Case 1 – Alkali-Silica Reaction

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Case 1 – Alkali-Silica Reaction

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Good Practices

Using low-alkali cement reduces the amount of alkalis available for the reaction.
Selecting non-reactive aggregates or using supplementary cementitious materials (such as fly ash or silica fume) can also mitigate ASR.
Proper concrete mix design and curing practices help reduce the risk of ASR.
Monitoring and maintenance, including sealing cracks and controlling moisture ingress, can help manage existing ASR-related issues.

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 seawater spray (directly or indirectly) Concrete surfaces exposed to corrosive fumes and heavy traffic
Most severe	Concrete surfaces frequently exposed to seawater spray (directly or indirectly) and heavy traffic
Abrasive	Concrete surfaces exposed to abrasive action.

Table 1: Classification of exposure conditions [2]

Material

Since this defect arises from the presence of alkali-silica reactive aggregates, it would be a good practice to minimize the alkali content in the concrete and use non-reactive aggregates.

Measures that can be taken include:

reduce the degree of saturation of the concrete such as impermeable membranes
use of any low alkali (less than 0.6% equivalent sodium oxide content) Portland cement; such a cement is available under BS 4027 (Withdrawn) [3]
limit the alkali content of the concrete mix to 3kg/m³ of equivalent sodium oxide content.
admixtures that would either preferentially replace the alkalis or immobilize them are used.

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 [6]. Table 2 shows the various concrete grades 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 [3]

Construction

Ready mix concrete is preferred over site mixed concrete to achieve consistency.
Check for quality of concrete before placing [4]. 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 meters.
Ensure thorough compaction of the concrete during placement.

Quality control

Avoid following during concreting to minimize 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.

The admissible concrete and steel stresses in the façade elements should not exceed the following indicative stresses. Admissible concrete and steel stress:

During demoulding: 10 to 15 N/mm²
In service: 45 N/mm² or 0.67 fc/m whichever is the lesser.
Admissible steel stresses: In order to decrease the risk of cracking, the stress of the main reinforcement near to the visual faces of the elements will be limited to maximum 120 N/mm².
It is recommended to use steel with deformed profile giving improved adhesion, and the diameters smaller or equal to 16mm.

Reinforcement

Design

Sufficient concrete cover should be provided to prevent corrosion of reinforcement.

Condition of exposure	Nominal cover
Mild	25	20	20	20	20
Moderate	–	35	30	25	20
Severe	–	–	40	30	25
Very severe	–	–	50	40	30
Most severe	–	–	–	–	50
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.

Table 3: Limiting values of the nominal cover of normal weight aggregate concrete [2]

Material

All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32 [3]. 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[5]
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

Alkali silicate-based mineral masonry coatings may be used. They are based on an alkali silicate solution (usually sodium or potassium silicate) in water and are pigmented with alkali-resistant pigments [9].

Alkali silicate-based masonry coatings may be modified by additions of aqueous polymer dispersions, such as acrylic copolymer having good resistance to alkali to modify their drying characteristics under adverse conditions and other physical properties [9].