by Eugene Goeb, P.E.
The fact that we are troubled by the same kinds of surface problems year after year isn’t due to the failure of concrete as a material. It’s much more likely due to lack of dicipline in applying commonly held knowledge of concrete mixtures, and placing and finishing practices. Deicer scaling, peeling, checking (crazing), dusting and lack of resistance to abrasion are the result.
Control of Air Entrainment:
Prevention of deicer scaling requires discipline to provide a proper testing program to verify air content in the mix; and to adjust the dosage rate of an air entraining admixture as changing conditions dictate. Air content is reduced when: mix temperature increases; slump is reduced; sand content is reduced or the gradation changes from fine to coarse; cement content is increased or fly ash is introduced as part of the cementitious material; carbon content in the fly ash increases. Frequent air tests will determine the need for adjustment.
Excess Water on the Surface During Finishing:
Wet (high slump) mixes are especially vulnerable to surface problems. For smaller jobs such as sidewalks, driveways or parking areas, a good average would be about a 4-to 5 in. slump. For floors it could be less. If vibration is used, it should be less than 4 in. If placing conditions demand a high slump, a superplasticizer can used rather than adding water to achieve the required slump.
Wet mixex will bleed readily with minimum manipulation, and sometimes with no manipulation at all. When an excessively wet surface is troweled, the opportunity for scaling, peeling, crazing or dusting is created. If we believe that to be true (and it is!) that should be enough to cause everyone connected with a job to exercise the dicipline needed to avoid facing that kind of situation.
In addition to limiting the slump, excess bleeding can be reduced by increasing fines in the mix with an increase in sand proportion, addition of cement, or introducing fly ash which is finer than cement and may reduce the total water required. Add an air entraining mixutre to obtain about 3 percent or 4 percent (in concrete where air entrainment is not specified for freeze-thaw, deicer exposure). Air entrainment reduces bleeding, net water requirement, and segregation.
In the early days of air entrainment, many cement finishers fought air entrained concrete. That’s still true but to a lesser degree today. The common complaint was that the surface was too “sticky”, making it difficult to float and trowel. That was because air entrainment does make it more difficult to work water and entrapped air to the surface. During floating of high-slump mixes, surface concrete adhered to the wood float. During troweling, because water couldn’t be worked to the surface as easily as before, the complaint was that there was “no paste to work with.” Both problems were greatly reduced with stiffer mixes, improved control of air content, and learning how to work with air entrainment.
One unfortunate habit that has developed out of that problem is the practice of sprinkling water on the surface just ahead of, and during, both hand and power finishing. Depending on the timing of finishing, the result can be a thin cement paste covering a hardened slab with very little bond. When the unbonded cement paste surface begins to peel of the slab (not to be confused with deicer scaling) it will look just like cement paste was trowel on to a hardened, flat finish. It can be mistaken for deicer scaling, at least until you see it on an indoor floor slab. In contrast, deicer scaling penetrates deeper with an unmistakable look of deterioration.
Surface checking (or crazing) is another product of prematurely troweling high slump concrete, made worse when combined with lack of curing. Troweling the wet surface leaves a high shrinkage paste at the surface that develops a network of fine, superficial cracks when allowed to dry without curing. Fortunately, the problem is usually only cosmetic, without making the concrete less serviceable.
The dusting floors that can’t be blamed on invented heaters in cold weather construction are most likely caused by a combination of wet concrete, excessive bleeding, premature troweling and lack of curing. The same kind of explanation would relate to a floor surface that was not resistant to abrasion.
Recommended Alternative to Troweled Finishes:
There are few outdoor surfaces that really require a trowed finish. For sidewalks, driveways, patios and parking lots, there are advantages in having a touble-free, non-skid, durable surface created with a broom or burlap drag. The contractor can recommend it just that way and sell it as a superior product. Troweling doesn’t improve the performance of outdoor flatwork, but it can be a part of a problem. While using other disciplined construction practices, substituting the broom or burlap drag finish for the trowel just might help eliminate some of these nagging problems that seem always to be with us.