Hard Steel Troweled (Burnished) Concrete Finish and
Floor Covering Installation – June, 2021
George Donnelly Testing and Inspections
1 Curso Lane, Hot Springs Village, AR 71909-3723
All too often conflicts between Division 3 and Division 9 of project manuals result in arguments or change orders. The worst case scenario is of course floor covering system failures that could have been avoided. While there are a number of issues that need discussion, this paper is intended to help the reader understand the potential impact of burnished concrete finishes on the installation of adhered floor coverings.
Without doubt concrete finishers are proud of their work and smooth, glossy concrete is considered pleasing to the eye of finishers, Architects, Owners and General Contractors. Concrete that has been hard steel troweled or “burnished” is typically the result of finishing with ride on trowels that deliver floors meeting stringent ff and fl specifications required in Division 3 of project manuals. However, this finish may be problematic for floor covering contractors and ignores the recommendations found in the American Concrete Institute (ACI) publication ACI 302.1R-15 “Guide to Concrete Floor and Slab Construction”. Table 4.1 of the ACI guide (page 5) lists nine (9) classes of floors, the following notes are found for classes that may be anticipated in buildings that that receive adhered floor coverings along with other uses:
Covered Surface –Foot Traffic
Offices, Churches, Commercial, Institutional with Floor Coverings
Flat level slabs, suitably dry for applied floor coverings
Light steel-troweled finish
Exposed or Covered Surface – Foot Traffic
Topping over base slab for commercial or nonindustrial buildings where construction type or schedule dictates
Topping – for exposed surface, normal steel-troweled finish, for covered surface, light steel-troweled finish
Exposed or Covered Surface - Foot and light Vehicular Traffic
Institutional or Commercial
Level and flat slab suitable for applied coverings…
Normal steel-troweled finish
Exposed Surface – Industrial Vehicular Traffic – Soft or Soft Solid Wheels
Industrial floors for manufacturing, processing and warehousing
Hard steel-troweled finish
Based on the ACI guide chart there is cause to suggest that light steel-troweled finishing should be the specified finish for all slabs intended to host adhered floor coverings and foot traffic. Under “Special Considerations” there is an implication that light steel troweled finishes may help deliver “suitably dry” concrete as a reason to specify this finish.
Three (3) attributes of hard-steel troweled (burnished) finishes that potentially interfere with the installation of adhered floor coverings are:
1. Hard troweled finishes impact concrete drying. The dense surface created by hard troweling impedes drying requiring substantial additional time to achieve a suitably dry slab.
2. Hard troweled finishes are typically non-porous and may impact the ability of water-based floor covering adhesives to cure and dry.
3. Hard troweled finishes inhibit natural carbonation and maintain pH levels in excess of that which is acceptable for most floor covering adhesives.
Concrete Construction (CC) magazine published a feature article on August 5, 2010 entitled “Self-Curing Warehouse Floors?”. The article describes a study performed by Concrete Construction and Scurto Cement intended to monitor slab curling of a warehouse floor. The study included approximately 60,000 square feet of floor slab comprised of 10 concrete mix designs. The concrete was burnished “in a manner consistent with what most big box owners currently require”. However, the study revealed significant data related to the effect of burnishing concrete surfaces, including:
1. In-situ relative humidity was never less than 94% over the first year and a half of the study, regardless of mix design.
2. Hard-troweling the floor with ride-on trowels primarily affected the top 1/8” of the floor slab, effectively dewatering by compression. This process left the surface layer with an extremely low w/c/ratio.
3. Hard-troweled surfaces greatly reduce the size of capillaries and pores
4. Dense, hard-troweled surfaces carbonate very slowly
Concrete Experts International (CEI) published an essay focused on the carbonation of concrete in 2006. While the essay discusses their research in depth, the finding most related to burnished concrete is the finding that carbonation cannot occur when in-situ RH exceeds 90%. At in-situ RH of 90% or greater carbon dioxide cannot enter the concrete and carbonation is prohibited.
As reported in the CC study across 10 mix designs of concrete none were below 94% in-situ RH after a year and a half!
When the CEI data is applied to the CC findings it becomes clear that burnished concrete surfaces will remain high in pH as carbonation is inhibited or prohibited.
The findings of these studies indicate:
1. Burnished concrete surfaces trap moisture beneath the hardened surface cap, dramatically affecting the time required to achieve a slab suitably dry for the installation of adhered floor coverings.
2. Burnished concrete surfaces become exceedingly non-porous, inhibiting the absorption of water/moisture form floor covering adhesives.
3. Carbonated concrete surfaces exhibit pH ranging between pH 7.5 and pH 9. In lieu of natural carbonation, the slab surface pH will range between pH 10 and pH 14.
Based on the information contained herein along with case studies of floor covering system bond failures, it is the opinion of this agency that hard-steel troweled or burnished concrete finishing is not suitable for concrete slabs intended to host adhered floor coverings. It is the hopeful intent of this information to cause a change in the way concrete finishing is specified and performed. A light steel trowel finish is the maximum density that should be specified for any concrete intended to host adhered floor coverings.
If a project is intended to have both adhered floor coverings and open concrete surfaces, it is understood that a single finishing method would be most practical. Under this condition, if slabs have been hard-steel troweled, a line item cost should be allowed to grind or brush-blast the slab surfaces permitting drying and carbonation at the earliest possible time after curing.
As noted at the outset of this paper there are numerous conflicts between the requirements in Division 3 and Division 9 related to concrete slab floors. This paper is intended only to address the potential effect of hard steel trowel finishing. Concrete contractors typically have no responsibility for achieving dry concrete that has achieved a thin layer of carbonation. Floor covering contractors require suitably dry concrete that has achieved a pH suitable for the application of adhesives. Coordination of trades and thoughtful design specification could help prevent floor covering system bond failures related to burnished concrete surfaces.
ACI 302.1R-15, American Concrete Institute, 38800 Country Club Drive,
Farmington Hills, MI 48331
Concrete Construction – Self Curing Warehouse Floors? – August 2010 – PO Box 3494
Northbrook, IL 60065
Carbonation of Concrete – Concrete Experts International – Gonghusvej 242, DK-2950
Copyright 2017 GEORGE DONNELLY TESTING and INSPECTIONS