Measures to reduce the risks
By Roderick Kabel
With any piece of wood and with any type of wooden construction project incorporating a miter joint, there are a number of factors to consider. First and foremost, any wooden miter joint must be tight, as seamless as possible and each miter cut properly secured to the other.
This post will be focused on epoxy river tables with preformed miter joint corners built into the mold/frame. We will not address the many proper and different ways to cut and secure miter joints, that’s totally a personal preference to each of you.
What we will discuss are a little physics and geometry regarding concrete, which coincidently correlates closely to the pouring and cure of preformed epoxy miter joints.
In the world of concrete there are 5 sayings:
Concrete will get hard
Concrete will turn color
Concrete won’t catch fire
Concrete will not get stolen
Concrete will crack
Four out of the above five aspects are true when it comes to epoxy. (Some jerk could actually steal your epoxy table…)
There is a known problem encountered when pouring concrete around what is called a “re-entrant” corner. Re-entrant corners are sharp angled inside and/or outside encircling walls or barriers. A re-entrant corner creates extreme tensile stress concentration at the corners. As the concrete cures, it tries to linearly shrink and move in two directions at right angles to each other.
In simple terms, concrete (and epoxy) can crack from factors such as the stress from its curing, shrinkage, thickness, settling, expansion, contraction, and pressure pushing back from its surrounding environment. Cracks can also occur as the result of temperature differences and changes, as well as curling tensile stresses in the top and bottom of the slab – building up in the first few hours after the pour – which are additive to the right-angle linear tensile stress and shrinkage.
The combination of these tensile stresses try to rip concrete apart, starting at the corner. As the tensile strength of the concrete is exceeded, cracks form diagonally at approximately 135-degrees from each edge of a 90-degree corner. As further shrinkage and curling occur over time, the crack widens and lengthens.
Epoxy bar tops, countertops and tables also utilize re-entrant corners in the form of preformed miter joints. This isn’t uncommon but the possibility of a crack at the miter joint is high, much like that in concrete. Hence, our correlation between concrete re-entrant corners and an epoxy thick pour miter joint.
Whereas cracking in concrete around a re-entrant corner can be managed and prevented with proper geometric joint sawing, we epoxy table builders do not have this luxury. We can’t and wouldn’t make stress relief cuts in our tables.
As such, our goal for solid epoxy around a wood slab miter joint doesn’t have many resolutions to prevent cracking. So, what do we do?
Adding a simple rounded nose to a miter joint may be a good solution. Geometry wise, if you consider the typical "point" a miter joint creates, it can be sharp and hard. The pressure of curing epoxy pushing on a 1-2mm miter joint point, will act like a giant scratch awl. (Envision an emergency window breaker for escaping a car.) This creates more than adequate pressure back against the epoxy in its final stages of cure hardening to make the epoxy crack.
If the miter joint point is slightly rounded over nose (10mm), the intensifying pressure from the epoxy will have improved displacement, lessening stress and the chances for a crack.
We believe this simple detail is the savior when we see successful epoxy pours with preformed miter joints. Additionally, assisting successful outcomes come from proper fastening and securing of the wood slabs to themselves, and to the frame, preventing the wood from moving. Which, moving wood slabs only exacerbates the issue. If the wood slab miter joint isn’t secured tightly at the seam and it is allowed to open up, this will also compound the overall stress on the epoxy.
Another very large hurdle is the physical moving of the entire table. Unless you are able to pour the miter joint corner on site, the unstable physics of moving a finished form/table - avoiding road bumps from your shop to the install location - tends to lean heavily towards the probability of epoxy cracking during travel.
To avoid epoxy miter corner cracks altogether we suggest building your deep pour river bar/countertop completely straight in your shop or on location. Then miter cut the table top at your preferred turn angle (90-degrees), and splice together properly at the installation site.
There can be a lot of money wrapped up in epoxy tables, bars and countertops. Use every precaution possible to ensure the best results possible for you and your customer.