According to ASTM C-309, the proper curing of concrete is “the control of moisture loss and temperature of freshly placed concrete.” Let's break this down into two parts: Moisture loss and temperature.
Most mix designs contain approximately twice as much water as needed to fully hydrate the cement in the mix. This excess water is often referred to as water of convenience. Water of convenience helps makes the concrete come down the chute and aids in its placement. After placement and finishing, this excess water has to go somewhere. If the water escapes too rapidly the cement particles do not have a chance to fully hydrate. Rapid drying causes strength loss, especially at the surface. Proper curing prevents this rapid loss.
Curing also relates to controlling the temperature of the freshly placed concrete. This means that we are trying to simulate keeping the concrete at approximately 70 degrees. This is done in several ways. Shading the concrete from a hot sun is usually impractical however, white pigmented, (Type 2), curing compounds reflect the suns rays and help keep concrete cool. Wet cure blankets are also used for this purpose. In cold temperature concrete work, the use of insulated blankets on the surface until the concrete reaches sufficient strength to resist freezing is often necessary.
Curing compounds were invented to reduce the cost of curing as well as increasing the flexibility of job site activities. Good quality curing compounds, properly applied, will allow moisture to slowly escape through a thin membrane thus ensuring effective curing. Cure & Seal products are made from acrylic or acrylic blended resins. These multi-function products cure the concrete as well as offer some long-term sealing protection from the elements.
Scaled exterior concrete is usually attributed to either no curing or improper curing; “Improper” meaning the wrong curing choice for the conditions of the job. In areas of high freeze / thaw weather, the concrete must be allowed to de-water prior being exposed to freezing weather. Usually, this means using a 3-day blanket cure after October 15th, followed by a period of air drying. Penetrating sealers such as Aquapel which protect the slab from de-icing salts can then be applied after a 14 day air drying period.
Controlling rapid moisture loss during the placement process is equally important. Frequent use of E-Con evaporation retardant will help retain valuable moisture during hot or cold weather, low humidity and/or windy placements.
Occasionally I hear, “I don't need to use a curing compound, I burn all of my floors really tight.” Frankly, I've heard this far too many times and it is simply not true. Tight troweling does reduce moisture loss but it also weakens the paste strength at the wear surface.
I also hear this one, “I only use a curing compound when it's below 70 degrees. If it's warm enough, you don't need one.” Huh? Are you joking? Hot weather is when you really need the water retention of a good curing method.
What to use and when to use it….
This guide will help you in choosing the right L&M Curing product for the job.
- L&M Cure “R”: You can never go wrong selecting this dissipating resin cure, inside or outdoors. After providing good curing protection over a period of time, it can be easily removed after the curing process allowing penetrating sealers such as Aquapel, Seal Hard, etc. to be applied.
- Cure W2: Highly recommended for exterior concrete pavements to control hot weather issues.
Curing and Sealing Compounds
- Dress & Seal does most of the heavy lifting for L&M. It is our most popular cure & seal product. It can be used both inside and outdoors.
- Dress & Seal 30 is a UV stable, high solids for that tough, high shine cure & seal.
- Lumiseal Plus is a 100% pure acrylic that is typically used on stamped, colored concrete to bring out the character of the concrete. Totally UV stable.
- Lumiseal FX is the most unique product we offer. It is a water based, pure acrylic cure & seal that can be used on its own or over previously applied sealers. It leaves a pleasing medium gloss finish or can be burnished to a deep shine. It will allow concrete to breathe more than other sealers and does not blush white when exposed to water.
All of the cure and seal products listed are available in both solvent based or water based formulas except Lumiseal FX.
Curing & Sealing…What's the difference?
Curing compounds, blankets or wet curing helps hydrate the cement particles in order to develop stronger paste. This is usually referred to as a 28-day process.
Cure & Seal products cure the concrete and leave a tough membrane of acrylic resins on the surface and some penetration into the pores of the concrete.
Sealing concrete from the elements and heavy usage presents quite different challenges. Let's break this down into two categories: indoors and outdoors.
Exterior concrete is often exposed to de-icing salts, acid rain, and all sorts of spills and stains. To truly protect it from weather damage, we recommend applying Aquapel to the surface after the curing process is complete. This is quite easily done using a simple garden type, hand pump sprayer.
For a quick lesson on how to apply the product and see the results, go to lmcc.com and click on “You Tube.” Click the Aquapel “how to” and watch this short video.
Indoors, we are faced with ugly tire marking, high abrasion and spills of all types. Membrane forming or topical products actually make the problems worse so we try to avoid their use and think more towards penetrating products.
Seal Hard, Chem Hard and Lion Hard are our three silicate based products. These liquid hardener products are typically called “densifiers.” They chemically react with the weak calcium hydroxide in the concrete, producing Calcium Silica Hydrate (CSH). Through this reaction, the pores and capillaries in the concrete are then “filled” so that the treated slab will resist fluid penetration as well as unsightly micro pitting (dusting) of the surface.
In applications where there are often oil spills or leaky vehicles, we offer a companion product to our densifiers called Petrotex. This, too, is a penetrating product. Treated concrete surfaces become oil phobic, greatly reduce oil staining of the concrete. It is typically spray applied over a surface that has been treated with Seal Hard, then leveled using a micro fiber applicator. On polished concrete, Petrotex can be burnished using a high-speed machine to enhance the shine of the floor.
The Unsolved Mysteries
I'm frequently asked “how soon can I put Seal Hard down on a floor?” I am wondering, is there a race out there to see how soon you can do something wrong? Why would you want to rush things and skip good curing practice? Wouldn't it make more sense to use the dissipating Cure “R” and build the building to substantial completion, then clean and then finally seal the floors? The more mature the concrete is, the more effective densifiers are. Don't rush it. Normal wait time for Seal Hard is 7 days after placement. Three days minimum is required in critical fast-track situations.
Some manufacturers falsly state that their silicates will “cure” the concrete. This is far from the truth and ASTM and ACI strictly prohibit stating that they do. Still others have “single step” type products where they blend silicates with resins and promote that they “cure, harden and densify” in one application. Miracles do happen, but not usually in concrete. Remember this advice: paper does not refuse ink. One can say or state anything on paper but that does not necessarily make it so. There is simply no good substitute for doing things correctly using proven methods.
We hope this helps clear up some of the mystery behind why and how we cure. Make sure to visit our website.
Bill Butler is a sales and tech rep for L&M Construction Chemicals. He has worked in the concrete industry since 1976 and has been involved with ready mix trucks, concrete admixtures and construction products for the concrete industry. His approach to helping contractors and installers “do things right the first time” or when necessary “doing things right the second time” is to ask good questions, be thorough, use common sense and logic.
© 2013 L&M Construction Chemicals, Inc. | ConcreteNews Fall 2013.