Name: concrete transition zone and its porosity
Uploaded: 2024-11-07T14:48:31.000Z
Duration: 1 min 28 s
Channel: JoVE

La zona di transizione, che si trova nel punto in cui l'aggregato incontra la pasta cementizia, è più porosa e, di conseguenza, più debole della pasta idrata oltre la zona di transizione.
In primo luogo, quando il calcestruzzo viene appena compattato, i film d'acqua circondano le particelle di aggregato più grandi, il che può comportare un maggiore rapporto acqua-cemento vicino a questi aggregati rispetto alle aree più lontane.
Successivamente, all'interno della matrice del calcestruzzo, ioni come calcio, solfato, ossidrile e alluminato provenienti dalla dissoluzione di vari composti precipitano per formare ettringite e idrossido di calcio.
Vicino agli aggregati grossolani, dove il rapporto acqua-cemento è elevato, questi composti cristallizzano in strutture più grandi e più porose di quelle della pasta cementizia più densa.
L'idrossido di calcio spesso cristallizza in strati orientati simili a piastre, tipicamente perpendicolari alle superfici dell'aggregato, contribuendo alla porosità complessiva.
Infine, con l'avanzare dell'idratazione, lo spazio tra i grandi cristalli iniziali di ettringite e idrossido di calcio inizia a riempirsi con un idrato secondario di silicato di calcio scarsamente cristallizzato, insieme a cristalli più piccoli di ettringite e idrossido di calcio.
Questa fase di riempimento secondario contribuisce a creare una zona di transizione interfacciale più densa e forte.

zona di transizione

Transition Zone

La zona di transizione nel calcestruzzo è un'area critica in cui l'aggregato incontra la pasta di cemento, ed è caratterizzata da una porosità e una debolezza distinte rispetto al materiale circostante. L'adesione attorno agli aggregati è dovuta principalmente alle forze di Van Der Waals. I vuoti all'interno di questa zona influenzano la sua robustezza; inizialmente, è meno durevole della malta circostante a causa dei vuoti più grandi. Quando il calcestruzzo viene compattato, un rapporto acqua-cemento più elevato vicino agli aggregati porta alla formazione di pellicole d'acqua attorno a loro, influenzando la porosità della zona. Quest'area vede la cristallizzazione di composti come calcio, solfato, idrossile e alluminato in ettringite e idrossido di calcio, che tendono a formare strutture più grandi e porose vicino agli aggregati grossolani a causa dell'elevato rapporto acqua-cemento. Queste strutture, in particolare l'idrossido di calcio, cristallizzano in strati orientati simili a piastre, comportando un ulteriore aumento della porosità.


 Nel tempo, con il progredire dell'idratazione, i cristalli inizialmente grandi di ettringite e idrossido di calcio iniziano a essere circondati da una matrice più densa di silicati idrati di calcio, o C-S-H secondario finemente cristallizzato, insieme a cristalli più piccoli degli stessi composti, portando a una densificazione della zona di transizione e a un aumento della sua resistenza.

Zona di transizione

La zona di transizione, che si trova dove l'aggregato incontra la pasta cementizia, è più porosa e, di conseguenza, più debole della pasta idrata oltre la pasta di transizione ...

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Strength of Concrete

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concrete transition zone and its porosity

The concrete transition zone is the thin region where aggregate meets cement paste. It has more pores than the surrounding material, so it is usually weaker at first. The bond around the aggregate comes mainly from Van der Waals forces, which are weak attraction forces between particles.

When concrete is compacted, this zone often has a higher water-cement ratio near the aggregate. That extra water can form thin water films around the coarse aggregate. These films increase porosity and help create larger voids in the transition zone.

As the cement hydrates, compounds that contain calcium, sulfate, hydroxyl, and aluminate crystallize into ettringite and calcium hydroxide. Near the coarse aggregate, these crystals can grow larger and form more porous structures because of the higher water-cement ratio. Calcium hydroxide can also form oriented, plate-like layers that add to the porosity.

Over time, hydration continues and the transition zone becomes denser. The early large crystals of ettringite and calcium hydroxide are surrounded by a finer, secondary C-S-H matrix, or calcium-silicate-hydrate. Smaller crystals of the same compounds also appear, and this densification increases the strength of the zone.

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transition zone

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key terms and concepts

Learn how the concrete transition zone forms, why it starts with larger voids, and how hydration and C-S-H make it denser and stronger over time.

Concrete Transition Zone and Its Porosity | 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.

civil engineering

Concrete Transition Zone and Its Porosity

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