Technical Bullentins & Memos

Finishing

The characteristics of the concrete surface alone determine the serviceability of floor slabs.  Without special precautions, the top surface of any concrete can easily be of lower quality than the remainder.  For instance, if bleed water is worked into the surface of a floor, the water/cement ratio of the surface concrete is increased, with resulting decreases in strength, durability, abrasion resistance, and other desirable properties.  Proper finishing of a floor slab therefore requires much more time and effort than does most other concrete.  Consequently, the producers of floors must be aware of all the details of the work and of the necessity for good craftsmanship.

Spreading and Compacting-Concrete from a chute, or conveyor must be delivered without segregation of course and fine aggregates.  Spreading, the first operation in producing a plane surface, should be performed with a short-handled shovel or a come-along.  Initial compaction of floors is usually accomplished in the first operations of spreading, vibrating, screeding, and darbying or bullfloating.

Screeding-Screeding is the act of striking off the surface of the concrete to a predetermined grade.  This must be done immediately after placement.  Of all the floor-placing and finishing operations, form setting and screeding have the greatest effect on floor levelness.  Vibrating screeds should be moved forward as rapidly as proper consolidation allows.  If not used in this manner, too much mortar will be brought to the surface.

Screeding must be completed before any excess moisture or bleeding water is present on the surface.

Bull Floating-if done, should immediately follow screeding and must be completed before any excess moisture or bleeding water is present on the surface.  Any finishing operation performed while there is excess moisture or bleeding water on the surface will cause dusting or scaling.  This is the basic rule of concrete finishing and cannot be overemphasized.  One of the bullfloat’s purposes is to eliminate the ridges and fill the   voids left by the straightedge.  In addition it should fill all surface voids and only slightly embed the course aggregate.  This prepares the surface for subsequent edging, jointing, floating, and troweling. 

Waiting-A slight stiffening of the concrete is necessary before proceeding further.  Depending on the job conditions, it is usually necessary to wait for this to occur.

Edging-is not required on most floors.  The purpose of the edger is to form a radius at the edge of the slab, to produce a neater looking edge that is less vulnerable to chipping.

Jointing-the slab is jointed immediately following edging, or at the same time.

Floating-after edging and hand jointing operations, the slab should be floated.  The purpose of floating is threefold: (1) to embed the large aggregates just beneath the surface of a mortar composed of the cement and fine aggregates from the concrete; (2) to remove slight imperfections, humps, and voids, and to produce a level or plane surface; and (3)  to compact the concrete and consolidate mortar at the surface in preparation for other finishing operations.

Many variables-concrete temperature, air temperature, relative humidity, and wind-make it difficult to set a definite time to begin floating.  The concrete is generally ready for floating when the water sheen has disappeared, or has been removed, and the concrete will support a finisher and knee boards without more than approximately ¼ inch indentation. 

Normally, concrete will be ready for floating in the order in which it was placed.  On a given placement, however, certain areas will become ready for floating before the general overall areas.  These areas must be floated first.  They include areas adjacent to screeds, forms, blockouts, walls, columns, doorways (from wind), or specific areas that may be exposed to sun as compared to the larger areas in the shade or under cover, and those exposed to wind as compared to other areas that are not. 

Troweling-the purpose of troweling is to produce a smooth, hard surface.  Troweling is done immediately following floating; no troweling should ever be done on a surface that has not been floated by power or by hand. 

If troweling is done by hand, it is customary for the concrete finisher to float and then steel trowel an area before moving his kneeboards. 

For first troweling, the trowel blade must be kept as flat against the surface as possible.  If the trowel blade is tilted or pitched at too great an angle, an objectionable “washboard” or “chatter” surface will result.  For first troweling, a new trowel or one less than 4 ¾  inch wide is not recommended.  An older trowel which has been “broken in” can be worked quite flat without the edges digging into the concrete.  The smoothness of the surface can be improved by timely additional trowelings.  There should be a time lapse between successive trowelings to permit concrete to become harder.  As the surface stiffens, each successive troweling should be made with smaller trowels tipped progressively more to enable the concrete finisher to use sufficient pressure for proper finishing.  Additional troweling increases the compaction of fines at the surface, giving greater density and more wear resistance.

During periods of hot, dry, windy weather it is recommended that troweling be kept to the minimum necessary to obtain the desired finish.  When ambient conditions create a high water loss from slab evaporation, any delay in protecting the slab with curing compounds or other water-retaining materials can result in plastic shrinkage cracking, increased adverse carbonation, low surface strength, dusting, and early deterioration.