QUESTION: Please settle an argument that several finishers on my crew have about hand tools and finishing hard-troweled floors. Some of my finishers use a fresno instead of a magnesium float to smooth the surface, saying that a fresno is faster and better. Others claim the surface should first be floated with a magnesium float before troweling or using a fresno. What are the differences and purposes of a magnesium float, trowel, and fresno? Does it really make a difference how or when these tools are used?
ANSWER: A hand-held magnesium float, steel trowel, and fresno are different tools and should be used in the proper sequence when finishing a hard-troweled floor. Improper or out-of-sequence use can cause surface defects, including blistering and delaminations.
Wood and magnesium floats
While hand floats are primarily available in wood and magnesium, the most popular are made of magnesium. This is because magnesium floats are extremely durable and easily slide across the surface. Wood floats are rougher and typically limited to floating non-air-entrained and high-slump concretes. (Always use non-air-entrained concrete for troweled floors. Air-entrainment significantly increases the risk of trapping escaping air bubbles below the top surface, which leads to blisters and delaminations.)
Both float types are available with surfaces 3 to 4 inches wide and 12 to 20 inches long. The purpose of floating with an actual float is to embed the coarse aggregate particles, remove surface imperfections such as ridges from bull floating, create a smooth surface, and bring some mortar to the surface. Floating also cuts down high spots and fills holes as well as reestablishes the moisture in the surface mortar that was lost to evaporation.
Always float the surface before finishing with a trowel or fresno. Start floating after the bleedwater sheen has disappeared and a finisher using kneeboards has left no more than about a 1/8-inch indentation. Trapping bleedwater beneath the top surface, or finishing bleedwater into the surface, can cause delaminations or dusting.
While floating with either a wood or magnesium float, do not tilt the float. Keep the blade as flat as possible or in full contact with the surface of the concrete to avoid densifying and sealing the surface. To avoid blisters and delaminations, the surface must be kept “open” so bleedwater and entrapped air can escape. Increasing the blade angle will reduce the contact area between the blade and concrete, which increases the blade pressure. Too much pressure can seal the surface by compacting and densifying the mortar along the surface.
Troweling is done in the final stages of finishing and only after the surface has been floated. Troweling makes the surface hard and dense. Hand finishing trowels should be made of steel, usually 3 to 5 inches wide and 10 to 20 inches long. Use large sizes for the first troweling to spread the blade pressure over a large area, which minimizes the risk of prematurely sealing the surface. Keep the trowel blade as flat as possible.
As the surface becomes harder, perform subsequent trowelings with smaller trowels (in width and length). Also, progressively increase the tilt angle of the trowel blade to increase blade pressure. Increasing the blade pressure compacts and densifies the surface mortar to create a smooth, hard, and more wear-resistant surface.
A fresno is a large, long-handled trowel that looks similar to a bull float, except the blade is made of tempered or “blue” steel. Blades are typically 5 inches wide and 18 to 48 inches long with rounded corners, but square corners are available. Handles are attached to the blade with adjustable or swivel brackets. A fresno is a steel trowel and should be used just like a hand-held steel trowel. In fact, most manufacturers refer to a fresno as a fresno trowel.
Just like a steel trowel, only use a fresno after the surface has been floated with a wood or magnesium float. Otherwise, a fresno may seal the surface too soon, causing blistering or delaminations. Fresnos are useful when troweling slabs that do not require a hard, steel-troweled surface but where speed of troweling is important.
Kim Basham, PhD, PE, FACI, is president of KB Engineering. He specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. E-mail email@example.com; visit www.kbengllc.com.