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

Aggregates naturally contain siliceous minerals that react with the alkaline hydroxides from cement alkalis.
The alkali-silica gel formed attracts water and swells, which is constrained by the cement paste around it, leading to internal pressure, causing the cement paste to crack and disrupt, evidenced by expansion.
Expansion is partly due to the alkali-silica reaction&#39;s osmotic hydraulic pressure and swelling pressure of solid products.
The size of the siliceous particles influences the reaction rate, with fine particles causing expansion in weeks, while larger ones may take years to cause damage.
The progression of the alkali-aggregate reaction is affected by factors like aggregate porosity, cement alkali content, water in the paste, and cement paste permeability.
This reaction occurs mainly on concrete&#39;s surface under continuous moisture or during wetting and drying cycles, particularly at temperatures between 50 and 100 degrees Fahrenheit.
If avoiding harmful cement-aggregate combinations is difficult, incorporating a pozzolan and at least 30 percent limestone coarse aggregate by mass is advisable. Using low-alkali cement also helps reduce the alkali aggregate reactions.

alkali-aggregaat reactie in beton

Alkali Aggregate Reaction in Concrete

De alkali-aggregaat reactie in beton omvat natuurlijke siliciumhoudende mineralen in aggregaten die reageren met alkalische hydroxiden afkomstig van cement alkaliën. Deze reactie vormt een alkali-silica gel die water absorbeert, opzwelt en in volume toeneemt, dat wordt ingesloten door de omringende cementpasta, waardoor interne druk ontstaat die het beton doet barsten en verstoren. De mate van uitzetting en schade kan gedeeltelijk worden toegeschreven aan de osmotische hydraulische druk van de alkali-silicareactie en de zwellingsdruk van de gevormde vaste producten.

De reactiesnelheid wordt beïnvloed door de grootte van de silicium deeltjes, fijnere deeltjes kunnen binnen enkele weken uitzetting veroorzaken, terwijl grotere deeltjes jaren nodig kunnen hebben om soortgelijke schade te vertonen. Factoren zoals de porositeit van de aggregaten, het alkalige gehalte van het cement, de hoeveelheid water in de pasta en de permeabiliteit van de cementpasta beïnvloeden allemaal de voortgang van deze reactie. Deze reactie vindt doorgaans plaats op het oppervlak van beton onder continue vochtigheid of tijdens bevochtigings- en droog cycli, vooral bij temperaturen tussen 10 en 37,78 graden Celsius. Om deze reacties te verzachten, is het raadzaam om cement met een laag alkalige gehalte te gebruiken, een puzzolaan op te nemen en ten minste 30 procent kalksteengrof aggregaat per massa in de betonmix te gebruiken.

Alkali-aggregaat reactie in beton

Aggregates naturally contain siliceous minerals that react with the alkaline hydroxides from cement alkalis.
The alkali-silica gel formed attracts water ...

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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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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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