Concrete Slab & Exposed Aggregate Floors

Exposed Aggregate Concrete - Cleaning and Sealing

Material Description: This page is about regular concrete slabs and exposed aggregate concrete slabs. They are treated the same way.
A regular concrete slab is finished by hand steel trowling, machine steel troweling, brooming, etc. A steel trowel finish would be a tighter, slicker finish and thus less absorbent than brooming or other type finish.
An exposed aggregate finish is created by using an acid wash to the surface of a freshly poured concrete slab to eat away the cement film at the surface and expose the faces of decorative pebbles that were used in the concrete batch. The pebbles are usually very dense (low absorption) and are now surrounded by relatively porous (more absorbent) concrete.

Penetrating type sealer: for a surface of very different absorption characteristics like this, the sealer choice is best if oriented to the lowest absorption part (the pebbles). It is easy to add another application of sealer if the higher absorption part (the concrete) needs it.
For a coating type sealer the absorption difference is not a concern.

Certain conditions that accelerate the drying process of freshly poured concrete (heat, wind, waterproof underlayment) can create two preventable conditions: Surface (or deeper) shrinkage cracks and lower ultimate strength concrete
Both are preventable by using a "curing compound" that slows down the water evaporation process.

If your exposed aggregate has loose pebbles and/or "powdering" cement, then the best choice is a petroleum solvent based acrylic sealer. It will create a light gloss on the pebbles, no gloss on the concrete, and strengthen the concrete and it's bond to the pebbles.

Further "Items of Interest" for this surfacing, and all others, are at the bottom of this page.

Your Choices for Sealing

This surface has two very different absorption characteristics. The pebbles are much denser and lower absorption than the concrete. It is usually best to make the choice based on the pebbles so there is not to much gloss buildup on them.

Our suggestions to test are:

Penetrating petroleum solvent formulas with acrylic solids. These can be sprayed to finish nicely, but solids level might need to be adjusted if manufacturer allows dilution.

Water based film forming that can flow out and self-level.

Water based penetrating that is usually invisible when dry. However, test for sealer residue (usually white) on the pebbles.

Click here to see What effects you can expect from each sealer type.
Click here to see our suggested sealers, cleaners, and application tools.

Most people have no idea about the history and characteristics of their flooring. You can click this link to see how to easily test with water drops and understand which sealer is most appropriate for your goals.

Some questions you may not be thinking to ask right now that could become important:
    . Was it sealed in the past? Does that matter?
    . Will a new sealer be compatible with whatever was used before?
    . What sealer will give the visual results you want?
    . Will you also be able to have a sealer solve problem(s)? ( Answer: yes. Just know which to pick.)

Items of Interest

You may have heard of concerns about Agglomerates. All of them would be prevented with the information we provide here. Therefore, you can have the floor you want!

How to select a sealer

To select a sealer it is good to have some idea of the absorption rate so you achieve the gloss level (none to high) and all the other benefits available without using more sealer than necessary. Also, you can test for (and protect against) acidic liquid sensitivity.
A sealer can do far more than just bead water and look pretty! To see what that is click here.

Important: If your project has had any sealer applied in the past, it must be evaluated differently. To see why, Click here!

Sealer "solids" levels?

This discussion applies only to the petroleum solvent based sealers. With the water based sealers, solids level is not a consideration.
A porous surface will require more gallons (more money) of a lower solids sealer than using a higher solids level sealer. That, plus different surfaces have different requirements. It is only a matter of which is best for your needs.
The more porous the surface, the more solids will be required to achieve the desired effects of gloss, strengthening, stopping efflorescence, etc.
The more porous the surface, generally the greater the need for the sealer to create a stronger surface.

An old sealer below the surface, even after stripping, will lower the absorption and porosity to some degree. Sometimes it is uneven below the surface and can create an uneven coloring effect with a color enhancing sealer applied later. Another reason to test first.

Do not believe yet that you have the type of surfacing you were told:

There is no need for confusion or problems brought about by misidentification of a surface type, yet it happens all too often. Sellers use fancy marketing names that can be misleading about the true nature of a surfacing. For instance, a customer was told they bought "Petite Granite" for a bar surface. But, unlike granite it was etching circles from wine drips. After simple testing, it proved to be a limestone which is treated very differently than granite. It was easily restored and protected after a 60 second test. Please review our "Surface Types" page to compare pictures, descriptions, and testing if needed, to confirm you have what you were told.

Colors fading?

The iron oxide pigments used in concrete products, and the colored clays in clay products, do not fade . The appearance of fading is actually from tiny efflorescence particles (white powder) in the pores. It can be removed and color restored with a good efflorescence cleaner and then stopped from returning, and color restored, by sealing with the appropriate "color enhancing" sealer.

Renew a glossy finish

Re: "penetrating sealers": do not apply thin layer upon thin layer. Apply a sufficient quantity to insure below surface penetration & bonding. A layer of sealer on top of another layer of sealer can result in poor bonding between layers and that can cause separation peeling that looks grayish.

Concerned about Doing It Yourself or what your contractor says?

If you are concerned about doing it yourself - consider that the satisfaction of a project is directly related to YOUR knowledge of what needs to be done and how. Who actually does the work is less important.
The goal for contractors is - NO CALL BACKS. A good contractor will understand the logic of not taking shortcuts.