Specifying broomed exterior concrete surfaces

Measured flatness on various broomed surface measurements
In 2009, Dave Schiable (Baker Concrete) and Scott Tarr and David Scott (Concrete Engineering Specialists [CES]) conducted a series of tests on broomed surfaces of an industrial paving project at Baker’s main office in Monroe, Ohio. The variables for the broomed surface measurements were:

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concrete mix: 28 mPa (4000 psi), 339 kg/m3 (564 lb/cy) of cementitious, four to seven percent air content, slump 150 to 200 mm (6 to 8 in.);
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weather conditions: air temperature of 21 C (70 F), RH 67 percent, sunny;
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broom texture: light, medium, heavy;
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finishing technique: (A) hand screed with 4.8-m (16-ft) long magnesium straightedge, 1.2-m (4-ft) wide magnesium bullfloat, 900-mm (36-in.) wide broom; (B) hand-held 3.6-m (12-ft) long vibrating screed, 1.2-m wide magnesium bullfloat, 900-mm wide fresno, and 900-mm wide broom; (C) hand-held 3.6-m (12-ft) vibrating screed, 4-ft wide magnesium bullfloat, 900-mm walk-behind machine with float shoes, 900-mm wide fresno, 900-mm wide broom; and
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measurement devices: dipstick with points and pads for feet, F-meter.

Although this study produced many different flatness measurements, only selected test results are shown here. Two separate placements were made, each on a different day. The first was a bullfloat-only surface—one section remained unbroomed, and another three sections received light, medium, or heavy brooming. The measured flatness for the bullfloat-only surface was FF 18.5, while subsequent light, medium, and heavy brooming over that bullfloated surface resulted in FF of 18.7, 17.5, and 14.5, respectively. Thus, it appears the bullfloat-only broom surface was close to the maximum FF of 20 predicted by Stephan’s data, but textures produced by medium or heavy broom finishing decreased the flatness.

The second placement featured finishing techniques A, B, and C, again with light, medium, and heavy broom textures. Figure 1 shows the flatness resulting from the three finishing techniques used during the second placement. As mentioned, the finishing technique for Type A involved the least amount of finishing consisting only of a screed, bullfloat, and broom while Types B and C included more finishing tools and techniques. Figure 1 shows the least amount of finishing, Type A, produced the highest flatness. Surprisingly, differences in broom texture did not have as great an effect on F-numbers in this set of data as they did on the concrete placements previously discussed.

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Figure 1: These two photos contrast the size effect difference in producing a broom finish for a 1.2-m (4-ft) wide sidewalk (left) versus a large-scale concrete placement for a parking garage (right). Photo at left courtesy of Portland Cement Association. Photo at right courtesy CECO Concrete Construction

Drainage
It is appropriate to consider drainage after the discussion 
of flatness because specifiers often state a flatness tolerance either by selecting a gap under a 3-m (10-ft) straightedge or an FF value—neither of which is consistent with another common specification related to drainage. For instance, a straightedge flatness tolerance of a 6-mm gap in 3 m (¼ in. in 10 ft) is essentially describing the allowable depth of water in a ‘bird bath’ (i.e. puddle). If the specification also prohibits bird baths or water ponding, the two requirements are incompatible.

The American Society of Concrete Contractors (ASCC) Position Statement 7, “Bird Baths on Concrete Slabs” showed bird baths are the unavoidable consequence of having a surface tolerance, even if the surface is sloped. This position statement was based on an article, “Birdbaths: Expectations vs. Reality,” by Suprenant in ACI’s Concrete International.

Even if there are some bird baths, drainage design is important in conveying rainwater from a parking lot or parking garage through a drain system to an appropriate discharge location. There are a few primary factors in drainage design affecting both the economics of the drainage design and performance of the drainage system. The two primary factors are direction and magnitude of surface slope.

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8 comments on “Specifying broomed exterior concrete surfaces”

  1. It seems like a lot more companies are using that ready-mix concrete. They are able to make up the mix twice as fast. It is yet to be determined if it is as good of quality. We’ll see once a few driveways get built.

  2. It is interesting that a slip-resistant surface is actually required for this type of thing. This makes sense especially in areas where snowy and rainy weather is really common. I know that where I live, parking lots would be treacherous if this type of thing wasn’t required. I appreciate your helpful information on how and why they do this type of thing to exterior concrete.

  3. There is a lot you can do with concrete. I remember laying concrete once and to see all of the work that goes into it is cool. If you want the job done right you have to make sure you lay it correctly and finish it with the broom finish to kept people from slipping.

  4. There are so many techniques and ways of putting in concrete. For something that seems so simple, it gets really complex. It seems like more and more methods are created all the time. Being someone who installs concrete for work, I have to stay on top of all the methods and figure out which one to use.

  5. Thanks for all of the insight about broomed exterior concrete surfaces. You talk about how this method is used to achieve the size effect because with larger surfaces, it becomes difficult to give the concrete a unified and consistent appearance, so the broom finish accomplishes this task. I can see how this is a fairly easy way to make sure that the surface ends up looking polished, smooth, and unified. Thank you again for the insight!

  6. Thanks for your article about concrete construction an safety. I didn’t know that exterior concrete surfaces must have some form of slip-resistant surface. I grew up in the Northwest, and it rained for most of the year. This makes perfect sense, though. You can get highly polished surfaces with concrete. With a slip resistant surface it should help to minimize falls.

  7. Thanks for sharing such an insightful information over here. A concrete driveway is considered to be the best part in housing decisions. If we compare to alternative driveway materials like cement or stone, Concrete offers numerous advantages. The above video shows all of the concrete benefits and its features. For Knowing further benefits of using concrete contact visit expert advice at the official site of paving contractors like Asphalt Paving Frederick MD

  8. Thanks a lot for sharing info about concrete surfaces in such detail. I found the size effect aspect especially interesting and relevant, being in the commercial and industrial flooring industry myself.

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