00:00Can I use rusty steel rebar in my concrete?
00:10Hey engineering enthusiasts!
00:12Today we are going to talk about a question that comes up on pretty much every construction
00:17site.
00:18Is it safe to use rebar that's rusty?
00:21You've probably seen those rebars left out in the weather for a few days or even weeks.
00:27And they end up getting that orange color.
00:32But does that mean they've lost strength and can't be used anymore?
00:35Or is there a type of rust that's normal and even expected on construction sites?
00:42To answer that, we first need to understand exactly what this rust that appears on steel
00:47is.
00:48The oxidation of iron is a natural chemical reaction that happens when iron comes into
00:54contact with oxygen and water.
00:56The basic reaction of rust formation is the one shown here on the screen, where this iron
01:02hydroxide, when it loses water, it turns into hydrated iron oxide, which is this orange layer
01:10that we see.
01:11The question that comes to many people's minds is, does this surface layer already represent
01:18any serious structural damage?
01:20When the rebar only has a thin and adhered layer of rust called surface oxidation, this usually
01:28does not represent any structural compromise.
01:32This type of rust is common when steel is exposed to air, especially in humid places.
01:39And in many cases, it appears within a few hours.
01:43The standard does not require the steel to be shiny or completely clean to be used.
01:49The big question is whether this surface rust interferes with the bond with the concrete.
01:56This bond between steel and concrete is essential for the structure to work as a single element.
02:04Concrete resists compression very well, but it needs steel to resist tension.
02:10When the rust is light and well adhered, it doesn't harm this interaction.
02:16In fact, the roughness caused by this film can even in some cases improve the bond between
02:23concrete and steel.
02:25The question that arises then is, how much oxidation is considered acceptable?
02:32To answer this, it's necessary to differentiate between surface oxidation and advanced corrosion.
02:39Light oxidation actually forms a thin, uniform and adherent layer on the steel without producing
02:45dust or creating flakes.
02:47Deep corrosion, on the other hand, is completely different.
02:52It forms thick, loose and flaky layers, and this indicates a real loss of the rebar's cross-section.
02:59When this corrosion happens, part of the bar's diameter is literally lost.
03:05And here comes a critical question.
03:08How can we ensure that a corroded rebar still has the same strength it was designed for?
03:15This is exactly where visual inspection becomes fundamental.
03:20If you use a wire brush and notice that only fine dust comes off, and that right underneath
03:27the rebar is intact, this usually indicates that the oxidation is superficial.
03:33But if the brush removes large scales, or if you notice deep irregularities, that's already
03:40a sign of severe corrosion.
03:42When this happens, it's hardly possible to guarantee that the bar still meets the diameter specified
03:50by ABNT NBR 7480.
03:54And when the actual diameter decreases, the strength decreases as well.
04:00And this puts the safety of the structure at risk.
04:03Another important point is to understand the role of concrete in protecting the steel.
04:09Concrete has a high pH, usually above 12, and this creates a passivation layer around the
04:17rebar, which stops oxidation.
04:21In other words, once the steel is properly surrounded by concrete, corrosion practically stops, as long
04:28as there is no ingress of aggressive agents such as chlorides or excess carbon dioxide.
04:35And is this alkaline protection enough when the rebar is already quite corroded?
04:40The answer is no.
04:42Concrete does not reverse pre-existing damage.
04:46If the rebar has already lost cross-sectional area, is already flaking or deformed, the concrete
04:53does not restore its original strength.
04:56That's why, ideally, the bars should go into the concrete pore with, at most, a superficial
05:02oxidation.
05:03Without any signs of deep corrosion, in major projects, it's even common to measure the
05:10diameter of the bars to ensure they still meet the nominal specification.
05:14It's also worth remembering that the environment where the structure will be built influences
05:20this decision.
05:22In coastal, industrial, or highly polluted regions, the presence of chlorides and other corrosive
05:29agents is higher, and the structure is more susceptible to corrosion over the years.
05:35In places like this, starting the construction with rebar that's already compromised is even riskier.
05:42And we can't forget about the bond to the concrete.
05:45When the rust is loose, in thick powder or flakes, it creates a barrier between the rebar
05:52and the fresh concrete.
05:54This barrier prevents full contact and reduces the concrete's ability to grip the steel.
06:00In extreme situations, this can cause cracks and even failures.
06:06But how can we ensure that the steel surface is suitable for a good bond?
06:10The most common practice on construction sites is simple.
06:15If the rust is light, just use a brush and check the condition of the bar.
06:20And if it looks intact and without any cross-sectional loss, it can be used.
06:26If the corrosion is deep or irregular, the rebar should be discarded.
06:30It's a practical, quick, and efficient assessment that follows the recommendations of standards
06:36and technical guidelines used in the industry.
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07:13And you, have you ever noticed the rusty rebar at construction sites?
07:17Did you also wonder whether they could be used?
07:20Leave a comment below.
07:21I want to know.
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07:38That's it, Tamões da Engenharia.
07:40Big hug, and I'll see you in the next video.
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