7 Common Issues to Avoid When Exposing Aggregate in Concrete

Learn what you can do to prevent these common issues that arise when exposing aggregate in concrete.

When exposing aggregate, there are several mistakes that contractors often make that can end up taking time and resources to fix. Here, you’ll learn about these common issues and how you can avoid them, with guidance from Brickform trainer Johnny D.

1. Inadequate or inconsistent exposure

When surface retarder is not applied immediately after finishing the concrete slab, you may not get great exposure on your surface. Additionally, if material isn’t applied adequately, you may end up with a surface that looks inconsistent. In other words, some spots on the concrete slab may have different depths of exposure than others.

Photos taken during a video shoot at Lumber Lane

Solution: Put surface retarder down immediately after finishing and make sure it’s evenly distributed across the entire surface. This will ensure that the level of exposure is the same throughout. Johnny D. recommends about 250 to 300 square feet per gallon. “I spray the entire job then go back and make sure I hit everything,” he says.

Brickform’s Select-Etch makes it easy to do this. Because it’s opaque, you can tell when you have 100% coverage.

2. Too much material coming off

If you begin spraying the surface and find that too much material is coming off, you could be moving into the removal step of the process too early. Or, you are being too aggressive in your removal method.

Solution: First, be sure to follow instructions closely to make sure you’re not removing material too early. If possible, have a test slab nearby where you can see if your timing for removal is right. “If you start taking material off your client’s surface and it’s not ready, you can’t reverse that,” says Johnny D.

When you first begin to remove the retarder, do not begin with a power washer. Always start with a brush and hose so you don’t inadvertently damage the surface. From there, if the material still isn’t coming off, try a swing buffer or pressure washer. But be careful, as any marks you make with a pressure washer are permanent.

3. Material not coming off

If you begin the process of spraying the surface to expose aggregate and it’s difficult to remove, you may have waited too long to take it off.

Solution: If you do leave the retarder on for too long and aren’t able to remove it using a brush and hose, try a swing buffer or pressure washer, being cautious not to scratch the surface.

4. Damaging the surface

When removing the top layer with a pressure washer, it’s common to inadvertently leave unwanted lines on the surface. This can happen when:

It’s too early to use a pressure washer and you should be using a brush and hose.
The pressure is turned up too high.
You get too close to the surface with the pressure washer.

Solution: When you first attempt to remove material, don’t begin by using a pressure washer. Instead, use a hose with a high-pressure nozzle and gently scrub the surface instead.

However, as mentioned above, there are times when a pressure washer is necessary—for example, in hot weather or when you must wait more than a couple hours to remove the surface retarder. “The more experience you get, the more you know what your limitations are,” says Johnny D. For best results, stay at a consistent distance from the surface of the concrete as you spray. Over time, you’ll get a feel for how close to get and how high you should turn up the pressure. Using a turbo nozzle is not recommended.

While Johnny D. doesn’t use a pressure washer for same-day removal, he does use one when doing a final rinse the following day.

5. Inappropriate mix design

The type of concrete mix you use impacts the way your final surface looks after exposing the aggregate. For example, if your client wants a fine, sandy look, you’ll need more sand in your mix.

Solution: Get to know your ready-mix suppliers. Discuss the outcome you’re looking for and the type of decorative mix you need. “Most ready mix companies are going to have a ton of different mix designs, and a lot of them will have mix designs specific for exposed decorative aggregate,” says Johnny D.

6. Unwanted marks on the surface

If you have uneven finishing or trowel marks, you will see those in the final product. The shallower the exposure, the more it will stand out.

Solution: Pay close attention during the finishing process, ensuring that nothing is sitting on the surface for any longer than it needs to. “Keep the bull float off the slab if it’s not moving,” says Johnny D. “If someone is resting the bull float in the middle of the slab, they’re pushing aggregate down and bringing more cream and moisture up to the surface.”

7. Weak surface

One of the main causes of a weak exposed aggregate surface is excess water diluting the cement paste. Unfortunately, this can make the concrete slab prone to issues such as cracking or scaling.

Solution: In addition to making sure you include the correct amount of water in your mix before pouring the concrete, it’s crucial to protect your surface from rain. An unexpected rain shower can ruin a pour job, so make sure to keep supplies on hand to properly protect your project site.

Best product for exposing concrete aggregate

For a surface retarder that makes the process of creating a high-quality exposed aggregate finish easier, try Select-Etch by Brickform. With twelve predetermined depths of exposure, this water-soluble retarder gives contractors more control over the process. Since the window of time between application and removal is longer than most chemical retarders, it really allows the underlying concrete to harden properly.

Additional tips for best exposed aggregate results

Here are a few additional tips for installing a beautiful exposed aggregate surface for your client.

Read all instructions on the TIS thoroughly before beginning your project.
Do a final rinse to make sure all paste is removed. If you have any stubborn spots, Johnny D. recommends using diluted muriatic acid (1 part acid to 10 parts water).
Seal the slab. For exposed aggregate, we recommend Brickform’s Stealth-Seal WB for a more natural look (no gloss). Alternatively, Gem-Guard SB can be used to get a little bit of a color enhancement.
Complete a test-run on a concrete slab to make sure you can practice creating the right level of exposure and verify it with your client.