I’m happy to say I had a summer without blogs. I don’t mind them, but being self-diagnosed with A.D.D., it’s impossible to write when the temperature outside climbs north of 20 degrees.

So as the summer wains throughout Alberta and temperatures fall more temperately in Southern Alberta, we make way for my favourite time of year-concrete season. When the temperature are blistering hot in the summer, concrete is temperamental as workers need to protect it ( pout it early in the day and protect it). Below 5 C degrees, workers have to do protect it from the cold. So the days that are 10-20 degrees are really made for concrete. I know what’s going through your head-concrete, really? How exciting is that? Creating architectural decorative concrete is our happy place. We don’t get excited about dies, finishes or sealers-although that is part of the game, rather more a pre-casting of sometimes complicated form-work.

Decorative concrete can be so many things. It’s truly a magical of all the construction activities. You invest in a pile of work creating a concrete form, have really one chance to place the concrete, wait, then more waiting, and only in the final unmasking will you know if you thought and worked everything through properly. It’s a bit embarrassing how much we like concrete.

The way we approach decorative concrete work is through the use of inverted form-work. We find cast in place forming requires skills in reading the concrete and trowel skills which we just don’t currently have. Plus concrete can be fickle. If you over shoot the slump even just a smidge (the water concrete ratio), there could be abnormal shrinkage (‘curling’ around the perimeter) as the slab cures and the excess water evaporates. Pouring the concrete up-side down also ensures and abnormalities due to evaporation are not visible (creates an alligator skin appearance). Success with this type of project relies on being able to think upside down and backward, there are a couple others tricks we use to help improve the chances of success.

All our forms are waterproof. We have used melamine in the past. We have also lined our forms with plastic laminate. On this build we decided to paint the inside with epoxy (multiple layers, sanding smooth in between coats). Bare wooden form-work will absorb water in the concrete too quickly, adversely affecting the cure. Fusing over the inside finish cannot be over-stated enough. Whatever existing on your form will telegraph onto the end product. The more pre-finishing of your form-work equals less finishing afterwards. We finish sand to 200-300 grit on our form-work.

We caulk the inside corners of the form-work. This accomplishes a few things. The outside corner of concrete is the weakest part-and having even the slightest bull-nose will give projects a stronger, more uniform look that will better withstand abuse. A carefully caulked bull-nose edge is nicer looking over a tooled edge in our opinion (most concrete tools are made for large crude projects. Also, concrete can be terribly sharp. A 90 degree angle after dehydration can be borderline dangerous. My 16 year old has a scare on his nose to prove it. Lastly, water will not leave the bottom of the form. This ensures the water cement ratio is intact and there is no spreading of the form when you start to vibrate it (reducing patterning for fast dehydration).

Our caulking is a different colour from the form-work. This is a practise that self corrects sloppy care in the preparation of the form. You can easily see where the caulk is, and if it is the shape you want it. Cheap caulking will shrink more that some of the more expensive types as it is more moisture from acrylic sources. If you have a choice opt for the one with more silicon as this will better retain its shape.

Placed concrete as a material is strongest when its final form is chunky and withstands compression forces. As it thins, it can be extremely weak in tensile strength. Sometimes we can work around this property by sneaking in steel deep on the inside. Depending on the application, we strengthen with reinforcing mesh, sometimes we use rebar. With this sink project, we were inspired by the stories of the old carpenters. Decades ago carpenters would throw a 2-5lb sack of 2-penny nails into their concrete mix. They did this for pours where the geometry discriminated against traditional reinforcing-as in a pile or where preparing the reinforcing was time intensive. With this project the geometry of the rectilinear reinforcing does not match the curvilinear form of the sink which is why we opted not for mesh. Using left over nails and screws from other projects is a good way to achieve strength and reuse materials (make sure they are clean of oil or rust-galvanized and textured are a plus). On larger projects, commercially produced metal or poly thread reinforcing might be the best choice.

The thinnest edge of the design was 1-1/4”(32mm). This has everything to do with the concrete mix we used. The maximum aggregate was 1/2”(12mm). Most commercial mixes I have seen employ four sizes of aggregate. I assume this target mix is similar. Extrapolating there would 25% of the aggregate would be 12mm, 25%-6mm aggregate, 25%-3mm aggregate and you guessed it 25%-1.5mm aggregate (sand). All this means is there should be ample room to have multiple layers of aggregate within the 32mm thickness. Part of the way concrete achieves its strength is having smaller particles fill between the voids left by larger particles (and be crushed-not washed). To be doubly sure all the voids are filled-I used 4 parts of the target heavy strength mix with an additional 1 part of the target parging mix (I have left over from another project). The parging mix is super fine so I am hoping for a void- free finish.

A fair amount of work goes into project after the forms are stripped. It is incredibly difficult to leave the project to rest in the form as it cures. There is an inherent eagerness to sneak a peek. The longer the cure is better. 3 days or more if your patience can handle it. Remember, concrete achieves its full strength after 28days of curing. 75% of concrete strength is usually achieved in the first week so man handling your project before then is not to your advantage.

Post stripping, a fair amount of time goes into sanding the project. We start with a 300 grit and work towards 800 grit wet dry sand-paper. We have also used honing or wet-stones in a similar fashion. Wet-stones are very useful on flat or outside edges. Grinders with a diamond head are used by others with success, but we found it is too easy lose attention and have the grinder run away on you. Holes are easily filled with epoxy augmented with concrete flour to a nut-butter consistency. We have used mortar with some water and ‘Weldbond’ but you really have to ensure the source material is well-hydrated (otherwise your parging will have the necessary moisture ‘sucked out’ before it has time to cure).

Sealing can be done with either a penetrating or topical product. There are a wide selection of food grade or inert products on the market due to the environmental shift of the world. 20 years ago it was near impossible to find these. Xylene based or Phenol based products were the standard and not recommended for interior spaces people occupy (definitely not food preparation surfaces). Remember the old days of the head-ache you could get from the old interior oil paints? Multiply that by 2.

We never seal concrete fully. I believe it needs to breathe somewhat. I have read studies that suggest it takes a year per inch of material for it to reach the indoor humidity(when uncovered). We are always reluctant to get in the way of nature. We opted to use our tried and true Ecopoxy, leaving the underside untreated to breath. There are likely many good options out there besides Ecopoxy-but we felt comfortable using it on our Covid 19 project

We tried using pigments and waxes over the years. We tend to prefer the plain concrete look. Interstar is a brand we had good success with. We learned a good deal prior due to the Cheng concrete countertops and the Concrete Exchange. We had good success with their penetrating food safe sealer and paste wax (for buffing). One trick we have used if pigment isn’t readily available, in lieu of the parging mix, tile grout mixes all a decent compromise. Experiment prior to final pour so that you can ensure the grout is compatible.

Managing Expectations

There is much cross-over information and skill between concrete work and modern rammed earth. In case this post has intrigued you to read more:)