Trowling
If a dense, smooth finish is desired, floating must be followed by steel troweling, as shown in Figure 74. Troweling should begin after the moisture film or sheen disappears from the floated surface and when the concrete has hardened enough to prevent fine material and water from being worked to the surface. Delay this step as long as possible. Troweling too early tends to produce crazing and lack of durability. Too long a delay in troweling, however, results in a surface too hard to finish properly.
The usual tendency is to start to trowel too soon. Troweling should leave the surface smooth, even, and free of marks and ripples. Spreading dry cement on a wet surface to take up excess water is never a good practice where a wear resistant and durable surface is required. Avoid wet spots if possible. When they do occur, do not resume finishing operations until the water has been absorbed, has evaporated, or has been mopped up.
Steel Trowel
An unslippery, fine textured surface can be obtained by troweling lightly over the surface with a circular motion immediately after the first regular troweling. In this process, keep the trowel flat on the surface of the concrete. Where a hard steel troweled finish is required, follow the first regular troweling by a second troweling. The second troweling should begin after the concrete has become hard enough so that no mortar adheres to the trowel, and a ringing sound is produced as the trowel passes over the surface. During this final troweling, the trowel should be tilted slightly and heavy pressure exerted to thoroughly compact the surface. Hairline cracks are usually due to a concentration of water and extremely fine aggregates at the surface. This results from overworking the concrete during finishing operations. Such cracking is aggravated by drying and cooling too rapidly. Cracks that develop before troweling can usually be closed by pounding the concrete with a hand float.
Mechanical Troweling
Machine The mechanical troweling machine shown in Figure 76 is used to good advantage on flat slabs with a stiff consistency. Mechanical trowels come with a set of float blades that slip over the steel blades. With these blades, you can float a slab with the mechanical trowels. The concrete must be set enough to support the weight of the machine and the operator. Machine finishing is faster than hand finishing, but it cannot be used with all types of construction. Refer to the manufacturer’s manual for operation and maintenance of the machine you are using.