|FHWA > Engineering > Pavements > Concrete > Alkali-Silica Reactivity (ASR)|
Alkalis (Na, K, and Ca) are imperative to ASR. The Na+ and K+ ions combine with the "freed" silica to form ASR gel; and the Ca++ ions play a key role in the expansion of the gel (How exactly Ca++ effects gel expansion is unclear, but there are many investigations that suggest the absence of Ca++ ions would cause the gel to be non-expansive). Without a sufficient supply of Na+ and K+ ions, the pore solution pH drops, the solubility of silica is reduced, and ASR gel formation hindered. Without Ca++ ions, the gel would not expand and deleterious deterioration would not occur.
Alkalis in cement
Figure A2a.F1. Alkalis in Cement are Represented in an Oxide Analysis Table.
In order to calculate the total alkali contribution of cement, the amount of K2O is converted to "equivalent sodium" by way of multiplying the reported amount by 0.658 (the ratio of molecular weight of K2O to Na2O) and combining it with the Na2O amount. The alkali equivalent, or equivalent soda, is calculated using the following equation:
Alkali Equivalent or "Equivalent Soda":
Na2Oe = Na2O + .0658 K2O
In 1995, Gebhardt showed that for one type of cement the chemistry could be very different depending on the manufacturer. He showed that for 69 North American Type I cements tested, just over half had alkali contents less than 0.60%. The other half had greater alkali contents.
In 1940 and 1952, Stanton showed that alkali content influenced the amount of expansion measured in his mortar bar testing method. He found that the more alkalis in a cement, the greater the expansion. The graph in Figure 5.9 shows Stanton's results. This work defined the concept of "low-alkali cement". It is still generally believed today (wrongly so) that the use of such cement only is a sufficient measure to avoid damaging ASR. Effective measures in avoiding ASR are discussed in the section titled Controlling ASR.
Alkalis in concrete
Although alkalis supplied by cement play an important role, it is now fairly well established that it is the alkali content in the concrete and not just the alkali content of the cement that influences the risk of damaging reaction for a particular aggregate. The concrete alkali content is calculated as follows:
Work done in Canada (Figure 5.12) shows that after two years, concrete containing alkalis at less than 3.0 kg/m3 (5lbs/yd3) had the potential of staying below the critical threshold limit set by Canadian Standards Association (CSA). Expansions of concrete with greater alkali content were well above the threshold limit and could potentially result in deleterious expansion in the field. The threshold alkali content - above which deleterious expansion occurs - varies with the type of reactive aggregate.