Case 3

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Good Practices

Several strategies can help prevent or minimize sulfate attack:

Ensure sufficient concrete thickness
Use cement that is resistant to sulfate attack
Utilize concrete with low permeability
Maintain a low water/cement ratio (less than 0.45)
Ensure proper compaction and curing
Eliminate salts from the mix components
Apply a protective membrane to concrete surfaces in contact with sulfate-contaminated soil or water

Concrete

Design

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

Environment	Exposure conditions
Mild	Concrete surfaces protected against weather or aggressive conditions
Moderate	Exposed 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
Severe	Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation Concrete surfaces occasionally exposed to light traffic
Very severe	Concrete surfaces occasionally exposed to sea water spray (directly or indirectly) Concrete surfaces exposed to corrosive fumes and heavy traffic
Most severe	Concrete surfaces frequently exposed to sea water spray (directly or indirectly) and heavy traffic
Abrasive	Concrete surfaces exposed to abrasive action.

Table 1: Classification of exposure conditions

Material

Recommendations for concrete to resist sulphate attacks are given in BRE Digest 250 and BS EN 1992-1-1+A1. Generally the concrete should be:

Good quality, well designed, well placed and compacted
Low water cement ratio
Use of air-entrainment to the extent of about 6%
High pressure steam cured
Provided with a protective coating
Sufficient cover

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. Table 2 shows the various concrete grade to be achieved.

Concrete Grade	30	35	40	45	50
Minimum cement content (kg per m3)	275	300	325	350	400
Maximum cement content (kg per m3)	550	550	550	550	550
Maximum % of Fine Aggregate to Total Aggregate	50	50	50	50	50
Maximum water to cement ratio	0.55	0.50	0.45	0.40	0.40

Table 2: Designed mix of concrete

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 control

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 sulphates (Table 3).

Conditions of exposure	Nominal cover
Mild	35	30	30	30	30
Moderate	–	45	40	35	30
Severe	–	–	50	40	35
Very severe	–	–	60	50	40
Most severe	–	–	–	–	60
Abrasive	–	–	–	see note 3	see note 3
Maximum free water/cement ratio	0.65	0.60	0.55	0.5	0.45
Minimum cement content (kg/m3)	275	300	325	350	400
Lowest grade of concrete	C30	C35	C40	C45	C50
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

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
use of metallic coatings such as zinc and nickel
Cathodic protection
use of corrosion inhibitors
use of corrosion resistance reinforcement (eg. stainless steel)
use of low permeability concrete, with improved resistance to chloride ion ingress

Surface protection systems

Surface protection systems may be applied to improve resistance of chemical attacks and enhance concrete durability.
Impregnation systems: function by lining the pores of concrete, reducing the capillary absorption capacity of substrate and lowering the amount of water that can be absorbed.
Prevent penetration of water and solutions into concrete without hindering the escape of internal moisture from concrete
Materials used:
- silicon organic solutions
- resins
- oils
Sealers
- Heavy barrier against penetrating solutions but may hinder the escape of internal moisture
Materials used:
- epoxy resins (EP)
- polyurethane resins (PU)
- unsaturated polyester resins (UP)
Coatings
- compared to sealers, they provide additional protection against mechanical influence.
- increased resistance to diffusion of internal moisture
Materials used:
- plastic modified cement systems and resins