Through my on-the-job training, Iíve experienced many finishing techniques and tools used by my bosses and relatives. Some of their methods Iíve adopted as my own and others Iíve hauled to the landfill of aobsolescence. Discovering what works and what doesnít has been a frustrating and expensive journey.
I realized interior concrete finishing is as different from exterior finishing, as snow skiing is different from water skiing. Durable interior concrete is abrasion resistant, and requires repeated trowel passes to produce a densified surface. Durable exterior concrete does not require intense finishing efforts. Why? Concrete air entrainment, moderate slump placement, and curing done by my crew are fundamentals of durable exterior concrete.
The primary difference between interior and exterior concrete is making efforts to densify interior concrete in late stages of finishing; and making no efforts to densify exterior concrete. I carefully train my crew to choose finishing tools and techniques, based on the environment where the concrete will serve.
Owners, builders and general contractors do not under-stand this distinction, and sometimes tell me, "Make the driveway and sidewalk just like those next door...dead flat with only the hint of a broom" This sets the stage for scaling and delamination (sheet scaling). I help people understand how "rough broomed" driveways and sidewalks are value added systems that provide traction for pedestrians, and minimize "slip and fall" lawsuits. I provide non-slip concrete surfaces by using a variety of specialty brooms.
Iím proud of my concrete tools and enjoy explaining their function. My magnesium strike-off bar (screed or straightedge) is one of my most important tools. I learned long ago, if my crew spends more time and effort leveling concrete surfaces during strike-off, we minimize birdbath puddles, and save time on finishing steps done later.
Do you know that anything done to improve and enhance exterior concrete surfaces has the potential to damage these surfaces? This includes work done with magnesium bullfloats. My crew routinely bullfloat a slab immediately after strikeoff, before excess water in the concrete begins to rise (bleed). Minimal bullfloating is all that is needed because magnesium tools efficiently bring cement paste to the surface. Excessive bullfloating may cause concrete surfaces to be sealed (closed or densified), which traps bleed water and sets the stage for surface problems.
My pitch-adjustment bullfloat is a good tool. However, if used with too much pitch (angle or tilt), it may seal concrete surfaces, which sets the stage for surface problems.
I am fortunate to have rediscovered wood concrete tools. Wood floats and bullfloats cannot accidentally seal concrete surfaces. In other words, wood tools keep concrete surfaces open. Keeping surfaces open is important during all stages of exterior finishing. Keeping surfaces open is also important during early stages of interior finishing.
I tell owners, builders and general contractors about the city sidewalk specification in Davenport Iowa. It reads; "After consolidation, the concrete surface shall be finished with a wood float to a uniform, fine, granular texture..." Although a wood float finish requires more labor than a broom finish, it appeals to concrete finishers who desire to hand trowel exterior concrete. I understand the City of Davenport has eliminated scaled city sidewalks by demanding wood float finishes.
The ACI 330-92, Guide for Design and Construction of Concrete Parking Lots, states in Section 4.5.2: "The use of hand or power floats and trowels is not necessary and is not recommended as their use may result in scaling." The 1995 edition of the Cement Masonís Guide, published by the PCA, contains the following statements, "Because of the danger of scaling or blistering, troweling of air-entrained exterior flatwork is not recommended," and "Donít hard trowel air-entrained concrete surfaces that will be exposed to freezing and thawing, deicing agents or both."
The use of a Fresno (walking steel trowel) on exterior concrete may set the stage for surface problems. As I understand, the Fresno was designed to be used after concrete bleeding is complete, and the concrete has almost achieved final set. The Fresnoís supposed to smooth the almost hardened concrete, so the finisher can produce Ďjust the hint of broom". The challenge is how to determine when bleeding is com-plete. Here is my rule of thumb, "Bleeding is com-plete when the surface is almost too hard to trowel."
I once had delamination occur on a small triangular area of a driveway. After much fact finding and debate, we determined that shade from a giant spruce tree caused concrete there to set slower and bleed longer. In those days, I used Fresno trowels. I believe my Fresno prematurely sealed the shaded area, trapped bleed water and caused delamination. I now prohibit the use of steel hand-trowels and steel Fresno trowels on my exterior jobs.
Some contractors use a Fresno to eliminate bullfloat lap marks, which mean they are using the tool when the concrete is still wet and still bleeding. Consequently, the concrete surface may be prematurely sealed, which sets the stage for surface problems. If you desire to reduce bullfloat lapmarks, consider using an eight foot bullfloat blade that has rounded ends. Also consider using bullfloats manufactured so the ends have a slight degree of upward bend.
Because concrete is a forgiving material, outdated techniques and unwise tool choices do not always result in concrete surface problems. It amazes me that there are so few problem driveways and sidewalks. I say this because I see many contractors using outdated, inappropriate, and un-wise finishing techniques. Some contractors have built their reputation on driveways that are "dead flat with only the hint of a broom". Their exterior work looks good, but their concrete is at risk if it was prematurely sealed, over-finished, or finished with steel tools.
At one point, I was reluctant to share the expensive lessons of my experience. If I had to learn the hard way, so should my competitors. I realize now that my on-the-job training has shortcomings. Seminars I skipped, concrete magazines I avoided, and supplier input I rejected, may have been unwise choices. Now I believe the occasional surface problems of my exterior work may have been caused by my crew using outdated, inappropriate or unwise finishing tools and techniques. This is my current attempt to share my wisdom with others, and I hope you will too.
This article was contributed by Edwin H. Gebauer, a cement sales representative for La-farge Corporation in Wisconsin and a member of the Wisconsin Ready Mixed Concrete AssociationĎs Technical Committee. In 1996, he was certified as a Concrete Flatwork Technician, sponsored by the American Concrete Institute (ACI) and the Associated Builders and Contractors (ABC) of Wisconsin.
Through his many years in the industry, first as a ready mix producer, then as a technical representative for the Davenport Cement Company, and now as a salesman with Lafarge Corp., the author has had countless conversations with contractors in several states. These conversations, combined with his exhaustive research and training, provide a framework of successful concrete flatwork finishing experience. Gebauer shares these guidelines as seen through the eyes of a fictitious successful contractor. This article could have been written by many of the successful concrete contractors you work with every day. We encourage you to make copies of this article and distribute it.