Scaling…. Why & Concrete Sealer Prevention
Problem Clinic - on surface concrete
Concrete Construction Jan. 2003, PCA, ACI & ICRI reference

Scaling means breaking off of surface concrete in small flakes because the concrete was not resistant to freezing and thawing while more than 91% saturated with water. Since presumably the action doesn't involve aggregate, which it can sometimes in the case of dolomite coarse aggregate { Calcium Magnesium Carbonate } , the quality of the aggregate is not addressed here. Hence there are only two criteria that must be met in order for concrete not to scale under this condition: 1) An air bubble not more than 0.2 mm from anywhere in the paste, and 2) a degree of maturity such that enough of the mixing-water-filled space has been filled with hydration products so that the air-void system can handle the 9% increase in volume when the freezable water becomes ice. This will be true if the strength reaches 3500 or 4000 psi.
The 0.2 mm bubble spacing factor (I) will be met with 9% air produced by proper dosage of an air-entraining admixture meeting the ASTM specification for AEA, which is about 7% air in most concrete. If you get the 7% air content in the concrete as it leaves the mixer, it is possible to cause a lot of it to leave the concrete without causing loss of frost resistance if the concrete is reasonably good to start with. If the mixture is not oversanded one can vibrate it for a long time, drive out a lot of air and still have frost resistance. Several investigators have vibrated 7% air content concrete down to 3 or 4% and I (the spacing factor) only went up from 0.005 to 0.007 inches (0.008 in. = 0.2 mm, so this is still within the acceptable range). What happens is simply the effect of buoyancy--the bigger bubbles that make up 2% or so of the air that is entrapped (not entrained) are expelled with little damage to the total number of bubbles and all the little bubbles are left. The problem is, if you should do this, most current produced mixtures would segregate because the volume of coarse aggregate is too small and it sinks leaving a thick layer of mortar with extra bleed water in it on the top of the slab.

Often what is done to the surface as part of the finishing operation is done before the cement hydration is complete and gravity is still pulling the coarse aggregate down and therefore the concrete is still bleeding and the bleed water ends up being reworked into the surface concrete (mortar). The paste at the surface then becomes thinner, and buoyancy allows more air bubbles to escape--if this goes too far the surface mortar will not only have lost more air bubbles than it can afford to lose to still be frost-resistant but also the extra water will raise the w/cm so this material will take longer to reach 3500 to 4000 psi when it can meet the second criteria noted above (enough maturity to handle the increase in volume when the freezable water becomes ice). Any process that makes the surface material wetter than intended can cause loss of frost resistance either by loss of air bubbles or increase in w/cm or both. This is where a good concrete sealer can bridge the gap.

The real question is whether or not it is cost effective to try to match the specific levels of concrete properties to the environment in which the concrete is to serve. All concretes have some capillary cavities in which freezable water can be present. Any cavity not 91% full will not cause trouble because water turning to ice only increases in volume 9%. Furthermore, as the size of the cavity gets smaller, the freezing point of the water gets lower--also as material dissolves into the water (the pure fluid) the freezing point gets lower, and hence it will take lower temperatures to cause damage. Then there is the degree to which the concrete can accommodate the 9% volume change. If there's only a few mL of pore fluid per unit volume of concrete then possibly the concrete is ductile and resilient enough (has a low enough modulus of elasticity) to expand enough without cracking to accommodate that much increase in volume. And so, for some concretes in some not very severe environments 3% air content may be enough to provide frost resistance. Needless to say that concrete placed correctly should be reasonably frost freeze-thaw resistant, follow that with a performance driven concrete sealer like a siliconate chemically reactive concrete sealer and it will greatly enhance the overall concrete quality and durabilty.
In theory, concrete with a water-cement ratio less than 0.4 that is continuously kept moist will have enough cement to fill all the capillary cavities and therefore won't have freezable water--but that might take an infinite amount of time.
And so the story could go on and on.

For more information and asistance go to our Surface Defects page, or call

V-SEAL / TARA Group at numbers below.