What percentage of alkalis should restricted aggregates contain when used with cement or a combination including fly ash?

Restricted aggregates used in concrete mixtures, especially when combined with cement or additives such as fly ash, must have low levels of alkalis to minimize the risk of alkali-silica reaction (ASR). This reaction can lead to serious durability issues in concrete, such as cracking and spalling, if reactive aggregates are used in combination with high-alkali cements.

The specified limit of 0.6% or less for alkali content in restricted aggregates is critical because it helps ensure that the concrete's resistance to ASR is maintained. Keeping the alkali levels below this threshold lowers the potential for expansive reactions between the alkalis in the cement and reactive silica within the aggregates. This standard is informed by empirical data and research that have demonstrated a clear link between higher alkali levels in aggregates and the increased likelihood of harmful ASR occurrences.

In contrast, the other percentages mentioned would either allow higher alkali levels in the aggregates than is considered safe and manageable or exceed recommended thresholds, which could contribute to increased risks of ASR and subsequent concrete degradation. Therefore, understanding the specification of 0.6% or less is essential for construction practices aimed at ensuring the longevity and performance of concrete structures.