the mix will pour like heavy paint.

The coating is thrown on the slab with a brush. The best method is to pick up a good quantity and apply it to the slab by snapping the brush. A quick flick of the wrist does it—a snap that causes the mix to leave the brush and strike the slab smartly. The idea is to make an uneven surface with depressions and ridges that are from 1/4 to 1/2 in. high. Allow the coat to harden until you can rest a board across it for working and use a steel trowel to flatten the ridges. The result is a kind of three-dimensional texture that is smooth in some areas and coarse-grained in the depressions. You can vary the results by the amount of final troweling you do and/or the amount of finish mortar applied. The final effect is most pleasing if the slab is then scored in a uniform or random geometrical pattern before the concrete cures.

Exposed Aggregate

The true exposed aggregate finish does not rely on exposure of materials in the mix itself. Instead, a specially selected ag­gregate is imbedded in the surface of the pour by tamping. Washing and brooming then reveals this attractive stone.

The Portland Cement Association rec­ommends colorful gravel aggregate which is quite uniform in size—ranging from 1/2 to 3/4in. The aggregate should not con­tain flat, sliver-shaped particles. Stones under 1/2 in. diameter may not bond properly and could become dislodged dur­ing the exposure procedure.

Immediately after the pour has been struck, spread the selected aggregate by hand over the entire surface until it is fully covered. To imbed the stones, use the flat side of a 2x4, working only as long as you have to to flatten the surface and sink the stones. As soon as the surface will support your weight on a kneeboard, hand-float the surface until all the added ag­gregate is thoroughly coated and settled to just beneath the surface of the pour. Be sure there are no holes or openings in the surface and that the cement fully en­cases each piece of new aggregate.

For exposed aggregate surface allow concrete to set and hose down to remove the finer surface particles. A stiff brush can be used to help expose more of the larger aggregates.

Straight, zigzag or wavy striations can be made with broom. Use stiff bristles for rough surface, soft bristles for a finer texture. Softer the concrete, the deeper the lines.

True aggregate surface is made by applying stone to surface of pour covering all open spaces. Size of stone should range between ½ in. and ¾ in.

Use flat side of 2x4 or special tamper like one shown to settle aggregate into the pour. Strike uniformly so all stones sink a similar distance.

Tamp until all stones are covered (below, 1.). Tool here can be used like float, or you can use hand float to further settle and even the aggregate.

After suitable curing time, surface is wet down and worked with broom so fine surface materials are removed to expose the colorful top aggregates.

Flagstone effect is achieved by scoring with piece of 1/2 in. copper pipe as below. Note that surface has been floated. Alter scoring, refloat surface to remove burrs. Go over lines once more with pipe, finish with steel trowel. Scored lines get final cleaning with a soft brush (see left photograph).

Circle effect (1.) is done with cans of various sizes; other materials can also be used. Leaves can be imbedded in surface, then removed later.

The proper time for exposing the aggre­gate is very critical. For the amateur, the best bet is to pour one or two small test slabs along with the main pour and wash and broom these after a suitable interval of time. If results here are successful, pro­ceed to the main job. It's difficult to give a specific time lapse due to the fact that weather conditions, consistency of the mix used, etc., all have a bearing. The impor­tant point is that the surface be suffi­ciently hard so that scraping with a broom and wetting with a hose will do nothing more than remove the thin coating of cement that covers the aggregate.

The same technique can be used indoors to obtain a surface that is similar to ter-razzo. In this case, the hosing and broom­ing operation is eliminated; the exposure and smoothing of the stones is done entirely by grinding followed by polishing.

Scored Surfaces

You can score a concrete slab or walk in many ways after it has been prepared and excess moisture has left the surface. Scoring can be done with a groover or edger or, as shown in the photos, with a bent piece of 1/2-in. copper pipe. The score marks should be about 3/8 in. deep and should be made while the concrete is still quite plastic so that the pipe will move aside any large pieces of aggregate it encounters.

This procedure will raise burrs, so float or trowel the surface once more and then go over the lines again with the scoring tool. Finish with a final, very careful troweling. After the concrete sets it can be worked very lightly with a fine-bristle brush. Clean the joints with a soft paint brush. Do not use any water for these last operations.

Photo Plasties photo

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