Windows of Finishability for Steel Troweled Slabs - Dr. Kim Basham<br />Registered Professional Engineer

Understanding the windows of finishability can help avoid surface defects and costly call backs caused by premature or late finishing.

The window of finishability as defined by ACI 302.1R-04, “Guide for Concrete Floor and Slab Construction” is the time period available for finishing operations after the concrete has been placed, consolidated, and struck-off, and before final troweling (Ref. 1). The ACI 302 definition includes the finishing operations of bull floating, restraightening, waiting, power floating and troweling as shown in Figure 1. This window of finishability, established by the behavior of the freshly placed concrete, must be large enough but not too large so finishers have time to achieve the desired floor flatness and surface finish.

In addition to the ACI 302 window of finishability, there is a second or “true” window of finishability as shown in Figure 1 (Ref. 2). It starts when the bleedwater sheen has disappeared and the concrete has stiffened sufficiently to support the weight of the finishers. The ACI 302 and the true window of finishability both end before the final troweling or when the concrete has hardened so that restraightening or manipulation of the surface mortar is no longer possible. As with the ACI 302 window, the size of the true window of finishability is also established by the behavior of the concrete.

To avoid finishing defects on steel trowel slabs, finishers must understand and deal with both windows of finishability, especially the true window of finishability.

Premature Finishing

Typically, the terms “premature finishing” and “late finishing” are associated with the true window of finishability. Premature and late finishing occurs when the power floating starts too soon or too late. Starting too soon may cause surface defects including premature surface wear, crazing, dusting, blistering or delaminations. If power floating starts too late, the surface mortar may become too stiff for finishers to achieve the desired floor flatness or surface finish.

Establishing the proper time to start power floating is based on the depth of a finisher's footprint. However, many finishers do not judge concrete stiffening with the depth of a footprint because footprints are too hard to repair and may reduce floor flatness. Instead they use their fingers to press on the surface.

Do not start power floating until bleedwater has stopped rising to the surface, the bleedwater sheen has disappeared, and the concrete has stiffened sufficiently to support a finisher with no more than about a ¼ in. footprint indentation. Otherwise, bleedwater may be finished into the top surface or the surface may be prematurely sealed. As shown in Figure 1, waiting is an official finishing step.

Mixing bleedwater into the surface typically results in a weakened top surface prone to premature surface wear, crazing, and dusting. Sealing the surface prematurely traps rising bleedwater and air bubbles just below the surface and typically causes surface blisters and delaminations to occur.

Finishing bleedwater into the surface

Bleedwater is extra mix water that rises and collects on the surface of the concrete when the bleed rate of the concrete exceeds the surface evaporation rate. Finishing bleedwater into the surface typically increases the water to cementitious material (w/cm) ratio along the surface which decreases the surface strength of the concrete making the surface more prone to premature wear.

Figure 1. The true window of finishability starts after the waiting period or after the bleedwater sheen has disappeared and the concrete has stiffened sufficiently to support the weight of a finisher. Essentially, the true window of finishability is a smaller window within the ACI 302 window of finishability. (Modified from Ref. 2)

If surfaces have stiffened or hardened sufficiently for power floating but the bleedwater has not evaporated, drag a rubber or compressor hose across the surface or use a floor squeegee to remove the water. Also, do not add and finish water into the surface to facilitate finishing because the surface has dried. Finishing “added” water into the top surface has the same detrimental effects as finishing bleedwater into the surface.

Dusting is the development of a fine powdery material consisting of water, cement and fine particles that easily rubs off the top surface. Mixing bleedwater or “added” water into the surface dilutes the mortar phase of the concrete and creates a thin, weak layer of mortar called laitance along the top surface.

Crazing is the chicken-wire-like pattern of fine cracks that are barely visible and sometimes only visible when the concrete is drying after the surface has been wet. Since crack depths are very shallow, this form of surface cracking is primarily an aesthetic concern. Crazing seldom creates structural or serviceability issues, even for floors exposed to heavy forklift traffic. Crazing is caused by minor surface shrinkage related to rapid surface drying and/or wetting-and-drying cycles.

Trapping bleedwater and air

When mix water migrates upward because the cement and aggregate particles are settling, the surface must be “open” so as not to trap the rising bleedwater and air bubbles directly beneath the top surface (Photo 1 previous page). Trapping bleedwater and air creates a thin, weak zone directly beneath the surface and results in surface blisters or delaminations (Photos 2a-c: previous page and photo 3: above). Blisters and delaminations typically form during the onset of troweling. Blisters, typically about 1/8 in. deep, range in diameter from 1/4 to 4 in. Whereas, surface areas for delaminations are larger and can range from a few square inches up to several square feet or more and have depths varying from about 1/8 to 3/8 in.

PHOTO 1: As the aggregates and cement settle, water and air bubbles rise up through the concrete and escape through the surface.
PHOTOS 2A, B, C: If the surface is prematurely finished and sealed, rising water and air bubbles can become trapped below the surface and blisters can occur.

PHOTO 3: Trapping bleedwater and air bubbles directly below the surface can cause large areas of the surface to delaminate.
PHOTO 4: Sounding with a hammer or chain is the common method to identify blisters and delaminations.
PHOTO 5: Monitor surface evaporation rates. If necessary, protect surface from rapid and premature drying throughout the window of finishability
PHOTO 6: Crusting caused by premature drying of the top surface commonly causes a wavy and cracked surface that is prone to delamination.

The easiest way to identify surface blisters and delaminations is by sounding or dragging a heavy, steel chain across the surface or sounding with a hammer (Photo 4). As the chain crosses over surface blisters or delaminations, a dull pitch or hollow sound occurs indicating the top surface is not completely attached to the underlying concrete.

Premature sealing often occurs when the top surface stiffens due to surface drying or the top surface is setting faster than the underlying concrete. Top down setting often occurs when the ground temperatures are cool but the ambient conditions are warm and sunny. When this happens, flatwork finishers mistakenly believe the slab is ready to be power floated or troweled. However, the bleedwater and air bubbles are still rising. If the finishers seal the surface prematurely, then rising water and air bubbles become trapped beneath the surface.

To offset early surface drying and top down setting, plan ahead and be prepared for these situations. Use water foggers and evaporation retarders to avoid early surface drying. Offset top down setting by warming the base material or accelerating concrete setting with chemical admixtures. Be aware of these challenging conditions and do not seal the surface during bull floating or restraightening and wait until bleeding has ceased before power floating.

Protect Throughout Window of Finishability

To avoid surface defects associated with drying of the top surface such as crusting and delaminations, favorable moisture and temperature conditions must be maintained throughout the finishing process or the ACI 302 window of finishability.

Crusting occurs when the top surface becomes stiff due to excessive surface drying but the underlying concrete is still plastic (Photo 5). Slabs that crust are difficult to finish and typically result in a wavy, cracked surface and are prone to premature surface wear and delaminations (Photo 6).

To offset early surface drying, use water foggers and evaporation retarders. Do not use evaporation retarders as a finishing aid. That is, do not apply evaporation retarders to facilitate finishing because the surface has dried and become too stiff to finish. After mixing, evaporation retarders are typically nine parts water so it is not acceptable to finish or mix an evaporation retarder into the top surface of the concrete. Start final curing immediately after troweling or saw cutting.

Late Finishing

Waiting too long to start power floating and troweling may result in unacceptable floor flatness and surface finishes. Premature finishing can lead to costly surface defects. The finishing challenge is hitting the true window of finishability regardless of the changing and adverse placing conditions.

Be prepared to adjust the concrete mix design (e.g., chemical admixtures) and finishing operations including timing and concrete protection so the finishing operations falls within the windows of finishability. To overcome the finishing challenges and consistently produce high quality and defect free floors you must understand the windows of finishability, especially the true window of finishability.

References

  1. ACI 302.1R-04, “Guided for Concrete Floors and Slab Construction,” American Concrete Institute, Farmington Hills, Mich., 2004, www.concrete.org.
  2. Suprenant, B.A. and Malisch, W.R., “The true window of finishability,” Concrete Construction, Hanley Wood LLC, C9810859, 1998

About the Author:

Dr. Kim Basham
Registered Professional Engineer: Wyoming, Colorado, Arizona, Texas, Rhode Island, Utah, Washington, Massachusetts
Education: B.S. & M.S. in Structural Engineering from Virginia Polytechnic Institute & State University, Ph.D. in Civil Engineering from the University of Wyoming.

Kim D. Basham can be reached by phone at 307-635-7240
e-mail: kbasham@ConcreteES.com


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