Name: how moisture cracks alkali-reactive concrete
Uploaded: 2024-11-07T14:48:46.000Z
Duration: 1 min 26 s
Channel: JoVE

Gli aggregati contengono naturalmente minerali silicei che reagiscono con gli idrossidi alcalini degli alcali cementizi.
Il gel alcalino-silice formato attira l'acqua e si gonfia, che è limitato dalla pasta cementizia che lo circonda, portando a una pressione interna, provocando la rottura e la rottura della pasta cementizia, evidenziata dall'espansione.
L'espansione è in parte dovuta alla pressione idraulica osmotica della reazione alcali-silice e alla pressione di rigonfiamento dei prodotti solidi.
La dimensione delle particelle silicee influenza la velocità di reazione, con le particelle fini che causano l'espansione in settimane, mentre quelle più grandi possono impiegare anni per causare danni.
La progressione della reazione alcali-aggregato è influenzata da fattori come la porosità dell'aggregato, il contenuto di alcali del cemento, l'acqua nella pasta e la permeabilità della pasta cementizia.
Questa reazione si verifica principalmente sulla superficie del calcestruzzo in condizioni di umidità continua o durante i cicli di bagnatura e asciugatura, in particolare a temperature comprese tra 50 e 100 gradi Fahrenheit.
Se è difficile evitare combinazioni dannose di cemento-aggregato, è consigliabile incorporare una pozzolana e almeno il 30% di aggregato grossolano di calcare in massa. L'uso di cemento a basso contenuto di alcali aiuta anche a ridurre le reazioni degli aggregati alcalini.

reazione alcali-aggregato nel calcestruzzo

Alkali Aggregate Reaction in Concrete

La reazione alcali-aggregato nel calcestruzzo coinvolge minerali silicei naturali in aggregati che reagiscono con idrossidi alcalini derivati ​​da alcali del cemento. Questa reazione forma un gel alcali-silice che assorbe acqua, si gonfia e aumenta di volume, e poiché è confinato dalla pasta di cemento circostante, crea pressioni interne che crepano e distruggono il calcestruzzo. L'entità dell'espansione e del danno può essere in parte attribuita alla pressione idraulica osmotica della reazione alcali-silice e alla pressione di rigonfiamento dei prodotti solidi formati.


 La velocità di reazione è influenzata dalla dimensione delle particelle silicee; le particelle più fini possono causare un'espansione nel giro di poche settimane, mentre le particelle più grandi potrebbero impiegare anni per manifestare danni simili. Fattori come la porosità degli aggregati, il contenuto di alcali del cemento, la quantità di acqua nella pasta e la permeabilità della pasta di cemento influenzano la progressione di questa reazione. Questa reazione si verifica in genere sulla superficie del calcestruzzo in condizioni di umidità continua o durante cicli di bagnatura e asciugatura, in particolare a temperature comprese tra 10 e 38 gradi Celsius. Per mitigare queste reazioni, è consigliabile utilizzare cemento a basso tenore alcalino, incorporare una pozzolana e utilizzare almeno il 30 percento di aggregato grossolano di calcare in massa nella miscela di calcestruzzo.

Reazione alcali-aggregato nel calcestruzzo

Gli aggregati contengono naturalmente minerali silicei che reagiscono con gli idrossidi alcalini degli alcali cementizi.
Il gel alcali-silice formato attira l'acqua ...

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how moisture cracks alkali-reactive concrete

Alkali-aggregate reaction in concrete can weaken a structure when siliceous minerals in the aggregate react with alkaline hydroxides from cement. The reaction makes an alkali-silica gel, which pulls in water and swells. As the gel expands inside the rigid cement paste, it creates pressure that can crack and disrupt the concrete.

The damage comes from both osmotic hydraulic pressure and the swelling of the solid reaction products. Particle size also matters. Fine siliceous particles can cause expansion in weeks, while larger particles may take years before the same kind of damage appears.

Several factors can speed or slow the reaction. These include the porosity of the aggregate, the alkali content of the cement, the amount of water in the paste, and the permeability of the cement paste. The reaction often shows up near the surface of concrete when moisture is present all the time or when the material goes through wetting and drying cycles, especially at temperatures between 50 and 100 degrees Fahrenheit.

Concrete can be protected by using low-alkali cement, adding a pozzolan, and including at least 30 percent limestone coarse aggregate by mass in the mix. These steps help limit the formation of the damaging gel and reduce the chance of cracking over time.

How Moisture Cracks Alkali-Reactive Concrete

how-moisture-cracks-alkali-reactive-concrete

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alkali aggregate reaction in concrete

Alkali-aggregate reaction in concrete can weaken a structure when siliceous minerals in the aggregate react with alkaline hydroxides from cement. The reaction makes an alkali-silica gel, which pulls in water and swells. As the gel expands inside the rigid cement paste, it creates pressure that can crack and disrupt the concrete. The damage comes from both osmotic hydraulic pressure and the swelling of the solid reaction products. Particle size also matters. Fine siliceous particles can cause...

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key terms and concepts

Learn how alkali-silica gel forms in concrete, why moisture makes it swell, and how low-alkali cement and pozzolans help reduce cracking.

How Moisture Cracks Alkali-Reactive Concrete | JoVE K12

JoVE Core Civil Engineering provides a foundational exploration of essential construction materials in civil engineering, focusing on bricks, stones, steel, wood, and concrete through easy-to-understand animated videos. This collection covers the properties, manufacturing processes, and practical applications of these materials. Additionally, the accompanied science-in-action videos demonstrate the research experiments performed in laboratories.

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How Moisture Cracks Alkali-Reactive Concrete

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