Spalling is a term used to describe cracking concrete due to rusting steel within the concrete. As the steel begins to rust it starts to expand and this expansion is what will cause the cracking of the concrete. This can occur in any concrete structure that has steel embedded within it. The reason for steel to be placed within the concrete is to give it structural strength as well as to prevent cracking. There are a few reasons that the internal steel may begin to rust.
#1The steel may have been placed within the forms for the new slab, column, or other structure and left exposed to the elements for some time before the concrete was poured. this can allow for the rusting process to begin.
#2 The steel was placed to close to the outer edge of the concrete and since concrete is porous ( Meaning air and water can pass through) the steel is not protected with a sufficient amount of coverage to lessen its exposure to the elements. The recommended amount of coverage from top, bottom, and sides of any concrete pour to the steel within is 1-1/2" inches.
Another common cause of spalling concrete is due to penetrations made into the concrete after the concrete has been poured. In the Florida Keys where I live we see this mostly on concrete docks where the homeowner or contractor has installed items such as cleats, ladders, or dock fenders using anchoring methods that allow the salt air and water to penetrate the slab. The most common types of anchors used ( but are not recommended are Lead shields, wedge anchors, and sleeve anchors ). The reason these anchors are not recommended and will cause spalling is due to the fact that to install them you must drill a hole into the concrete thus creating an entry point for air and water to enter into the slab. These anchors do not make the hole they are in water tight. You may be asking yourself at this point " So if I can't drill a hole in the concrete to mount my new fish cleaning table then how can I install it?" Well that's a great question. And the answer is to drill a hole in the concrete. The problem is that the hole you drilled is not sealed. In our business we only use epoxy anchors meaning no matter what it is we are mounting to the concrete we will drill our hole into the slab, blow out the concrete dust from within the newly drilled hole, we then will inject a 2 part construction epoxy into the hole, and then install our stainless steel all thread. The average cure time on this epoxy is 24 hours. After the epoxy has cured you can then install your new item using washers and nuts on the all thread. this will give you a watertight anchor that will not cause you to have to make expensive repairs in the future.
So how strong is an epoxy anchor? There are may brands of epoxy and within those brands there are many grades of epoxy. Our preferred epoxy is Red Head G-5 which will achieve bonding strength of 1600 PSI and is used for attaching everything from cleats to boat lifts.
So what do I do if I already have spalling concrete?
Well my friend this is a tough one. First of all how bad is the spalling? There comes a point where it is just better to remove and replace the concrete and steel as rust is like a cancer and once it begins it will continue until it completely consumes or has been stopped in its tracks.
One of the most common questions I am asked when meeting with a customer is " So how much is it going to cost to repair my spalling?'
When it comes to giving an estimates this is the one that is the hardest to answer and here is why. As you now know spalling is due to the steel (rebar, Welded wire mesh, or other) rusting within the concrete and due to its expansion is causing the concrete to crack.
We see in a lot of cases where a homeowner or a contractor is not fully aware of how to go about the repair process and simply break off the loose concrete and cover the rusted steel with a new finish. This type of repair is not a solution it is a problem as the cracks will soon be back and the steel will still be rusting. Here is the challenge in trying to give a fair estimate on a spalling repair. You cannot tell by just looking at it what all is entailed. In order to do a proper repair you need to cut and chip away the concrete to expose the rusted steel. However just because the crack in the concrete is 1' long this doesn't mean that the steel is only rusting where you see the crack. When we do a spalling repair, we will start cutting and chipping the concrete where we see obvious signs of spalling. In order to make this a permanent repair we need to continue exposing the steel until we find what is called blue steel (steel that has no sign of rust). Once we get to this blue steel we then can make a determination of the condition of the steel and come up with a course of action. In some cases the steel has rusted to the point where the rebars diameter has been reduced to nothing. If the rebar diameter is less than 75% of its original diameter this bar will need to be replaced. At this point we will cut the old rebar out and epoxy in new rebar, making sure to leave enough of the original bar to do a proper lap splice. Meaning we need to tie the new rebar to the original to maintain proper structural support after the repairs have been completed. The length of the splice or length of bar to be tied together needs to be at least 40 times the diameter of the bar being used (#5 rebar Nominal diameter: 0.625 inches requires a minimum lap splice of 25"). Once the steel is spliced together we will then coat the steel in a zinc chromate primer and or epoxy paint to seal the steel and protect it from future rusting, "Remember no oxygen no rust". If the steel does not need to be cut out and replaced we will use a wire wheel, sand blaster, or other method to remove the heavy rust and then proceed to apply a rust converter such as Ospho or Corroseal. Once the oxidation or rusting process begins it will not stop unless you treat the steel with a product meant to stop the rust. The final step in this repair is to build forms and pour the new concrete. Now you may remember i said earlier that concrete is porous so now that you have no unprotected holes in the concrete and no rusted rebar within the concrete this does not mean you will have no more spalling. You will want to put a coating on the concrete to seal the entire surface. There are several products on the market that will do the job. You can use a clear concrete sealer such as H&C Concrete sealer or there are sealers that come in many color options. Regardless of what you chooseit is recommended that you protect your concrete and it will last with very little maintenance.