Homeowner’s Guide to Healthy Concrete

Concrete driveways and walkways can greatly enhance the appearance and value of a property. Healthy concrete, however, does not happen by accident. Thoughtful planning, a quality mix, professional placement and proper curing and maintenance can produce beautiful concrete that will last for years. The time to think about what you want from your concrete in terms of appearance, performance, and maintenance is NOW – before the concrete is placed.

You may hear many things about what to do with your newly placed concrete – some are good ideas, others are myths and misconceptions. this information is to help you, the homeowner, understand what is needed for healthy concrete and to separate these myths from reality. While the information presented will not make you a concrete engineer or contractor, it will help you to make informed choices when planning your concrete driveway and walkways. In particular, this information is designed to help you understand the curing and sealing processes as these are often the homeowner’s responsibility once the concrete has been placed and finished.

Concrete Construction Responsibilities

Concrete construction is a complex set of activities requiring professional skills and an extensive understanding of concrete. Usually several parties are involved – the general contractor, the home builder, the concrete contractor, the ready-mix producer and the homeowner. The technical aspects of installing concrete such as the planning, preparation, mix specification, placing and finishing are the responsibility of the home builder, ready-mix producer, and largely, the concrete contractor. Concrete curing should be done based on a joint decision between the homeowner and the concrete contractor. Ultimately, any ongoing maintenance is the homeowner’s choice and responsibility.

SEALING AND CURING CONCRETE BASICS

Concrete Concerns

Concrete is a blend of cement, mineral aggregates and other natural materials. Therefore, it may have some natural imperfections.

The most prevalent concerns of homeowners are scaling and uniform appearance. Because of concrete’s complexity, it takes a team effort to fight these potential problems.

Scaling

This is also called flaking. Most scaling can be prevented by:

The use of air entrained concrete
Your ready-mix producer and concrete contractor control the amount of air entrained in the concrete and the mix quality.

Proper finishing
The concrete contractor must be aware of the proper timing of the finishing operations, which can vary greatly depending on the weather. Finishing too early or over finishing can result in a weak concrete surface, susceptible to scaling.

Judicious use of deicing salts
Calcium or sodium chloride salts on their own will not chemically damage or etch your concrete, but the fact that they do allow the surface to stay saturated with water can damage your concrete. Deicing products and fertilizers made with ammonium nitrate or ammonium sulfate can chemically attack the concrete, causing severe damage.

Deicing salts are not recommended in the first year and over the long haul should always be used judiciously. Use plain sand instead.

Insufficient or no curing
Failure to properly cure your concrete often results in a weak surface skin which will be prone to scaling. Although proper curing should be done by your concrete contractor, it is absolutely necessary that you and your contractor work together on this because the curing method is determined by your plans for ongoing maintenance.

Uniform Appearance

It seems that most homeowners are looking for their concrete to turn white quickly and evenly. In reality, the longer our concrete keeps its darker color, the better it is retaining moisture. this will ultimately result in stronger, healthier concrete.

Your entire drive or walkway should even out in appearance over the first month after placement. Variations of dark and light areas during this time are normal.

After reading this post, you will discover that you need to start at the end – that is the end result you want – and plan back to the beginning of your concrete construction to produce healthy, durable concrete. For technical information on concrete mix, design and placement, refer to Ohio Ready Mixed Concrete Association’s Industry Recommendation for Exterior Residential Concrete Flatwork. If you have any additional questions regarding curing and sealing, we would welcome your call.

CURING

Curing Concrete – What does it mean?

When there’s an unsightly concrete driveway, everyone would like some way to ‘cure’ the problem, but that’s not what we mean by curing concrete. It is true, however, that had the problem concrete been properly cured, it may never have gotten sick.

Curing is the maintaining of a satisfactory moisture content and temperature in concrete for a sufficient period of time during its early stages so that its desired properties may develop.

Strength

The amount of water in the concrete while it is being placed is normally more than must be retained for curing. However, concrete that dries out too quickly may not retain enough water needed for the hardening process – a chemical reaction called hydration.

Temperature also greatly affects the hydration process. While hot weather can make the concrete harden and gain strength faster, it ultimately leads to a weaker concrete than one that has been kept cool (but not within freezing range) during its first few days.

Thus, the goal in curing is to keep the concrete cool and moist so that it gains its strength slowly, but efficiently. Laboratory tests show that moist cured concrete can be twice as strong as concrete cured in a dry environment.

Freeze-Thaw Resistance

Curing can also help concrete to be more durable, meaning resistant to damage caused by freezing and thawing. As long as the hydration process continues in concrete, the cement portion hardens and becomes more dense. If the concrete is properly cured, it will be less porous than uncured concrete thus making it more difficult for water and salts to penetrate.

Properly cured concrete is also more wear resistant and less susceptible to dusting and scaling.

Curing Concrete – How is it done?

There are numerous methods for curing concrete from covering with plastic sheeting or wet burlap to use of straw or ponded water.

Most Curing

Probably the best method for curing concrete is to flood the surface continuously with water for the first seven (7) days after placement. However, it is important that the concrete not be allowed to dry out. Often, contractors will recommend to the homeowner  to wet the new driveway for the first week after its completion. But, if the concrete is allowed to dry between soakings, this alternate wetting and drying may actually damage the concrete. So if you are going to water cure, plan on keeping the sprinkler going for at least a week.

Membrane Curing

The most common method of curing new concrete driveways is the use of a liquid membrane-forming compound normally called a curing compound or a ‘cure and seal’. These materials are usually sprayed or rolled on the concrete surface. Once dry, they form a thin film like varnish on wood which restricts the evaporation of moisture from the concrete.

The most important thing to remember regarding the use of a curing compound is timing. The application of these products should be done as soon as the final finishing operations are complete or as soon as their application won’t mar the concrete’s surface. So if someone says, “Let’s wait until tomorrow,” you will know it’s not a good idea.

The next most important thing is application rate. A light sprinkling or dusting on the surface will not do the trick. A sufficient coat according to the manufacturer’s recommended coverage rate is critical.

Curing with sealing in mind

When choosing the curing method and materials that will be used on your driveway, one important aspect to consider is how you intend to seal and maintain your concrete in the future. Sealing your concrete is addressed in the next section, but for now let’s look at how the curing method can affect your sealing decision.

The most common type of membrane cure used is referred to as a ‘cure and seal’. But let’s make one thing clear, this is not a one step process for permanent concrete sealing. It does, however, dictate the use of a film-forming sealer unless the ‘cure and seal’ is chemically removed or sand blasted away.

If you plan to use a penetrating sealer for ongoing maintenance, then your concrete should either be moist cured or cured with an easily removed concrete curing compound.

By making the sealing choice before the driveway is installed, you can then inform your contractor on the curing method that you would prefer.

SEALING

Sealing Concrete – Ongoing protection

Just as you paint the trim on your home or wax your car to keep them looking nice and protect their base structures 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 the 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 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 the 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 Sealer

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.

The ‘Wet Look’ vs the ‘Dry Look’

Wet Look – Film Formers

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

Dry Look – Penetrating Sealers

  • Advantages
    • should not change the concrete’s appearance
    • less frequent application needed
  • Disadvantages
    • usually more costly
    • not as good of a stain protector
    • cannot be applied over a film former

Film Formers – ‘Wet Look’

The film formers are usually made from acrylic or rubber based compounds. They form a thin coating on the surface of your concrete, leaving a wet look, much like varnish does on wood. These products generally tend to be less expensive on a per gallon basis than their penetrating counterparts, but you’ll probably find that they will need more frequent application, since they will weather and wear away more quickly.

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

The biggest problem that can develop with the 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 the 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.

One other potential problem is too much of a film build up on the surface may reduce the friction that keeps feet or tires from slipping.

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 the concrete, and once there, react with the alkaline materials and moisture present to form silicone. The silicone filled pores then make your concrete water repellent.

While the 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, is that, 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 in solvent. These are very effective concrete sealers. However, linseed oil is a very dark material and many people find the dark, blotchy result objectionable.