Specifying broomed exterior concrete surfaces

Directional slopes for drainage
There are four different options for choosing the slope needed for water to reach a drain or drain off an edge of a parking lot or parking structure surface:

  • 
one-way top-surface slope;
  • 
one-way top and bottom surface slope;
  • 
two-way top and bottom surface slope; and
  • 
warps.
broom_figure2
Figure 2: A one-way, top-surface slope is the lowest cost option because it maintains a constant base elevation for a parking lot or a constant soffit elevation for parking structures. Concrete thickness is variable—
thicker at the top of the slope and thinner at drain locations. Images courtesy CECO Concrete Construction

One-way top-surface slope
This is the lowest cost option because it maintains a constant base elevation for a parking lot or a constant soffit elevation for a parking structure. This top-surface slope uses variable concrete thickness—thicker at top of slope and thinner at the drain locations (Figure 2).

One-way top and bottom surface slope
To reduce the volume of concrete needed, the base of a parking lot or the bottom of suspended slabs may be sloped parallel to the top surface. This may be more expensive than the one-way top-surface slope option. The parking lot base may be easy to slope with laser-guided fine-grading equipment, but positioning the suspended formwork at varying elevations is labor-intensive and costly. Beams should also be sloped to be parallel to the top slab and avoid variable beam depths.

broom_figure3
Figure 3: The two-way top and bottom surface slope option is costly and should be avoided. For a parking lot base, 3D laser-guided fine-grading equipment would be needed, preventing many contractors without the equipment from bidding. For a parking structure, the high (ridges) and low (valleys) spots run in two directions, slowing formwork productivity at each slope change.

Two-way top and bottom surface slope
Figure 3 shows this two-directional drainage slope option is costly and should be avoided. For a parking lot base, 3D laser-guided fine-grading equipment would be needed, preventing many contractors without the equipment from bidding. For a parking structure, the high (ridges) and low (valleys) spots run in two directions, slowing formwork productivity at each slope change.

Warps
Of all slope designs, warps are the most expensive slabs to construct with either a parking lot base or with formwork 
for suspended parking structure slabs. Other possible slope designs should be considered to reduce the owner’s costs for water drainage (Figure 4).

broom_figure4
Figure 4: Of all slope designs, warps are the most expensive slabs to construct with either a parking lot base or with formwork for suspended parking structure slabs. Other possible slope designs should be considered to reduce the owner’s costs for water drainage.

Most specifiers find a way to incorporate the one-way top surface slope or the one-way top and bottom surface slope. While warps are rarely specified, some specifiers still require the expensive two-way top and bottom surface slope option.

How much to slope?
It is important to recognize the effect of the chosen slope on cost and performance. A two percent slope (1⁄4 in./ft) will be shown to be the most effective slope for water drainage. However, a two percent slope over a 18-m (about 60-ft) wide parking structure (i.e. a drive lane and two parking stalls) requires an elevation change of 380 mm (15 in.). A 90-m (300-ft) long parking lot with a two percent slope requires an elevation change of 1900 mm (75 in.). Accommodating these elevation changes can increase costs, so some specifiers reduce the drainage slope as a compromise to reduce the cost of handling the elevation change. However, what effect does the compromise have on drainage?

The following are ACI recommendations for drainage slopes for parking structures:

ACI 362.1R-97:

A minimum slope in any direction of 1½ percent is recommended with 2 percent being preferred. This slope does not usually require special slab tolerances, and will generally overcome inaccuracies in construction and deflection estimates. Camber and deflections, however, should be taken into consideration when establishing a drainage pattern.

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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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