Spalling Concrete – How and Why?

Hey, hey, my, my. If concrete’s spalling, do you know why? Ok, so maybe Neil Young didn’t really have concrete in mind when he wrote his iconic song. But he sure did know what he was talking about when he said “Rust Never Sleeps”. While rust may look like it is simply brown and dormant (as well as falsely rumored to give you tetanus if you cut yourself on it), rust is nothing but continuous, relentless corrosion. When rust attacks reinforcing steel within concrete, it can do an incredible amount of damage.

Have you ever seen a concrete slab, bridge, parking garage roof, sidewalk, curb or wall that looks like a chunk of it has just blown off, as if it was spit out from the inside? A piece may even still be lying close by as if begging to be put back in it’s place, like a lonely Jenga block that was removed too soon. (And we all know what happens in Jenga when you take out the wrong block…so read on.)

This phenomenon is called spalling. It is unsightly at best and potentially dangerous to the structure of the concrete at worst. It can be caused by a variety of reasons:  freeze-thaw cycles, deicing chemicals, salt (think about the effects of salt water in harsh coastal areas).

One villain is corrosion, commonly referred to as rust when it involves iron and its alloys, such as steel. Rebar is cylindrical lengths of steel placed within concrete to make it stronger. Steel mesh, beams and other types of support can be used as well. The high alkaline environment of concrete (pH 12 to 13) should cause a thin oxide layer to form a passive film around embedded steel to help protect it. But concrete naturally has pores and capillaries, which allow air and moisture to penetrate and find its way to the rebar to react with chloride ions even in the thickest of concrete slabs, which compromises this film.  Rebar is often left even more vulnerable when the depth of the concrete to be used is thin and therefore doesn’t offer enough coverage of the reinforcement within.

Here’s a fun fact takeaway: The intrusion of chloride ions, present in deicing salts and seawater, into reinforced concrete can cause steel corrosion if oxygen and moisture are also available to sustain the reaction. Chlorides dissolved in water can permeate through sound concrete and reach the steel. Chloride-containing admixtures can also cause corrosion.

SPALLING: When steel corrodes, the resulting rust occupies a higher volume than the steel, which creates expansion and causes tensile stress to the concrete. Expanding corrosion of concrete-covered steel is like an internal pressure cooker to the surrounding concrete, causing it to spall.  When spalling occurs, pieces of the concrete basically pop off (like an unsecured lid on said pressure cooker). Spalling can ultimately create more severe problems, because it perpetuates a vicious cycle.

If left untreated, spalls can accelerate deterioration by leaving even more steel exposed to the environment (see the ghastly photo below). In fact, rusting is one of the most common failure modes of reinforced concrete in bridges and buildings. Would you feel comfortable driving over a bridge or parking garage if you knew the underside looked like this? Many bridges have literally collapsed due to to the damaging effects of rust.

A high permeability rate (water migration through concrete when the water is under pressure) and air/water penetration (air and water passing through concrete) are a couple of the worst things that can happen to an innocent piece of rebar just doing its job to help sustain the strength of the structure. The answer is to stop oxygen and water ingress – but how?

One commonly suggested solution to prevent corrosion is to use coated or galvanized steel, but this can be expensive. Another protective option is to apply a topical membrane to the surface of the concrete, which is also costly and time consuming plus difficult to apply correctly. With topical treatments, a mere pinhole caused by hydro-static pressure or traffic abrasion can cause bubbles, peeling or cracking, providing yet another source of ingress for air and moisture to penetrate. This equates to repairs and the potential for removal, then re-installation of the membrane. Even at their best, topical membranes need to be re-applied over time. From a cost of ownership standpoint, if the first topical membrane application is expensive, factor in taking it off and putting it back on again! A third option is concrete admixes, but admixes can’t seal up the pores and capillaries that allow air and moisture intrusion because these ingress routes are actually caused by bleedwater rising to the surface as the concrete hardens, which occurs AFTER admixtures are introduced into the design mix.

A PERMANENT SOLUTION: Rest assured that there is a “one-time application” solution to prevent reinforced steel corrosion from happening in new concrete, and halt the process in existing concrete as well.

When used at time of pour, Spray-Lock Concrete Protection penetrates the pores and capillaries in the matrix of the concrete and seals them up with a “harder-than-the-concrete-itself” colloidal gel that stays in place permanently. SCP 327 effectively stops moisture from attacking the reinforcing steel, protecting it from ingress of air and water by acting as a “pore-blocker”. Used proactively, prevention is a fraction of the cost of repairing troubled concrete. But if the concrete slab in question is already existing, Spray-Lock has a product for that as well, SCP 578, which halts the destructive process. Finally, SCP 743 is designed for concrete remediation.

When it comes to concrete, an ounce of prevention is definitely worth a pound of cure. View the video below for a microscopic look at how SCP works. Contact me via LinkedIn or comment below to ask for more information about Spray-Lock Concrete Protection.