Case 2 – Chloride Ion Attack

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Good Practices

Concrete

Design

An understanding of the environment /atmospheric conditions should be taken into consideration during the design stage.

EnvironmentExposure conditions
MildConcrete surfaces protected against weather or aggressive conditions
ModerateExposed concrete surfaces but sheltered from severe rain or severe traffic
Concrete surfaces continuously under non-aggressive water
Concrete in contact with non aggressive soil
Concrete subject to condensation
SevereConcrete surfaces exposed to sever rain, alternate wetting and drying or occasional freezing or severe condensation
Concrete surfaces occasionally exposed to light traffic
Very severeConcrete surfaces occasionally exposed to sea water spray (directly or indirectly)
Concrete surfaces exposed to corrosive fumes and heavy traffic
Most severeConcrete surfaces frequently exposed to sea water spray (directly or indirectly) and heavy traffic
AbrasiveConcrete surfaces exposed to abrasive action.

Table 1: Classification of exposure conditions

Material

Good quality and impermeable by good selection of aggregates and minimum cement content.

Make sure concrete is well compacted and dense so that the rate at which chloride salts penetrate into the concrete is reduced. Table 2 shows the various concrete grades.

Concrete aggregates should have minimal salt content. Cement should comply with SS EN 197 series while coarse and fine aggregates used should comply with SS EN 12620. All aggregates shall be stored in clean places.

Concrete Grade3035404550
Minimum cement content (kg per m3)275300325350400
Maximum cement content (kg per m3)550550550550550
Maximum % of Fine Aggregate to Total Aggregate5050505050
Maximum water to cement ratio0.550.500.450.400.40

Table 2: Designed mix of concrete [7]

Addition of non-chloride admixtures to concrete mix [1]

  • Introduction of chemical admixtures with an insignificant chloride content
  • These non-chloride admixtures and non-chloride accelerators would offer benefit water-reduction and set control without addition of the objectionable chloride ion.
  • Depending on how these are incorporated in concrete mix, they can reduce permeability without increasing the chloride ion content.

Construction

  • Ready mix concrete is preferred over site mixed concrete to achieve consistency.
  • Check for quality of concrete before placing [8]. e.g. water cement ratio, slump test, etc.
  • Place the concrete carefully. If concrete is placed directly from a truck or concrete pump, place concrete vertically into position. Do not allow the concrete to fall more than 1 to 1.5 metres.
  • Ensure thorough compaction of the concrete during placement.

Quality

Avoid following during concreting to minimise cracks:

  • Avoid excessive manipulation of the surface, which can depress the coarse aggregate, increase the cement paste at the surface, or increase the water-cement ratio at the surface.
  • DO NOT finish the concrete before it has completed bleeding.
  • Do not dust any cement onto the surface to absorb bleed water.
  • Do not sprinkle water on the surface while finishing.

Reinforcement

Design

Sufficient concrete cover should be provided to prevent corrosion of reinforcement due to the ingress of chlorides.

Conditions of exposureNominal cover    
Mild3530303030
Moderate–45403530
Severe––504035
Very severe––605040
Most severe––––60
Abrasive–––see note 3see note 3
Maximum free water/cement ratio0.650.600.550.50.45
Minimum cement content (kg/m3)275300325350400
Lowest grade of concreteC30C35C40C45C50
1) This table relates to normal-weight aggregate of 20mm nominal size. Adjustments to minimum cement contents for aggregates other than 20 mm nominal maximum size are detailed in BS EN 206+A2.
2) Cover should be not less than the nominal value corresponding to the relevant environmental category plus any allowance for loss of cover due to abrasion.

Table 3: Limiting values of the nominal cover of normal weight aggregate concrete [3]

Material

All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32. Reinforcement can be protected further by using following methods:

  • removal of rust and mill scale before embedment
  • use of non-metallic coatings such as epoxy resins and solvent containing acrylic resins[6]
  • use of metallic coatings such as zinc and nickel
  • Cathodic protection
  • use of corrosion inhibitors
  • use of corrosion resistance reinforcement (e.g. stainless steel)
  • use of low permeability concrete, with improved resistance to chloride ion ingress

Surface Coating

Apply coating to concrete surfaces to prevent chlorides attack via water (Table 4).

CoatingProtective materialEfficiencyExpected lifespan
Liquid surface coatings1. Pitch or coal tar epoxy with 250µm coating thickVery high if well applied and free from pin holesup to 20 yrs depending on exposure
2. Epoxy resin, two pack polyurethane 250µm thick Very good, should be applied in two coatsdepending on thickness and conditions. Thick coatings can have long life span of more than 10 yrs
3. Solvent-based acrylic, methacrylate, styrene acrylic Good, paints combine protective and decorative qualitiesUp to 10 yrs depending on exposure
4. Emulsion based acrylic or styrene-butadiene polymers and co-polymers with or without materials Fair, but may not be truly impermeableUp to 10 yrs depending on exposure
Water repellentSilicone, silane, siloxaneMake surface water repellent without making it waterproof.
These materials are easy to reapply after the years and some form good primers for other types of coating.
Concrete to be dry to ensure good penetration		About 10 years

Table 4: Surface coatings to resist chlorides attack.