**3. Corrosion causes in steel reinforcement**

Concrete is a construction material composed of fine and coarse aggregate bonded with cement and water, which hardened over time. Concrete has very high compressive strength, but its low tensile strength limits the utilization of concrete in the infrastructure sector. In the fifteenth century, the introduction of steel reinforcement changed the infrastructure sector forever.

Steel reinforcement, also known as rebar, is the steel products such as mesh, wire, or bars used in the concrete to increase the tensile strength by holding the concrete in tension. As the steel reinforcement acts as a tensile device and significantly increases the tensile strength of concrete, the utilization of steel-reinforced concrete in the infrastructure sector also increased. However, this reinforcement inside the reinforced concrete structure is susceptible to corrosion damage.

When the iron gets oxidized to iron oxide, this iron oxide forms a layer around the bar, causing the expansion of the rebar. This expansion set up internal stress in the concrete, leading to cracks in the concrete. This corrosion seriously damages the structure and may lead to total structural collapse.

To understand the causes, it is recommended to understand the concrete cement process. Hydration or curing is the process of hardening concrete with water. **Figure 16** shows the basics of the hydration process. During the hydration process, the compounds in the cement form chemical bonds with water molecules and become hydrates or hydration products. The following chemical reactions occur during the hydration of cement.

**Figure 16.** *Hydration of concrete.*

The tricalcium aluminate reacts with the gypsum and added water to produce ettringite and heat. The liberated heat heats the concrete structure.

Tricalcium aluminate þ Gypsum þ Water ! Ettringite þ Heat 866 kJ ð Þ (2)

After the consumption of all gypsum by the tricalcium aluminate, the produced ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals. In this reaction, no heat is liberated.

```
Tricalcium Aluminate þ Ettringite þ Water ! Monosulphate Aluminate Hydrate
                                                                        (3)
```
The tricalcium silicate present in the cement reacts with added water to produce calcium silicate hydrates, lime (Calcium hydroxide), and heat.

Tricalcium Silicate þ Water ! Calcium Silicate Hydrate þ Lime þ Heat 174 kJ ð Þ (4)

The liberated heat heats the concrete structure. Moreover, the crystals of monosulfate are only stable in a sulfate-deficient environment. In the presence of sulfates, the monosulfate crystals become the ettringite crystal.

```
Calcium aluminate monosulphate þ Sulphates þ Water ! Ettringite (5)
```
The ettringite crystals are about two-and-a-half times the size of the monosulfate crystals. This increase in the size of crystals causes concrete cracking, and this process is known as a Concrete Sulfate Attack. The dicalcium silicate (belite) present in the cement reacts with added water to form calcium silicate hydrates and heat. The liberated heat heats the concrete structure.

Dicalcium silicate þ Water ! Calcium silicate hydrate þ Lime þ Heat 58 ð Þ *:*6 kJ (6)

*Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

However, the hydration of dicalcium silicate generates less heat and has a slow reaction rate. The contribution of calcium silicate hydrate to the cement strength is comparatively slow initially. Moreover, in the long term, it strengthens the concrete.

Further, the ferrite present in the cement undergoes two progressive chemical reactions with the gypsum present in the cement.

Initially, the ferrite reacts with the gypsum and added water to form ettringite, lime, and alumina hydroxide.

Ferrite þ Gypsum þ Water ! Ettringite þ Ferric Aluminium Hydroxide þ Lime (7)

Secondly, the ferrite further reacts with the ettringite to form garnets.

Ferrite þ Ettringite þ Lime þ Water ! Garnets þ Ferric Aluminium Hydroxide (8)

The formed garnets are responsible for space filling only and do not contribute to the strength of cement in any way.

The rebars get corroded due to various reasons. The following are the reasons which cause the rebar's corrosion.

1.Seepage or leakage


Let's discuss the above-said causes in detail.

#### **3.1 Seepage or leakage**

Due to liberated heat and voltage present in the concrete, the pours were formed. When these pours contain moisture, the contained moisture acts like an electrolyte and reacts with cement, causing the corrosion of rebars. The highly permeable concrete having seepage and leakage leads to the corrosion of rebars. Water seepage or leakage is the principal cause of rebar corrosion and concrete deterioration. **Figure 17** shows the basic types of leakages in the concrete structure.

#### **3.2 Inadequate concrete cover**

The inadequate concrete cover provides a clear passage for moisture to reach the rebars. Further, this also encourages corrosion due to carbonation and the ingress of chlorides. **Figure 18** shows the basic electrochemical reaction that happen due to inadequate concrete cover. The general corrosion products of rebars are α-Fe, FeO, Fe3O4, α-Fe2O3, γ-Fe2O3, δ-FeOOH, α-FeOOH, γ-FeOOH, β-FeOOH, Fe(OH)2, Fe (OH)3, and Fe2O3.3H2O.
