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ASR ASR

Subject: Alkali-Aggregate Subject: Alkali-Aggregate ReactivityReactivity

Certain constituents in aggregates can react harmfully with alkali hydroxides in concrete and cause significant expansion. There are two forms of this reaction:– Alkali silica reaction (ASR)– Alkali-carbonate reaction (ACR)

Alkali silica reaction (ASR)– Develops by aggregates containing reactive silica

minerals. This form is more serious and common than ACR.

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• ASR has been recognized as a potential source of distress in concrete since the late 1930s

Alkali carbonate reaction (ACR)– The aggregates [dolomitic (calcium-

magnesium carbonate)] have specific composition that is not very common.

Alkali silica reaction (ASR)Alkali silica reaction (ASR)

Mechanism

• The reaction can be visualized as a two-step process:– Alkali hydroxide + reactive silica gel → alkali-

silica gel– Alkali-silica gel + moisture → expansion


The amount of gel formed in the concrete depends on – Amount of and type of silica in aggregate.– Alkali hydroxide concentration.– Sufficient moisture.


• The ASR forms a gel that swells as it draws water from the surrounding cement paste (has great affinity to moisture). In absorbing water, these gels can induce pressure, expansion, and cracking of the aggregate and the surrounding paste.

• The alkali silica gels will fill the microcracked regions both within the aggregate and concrete. Continued availability of moisture to the concrete causes enlargement and extension of the microcracks which eventually reach the outer surface of the concrete. The crack pattern is irregular and referred to as map cracking (see Figure 5-20). Or fragments breaking out of the surface of the concrete (popouts) as in Figure 5-21.

PopoutsPopouts


• List of most reactive substances:– Opal (SiO2 nH2O)

– Chalcedony (SiO2)

– Certain forms of quartz (SiO2)

– Cristobalite (SiO2)


• The most important harmful alkali reactive aggregates:– Opaline cherts– Chalcedonic cherts– Siliceous limestones– Siliceous dolomite


• Identification of Potentially Reactive Aggregates:– Field performance history of structures in

service for more than 15 years.– Different tests can be conducted for initial

screening and evaluating potential alkali-silica reactivity.


• Control of ASR– Use of low-alkali Portland cement (less than 0.6%

equivalent Na2O) when alkali-silica reactive constituents are suspected to be present in the aggregate.

– If low-alkali cement is not available, the total alkali content can be reduced by replacing a part of high-alkali cement with supplementary cementitious materials such fly ash, ground blast furnace slag, and silica fume, or use blended cement.


• Control of ASR– Wash beach sand and gravel with sweet

water to insure that the total alkali content from the cement and aggregates in concrete does not exceed 3 kg/m3.

– Control the access of water to concrete.– Replacing 25% - 30% of the reactive sand

gravel aggregate with crushed limestone (known as limestone sweetening).


• Utilization of silica fume, fly ash, and blast furnace slag as partial replacement of cement will reduce the expansion as shown in Figure 5-23.

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