Just as you paint the rim on your home or was your car to keep them looking nice and protect their base structure from detrimental elements, you should seal your concrete regularly to protect it from moisture penetration and prolong its life too.  Although it seems ironic, it is true that when concrete is first placed, we want to keep the moisture in; once it has matured, we want to keep moisture out.

This is especially true for concrete that will be subjected to freezing and thawing.  The single most important factor in making concrete more resistant to freeze-thaw cycles is air entrainment.  Air entrainment is the purposeful addition of tiny air bubbles in the concrete during batching. After the concrete hardens, the air bubbles are still in the concrete. These air bubbles very simply allow room for the expansion of freezing water that may be in the hardened concrete.

Imagine blowing up a balloon.  Once you have filled the balloon to its capacity, it will burst if you force more air into it, leaving you with a broken balloon. Concrete's air entrainment system works in much the same way.  The microscopic air bubbles or voids in the hardened concrete will provide room for expansion of some water in the concrete as it turns to ice during freezing.  However, if there is more water in the hardened concrete than the air bubbles can accommodate, the water's expansion during freezing will cause damage to the saturated concrete.  Thus an adequate air void system is necessary.

If the surface of hardened concrete is wetted for a long period of time, some of the water will be absorbed into the concrete surface causing it to become saturated.  While salt itself is not harmful to the concrete's surface, salt's melting of ice and snow keeps the surface saturated with water. This allows excess water into the concrete.  When this saturated concrete freezes, the air entrainment system is overloaded causing the walls of air pockets to rupture, and damaging the concrete.

Although the air bubbles may be enough, you can add some additional insurance by limiting the amount of water that can get into your concrete.  You can do this by maintaining your concrete with a sealer designed to keep out water and deicing salts.

Choosing a product to seal your concrete can seem like a complicated process, but let's try to cut through it all to some simple choices.  There are really only two types of concrete sealers - those that form a film on the surface of your concrete, giving it a wet look, and those that are designed to penetrate the concrete leaving it dry looking, yet water repellent.  Like any choices, each has its advantages and disadvantages.

Advantages

• tend to be less costly
• better stain protection (i.e., oil, grease, etc.)
• usually compatible with curing method used glossy to medium gloss look

• should not change the concrete's appearance
• less frequent application needed

Disadvantages

• can darken the concrete
• may appear blotchy if not evenly applied
• will wear away, requiring more frequent applications
• may create a slippery surface

• usually more costly
• not as good of a stain protector
• cannot be applied over a film former

One significant advantage of the film formers is that there is usually not a compatibility concern with the method of curing used or whatever pervious sealer might have been applied.

The biggest problem that can develop with film formers is that they tend to darken the color of your concrete.  Just like varnish will darken or enrich the color of wood, these will do the same to concrete.  And just like it may take several coats of varnish to provide an even, rich color, don't expect the film forming concrete sealer to perform differently.  If after one coat you get some dark areas and some light, you may want to apply another coat to make it evenly dark.  These initial variations in color may be caused by natural variances in the porosity of the concrete and/or uneven application, but they are quite normal.

Penetrating Sealers - 'Dry Look'

Most penetrating sealers are made from derivatives of silicone called silanes or siloxanes. These materials are designed to penetrate into the pores of concrete, and once there, react with alkaline materials and moisture present to form silicone.  The silicone filled pores then make your concrete water repellent.

While silane and siloxane penetrating sealers are usually more expensive than the film formers, they should last longer too.  Another reason that the penetrating sealers are gaining in popularity in spite of their price, that is, when properly applied, they don't change the appearance of the concrete. Their biggest disadvantage, or at least the major concern in their application, is that there can be no other membrane cure or sealer on the concrete when applying and the concrete must be at least 28 days old.

A note on linseed oil sealers

Other commonly sold sealers for concrete are based on linseed oil and solvent.  They are very effective concrete sealers.  However, linseed oil is a very dark material and many people find the dark, blotchy result objectionable.