Exposed Aggregate DIY Home

Introduction

Exposed aggregate concrete is a decorative style of concrete suitable for use as a driveway, sidewalk, or patio. The appeal is in the finish, which exposes the smooth textured small stones and pebbles that are part of the concrete. This is in contrast to a smooth concrete finish, where the visible surface is primarily composed of the 'fines' within the concrete. Exposed aggregate provides some visual interest by creating random patterns, and a variety of colours. Very little of the exposed surface is the characteristic dull grey of the concrete fines.

Traditionally, exposed aggregate concrete has not been done on a do-it-yourself basis. I don't know whether this has been due to a shortage of information aimed at the do-it-yourselfers, or if there aren't enough DIYers to warrant a large body of literature. Regardless, after scouring the web, libraries, home improvement centres, and whatever other resources I could find, I was able to come up with a method of doing my own exposed aggregate concrete.

My project was a largish patio, approximately 1200 square feet, plus an attached sidewalk, about 18 feet long. I had previously done very little concrete work, other than bedding a few fence posts in concrete, a small section of sidewalk, and a small water collection box that is part of the drainage system on my property. I wasn't completely convinced that exposed aggregate was a viable option as a DIYer, so I did a test pour on a small sidewalk in an inconspicuous section of my property. There, I tried four separate sections of sidewalk, each finished in a different way. Two of the four sections were finished in exposed aggregate, but using different methods of work. After evaluating the four different sections, I chose to do the patio project using the method of exposed aggregate finishing described here. The experience gained from this testbed pour was extremely valuable, and I would urge anyone with little concrete experience to do the same thing. Even if you have to break up the test patch and throw it away or bury it, it will be a worthwhile experience.

When one uses the term, exposed aggregate, it suggests that the surface finish is representative of the overall content of the concrete. Indeed, this may have been the case at one point in the evolution of the technique. Today, however, the aggregate that is exposed on the visible surface is deliberately seeded there for it's aesthetic purposes. The placement of the aggregate on the surface of the uncured, soft concrete is the focus of this article.

This article is not about mixing concrete, setting forms, or other issues not directly related to the exposed aggregate technique. That information is readily available from numerous other sources. Some of what is written here is a combination of information from books and a very few bits of information I was able to find on the web. Mostly, however, it is of my own contrivance and experience. There are other techniques, and I can't vouch for how well or how poorly they work, or whether other techniques are easier or more difficult. I'm just giving an accounting of a method that worked (very well) for me. I've got a very nice 1300 square foot patio to show for it.

Step 1 - Planning.

Don’t underestimate the amount of time and effort required to do a good job of finishing concrete in exposed aggregate. I think it takes a DIYer about double the amount of time it would take to apply a conventional finish, not including acid washing, and clear-coating. Don’t attempt to do too much in one day, or without an adequate number of workers. I have found that using two helpers (total 3 workers), with everyone working pretty hard all day, we could do about 125 square feet on a hot day at the very most. On a cool cloudy day, we could manage about 50 square feet more. On a very hot day, the concrete sets nearly as fast as we could mix and pour it, and it was taxing to manage 80 – 100 square feet per day. Using less than two helpers drastically reduces the amount of surface that can be poured and finished in a day.
Make sure your equipment and material is at the job site before you begin. Make sure all forms are fully set, and that you are not going to be distracted by last-minute details like cutting and installing rebar or remesh. Have any ramps and planking necessary for maneuvering wheelbarrows in place beforehand. Clear the workspace of any unnecessary obstacles and hazards. If you are using water from a well, consider whether the well has the capacity to supply the large amounts of water you will need for mixing and washing the finished surface (also consider the upshot of underestimating this). Also consider that you will need water for cleaning tools, and after a hard day’s work, you’re going to want a shower.
If your project requires you to pour in sections, consider which sections are going to be poured first, and arrange to protect the already finished surface(s) against spills and other workplace damage. Consider where the wash-water is going to go, and try to avoid washing across an already finished surface.


Tools
1. Mixing tools, unless using 'ready-mix' concrete.
2. Shovels, rakes, hoes, etc. for placing wet concrete into the forms.
3. Wheelbarrow.
4. Wood float(s).
5. Steel trowel(s)
6. Length of 2-by-4, long enough to reach across the forms, less about 8-12 inches, to be used as a screed.
7. Brushes with handles. Any fairly soft bristled brush with a handle that keeps fingers out of the concrete will do. The type used for brushing snow off of cars is ideal (these are like giant toothbrushes, and hard to find in July). Get enough for everyone to use one.
8. A lawn roller. I made mine out of concrete using a 24 inch long section of 10 inch diameter tubing used for forms for footings, and a few pieces of steel for an axle and handle.
9. One or more (3 or 4 is ideal) plastic pails, about two-gallon size (I used empty dishwasher soap pails).
10. Protective clothing, suitable for working on wet concrete (gloves for sure, rubber boots)
11. A piece of plywood about 2 feet square, for kneeling/standing on uncured concrete.
12. A few blocks of 2-by-4, 12 inch - 24 inch long
13. A mason's hammer, about 2 lb.

Materials
1. Concrete, either mixed on site with a portable mixer, or delivered from a 'ready-mix' supplier (I mixed my own).
2. Pea-gravel (bullet gravel, 3/8" drain rock), about 1 US gallon per 3-5 square feet of surface area.
3. A good supply of water from a garden hose with a spray nozzle.
4. Cement and sand, enough to make about 1 gallon of 'mortar'.

5. Laborers.If you are mixing concrete in batches, I suggest three persons as a minimum. Four is better. The day will pass quickly, but you will be tired at the end of it. Get an early start, especially if you are planning to do an area more than about 100 square feet.

Step 2 - Prepare the aggregate material.

The pea gravel used will have a strong influence on the final appearence of the project. Spend some time selecting the pea gravel that best suits your sense of asthetics. Selection can be based on any combination of shape, colour, texture (smooth, fractured, shiny, dull), and to a lesser extent, size. Some landscape supply outlets may have a range of choices, and may be able to order material to meet your specification. If you are attempting to match an existing exposed aggregate slab, this step will be particularly importent. Thoroughly wash the pea-gravel, about 6-8 gallons at a time in a wheelbarrow, until the water is clear when mixed around with the gravel. Keep the cleaned gravel nearby, and fill the plastic pails with the gravel. This is best done the day before.

Step 3 - Pour the base.

Pour the concrete into the forms, tamping, spreading, etc. as for an ordinary poured surface. If the concrete is being mixed on site in batches, leave about 1" of the forms unfilled. When the forms are completely filled to within 1" of the top, finish filling the forms to within about 1/4" from the top. Use the 2-by-4 screed to level the top surface, and then smooth with the wooden float and the steel trowel. 

You want to have a top surface that is flat, and is rich with concrete 'cream'. Work the concrete only enough to bring up a good layer of cream, and avoid excessive working. You will have to use some judgment as to how flat the surface is, as the screed cannot ride on the top edge of the forms as is normally done. The reason for leaving the top surface below the top of the forms is to allow room for the final layer of pea-gravel. The top layer is best poured in as few batches as possible, so that there is a consistent degree of wetness to the entire top surface. This aids in the following steps. Allow the concrete to sit until there is no more water on the surface. You want the surface to be stiff enough to support a bit of weight, like a pail of rocks on your 2-by-2 foot piece of plywood, but soft enough to press the pea-gravel into without too much effort. As the water rises to the surface, you want to take notice of any low areas that tend to accumulate water, and trowel these out so the surface is as flat as possible.

 

 

 

 

 

Step 4 - Seed the aggregate surface.

 When the concrete has just started to set, it's time to seed the aggregate into the top surface. Fill a pail with the clean pea gravel, and using a gentle sweeping motion, cast a layer of pea gravel onto the wet concrete. The gravel should not sink under it's own weight, but remain on the surface. Use a brush to distribute the gravel evenly, so that the concrete underneath is almost entirely concealed. For a large surface, you may need to set up some type of scaffolding to gain access to the areas away from the perimeter. Try to achieve a very uniform spreading of the gravel, and avoid leaving any areas with too much concrete exposed. Any area of exposed concrete larger than a nickel is too big. Also avoid regions of excess gravel, as there will not be enough concrete cream to bond the pea gravel to the substrate, and this will result in a hollow area with unexposed pea gravel. At this stage, you may notice some areas that do not have enough cream at the surface to bed the pea gravel in. Mix a batch of sand and cement, about 3:1 or 4:1, fairly wet, and apply it over the pea gravel as needed to completely surround any loose gravel.


Step 5 - Press the stones into the concrete.

 This step can begin as soon as an area is uniformly covered with the pea gravel. Start by pressing the gravel into the surface with a wood float, or just a block of wood, such as a 12"-18" piece of 2-by-4. Use a pressing or gentle tapping motion. At this stage you may start to see areas that require a few additional stones, so drop a few stones in place as necessary, and start pressing them in. By now, the concrete should be becoming fairly stiff, so that you can lean on it with your hand without leaving an impression. You will probably need to apply some small patches of the mortar mixture in various places as you start placing and pressing the gravel. Make sure there are no lumps of mortar sticking up above the overall surface. Use the brushes to gently spread the mortar over a wide area. Brushes of the right stiffness can be used in a tapping motion, where the ends of the bristles tap the stones into the concrete. This has been found to be an effective way of bringing up a bit of moisture and cream, as well as working any applied mortar around loose stones.
When you have pressed the gravel into a large area and are satisfied that there are no voids or piles of excess gravel, begin rolling the gravel into the concrete with a lawn roller. If the weather is hot, you will have to start working fast at this stage. Roll the gravel until it is completely pressed into the concrete, and the roller begins to draw up a thin layer of concrete fines around the pea gravel. Don't worry about the gravel disappearing into the concrete. It should be pressed in so that the top surface looks uniformly covered with the concrete fines.
At this stage you should be able to tread gently on the surface to push the roller. Avoid any twisting motions with your feet, and try not to dig in your heels or toes, which may leave impressions in the surface. As you roll the surface, try to avoid steering the roller while it is actually on the concrete, as the twisting motion will tend to lift stones out and leave pits. If the concrete is too soft at this stage you may see ridges forming along the edge of the roller. If this happens, stop rolling and wait a few minutes, until the concrete has stiffened enough to fully support the roller. If the gravel becomes difficult to roll into the concrete, you can give the concrete a very light mist of water to loosen it up slightly. Don't overdo this, as it will substantially reduce the strength of the concrete and reduce the bonding strength of the cement to the stones. All of your workers should be very busy at this stage, pressing, rolling, and always scanning for imperfections. Eventually, the stones will all be pressed into the concrete, and the entire top surface will be covered with a thin layer of cream. No bare stones should be exposed.
Sometimes, if the concrete becomes too dry, it will tend to cause the stones to stick to the roller. If this happens, make sure the stones are removed from the surface of the roller before the roller makes a full revolution, or the ‘extra’ stone will cause an indentation in the surface. A very fine mist of water applied to the surface will generally cure this problem.
Now that the stones are all pressed into place, get down low to the surface, and sight across it, looking for any ridges left by the roller. If you see any, and you probably will, either roll them out, or get a 2 foot block of 2-by-4 and lay it flat across the high spot. Give it a few whacks with the mason's hammer, up and down the board. Move the board around and repeat until the high spot is flattened. Keep scanning across the surface and get it as flat as possible.


Take a short break. If it is a cool day, this might be as long as an hour. If it is a hot day, it might be only 5 or 10 minutes.

Step 6 - Expose the aggregate.

When the concrete fines on the surface have lost the shiny wet appearance, and have become a dull colour, it is time to start exposing the aggregate. Begin by gently brushing the surface with a soft bristled brush or broom. Gather the fine concrete into small piles, as you begin to reveal the shapes of the stones in the concrete. If any stones get lifted out by brushing, stop and wait a little longer. As you brush, remove just enough material so that the shape of each stone becomes clearly defined, but not so much that the stones are standing up above the surrounding concrete. You aren't trying to remove all of the concrete, just enough to be able to see the shape of the individual stones. The colour is going to remain the colour of wet concrete. To gain access to the interior surface, use the piece of plywood to kneel on. Be sure to keep your toes from digging into the soft concrete, and dislodging any of the stones or leaving impressions in the surface. Brush the piles of sand and cement off of the surface. At this point the concrete should be becoming pretty hard; hard enough to walk on, and the brushing may start to require some effort to reveal the shapes of the stones. Eventually, you will have removed all of the excess material, and the surface texture will have a pebbled appearance (big surprise!), but will still be the colour of wet concrete.


Next is the moment of transformation. Gently spray the surface with a garden hose sprayer. Wash at a low angle across the surface to reveal the stones, and gently brush with a soft broom to move the remaining bits of sand and cement from the surface. The transformation from the dull grey of the wet concrete to the brilliant wet clean stones is a remarkable sight. Your hard work is being rewarded before your very eyes. As you brush off the last remnants, look for any areas that are not sufficiently exposed. Used a brush to finish these areas and wash it some more. Avoid using a strong spray, as this will dislodge the stones, and leave small pits in the surface. By now, you should be able to walk freely on the surface, but continue to use caution not to kick any stones out of the substrate. If any do get accidentally removed, they cannot be put back in at this point. You will notice the effect of the roller on the stones, at this stage. The stones will have aligned themselves so that a flat section of each stone is at the surface, and the top plane of the finished surface is quite smooth. My patio is quite comfortable to walk on with bare feet.

When the aggregate is fully exposed, it will still have a slightly hazy coating of cement on it. This will weather off over time, but may be etched off using muriatic acid. The aggregate will have a dull grayish coating on it, which will seem to disappear when it is wet. It is a good idea to keep the surface damp for a few days, if possible, after it is poured. This will allow the slab to reach its maximum strength. A variety of clear-coat finishes are available at home centres, plastics suppliers, and some paint stores. These should be applied according to the manufacturer’s directions, which usually stipulates that the surface should be kept dry for some period before and after application. Most will tend to leave a glazed appearance, similar to the effect of water on the exposed aggregate, which most people find pleasing.

Cautions

One important point about washing the cement off of the surface; the wash water is extremely high pH, and full of suspended solids. You must not allow this water to enter storm drains that empty into fish habitat. The cement is deadly to fish and their eggs. Before doing the project, dig a trench around the area deep enough to contain all of the water that runs off during the cleaning stage. The water can then soak into the soil, trapping the solids.


Your comments and questions are invited. If you have experience using this method and would like to add your comments to this page, I will be happy to make your comments available.


This page last updated 01-Jan-2006 by
Rod Nussbaumer bomr@trmail.triumf.ca

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