After my last installment of Engineering for Dipsticks, I got some feedback from a number of structural engineers on topics they’d like me to cover. One of them asked if I could cover the topic of compatibility in concrete repair specifically, “The compatibility of “stiffness” when it comes to repair material selection (i.e. is there such a thing as a material being too strong for an application).
An age-old aphorism within the concrete repair community is “Repair like with like.” This means that if you are going to repair an existing concrete slab that has a compressive strength of 4,300 psi, then your repair mortar should be relatively close to the original concrete in strength. The International Concrete Repair Institute has even spelled out these guidelines in their Technical Document: Spall Repair in Horizontal Concrete. “Compressive strength should be greater than the original concrete and should not be less than 4000 psi.”
On it’s surface, it makes sense that you wouldn’t repair concrete with weaker material. But there are some occasions where a “stronger” material can actually make repairs less durable.
A great example of this is in the treatment of control joints in warehouse environments. Often times, the shoulders of control joint are one of the first things to spall. This is because they are subjected to lots of wear from forklifts, hard-wheeled hand carts, etc. rolling over them all day long.
Overhead view of a typical deteriorated warehouse control joint.
A lot of designers and/or contractors say, “That thing is a pain in the butt to maintain. I’m going to fill it with the hardest, most durable material I can find.” This leads many to use a structural epoxy to fill the cracks. However, typical structural epoxy will have three times the compressive strength and potentially twenty times the tensile strength of the existing concrete!
“Great!” you might say. “That should give me a long lasting repair.” However, there are other factors to consider. What if the original cause of the cracking is due to movement or deflection between the slabs? Now the control joint, which was designed to crack, is filled with an ultra-hard, ultra-stiff material. If the movement continues, then the stress will migrate to the areas adjacent to the control joint, causing the concrete to crack further away from the original control joint.
Movement cracks will likely re-occur near dotted lines if control joints are filled with a structural epoxy. The control joint isn’t allowed to crack because of the extra-hard material, so it cracks somewhere else and makes the spall worse.
If the control joint was filled with the appropriate material [COUGH Euclid’s QwikJoint UVR polyurea COUGH], then we might not have that problem. A slightly softer material could still provide adequate load transfer yet allow for some movement. This would keep the cracking where it’s supposed to be, in the control joint. On paper, a softer polyurea might not be as impressive as a structural epoxy (less than 15x the tensile strength, etc), but it’s more likely to be the right choice for long repair.
Investigation into the root cause of the failure should determine the approach to repair. Stronger is not always longer.