Why reinforced concrete crack?

Crack is common in reinforced concrete. Although concrete is strong in compression, it is weak in tension. When the tensile stress exceeds the concrete's tensile capacity, cracks begin to form.

N.A. x As Section M wk Side View

Will the crack affect the performance of reinforced concrete?

In Ultimate Limit State (ULS) design, concrete in the tension zone is assumed to have cracked, and the reinforcing steel carries the tensile forces. Cracking is therefore expected in reinforced concrete.

N.A. x As Section M εcc εs Strain Fcc Fst Stress Block σs Under Working Load

Rather than preventing cracks completely, reinforced concrete design aims to control crack widths to satisfy durability, appearance, and serviceability requirements throughout the structure's design life.

Why is Crack Control Important?

Small cracks are generally harmless. However, excessive crack widths can lead to several problems:

  • Water penetration
  • Corrosion of reinforcing steel
  • Reduced durability
  • Poor appearance
  • Increased maintenance costs

The purpose of crack control is not to eliminate cracking, but to limit the crack width to an acceptable value so the structure continues to perform as intended.

How Does Eurocode 2 Control Cracking?

Eurocode 2 treats crack control as a Serviceability Limit State (SLS) requirement. Crack control ensures that the structure remains durable and functional during normal service conditions.

Eurocode 2 Clause 7.3 provides methods for crack control, including:

  • Minimum reinforcement requirements
  • Bar spacing limitations
  • Direct crack width calculation

The calculated characteristic crack width must be smaller than the allowable crack width .

Condition for crack control
where
wk = calculated characteristic crack width
wmax = allowable crack width specified by Eurocode 2

How is the Allowable Crack Width (wmax) Determined?

The allowable crack width is not a fixed value.

It depends mainly on:

  • Exposure conditions
  • Durability requirements
  • Different country may also have additional own requirement to it

Eurocode 2 provides recommended crack width limits based on the exposure class.

Generally,

Exposure Condition Typical Crack Width Limit
Mild indoor environments 0.4 mm
Moderate exposure 0.3 mm

The harsher the environment, the smaller the allowable crack width.

This reduces the likelihood of moisture and harmful substances (chlorides, carbonation, freeze/thaw cycling) reaching the reinforcement.

Step 1 – Determine the Exposure Class

The first step is to determine the exposure class of the structure.

Eurocode 2 refers to the exposure classes defined in Table 4.1, which describe the environmental conditions surrounding the concrete.

Typical examples include:

Exposure Class Typical Environment
X0 Dry internal concrete with negligible corrosion risk
XC1–XC4 Carbonation-induced corrosion
XD1–XD3 Chloride exposure (excluding seawater)
XS1–XS3 Marine environments
XF Freeze–thaw exposure
XA Chemical attack
Concrete Exposure Class from BSEN1992-1-1:2004
Concrete Exposure Class from BSEN1992-1-1:2004
Details of each the exposure class can be seens in below:
EN 206 concrete exposure classes reference table Monochrome spec-sheet style table listing exposure classes X0, XC1-4, XD1-3, XS1-3, XF1-4, XA1-3 with simple line icons and short descriptions class description of environment 1 — no risk of corrosion or attack X0 no exposure risk (inside buildings with very low air humidity) 2 — corrosion induced by carbonation XC1 dry or permanently wet(inside buildings with low air humidity) XC2 wet, rarely dry(surfaces subject to long-term water contact, foundations) XC3 moderate humidity(inside buildings with moderate/high humidity) XC4 cyclic wet and dry(surfaces subject to water contact) 3 — corrosion induced by chlorides XD1 moderate humidity (surface exposed to airborne chlorides) XD2 wet, rarely dry, chloride water(swimming pools, exposed to industrial waters) XD3 cyclic wet/dry, chloride spray(bridge exposed to spray with chlorides, pavement) 4 — corrosion induced by chlorides from sea water XS1 airborne sea salt (Structures near to on the coast) XS2 permanently submerged (Parts of Marines Structures) XS3 tidal, splash, spray zone (Parts of Marines Structures) 5 — freeze/thaw attack XF1 moderate saturation, no de-icing (Vertical surface expose to rain and freezing) XF2 moderate saturation, with de-icing agent (Vertical surface of road expose to freezing) XF3 high saturation, no de-icing (Horizontal surface expose to rain and freezing) XF4 high saturation, de-icing or sea water (Road and bridge decks expose to de-icing) 6 — chemical attack XA1 slightly aggressive chemical env. (Natural soils and ground water) XA2 moderately aggressive chemical env. (Natural soils and ground water) XA3 highly aggressive chemical env. (Natural soils and ground water)


Each exposure class reflects a different durability requirement.

Step 2 – Select the Recommended Crack Width Limit

Once the exposure class has been identified, Eurocode 2 Clause 7.1N recommends the maximum allowable crack width, where the used limits are:

Exposure Class Recommended Maximum Crack Width
X0 0.4 mm
XC1 0.4 mm
XC2, XC3, XC4 0.3 mm
XD, XS 0.3 mm (or more stringent where required)

For structures where watertightness or appearance is critical, designers may specify smaller crack width limits such as 0.2 mm or 0.1 mm.

What Happens Next?

After determining the allowable crack width, then we need to calculate the expected crack width of reinforced concrete section.

Calculation of Flexural Crack
Calculation of Flexural Crack

Details of calculation of crack width can be found in other articles such as the baisc idea and formula of crack width and also work example for calcualtion of crack width.

Summary & Key Takeaways

  1. Crack control is a Serviceability Limit State (SLS) check that limits crack widths to ensure the durability, appearance, and long-term performance of reinforced concrete structures.
  2. The allowable crack width (wmax) depends on the exposure class. More aggressive environments generally require smaller crack width limits to better protect the reinforcing steel from corrosion.
  3. The crack control process is: 1. determine the exposure class, 2. obtain the allowable crack width from Eurocode 2, 3.calculate the expected crack width, and 4.verify that it does not exceed the allowable limit.
#rc design #crack width calculation #formula #structural
CivilSimple Team

CivilSimple Team

The CivilSimple Team writes practical engineering guides for the profession and the curious. All articles are reviewed for technical accuracy before publication.