Getting the most from your Power Trowel

Finishing concrete has always been about timing: being in the right place at the right time with the right tool. Good power troweling techniques are essential since floor flatness depends directly on a finisher's ability to run trowel machines. The right tool and time The purposes of power floating are: to embed the large aggregate just be­neath the surface of mortar to remove slight imperfections, humps, or voids to compact the concrete and consol­idate mortar at the surface in prepara­tion for other finishing operations. As I mentioned, timing is every­thing in finishing. The rule of thumb when to power float a floor is that your footprint should be 1/4 inch deep or less, with little or no bleed water present. Most floors that result in low F-numbers are the direct result of fin­ishers getting on a floor too early with power trowels and creating lumps and bumps. Remember, this is the most plastic state that the floor will be in during a power floating sequence. Timing is everything — poor timing causes finishing problems. Also remember that any finishing operation done while there is excess moisture or bleed water on the surface can cause dusting or scaling.

		Choosing a walk-behind trowel When choosing a walk-behind power trowel, you should consider square footage/meterage, number of floor penetrations, concrete temperature, air temperature, wind, and relative humidity, along with mix design. Here are the recommended uses for various types of walk-behind trowels: Rotor edging trowel (24-inch diameter): used on edges; pan recommended Rotor edging trowel (30-inch diam­eter): used mainly on edges, patios, basements, and driveways; pan rec­ommended Rotor power trowel (36-inch diame­ter): used on small to medium pours, all types of floors; pan recommended Rotor power trowel (42-inch diame­ter): used on medium to large pours, all types of floors; pan recommended 2 Rotor power trowel (46-inch diam­eter): used on medium to large pours, all types of floors; pan not recommended
Blades and pans Once a machine is chosen, you must select carefully what type of blade or pan to use. Here are some tips to help with that decision: Clip-on float blades: Used on first pass over floor Used to embed aggregate Used to compact and consolidate con crete for other finishing operations Used for low FF values or when F- numbers are not specified
Combination blades: For residential use Good transition: floating to troweling s Good finish blades (It is best to finish with what you learned with, and this is generally a regional preference.)
Finish blades: Not recommended for floating Used for burnished finish Reversible
Float pans: Used on first pass Used to embed aggregate Used to compact and consolidate concrete Used on most floors with an FF spec­ification Will smear concrete flat while clip-on float shoes ride over humps Give more surface contact Using float pans with both walk- behind and ride-on trowels is now an accepted practice because float pans can improve floor flatness dra­matically by removing surface imperfec­tions and by improving the consolida­tion and compaction of the concrete sur­face. Float pans are used on the majori­ty of floors when a high spec floor is required.  When using float pans: Make a minimum of two passes Make each pass perpendicular to the previous pass The more pan passes, the flatter the floor Run a pattern—don't wander all over the floor After the last float pass on a floor, immediately begin the initial trow­eling sequence at 90 degrees to the pre­vious pass. For this transition—float­ing to troweling—combination blades are very helpful. Be sure to run the blades of the troweling machine flat or with a very slight pitch on the first pass (and at a slow speed). Each additional troweling pass increases the compaction of the fines at the surface and decreases the water­cementitious materials ratio of the con­crete near the slab surface. The trow­el blades agitate surface paste and hasten the evaporation rate of water within the paste. With each successive pass, raise the blades of the troweling machine to enable the finisher to apply suffi­cient pressure for proper finishing. Chatter marks on a floor are a result of the blades being raised too high, too early, or too fast for floor condi­tions.
For the best results, the high per­formance and hydraulic-driven ride-on trowels are the most powerful on the market and are used on most large commercial jobs. These riders produce good horsepower-to-weight ratios for pans and will provide superior finishes.											Choosing a ride-on trowel What size ride-on trowel should you choose? Use the following tips as a guide: Nonoverlapping (NOL) double 30­inch-diameter rotor ride-on trowel Good in very confined areas Covers small square footage Overlapping (OL)/NOL double 36­inch-diameter rotor ride-on trowel Perfect size for a first-time buyer Easiest machine to control Good horsepower-to-weight ratio Ideal for finishing small to medium sized floors NOL double 42-inch-diameter rotor ride-on trowel Good horsepower-to-weight ratio Can be used on small, medium, and large pours Can pan float and finish high FF numbers OL/NOL double 46/48-inch-diameter rotor ride-on trowel Medium to large pours OL machines can't be used with pan floats NOL double 58/60-inch-diameter rotor ride-on trowel Large placements Provides highest FF numbers Counter-rotational machine allows forward travel along edges and reduces windrow effect
Ride-on trowel tips Once the floating process begins with a ride-on trowel, look for the win­drow (the area between the two pans on a non-overlapping rider). Use the height of the windrow to determine when to start machining. Most finish­ers wait until the float pan creates a 1/4- inch to 1/2-inch windrow, therefore not displacing too much concrete. Signs of premature finishing would be: Windrow too high Machine splashing water Machine covered with concrete As with all concrete floors, outside temperature, wind, relative humidity, and other factors determine how many passes one should make with a ride-on trowel. Here are a few guidelines expe­rienced concrete finishers use: Make at least two passes—the more, the better Run the second pass perpendicular to the previous pass Run a diagonal on the third pass to help the flattening process Run a minimum of two riders with pans on larger jobs After panning with a ride-on trowel, move ahead with the finishing procedure. A slight color change in the concrete signals that the floor is ready for the finishing process. To start the finishing process (remember combo blades are good for this transition), run two passes in opposite directions to remove bug holes left by the pan machine. As the concrete dries and hardens, increase the blade pitch to provide greater pressure to the con­crete surface and to increase surface density. The final finish depends on the type of job and the specification. Always remember, entrained air is not recommended in concrete for slabs to be given a smooth, dense, hard-trow­eled finish because blistering and delamination may occur.
		Maintaining power trowels When the pour is complete, proper maintenance of equipment is important. Inspect and clean trowels daily. A part covered with concrete can't be inspected. If your daily inspection shows something that appears to be worn out, make sure a trained mechanic checks it before you start working with the machine. Not only can you damage the machine, but you can injure another person working around you if you run equipment that is damaged, and worn blades or pans, bent arms, and other conditions can have neg­ative effects on the final product. You can check trowel arms with a straight­edge or level. It is always advisable to have extra equipment available in case the equipment or the weather cause problems, or the concrete requires more equipment than anticipated. A maintenance checklist for ride- on power trowels (most of this also applies to walk-behind trowels) should include: Daily: Grease the trowel arms Check the air filter Check the oil level in the gearbox Apply some type of concrete release agent to keep the concrete from ad­hering Check belt tension and condition Visually inspect all parts of the ma­chine for any wear or loose nuts or bolts Every 20 hours: Grease the steering control linkage Every 50 hours: Check and adjust the valve clearance on the engine Check and adjust the drive belts Every 100 hours: Change the engine oil a Replace oil filters Oil the crosshead Every 300 hours: Replace the fuel filter Grease the trowel gearbox a Replace the spark plugs Handling and transporting the equipment also must be monitored. Often more damage occurs while mov­ing machines than while finishing a concrete floor. Purchasing a walk-behind or ride- on trowel is a major investment. If you depend on your equipment to run day in and day out, it must be maintained and serviced properly.
References Bob Simonelli is the precision flooring con­sultant for Allen Engineering Corp. Bob conducts training institutes for Allen Engineering, focusing on high-tolerance floor construction. His teaching method includes consulting and working "hands on" with crews to teach techniques. He has 20 years of field experience and is an ACI certified technician for flatwork finishing. Extracts taken from Contractors Guide to quality concrete floors.