Concrete, Cement, Corrosion - IA1 Engineering

Concrete/Cement

Cement:

-Binding agent used to bind various construction materials.

-Essential ingredient of concrete and mortar.

-Mixed with water to form paste that binds aggregates like sand or crushed rocks.

-Calcium, silicon, iron, aluminium are closely grounded to form fine powdered product, cement.

Concrete (rock like material):

-Composite material:

-Binding material (cement or lime), aggregate (gravel, sand), water.

Reinforced cement concrete (RCC):

-Usually with steel (takes some of the load).

-Sometimes ‘prestressed’→ apply initial compressive forces, before concrete experiences tensile stress.

Steel reinforcing is sealed inside concrete

↓

If exposed (unsealed) to air and water

=Corrosion

↓

Causes swelling + porous

Swelling= stress and cracking

Porous= water ingress → ice and expansion

Ice and expansion= stress

‘Pre-stressed’- cables are in tension before concrete is poured.

‘Post-tensioned’- cable is threaded through holes in the cured (hard) concrete, then tensioned.

Corrosion

-An attack on a material because of chemical, frequently electromagnetic reactions with the surrounding medium.

Chemical corrosion- oxidation that occurs by the action of dry gases at high temperatures.

Electrochemical corrosion- takes place in electrode reactions, often in humid environments.

Electrode potential- Reactivity:

-Metals with a smaller electrode potential value (volts), are an anode, which will corrode. Whilst the metal with the higher electrode potential value (volts), are a cathode, which won’t rust.

Parameters affecting corrosion rate:

-Oxidising agents

-Electrical conductivity of the electrolyte

-Increase in temperature

-Increase in concentration

Types of corrosion:

General attack- attacks the entire surface of a metal structure, cause by both chemical and electrochemical reactions.

Localized, only attacks portions of a metal structure:

Pitting- the creation of small holes in surface of metal.

Crevice- occurs in stagnant locations (EX. Under gaskets).

Filiform- occurs when water gets under a coating like paint.

Galvanic corrosion- when 2 different metals are located in a liquid electrolyte such as salt water.

Environmental cracking- when stressful environment conditions begin to crack, fatigue, brittle and weaken metals.

How to prevent corrosion:

-Metal type

-Protective coating (zinc coating)

-Environmental measures

-Sacrificial coatings

- Corrosion inhibitors

-Design modifications