11.5
Alkali Aggregate Reaction in Concrete
The alkali-aggregate reaction in concrete involves natural siliceous minerals in aggregates reacting with alkaline hydroxides derived from cement alka…
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'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'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.
View the full transcript and gain access to JoVE Core videos
Q1: What causes alkali-aggregate reaction in concrete?
Alkali-aggregate reaction occurs when siliceous minerals naturally present in aggregates react with alkaline hydroxides from cement alkalis. This reaction forms an alkali-silica gel that absorbs water and swells. The swelling is confined by surrounding cement paste, creating internal pressure that cracks and disrupts the concrete structure.
Q2: How does particle size affect the rate of alkali-silica reaction?
Siliceous particle size significantly influences reaction progression. Fine particles cause expansion within weeks, while larger particles may take years to produce similar damage. This variation in reaction rate depends on the surface area available for chemical interaction between the aggregate and cement alkalis.
Q3: What factors control the progression of alkali-aggregate reaction?
The alkali-aggregate reaction's progression is affected by aggregate porosity, cement alkali content, water present in the paste, and permeability of concrete. These factors work together to determine how quickly and severely the reaction develops, influencing the timeline and extent of concrete damage.
Q4: Under what environmental conditions does alkali-aggregate reaction occur?
Alkali-aggregate reaction primarily occurs on concrete surfaces under continuous moisture or during wetting and drying cycles. The reaction is most active at temperatures between 50 and 100 degrees Fahrenheit. These conditions provide the moisture and temperature environment necessary for the silica-alkali interaction to proceed.
Q5: What mechanisms cause concrete expansion from alkali-silica gel?
Expansion results from two mechanisms: osmotic hydraulic pressure and swelling pressure of solid products formed during the reaction. The alkali-silica gel absorbs water and expands, but confinement by cement paste creates internal stresses. These pressures accumulate until they exceed the concrete's tensile strength, causing cracking.
Q6: How can alkali-aggregate reaction be prevented in concrete?
Prevention strategies include using low-alkali cement to reduce available alkalis for reaction. Incorporating a pozzolan and at least 30 percent limestone coarse aggregate by mass also mitigates the reaction. These measures either reduce reactivity or dilute the harmful cement-aggregate combinations that trigger expansion.
Q7: Why does alkali-silica gel cause concrete to crack?
Alkali-silica gel attracts water and swells, increasing in volume. Since the gel is confined within the cement paste matrix, this expansion generates internal pressure. When internal pressure exceeds the concrete's tensile strength, the cement paste cracks and disrupts, leading to visible expansion and structural damage.