It is important for Homeowners to be able to know the real facts about concrete stem wall repair in order to understand the claims made by some “Foundation / Concrete Repair Experts”. Most Homeowners are not knowledgeable enough to know if these claims are factual or not. So I have taken a few minutes to talk about the facts. Feel free to fact check me.
One “expert” claims he is using 10,000 psi repair mortar and claims that this can fix a deteriorating stem wall that is being cracked up from rusting rebar. Let’s look at this claim:
Rusting rebar exerts up to 9000 psi pressure on concrete as the iron undergoes oxidization to become iron oxide (rust). Even though the iron in the rebar gets smaller, the iron oxide (though it is much weaker than iron) takes up much more room, expanding to 5 times more than the original rebar size! This is what causes the cracking and even chunking off of concrete around it.
It is important for Homeowners to be able to know the real facts about concrete stem wall repair in order to understand the claims made by some “Foundation / Concrete Repair Experts”.
Ok you must be thinking…..well 10,000 psi repair mortar is 1,000 psi stronger than the pressure on it by the concrete expansion, commonly called rust jacking. No problem right? Not so fast. The 10,000 psi concrete refers to it compressive strength. This is the strength as you compress it… basically squish it. When rust jacking occurs, it puts tensile pressure on the concrete around it. Tensile forces are the opposite of compression. It the strength of concrete to resist being pulled apart… like if you grabbed it with bionic hands and stretched it like taffy.
Concrete is great in compressive strength - it is really weak in tension - it is the reason we put rebar in it. Rebar (steel) is really good in tension and helps concrete overcome its weakness in tension….. until the rebar rusts. The compressive strength of the concrete is relatively irrelevant for this problem. Its tensile strength is what will resist rust jacking as the rebar on the inside splits it apart. So what is the tensile strength of concrete?
Typically tensile strength is often calculated as 7.5 times the square root of the compressive strength. In this case it would be the square root of 10,000 which is 100…. times 7.5 which is 750 psi.
Well you can see now that 10,000 psi concrete with a 750 psi tensile strength…. is no match for 9000 psi rust jacking.
This problem can easily be overcome by understanding the science of the problem and overcoming the nature of the problem of its roots. I discuss these in more detail in my other blogs:
Composite Stem Wall Repair System
Stem Wall Repair Phoenix: Best Practices
Mitigated: Sulfate Chloride Attack on a Stem Wall
As always feel free to check the facts presented here. And remember above all no matter how overconfidently presented, what really counts are the facts and science that can be independently verified.
