ASTM C 309 is the most common concrete curing specification in use today because it sets the amount of moisture loss that comes out of concrete during the curing process. According to ASTM.org, ASTM C 309 states that the curing compound must form a membrane that has moisture retention properties equaling 0.55 Kg/M2 in 72 hours when applied at 200 square feet per gallon.

 

ASTM C 309 It is broken down to Types and Class. Types are referring to the physical appearance of the curing compound while the Class refers to the type of solids.

 

Physical Appearance

 

The physical appearance of the curing compound is described by three types:

 

Type 1 – The curing compound is clear or translucent without dye

 

Type 1-D – The curing compound is clear or translucent with fugitive dye

 

Type 2 – The curing compound is pigmented white

 

Solids

 

The type of solids in the curing compound is characterized into one of two classes:

 

Class A – No restrictions on the type of solids

 

Class B – Solids must be a resin (acrylic, acrylic blend, epoxy, and all other blends (wax based solids excluded)). If a curing compound is said to be Class B, it is also considered to be a Class A.

 

Why ASTM C 309 Is Important

 

As mentioned above, ASTM C 309 sets a standard for the amount of moisture loss that comes out of concrete. In order for concrete to achieve a desired level of strength, the moisture content in the concrete during the curing process needs to be kept consistent which is where a cure and seal comes into play. If too much moisture escapes, the concrete will be weak and brittle. If too much moisture is trapped within the mixture, it could take the concrete an extended period of time to cure causing more issues.  When a cure and seal that meats ASTM C 309 standards is applied to the concrete it will help to retain the proper level of moisture needed so that the concrete can achieved desired strength.

 

When concrete is able to properly cure, it does more than just increase strength. When moisture is kept in the concrete by a cure and seal it allows for the concrete to continue reacting before the water evaporates. The more time the concrete has to react, the more time it has to fill in the voids. Fewer voids result in decreased permeability which leads to better freeze-thaw resistance and over abrasion resistance on the surface. 

 

ASTM C 309 Sealers and How to Choose the Best One

 

The most common type of cure and seal that meets ASTM C 309 standards in an acrylic cure and seal. They are popular for a two reasons:

 

  1. They help concrete to properly cure and reduce premature moisture loss in concrete for the first 28 days.
  2. They leave behind a protective film on the surface that reduces deterioration caused by surface abrasion, foot traffic, and spills.

 

The three sealers that best meet this are the Armor LV25, the Armor WB25, and the Armor AX25 (to purchase products, visit FoundationArmor.com). The LV25 is a solvent based acrylic, the WB25 is a water based acrylic, and the AX25 is a Siloxane water repellent infused solvent based acrylic. All three meet ASTM C 309 specifications and will protect the surface of the concrete while helping them cure. How to you choose which one is best? The best way to choose a cure and seal is to fast forward three years when you will need to touch up the coating. Here are a few things to consider:

 

  1. Solvent based acrylic sealers are very easy to maintain. When it comes time to recoat in a 2-5 years, you simply apply more to a clean, dry surface. Solvent based acrylics are self-priming and when a new solvent based acrylic is applied to an old one, they will bond together forming a consistent finish.
  2. Water based acrylics are slightly harder to maintain as they require an extra step. When it comes time to recoat in 2-5 years you will have to clean and scuff the surface. The surface must be screen sanded before the new coat can be applied or you run the risk or having the two not bond. While screen sanding may not be hard work on a small area, it can add hours of extra work to large areas.
  3. Enhance acrylics, like the Armor AX25 which is infused with a Siloxane sealer, are for areas that are constantly exposed to large amounts of water, such as a pool deck. Other than sealing areas like pool decks, enhanced acrylics may not be necessary.

 

Now, it is important to understand the difference between a cure and seal, and a product that can be applied to new, uncured concrete. Take for example the Armor LV25 and the Armor SC25. The Armor LV25 is a cure and seal product that has passed ASTM C 309 standards and is approved for use as a cure and seal. It helps to properly cure the concrete and leaves behind a protective film on the surface. The Armor SC25 is not a cure and seal product, but it can be applied to newly poured concrete to help protect the surface during the curing process. It is classified as a penetrating water repellent sealer and will not leave behind a film or coating on the surface. The Armor SC25 is a great option for people looking to protect their concrete during the curing process, but don’t want to put a coating down and maintain it.

 

How To Apply a Cure and Seal

 

Not all cure and seal products are the same, and for that reason, application instructions will vary from one manufacturer to the next. It is important to refer to the Technical Data Sheet and Application Instructions for the product in use.

 

Foundation Armor cure and seals are fairly similar in their application instructions, and very easy to apply. Simply apply the cure and seal with a roller or sprayer to clean concrete 24+ hours after the concrete has been cured.