March 27, 2015
by Bruce A. Suprenant, PE, PhD,
and Frank Salzano, PE
As a safety requirement, all exterior concrete surfaces must have a slip-resistant surface. The most common method for this is specifying a ‘broom finish.’ But, is that all there is to it? Specifications must also satisfy owner requirements for appearance, flatness, texture, as well as drainage.
These requirements vary from project to project because of the wide variety of exterior concrete surfaces with a broom finish, including:
Contractors can achieve many different specified broom finish characteristics, but the most important issue often affecting achievability is the size effect. With smaller concrete placements, more consistent brooming results can be expected. As the concrete placement size increases—from a 1.2 x 30.5-m (4 x 100-ft) sidewalk to a large-scale parking garage placement of 930 m2 (10,000 sf), for example—it becomes more difficult to produce a consistent appearance, flatness, texture, and drainage. What is possible to produce on a 1.2-m (4-ft) wide sidewalk surface is impossible for a large-scale parking garage surface.
Specifiers must recognize this inherent difficulty with different-sized concrete placements and owners must realize that while the larger parking garage placements cost less and shorten the schedule, they will not have the same appearance, flatness, texture, and drainage as a sidewalk. If the owner elects to place narrow strips for the parking garage slab to achieve a more consistent result, the trade-off is increased cost and a longer schedule.
The size effect
Specifiers sometimes create their own reference sample, usually about 0.3 to 0.6 m x 0.3 to 0.6 m (1 to 2 ft x 1 to 2 ft), to show the contractor the texture and appearance specified for the project. Although these sample panels exhibit a consistent texture and appearance, they give the contractor little beneficial information. The contractor does not know the concrete mixture proportions, setting time, or bleeding rate and duration. The finishing tools and procedures used to obtain the sample texture and appearance are also unknown. The sample’s small size makes it easy to obtain a consistent result.
However, the size of these samples makes them inappropriate for comparison with concrete surfaces produced on a jobsite, even including a 3 x 3-m (10 x 10-ft) mockup panel. A small sample panel should not be used in specifying a desired finish. If a reference sample is desired, one should choose a broom-finished surface matching the project’s scope of work. If it is a parking garage project, for instance, a satisfactory as-built broom-finished surface for another parking garage should be chosen as a sample. A sidewalk panel bounded by contraction joints, conversely, should not be selected to represent the expected broomed-surface finish for this garage.
Specifiers sometimes require a 3 x 3-m (10 x 10-ft) mockup demonstrating typical joints, surface finish, texture, tolerances, and standard of workmanship produced by the contractor’s methods. This is a better reference than a small sample panel, but is still not indicative of what can be achieved for a large-scale concrete placement in a parking garage. For instance, this mockup usually contains only one truckload of concrete and thus does not represent batch-to-batch variations in setting time or bleeding rates and duration that contractors must contend with. Additionally, the weather—temperature, wind, sun, relative humidity (RH), rain, or snow—will have little effect on a mockup as it is easy to protect a concrete placement of this size from the elements.
Most importantly, one broom pass can be used to texture the surface while the finisher stands on the perimeter of the mockup. The results of brooming in one pass while standing on the placement perimeter differ dramatically from those for large-scale concrete placements.
Unfortunately, the appearance, flatness, texture, and drainage characteristics of a well-built sidewalk create an unreasonable expectation that the same results can be achieved on large-scale concrete placements, but this is untrue. A sidewalk has nearly the same ease-of-placement benefits as a mockup. Further, it is easier to control a short-handled broom used on a 1.2-m (4-ft) wide sidewalk than it is to control a broom with a 6-m (20-ft) long handle used on large placements.
The main differences in providing a broom finish for a large-scale concrete placement as opposed to a sample, mockup, or sidewalk are:
The differences in the size effect are shown in the photos above, which compare the finishers brooming a 1.2-m wide sidewalk and a large-scale concrete placement for a parking garage.
Due to the size effect, specifiers, architects, and owners must have realistic, but different, expectations for sidewalks and large-scale concrete placements. While a sample panel and a mockup both provide an indication of the texture desired, these are not useful for evaluating how uniform or consistent the texture will be on a large-scale concrete placement.
Broom specification requirements
Typical broom specification clauses from MasterSpec and American Concrete Institute (ACI) specifications or guides are as follows:
MasterFormat Section 03 30 00–Cast-in-place Concrete:
Immediately after float finishing, slightly roughen trafficked surface by brooming with fiber-bristle broom perpendicular to main traffic route. Coordinate required final finish with the architect before application.
MasterFormat Section 32 13 13–Concrete Paving:
(A) Medium-to-fine-textured Broom Finish: Draw a soft-bristle broom across float-finished concrete, perpendicular to line of traffic, to provide a uniform, fine-line texture. (B) Medium-to-coarse-textured Broom Finish: Provide a coarse finish by striating float-finished concrete surface 1⁄16 to 1⁄8 inch deep with a stiff-bristled broom, perpendicular to line of traffic.
ACI 301-10, Specifications for Structural Concrete:
Immediately after concrete has received a floated finish, give the concrete surface a coarse transverse scored texture by drawing a broom or burlap belt across the surface.
ACI 330.1-03, Specification for Unreinforced Concrete Parking Lots:
Broom concrete surface with a steel or fiber broom to produce corrugations between 1⁄16 and 1⁄8 in. deep. Broom perpendicular to nearest edge of pavement. Broom all areas of a panel in the same direction. Use the same type and manufacture of broom for all paved surfaces to provide a consistent appearance.
ACI 362.1R-12, Guide for the Design and Construction of Durable Parking Structures:
Deep broom finishes are not recommended. A light to medium broomed or float swirl finish should be applied to driving and parking surfaces except where an alternate finish is required to install joint materials.
Avoiding escape clauses
Out of the five specification provisions cited, one requires a “uniform fine-line texture” and another requires a “consistent appearance” for a broomed surface. Yet, another clause requires the contractor to “co-ordinate” the final finish with the architect. Unfortunately, the words “uniform,” “consistent,” or even “co-ordinate” are often interpreted to mean “to the satisfaction of the architect.”
CSI, ACI, and Federal Highway Administration (FHWA) caution specifiers not to use the phrase “to the satisfaction of the architect.” FHWA states these are “escape clauses” that do not convey a measureable standard and should not be used in specifications. The terms “uniform” and “consistent” do not describe measurable standards, but are subject to interpretation by the specifier or architect, and thus could be considered escape clauses.
As mentioned, two of the five cited specification clauses require a 2 to 3-mm (1⁄16 to 1⁄8-in.) depth of the broom striations. Measuring the striation depth for a broomed surface is uncommon, and it is unclear as to how this would be done. In the absence of any data, the authors also cannot determine if the 2 to 3-mm range describes variations that can realistically be achieved on a large-scale concrete placement.
At first it might seem simple—a broom finish is a broom finish. However, the five specification clauses cited contained six different variations in describing the broom finish:
The inconsistency in defining a broom finish makes it difficult for the contractor to decide what is being specified and how to bid the specified broom finish.
Two differing finishing techniques used for exterior concrete surfaces include:
For pavements in cold climates, some specifiers prefer the bullfloat and broom technique because they believe using fewer finishing steps reduces the probability of altering the air-void system that helps prevent damage to the surface caused by freezing and thawing. Other specifiers believe a flatter surface can be obtained with the use of float pans. However, because the power floating leaves a grainy or granular floated surface, a fresno is sometimes used to smooth the surface before brooming.
A highway straightedge is sometimes considered useful on exterior concrete surfaces to improve the surface flatness and reduce any water ponding on the pavement after a rain. A straightedge, however, might affect the air-void system in the surface concrete and thus reduce the surface durability. Further, when slopes are set with wet screeds instead of hard forms, the scraping action of the straightedge can decrease the elevation of the high wet screed and increase the elevation of the low wet screed. This loss of elevation control means the slab will not drain as well. Thus, use of the straightedge to improve flatness may decrease freeze-thaw resistance and have a harmful affect on drainage. While a few specifiers require use of a straightedge for finishing, it is not specified for most exterior broomed surfaces.
Exterior surface flatness
There are two methods for specifying floor flatness for exterior concrete surfaces: the gap under a 3-m (10-ft) straightedge and the F-number system. The oldest method of measuring floor flatness is with a 3-m straightedge.This method measures only floor flatness and not floor levelness. It is generally a less satisfactory way of measuring interior floor flatness than the F-number system. Nevertheless, straightedge requirements are still found in some project specifications for exterior concrete surfaces. The F-number system measures both floor flatness and levelness.
When non-residential floor installations exceed 900 m2 (about 10,000 sf) in total project area, ACI 301, Specifications for Structural Concrete, requires the floor flatness and levelness to be measured using the F-number system. This specification, however, addresses only surface requirements for a trowel finish, so the need for using F-numbers based on floor size would appear to be for interior concrete only.
ACI 302.1R-04, Guide for Concrete Floor and Slab Construction, includes a more detailed discussion on the use of F-numbers and the straightedge measurements. This document also recommends appropriate F-numbers for typical floor uses, including a composite overall floor flatness of FF 20, and a composite overall floor levelness of FL 15 for parking structure slabs.
ACI 302.1R states the minimum local F-number values are set at 67 percent of the specified overall values; and thus the minimum local value for flatness is FF 13 and for levelness is FL 10. However,
ACI 302.1R also states levelness does not apply to sloped surfaces and, since most exterior concrete surfaces are sloped, the levelness requirement would not apply.
Flatness specification clauses
Flatness specification requirements from MasterSpec and ACI specifications and guides are summarized as follows.
MasterFormat Section 03 30 00–Cast-in-place Concrete
This requires a straightedge or F-number tolerance for trowel finish but no tolerances for scratch, float, or broom finish.
MasterFormat Section 32 13 13–Concrete Paving: Surface
This requires a gap below 3 m (10 ft) long, unleveled straightedge not to exceed 13 mm (1⁄2 in.).
ACI 117-10, Standard Specifications for Tolerances for Concrete Materials and Construction, Section 12, Exterior Pavements and Sidewalks
Surface tolerances specified only for ramps, sidewalks, and intersections. The commentary indicates conventional floor surface tolerances are appropriately applied to areas such as mechanical rooms, non-public areas, or surfaces under raised computer flooring or thick-set tile. Unlike ACI 302.1R, it does not mention parking garage structures.
ACI 301-10, Specifications for Structural Concrete
This requires flatness surface tolerances for scratch, float, and trowel finish, but not for broom finish.
ACI 302.1R-04, Guide to Concrete Floor and Slab Construction
This provides recommendations for trowel finish for different applications, but no tolerance recommendations for scratch, float, or broom finish.
ACI 330.1-03, Specification for Unreinforced Concrete Parking Lots
In any direction, the gap below a 3-m unleveled straightedge resting on high spots shall not exceed 13 mm.
ACI 362.1R-12, Guide for the Design and Construction of Durable Parking Structures
This does not recommend any surface tolerances.
Straightedge vs. F-number
The Commentary for ACI 117-10 indicates an FF of 20 defined as a “conventional” floor classification is approximately equal to a 6 to 15-mm (1⁄4 to 5⁄8-in.) gap under a 3-m (10-ft) straightedge. Conversely, the Commentary states a 13-mm (1⁄2-in.) gap under a 3-m straightedge would be equal to a floor flatness ranging from FF 17 to 28. The Commentary for ACI 117-90 had previously stated a “rough correlation” for a 13-mm gap under a 3-m unleveled straightedge would be an FF of 12.
F-numbers are used extensively for specifying flatness and levelness of interior floor slabs. It appears because of this, and limitations of the straightedge method, some specifiers use F-number tolerances for exterior concrete slabs. Currently, there are no industry recommendations for F-numbers on exterior broomed concrete surfaces. These authors have seen a specification requiring an overall FF/FL of 35/25 for a project. It should be noted that the levelness, FL, specification would be inappropriate if the surface was sloped. However, what is a reasonable FF requirement for a sloped and broomed surface?
Measured flatness on a bullfloat surface for buildings
The lowest FF for a broomed surface would be that resulting from a bullfloat and broom finishing technique. Released in 1989, “Measuring the Quality of Floor Finishes” by D.E. Stephan reported floor flatness measurements from over 4000 readings taken on 11 buildings representing bullfloated concrete surfaces. The average floor flatness for all the buildings was an FF of 20 with individual building measurements ranging from 15 to 25.
Brooming the bullfloated surface imparts a surface texture that reduces the floor flatness below that for just a bullfloated surface. Thus, Stephan’s data sets an upper limit on floor flatness that can be achieved with a bullfloat and broom surface.
Measured flatness on a broomed surface
for parking structures
Floor flatness measurements for broomed slab-on-ground and suspended slabs for two parking structures in Miami, Florida, and Quincy, Washington, are shown below:
Measured flatness on various broomed
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:
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.
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.
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
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.
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.
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).
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:
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.
A minimum design slope of 1½ percent in any direction, parallel to grid lines and diagonally, should be provided between the floor drain and any point on the floor slabs, except as indicated in 220.127.116.11 [pre-topped double tees] and 18.104.22.168 [corners]. The minimum slope as constructed should be 1 percent. Consider camber and deflections when establishing a drainage pattern. Drainage slope design should take into account dead and live load deflections. The minimum effective constructed drainage slope after volume changes and deflections have taken place should not be less than 1 percent.
The design drainage slope for suspended slabs should be chosen so the as-constructed slope can drain water even when allowing for construction tolerances and deflection. ACI recommends a two percent slope for drainage of suspended slabs in parking structures. Unfortunately, this advice is sometimes not considered when selecting slopes for suspended slabs such as balconies and plaza decks. Often, these slopes are chosen so shallow—1⁄16 in./ft or less—that water will not drain because construction tolerances and deflection offset the shallow slope. Even worse, the water may flow in the wrong direction—in the case of a balcony, toward the door. If a design slope less than 1.5 percent is chosen for a suspended slab, the owner should be cautioned that water drainage will be compromised.
Before choosing the final drainage design slope, the specifier should ask the structural engineer to evaluate the effect of deflection on drainage. This is especially important for post-tensioned cantilevered balconies where the post-tensioning forces often raise the unsupported balcony edge.
Based on many years of parking garage design experience, recommendations have been made with respect to drainage of parking structures and plaza decks. Their recommendations are:
ACI 302.1R-04 recommends a two percent (1⁄4 in./ft) slope, for positive drainage of exterior slabs.
A Texas Transportation Institute study showed the effect of rainfall intensity, pavement cross-slope, surface texture, and drainage length on the depth of water on the concrete surface. Nine test surfaces were placed on individual 8.7 x 1.2-m (28.5 x 4-ft) prestressed concrete double tees. A leveling course with the desired textures was placed on the surface of each double tee. The following variables were considered:
The rainfall was applied evenly on top of the concrete surface by nozzles calibrated for each rainfall intensity. During the test, five water depth measurements, spaced equidistant across the width of the surface, were taken at four locations (about 1.8-m intervals) along the surface’s drainage length. After reviewing the 500 water depth measurements for each test, the authors concluded the slope was the most important variable affecting the amount of water on the surface, with the effect of slope changes being more pronounced at flatter slopes. For example, increasing the slope from 0.5 to two percent (1⁄16 to 1⁄4 in./ft), decreased the amount of water on the surface by 62 percent. The authors also noted that any slope increases beyond two percent (1⁄4 in./ft) did not provide the commensurate drainage benefits.
Galloway’s test results support the American Concrete Institute and Chrest recommendations for a two percent (1⁄4 in./ft) slope for drainage of broomed surfaces.
|For more information on exterior concrete surfaces, see “Tips for specifying exterior concrete surfaces.”|
Frank P. Salzano, PE, is concrete frame manager/quality control manager for Ceco Concrete Construction. He has worked in concrete construction for more than 30 years in his own concrete construction business and for both self-perform general contractors as well as concrete contractors. Salzano has a B.S. in civil engineering from Virginia Tech and a MS. in construction management. He can be contacted at email@example.com.
Bruce A. Suprenant, PE, PhD, is the technical director for the ASCC and a Fellow of ACI. He has taught concrete materials, construction, and structures for 15 years in universities and has been a consultant in that field for 20 years. Suprenant received ACI’s Roger Corbetta Construction Award and has authored or coauthored more than 100 articles and papers, including one that received ACI’s Construction Award in 2011. He can be reached via e-mail at firstname.lastname@example.org.
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