Is your exposed aggregate patchy?


Looking for a fix/solution for patchy concrete?

You are likely here because you suffer ‘patchy exposed aggregate’ anxiety.  

  •  Why does my new concrete look blotchy/spotty
  • Fixing exposed aggregate concrete
  • How to fix blotchy concrete

However, inconsistent finishes in the exposed aggregate are more common than you think.
Whether you are;

  • A concretor looking for a solution to some common problems
  • A homeowner who is unhappy with the finish
  • Or a builder is stuck in the middle.

We can help

All that hard work

  • planning
  • preparation
  • cutout and form-work
  • pouring the slab, screeding, etc.
  • washing off the slurry

And it is PATCHY!! Fear not. It can happen to the best Concretors. We can help!

At one end, we have Concretors trying to remedy a job and doing the right thing. At the other end, we see homeowners left with a substandard job, and the contractor has ‘done a runner’. We understand that things go wrong, and we are here to help.

Exposed aggregate concrete has, by nature, a random finish where colours and density can vary but, as a whole, looks amazing. The issue is when the expected finish gives way to obvious areas of inconsistency and patches.

What is ‘Patchy exposed aggregate’?

The term ‘patchy’ describes the look of under-exposed areas resulting in an uneven appearance. As a builder, owner, concretor or ourselves looking at a concrete slab, we are trying to find any problems. It’s normal, and it is what we do regularly. However, if a fresh pair of eyes can easily see a problem with the concrete, then there is an issue.

Patchy exposed aggregate concrete sandblasted at the top and untreated at the bottom
Image of blasted and unblasted exposed aggregate to help describe ‘percentages’

Try to describe the surface of the exposed aggregate slab in percentages. Stone (aggregate) versus slurry (sand and cement). The higher, the better.

The image above shows the freshly blasted area at the top showing about 90% stone vs slurry. In the unblasted area, at the bottom, it varies from as high as 80% down to 10%

What causes bad exposed aggregate concrete?

There can be many causes of a patchy finish. Some include:

  • Pressure washer issues or just not washing the slurry off quickly enough.
  • Problems with the retarding agent, washing off in the rain or not an even coverage
  • Issues with the batching plant or sitting in the delivery truck for too long
  • Difficulty screening the concrete leaving ‘screed lines’ of slurry on the surface
  • Having an inexperienced contractor or one that does not care

We have done enough exposed aggregate slabs to hear most mistakes, problems, excuses, and bad luck. We are sometimes shocked and amused by what we hear. This, however, does not change what has happened, and we strive to deliver what was originally expected.

What can I do?

How to fix an under-exposed aggregate concrete job?

DO NOT put any sealer on the concrete! It only makes it harder to rescue the job and will cost more time and effort. You can try an acid wash if the patches are only light, but it’s unlikely to work if they are large or heavy.

Call us as soon as you see the problem, and we will look at the scale of the task. The fresher it is, the easier it will be to sandblast the exposed aggregate and remove the patchy sections. However, even if it is weeks or months later, we can still help. We have done it many times before.

We often get calls from property owners, builders and concretors, all looking for a way to salvage a good result.


Sometimes it is a patchy sealer

Picture shows the sealer has lifted off the slurry between the aggregate and where we have blasted it cleanOther causes of a patchy exposed aggregate are the poor application of a sealer or applying the sealer too soon. There are some exceptions; however, sealers should only be applied after the concrete has cured.

Bad sealer application on an exposed aggregate driveway

Concrete needs a minimum of 28 days to cure. Moisture is still rising to the top during the curing process. Trapping the moisture with a sealer applied too early will cause the paint to lift. Commonly we see white patches of sealer where this has occurred.

There are different sealers, but only the permeable sealers can be applied earlier than others. These allow some moisture to escape and won’t cause delamination at the surface.

Occasionally we see a bad application of sealer. Imagine painting a wall but doing it in a rush, not applying an even coverage and leaving the paint to run down the wall. We surprisingly see this happen to concrete slabs as well. A bad concrete sealing job can really detract from the overall appearance.

How to fix blotchy concrete sealer?

We can remove several layers of blotchy concrete sealer from concrete slabs that may have been applied over many years. Recoating the concrete with a fresh layer of sealer can help, but it can eventually fail. The only way to restore the concrete to a fresh appearance is to start over. Contact us for help


Remove sealer or paint from an old slab?

Traditional methods for removing concrete sealers used chemical strippers, Xylene or acid. Neither of these is a favourable method due to some inherent dangers and impact on surrounding areas. We can remove the sealer without chemicals and prepare the surface for a new finish. Working with many contractors that apply concrete sealer, we recommend 1300 RESEAL in our main area of operations.

We are often asked to remove paint from a driveway, and we first check why it was painted in the first place. Often paint is used to cover and hide different concrete colours.

Different coloured and textured concrete is often hidden by using paint.

This driveway was widened at some point and coated with house paint before the new homeowners moved in. It is easy to see why it was painted as the extra concrete to make the driveway wider looks quite strange without a perfect colour match. Paint it, absolutely, and no issue with that, BUT use the correct paint, please! Good quality concrete paint will last many years without fail while regular house paint just won’t work. We removed all the flaky paint and gave the concrete a good clean surface ready for the correct product to be applied.

Some pics of past jobs

A job where the concrete steps were poured unevenly. The riser heights were made even again by grinding the lower treads. We then came in to expose the aggregate.

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A badly exposed aggregate concrete job that needed to be fixed. Large patches of underexposed aggregate were causing some big headaches for the client. We were able to sort this out quickly, so everybody was happy.



Exposed aggregate needs a good clean

MP BLAST also provides a high-pressure washing service. We use high-pressure pumps to remove years of grime, organic matter, and concrete stains. Home-grade pressure washers can’t remove all the dirt from an exposed aggregate slab. Sandblasting exposed aggregate is not always the only solution, but we can tailor a plan to suit your predicament and budget.