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How to prevent the occurrence of Alkali Aggregate Reaction (AAR) in bridge structures?

How to solve the problem?

What is best method of repair or solution to overcome the defect?

in Bridge 48,780 points 106 195 351

3 Answers

5 votes
 
Best answer

How to prevent the occurrence of Alkali Aggregate Reaction (AAR) in bridge structures?

One of the products of cement hydration is calcium hydroxide which is high alkalinity in nature. This alkalinity environment is ideal to protect the corrosion of rebar in the concrete. However, with the present of silicate, chloride and sulphate in the aggregates, chemical reaction occurred and produces reaction product such as alkali silicate gel. This reaction product is expansive and may cause crack to form.

To prevent such reaction to occurs, testing on the aggregate such as alkalinity test, chloride and sulphate test are necessary to contain these substances below the limit level. Our JKR road spec stipulates these requirements and must comply to the given limits (clauses of testing of aggregates). The alkalinity test can also be referred to relevant ASTM test.

How to solve the problem?

The reaction product gel of alkali-aggregate reaction is expansive and causing the concrete to crack. Once the reaction product has formed, it will then stabilized and the concrete becoming weakened. The treatment of repair will be similar as other concrete attack remedy.

What is best method of repair or solution to overcome the defect?

Repair solution will depend on the deplorable situation of defects ranging from durability rehab to major strengthening.

Fairul

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1

Thank you Dr @fairul for the valuable info! 

7 votes
As you know that the AAR is happen to all bridge structure not in Malaysia but problem for entire of world. This is occur when quartz in the aggregate react directly to the existing moisture in the concrete and produce alkaline reaction. This alkaline will effect to the concrete structure and produce spalling and crack to the concrete body. A lot of studies had been carried by the researchers on this problem and our local university also take part on this research like UTM and UKM itself (can refer to the journal). From my reading, it could be say that most of the studies focus on identify reactive aggregates (alkaline present) through the testing like Mortar Bar Test (ASTM C1260) and Petrographic Examination (ASTM C295). To be proven by study and research thats Malaysian granite which produced aggregates are contain quartz and surely AAR occured (refer journal and paper by UTM).

From my view it can summarize that the effect of quartz in aggregate with moisture will produce the alkaline reaction and it can be control the reaction through recommendation as per belows:

1) Design of abutment need to reconsider by build as far as possible from the edge of river which to minimize the effect of moisture to the concrete body (effect up and down of river water level).

2) Using the blended cement together with OPC in concrete mixture such as fly ash or something else that prevention of water infiltration (moisture) in the concrete.

3) Coating the concrete structure (external) using local product like waterproofing base to prevent water infiltration.
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1

Many thanks for the insights, Tn @mhafizs yes

1 vote
Why is that AAR is commonly found in bridges and quite rarely found in building structures?
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1
Actually it do found in buildings......just that most of building structures are cover with plastering works, as a result is hard for us to detect and reports the occurrence.
1
The crack pattern found on structures suffering from AAR is quite distinct and rarely found in building structures. Building deteriorations are more prone to carbonation rather than AAR. It is a good subject to study for future PhD candidates.
1
Actually we cant' determine the actual crack pattern until all the plastering works had been removed even though all the aggregates is of the same source.
0
My comments are based on our forensic investigations made to buildings suffering from deterioration. Of course, the trigger factor will be the presence of cracks on the plastered surface. Based on that, most problems are carbonation based not AAR... Back to my question...
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