Name: why concrete can fail around rebar
Uploaded: 2024-11-07T14:48:46.000Z
Duration: 1 min 27 s
Channel: JoVE

Dentro de uma massa de concreto, a alta alcalinidade do hidróxido de cálcio com um pH de cerca de 13 protege o reforço do aço contra a corrosão, criando uma camada protetora de óxido de ferro, um estado conhecido como passividade.
Devido aos potenciais eletroquímicos variáveis, regiões anódicas e catódicas são formadas na superfície do aço.
Essas regiões são superadas pela solução salina em cimento hidratado, atuando como eletrólito.
Os íons ferrosos carregados positivamente do ânodo se dissolvem na solução salina.
Em contraste, os elétrons carregados negativamente se movem para o cátodo, são absorvidos pelo eletrólito e reagem com água e oxigênio para formar íons hidroxila.
O hidróxido férrico é formado quando os íons hidroxila se ligam aos íons ferrosos. A oxidação subsequente do hidróxido férrico leva à formação de ferrugem.
Além disso, os íons cloreto na pasta de cimento circundante formam ácido clorídrico em pontos anódicos, quebrando a camada protetora do aço e causando corrosão por pites.
Depois que o aço enferruja, ele aumenta de volume e exerce pressão que pode levar a rachaduras e lascas no concreto.
O uso de aço revestido com epóxi ou zinco ou a redução da permeabilidade do concreto pode reduzir a corrosão do aço.

corrosão de reforço

Corrosion of Reinforcement

A corrosão do reforço de aço dentro do concreto é um processo influenciado pelas propriedades inerentes do material e fatores externos. O alto nível de pH de cerca de 13, fornecido pelo hidróxido de cálcio presente no concreto, protege inicialmente o reforço de aço promovendo a formação de uma camada passiva de óxido de ferro em sua superfície.

No entanto, ao longo do tempo e sob certas condições como carbonatação, entrada de cloreto e rachaduras, esse estado protetor pode ser comprometido. O aço tem áreas com diferentes potenciais eletroquímicos, conhecidas como regiões anódicas e catódicas. A pasta de cimento hidratada, contendo sais, atua como um eletrólito que conecta essas regiões. Nas regiões anódicas, o ferro do aço se dissolve como íons carregados positivamente no eletrólito, enquanto nas regiões catódicas, os elétrons são consumidos em reações que envolvem água e oxigênio do concreto circundante para produzir íons hidroxila.

Esses íons hidroxila se combinam com o ferro dissolvido para criar hidróxido férrico, que oxida ainda mais para se tornar ferrugem. A presença de íons cloreto exacerba esse processo, levando à corrosão localizada por pites, formando ácido clorídrico que ataca a camada protetora de óxido de ferro. Essa corrosão pode expandir o aço, fazendo com que o concreto rache e lasque.

Medidas preventivas contra a corrosão do reforço de aço incluem o uso de revestimentos protetores, como epóxi ou zinco, no próprio aço ou a melhoria da qualidade do concreto para reduzir sua permeabilidade, dificultando assim a entrada de agentes corrosivos.

Corrosão de Reforço

Dentro de uma massa de concreto, a alta alcalinidade do hidróxido de cálcio com um pH de cerca de 13 protege o reforço do aço contra a corrosão, criando uma proteção ...

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JoVE Core

Civil Engineering

Engenharia Civil

Unit 1

Unidade 1

Permeability of Concrete

Permeabilidade do Concreto

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

why concrete can fail around rebar

Steel reinforcement in concrete can corrode when its protective environment changes. Fresh concrete is highly alkaline because of calcium hydroxide, with a pH of about 13. That high pH helps form a passive iron oxide layer on the steel surface, which slows corrosion.

Over time, carbonation, chloride ingress, and cracking can weaken this protection. Concrete contains hydrated cement paste with salts, and it acts as an electrolyte. Different areas of the steel then behave as anodic and cathodic regions, which creates a small electrochemical cell.

At the anodic regions, iron dissolves into the electrolyte as positively charged ions. At the cathodic regions, electrons are used in reactions with water and oxygen in the surrounding concrete to form hydroxyl ions. These hydroxyl ions combine with the dissolved iron to make ferric hydroxide, which then oxidizes into rust.

Chloride ions make the problem worse. They can cause localized pitting corrosion by forming hydrochloric acid that attacks the protective iron oxide layer. As rust builds up, the steel expands and can crack the concrete, leading to spalling.

Corrosion can be reduced in a few ways. Protective coatings such as epoxy or zinc can be applied to the steel. Another option is to improve concrete quality so it is less permeable and slows the entry of corrosive agents.

Why Concrete Can Fail Around Rebar

permeability of concrete

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corrosion of reinforcement

Steel reinforcement in concrete can corrode when its protective environment changes. Fresh concrete is highly alkaline because of calcium hydroxide, with a pH of about 13. That high pH helps form a passive iron oxide layer on the steel surface, which slows corrosion. Over time, carbonation, chloride ingress, and cracking can weaken this protection. Concrete contains hydrated cement paste with salts, and it acts as an electrolyte. Different areas of the steel then behave as anodic and cathodic...

key terms and concepts

Learn how concrete’s high pH protects steel rebar, why carbonation and chlorides trigger rust, and how coatings and dense concrete help prevent cracking.

Why Concrete Can Fail Around Rebar | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

civil engineering

Why Concrete Can Fail Around Rebar

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