Power Trowelling Hints
For final finishing of concrete floors, power trowelling gives by far the best results for strength, durability and flatness. Using a power trowel, concrete can be finished off to a very hard and durable surface.
Remember that a power trowel will not level a concrete floor. A power trowel is a finishing machine designed to put a hard skin onto the surface of the concrete which is smooth and reasonably dust free.
Small holes and slight high spots can be levelled off by the action of the trowel, but a floor should generally be laid, levelled and compacted before a trowel is used. Double beam screeds, straight edges, and hand troweling of edges and main joints should be done prior to power trowelling.
Selecting power trowels
Power trowels are almost always either petrol or electrically driven; the choice depends on the power supply available and the area in which the machine is being used. (Trowels are designed for one or the other; fitting a petrol drive to an electric trowel, or vice-versa, will give poor results).
Drive units vary between about 5hp and 25hp. Usually 5-10hp are used for walk-behind trowels, and 10-25hp for the larger ride-on trowels.
Trowels are available in various sizes. The most popular sizes are:
|600mm or less||final screeds and refurbishment work; for working through doorways in finished buildings|
|800-1200mm||most common applications such as factories, warehouses, airport hangars, etc|
|900-1200mm double head||larger floors areas where high quality finish is required|
The most important feature in a power trowel is its ability to run smoothly and be well balanced. Make sure there is a simple method of ensuring that the blades are level, and a quick method of adjusting them if they do become out of line. A trowel with non-aligned blades becomes unstable, and will make it impossible for the operator to achieve a good finish which meets specifications.
Safety standards are becoming more and more strict, so ensure trowels have adequate guards. Triple guard rings, engine cutout switches, and electrical protection are to be recommended.
Trowels can be fitted with various types of blades or discs:
|float blades||used first for floating|
|float disc||alternative to float blades|
|finish blades||used for two or more passes for final finish|
|combination blades||one side used for floating, the other side for finishing|
Most commonly, float blades or a disc are used followed by finish blades.
Combination blades are normally only used on small contracts – using two types of blade usually ends up cheaper.
It is essential to trowel at the correct time. The operator must wait until the soft concrete hardens sufficiently to take the weight of the machine and in most cases the operator as well.
If the concrete is too soft the machine will tear up the surface; if it is too hard, the machine will have little effect on the surface.
A useful guideline is when the operator can stand on the concrete surface and leave footprints of about 3mm depth.
Floating: the first pass is called the ‘floating’ pass. The trowel is fitted with either float blades or floating disc. Trowelling starts at the top of the slab and moves from one side of the concrete bay to the other, with the operator walking backwards (the trowel will take out the operators footprints). The bay should be given one initial pass and the speed of travel should be slow and uniform.
For the floating pass, the trowel blades should be almost flat to create suction between float blade and concrete surface. After the first pass there will be circular marks left on the surface. These should not be deep marks, and will be removed on the second pass. The trowel should pass over the form work to ensure level joints.
Finishing: Once the surface has been ‘floated’, the blades of the trowel are changed to finish blades. The finish pass should be started when the surface has dried off, and walking on the surface leaves no appreciable indentations. A good guideline is when the operator can place his hand on the concrete surface and no sand/cement particles are left on his hand.
For the finishing pass the blades are angled to suit the concrete, increasing the angle after each pass – an angle of around 5-10º is usual, the steeper the angle the harder the finish. Several passes with the finish blades may be required to achieve the specified finish.
Care must be taken at all stages of trowelling to ensure that all areas of the concrete are covered by the trowel. In most cases hand trowelling of the edges, joints, around columns and at walls should be done prior to floating. This makes sure that no large high spots are evident (power trowels will not take off high points).
Where levels are critical, hand levelling beams are often passed over the surface to ensure that there are no high spots. This hand level (usually 3-5 metres wide) will also remove surplus material after the floating pass, making the final finishing pass much easier to do.
Weather: A warm dry day with slight wind gives the best curing conditions. A cold damp day is the worst.
There is no substitute for good quality concrete, care in preparing the formwork, correct compaction, initial levelling and care and patience in the final finishing of all the surface area.
Using the correct tools, an extremely good quality product can be achieved.