Concrete durability

Physical causes act by mechanical, thermal or moisture processes without changing the chemical composition of cement paste.

All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them.

Mechanical causes

Mechanical causes are those from applied loads and impacts:

• Overloading and structural defects, causing cracks beyond design level.
• Impact (vehicle collision, pile driving, falling objects), vibration and fatigue from machinery or traffic, causing cracks and spalling.
• Settlement and movement of supports, and earthquakes.
• Abrasion and erosion by traffic, water flow with silt and cavitation, which wear the surface.
• Explosion, blasting.

Control: correct design for loads, joints, stiff foundations, hard aggregate and high strength for wearing surfaces.

Physical causes

All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them.

Electrochemical process of corrosion

Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by carbonation reducing pH below about 9, or by chlorides above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery:

   concrete (electrolyte, pore water)
     anode               cathode
   Fe -> Fe2+ + 2e-    O2 + 2H2O + 4e- -> 4OH-
 ===[ steel bar ]==============[ steel bar ]===
         \-- e- flow in the steel -->--/
        Fe2+ ------- ion flow ------- OH-

1. Anode (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$
2. Cathode (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$
3. Ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, then it oxidises to $\text{Fe(OH)}_3$ and finally hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust).

The rust has 2 to 6 times the volume of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones.

Effect of corrosion on the concrete element

1. Expansive rust creates tensile stress; cracks run along the bars, followed by spalling of the cover and delamination.
2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member.
3. Loss of bond between steel and concrete, so anchorage fails.
4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process).
5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases.

Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete.

The rate is controlled by permeability: the denser the concrete, the slower the attack.

Physical causes

Physical causes act by mechanical, thermal or moisture processes without changing the chemical composition of cement paste.

All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them.

Chemical causes

Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete.

The rate is controlled by permeability: the denser the concrete, the slower the attack.

Definition

Permeability is the ease with which a fluid (water, gas, dissolved ions) can flow through concrete under a pressure difference. By Darcy's law for saturated flow,

$$\frac{q}{A} = K\,\frac{\Delta h}{L}$$

where $K$ is the coefficient of permeability (m/s), $\Delta h$ is the head and $L$ the thickness. Dense concrete has $K$ of $10^{-12}$ to $10^{-10}$ m/s.

Agree: permeability is the root cause of lack of durability

Almost every cause of deterioration needs a fluid to move into the concrete:

• Water carries sulphates, chlorides, and acids inside (chemical attack, corrosion).
• Oxygen and CO$_2$ gas penetration cause corrosion and carbonation.
• Water in pores freezes and causes freeze-thaw damage; it also supports alkali-silica reaction and leaching.

If the permeability is low, these agents enter very slowly, so concrete lasts longer. Permeability depends on w/c, curing, compaction, cement and microcracks. Hence IS 456 limits w/c and requires minimum cement and curing (Table 5).

Porosity and permeability are different

Porosity is the total volume of voids; permeability depends on whether the pores are connected and how large they are.

 Low permeability           High permeability
 (isolated pores)           (connected pores)
 +----------------+         +----------------+
 | o   o    o   o |         | o==o==o   o=o  |
 |   o   o   o    |         |  \\  ||  //   |
 | o    o    o  o |         | o=o=o=o===o    |
 +----------------+         +----------------+
 same porosity (volume of pores), flow differs

For example, lightweight foamed concrete has high porosity but low permeability if pores are discrete; a paste with continuous capillary pores (w/c above 0.7) has similar porosity but is highly permeable.

Preventive measures against corrosion of reinforcement

1. Dense, impermeable concrete: low w/c (max 0.40 to 0.55 by exposure), enough cement (min 300 to 360 kg/m$^3$), good grading, full compaction and curing (IS 456, Table 5, Cl. 8).
2. Adequate cover: nominal cover of 20 mm (mild) to 45 mm (severe) and 75 mm (extreme) for RCC members, using cover blocks (IS 456, Table 16 and Cl. 26.4).
3. Limit chlorides and sulphates in aggregate, water and admixtures (IS 456 Table 7; no sea water for RCC); avoid calcium chloride.
4. Cement type: use PPC, slag or sulphate-resisting cement in aggressive areas, with fly ash or silica fume to reduce permeability.
5. Crack control: limit crack width to 0.3 mm (IS 456, Cl. 35.3.2) by detailing and joints.
6. Protected steel: epoxy-coated or galvanised bars, stainless steel, or corrosion-resistant (CRS / Fe 500D) TMT bars.
7. Corrosion inhibitors (calcium nitrite) in the concrete mix.
8. Surface protection: waterproof coating, sealers or cladding, proper drainage with no ponding, damp-proof course.
9. Cathodic protection (sacrificial anode or impressed current) for existing structures; and timely repair of cracks.
10. Regular inspection and maintenance.

Physical causes

Physical causes act by mechanical, thermal or moisture processes without changing the chemical composition of cement paste.

All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them.

Chemical causes

Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete.

The rate is controlled by permeability: the denser the concrete, the slower the attack.

Effects of corrosion of steel in concrete

1. Expansive rust creates tensile stress; cracks run along the bars, followed by spalling of the cover and delamination.
2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member.
3. Loss of bond between steel and concrete, so anchorage fails.
4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process).
5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases.

Porosity

Porosity is the ratio of the volume of voids (gel pores, capillary pores, air voids) to the total volume of the material, expressed in percent. It affects strength and the amount of water that can be absorbed. The pore sizes: gel pores (1 to 3 nm), capillary pores (10 nm to 10 µm), air voids (above 50 µm).

Permeability

Permeability is the ease of fluid flow through the material under pressure ($q/A = K\,\Delta h/L$). It depends on connection and size of the pores, not only on the volume.

 same porosity            different permeability
 +--------------+         +--------------+
 | o  o   o  o  |  low    | o=o=o   o=o  |  high
 |  o   o   o   |  perm.  |  \\ ||  //   |  perm.
 | o   o   o  o |         | o=o=o=o==o   |
 +--------------+         +--------------+
 isolated pores            connected pores

Corrosion of steel

Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by carbonation reducing pH below about 9, or by chlorides above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery:

   concrete (electrolyte, pore water)
     anode               cathode
   Fe -> Fe2+ + 2e-    O2 + 2H2O + 4e- -> 4OH-
 ===[ steel bar ]==============[ steel bar ]===
         \-- e- flow in the steel -->--/
        Fe2+ ------- ion flow ------- OH-

1. Anode (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$
2. Cathode (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$
3. Ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, then it oxidises to $\text{Fe(OH)}_3$ and finally hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust).

The rust has 2 to 6 times the volume of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones.

Carbonation

Carbonation is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

• The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
• The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and starts corrosion once the carbonation front reaches the bar.
• Rate is highest at relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
• It is detected with phenolphthalein spray: the uncarbonated concrete (pH above 9.2) turns pink, carbonated concrete stays colourless.
• It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
• Prevent by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.

Effect on durability: carbonation and corrosion together shorten the service life, because the cracking, spalling and loss of steel section of the cover follow initiation time (carbonation to the bar) and propagation time (rusting).

Carbonation is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

• The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
• The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and starts corrosion once the carbonation front reaches the bar.
• Rate is highest at relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
• It is detected with phenolphthalein spray: the uncarbonated concrete (pH above 9.2) turns pink, carbonated concrete stays colourless.
• It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
• Prevent by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.

Effect of carbonation on durability

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

• The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).

• The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and starts corrosion once the carbonation front reaches the bar.

• Rate is highest at relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).

• It reduces alkalinity, so the steel passive layer breaks down and corrosion begins; rust expansion cracks and spalls the cover.

• Surface shrinkage cracks may form, giving access to more CO$_2$, water and chlorides.

• It slightly increases surface hardness, but this does not offset the corrosion risk.

Effect of permeability on durability

• Permeability controls how quickly water, oxygen, CO$_2$, chloride and sulphate ions enter the concrete.
• Low permeability (low w/c, good curing and compaction, supplementary cementitious materials) delays carbonation, corrosion, sulphate attack, freeze-thaw, and leaching, and so extends service life.
• High permeability makes the concrete deteriorate quickly.

Link between them

Carbonation depth is proportional to the permeability to CO$_2$: dense concrete of w/c 0.45 carbonates a few mm in 50 years, while w/c 0.7 concrete may carbonate to 30 mm or more. Both are therefore controlled by the same measures: w/c not more than 0.45 to 0.55, minimum cement content, full compaction, curing for 7 to 10 days and adequate cover (IS 456 Tables 4, 5 and 16).

Water, permeability and durability

Water is the vehicle for almost all deterioration processes: it carries aggressive ions (chlorides, sulphates), enables carbonation and corrosion, and freezes in pores. The mixing water decides the pore system: excess water beyond that needed for hydration (w/c above about 0.38) is left as capillary pores which become interconnected, so permeability increases rapidly with w/c (e.g. between w/c 0.4 and 0.7 it may rise by several hundred times). Higher permeability means faster ingress and less durable concrete, hence IS 456 (Table 5) limits w/c to 0.40 to 0.55 by exposure, and requires full compaction and curing.

Corrosion of steel

Steel is protected by a passive film in the alkaline concrete (pH above 12.5). Chlorides or carbonation break it; with moisture and oxygen, an electrochemical cell forms: anode $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$, cathode $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$, giving rust of 2 to 6 times the volume, which cracks and spalls the cover and reduces the bar area.

Remedial measures

• Prevent: low w/c, enough cement, cover (Table 16), limit chloride, PPC, coatings, epoxy-coated or galvanised bars, inhibitors, crack control.
• Repair: remove cracked cover, clean bars to bright metal, apply anti-corrosive primer, patch with polymer-modified mortar or micro-concrete, apply protective coating, or cathodic protection.

Water-cement ratio is the main factor controlling the durability of concrete. Water more than that needed to hydrate cement (w/c above about 0.38) leaves capillary pores; a higher w/c gives more and larger interconnected pores, so higher permeability, strength lower, and faster ingress of water, chlorides, sulphates and CO$_2$. This leads to corrosion, carbonation, freeze-thaw and chemical attack.

Hence IS 456 (Table 5) limits the maximum w/c for reinforced concrete: 0.55 (mild), 0.50 (moderate), 0.45 (severe and very severe) and 0.40 (extreme exposure), with a minimum cement content of 300 to 360 kg/m$^3$.

Spalling is the breaking away of pieces of concrete from the surface in the form of flakes or chunks, leaving a rough, shallow cavity. It is caused by corrosion of reinforcement (expansion of rust), freeze-thaw, fire, impact or excess local stress.

Delamination is the separation of the concrete cover into thin layers parallel to the surface, at or near the level of reinforcement, without the layers falling off yet. It is caused by rust expansion of bars, poor finishing (over-trowelling), or fire. The area gives a hollow sound on tapping with a hammer, and if it is not repaired it ends in spalling.

 sound concrete   delaminated zone       spalled zone
 ______________  ______________________  _____________
 |            |  |  .  .  . crack  .   |  |   ____    |
 |  cover     |  | . . . . . . . . . . |  |  /    \   |
 |  o rebar   |  |  o rust  (hollow)   |  | | o     |  |
 |____________|  |_____________________|  |__\_____/__|
                                         exposed, rusted bar

Delamination (layer parting at bar level) comes first, then spalling (loss of the layer). Repair: remove the unsound concrete, clean the bar, coat, and patch.